Functional specializations within the tectum defense systems of the rat

Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298 www.elsevier.com/locate/neubiorev

Review

Functional specializations within the tectum defense systems of the rat L.C. Schenberga,*, R.M.F. Po´voaa, A.L.P. Costaa, A.V. Caldellasa, S. Tufikb, A.S. Bittencourta a

Departamento de Cieˆncias Fisiolo´gicas – Centro Biome´dico (Edifı´cio do Programa de Po´s-Graduac¸a˜o em Cieˆncias Fisiolo´gicas), Universidade Federal do Espı´rito Santo, Av. Marechal Campos 1468 (Maruı´pe), 29043-125, Vito´ria, ES, Brazil b Departamento de Psicobiologia, Escola Paulista de Medicina, Universidade Federal de Sa˜o Paulo, Sa˜o Paulo, SP, Brazil

Abstract Here we review the differential contribution of the periaqueductal gray matter (PAG) and superior colliculus (SC) to the generation of rat defensive behaviors. The results of studies involving sine-wave and rectangular pulse electrical stimulation and chemical (NMDA) stimulation are summarized. Stimulation of SC and PAG produced freezing and flight behaviors along with exophthalmus (fully opened bulged eyes), micturition and defecation. The columnar organization of the PAG was evident in the results obtained. Defecation was elicited primarily by lateral PAG stimulation, while the remaining defensive behaviors were similarly elicited by lateral and dorsolateral PAG stimulation, although with the lowest thresholds in the dorsolateral column. Conversely, the ventrolateral PAG did not appear to participate in unconditioned defensive behaviors, which were only elicited by high intensity stimulation likely to encroach on adjacent regions. In the SC, the most important differences relative to the PAG were the lack of stimulation-evoked jumping in both intermediate and deep layers, and of NMDA-evoked galloping in intermediate layers. Therefore, we conclude that the SC may be only involved in the increased attentiveness (exophthalmus, immobility) and restlessness (trotting) of prey species exposed to the cues of a nearby predator. These responses may be distinct from the full-blown flight reaction that is mediated by the dorsolateral and lateral PAG. However, other evidences suggest the possible influences of stimulation schedule, environment dimensions and rat strain in determining outcomes. Overall our results suggest a dynamically organized representation of defensive behaviors in the midbrain tectum. q 2005 Elsevier Ltd. All rights reserved. Keywords: Periaqueductal gray; Superior colliculus; NMDA; Freezing; Flight; Micturition; Defecation; Wistar rats; Wild rats

Contents 1. 2. 3. 4. 5. 6. 7. 8. 9.

The defence reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The periaqueductal gray matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The superior colliculus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The threshold logistic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The nature of tectum-evoked defensive behaviors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Topography of electrically-evoked components of defence reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NMDA receptor mediated glutamate transmission within the periaqueductal gray and superior colliculus . . . . . . . . . . . . . . Topography of NMDA-evoked single components of defence reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functional specializations within the tectum defense systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1280 1280 1281 1283 1284 1288 1289 1291 1291

Abbreviations AC, anterior cingulate area; cGMP, cyclic guanosine monophosphate; CnF, cuneiform nucleus; D, defecation; DLPAG, dorsolateral periaqueductal gray; DMPAG, dorsomedial periaqueductal gray; E, exophthalmus; EAA, excitatory amino acids; ED50, median effective dose; F50, median effective frequency; G, galloping; I, immobility; I50, median effective intensity; J, jumping; LC, locus coeruleus; LPAG, lateral periaqueductal gray; NADPH, reduced nicotinamide adenine dinucleotide phosphate; NMDA, n-methyl-D-aspartic acid; nNOS, neuronal nitric oxide synthase; NO, nitric oxide; PAG, periaqueductal gray matter; SC, superior colliculus; T, trotting; VLPAG, ventrolateral periaqueductal gray. * Corresponding author. Tel.:C55 27 3335 7332; fax: C55 27 3335 7330. E-mail address: [email protected] (L.C. Schenberg). 0149-7634/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.neubiorev.2005.05.006

1280

10.

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294

The role of periaqueductal gray matter (PAG) and superior colliculus (SC) in the expression of the rat defensive behaviors was appraised in the present study. In contrast to the single-neuron approach most often employed in the neuroethology of lower species (Ewert, 1985; Ingle and Crews, 1985), or the present-day methods of c-fos immunohistochemistry used in higher vertebrates (Canteras and Goto, 1999; Dielenberg et al., 2001), mechanisms underlying the unconditioned defensive behaviors were primarily studied through the traditional methods of brain stimulation and lesion. For that reason, this study reviews the data on electrically-and chemicallyevoked defensive behaviors, particularly, those accomplished using the technique of threshold logistic analysis (Bittencourt et al., 2004, 2005). Consequently, the review is heavily focused on PAG-and SC-evoked freezing and flight behaviors of the rat. Apart from briefly discussing few seminal studies, we did not provide a detailed appraisal of the classic literature on amygdala- and hypothalamusevoked defensive aggression in cats (for a review on this issue, see Gregg and Siegel, 2001).

1. The defence reaction Hess and Bru¨gger (1943) were the first to show that the electrical stimulation of the hypothalamus of the cat produces a complex set of behaviors comprising an immobile aggressive display with slight hunching of the back, flattening of the ears, teeth baring, hissing, growling, unsheathed claws, defecation, piloerection and marked mydriasis. This multifaceted but integrated set of responses was called the ‘affective defense reaction’ (affektiven Abwehrreaktion). Hess also pointed out that these responses were preceded by arousal and progressive restlessness and that with stronger stimulation could culminate in either attack or flight (Hess, 1954). However, the lack of appropriate methods for measuring these behaviors hindered their functional analysis. Therefore, Hess’ painstaking mapping of the cat brain stem was mostly descriptive and also had fairly low spatial resolution (see plate VIII of Hess, 1947). Fernandez de Molina and Hunsperger (1962) presented the first functional model of the brain defense system and this was a major breakthrough. Combining both stimulation and lesion techniques, these authors presented evidence that the cat defense system was made up of two ‘core zones’ controlling the immobile aggressive display. These were the perifornical hypothalamus (already described by Hess and Bru¨gger (1943)), and the PAG. According to these authors, the core zones were surrounded by a more diffuse ‘outer

flight zone’ extending from the preoptic area to the caudal midbrain, and including the SC. More importantly, because lesions of PAG attenuated both the amygdala- and hypothalamus-evoked defensive reactions, these authors suggested that the rostrally-evoked defensive behaviors were mediated through the PAG. These studies provided a still very influential model of a PAG-centered brain defense system.

2. The periaqueductal gray matter Research on intracranially-induced defensive behaviors was substantially advanced in the early 1980s with the introduction of neuron-selective stimulation with excitatory amino acids (EAA) (Bandler, 1982; Bandler and Carrive, 1988; Bandler and Depaulis, 1988). However, except for the prominent role of PAG in the expression a wide variety of both unconditioned and conditioned defensive behaviors (Blanchard et al., 1981; Ledoux et al., 1988), Hunsperger’s model of the cat defense system was not corroborated in the rat. Thus, while the microinjection of kainate into the rostral PAG was reported to produce postures resembling the defensive aggression of rats (boxing), the full-blown attack observed in cats were never reported in rats (Bandler and Depaulis, 1988). Also, instead of the flight ‘outer zone’, trotting, galloping and jumping were reliably produced by low-intensity stimulation of the PAG ‘core zone’ properly (Bandler and Depaulis, 1991; Schenberg et al., 1990; Sudre´ et al., 1993). Yet, Hunsperger’s model of cat defense system has been frequently adopted as an underlying model in rat studies. More recently, the PAG has been conceptualized as four functionally specialized columns extending for varying distances along the aqueduct, namely, the dorsomedial (DMPAG), dorsolateral (DLPAG), lateral (LPAG) and ventrolateral (VLPAG) columns (Bandler and Depaulis, 1991; Bandler and Keay, 1996; Carrive, 1993). The PAG columnar parcellation was firmly established by tracttracing, neurochemical and immunohistochemical methods (Bandler et al., 1991; Bandler and Keay, 1996; Carrive, 1991, 1993; Floyd et al., 2000; Onstott et al., 1993). According to this parcellation, the defensive behaviors (freezing, flight or fight) and aversion-related responses (switch-off behavior) were ascribed to the DMPAG, DLPAG and LPAG (usually named the ‘dorsal’ PAG), while the quiescent behavior (Bandler and Depaulis, 1991; Bandler and Keay, 1996; Carrive, 1993), opioid-dependent analgesia and rewarding properties (self-stimulation) (Besson et al., 1991; Dennis et al., 1980; Schenberg and Graeff, 1978) were attributed to the VLPAG.

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

Similarly, the hemodynamic correlates of defensive behavior (hypertension, tachycardia) and recuperative-like behaviors (hypotension, bradicardia) were ascribed to the dorsal and ventrolateral PAG, respectively (Carrive, 1991; Carrive and Bandler, 1991; Lovick, 1991). Bandler and Depaulis (1991) also suggested that the ‘forward’ and ‘backward’ avoidance behaviors are represented in caudal and rostral districts of LPAG, respectively. The connections of PAG are most often evident within the brainstem. The main DLPAG afferents come from the dorsal premammillary nucleus of hypothalamus, the deeper layers of the SC, the cuneiform nucleus (CnF), and nucleus prepositus hypoglossi (Bandler et al., 1991; Canteras and Swanson, 1992; Redgrave and Dean, 1991). However, Floyd et al. (2000) showed that the DLPAG is also the target of projections from the orbitomedial prefrontal cortex, particularly, from caudal sectors of prelimbic area and from both the dorsal and ventral anterior cingulate (AC) areas which are known to receive visual inputs (Vogt and Miller, 1983). In turn, the DLPAG projects to the deeper layers of the SC (Redgrave and Dean, 1991), the CnF and the locus coeruleus (LC) (Sandku¨hler, 1991; Sandku¨hler and Herdegen, 1995). The functions of the DLPAG have remained elusive until relatively recently (Bandler et al., 1991). In contrast to the lack of spinal afferents of DLPAG, the LPAG receives projections from the spinal trigeminal nucleus and from both enlargements of the spinal cord (Bandler et al., 1991; Blomqvist and Craig, 1991). Additional afferents arise from the rostral nuclei of medial hypothalamus, and the central amygdala (Floyd et al., 2000; Shipley et al., 1991; Veening et al., 1991). The efferents of these columns are quite different inasmuch as the LPAG projects extensively to the ventromedial, ventrolateral and dorsal medulla (Bandlers et al., 1991). These projections have been associated with the increased somatic (ventromedial medulla) and sympathetic (ventrolateral medulla) activities associated with the defensive behaviors (Bandler et al., 1991). The VLPAG extends for the entire length of the caudal third of PAG. Although this column has the same pattern of connections of the adjoining LPAG (Bandler et al., 1991), injections of EAA into the VLPAG were reported to evoke both a decrease in somatomotor activity and hypotension, i.e. effects which are the opposite of those evoked from the LPAG (Bandler et al., 1991; Carrive, 1991; Carrive and Bandler, 1991; Lovick, 1991). However, VLPAG-evoked hypotension has not been confirmed in awake rats (Morgan and Carrive, 2001). As already mentioned, this column has been related to quiescent behaviors, to opioid analgesia and to the rewarding properties of PAG stimulation. Intrinsic connections are of paramount importance in the mapping of behavioral functions in the distinct columns of the PAG. Studies using small-volume injections (50 nl) of bicuculline (a GABA antagonist) into separate columns of PAG followed by c-fos immunohistochemistry revealed a rather specific pattern of connections (Sandku¨hler, 1991;

1281

Sandku¨hler and Herdegen, 1995). Thus, whereas the stimulation of dorsal columns produced an extensive labeling along the entire rostrocaudal length of PAG, encroaching the CnF, microinjections into the ventral districts produced a much more restricted labeling (Sandku¨hler, 1991; Sandku¨hler and Herdegen, 1995). Moreover, whereas the stimulation of DMPAG produced an intense bilateral labeling of the PAG, injections into the rostral third of DLPAG produced the ipsilateral labeling of DMPAG and DLPAG at the level of injection and throughout the ipsilateral extension at more caudal levels. The LC was densely labeled as well, confirming data of other studies (Cameron et al., 1995). In contrast, stimulation of the VLPAG labels only the adjoining reticular formation and a restricted region of rostral LPAG, but not areas caudal to the injection site (Sandku¨hler, 1991; Sandku¨hler and Herdegen, 1995). Therefore, there seems to be an intimate connection between the DMPAG and the DLPAG, as well as between both of these columns and (1) the deeper layers of SC, (2) the caudal LPAG, (3) the CnF and (4) the LC. Projections to the latter nuclei may be of paramount importance in defensive behaviors. Indeed, the CnF is a wedge-like structure below the inferior colliculus that is believed to be part of the so called ‘mesencephalic locomotor region’ (Armstrong, 1988; Garcia-Rill and Skinner, 1988). In turn, the LC has been suggested as an important substrate of both attentional and emotional states (Aston-Jones, 2000; Bunney and Aghajanian, 1976; Servan-Schreiber et al., 1990). The possible functional importance of these PAG projections are indicated by the threshold logistic analysis of PAG-evoked defensive behaviors that are reported below.

3. The superior colliculus The main visual organ of lower vertebrates is the optic tectum, a midbrain structure that receives retinal input and contributes to the generation of motivated behaviors (Ewert, 1985; Ewert et al., 1999; Patton and Grobstein, 1998). In mammals, the tectum comprises the mesencephalic structures dorsal to the aqueduct, i.e. the dorsal half of the PAG and both colliculi. As far as the SC is concerned, the tectum retained a prominent role in most, if not all, visually guided behaviors of mammals. In addition, the SC has been suggested to participate in both the defensive behavior of the rat (Blanchard et al., 1981; Dean et al., 1988b and Mitchell, 1988; Redgrave and Dean, 1991; Sahibzada et al., 1986; Sudre´ et al., 1993; Vargas et al., 2000) and in the recognition of fearful expressions in humans (de Gelder et al., 1999; Linke et al., 1999; Morris et al., 1999, 2001, 2002; Vuilleumier et al., 2003). Nevertheless, the exact contribution of SC to the production and organization of defensive behaviors remains uncertain. The SC is divided in 7 layers, namely, zonal layer (lamina I), superficial gray layer (lamina II), optic layer

1282

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

(lamina III), intermediate gray layer (lamina IV), intermediate white layer (lamina V), deep gray layer (lamina VI), and deep white layer (lamina VII) (Huerta and Harting, 1984; Kanaseki and Sprague, 1974). Layers above and below the intermediate gray layer are referred to as the ‘superficial’ (I-III) and ‘deeper’ (IV-VII) layers of SC, respectively. However, in the present study the ‘intermediate’ (ILSC) and ‘deep’ (DLSC) layers of SC refer to both strata (white and gray) of these levels. Extensive investigation of SC suggests that its organization follows a general plan in vertebrate species (Stein, 1981; Stein and Gaither, 1983). The superficial gray and upper optic layers are primarily visual, receiving massive inputs from retinal ganglion cells, primary visual cortex and parabigeminal nuclei (Harvey and Worthington, 1990; Kasper et al., 1994; Linden and Perry, 1983; Serizawa et al., 1994). In turn, they project to the deeper layers, the thalamus (mainly to the lateral posterior/pulvinar complex) and the DLPAG (Doubell et al., 2003; Lane et al., 1993; Lee et al., 1997; Rhoades et al., 1989; Stepniewska et al., 2000). In contrast, visual inputs of the ILSC arise predominantly from the secondary visual cortices 18/18a (Harvey and Worthington, 1990; Serizawa et al., 1994). In fact, Benzinger and Massopust (1983) showed that the area 18 projects to the four deepest laminae of the SC and, most notably, the DLPAG. The ILSC are also the recipient of somatosensory and auditory inputs (Cadusseau and Roger, 1985; Edwards et al., 1979; Stein et al., 1975; Sterbing et al., 2002; Telford et al., 1996; Wallace et al., 1996). Moreover, the ILSC and DLSC are targeted by somatosensory inputs from spinotectal afferents involved in turning movements of the head and body, as well as by collaterals of trigeminothalamic projections of multi-whisker sensory neurons (Grunwerg and Krauthamer, 1990; Veinante and Deschenes, 1999; Wang and Redgrave, 1997). Nonetheless, SC laminae become progressively less conspicuous as one moves from the most superficial layer to the deepest one. Accordingly, Edwards (1980) pointed out many years ago that the distinct reticular properties of the deeper collicular layers sharply distinguish them from the superficial ones whose structural and functional characteristics are more like those of a sensory structure. In the same vein, some authors suggested more recently that the deep gray layer of SC and the dorsal PAG should be considered as a single structure split up by the deep white layer bordering the PAG (Holstege, 1991). In spite of some species variability (Nudo et al., 1993), the upper and lower visual fields are mapped onto the medial and lateral sectors of SC, whereas the nasal and temporal fields are represented in the rostral and caudal districts, respectively (Lane et al., 1971, 1973; Nudo et al., 1993). Retino and corticotectal projections as well as the auditory and somatosensory inputs are spatially aligned across the collicular layers, with a rough overlap of visual and auditory meridians (Dra¨ger and Hubel, 1975; King and Hutchings, 1987; King and Palmer, 1983; Stein et al.,

1975**; Stein and Clamann, 1981; Sterbing et al., 2002). Consequently, each colliculus integrates visuotopic, audiotopic and somatotopic information of the contralateral side of body and space. Neurons of deeper layers are thus in a favorable position to provide the sensory synthesis that plays a crucial role in attentive and orienting behaviors that are universal components of defensive reactions. Indeed, neurons of these layers were shown to perform ‘multisensory integration’, i.e. a nonlinear enhancement (or inhibition) of responses from two sensory modalities (Stein, 1998; Stein et al., 1995, 2001; Wallace et al., 1998). These multisensory neurons are quite often motor units. Indeed, neurons of deeper collicular layers are the source of tecto-reticulo-spinal, tecto-cuneiform and tecto-pontine pathways controlling a multitude of motor responses (Huerta and Harting, 1984; King et al., 1996; Nudo et al., 1993; Redgrave et al., 1986, 1987, 1988). The tectoreticulo-spinal pathway arises from large somata of the lateral sectors of the ILSC (Huerta and Harting, 1984; Redgrave et al., 1986) and has been classically related to the control of rapid eye movements (saccades) and head/body turns (orienting reflex) that shift the line-of-sight (gaze) onto a visual target (Sparks, 1999). In turn, the tecto-cuneiform pathway is a largely uncrossed projection to the CnF (Huerta and Harting, 1984; Mitchell et al., 1988; Redgrave et al., 1987, 1988). The somata of neurons forming the tectocuneiform pathway are concentrated in the medial sectors of deeper collicular layers. Consequently, it was proposed that whereas the crossed pathway is involved with nonthreatening events in the lower visual field, e.g. those of foraging, the tecto-cuneiform pathway is most likely dedicated to events in the upper visual field, such as the sight of a distant predator (Redgrave et al., 1986, 1987, 1988; Redgrave and Dean, 1991). However, because rats must defend from both poisonous invertebrates (spiders and scorpions) and ground predators (carnivores and snakes) in nearby surroundings, the presumed defensive specialization of medial sectors can be questioned. On the other hand, the tecto-pontine pathway is an ipsilateral projection to the ventrolateral pons that appears to be involved in circling behavior (Buckenham and Yeomans, 1993; Dean et al., 1988a; DiChiara et al., 1982). Moreover, the deeper collicular layers relay information from the orofacial/vibrissae motor cortex to facial and trigeminal nuclei (Miyashita et al., 1994; Miyashita and Mori, 1995) and participate in the control of pinnae (Henkel and Edwards, 1978; Henkel, 1981; Stein and Clamann, 1981). These pathways are likely to be involved in defensive behaviors. Thus, while fast circling/ rotation is quite often the behavior of the cornered rat, SCmediated projections to the facial and trigeminal motor nuclei may be the substrate of defensive biting. In addition, pinnae movements are of paramount importance for soundlocation of both the predator and prey. While a decisive role for the PAG in the expression of defensive behaviors has been long established (see above),

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

conclusive evidence on the contribution of SC to defensive behaviors came much later with the publication of Blanchard’s influential studies on the defensive reactions of wild rats (Blanchard et al., 1981). These authors showed that wild rats were ‘tamed’ by lesions of PAG. Conversely, lesions of the SC that produced a marked deficit in the defensive reactions to an approaching experimenter had no effect in the vigor of escape to the tactile stimulation of the dorsum or vibrissae. Therefore, as far as the physical contact is concerned, neither the motivational, nor the executive mechanisms of defense were affected by lesions of SC. It remained unsolved, however, whether the SC is merely a relay of visual information processed elsewhere in the brain (e.g. lateral posterior/pulvinar complex), or if it is specifically involved in the attentional and motivational mechanisms towards a visual threat. Indeed, defensive responses to visual and physical contact could be processed separately through the SC and LPAG, respectively. Finally, the well known problems of specificity with electrolytic lesions do not allow strong conclusion to be made about a differential participation of the DLSC and DLPAG in defensive behaviors. Nonetheless, other evidence amassed over the last two decades suggest that the SC may indeed take an active part in the production of defensive behaviors. For instance, electrical and chemical stimulation of the SC are longknown to produce defensive behaviors in the rat (Dean et al., 1988b; Keay et al., 1990; Sahibzada et al., 1986; Schenberg et al., 1990; Sudre´ et al., 1993; Vargas et al., 2000). The question then arises whether the SC has the intrinsic capability of producing these behaviors (Redgrave and Dean, 1991) or if it merely relays sensory information to other key centers for the organization of defense, such as the PAG (Blanchard et al., 1981), the CnF (Mitchell et al., 1988), the LC (Cameron et al., 1995) or the amygdala (de Gelder et al., 1999; Ledoux et al., 1988; Morris et al., 1999, 2001, 2002; Vuilleumier et al., 2003).

4. The threshold logistic analysis Ever since the epoch-making report of Hess and Bru¨gger (1943), the lack of a clear quantitative approach has been a major drawback in the study of behaviors evoked by intracranial stimulation. Because brain structures can be involved in the encoding of many different responses, mapping studies should allow the description of the hierarchy of these behaviors across a range of stimulation intensities. On the other hand, the very possibility of a topographic representation of single components of defensive behaviors (e.g. immobility, trotting, galloping, jumping, attack, etc.) has been greatly overlooked, if not neglected. Thus, in spite of the recent data pertaining to this issue (Bittencourt et al., 2004, 200), there are many more questions than answers on this topic. For instance, it

1283

remains unclear whether different defensive behaviors are brought about by the activation of different structures or if the same structure can express different modes of functioning. Also, it is not known whether these responses are triggered altogether through a single structure but executed separately elsewhere in the brain, or if they are both triggered and executed through separate channels. Alternatively, several defensive behaviors could be the expression of the combined activity of diverse structures. Finally, we are just beginning to understand the processing of information that takes place upstream and downstream to tectum defense systems. The cellular and molecular substrates of brainstem defense machinery are even more uncertain than their topography. The results described below represent fifteen years of work by the authors using the threshold logistic analysis. The method was originally developed as a convenient way of modeling the probabilities of intracranially-evoked behaviors (Schenberg et al., 1990). Its basic assumption is that whenever a brain structure is involved in the production of a given behavior (executive or command function), stimulation of this structure with low-magnitude stimuli should produce the behavior in a stimulusdependent manner. On the other hand, low-intensity stimulation of a brain structure the function of which is merely modulatory might modify an ongoing behavior without producing it by its own. Therefore, the main criterion in defining the defensive repertoire of tectum structures was the stimulus-dependent elicitation of defensive behaviors with stimulation of low-magnitude. Consequently, responses requiring high magnitude stimulation were considered to be a by-product of the stimulation spreading to other regions and excluded from the likely repertoire of the stimulated regions. However, because the volume of neuronal depolarization is proportional to the stimulation intensity, a second important criterion was the production of a given behavior following the high-resolution stimulation with frequencyvarying square-wave pulses of fixed intensity. The failure to elicit a given behavior with this kind of stimulus was regarded as evidence enough for its exclusion from the likely repertoire of the structure under consideration. Yet, as discussed below, the simple elicitation of the behavior does not assure the participation of the stimulated structure. So far, this method has not been applied to the concomitant stimulation of two structures or more. Technically, the threshold logistic analysis is an extension of regression methods involving binary variables commonly employed in the determination of drug effective dose (ED50, LD50, etc) and relative potency (drug quantal assay). As such, it yields both the sigmoidal (probability distribution) and Gaussian-like (probability density) logistic functions of threshold responses. Moreover, it provides the unbiased estimates of median effective stimuli (m50GSE), in the present case, the intensity (I50), frequency (F50) and dose (ED50) which are expected to produce a given response in

1284

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

50% of the population. Because these medians are estimates of the logistic function, they may be higher than the cutoff stimulus if the response was presented by less than 50% of rats. In addition, the behavior hierarchy of a given structure was analyzed through the ratio of the median threshold of each response (e.g. I50, F50 or ED50 of immobility, galloping, etc) to the median threshold of least magnitude, in the present experiments, that of exophthalmus (I50E, F50E or ED50E). Accordingly, the threshold ratio, herein termed the ‘relative threshold’ (RTZI 50 /I 50E, F 50/F50E or ED50/ED50E), ranks all responses in function of exophthalmus. Likewise, the threshold ratio between structures may provide evidence of behavior topography. Details of these procedures can be found in our recent published studies (Bittencourt et al., 2004, 2005; Schenberg et al., 2002; Vargas and Schenberg, 2001). The precise quantification of intracranially-evoked behaviors is expected to help in the understanding of underlying neural mechanisms, in finding the regularities among different approaches and brain structures, and in limiting the range of stimulus magnitude. Yet, because intracranial stimuli are fixed variables, it is quite surprising that the regression analysis of intracranially-evoked behaviors was never employed before. Finally, the logistic threshold analysis emphasizes the critical role of thresholds in the timing of defensive reactions. In this regard, it is often unnoticed that whereas the behavioral features (duration, speed, force, amplitude, etc) express the vigor of an ongoing behavior, thresholds control the probability of a behavioral outcome. Therefore, the threshold logistic analysis provides the structure

‘preferential repertoire’ under a given set of internal as well external conditions.

5. The nature of tectum-evoked defensive behaviors Analysis of behavior probabilities supposes that the behavioral outcomes are both conspicuous and discrete. Yet, PAG- and SC-evoked behaviors have a perplexing wealth of labels. Perusal of the literature reveals ‘behaviors’ as varied as explosive or oriented flight, forward and backward avoidances, darting, running, jumping, circling, rotation, freezing, flinching, cringing, shuffling, shying, fighting, boxing, quiescence, motionless, alerting, orienting and approaching, among others. Ethologically-based descriptions are nevertheless scarce. Chosen descriptions have far reaching consequences in behavior interpretation. Thus, while trotting serves for both a long-range exploration (intra-territorial or migratory) and defensive purposes (shelter-seeking behavior), jumping may be either exploratory (climbing), defensive (escape or jump attack) or offensive (jump attack). Freezing (tense immobility plus exophthalmus) is also a multipurpose behavior. Therefore, while the immobile prey may pass unnoticed to a predator, immobility is also a conspicuous component of both defensive and non-defensive attentive behaviors. In contrast, rotation and galloping seem to be the strict defensive behaviors of a prey either cornered or chased, respectively. However, behavior functions are also dependent on both the species and context. Thus, immobility and galloping are also

Table 1 Ethogram of spontaneous and stimulus-dependent behaviors of a single rat in an open-field Sleeping Resting

Grooming Sniffing Scanning Walking Raising Mystacioplegia Exophthalmus Immobility

Trotting Galloping Jumping Rotation Defecation, Micturition

Quiescent horizontal posture with eyes closed, no sniffing activity and overall muscle relaxation as suggested by the flexion of the limbs and lowering of the trunk, head, neck and tail Quiescent horizontal posture with open or half-open eyes, reduced sniffing activity and muscle relaxation as suggested by the flexion of the limbs and lowering of head, trunk and tail. Otherwise, the head may be raised and the rat displays a ‘sphinx-like’ posture Self-directed behavior consisting of the repetitive manipulation of the fur of the head and trunk as well as the manipulation and/or licking of genitals. The rat displays a ‘sitting-like’ posture (upright posture on flexed hind limbs Bursts of olfactory exploration of the environment as indicated by nostril movements, vibrissae fluttering and orientation movements of the head Lateral displacements of the head suggesting a visually guided exploration of environment Slow locomotion of the rat with lowering of trunk and tail and out of phase stance and swing movements of contralateral limbs Upright posture with extended hind limbs Stimulus-induced arrest of ongoing vibrissae movements Term borrowed from medicine to name the eyeball protrusion and fully opened eyelid. The eyes take on a spherical shape and brilliant appearance suggestive of an increased entrance of light Overall behavioral arrest accompanied by increased muscle tonus as suggested by the extension of neck and/or limbs and raising of head, trunk and/or tail. Except for the visible tachypnoea, the rat looks like a ‘statue’ for periods as short as 3 s or lasting the whole stimulation trial (20 s). The sudden behavioral arrest may produce anomalous postures Fast locomotion with elevation of trunk and tail and out of phase stance and swing movements of contralateral limbs Running alternating stance and swing movements of anterior and posterior limb pairs Upward leaps directed to the border of the open-field Spinning body movements Ejection of feces and urine (specific behaviors, such as the search of a suitable place observed in dogs and cats, or feces concealment of cats, were not described in rats

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

etc). To our knowledge this is the only description of intracranially-induced behaviors based on the established patterns of quadrupedal locomotion, namely, walking, trotting and galloping (Armstrong, 1988; Garcia-Rill and Skinner, 1988). Further, it includes jumping, the physiology of which has been barely studied. The ethogram is also the outcome of successive improvements of our earlier catalogs (Schenberg et al., 1990; Sudre´ et al., 1993). Thus, whereas darting was excluded on purely technical grounds (the short periods of immobility and galloping were not recorded unambiguously), whisker paralysis (mystacioplegia) was added as a component of freezing. Improvements were also based on both the observation of wild rats in the field and laboratory, and on the distinct pharmacological sensitivity of intracranially-induced defensive behaviors (Schenberg et al., 2001, 2002; Vargas and Schenberg, 2001). These observations have lead to the splitting of running into trotting and galloping in our recent studies (Bittencourt et al., 2004, 2005). The threshold logistic analysis revealed a close relationship between the freezing and flight ‘threshold distances’ of wild rats to an approaching predator (Blanchard et al., 1990) (Fig. 1) and the immobility and galloping ‘threshold intensities’ of Wistar rats to intracranial stimuli, respectively (Fig. 2) (Bittencourt et al., 2004, 2005; et al., 1993). Indeed, the latter studies showed that tectum-evoked

Fig. 1. Defensive behavior as a joint function of prey-predator distance and the availability of an escape route. Redrawn from Blanchard et al. (1990).

key components of predator’s ambushing and chasing behaviors, respectively. The ethogram of present studies (Table 1) is an attempt to describe the behavior of a single rat in the open-field as objectively as possible, sometimes merging single responses into high-order behaviors (e.g. sleeping, resting, grooming, etc), sometimes splitting-down complex behaviors, such as freezing and flight, into clear-cut components (exophthalmus, immobility, trotting, galloping, jumping, Immobility

Running

Response Probability ( r/n )

1.0

0.5

0.5

0

0

2.4

2.4

1.2

1.2

30

Jumping Random Intensities

Response Frequency ( df/dx )

Increasing Intensities 1.0

10

1285

70

0

10

30

70

10

30

70

0 10

30

70

Log-Intensity (µA) Fig. 2. Threshold curves of hierarchical (left) and random (right) patterns of somatic defensive behaviors elicited by electrical stimulation of the same tectum structure with either stepwise increasing or random intensities, respectively (0–55 mA, 60 Hz, AC). Increasing and random intensities were applied in intervals of 5 min and 24 h, respectively. Sigmoidal curves are the best-fitting logistic functions of responses across accumulated frequencies. Bell-shaped curves are the probability density functions of corresponding logistic distributions. Vertical dotted lines indicate the median thresholds (I50). Running represents the accumulated frequency of either the galloping or trotting response of the least magnitude threshold. r, responders, n, number of stimulated rats. Redrawn from Sudre´ et al. (1993).

1286

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

∆ Blood Pressure (mmHg)

freezing behavior was invariably produced with the least magnitude stimuli much in the same way as rodents and hares freeze when mildly alarmed by distal threats (Blanchard et al., 1990; Kingdon, 1974). Blanchard et al. (1990) also showed that wild rats present jump attacks if physical contact is imminent and escape is not available (Fig. 1). Although the albino rat rarely displays jump attacks, PAG-evoked jumping is a closely related behavior evoked at higher intensities. Remarkably, freezing and flight opposite motor patterns were shown to be produced by small variations in the intensity (10–15 mA) of a sine-wave pulse applied through a single electrode in either the PAG or SC (Schenberg et al., 1990; Sudre´ et al., 1993; Vargas et al., 2000) (Fig. 2). Notably, these behaviors were accompanied by distinct cardiovascular patterns (Schenberg et al., 1993) that reproduce the hemodynamic changes seen in corresponding natural behaviors (Adams et al., 1971; Hofer, 1970) (Fig. 3). The capability of tectum structures to integrate both freezing and flight, as well as the respective cardiovascular patterns, is a remarkable but barely studied phenomenon. Immobility, running (trotting and/or galloping) and jumping were found to be produced in either sequential or random patterns, provided that the intensities were Subliminal Intensity Freezing Threshold Flight Threshold

40

* 20

*

*

*

*

*

*

30

45

60

0 –10 0

15

*

60 ∆ Heart Rate (bpm)

*

30

0

–30 0

15 30 45 Stimulus Duration (s)

60

Fig. 3. Time-course of blood-pressure and heart-rate changes seen throughout 1 min of electrical stimulation of the dorsal PAG of nonanesthetized freely moving rats. The gray area represents the mean values (GSEM) of sham-stimulation trials. Symbols depict either a subliminal stimulus (;) or the threshold intensities of freezing (C) and flight (&) reactions. The distinct hemodynamic patterns of electrically-induced freezing and flight reactions reproduce those seen in natural behaviors. *P!0.05, significantly different from both sham and subliminal stimuli (repeated measures ANOVA). Redrawn from Schenberg et al. (1993).

presented in a stepwise increasing order or randomly, respectively (Sudre´ et al., 1993) (Fig. 2). These patterns are easily recognized in the wild. Thus, whereas the sequential output reproduces the defensive tactics of the rat against a slowly approaching predator (distal threat) (Blanchard et al., 1990), the random output is most likely the behavior of a prey that was either driven into a corner or surprised by an ambush attack (proximal threat) (Driver and Humphreys, 1988). Random output is typically a ‘protean behavior’, i.e. an unpredictable sequence of responses that bewilders the predator (Driver and Humphreys, 1988; Eibl-Eibesfeldt, 1975). For example, cornered rats may alternate between freezing, rotation, flight and fight. Also, fleeing in the wild is frequently terminated by sudden cryptic immobility, particularly, when the hunted animal is less swift than its pursuer (Curio, 1976; Driver and Humphreys, 1988). The prey may also lie down until the predator comes closer, and then get up, fresh as possible, to resume a headlong flight (Kingdon, 1982). PAG-evoked defensive responses have been also characterized as either stereotyped (robot-like) or explosive (chaotic) behaviors. However, besides never colliding against the wall, PAG-stimulated rats present jumps directed to the edges of an open-field that very often result in successful escapes. These observations suggest a powerful control of these behaviors by environmental stimuli. To confirm this, experiments were carried out in which rats with electrodes in the PAG were stimulated in four environments, namely, 50 or 20 cm diameter arenas with or without a roof placed 25 cm high. Remarkably, narrowing of the arena lead to the replacement of running by a fast rotation behavior absent in the large arena (Fig. 4). These data suggest that narrowing of the arena shift the behavior control from tecto-cuneiform (running) to tectopontine (circling/rotation) pathways. Moreover, because thresholds for rotation were much higher than those for running, rats remained frozen to a wider range of stimuli, apparently, ‘refraining’ from escape. Interestingly, wild rodents may also freeze for up to 60 min when confined in a small box with a snake (Hofer, 1970). Because snakes lose contact with an immobile prey (Curio, 1976), freezing retains a survival value even after the detection of the rat by a snake. Therefore, contrary to the prevalent idea that freezing is solely a response to distal threats (Gray, 1991), it is also effective against a nearby predator. For instance, when a hare is approached, it relies on its camouflage until the last possible moment, crouching lower and lower and apparently drawing its fur tighter and tighter to its body as the enemy draws nearer. Leverets are very tight sitters and depend on this for their survival (Kingdon, 1974). Blanchard et al. (1990) also showed that freezing is the preferential (first shown) response of wild rats to an approaching predator when escape is not available (Fig. 1). The hierarchical repertoire seen with stepwise stimulation of the tectum reproduces this situation. It is

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

ARENA Open Large Small 1.0

Immobility =

=

1287

Roofed

Exophthalmus =

=

=

=

0.5

RESPONSE PROBABILITY (rsp/n)

0 1.0

Running =

Jumping

Rotation

<

=

=

=

=

0.5

0 1.0

Defecation <

=

Micturition =

=

=

=

0.5

0 3

10

30

70 3 INTENSITY (µA)

10

30

70

Fig. 4. Exteroceptive modulation of PAG-evoked defensive behaviors. Rats were stimulated with stepwise increasing intensities in either large or small arenas (50 or 20 cm diameter), with or without a roof placed 25 cm high. Arena narrowing suppressed running and lead to the appearance of a fast rotation. Nevertheless, galloping occurred at intensities much lower than those of trotting (I50 of 35 and 110 mA, respectively). Significant differences in curve location are represented by symbol inequalities (likelihood-ratio c2-test, Bonferroni’s criterion 5% level).

worth noting here that a 50 cm high ‘open-field’ is in fact a blind alley. Very unexpectedly, neither the narrowing of the arena nor the presence of the roof had any effects in the thresholds of other defensive somatic behaviors, including jumping (Fig. 4). Indeed, wild rats try to escape from both traps and cages jumping against the roof. This behavior may last for long periods, bringing about injury of the head. Rat strain may be another variable influencing the thresholds of intracranially-evoked defensive behaviors. Recent studies from our laboratory compared the thresholds of PAG-evoked behaviors of Wistar and wild Rattus norvegicus (Fig. 5). Because the wild rats were handled under ethyl ether anesthesia, Wistar rats were either preanesthetized (controls) or not (naı¨ve). Rats were stimulated 1 h following the recovery from anesthesia. Ether had facilitatory aftereffects on both galloping and jumping, reducing their thresholds by 23 and 33%, respectively.

Surprisingly, thresholds for wild rats were 23% (exophthalmus) to 80% (micturition) higher than those of naı¨ve Wistar rats. Most important, galloping and jumping of wild rats had thresholds 52 and 34% higher than naı¨ve Wistar rats, respectively. Because of the ether facilitatory aftereffect, thresholds of these responses were about twice those of pre-anesthetized Wistar rats. Thus, the high defensiveness of wild rats to natural stimuli does not seem to be due to differences in their PAG. As a matter of fact, whereas the medulla of wild rats is only slightly heavier (1.6%) than that of the Wistar rats, other structures are markedly heavier, particularly, the cerebellum (10.3%), striatum (10.9%) and telencephalon (10.6%). Wild rats also have a bigger hipoccampus (10.2%) and neocortex (12.5%) (Kruska, 1975a,b). The higher development of forebrain structures in wild rats is likely to underlie the higher defensiveness of this strain to natural stimuli.

1288

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

Response Probalility (rsp/n)

Wistar 1.0

Wistar post-ether

Wild post-ether

Defecation

Exophthalmus

Immobility

Micturition

0.5

0 10

30 1.0

70

10

30

70

Trotting

10

30

Galloping

70

10

30

70

Jumping

0.5

0 10

30

70

10

30

70

10

30

70

Intensity (µA) Fig. 5. PAG-evoked defensive behaviors of Wistar and wild Rattus norvegicus. Wild rats had to be pre-anesthetized with ethyl ether to be handled. Therefore, Wistar rats were either pre-anesthetized (controls) or not (naı¨ve). Rats were stimulated 1 h after recovery from anesthesia. Dotted lines, significantly different from naı¨ve group. Dashed lines, significantly different from remaining groups (likelihood ratio c2-test, Bonferroni’s criterion 5% level).

Finally, it should be added that neither the appetitive, nor offensive, mouse-killing or reproductive behaviors were observed following electrical and chemical stimulation of tectum of either Wistar (Bittencourt et al, 2004, 2005) or wild rats in presence of water, food, a mouse or a female rat. The above results suggest that the elicitation of many PAG- and SC-mediated behaviors may depend upon the experimental apparatus (open-field, Skinner box, etc.) or the schedule of stimulation used. Moreover, they also suggest that forebrain influences cannot be discarded when mapping the defense system. Therefore, rather than a topographically rigid map, the tectum map needs to be considered as being dynamically organized.

6. Topography of electrically-evoked components of defence reaction Intensity-varying sine-wave stimulation of the DLSC, DLPAG and LPAG produced repertoires virtually identical in both their thresholds and hierarchy (Figs. 6 and 7). Consequently, instead of a column specialization, the effects of sine-wave stimulation seems to be the result of either the differential amount of tissue depolarized with increasing intensities (‘mass effect’) or the intrinsic properties of neurons and axons at different intensity levels (e.g. diameter-, channel- or transmittercontrolled firing rates).

However, in spite of the low precision of sine-wave stimulation, ILSC and VLPAG thresholds were much higher than those of other structures and the VLPAG repertoire did not include defecation. ILSC-evoked jumping presented thresholds that were markedly higher and variable (Fig. 6). VLPAG-evoked trotting, galloping and jumping showed a non-hierarchical output (Fig. 7). Hence, ILSCand VLPAG-evoked responses were most probably produced by current spreading to nearby active areas. Besides supporting the functional specialization of tectum structures, these data suggest that sine-wave pulses of up to 50 mA (w17 mA, r.m.s.) may depolarize an area of 2 mm diameter or more. The repertoires elicited by frequency-varying squarewave stimulation applied to the DLPAG and LPAG were virtually identical (Figs. 6 and 7). Because the frequency stimulation depolarizes a constant volume of tissue, the hierarchy of frequency-evoked behaviors is unlikely to be due to stimulus spreading. On the other hand, the relative threshold of jumping for both PAG columns was remarkably higher than those of galloping and trotting (Fig. 7). Because frequency stimulation depolarizes a small volume of tissue, its lower effectiveness in eliciting jumps suggests that this response may require the depolarization of a larger extent of PAG, perhaps involving more than one column. Notably, the VLPAG repertoire lacked jumping, galloping, micturition and defecation. Moreover, frequency thresholds of VLPAG-evoked exophthalmus and immobility

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

EXOPHTHALMUS

1 2 3 4 5 IMMOBILITY 1 2 3 4 5 TROTTING 1 2 3 4 5 GALLOPING 1 2 3 4 5 JUMPING 1 2 3 4 5 MICTURITION 1 2 3 4 5 DEFECATION 1 2 3 4 5#

1 - ILSC 2 - DLSC 3 - DLPAG 4 - LPAG 5 - VLPAG

10

30

100

300 µA

EXOPHTHALMUS

1 2 3 4 5 IMMOBILITY 1 2 3 4 5 TROTTING 1 2 3 4 5 GALLOPING 1 2 3 4 5# JUMPING 1# 2# 3 4 5# MICTURITION 1 2 3 4 5# DEFECATION 1 2 3 4 5#

10

were about 3.5 times higher than those of LPAG. Thus the latter responses were apparently produced by the depolarization of LPAG at higher frequencies. Indeed, because the current density decays towards the periphery of an electric field, regions inside the fringe of the field that were not depolarized with low-frequency stimulation might have been depolarized with the higher frequencies. In any event, these data suggest that neither reactive immobility (freezing), nor active defensive behaviors (flight) are organized in VLPAG. Remarkably, frequency stimulation of both the ILSC and DLSC failed in produce jumps. Moreover, frequency thresholds of SC-evoked galloping were markedly higher (F50O216 Hz) than those of both the DLPAG and LPAG (F50!125 Hz) (Fig. 6). As well, SC maximum output of galloping (23% of rats) was only half that of PAG (46%). Therefore, SC-evoked galloping was most likely produced by depolarization of dorsal PAG. In contrast, frequency thresholds of immobility, trotting, exophthalmus, defecation and micturition in the SC showed values comparable to those of PAG, supporting a genuine participation of deeper collicular layers in these responses. However, because the thresholds of the DLPAG were always lower than those of the SC, the deeper collicular layers seem only to prime the defensive circuits inside that column. These data also confirm the more precise spatial resolution of frequency stimulation, suggesting that a monophasic square-wave pulse of about 40 mA and 0.1 ms depolarizes a volume 0.5 mm across or less.

7. NMDA receptor mediated glutamate transmission within the periaqueductal gray and superior colliculus

100

30

300

1000 Hz

EXOPHTHALMUS

1 2 3 4 5 IMMOBILITY 1 2 3 4 5 TROTTING 1 2 3 4 5 GALLOPING 1 2 3 4 5 JUMPING 1 2 3 4 5# MICTURITION 1 2 3 4 5# DEFECATION 1 2 3# 4 5#

0.3

1289

The PAG is rich in the neurotransmitters glutamate and aspartate and their respective receptors. The density of EAA receptors is rather uneven, with the DLPAG presenting the highest density of all receptors, including the n-methyl-D-aspartic acid (NMDA) type, whereas the lowest densities are found in the VLPAG (Albin et al., 1990; Beitz and Williams, 1991; Gundlach, 1991). The NMDA receptor-induced Ca2C entry into the neuron is the main signal for the neuronal production of nitric 3

1

3

95% Confidence Interval (median±1.96SE)

10 nmol

Fig. 6. Thresholds of tectum-evoked defensive behaviors produced by either the electrical stimulation with stepwise increasing intensities (top) and frequencies (middle), or by a slow microinfusion of NMDA (bottom). Horizontal lines represent the median confidence intervals (95%C.I.Z medianG1.96SE) as determined by the threshold logistic analysis. Population median estimates were connected by thin lines. Medians higher than the cutoff stimulus (dashed lines) indicate that the response was produced in less than 50% of rats. Non-overlapping C.I. indicate thresholds significantly different at 5% level (confidence probabilities). Current stimulation: 20 s trains of 60 Hz sine-wave pulses (cutoffZ70 mA). Frequency stimulation: 20 s trains of 1 ms square-wave pulses (group mean intensityZ37 mA, cutoffZ130 Hz). Chemical stimulation: 0. 5 nmol/min microinfusion of NMDA (maximum volumeZ230 nl). # lack of significant fitting due to the virtual absence of the response.

1290

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

5 4 Current Relative Threshold (I 50/I50E)

D

DLPAG LPAG VLPAG ILSC DLSC

J

D D

3

MT MM D J GM J GJ T GG

2

T T II I 1

T G J

I

EEE E

M

I E

20

50

100

200 µA

20 Frequency Relative Threshold (F 50/F50E)

J

15 J

10

M G

5

D M DMD G G GT D T TM T I

I II

1

IT

EEE E

10

E

40

100

400 Hz

5 NMDA Relative Threshold (ED 50/ED50E)

D 4

DM M

3

M JG DT

TJ 2

G T I T I

1

E 0.4

EEE 1

G MI I

oxide (NO). In turn, NO activates guanylate cyclase and promotes the synthesis of cyclic guanosine monophosphate (cGMP) (Garthwaite, 1991; Snyder and Bredt, 1992). The DLPAG is also notable for its high diaphorasic activity, indicating the presence of considerable amounts of nicotinamide adenine dinucleotide phosphate (NADPH), a cofactor of neuronal NO synthase (nNOS) (Onstott et al., 1993; Paxinos et al., 1999). Therefore, NMDA-receptor induced synthesis of NO is likely to be amplified by the DLPAG high content of NADPH. This notable feature suggests a key role of NMDA-NO-cGMP signaling pathway in this column (Carobrez et al., 2001; De-Oliveira et al., 2000, 2001). Indeed, PAG dorsal columns of rats which were exposed to a cat showed a marked increase in both the c-fos immunoreactivity and nNOS/cGMP levels (Canteras and Goto, 1999; Chiavegatto et al., 1998). Further, the intraperiaqueductal microinjection of NO donors was reported to produce both running and jumping behaviors (DeOliveira et al., 2000, 2001). Finally, whereas the NOevoked flight was attenuated by methylene blue, an inhibitor of soluble guanylate cyclase, the intra-periaqueductal injection of 8-bromo-cGMP, a membranepermeable analogue of cGMP, produced both running and jumping (De-Oliveira et al., 2000). Likewise, stimulation of deeper collicular layers with either glutamate or NMDA produces defensive-like behaviors and increases in blood pressure and heart rate (Dean et al., 1988b; Keay et al., 1990). The ILSC are characterized by a periodic architecture of cell patches forming overlapping lattices of high immunoreactivities for NADPH-diaphorase (Soares et al., 2003) and choline acetyltransferase (Jeon and Mize, 1993; Wallace and Fredens, 1989). These lattices bear a specific relationship to the cell groups that give rise to tecto-reticulo-spinal and tecto-cuneiform pathways (Illing, 1992; Jeon and Mize, 1993). Moreover, neurons of superficial collicular layers of ferrets project to tecto-reticulo-spinal neurons that stain prominently for the NR1 subunit of the NMDA receptor (Doubell et al., 2003). Kobayashi et al. (2001) presented evidence that NMDA and nicotinic receptors may cooperate within the ILSC. Extrinsic afferents to this lattice comprise both the cholinergic inputs from the pedunculopontine tegmental nucleus and presumptive glutamatergic inputs from the precommissural nucleus and DLPAG (Beitz, 1989; Wallace and Fredens, 1989). The dorsal premammillary nucleus of the hypothalamus, a key structure of rat defense system 3

GT J

I E 3

Median Threshold

6 nmol

Fig. 7. Ethocharts of tectum-evoked defensive responses produced by electrical stimulation with stepwise increasing intensities (top) and frequencies (middle), or the slow microinfusion of NMDA (bottom). Letters represent the relative threshold, i.e. the ratio of the median threshold (I50, F50 or ED50) of each response to the least magnitude threshold, namely, that of exophthalmus (I50E, F50E or ED50E). Abscissas represent the median absolute values. E, exophthalmus, I, immobility, T, trotting, G, galloping, J, jumping, M, micturition, D, defecation.

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

(Canteras et al., 1997; Ceza´rio, 2001; Dielenberg et al., 2001) is another potential source of glutamatergic afferents to deeper collicular layers (Canteras and Swanson, 1992). The role of NMDA glutamate receptors in mediating both the PAG- and SC-evoked defensive responses was appraised through dose-response logistic analysis. However, in contrast to electrical stimuli, drug effects cannot be halted at will. Accordingly, chemical stimulation employed a pump-driven microinfusion of 10 mM NMDA at the rate of 50 nl/min (0.5 nmol/min) until either the presentation of an intense flight behavior or up to a maximum volume of 230 nl (2.3 nmol). These procedures allowed the discrete presentation of defensive behaviors across an increasing dose range of NMDA.

8. Topography of NMDA-evoked single components of defence reaction Interestingly, the repertoires of the DLPAG and LPAG were only discriminated by NMDA stimulation in a way that was not apparent with electrical stimulation (Figs. 6 and 7). In general, the effective doses for the DLPAG were markedly lower and much less variable than those of other structures including the LPAG (Fig. 6). The only exceptions were the lack of NMDA-evoked defecation in the DLPAG and the similar effective doses for DLPAG- and LPAGevoked micturition. Accordingly, because defecation was also absent in VLPAG, PAG representation of this response was apparently restricted to the lateral column. The resolution of NMDA microinfusion appeared to be lower than that of frequency stimulation. For instance, galloping and jumping were both produced following NMDA stimulation of VLPAG. Doses were nevertheless twice those of DLPAG. Indeed, Nicholson (1985) estimated that a micropipette injection of only 10 nl may spread 130– 230 mm away from the injection site. Likewise, Sandku¨hler and Herdegen (1995) were able to selectively stimulate the PAG columns with volumes not greater than 50 nl. Not surprisingly, NMDA-evoked defensive responses from the VLPAG were nearly absent with volumes lower than 100 nl (1 nmol). Also, VLPAG effective doses for producing exophthalmus, immobility and trotting were 3.2, 2.2 and 2.5 times higher than those of DLPAG and 1.5 times higher than those LPAG (Bittencourt et al., 2004). VLPAG effective dose range were mostly rather narrow and lacked the hierarchy seen in the LPAG and DLPAG profiles (Fig. 7). Therefore, VLPAG-evoked exophthalmus, immobility, trotting, galloping and jumping were most probably produced by NMDA dorsalward diffusion into the LPAG. Some DLPAG-evoked responses could have conceivably been due to NMDA spreading to the deeper layers of SC. However, NMDA stimulation of these layers failed in producing either the galloping (ILSC) or jumping (ILSC and DLSC) responses. Moreover, in spite of the proximity of

1291

DLPAG and DLSC, galloping and jumping effective doses were markedly higher in the latter structure. Overall, these results suggest a major role of both the DLPAG and respective NMDA receptors in the initiation of defensive behaviors.

9. Functional specializations within the tectum defense systems Immobility and exophthalmus, the responses which characterize freezing behavior, were elicited across the entire dorso-ventral extent of the SC and PAG. However, while the thresholds of electrical stimuli were lower in the DLPAG and LPAG, the minimally effective doses of NMDA were found in the DLPAG. These data suggest both a more extensive representation of freezing in tectum structures and a major role of DLPAG and respective NMDA receptors in its expression. Immobility has been often regarded as a cryptic display of insects and lower vertebrates, less being known about its adaptive role in higher species (Curio, 1976; Driver and Humphreys, 1988). Yet, as mentioned earlier, freezing seems to be a suitable defensive tactic against either distal or proximal threats. Freezing also helps in keeping the prey in shelter (Blanchard and Blanchard, 1989; Blanchard et al., 1991) and is particularly effective as a camouflage in darkness. Finally, freezing greatly reduces the prey’s visibility and noise, which are the major clues of night predators, such as cats and raptors (Curio, 1976; Kingdon, 1977; Konishi, 1997). Even prey of medium size make use of freezing as a defensive tactics. For instance, the steinbuk (Raphicerus campestris), an African non-gregarious secretive antelope, hides in tall grass and freezes instead of flying to an approaching predator. Flight only ensues if the predator comes very close (Kingdon, 1982). The present results suggest that the DLPAG- and LPAG are key structures underlying these responses. Prey species also make extensive use of passive immobility (motionless). Thus, newborn antelopes that have no stamina for a headlong flight stay most of their time hidden or lying motionless (Kingdon, 1982). Indeed, there is evidence that ‘quiescence’ is the main defensive response mediated by the VLPAG. Quiescence, along with analgesia and hypotension, is seemingly related to postdefense recuperative-like behaviors (Bandler and Depaulis, 1991; Bandler and Keay, 1996; Carrive, 1993; but see Morgan and Carrive, 2001). Because the focus of the present study was the organization of defensive behaviors, we did not record the eye, pinnae, head and body responses classically related to the SC (Sparks, 1999). However, protrusion and full opening of the eye, a response we termed ‘exophthalmus’ (Schenberg et al., 1990), was recorded as an integral component of the rat defense reaction. Although the exophthalmus was first described following the stimulation

1292

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

of hypothalamic defense areas (Morpurgo, 1968) and the deeper layers of the SC (McHaffie and Stein, 1982) of the rat, the present data and other studies show that this response may be produced by stimulation of other brain areas, including the PAG itself and the dorsal premammillary nucleus of the hypothalamus (Ceza´rio, 2001), which is the main hypothalamic nucleus projecting to the deeper collicular layers and dorsal PAG (Canteras and Swanson, 1992). Although McHaffie and Stein (1982) did not anticipate any possible function of exophthalmus, the concomitant development of exophthalmus and mydriasis increases the visual capabilities of the prey. Consequently, these responses may well play a prominent role in the attentive behavior of the rat, particularly, during the night. It remains to be verified whether the rat exophthalmus has any functional relationship to the SC rostral fixation cells that are activated during visual attention in foveated species (Begeron and Guitton, 2001; Munoz and Fecteau, 2002). As a matter of fact, Bell et al. (2003) showed that the visual fixation reduces both the sensory and multisensory processing of the SC. A reconciling hypothesis on the function of these processes in attention suggests that whereas the multisensory integration plays a prominent role in the screening of relevant stimuli, visual fixation suppresses the disturbing interferences of extraneous stimuli. These processes could be modulated by tectopulvino-cortical pathways that seem to participate in the appraisal of relevant stimuli (Jiang et al., 2002; Jiang and Stein, 2003; Posner, 1995; Stein et al., 1995). McHaffie and Stein (1982) also note that while pinnae orienting movements were solely produced by stimulation of caudal sectors of deeper collicular layers, whisker movements were evoked across all collicular layers, including the most superficial. However, in the present experiments stimulation of deeper collicular layers resulted in paralysis of ongoing whisker movements (mystacioplegia). Even though this response was not examined systematically, it was considered to be a component of the freezing reaction. Indeed, it makes good sense to stop the tactile exploration of nearby surroundings when predators are visually detected. The emotional nature of PAG-evoked freezing reaction remains unclear. In fact, although freezing has been long regarded as one of the most salient indexes of fear (or anxiety) in both rats and humans (Darwin, 1872; Fanselow, 1991; Gray, 1991; Gray and McNaughton, 2000), pharmacological data suggest that the DLPAGevoked freezing is most probably an attentional response largely mediated by norepinephrine (NE) (Schenberg et al., 2001). Indeed, electrophysiological studies support the prominent role of NE in attention (Arnsten et al., 1996; Aston-Jones et al., 2000; Bunney and Aghajanian, 1976; Franowitz et al., 2002; Servan-Schreiber et al., 1990). Accordingly, the building up of immobility and exophthalmus along with mystacioplegia suggests an attentional shift from ‘tactile’ proximal targets to ‘visual’

distal ones, a response which is supposed to be mediated by LC noradrenergic projections to prefronto cortical areas (Aston-Jones et al., 2000; Delagrange et al., 1993). Although the electrophysiological studies in anesthetized rats failed in demonstrating the PAG direct projection to LC somata, they did not exclude the PAG projections to rostral dendrites of LC (Ennis et al., 1991). In addition, the activity of the LC of freely moving cats is robustly increased during intra-specific aggressive behavior (Levine et al., 1990). Consistent with this, there is evidence showing that the caudal PAG projects to the LC (Cameron et al., 1995; Sandku¨hler, 1991; Sandku¨hler, and Herdegen, 1995). Because the AC was proposed to be involved in executive attention, i.e. in attention-to-action mechanisms (Devinsky et al., 1995; Posner, 1995), it is tempting to speculate that the LC projections to the medial prefrontal cortex and back to the DLPAG (Floyd et al., 2000) and LC makes up an attention/alarm circuit (LC-AC-DLPAG). The ascending projections of the LC have been also suggested to be involved in anxiety disorders (Gray et al., 1975; Redmond et al., 1976; Soutwick et al., 1999; Sullivan et al., 1999). Nevertheless, stimulation of the LC of humans that lead to a 16 fold increase in NE metabolites produced only an increase in alertness (Libet and Gleason, 1994) rather than fear or anxiety. Thus, DPAG-evoked freezing is most probably the expression of a heightened attentiveness to threatening visual stimuli (alarm reaction). Eventually, the PAG-evoked freezing behavior was often accompanied by defecation and/or micturition. These responses have been also regarded as reliable indexes of fear (Gray, 1991). Interestingly, Hess (1954) reported that whereas the stimulation of the hypothalamus produces the entire pattern of defecation in cats, including the search of an appropriate place and the subsequent concealment of feces, stimulation of caudal brainstem produces solely the ejection of feces. Thresholds of these responses were nevertheless much higher than those of immobility and exophthalmus, suggesting the relative independence of these systems (Bittencourt et al., 2004, 2005). Actually, their thresholds were even higher than those for galloping and jumping. More importantly, the lack of defecation following NMDA stimulation of DLPAG may be a physiological substrate of the antagonism of the abdominal contractions of defecation and flight responses. Because frequency and NMDA stimulation of VLPAG failed to elicit defecation, this response seems to be organized in the LPAG, exclusively. Indeed, transneuronal retrograde labeling following the injection of pseudorabies virus into the distal colon of rats labeled a restricted region of the LPAG (Valentino et al., 2000). On the other hand, micturition was similarly evoked in the DLPAG and LPAG. Even though there is no consensus about the projections of PAG discrete columns to Barrington’s nucleus, available evidence suggests

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

extensive projections from DMPAG and central sectors of PAG abutting both the LPAG and VLPAG (Blok and Holstege, 1994; Ennis et al., 1991; Matsuura et al., 2000; Valentino et al., 1994, 2000). These projections may be related to the wider areas from which the micturition can be elicited. Although, the survival value of fear-evoked defecation and micturition is not yet understood, a large part of rodent’s life revolves around scents, some of which could be carried out by feces and urine (e.g. released by anal sac glands). Overall, the present studies suggest a genuine participation of SC in exophthalmus, immobility, defecation, micturition and, less evidently, trotting. It is notable that the effective currents (!70 mA) and doses (!2.3 nmol) used in the present experiments were much lower than those reported in previous studies using either electrical (Sahibzada et al., 1986) or chemical stimulations of SC (Keay et al., 1988, 1990). Particularly, the lower potency of glutamate may be explained by its highly efficient uptake. In fact, while the SC-evoked defensive behaviors were effectively produced by a 10 nmol glutamate microinjection (Dean et al., 1988b, Keay et al., 1988), Krieger and Graeff (1985) failed in inducing the defense reaction following the microinjection of a 5 nmol glutamate dose into the DLSC. Nevertheless, NMDA doses used here were also much lower than the 20 nmol dose reported by Keay et al. (1990). Accordingly, the longer stimulation trials (20 s pulse trains and up to 4.6 min microinfusion), that were about ten times longer than those of other studies, might explain the lower magnitude of our effective stimuli. While the progression from walking to trotting and galloping has been related to the CnF and pedunculopontine nuclei of the ‘mesencephalic locomotor region’ (Armstrong, 1988; Garcia-Rill and Skinner, 1988), the mechanisms underlying jumping behavior remain obscure. Anyway, because jumping was defined as a vertical leap directed to the border of the open-field, its execution must involve the sudden replacement of trotting and galloping programs by those of upward gaze and jumping. Interestingly, the deeper collicular layers send projections to the nucleus of posterior commissure and the intersticial nucleus of Cajal, structures that are thought to be involved in eye vertical movements (Huerta and Harting, 1984).

10. Conclusions The studies reported here are the first mapping of the tectum organization of single components of freezing and flight behaviors of the rat. Threshold logistic analysis largely corroborated the columnar organization of these responses in caudal PAG. Thus, whereas the PAG-evoked defecation was found to be restricted to the LPAG, exophthalmus, micturition and somatic defensive responses were similarly organized in the DLPAG and LPAG, but not VLPAG. Importantly, DLPAG-evoked defensive responses

1293

required NMDA doses significantly lower than those of LPAG. Accordingly, NMDA receptors within the DLPAG are likely to play a major role in the initiation of both freezing and flight responses. Because the DLPAG does not send direct projections to caudal brainstem, projecting instead to dorsomedial and lateral PAG, LC and CnF, DLPAG-evoked responses are most probably mediated through these structures (Cameron et al., 1995; Holstege, 1991; Sandku¨hler, 1991; Sandku¨hler and Herdegen, 1995). In contrast, present data do not support the VLPAG as a substrate of either unconditioned flight or freezing behaviors. While the tecto-cuneiform pathway has been implicated in the defensive behaviors of the rat (Redgrave and Dear, 1991; King et al., 1996; Dean et al., 1989), the recognition of fearful expressions in humans has been proposed to involve a tecto-pulvino-amygdalar pathway (de Gelder et al., 1999; Linke et al., 1999; Morris et al., 1999, 2001, 2002; Vuilleumier et al., 2003). The present data agree with those of Blanchard et al. (1981) inasmuch as they suggest that the deeper layers of SC are mainly involved in increased attentiveness (exophthalmus, immobility) and, perhaps, restlessness (trotting) behaviors, but not in flight (galloping, jumping). Thus, although the unconditioned defensive behaviors of the rat are most probably executed through the PAG and its descending efferents, these data do not discard the parallel contribution of tecto-pulvino-amygdalar pathway in the visual detection of predators, much in the same way as that of the ‘unconscious’ recognition of fearful human faces. Also, the present data do not exclude the participation of the ascending collicular efferents in the generation of both exploratory and defensive trotting. Particularly, projections to the substantia nigra and ventral tegmental area of Tsai may play an important role in this behavior (Coizet et al., 2003; Comoli et al., 2003). Finally, although the ultimate execution of PAG-evoked defensive responses might involve premotor structures in the brainstem, there must be a PAG intrinsic mechanism that determines which, when and how these structures must be activated in response to specific environmental stimuli. The present and previous studies strongly suggest the DLPAG as the best candidate. Indeed, the DLPAG visual vestigial inputs from area 18 (Benzinger and Massopust, 1983) suggest that this structure might be evolved as a specialization of the tectum primitive defensive systems of lower vertebrates (Ewert, 1985).

Acknowledgements The present study was supported by research grants from FINEP and AFIP-UNIFESP. L.C. Schenberg and S. Tufik were recipients of CNPq research fellowships. A.S. Bittencourt was recipient of a CAPES PhD fellowship and A.L.P. Costa and A.V. Caldellas of CNPq undergraduate fellowships.

1294

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

References Adams, D.B., Baccelli, G., Mancia, G., Zanchetti, A., 1971. Relation of cardiovascular changes in fighting to emotion and exercise. J. Physiol. 212, 321–335. Albin, R.L., Makowiec, R.L., Hollingsworth, Z., Dure, L.S., Penney, J.B., Young, A.B., 1990. Excitatory amino acid binding sites in the periaqueductal gray of the rat. Neurosci. Lett. 118, 112–115. Armstrong, D.M., 1988. The supraspinal control of mammalian locomotion. J. Physiol. 405, 1–37. Arnsten, A.F., Steere, J.C., Hunt, R.D., 1996. The contribution of alpha 2-noradrenergic mechanisms of prefrontal cortical cognitive function. Potential significance for attention-deficit hyperactive disorder. Arch. Gen. Psychiat. 53, 448–455. Aston-Jones, G., Rajkowski, J., Cohen, J., 2000. Locus coeruleus and regulation of behavioral flexibility and attention. Prog. Brain Res. 126, 165–182. Bandler, R., 1982. Induction of ’rage’ following microinjections of glutamate into midbrain but not hypothalamus of cats. Neurosci. Lett. 30, 183–188. Bandler, R., Carrive, P., 1988. Integrated defence reaction elicited by excitatory amino acid microinjection in the midbrain periaqueductal grey region of the unrestrained cat. Brain Res. 439, 95–106. Bandler, R., Depaulis, A., 1988. Elicitation of intraspecific defence reactions in the rat from midbrain periaqueductal grey by microinjection of kainic acid, without neurotoxic effects. Neurosci. Lett. 88, 291–296. Bandler, R., Carrive, P., Depaulis, A., 1991. Emerging principles organization of the midbrain periaqueductal gray matter, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 1-8. Bandler, R., Depaulis, A., 1991. Midbrain periaqueductal gray control of defensive behavior in the cat and rat, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 175-198. Bandler, R., Keay, K.A., 1996. Columnar organization in the midbrain periaqueductal gray and the integration of emotional expression. Prog. Brain Res. 107, 285–300. Beitz, A.J., 1989. Possible origin of glutamatergic projections to the midbrain periaqueductal gray and deep layer of the superior colliculus of the rat. Brain Res. Bull. 23, 25–35. Beitz, A.J., Williams, F.G., 1991. Localization of putative amino acid transmitters in the PAG and their relationship to PAG-raphe magnus pathway, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 305-327. Bell, A.H., Corneil, B.D., Munoz, D.P., Meredith, M.A., 2003. Engagement of visual fixation suppresses sensory responsiveness and multisensory integration in the primate superior colliculus. Eur. J. Neurosci. 18, 2867–2873. Benzinger, H., Massopust, L.C., 1983. Brain stem projections from cortical area 18 in the albino rat. Exp. Brain Res. 50, 1–8. Bergeron, A., Guitton, D., 2001. The superior colliculus and its control of fixation behavior via projections to brainstem omnipause neurons. Prog. Brain Res. 134, 97–107. Besson, J.M., Fardin, V., Olive´ras J.L., 1991. Analgesia produced by stimulation of the periaqueductal gray matter, true antinociceptive effects versus stress effects, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 121-138. Bittencourt, A.S., Carobrez, A.P., Zamprogno, L.P., Tufik, S., Schenberg, L.C., 2004. Organization of single components of defensive behaviors within distinct columns of periaqueductal gray matter of the rat, role of N-methyl-D-aspartic acid glutamate receptors. Neuroscience 125, 71–79.

Bittencourt, A.S., Nakamura-Palacios, E.M., Mauad, H., Tufik, S., Schenberg, L.C., 2005. Organization of electrically- and chemicallyevoked defensive behaviors within the deeper collicular layers as compared to the periaqueductal gray matter of the rat. Neuroscience 133, 873–892. Blanchard, D.C., Williams, G., Lee, M.C., Blanchard, R.J., 1981. Taming of wild Rattus norvegicus by lesions of the mesencephalic central gray. Physiol. Psychol. 9, 157–163. Blanchard, R.J., Blanchard, D.C., 1989. Antipredator defensive behaviors in a visible burrow system. J. Comp. Psychol. 103, 70–82. Blanchard, D.C., Blanchard, R.J., Rodgers, R.J., 1990. Pharmacological and neural control of anti-predator defense in the rat. Aggr. Behav. 16, 165–175. Blanchard, D.C., Blanchard, R.J., Rodgers, R.J. 1991. Risk assessment and animal models of anxiety, in: Olivier, B., Mos, J., Slangen, J.L., (Eds.), Animal Models in Psychopharmacology. Birkha¨user Verlag, Basel, pp. 117-134. Blok, B.F.M., Holstege, G., 1994. Direct projections from the periaqueductal grey to the pontine micturition center (M-region). An anterograde and retrograde tracing study in the cat. Neurosci. Lett. 166, 93–96. Blomqvist, A., Craig, A.D., 1991. Organization of spinal and trigeminal input to the PAG, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 345-363. Buckenham, K.E., Yeomans, J.S., 1993. An uncrossed tectopontine pathway mediates ipsiversive circling. Behav. Brain Res. 54, 11–22. Bunney, B.S., Aghajanian, G.K., 1976. Dopamine and norepinephrine innervated cells in the rat prefrontal cortex, pharmacological differentiation using microiontophoretic techniques. Life Sci. 19, 1783–1789. Cadusseau, J., Roger, M., 1985. Afferent projections to the superior colliculus in the rat, with special attention to the deep layers. J. Hirnforsch 26, 667–681. Cameron, A.A., Khan, I.A., Westlund, K.N., Willis, W.D., 1995. The efferent projections of the periaqueductal gray in the rat, a Phaseolus vulgaris leucoagglutinin study. II- Descending Projections. J. Comp Neurol 351, 585–601. Canteras, N.S., Swanson, L.W., 1992. The dorsal premammillary nucleus, an unusual component of the mammillary body. Proc. Natl Acad. Sci. U.S.A. 89, 10089–10093. Canteras, N.S., Chiavegatto, S., Valle, L.E., Swanson, L.W., 1997. Severe reduction of rat defensive behavior to a predator by discrete hypothalamic chemical lesions. Brain Res. Bull. 44, 297–305. Canteras, N.S., Goto, M., 1999. Fos-like immunoreactivity in the periaqueductal gray of rats exposed to a natural predator. Neuroreport 10, 413–418. Carobrez, A.P., Teixeira, K.V., Graeff, F.G., 2001. Modulation of defensive behavior by periaqueductal gray NMDA/glycine-B receptor. Neurosci. Biobehav. Rev. 25, 697–709. Carrive, P., Bandler, R., 1991. Viscerotopic organization of neurons subserving hypotensive reactions within the midbrain periaqueductal grey, a correlative functional and anatomical study. Brain Res. 541, 206–215. Carrive, P., 1991. Functional organization of PAG neurons controlling regional vascular beds, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 67-100. Carrive, P., 1993. The periaqueductal gray and defensive behavior. Functional representation and neuronal organization. Behav. Brain Res. 58, 27–47. Ceza´rio, A.F., 2001. Characterization of behavioral functions of dorsal premammillary nucleus of hypothalamus. MSc. Thesis, Universidade Federal do Espı´rito Santo, Vito´ria, ES, Brazil. Chiavegatto, S., Scavone, C., Canteras, N.S., 1998. Nitric oxide synthase activity in the dorsal periaqueductal gray of rats expressing innate fear responses. Neuroreport 9, 571–576.

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298 Coizet, V., Comoli, E., 2003. Max Westby. G.W., Redgrave, P., Phasic activation of substantia nigra and the ventral tegmental area by chemical stimulation of the superior colliculus, an electrophysiological investigation in the rat. Eur. J. Neurosci. 17, 28–40. Comoli, E., Coizet, V., Boyes, J., Bolam, J.P., Canteras, N.S., Quirk, R.H., Overton, P.G., Redgrave, P., 2003. A direct projection from superior colliculus to substantia nigra for detecting salient visual events. Nature Neurosci. 6, 974–980. Curio, E., 1976. The Ethology of Predation. Springer-Verlag, New York. Darwin, C., 1872. The Expression of Emotions in Man and Animals. John Murray, London. de Gelder, B., Vroomen, J., Pourtois, G., Weiskrantz, L., 1999. Nonconscious recognition of affect in the absence of striate cortex. Neuroreport 10, 3759–3763. Dean, P., Mitchell, I.J., Redgrave, P., 1988a. Contralateral head movements produced by microinjection of glutamate into superior colliculus of rats, evidence for mediation by multiple output pathways. Neuroscience 24, 491–500. Dean, P., Mitchell, I.J., Redgrave, P., 1988b. Responses resembling defensive behaviour produced by microinjection of glutamate into superior colliculus of rats. Neuroscience 24, 501–510. Dean, P., Redgrave, P., Wetsby, G.W.M., 1989. Event or emergency? Two response systems in the mammalian superior colliculus. Trends Neurosci. 12, 137–147. Delagrange, P., Canu, M.H., Rougeul, A., Buser, P., Bouyer, J.J., 1993. Effects of locus coeruleus lesions on vigilance and attentive behaviour in cat. Behav Brain Res. 53, 155–165. Dennis, S.G., Choiniere, M., Melzack, R., 1980. Stimulation-produced analgesia in rats, assessment by two pain tests and correlation with selfstimulation. Exp. Neurol. 68, 295–309. De-Oliveira, R.M.W., Del Bel, E.A., Guimara˜es, F.S., 2000. Behavioral and c-fos expression changes induced by nitric oxide donors microinjected into the dorsal periaqueductal gray. Brain Res Bull 51, 457–464. De-Oliveira, R.M.W., Del Bel, E.A., Guimara˜es, F.S., 2001. Effects of excitatory amino acids and nitric oxide on flight behavior elicited from the dorsolateral periaqueductal gray. Neurosci. Biobehav. Rev. 25, 679–785. Devinsky, O., Morrell, M.J., Vogt, B.A., 1995. Contributions of anterior cingulate cortex to behaviour. Brain 118, 279–306. DiChiara, G., Morelli, M., Imperato, A., Porceddu, M.L., 1982. A reevaluation of the role of superior colliculus in turning behavior. Brain. Res. 237, 61–77. Dielenberg, R.A., Hunt, G.E., McGregor, I.S., 2001. “When a rat smells a cat”. the distribution of Fos immunoreactivity in rat brain following exposure to a predatory odor. Neuroscience 104, 1085–1097. Doubell, T.P., Skaliora, I., Baron, J., King, A.J., 2003. Functional connectivity between the superficial and deeper layers of the superior colliculus, an anatomical substrate for sensorimotor integration. J. Neurosci. 23, 6596–6607. Dra¨ger, U.C., Hubel, D., 1975. Responses to visual stimulation and relationship between visual, auditory and somatosensory inputs in mouse superior colliculus. J. Neurophysiol. 38, 690–713. Driver, P.M., Humphreys, D.A., 1988. Protean Behavior, The Biology of Unpredictability. Oxford University Press, New York. Edwards, S.B., Ginsburgh, C.L., Henkel, C.K., Stein, B.E., 1979. Sources of subcortical projections to the superior colliculus in the cat. J. Comp. Neurol. 184, 309–329. Edwards, S.B., 1980. The deep cell layers of the superior colliculus, their reticular characteristics and structural organization. In: Hobson, J.A., Brazier, M.A.B. (Eds.), The Reticular Formation Revisited. Raven Press, New York. Eibl-Eibesfeldt, I., 1975. Ethology, the Biology of Behavior. Holt, Rinehart and Winston, Inc., New York. Ennis, M., Behbehani, M., Shipley, M.T., Van Bockstaele, E.J., AstonJones, G., 1991. Projections from the periaqueductal gray to the rostromedial pericoerulear region and nucleus locus coeruleus, anatomic and physiologic studies. J. Comp. Neurol. 306, 480–494.

1295

Ewert, J.-P., 1985. Concepts in vertebrate neuroethology. Anim. Behav. 33, 1–29. Ewert, J.-P., Buxbaum-Conradi, H., Glagow, M., Rottgen, A., SchurgPfeiffer, E., Schwippert, W.W., 1999. Forebrain and midbrain structures involved in prey-catching behaviour of toads, stimulus-response mediating circuits and their modulating loops. Eur. J. Morphol. 37, 172–176. Fanselow, M.S., 1991. The midbrain periaqueductal gray as a coordinator of action in response to fear and anxiety, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 151-173. Fernandez de Molina, A., Hunsperger, R.W., 1962. Organization of the subcortical system governing defence and flight reactions in the cat. J. Physiol. 160, 200–213. Floyd, N.S., Price, J.L., Ferry, A.T., Keay, K.A., Bandler, R., 2000. Orbitomedial prefrontal cortical projections to distinct longitudinal columns of the periaqueductal gray in the rat. J. Comp. Neurol. 422, 556–578. Franowitz, J.S., Kessler, L.E., Borja, C.M., Kobilka, B.K., Limbird, L.E., Arnsten, A.F., 2002. Mutation of the alpha2A-adrenoceptor impairs working memory performance and annuls cognitive enhancement by guanfacine. J. Neurosci. 22, 8771–8772. Garcia-Rill, E., Skinner, R.D., 1988. Modulation of rhythmic function in the posterior midbrain. Neuroscience 27, 639–654. Garthwaite, J., 1991. Glutamate, nitric oxide and cell-cell signalling in the nervous system. Trends Neurosci. 14, 60–67. Gray, J.A., McNaughton, N., James, D.T.D., Kelly, P.H., 1975. Effect of minor tranquillizers on hippocampal theta rhythm mimicked by depletion of forebrain noradrenaline. Nature 258, 424–425. Gray, J.A., 1991. The Psychology of Fear and Stress, 2nd ed. Cambridge University Press, Cambridge. Gray, J.A., McNaughton, N., 2000. The Neuropsychology of Anxiety. Oxford University Press, Oxford. Gregg, T.R., Siegel, A., 2001. Brain structures and neurotransmitters regulating aggression in cats, implications for human aggression. Prog. Neuropsychopharmacol. Biol. Psychiatry 25, 91–140. Grunwerg, B.S., Krauthamer, G.M., 1990. Vibrissa-responsive neurons of the superior colliculus that project to the intralaminar thalamus of the rat. Neurosci. Lett. 111, 23–27. Gundlach, A.L. 1991. Regional subdivisions in the midbrain periaqueductal gray of the cat revealed by in vitro receptor autoradiography, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York pp. 449-464. Harvey, A.R., Worthington, D.R., 1990. The projection from different visual cortical areas to the rat superior colliculus. J. Comp. Neurol. 298, 281–292. Henkel, C.K., Edwards, S.B., 1978. The superior colliculus control of pinna movements in the cat, possible anatomical connections. J. Comp. Neurol. 182, 763–776. Henkel, C.K., 1981. Afferent sources of a lateral midbrain tegmental zone associated with the pinnae in the cat as mapped by retrograde transport of horseradish peroxidase. J. Comp. Neurol. 203, 213–226. Hess, W.R., Bru¨gger, M., 1943. Das subkortikale Zentrum der affectiven Abwerreaktion. Helv. Physiol. Pharmacol. Acta 1, 33–52. Hess, W.R., 1947. Vegetative Funktionen und Zwischenhirn. Helv. Physiol. Pharmacol. Acta 5 (Suppl. IV), 1–38. Hess, W.R., 1954. Diencephalon. Grune and Stratton, New York. Hofer, M.A., 1970. Cardiac and respiratory function during sudden prolonged immobility in wild rodents. Psychosom. Med. 32, 633–647. Holstege, G., 1991. Descending pathways from the periaqueductal gray and adjacent areas, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 239-265.

1296

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

Huerta, M.F., Harting, J.K., 1984. The mammalian superior colliculus, studies of its morphology and connections. In: Vanegas, H. (Ed.), Comparative Neurology of the Optic Tectum. Plenum Press, New York. Illing, R.B., 1992. Association of efferent neurons to the compartmental architecture of the superior colliculus. Proc. Natl Acad. Sci. USA 89, 10900–10904. Ingle, D., Crews, D., 1985. Vertebrate neuroethology, definitions and paradigms. Annu. Rev. Neurosci. 8, 457–494. Jeon, C.J., Mize, R.R., 1993. Choline acetyltransferase-immunoreactive patches overlap specific efferent cell groups in the cat superior colliculus. J. Comp. Neurol. 337, 127–150. Jiang, W., Jiang, H., Stein, B.E., 2002. Two corticotectal areas facilitate multisensory orientation behavior. J. Cogn. Neurosci. 14, 1240–1255. Jiang, W., Stein, B.E., 2003. Cortex controls multisensory depression in superior colliculus. J. Neurophysiol 90, 2123–2135. Kanaseki, T., Sprague, J.M., 1974. Anatomical organization of pretectal nuclei and tectal laminae in the cat, J. Comp. Neurol. 1974. 158 1974 pp. 319–338. Kasper, E.M., Larkman, A.U., Lubke, J., Blakemore, C., 1994. Pyramidal neurons in layer 5 of the rat visual cortex. I. Correlation among cell morphology, intrinsic electrophysiological properties, and axon targets. J. Comp. Neurol. 339, 459–474. Keay, K.A., Redgrave, P., Dean, P., 1988. Cardiovascular and respiratory changes elicited by stimulation of rat superior colliculus. Brain Res. Bull. 20, 13–26. Keay, K.A., Dean, P., Redgrave, P., 1990. N-methyl-D aspartate (NMDA) evoked changes in blood pressure and heart rate from the rat superior colliculus. Exp. Brain Res. 80, 148–156. King, A.J., Palmer, A.R., 1983. Cells responsive to free-field auditory stimuli in the guinea pig superior colliculus, distribution and response properties. J. Physiol. 342, 361–381. King, A.J., Hutchings, M.E., 1987. Spatial response properties of acoustically responsive neurons in the superior colliculus of the ferret, a map of auditory space. J. Neurophysiol. 57, 596–624. King, S.M., Shehab, S., Dean, P., Redgrave, P., 1996. Differential expression of fos-like immunoreactivity in the descending projections of superior colliculus after electrical stimulation in the rat. Behav. Brain Res. 78, 131–145. Kingdon, J., 1974. East African Mammals, An Atlas of Evolution in Africa, Volume II B (Hares and rodents). Academic Press, London. Kingdon, J., 1977. East African Mammals, An Atlas of Evolution in Africa, Volume III A (Carnivores). Academic Press, London. Kingdon, J., 1982. East African Mammals, An Atlas of Evolution in Africa, Volume III C (Bovids). Academic Press, London. Kobayashi, Y., Saito, Y., Isa, T., 2001. Facilitation of saccade initiation by brainstem cholinergic system. Brain Dev. 23 (Suppl. 1), S24–S27. Konishi, M., 1997. Neural mechanisms of auditory image formation. In: Gazzaniga, M.S. (Ed.), The Cognitive Neurosciences. MIT Press, Cambridge. Krieger, J.E., Graeff, F.G., 1985. Defensive behavior and hypertension induced by glutamate in the midbrain central gray of the rat. Braz. J. Med. Biol. Res. 18, 61–67. Kruska, D., 1975a. Comparative quantitative study on brains of wild and laboratory rats. I. Comparison of size of allocortical brain centers. J. Hirnforsch. 16, 469–483. Kruska, D., 1975b. Comparative quantitative study on brains of wild and laboratory rats. II. Comparison of size of allocortical brain centers. J. Hirnforsch. 16, 485–496. Lane, R.H., Allman, J.M., Kaas, J.H., 1971. Representation of the visual field in the superior colliculus of the ground squirrel (Sciurus carolinensis) and the tree shrew (Tupaia glis). Brain Res. 26, 277–292. Lane, R.H., Allman, J.M., Kaas, J.H., 1973. The visuotopic organization of the superior colliculus of the owl monkey (Aotus trivirgatus) and the bush baby (Galago senegalensis). Brain Res. 60, 335–349. Lane, R.D., Bennett-Clarke, C.A., Allan, D.M., Mooney, R.D., 1993. Immunochemical heterogeneity in the tecto-LP pathway of the rat. J. Comp. Neurol. 333, 210–222.

Ledoux, J.E., Iwata, J., Cicchetti, P., Reis, D.J., 1988. Different projections of the central amygdaloid nucleus mediate autonomic and behavioral correlates of conditioned fear. J. Neurosci. 8, 2517–2529. Lee, P.H., Helms, M.C., Augustine, G.J., Hall, W.C., 1997. Role of intrinsic synaptic circuitry in collicular sensorimotor integration. Proc. Natl Acad. Sci. USA 94, 13299–13304. Levine, E.S., Litto, W.J., Jacobs, B.L., 1990. Activity of cat locus coeruleus noradrenergic neurons during the defense reaction. Brain Res. 531, 189–195. Libet, B., Gleason, C.A., 1994. The human locus coeruleus and anxiogenesis. Brain Res. 634, 178–180. Linden, R., Perry, V.H., 1983. Massive retinotectal projection in rats. Brain Res. 272, 145–149. Linke, R., De Lima, A.D., Schwegler, H., Pape, H.C., 1999. Direct synaptic connections of axons from superior colliculus with identified thalamo-amygdaloid projection neurons in the rat, possible substrates of a subcortical visual pathway to the amygdala. J. Comp. Neurol. 403, 158–170. Lovick, T.A., 1991. Interactions between descending pathways from the dorsal and ventrolateral periaqueductal gray matter in the rat, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 101-120. Matsuura, S., Downie, J.W., Allen, G.V., 2000. Micturition evoked by glutamate microinjection in the ventrolateral periaqueductal gray is mediated through Barrington’s nucleus in the rat. Neuroscience 101, 1053–1061. McHaffie, J.G., Stein, B.E., 1982. Eye movements evoked by electrical stimulation in the superior colliculus of rats and hamsters. Brain Res. 247, 243–253. Mitchell, I.J., Dean, P., Redgrave, P., 1988. The projection from superior colliculus to cuneiform area in the rat. II. Defence-like responses to stimulation with glutamate in CnF and surrounding structures. Exp. Brain Res. 72, 626–639. Miyashita, E., Keller, A., Asanuma, H., 1994. Input-output organization of the rat vibrissal motor cortex. Exp. Brain Res. 99, 223–232. Miyashita, E., Mori, S., 1995. The superior colliculus relays signals descending from the vibrissal motor cortex to the facial nerve nucleus in the rat. Neurosci. Lett. 195, 69–71. Morgan, M.M., Carrive, P., 2001. Activation of the ventrolateral periaqueductal gray reduces locomotion but not mean arterial pressure in awake, freely moving rats. Neuroscience 102, 905–910. Morpurgo, C., 1968. Pharmacological modifications of sympathetic responses elicited by hypothalamic stimulation in the rat. Brit. J. Pharmacol. 34, 532–542. Morris, J.S., Ohman, A., Dolan, R.J., 1999. A subcortical pathway to the right amygdala mediating unseen fear. Proc. Natl Acad. Sci. USA 96, 1680–1685. Morris, J.S., de Gelder, B., Weiskrantz, L., Dolan, R.J., 2001. Differential extrageniculostriate and amygdala responses to presentation of emotional faces in a cortically blind field. Brain 124, 1241–1252. Morris, J.S., de Bonis, M., Dolan, R.J., 2002. Human amygdala responses to fearful eyes. Neuroimage 17, 214–222. Munoz, D., Fecteau, J.H., 2002. Vying for dominance, dynamic interactions control visual fixation and saccadic initiation in the superior colliculus. Prog. Brain Res. 140, 3–19. Nicholson, C., 1985. Diffusion from an injected volume of a substance in brain tissue with arbitrary volume fraction and tortuosity. Brain Res. 333, 325–329. Nudo, R.J., Sutherland, D.P., Masterton, R.B., 1993. Inter and intralaminar distribution of tectospinal neurons in 23 mammals. Brain Behav. Evol. 42, 1–23. Onstott, D., Mayer, B., Beitz, A.J., 1993. Nitric oxide synthase immunoreactive neurons anatomically define a longitudinal dorsolateral column within the midbrain periaqueductal gray of the rat, analysis using laser confocal microscopy. Brain Res. 610, 317–324.

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298 Patton, P., Grobstein, P., 1998. The effects of telencephalic lesions on visually mediated prey orienting behavior in the leopard frog (Rana pipiens. I. The effects of complete removal of one telencephalic lobe, with a comparison to the effects of unilateral tectal lobe lesions. Brain Behav. Evol. 51, 123–143. Paxinos, G., Carrive, P., Wang, H., Wang, P-Y., 1999. Chemoarchitectonic Atlas of the Rat Brainstem. Academic Press, San Diego. Posner, M.I., 1995. Attention in cognitive neuroscience, an overview. In: Gazzaniga, M.S. (Ed.), The Cognitive Neurosciences. MIT Press, Cambridge. Redgrave, P., Odekunle, A., Dean, P., 1986. Tectal cells of origin of predorsal bundle in rat, location and segregation from ipsilateral descending pathway. Exp. Brain Res. 63, 279–293. Redgrave, P., Mitchell, I.J., Dean, P., 1987. Further evidence for segregated output channels from superior colliculus in rat, ipsilateral tecto-pontine and tecto-cuneiform projections have different cells of origin. Brain Res. 413, 170–174. Redgrave, P., Dean, P., Mitchell, I.J., Odekunle, A., Clark, A., 1988. The projection from superior colliculus to cuneiform area in the rat. I. Anatomical studies. Exp. Brain Res. 72, 611–625. Redgrave, P., Dean, P., 1991. Does the PAG learn about emergencies from the superior colliculus? in, Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 199-209. Redmond, D.E., Huang, Y.H., Snyder, D.R., Maas, J.W., 1976. Behavioral effects of stimulation of the nucleus locus coeruleus in the stump-tailed monkey Macaca arctoides. Brain Res. 116, 502–510. Rhoades, R.W., Mooney, R.D., Rohrer, W.H., Nikoletseas, M.M., Fish, S. E., 1989. Organization of the projection from the superficial to the deep layers of the hamster’s superior colliculus as demonstrated by the anterograde transport of phaseolus vulgaris leucoagglutinin. J. Comp. Neurol. 283, 54–70. Sahibzada, N., Dean, P., Redgrave, P., 1986. Movements resembling orientation or avoidance elicited by electrical stimulation of superior colliculus in rats. J. Neurosci. 6, 723–733. Sandku¨hler, J., 1991. Induction of the proto-oncogene c-fos as a cellular marker of brainstem neurons activated from the PAG, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 267-286. Sandku¨hler, J., Herdegen, T., 1995. Distinct patterns of activated neurons throughout the rat midbrain periaqueductal gray induced by chemical stimulation within its subdivisions. J. Comp. Neurol. 357, 546–553. Schenberg, L.C., Graeff, F.G., 1978. Role of the periaqueductal gray substance in the antianxiety action of benzodiazepines. Pharmacol. Biochem. Behav. 9, 287–295. Schenberg, L.C., Costa, M.B., Borges, P.C.L., Castro, M.F.S., 1990. Logistic analysis of the defense reaction induced by eletrical stimulation of the rat mesencephalic tectum. Neurosci. Biobehav. Rev. 14, 473–479. Schenberg, L.C., Vasquez, E.C., Costa, M.B., 1993. Cardiac baroreflex dynamics during the defence reaction in freely moving rats. Brain Res. 621, 50–58. Schenberg, L.C., Bittencourt, A.S., Sudre´, E.C., Vargas, L.C., 2001. Modeling panic attacks. Neurosci. Biobehav. Rev. 25, 647–659. Schenberg, L.C., Capucho, L.B., Vatanabe, R.O., Vargas, L.C., 2002. Acute effects of clomipramine and fluoxetine on dorsal periaqueductal grey-evoked unconditioned defensive behaviours of the rat. Psychopharmacology 159, 138–144. Serizawa, M., McHaffie, J.G., Hoshino, K., Norita, M., 1994. Corticostriatal and corticotectal projections from visual cortical areas 17, 18 and 18a in the pigmented rat. Arch. Histol. Cytol. 57, 493–507. Servan-Schreiber, D., Printz, H., Cohen, J.D., 1990. A network model of catecholamine effects gain, signal-to-noise ratio, and behavior. Science 249, 892–895.

1297

Shipley, M.T., Ennis, M., Rizvi, T.A., Behbehani, M.M., 1991. Topographical specificity of forebrain inputs to the midbrain periaqueductal gray, evidence for discrete longitudinally organized input columns, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 417-448. Snyder, S.H., Bredt, D.S., 1992. Biological roles of nitric oxide. Sci. Am. 268, 22–29. Soares, J.G., Mendez-Otero, R., Gattass, R., 2003. Distribution of NADPH-diaphorase in the superior colliculus of Cebus monkeys, and co-localization with calcium-binding proteins. Neurosci. Res. 46, 475–483. Southwick, S.M., Bremner, J.D., Rasmusson, A., Morgan, C.A., Arnsten, A., Charney, D.S., 1999. Role of norepinephrine in the pathophysiology and treatment of posttraumatic stress disorder. Biol. Psychiatry 46, 1192–1204. Sparks, D.L., 1999. Conceptual issues related to the role of the superior colliculus in the control of gaze. Curr. Opin. Neurobiol. 9, 698–707. Stein, B.E., Magalha˜es-Castro, B., Kruger, L., 1975. Superior colliculus, visuotopic-somatotopic overlap. Science 189, 224–226. Stein, B.E., 1981. Organization of the rodent superior colliculus, some comparisons with other mammals. Behav. Brain Res. 3, 175–188. Stein, B.E., Clamann, H.P., 1981. Control of pinna movements and sensorimotor register in cat superior colliculus. Brain Behav. Evol. 19, 180–192. Stein, B.E., Gaither, N.S., 1983. Receptive-field properties in reptilian optic tectum, some comparisons with mammals. J. Neurophysiol. 50, 102–124. Stein, B.E., Wallace, M.T., Meredith, M.A., 1995. Neural mechanisms mediating attention and orientation to multisensory cues. In: Gazzaniga, M.S. (Ed.), The Cognitive Neurosciences. MIT Press, Cambridge. Stein, B.E., 1998. Neural mechanisms for synthesizing sensory information and producing adaptive behaviors. Exp. Brain Res. 123, 124–135. Stein, B.E., Jiang, W., Wallace, M.T., Stanford, T.R., 2001. Nonvisual influences on visual-information processing in the superior colliculus. Prog. Brain Res. 134, 143–156. Stepniewska, I., Ql, H.X., Kaas, J.H., 2000. Projections of the superior colliculus to subdivisions of the inferior pulvinar in New World and Old World monkeys. Vis. Neurosci. 17, 529–549. Sterbing, S.J., Hartung, K., Hoffmann, K.P., 2002. Representation of sound source direction in the superior colliculus of the guinea pig in a virtual auditory environment. Exp. Brain Res. 142, 570–577. Sudre´, E.C.M., Barros, M.R., Sudre´, G.N., Schenberg, L.C., 1993. Thresholds of electrically induced defence reaction of the rat, short- and long-term adaptation mechanisms. Behav. Brain Res. 58, 141–154. Sullivan, G.M., Coplan, J.D., Kent, J.M., Gorman, J.M., 1999. The noradrenergic system in pathological anxiety, a focus on panic with relevance to generalized anxiety and phobias. Biol. Psychiatry 46, 1205–1218. Telford, S., Wang, S., Redgrave, P., 1996. Analysis of nociceptive neurones in the rat superior colliculus using c-fos immunohistochemistry. J. Comp. Neurol. 375, 601–617. Valentino, R.J., Page, M.E., Luppi, P.H., Zhu, Y., Van Bockstaele, E., Aston-Jones, G., 1994. Evidence for widespread afferents to Barrington’s nucleus, a brainstem region rich in corticotropin-releasing hormone neurons. Neuroscience 62, 125–143. Valentino, R.J., Kosboth, M., Colflesh, M., Miselis, R.R., 2000. Transneuronal labeling from the rat distal colon, anatomic evidence for regulation of distal colon function by a pontine corticotropinreleasing factor system. J. Comp. Neurol. 417, 399–414. Vargas, L.C., Marques, T.A., Schenberg, L.C., 2000. Micturition and defensive behaviors are controlled by distinct neural networks within the dorsal periaqueductal gray and deep gray layer of the superior colliculus of the rat. Neurosci. Lett. 280, 45–48.

1298

L.C. Schenberg et al. / Neuroscience and Biobehavioral Reviews 29 (2005) 1279–1298

Vargas, L.C., Schenberg, L.C., 2001. Long-term effects of clomipramine and fluoxetine on dorsal periaqueductal grey-evoked innate defensive behaviours of the rat. Psychopharmacology 155, 260–268. Veening, J., Buma, P., Ter Horst, G.J., Roeling, T.A.P., Luiten, P.G.M., Nieuwenhuys, R., 1991. Hypothalamic projections to the PAG in the rat, Topographical, immuno-electronmicroscopical and functional aspects, in: Depaulis, A., Bandler, R., (Eds.), The Midbrain Periaqueductal Gray Matter. Functional, Anatomical and Neurochemical Organization. Plenum Press, New York, pp. 387-415. Veinante, P., Deschenes, M., 1999. Single- and multi-whisker channels in the ascending projections from the principal trigeminal nucleus in the rat. J. Neurosci. 19, 5085–5095. Vogt, B.A., Miller, M.W., 1983. Cortical connections between rat cingulate cortex and visual, motor, and postsubicular cortices. J. Comp. Neurol. 216, 192–210.

Vuilleumier, P., Armony, J.L., Driver, J., Dolan, R.J., 2003. Distinct spatial frequency sensitivities for processing faces and emotional expressions. Nat. Neurosci. 6, 624–631. Wallace, M.N., Fredens, K., 1989. Relationship of afferent inputs to the lattice of high NADPH-diaphorase activity in the mouse superior colliculus. Exp. Brain Res. 78, 435–445. Wallace, M.T., Wilkinson, L.K., Stein, B.E., 1996. Representation and integration of multiple sensory inputs in primate superior colliculus. J. Neurophysiol. 76, 1246–1266. Wallace, M.T., Meredith, M.A., Stein, B.E., 1998. Multisensory integration in the superior colliculus of the alert cat. J. Neurophysiol. 80, 1006– 1010. Wang, S., Redgrave, P., 1997. Microinjections of muscimol into lateral superior colliculus disrupt orienting and oral movements in the formalin model of pain. Neuroscience 81, 967–988.