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Medial prefrontal lesion deficits involving or sparing the prelimbic area in the rat
Physiology & Behavior, Vol. 64, No. 3, pp. 373–380, 1998 © 1998 Elsevier Science Inc. All rights reserved. Printed in the U.S.A. 0031-9384/98 $19.00 1 .00
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Medial Prefrontal Lesion Deficits Involving or Sparing the Prelimbic Area in the Rat MARY E. FRITTS,1* E. TREY ASBURY,* JAMES E. HORTON† AND WALTER L. ISAAC† *Department of Psychology, Texas Christian University, TCU Box 298920, Ft. Worth, TX 76129; and †Department of Psychology, East Tennessee State University, Box 70649, Johnson City, TN 37614, USA Received 17 September 1997; Accepted 5 March 1998 FRITTS, M. E., E. T. ASBURY, J. E. HORTON AND W. L. ISAAC. Medial prefrontal lesion deficits involving or sparing the prelimbic area in the rat.PHYSIOL BEHAV 64(3) 373–380, 1998.—The rat medial prefrontal cortex (PFC) is believed to play a central role in working memory and selective attention processes. More recently, it has been shown that the effects of large PFC lesions on working memory may be due to the prelimbic area of the PFC. The aim of the present study was to compare the effects of lesions of the prelimbic area with PFC lesions that involved or spared the prelimbic area on shuttlebox avoidance and radial maze learning in rats. The findings indicate that rats with PFC lesions that spared the prelimbic area were impaired at avoidance but not radial arm maze learning, whereas rats with prelimbic lesions or PFC lesions that included this area were impaired on the radial arm maze but not the avoidance learning task. Results support the notion that the medial frontal cortex of the rat is a functionally dissociable region and suggest that the prelimbic area appears to be critical for working memory, but less so for attention processes. © 1998 Elsevier Science Inc. Spatial learning
Working memory
Avoidance learning
Prelimbic area
medial precentral and dorsal anterior cingulate cortices, whereas the ventral area includes the prelimbic and infralimbic cortices (7,32). Although the PFC of the rat has often been considered as a global structure, there is some evidence to suggest a possible functional dissociation between the dorsal and ventral areas (8,10,15,27,36). One subfield of the PFC that has gained considerable attention in this regard is the prelimbic area. The recent interest in the prelimbic area is based on its extensive anatomical connections as well as on growing evidence that most of the effects of large PFC lesions may be due to damage of this subfield (4,5). Numerous studies have shown that large PFC lesions impair performance on measures of delayed alternation, spatial reversals, locomotor activity, and emotionality in rats (10,11,39), whereas small PFC lesions have resulted in control-like performance on similar measures (15,33,39). In these studies, large lesions appear to damage the prelimbic area of the PFC whereas small PFC lesions do not. Unfortunately, it is not possible to make direct lesion comparisons across such findings. Although there may be a number of reasons for the inconsistent behavioral effects of PFC lesions in previous studies (e.g., procedural differences, demand characteristics), serious consideration must be given to variations in lesion size and locus. According to anatomical studies, the prelimbic area of the PFC is considered to be an area homologous to the dorsolateral prefrontal cortex of primates. In both species, the prelimbic area receives massive projections from the MDN (21,23,35). The prelimbic area extends from the frontal pole through the genu of the
CONSIDERABLE evidence indicates that lesions to the dorsolateral area of the prefrontal cortex induce a diverse pattern of impairments in humans and primates. Such deficits are thought to be due to a working memory impairment (12,26); that is, from a dysfunction in short-term memory systems needed for the manipulation of information (1,34). However, equally often is reported an inability of humans and primates with prefrontal lesions to use divided and/or selective attention in order to solve complex cognitive tasks (12,26). On the basis of such findings, it has been concluded that the prefrontal cortex is involved in attention and working memory processes. In a similar fashion, damage to the medial prefrontal cortex (PFC) of the rat has been shown to produce a general impairment on spatial working memory tasks such as the radial arm maze (2,16), the Morris water maze (10,13,22), and spatial alternation (14,20,24). Rats with lesions of the PFC have also been found to be impaired in instrumental tasks which do not require working memory but which involve selective attention. Such tasks require the organism to attend simultaneously toward various pieces of information and select the information that is pertinent for the solution (6,28). Such results provide further evidence that the prefrontal cortex is involved in attention as well as working memory. Traditionally, the PFC of the rat has been assumed to be a functionally homogenous area. In the rat, the PFC is defined as the cortical projection area of the mediodorsal nucleus of the thalamus (MDN; 23) and comprises two distinct regions, a dorsal and a ventral area (7,32). The dorsal area of the PFC is composed of the 1
Medial prefrontal cortex
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corpus callosum just underneath the anterior cingulate cortex. It receives afferent input from sensory structures (secondary visual and auditory cortices) and from primary motor cortex. In addition, it also receives extensive direct and indirect afferent inputs from limbic structures (amygdala, hippocampus, entorhinal and piriform cortices) as well as from autonomic centers (e.g., lateral hypothalamus; 7). The prelimbic area sends efferent projections to several motor structures (striatum, pallidum) as well as to numerous limbic structures (amygdala, septum, mamillary bodies, entorhinal cortex) and autonomic centers (lateral hypothalamus, nucleus of the solitary tract; 32). In addition, a direct pathway between the hippocampus (CA1, CA2, subiculum) and the prelimbic area has been found (19). The functions of the prelimbic area in the rat, however, are poorly understood. Because of its extensive connections with the limbic system and the hippocampus, a structure known to be involved in memory processes (9,34), the prelimbic area holds significant potential as a key structure involved in neural circuits subserving learning and memory functions. Studies have shown that damage to the prelimbic area produces a deficit in selecting appropriate responses from working memory, but not reference memory (i.e., memory for constant information; 31). Other findings suggest that the function of the prelimbic area is to update appropriate reactions depending upon the organism’s internal state or by the emotional significance of stimuli (27). At the same time, some reports have indicated a crucial role of the prelimbic area in activities which require high attentional control and effortful information processing (4,13,14). The present study was aimed at examining the effects of prelimbic area lesions with medial prefrontal cortex lesions that involved or spared the prelimbic area on the acquisition of a shuttlebox avoidance task and a radial arm maze task in rats. Given that the rat prelimbic area may modulate attentional processes and working memory, we expected quantitative differences in the performance of rats with frontal damage that involved or spared this subfield. METHOD—EXPERIMENT 1
Animals Thirty-six female albino Sprague-Dawley rats (300 to 350 g) bred in the departmental vivarium were used. All animals were individually housed in plastic cages (30 3 35 3 18 cm) with free access to food and water. Before surgery, the animals were handled for 15 min daily for 14 days. Experimental testing began after a 2-week recovery period following surgery, during which the animals were handled daily. Testing was conducted during the light phase of a 12:12 h light-dark cycle. Avoidance Box A four-way shuttlebox (61 cm2 3 57 cm high) was used (for details, see 18). It was divided into four quadrants by 3.5-cm wooden hurdles. The shuttlebox contained a hardware cloth floor and was covered by a removable double glass plate. The glass cover contained a layer of cheesecloth between the plates to reduce extraneous visual cues. The box was painted white and contrasted by black hurdles. For both the conditioned stimulus (CS) and unconditioned stimulus (US), a compound light and tone stimulus was used. The intensity of this compound stimulus was greater for the US than the CS. The CS was a 15-W light bulb (5 fc) suspended 5.0 cm above the corner of each quadrant along with a standard piezo buzzer (71 db SPL, C scale) placed 2.5 cm above each corner. The US consisted of two 90-W halogen bulbs and was centered above
each quadrant as well as two door buzzers (99 db SPL, C scale) located on the outer edges of each corner. All testing was conducted in a sound-attenuated chamber. Surgery All operations were performed under aseptic conditions. Each animal was anesthetized with Nembutal (50 mg/kg, intraperitoneally (i.p.)) and given atropine sulfate (0.15 cc, 10 mg/cc, i.p.) to minimize secretions during surgery as well as Combiotic (0.10 cc, 300,000 units/mL, intramuscularly (i.m.)) to prevent infection. Sixteen animals sustained single-stage bilateral medial prefrontal cortex lesions (that included or spared the prelimbic area) by aspiration (n 5 8 per group). The lesion area for the PFC that spared the prelimbic area (SMF) included the medial precentral, anterior cingulate, and ventral cingulate areas (23). The lesion area for the PFC that included the prelimbic area (LMF) encompassed the SMF areas as well as the infralimbic and prelimbic areas (23). Eight animals received electrolytic lesions of the prelimbic area (PLA). The electrode was inserted through a hole in the scull and lowered vertically to the following coordinates: 10.6 mm lateral, 3.2 mm anterior to bregma, and 4.3 mm ventral to the top of the skull (30). Electrolytic lesions of the PLA were made by applying a 1.0-mA anodal current for 10 s through a stainless steel electrode (insulated except at tip). Eight animals served as sham controls (CON). Shams were anesthetized and their scalps cut and sutured. Avoidance Training Each animal was habituated to the shuttlebox for 40 min for 2 consecutive days prior to training. Activity was measured by the number of entries into each quadrant across four 10-min observation periods. The animal was placed in the center of the shuttlebox and allowed to explore freely. During avoidance training, the animal was required to learn to leave the quadrant it was located in and to move into any other quadrant when the CS came on in order to avoid the US. The criterion for avoidance learning was defined as having the animal make eight avoidance responses within a series of ten trials (8/10). Each animal was given 50 trials. All training was conducted in one session. On the day of training, the animal was allowed to habituate to the apparatus for 5 min before the initiation of the first trial. All trials were started and ended manually by the experimenter. Each trial began with the onset of the CS in the quadrant where the animal was located. The CS remained on for 3 s or until the rat crossed the hurdle into a different quadrant. If the animal did not make this response during the CS, the CS would end and a 3-s interstimulus interval (ISI) would follow, during which the animal could respond and end the trial. If the animal responded during the CS or the ISI, an avoidance response was recorded. If the animal did not respond within this time, the US (localized to the quadrant the animal was located) would be turned on until the animal crossed the hurdle. An escape response was then recorded for the trial. When either an avoidance or an escape response was made, all stimuli were terminated and the trial ended. A 1-min intertrial interval (ITI) was used. Histology After completion of behavioral testing, animals were given a lethal dose of Nembutal (1.0 mg/kg) and then perfused transcardially with saline followed by 10% buffered formalin. The brains were removed and placed in a 10% buffered formalin solution. Frozen sections (40 um) were taken throughout the extent of the lesion. Every tenth slice was placed on a microscope slide in preparation for staining. Slices were stained with cresyl violet and
PREFRONTAL LESIONS counterstained with luxol fast blue, and viewed with a microprojector (Ken-a-vision). Stained sections through the lesions were drawn and compared with the corresponding coordinates from Paxinos and Watson (30) to determine the extent of the lesion. Lesion reconstruction included determination of remaining cortical areas through cytoarchitectonic analysis. Area measurements of lesion size were made (in mm) by tracing the image (1.253) at 0.5-mm increments throughout the lesion on a 7100 PowerPC using MacMeasure software (17). Analysis of secondary subcortical atrophy was accomplished by examining sections at several coordinates (22.3, 22.8, 23.3) rostral-caudal to bregma with an optical microscope to determine neuronal density of the lateral aspect of the MDN and the lateral dorsal thalamic nucleus (LDN). The MDN has major reciprocal connections with the medial frontal cortex and was chosen to assess subcortical degeneration. The LDN, although located at a similar distance from the frontal cortex, does not have such connections with the lesion area and would not be expected to show secondary degeneration. It was therefore chosen as a control structure for neuronal counts. At a magnification of 4, a 5 3 5 ocular square grid was centered over the lateral MDN on the left side of the brain. The magnification was increased to 20 and all healthy stained neurons within alternating squares for each row were counted. Neurons were considered healthy and were counted if the nucleus was visible and was inside the square, staining was even, and no signs of swelling or degeneration were visible. This procedure was repeated for the right side of the brain and counts from the two sides were averaged for each level. The same methods were then applied to the LDN. All counting was done without knowledge of experimental condition. METHOD—EXPERIMENT 2
Animals Thirty-six female Sprague–Dawley albino rats bred in the departmental vivarium were used. The animals were maintained under identical conditions as Experiment 1.
375 when the rat had gone down an arm far enough that his forepaws were at least 25 cm from the center platform. A correct response was a response to an arm that had not been chosen previously during that session. An error was recorded when a rat went into an arm that had previously been visited during that session. The total number of entries per day over testing was also collected as an index of general activity. Histology Histology was conducted as described in Experiment 1. RESULTS
Histological Verification Figure 1 shows a schematic representation of the SMF, LMF, and PLA lesions in coronal sections adapted from the Paxinos and Watson (30) stereotaxic atlas. The cytoarchitectonic nomenclature of Krettek and Price (23) was used in the description of the lesions in the present study. Damage in the SMF lesions extended to include the medial precentral, dorsal, and ventral anterior cingulate areas. LMF damage included the SMF areas and extended into the prelimbic and infralimbic areas. In PLA damage, the lesions began anteriorly from the prelimbic cortex and extended posteriorly to where the corpus callosum crosses the sagittal line. Common to all PLA lesions was total damage of the prelimbic area. Thalamic Degeneration A one-way ANOVA conducted on the MDN cell count data revealed a significant differences in neuronal density among the lesion groups [F(2,33) 5 4.09, p , 0.05]. Post hoc analysis by Tukey’s Honestly Significant Difference (HSD) test revealed that the MDN atrophy was significantly greater in LMF and PLA groups when compared to CON or SMF groups. Moreover, LMF lesions had lower neuronal counts than PLA lesions, whereas SMF lesions did not differ from CON. None of the groups showed any loss of neurons in the LDN. Neuronal counts for both nuclei across groups are shown in Fig. 2.
Apparatus A radial eight-arm maze was used. Each arm (9 3 66 cm) was attached to an octagonal center platform (30.5-cm diameter) and contained molding (2 cm) around the perimeter to prevent the animals from falling. At the end of every arm, a recessed food well (1-cm diameter; 0.5-cm deep) was located. The entire maze was elevated 96 cm above the floor and placed in a well-lit room. Surgery The surgical conditions were identical to Experiment 1. Eight Arm Maze Testing The animals were placed on a restricted diet 5 days after surgery and maintained at 85% of their ad lib. weight. One week after surgery, the animals were habituated to the maze for 7 days. During habituation, the animals were placed on the maze and allowed to freely roam all eight arms for 5 min. The arms were rebaited continuously with food pellets (Cocoa Pebbles) during this time. Testing began 14 days after surgery. The animals were tested once a day for 12 days. At the beginning of each test session, a single food pellet was placed at the end of each goal arm. Each rat was placed on the center platform of the maze at the start of each test session. A test session ended when the rat had either visited all eight arms or spent 10 min on the maze. A response was recorded
General Activity The data for the number of quadrant entries during the last day of habituation are shown in Fig. 3. There were significant differences among groups [F(2, 21) 5 4.69, p , 0.05]. Comparisons between groups (Tukey’s HSD, p , 0.05) revealed that rats in the LMF and PLA groups were more active than rats in the SMF and CON groups. LMF animals did not differ from PLA animals in activity. SMF animals did not differ from CON animals in activity. Avoidance Behavior Significant differences were found in the number of avoidances across groups [F(2, 21) 5 15.74, p , 0.05]. Comparisons between groups (Tukey’s HSD, p , 0.05) revealed marked differences (See Fig. 4, top). CON, LMF, and PLA lesioned animals had a greater number of avoidances than SMF animals. CON animals did not differ in avoidance responses from LMF animals. PLA animals had more avoidances than LMF and CON animals. Trials to Criterion Significant differences were found in the number of trials to criterion across control and surgical groups [F(2, 21) 5 25.20, p , 0.05; see Fig. 4, bottom]. Comparisons between groups (Tukey’s HSD, p , 0.05) revealed similar patterns to avoidance behavior. The CON, LMF, and PLA animals achieved criterion in fewer
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FIG. 1. Serial reconstructions of the smallest (darkened) and largest lesions of the SMF, LMF, and PLA lesion groups. Numbers adjacent to coronal sections represent coordinates in mm relative to bregma.
trials than SMF animals. CON animals did not differ in the number of trials to criterion from LMF or PLA animals. LMF animals did not differ from PLA animals. Radial Arm Maze Performance Figure 5 shows total entry errors (top) and entries/days (bottom) in the radial maze. ANOVAs were used to analyze the total number of days and total number of entry errors to criterion (no re-entry errors before entering all eight arms of the maze in a single session). A significant group effect was found for days to criterion [F(2, 31) 5 27.50, p , 0.05], and Tukey’s tests revealed that the CON and SMF rats were able to reach criterion faster than LMF or PLA rats. PLA rats were able to reach criterion faster than LMF rats. Total entry errors to criterion also differed significantly according to group [F(2, 31) 5 30.32, p , 0.05], and Tukey’s tests showed that the LMF and PLA animals made more errors than either the SMF or CON animals. PLA animals had fewer errors than LMF animals, whereas SMF and CON animals did not differ from each other. DISCUSSION
The present results showed that LMF and PLA lesions produced impairments on the acquisition of a radial arm maze task, whereas SMF lesions did not. In contrast, SMF lesions produced impairments on the acquisition of a four-way shuttle avoidance task, but no deficits in performance were observed following LMF or PLA lesions. Degeneration in the MDN was evident following either LMF or PLA lesions, although the atrophy was somewhat less in the PLA group. No significant MDN cell loss was observed in SMF lesions. Taken together, these findings support and extend prior suggestions that the PFC of the rat is a functionally dissociable region involved in both working memory and attention processes (8,10,27,36). The PFC has often been considered as a part of the neural circuitry subserving working memory processes (12,21). The same
hypothesis has been formulated for the function of the prelimbic area (4,5). Clearly, disruptions in working memory can account for a large part of the results obtained in the present study. The radial arm maze task requires working memory, or the capacity to respond based on information held “on line” in short-term memory that must be updated on a trial by trial basis. Accordingly, LMF lesioned animals demonstrated difficulties every time they needed to hold specific information over a time delay before being able to use it (i.e., which goal areas had been visited). Similarly, PLA lesioned animals showed impairments, although weaker, on the radial arm maze task. These findings are consistent with other reports describing radial arm maze deficits in rats with PFC lesions affecting the region of the prelimbic area (2,22,33,39). Some of the results obtained in the present study, however, do not completely support such an interpretation. We observed no impairments in performance by SMF lesioned rats on the radial arm maze. If, as is traditionally viewed, the PFC as a whole is involved in working memory processes, then one would expect SMF lesions to also produce an impairment on a typical working memory task such as the radial arm maze. However, SMF-lesioned animals were able to successfully acquire the spatial learning habit. If instead it is assumed that the PFC is composed of functionally different subregions, then the observed lack of deficits on the radial arm maze by SMF-lesioned animals occurred because the lesion area spared the circuitry of the prelimbic area. Considering that the prelimbic area has direct connections with structures known to be essential for memory (i.e., hippocampus, 9,19), the observed deficits may have been due to the disruption of efferent projections subserving learning and memory functions. For instance, damage to the MDN has been shown to produce working memory impairments on tasks such as the radial arm maze (5,16,20). We observed neuronal loss in the MDN for both LMF and PLA groups; damage to this circuitry may thus have contributed to impairments in radial arm maze learning by these animals. An obvious question then is why do SMF, not LMF or PLA, lesions impair the acquisition of avoidance behavior? The working
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377 priate response (entering a goal area for food) and inhibiting incorrect responses (re-entering goal areas already visited). LMF and PLA lesion-induced errors in the radial arm maze might be attributable to deficits in response inhibition (i.e., errors). Several anatomical studies show that the prelimbic area shares extensive connections with autonomic centers involved in appetative behaviors as well as several motor structures (7,32,37). Damage to these circuits may have disrupted motor response planning, which was compounded by food deprivation. Some evidence indicates that lesions to the PFC (mostly including the prelimbic area) induce emotionality and hyperactivity in rats (20,25,27). In the case of SMF lesions, the existing prelimbic circuitry enabled these animals to correctly plan their motor responses on the radial arm maze and thus no impairments in performance were observed. However, the response selection hypothesis cannot fully account for the lack of an impairment by SMF animals on the avoidance task if it is assumed that the prelimbic area is involved in response selection. According to this idea, a disruption of avoidance performance should occur from the initial acquisition phase of the task, even when the conditioned stimulus is available at the time of responding. Thus, if the prelimbic area is critically involved in response selection, no impairments in performance should be expected if the circuitry is undamaged. But that was not the case. Instead, it appears that the dorsal subfields of the PFC (i.e., medial precentral and dorsal anterior cingulate cortices) are critically involved in response selection and planning. Considering that these areas contain some features of a premotor cortex (7), it is possible that impairments in executing motor responses (i.e., move to a new quadrant) could be observed on a shuttle avoidance task. The four-way shuttle avoidance task requires the animal to respond readily (i.e., move from quadrant to quadrant) before the onset of the US. It might be that in LMF and PLA rats this impairment is masked by the demonstrated increase in activity. Indeed, we observed LMF and PLA animals to be more active than SMF animals during habituation. Such disruptions in avoidance learning have been reported previously with dorsal PFC lesions damaging the anterior cingulate and precentral medial cortex but sparing most of the prelimbic area (3,38). Similarly, other reports using CRF/extinction para-
FIG. 2. Mean neuronal counts in the MDN (top) and the LDN (bottom). LMF and PLA lesions produced significantly (p) greater atrophy in the MDN than SMF lesions, but LMF lesions induced more degeneration than PLA damage. No atrophy was observed in the LDN in any lesion group. Error bars represent standard errors.
memory hypothesis can account for the absence of effect by LMF and PLA lesions in the four-way shuttle avoidance task because it does not involve any working memory component, but the idea does not adequately explain the SMF-lesion deficit on this task. Two other theories may alternatively explain the observed discrepancies among lesion groups. Both are based on earlier studies which have shown that PFC-lesioned rats are unable to plan a response correctly, even though they correctly remembered the previous information (20,38), and findings which indicate a deficit in attentional processes (11,28,29). The PFC is believed to play a determinant role in response selection as well as in the planning of an action (20). The radial arm maze requires different types of responses (going left or right, going to various goal locations) that involve selecting the appro-
FIG. 3. Mean locomotor activity (number of quadrants crossed in 10-min blocks) of CON, SMF, LMF, and PLA groups during habituation. LMF and PLA animals were significantly (p) more active than either SMF or CON animals. Error bars represent standard errors.
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FRITTS ET AL. the animal’s ability to adequately plan actions involved in fear responding. In terms of the PLA and LMF animals, the common PLA circuit damage may have increased emotionality and activity, thus masking a deficit on a task that relies on continuous movement for successful fear reduction. At the same time, the PFC has been described as critically involved in attention (6,29). The hypothesis may account for the disruptions of performance obtained in tasks requiring high attentional control and time delays. Interestingly, it may help understand the observed deficit of SMF-lesioned animals on the avoidance task. Because fear is largely based on an innate tendency,
FIG. 4. Postsurgical performance on the four-way shuttlebox by CON, SMF, LMF, and PLA groups. Performance is plotted as the mean number of avoidances made during testing (top) and the mean number of trials needed to achieve criterion (eight avoidance responses in a series of ten trials) for avoidance learning (bottom). CON, LMF, and PLA animals made significantly (p) more avoidances and achieved criterion faster than SMF animals. Error bars represent the standard error of the mean.
digms have shown that either prelimbic lesions or large PFC lesions that include at least part of the prelimbic area do not disrupt one-way active avoidance conditioning (4). It is also known that animals with what appear to be small lesions of the PFC that spare the prelimbic area show fewer fear-associated behaviors (less freezing, smaller increases in blood pressure), at least in response to cues previously associated with shock (27,37). Because the prelimbic area sends efferent projections to several motor structures as well as to numerous limbic structures and autonomic centers, it is possible that a reduction in perceived fearfulness (thus perceived salience of the US) interfered with appropriate response selection. That is, the removal of dorsal PFC may have impaired
FIG. 5. Postoperative performance on the radial arm maze task by CON, SMF, LMF, and PLA groups. Performance is shown as the total number of arm entry errors made in each daily session (one trial of the eight-arm maze/session; top) and the mean number of entries per day over testing (bottom). CON and SMF animals achieved criterion significantly faster (p) than LMF or PLA animals, but PLA animals acquired the spatial habit faster than LMF animals. In addition, CON and SMF animals made significantly (p) fewer errors than either LMF or PLA animals. PLA animals made fewer errors than LMF animals. Error bars indicate the standard error of the mean.
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only longer ITIs would require a greater attentional control and then lead to a performance deficit in LMF- and PLA-lesioned rats. PLA and LMF animals, like CON animals, would reliably sit and stare in the direction of the CS or orient to it during its presentation. In contrast, SMF animals behaved similarly yet would pause and did not follow through with an avoidance. Instead, SMF animals remained on the hurdle until the US was presented. Thus, it seems that SMF-lesioned rats could not learn the avoidance task, even though they are as prone to respond as control rats, presumably because they lacked an attention processing mode. It has been suggested that the PFC plays a role in finding appropriate coping strategy in an aversive situation by influencing attentional processes in an attempt to deal with a stressor (27). Our data would suggest more specifically that damage to frontal cortex subfields (e.g., anterior cingulate, infralimbic, medial precentral) may be the basis for the inability of PFC animals to attend and sequence relevant information when required to do so. The idea is partially verified by our data with PLA animals. PLA-lesioned animals were better at attending to the CS and responding correctly to avoid the US than either the LMF- or SMF-lesioned animals. In fact, PLA animals made more avoidances than LMF animals. Activity alone does not adequately account for the group differences because both PLA and LMF animals had equivalently high levels of activity. Considering that LMF and SMF lesions contained some common area damage not shared by PLA lesions, these similarities along with the behavioral data suggest that impairments in attention processing may be involved in the observed findings. That is, the attention deficit in LMF animals may have been masked by increased activity induced from PLA damage. One final point to consider is that the differences between small
and large PFC lesions is due to a mass action effect. The critical point of this view is that the SMF lesions simply left more tissue intact and that the spared tissue is capable of mediating behaviors that approximate control performance. The possibility that the observed differences between lesion groups may be related to the volume of tissue destroyed within this brain region seems unlikely. PLA lesions induced a behavioral pattern of deficit close to, but weaker than, the ones described with larger PFC lesions. This, however, does not seem to be due to a mass action effect given that total PFC damage did not correspond with total task impairments. The fact that lesions restricted to the PLA area reproduce most of the effects generally obtained with larger PFC damage does not mean that prelimbic area is the most determinant part of the PFC. An alternative view to the mass action model instead is to stress the importance of the specific projection areas damaged and spared by the lesions, rather than lesion volume per se. The emphasis in this case would be the fact that LMF rats sustained greater damage than SMF rats to specific projection areas (i.e., PLA), such as was evident in the MDN (7,23,32). In conclusion, our data that lesions of the PFC that include the prelimbic area impair the performance of spatial, but not avoidance, learning are consistent with the hypothesis that the PLA is involved in working memory (2). In addition, the PLA appears to be less involved in attention processes. At present, the basis of our observed differences between lesion groups indicate that damage to specific subfields of the PFC is more critical for the emergence of either spatially organized behaviors or selective attention processes. Thus, the PFC does not appear to be a functionally homologous region.
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navigation: Evidence for impaired planning. Behav. Neurosci. 109: 474 – 484; 1995. Granon, S.; Vidal, C.; Thinus-Blanc, C.; Changeux, J. P.; Poucet, B. Working memory, response selection, and effortful processing in rats with medial prefrontal lesions. Behav. Neurosci. 108:883– 891; 1994. Hannon, R.; Bader, A. A comparison of frontal pole, anterior medial caudate nucleus lesions in the rat. Physiol. Behav. 13:513–521; 1974. Harrison, L. M.; Mair, R. G. A comparison of the effects of frontal cortical and thalamic lesions on measures of spatial learning and memory in the rat. Behav. Brain Res. 75:195–206; 1996. Hook, G. R.; Rasband, W. MacMeasure. A low-cost, easy-to-operate quantitative morphometrics system for the Macintosh computer. In: Bailey, G. W., ed. Proceedings of the 45th Annual Meeting of the Electron Microscopy Society of America. San Francisco: San Francisco Press; 1987. Isaac, W. Role of stimulation and time in the effects of spaced occipital ablation. J. Exp. Psych. 99:249 –355; 1964. Jay, T. M.; Glowinski, J.; Thierry, A. M. Selectivity of the hippocampal projection to the prelimbic area of the prefrontal cortex in the rat. Brain Res. 505:337–340; 1989. Koger, S. M.; Mair, R. G. Comparison of the effects of frontal cortical and thalamic lesions on measures of olfactory learning and memory in the rat. Behav. Neurosci. 108:1088 –1100; 1994. Kolb, B. Prefrontal cortex. In: Kolb, B.; Tees, R., eds. The cerebral cortex of the rat. Cambridge, MA: MIT Press; 1990:437– 458. Kolb, B.; Sutherland, R. J.; Whishaw, I. Q. A comparison of the contributions of the frontal and parietal association cortex to spatial localization in rats. Behav. Neurosci. 97:13–27; 1983. Krettek, J.; Price, J. The cortical projections of the mediodorsal nucleus and adjacent thalamic nuclei in the rat. J. Comp. Neurol. 171: 157–192; 1977. Larsen, J. K.; Divac, I. Selective ablations within the prefrontal cortex of the rat and performance of delayed alternation. Physiol. Psych. 6:15–17; 1978. Maswinkel, H.; Gispen, W. H.; Spruijt, B. M. Effects of an electrolytic
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33. Silva, M. G.; Boyle, M. A.; Finger, S.; Numan, B.; Bouzrara, A.; Almli, C. R. Behavioral effects of large and small lesions of the rat medial frontal cortex. Exp. Brain Res. 65:176 –181; 1986. 34. Squire, L. R. Memory and brain. New York: Oxford University Press; 1987. 35. Uylings, H. B. M.; van Eden, C. G. Qualitative and quantitative comparisons of prefrontal cortex in rat and in primates, including humans. Prog. Brain Res. 85:31– 62; 1990. 36. van Haaren, F.; de Bruin, J. P. C.; Heinsbroek, R. P. W.; van de Poll, N. E. Delayed spatial response alternation: Effects of delay-interval duration and lesions of the medial prefrontal cortex on response accuracy of male and female Wistar rats. Behav. Brain Res. 19:41– 49; 1985. 37. Verberne, A. J. M.; Lewis, S. J.; Worland, P. J.; Beart, P. M.; Jarrott, B.; Christie, M. J.; Louis, W. J. Medial prefrontal cortical lesions modulate baroreflex sensitivity in the rat. Brain Res. 426:243–249; 1987. 38. Winocur, G. Functional dissociations of the hippocampus and prefrontal cortex in learning and memory. Psychobiology 19:11–20. 39. Wolf, C.; Waksman, D.; Finger, S.; Almli, C. R. Large and small medial frontal cortex lesions and spatial performance of the rat. Brain Res. Bull. 18:1–5; 1987.
PII S0031-9384(98)00096 –1
Medial Prefrontal Lesion Deficits Involving or Sparing the Prelimbic Area in the Rat MARY E. FRITTS,1* E. TREY ASBURY,* JAMES E. HORTON† AND WALTER L. ISAAC† *Department of Psychology, Texas Christian University, TCU Box 298920, Ft. Worth, TX 76129; and †Department of Psychology, East Tennessee State University, Box 70649, Johnson City, TN 37614, USA Received 17 September 1997; Accepted 5 March 1998 FRITTS, M. E., E. T. ASBURY, J. E. HORTON AND W. L. ISAAC. Medial prefrontal lesion deficits involving or sparing the prelimbic area in the rat.PHYSIOL BEHAV 64(3) 373–380, 1998.—The rat medial prefrontal cortex (PFC) is believed to play a central role in working memory and selective attention processes. More recently, it has been shown that the effects of large PFC lesions on working memory may be due to the prelimbic area of the PFC. The aim of the present study was to compare the effects of lesions of the prelimbic area with PFC lesions that involved or spared the prelimbic area on shuttlebox avoidance and radial maze learning in rats. The findings indicate that rats with PFC lesions that spared the prelimbic area were impaired at avoidance but not radial arm maze learning, whereas rats with prelimbic lesions or PFC lesions that included this area were impaired on the radial arm maze but not the avoidance learning task. Results support the notion that the medial frontal cortex of the rat is a functionally dissociable region and suggest that the prelimbic area appears to be critical for working memory, but less so for attention processes. © 1998 Elsevier Science Inc. Spatial learning
Working memory
Avoidance learning
Prelimbic area
medial precentral and dorsal anterior cingulate cortices, whereas the ventral area includes the prelimbic and infralimbic cortices (7,32). Although the PFC of the rat has often been considered as a global structure, there is some evidence to suggest a possible functional dissociation between the dorsal and ventral areas (8,10,15,27,36). One subfield of the PFC that has gained considerable attention in this regard is the prelimbic area. The recent interest in the prelimbic area is based on its extensive anatomical connections as well as on growing evidence that most of the effects of large PFC lesions may be due to damage of this subfield (4,5). Numerous studies have shown that large PFC lesions impair performance on measures of delayed alternation, spatial reversals, locomotor activity, and emotionality in rats (10,11,39), whereas small PFC lesions have resulted in control-like performance on similar measures (15,33,39). In these studies, large lesions appear to damage the prelimbic area of the PFC whereas small PFC lesions do not. Unfortunately, it is not possible to make direct lesion comparisons across such findings. Although there may be a number of reasons for the inconsistent behavioral effects of PFC lesions in previous studies (e.g., procedural differences, demand characteristics), serious consideration must be given to variations in lesion size and locus. According to anatomical studies, the prelimbic area of the PFC is considered to be an area homologous to the dorsolateral prefrontal cortex of primates. In both species, the prelimbic area receives massive projections from the MDN (21,23,35). The prelimbic area extends from the frontal pole through the genu of the
CONSIDERABLE evidence indicates that lesions to the dorsolateral area of the prefrontal cortex induce a diverse pattern of impairments in humans and primates. Such deficits are thought to be due to a working memory impairment (12,26); that is, from a dysfunction in short-term memory systems needed for the manipulation of information (1,34). However, equally often is reported an inability of humans and primates with prefrontal lesions to use divided and/or selective attention in order to solve complex cognitive tasks (12,26). On the basis of such findings, it has been concluded that the prefrontal cortex is involved in attention and working memory processes. In a similar fashion, damage to the medial prefrontal cortex (PFC) of the rat has been shown to produce a general impairment on spatial working memory tasks such as the radial arm maze (2,16), the Morris water maze (10,13,22), and spatial alternation (14,20,24). Rats with lesions of the PFC have also been found to be impaired in instrumental tasks which do not require working memory but which involve selective attention. Such tasks require the organism to attend simultaneously toward various pieces of information and select the information that is pertinent for the solution (6,28). Such results provide further evidence that the prefrontal cortex is involved in attention as well as working memory. Traditionally, the PFC of the rat has been assumed to be a functionally homogenous area. In the rat, the PFC is defined as the cortical projection area of the mediodorsal nucleus of the thalamus (MDN; 23) and comprises two distinct regions, a dorsal and a ventral area (7,32). The dorsal area of the PFC is composed of the 1
Medial prefrontal cortex
To whom requests for reprints should be addressed. E-mail: [email protected]
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corpus callosum just underneath the anterior cingulate cortex. It receives afferent input from sensory structures (secondary visual and auditory cortices) and from primary motor cortex. In addition, it also receives extensive direct and indirect afferent inputs from limbic structures (amygdala, hippocampus, entorhinal and piriform cortices) as well as from autonomic centers (e.g., lateral hypothalamus; 7). The prelimbic area sends efferent projections to several motor structures (striatum, pallidum) as well as to numerous limbic structures (amygdala, septum, mamillary bodies, entorhinal cortex) and autonomic centers (lateral hypothalamus, nucleus of the solitary tract; 32). In addition, a direct pathway between the hippocampus (CA1, CA2, subiculum) and the prelimbic area has been found (19). The functions of the prelimbic area in the rat, however, are poorly understood. Because of its extensive connections with the limbic system and the hippocampus, a structure known to be involved in memory processes (9,34), the prelimbic area holds significant potential as a key structure involved in neural circuits subserving learning and memory functions. Studies have shown that damage to the prelimbic area produces a deficit in selecting appropriate responses from working memory, but not reference memory (i.e., memory for constant information; 31). Other findings suggest that the function of the prelimbic area is to update appropriate reactions depending upon the organism’s internal state or by the emotional significance of stimuli (27). At the same time, some reports have indicated a crucial role of the prelimbic area in activities which require high attentional control and effortful information processing (4,13,14). The present study was aimed at examining the effects of prelimbic area lesions with medial prefrontal cortex lesions that involved or spared the prelimbic area on the acquisition of a shuttlebox avoidance task and a radial arm maze task in rats. Given that the rat prelimbic area may modulate attentional processes and working memory, we expected quantitative differences in the performance of rats with frontal damage that involved or spared this subfield. METHOD—EXPERIMENT 1
Animals Thirty-six female albino Sprague-Dawley rats (300 to 350 g) bred in the departmental vivarium were used. All animals were individually housed in plastic cages (30 3 35 3 18 cm) with free access to food and water. Before surgery, the animals were handled for 15 min daily for 14 days. Experimental testing began after a 2-week recovery period following surgery, during which the animals were handled daily. Testing was conducted during the light phase of a 12:12 h light-dark cycle. Avoidance Box A four-way shuttlebox (61 cm2 3 57 cm high) was used (for details, see 18). It was divided into four quadrants by 3.5-cm wooden hurdles. The shuttlebox contained a hardware cloth floor and was covered by a removable double glass plate. The glass cover contained a layer of cheesecloth between the plates to reduce extraneous visual cues. The box was painted white and contrasted by black hurdles. For both the conditioned stimulus (CS) and unconditioned stimulus (US), a compound light and tone stimulus was used. The intensity of this compound stimulus was greater for the US than the CS. The CS was a 15-W light bulb (5 fc) suspended 5.0 cm above the corner of each quadrant along with a standard piezo buzzer (71 db SPL, C scale) placed 2.5 cm above each corner. The US consisted of two 90-W halogen bulbs and was centered above
each quadrant as well as two door buzzers (99 db SPL, C scale) located on the outer edges of each corner. All testing was conducted in a sound-attenuated chamber. Surgery All operations were performed under aseptic conditions. Each animal was anesthetized with Nembutal (50 mg/kg, intraperitoneally (i.p.)) and given atropine sulfate (0.15 cc, 10 mg/cc, i.p.) to minimize secretions during surgery as well as Combiotic (0.10 cc, 300,000 units/mL, intramuscularly (i.m.)) to prevent infection. Sixteen animals sustained single-stage bilateral medial prefrontal cortex lesions (that included or spared the prelimbic area) by aspiration (n 5 8 per group). The lesion area for the PFC that spared the prelimbic area (SMF) included the medial precentral, anterior cingulate, and ventral cingulate areas (23). The lesion area for the PFC that included the prelimbic area (LMF) encompassed the SMF areas as well as the infralimbic and prelimbic areas (23). Eight animals received electrolytic lesions of the prelimbic area (PLA). The electrode was inserted through a hole in the scull and lowered vertically to the following coordinates: 10.6 mm lateral, 3.2 mm anterior to bregma, and 4.3 mm ventral to the top of the skull (30). Electrolytic lesions of the PLA were made by applying a 1.0-mA anodal current for 10 s through a stainless steel electrode (insulated except at tip). Eight animals served as sham controls (CON). Shams were anesthetized and their scalps cut and sutured. Avoidance Training Each animal was habituated to the shuttlebox for 40 min for 2 consecutive days prior to training. Activity was measured by the number of entries into each quadrant across four 10-min observation periods. The animal was placed in the center of the shuttlebox and allowed to explore freely. During avoidance training, the animal was required to learn to leave the quadrant it was located in and to move into any other quadrant when the CS came on in order to avoid the US. The criterion for avoidance learning was defined as having the animal make eight avoidance responses within a series of ten trials (8/10). Each animal was given 50 trials. All training was conducted in one session. On the day of training, the animal was allowed to habituate to the apparatus for 5 min before the initiation of the first trial. All trials were started and ended manually by the experimenter. Each trial began with the onset of the CS in the quadrant where the animal was located. The CS remained on for 3 s or until the rat crossed the hurdle into a different quadrant. If the animal did not make this response during the CS, the CS would end and a 3-s interstimulus interval (ISI) would follow, during which the animal could respond and end the trial. If the animal responded during the CS or the ISI, an avoidance response was recorded. If the animal did not respond within this time, the US (localized to the quadrant the animal was located) would be turned on until the animal crossed the hurdle. An escape response was then recorded for the trial. When either an avoidance or an escape response was made, all stimuli were terminated and the trial ended. A 1-min intertrial interval (ITI) was used. Histology After completion of behavioral testing, animals were given a lethal dose of Nembutal (1.0 mg/kg) and then perfused transcardially with saline followed by 10% buffered formalin. The brains were removed and placed in a 10% buffered formalin solution. Frozen sections (40 um) were taken throughout the extent of the lesion. Every tenth slice was placed on a microscope slide in preparation for staining. Slices were stained with cresyl violet and
PREFRONTAL LESIONS counterstained with luxol fast blue, and viewed with a microprojector (Ken-a-vision). Stained sections through the lesions were drawn and compared with the corresponding coordinates from Paxinos and Watson (30) to determine the extent of the lesion. Lesion reconstruction included determination of remaining cortical areas through cytoarchitectonic analysis. Area measurements of lesion size were made (in mm) by tracing the image (1.253) at 0.5-mm increments throughout the lesion on a 7100 PowerPC using MacMeasure software (17). Analysis of secondary subcortical atrophy was accomplished by examining sections at several coordinates (22.3, 22.8, 23.3) rostral-caudal to bregma with an optical microscope to determine neuronal density of the lateral aspect of the MDN and the lateral dorsal thalamic nucleus (LDN). The MDN has major reciprocal connections with the medial frontal cortex and was chosen to assess subcortical degeneration. The LDN, although located at a similar distance from the frontal cortex, does not have such connections with the lesion area and would not be expected to show secondary degeneration. It was therefore chosen as a control structure for neuronal counts. At a magnification of 4, a 5 3 5 ocular square grid was centered over the lateral MDN on the left side of the brain. The magnification was increased to 20 and all healthy stained neurons within alternating squares for each row were counted. Neurons were considered healthy and were counted if the nucleus was visible and was inside the square, staining was even, and no signs of swelling or degeneration were visible. This procedure was repeated for the right side of the brain and counts from the two sides were averaged for each level. The same methods were then applied to the LDN. All counting was done without knowledge of experimental condition. METHOD—EXPERIMENT 2
Animals Thirty-six female Sprague–Dawley albino rats bred in the departmental vivarium were used. The animals were maintained under identical conditions as Experiment 1.
375 when the rat had gone down an arm far enough that his forepaws were at least 25 cm from the center platform. A correct response was a response to an arm that had not been chosen previously during that session. An error was recorded when a rat went into an arm that had previously been visited during that session. The total number of entries per day over testing was also collected as an index of general activity. Histology Histology was conducted as described in Experiment 1. RESULTS
Histological Verification Figure 1 shows a schematic representation of the SMF, LMF, and PLA lesions in coronal sections adapted from the Paxinos and Watson (30) stereotaxic atlas. The cytoarchitectonic nomenclature of Krettek and Price (23) was used in the description of the lesions in the present study. Damage in the SMF lesions extended to include the medial precentral, dorsal, and ventral anterior cingulate areas. LMF damage included the SMF areas and extended into the prelimbic and infralimbic areas. In PLA damage, the lesions began anteriorly from the prelimbic cortex and extended posteriorly to where the corpus callosum crosses the sagittal line. Common to all PLA lesions was total damage of the prelimbic area. Thalamic Degeneration A one-way ANOVA conducted on the MDN cell count data revealed a significant differences in neuronal density among the lesion groups [F(2,33) 5 4.09, p , 0.05]. Post hoc analysis by Tukey’s Honestly Significant Difference (HSD) test revealed that the MDN atrophy was significantly greater in LMF and PLA groups when compared to CON or SMF groups. Moreover, LMF lesions had lower neuronal counts than PLA lesions, whereas SMF lesions did not differ from CON. None of the groups showed any loss of neurons in the LDN. Neuronal counts for both nuclei across groups are shown in Fig. 2.
Apparatus A radial eight-arm maze was used. Each arm (9 3 66 cm) was attached to an octagonal center platform (30.5-cm diameter) and contained molding (2 cm) around the perimeter to prevent the animals from falling. At the end of every arm, a recessed food well (1-cm diameter; 0.5-cm deep) was located. The entire maze was elevated 96 cm above the floor and placed in a well-lit room. Surgery The surgical conditions were identical to Experiment 1. Eight Arm Maze Testing The animals were placed on a restricted diet 5 days after surgery and maintained at 85% of their ad lib. weight. One week after surgery, the animals were habituated to the maze for 7 days. During habituation, the animals were placed on the maze and allowed to freely roam all eight arms for 5 min. The arms were rebaited continuously with food pellets (Cocoa Pebbles) during this time. Testing began 14 days after surgery. The animals were tested once a day for 12 days. At the beginning of each test session, a single food pellet was placed at the end of each goal arm. Each rat was placed on the center platform of the maze at the start of each test session. A test session ended when the rat had either visited all eight arms or spent 10 min on the maze. A response was recorded
General Activity The data for the number of quadrant entries during the last day of habituation are shown in Fig. 3. There were significant differences among groups [F(2, 21) 5 4.69, p , 0.05]. Comparisons between groups (Tukey’s HSD, p , 0.05) revealed that rats in the LMF and PLA groups were more active than rats in the SMF and CON groups. LMF animals did not differ from PLA animals in activity. SMF animals did not differ from CON animals in activity. Avoidance Behavior Significant differences were found in the number of avoidances across groups [F(2, 21) 5 15.74, p , 0.05]. Comparisons between groups (Tukey’s HSD, p , 0.05) revealed marked differences (See Fig. 4, top). CON, LMF, and PLA lesioned animals had a greater number of avoidances than SMF animals. CON animals did not differ in avoidance responses from LMF animals. PLA animals had more avoidances than LMF and CON animals. Trials to Criterion Significant differences were found in the number of trials to criterion across control and surgical groups [F(2, 21) 5 25.20, p , 0.05; see Fig. 4, bottom]. Comparisons between groups (Tukey’s HSD, p , 0.05) revealed similar patterns to avoidance behavior. The CON, LMF, and PLA animals achieved criterion in fewer
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FIG. 1. Serial reconstructions of the smallest (darkened) and largest lesions of the SMF, LMF, and PLA lesion groups. Numbers adjacent to coronal sections represent coordinates in mm relative to bregma.
trials than SMF animals. CON animals did not differ in the number of trials to criterion from LMF or PLA animals. LMF animals did not differ from PLA animals. Radial Arm Maze Performance Figure 5 shows total entry errors (top) and entries/days (bottom) in the radial maze. ANOVAs were used to analyze the total number of days and total number of entry errors to criterion (no re-entry errors before entering all eight arms of the maze in a single session). A significant group effect was found for days to criterion [F(2, 31) 5 27.50, p , 0.05], and Tukey’s tests revealed that the CON and SMF rats were able to reach criterion faster than LMF or PLA rats. PLA rats were able to reach criterion faster than LMF rats. Total entry errors to criterion also differed significantly according to group [F(2, 31) 5 30.32, p , 0.05], and Tukey’s tests showed that the LMF and PLA animals made more errors than either the SMF or CON animals. PLA animals had fewer errors than LMF animals, whereas SMF and CON animals did not differ from each other. DISCUSSION
The present results showed that LMF and PLA lesions produced impairments on the acquisition of a radial arm maze task, whereas SMF lesions did not. In contrast, SMF lesions produced impairments on the acquisition of a four-way shuttle avoidance task, but no deficits in performance were observed following LMF or PLA lesions. Degeneration in the MDN was evident following either LMF or PLA lesions, although the atrophy was somewhat less in the PLA group. No significant MDN cell loss was observed in SMF lesions. Taken together, these findings support and extend prior suggestions that the PFC of the rat is a functionally dissociable region involved in both working memory and attention processes (8,10,27,36). The PFC has often been considered as a part of the neural circuitry subserving working memory processes (12,21). The same
hypothesis has been formulated for the function of the prelimbic area (4,5). Clearly, disruptions in working memory can account for a large part of the results obtained in the present study. The radial arm maze task requires working memory, or the capacity to respond based on information held “on line” in short-term memory that must be updated on a trial by trial basis. Accordingly, LMF lesioned animals demonstrated difficulties every time they needed to hold specific information over a time delay before being able to use it (i.e., which goal areas had been visited). Similarly, PLA lesioned animals showed impairments, although weaker, on the radial arm maze task. These findings are consistent with other reports describing radial arm maze deficits in rats with PFC lesions affecting the region of the prelimbic area (2,22,33,39). Some of the results obtained in the present study, however, do not completely support such an interpretation. We observed no impairments in performance by SMF lesioned rats on the radial arm maze. If, as is traditionally viewed, the PFC as a whole is involved in working memory processes, then one would expect SMF lesions to also produce an impairment on a typical working memory task such as the radial arm maze. However, SMF-lesioned animals were able to successfully acquire the spatial learning habit. If instead it is assumed that the PFC is composed of functionally different subregions, then the observed lack of deficits on the radial arm maze by SMF-lesioned animals occurred because the lesion area spared the circuitry of the prelimbic area. Considering that the prelimbic area has direct connections with structures known to be essential for memory (i.e., hippocampus, 9,19), the observed deficits may have been due to the disruption of efferent projections subserving learning and memory functions. For instance, damage to the MDN has been shown to produce working memory impairments on tasks such as the radial arm maze (5,16,20). We observed neuronal loss in the MDN for both LMF and PLA groups; damage to this circuitry may thus have contributed to impairments in radial arm maze learning by these animals. An obvious question then is why do SMF, not LMF or PLA, lesions impair the acquisition of avoidance behavior? The working
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377 priate response (entering a goal area for food) and inhibiting incorrect responses (re-entering goal areas already visited). LMF and PLA lesion-induced errors in the radial arm maze might be attributable to deficits in response inhibition (i.e., errors). Several anatomical studies show that the prelimbic area shares extensive connections with autonomic centers involved in appetative behaviors as well as several motor structures (7,32,37). Damage to these circuits may have disrupted motor response planning, which was compounded by food deprivation. Some evidence indicates that lesions to the PFC (mostly including the prelimbic area) induce emotionality and hyperactivity in rats (20,25,27). In the case of SMF lesions, the existing prelimbic circuitry enabled these animals to correctly plan their motor responses on the radial arm maze and thus no impairments in performance were observed. However, the response selection hypothesis cannot fully account for the lack of an impairment by SMF animals on the avoidance task if it is assumed that the prelimbic area is involved in response selection. According to this idea, a disruption of avoidance performance should occur from the initial acquisition phase of the task, even when the conditioned stimulus is available at the time of responding. Thus, if the prelimbic area is critically involved in response selection, no impairments in performance should be expected if the circuitry is undamaged. But that was not the case. Instead, it appears that the dorsal subfields of the PFC (i.e., medial precentral and dorsal anterior cingulate cortices) are critically involved in response selection and planning. Considering that these areas contain some features of a premotor cortex (7), it is possible that impairments in executing motor responses (i.e., move to a new quadrant) could be observed on a shuttle avoidance task. The four-way shuttle avoidance task requires the animal to respond readily (i.e., move from quadrant to quadrant) before the onset of the US. It might be that in LMF and PLA rats this impairment is masked by the demonstrated increase in activity. Indeed, we observed LMF and PLA animals to be more active than SMF animals during habituation. Such disruptions in avoidance learning have been reported previously with dorsal PFC lesions damaging the anterior cingulate and precentral medial cortex but sparing most of the prelimbic area (3,38). Similarly, other reports using CRF/extinction para-
FIG. 2. Mean neuronal counts in the MDN (top) and the LDN (bottom). LMF and PLA lesions produced significantly (p) greater atrophy in the MDN than SMF lesions, but LMF lesions induced more degeneration than PLA damage. No atrophy was observed in the LDN in any lesion group. Error bars represent standard errors.
memory hypothesis can account for the absence of effect by LMF and PLA lesions in the four-way shuttle avoidance task because it does not involve any working memory component, but the idea does not adequately explain the SMF-lesion deficit on this task. Two other theories may alternatively explain the observed discrepancies among lesion groups. Both are based on earlier studies which have shown that PFC-lesioned rats are unable to plan a response correctly, even though they correctly remembered the previous information (20,38), and findings which indicate a deficit in attentional processes (11,28,29). The PFC is believed to play a determinant role in response selection as well as in the planning of an action (20). The radial arm maze requires different types of responses (going left or right, going to various goal locations) that involve selecting the appro-
FIG. 3. Mean locomotor activity (number of quadrants crossed in 10-min blocks) of CON, SMF, LMF, and PLA groups during habituation. LMF and PLA animals were significantly (p) more active than either SMF or CON animals. Error bars represent standard errors.
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FRITTS ET AL. the animal’s ability to adequately plan actions involved in fear responding. In terms of the PLA and LMF animals, the common PLA circuit damage may have increased emotionality and activity, thus masking a deficit on a task that relies on continuous movement for successful fear reduction. At the same time, the PFC has been described as critically involved in attention (6,29). The hypothesis may account for the disruptions of performance obtained in tasks requiring high attentional control and time delays. Interestingly, it may help understand the observed deficit of SMF-lesioned animals on the avoidance task. Because fear is largely based on an innate tendency,
FIG. 4. Postsurgical performance on the four-way shuttlebox by CON, SMF, LMF, and PLA groups. Performance is plotted as the mean number of avoidances made during testing (top) and the mean number of trials needed to achieve criterion (eight avoidance responses in a series of ten trials) for avoidance learning (bottom). CON, LMF, and PLA animals made significantly (p) more avoidances and achieved criterion faster than SMF animals. Error bars represent the standard error of the mean.
digms have shown that either prelimbic lesions or large PFC lesions that include at least part of the prelimbic area do not disrupt one-way active avoidance conditioning (4). It is also known that animals with what appear to be small lesions of the PFC that spare the prelimbic area show fewer fear-associated behaviors (less freezing, smaller increases in blood pressure), at least in response to cues previously associated with shock (27,37). Because the prelimbic area sends efferent projections to several motor structures as well as to numerous limbic structures and autonomic centers, it is possible that a reduction in perceived fearfulness (thus perceived salience of the US) interfered with appropriate response selection. That is, the removal of dorsal PFC may have impaired
FIG. 5. Postoperative performance on the radial arm maze task by CON, SMF, LMF, and PLA groups. Performance is shown as the total number of arm entry errors made in each daily session (one trial of the eight-arm maze/session; top) and the mean number of entries per day over testing (bottom). CON and SMF animals achieved criterion significantly faster (p) than LMF or PLA animals, but PLA animals acquired the spatial habit faster than LMF animals. In addition, CON and SMF animals made significantly (p) fewer errors than either LMF or PLA animals. PLA animals made fewer errors than LMF animals. Error bars indicate the standard error of the mean.
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only longer ITIs would require a greater attentional control and then lead to a performance deficit in LMF- and PLA-lesioned rats. PLA and LMF animals, like CON animals, would reliably sit and stare in the direction of the CS or orient to it during its presentation. In contrast, SMF animals behaved similarly yet would pause and did not follow through with an avoidance. Instead, SMF animals remained on the hurdle until the US was presented. Thus, it seems that SMF-lesioned rats could not learn the avoidance task, even though they are as prone to respond as control rats, presumably because they lacked an attention processing mode. It has been suggested that the PFC plays a role in finding appropriate coping strategy in an aversive situation by influencing attentional processes in an attempt to deal with a stressor (27). Our data would suggest more specifically that damage to frontal cortex subfields (e.g., anterior cingulate, infralimbic, medial precentral) may be the basis for the inability of PFC animals to attend and sequence relevant information when required to do so. The idea is partially verified by our data with PLA animals. PLA-lesioned animals were better at attending to the CS and responding correctly to avoid the US than either the LMF- or SMF-lesioned animals. In fact, PLA animals made more avoidances than LMF animals. Activity alone does not adequately account for the group differences because both PLA and LMF animals had equivalently high levels of activity. Considering that LMF and SMF lesions contained some common area damage not shared by PLA lesions, these similarities along with the behavioral data suggest that impairments in attention processing may be involved in the observed findings. That is, the attention deficit in LMF animals may have been masked by increased activity induced from PLA damage. One final point to consider is that the differences between small
and large PFC lesions is due to a mass action effect. The critical point of this view is that the SMF lesions simply left more tissue intact and that the spared tissue is capable of mediating behaviors that approximate control performance. The possibility that the observed differences between lesion groups may be related to the volume of tissue destroyed within this brain region seems unlikely. PLA lesions induced a behavioral pattern of deficit close to, but weaker than, the ones described with larger PFC lesions. This, however, does not seem to be due to a mass action effect given that total PFC damage did not correspond with total task impairments. The fact that lesions restricted to the PLA area reproduce most of the effects generally obtained with larger PFC damage does not mean that prelimbic area is the most determinant part of the PFC. An alternative view to the mass action model instead is to stress the importance of the specific projection areas damaged and spared by the lesions, rather than lesion volume per se. The emphasis in this case would be the fact that LMF rats sustained greater damage than SMF rats to specific projection areas (i.e., PLA), such as was evident in the MDN (7,23,32). In conclusion, our data that lesions of the PFC that include the prelimbic area impair the performance of spatial, but not avoidance, learning are consistent with the hypothesis that the PLA is involved in working memory (2). In addition, the PLA appears to be less involved in attention processes. At present, the basis of our observed differences between lesion groups indicate that damage to specific subfields of the PFC is more critical for the emergence of either spatially organized behaviors or selective attention processes. Thus, the PFC does not appear to be a functionally homologous region.
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