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Single midline thalamic neurons projecting to both the ventral striatum and the prefrontal cortex in the rat
Pergamon
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Neuroscience Vol. 86, No. 2, pp. 635–649, 1998 Copyright 1998 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0306–4522/98 $19.00+0.00 S0306-4522(98)00062-1
SINGLE MIDLINE THALAMIC NEURONS PROJECTING TO BOTH THE VENTRAL STRIATUM AND THE PREFRONTAL CORTEX IN THE RAT K. OTAKE* and Y. NAKAMURA Department of Anatomy, Faculty of Medicine, Tokyo Medical and Dental University, Yushima 1-5-45, Bunkyo-ku, Tokyo 113-8519, Japan Abstract––The midline thalamic nuclei have been known to send projection fibres to the ventral striatum and the autonomic/limbic-associated areas of the prefrontal cortex. In the present study, we sought to determine whether or not single midline thalamic neurons project both to the ventral striatum and to the cerebral cortical areas. Experiments were performed on chloral hydrate-anaesthetized male Sprague– Dawley rats; two fluorescent retrograde tracers were centred on the medial or lateral part of the nucleus accumbens—the major part of the ventral striatum—and the medial or lateral prefrontal viscerolimbic cortex. Our retrograde double-labelling study revealed that a subset of midline thalamic neurons send projection fibres to both the nucleus accumbens and the cerebral cortex. Such neurons projecting to both targets were principally identified in the paraventricular thalamic nucleus. The majority of the duallylabelled neurons in the paraventricular thalamic nucleus projected to the lateral part of the nucleus accumbens and the medial wall of the prefrontal cortex. Dually-labelled neurons were additionally found in other midline nuclei, including the paratenial, intermediodorsal, rhomboid, and reuniens nuclei, as well as in the medial part of the parafascicular thalamic nucleus. Dually-projecting neurons identified in the present study may represent a potential link between the limbic striatum and the viscerolimbic-associated cortex, thus suggesting that non-discriminative information relayed to the prefrontal cortex might exert an influence through the same neurons on the nucleus accumbens implicated in affective behaviour. 1998 IBRO. Published by Elsevier Science Ltd. Key words: paraventricular thalamic nucleus, branching projection, retrograde double labelling, nucleus accumbens, infralimbic cortex, affective behaviour.
The ventral striatum, comprising the nucleus accumbens, the medium-cell parts of the olfactory tubercle and the fundus striati,24 has been associated with the limbic system due to its dense afferents from limbic-related structures including the amygdala, the hippocampal formation and the limbic-affiliated areas of the cerebral cortex.20,35,64 The ventral striatum receives large projections also from the thalamus, primarily from the intralaminar and midline nuclei,4,19,30,41,53 as does the dorsal striatum.23,36,49,61,68 The nucleus accumbens, a major sector of the ventral striatum implicated in various types of behaviour (see Ref. 51, for review), has been shown to exhibit a highly compartmental organization; it is now widely accepted that this nucleus is subdivided into a ventromedial part (referred to as the ‘‘shell’’ together with the olfactory tubercle) and a dorsolateral part (referred to as ‘‘core’’) in the rat, based on afferent/efferent relationships and/or differences in immunoreactivity for a variety of substances.6,7,21,25,26,69,74–76
*To whom correspondence should be addressed. Abbreviations: FG, FluoroGold; FR, FluoroRed; PVT, paraventricular thalamic nucleus. 635
The midline thalamic nuclei innervate the specific regions of the prefrontal cortex, which largely overlap the cortical areas sending projection fibres to the thalamic targets in the ventral striatum, and thus constitute the ‘‘parallelly arranged’’ basal gangliathalamocortical circuits.4,5,18,37,53 A critical question arising here is whether or not single midline thalamic neurons sending projection fibres to the ventral striatum also project directly to the cerebral cortex so as to influence the prefrontal cortical system at both the cortical and striatal levels. The possibility of such projections was investigated as for the thalamic neurons sending axons to both the dorsal striatum and the cortex, and the thalamostriatal projection fibres were shown to be, at least, partly derived from branching axons of the thalamocortical fibres.12,31,36,59,63 On the other hand, the existence of thalamic neurons sending projection fibres to both the ventral striatum and the cerebral cortex was denied in a recent study.61 The present study was therefore designed to re-evaluate the topographical organization of midline thalamic neurons which project to the nucleus accumbens and the prefrontal cortical areas with special emphasis on branching projections to these targets. Of particular interest is the possibility of branching projections from the
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paraventricular thalamic nucleus (PVT), a dorsal midline thalamic constituent which was demonstrated as a major source of thalamic input to the nucleus accumbens.4 This midline nucleus has long been considered as epithalamic due to the experimental data suggesting the lack of cortical projections, as well as from a developmental point of view.1,32 Recent anterograde transport studies verified the PVT as clearly sending substantial projection fibres to the medially-located infralimbic as well as to laterallay-located agranular insular areas of the prefrontal cerebral cortex.5,38 Although the PVT reportedly send projection fibres to both the core and shell of the nucleus accumbens, previous works suggested that the projections to these two accumbal subdivisions might originate from separate populations of the PVT neurons.4,16,53 Therefore, the present study also focused on the topographical organization of the core-projecting populations and shell-projecting populations of the PVT neurons. A preliminary report of this study was presented in abstract form.43 EXPERIMENTAL PROCEDURES
Data were obtained from eight adult male Sprague– Dawley rats (Nippon Bio-Supp. Center, Tokyo, Japan) weighing 200–250 g. Rats were anaesthetized with chloral hydrate (400 mg/kg, i.p.) and then mounted on a stereotaxic apparatus. The brain was exposed through small burr holes, and injections of retrograde tracers were made at coordinates derived from the atlas of Paxinos and Watson.50 Each rat received two kinds of fluorescent retrograde tracers, FluoroRed15 (FR, generously provided by Tombow Co., Ltd., Tokyo, Japan) and FluoroGold (FG, Fluorochrome, Englewood, CO, U.S.A.) in the following structures unilaterally on the same side: FR was injected into the shell or core subdivision of the nucleus accumbens, and FG into the infralimbic or agranular insular area of the prefrontal cortex. Since the thalamocortical fibres arising in the PVT appear to reach their target area in the medial prefrontal cortex, in part, through the dorsal part of the shell of the nucleus accumbens,38,71 coronal knife cuts (1 mm wide) were made just rostral to the shell of the nucleus accumbens before each injection.37,48 Injections of tracers were then made by pressure with glass micropipettes filled with an approximate volume of 50 nl of a 2% aqueous solution of FG and an approximate volume of 100 nl of undiluted FR.
The micropipettes were left in situ for 5–10 min following each injection. After a survival period of five to seven days, the rats received an overdose of chloral hydrate and were perfused transcardially with heparinized saline, followed by 4% paraformaldehyde in 0.1 M phosphate buffer. The brains were removed from the skull, postfixed for 2–3 h in the same fixative, and immersed in 0.1 M phosphate buffer containing 10% sucrose overnight at 4C. The brains were sectioned transversely at 40 µm on a freezing microtome, mounted on gelatin-coated slides, and air dried. The sections were examined with an epifluorescence microscope (Nikon FX) and photographed on T-MAX 400 film (Kodak). RESULTS
Study I: Combined injections into the lateral part of the ventral striatum and the medial prefrontal cortical area Two rats (ACC-15 and 16) received injections of FR and FG into the lateral part of the ventral striatum and the medial prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 1 (ACC-15). The FR deposit involved the lateral part of the core of the nucleus accumbens and the lateral part of the anterior limb of the anterior commissure and did not spread into adjacent part of the caudate–putamen, which was nevertheless penetrated by micropipette tracks. The FG deposit was centred on the infralimbic area, where virtually all cortical laminae were involved. The adjacent cortical areas, including the prelimbic area and dorsal peduncular cortex, are also involved to a variable degree, but the tracer did not spread into the nucleus accumbens. The ACC-16 revealed similar injection sites to the ACC-15, resulting in almost identical patterns of distribution of single- and/or double-labelled cell bodies. FluoroRed-labelled neurons. Neurons labelled retrogradely with FR were distributed in the midline and medial portions of the thalamus, exclusively on the side ipsilateral to the injection site. Many retrogradely-labelled neurons were found in the PVT throughout its rostrocaudal extent including its rostroventral extension (Figs 1a–e, 2a,c). A small
Abbreviations used in the figures ac Acb AcbC AcbSh AI CeM CL CP DI f fr IAM IL IMD LHb MD
anterior commissure nucleus accumbens nucleus accumbens, core nucleus accumbens, shell agranular insular cortex central medial thalamic nucleus central lateral thalamic nucleus caudate–putamen dysgranular insular cortex fornix fasciculus retroflexus interanteromedial thalamic nucleus infralimbic cortex intermediodorsal thalamic nucleus lateral habenular nucleus mediodorsal thalamic nucleus
MHb mt PC PFl PFm Pir PL PT PVT rf Re Rh sm SPF V3
medial habenular nucleus mammillothalamic tract paracentral thalamic nucleus parafascicular thalamic nucleus, lateral part parafascicular thalamic nucleus, medial part piriform cortex prelimbic cortex paratenial thalamic nucleus paraventricular thalamic nucleus fasciculus retroflexus reuniens thalamic nucleus rhomboid thalamic nucleus stria medullaris subparafascicular nucleus third ventricle
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Fig. 1. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-15. The tracer FG was injected into the medial prefrontal cortex (open arrow) and the FR into the core subdivision of the nucleus accumbens (solid arrow). Five representative transverse sections are arranged rostocaudally in a–e. Cells single-labelled with FG are indicated by dots; cells single-labelled with FR by open circles and dually-labelled cells by solid circles.
to moderate number of labelled neurons were found in other midline/intralaminar nuclei, including the paratenial (Fig. 1a, b), the central medial (Fig. 1b–d), intermediodorsal (Fig. 1c,d), reuniens (Figs 1b–d, 3a,c) and rhomboid thalamic nuclei (Fig. 1b–d and Fig. 4a). The medial part of the parafascicular nucleus were extensively labelled (Figs 1e, 4c). Retrogradely-labelled neurons were also found in mediodorsal nucleus (Fig. 1b–d).
FluoroGold-labelled neurons. Neurons retrogradely labelled from the infralimbic area were distributed in the midline and medial portions of the thalamus, exclusively on the side ipsilateral to the injection site. A major source of infralimbic afferents was mapped to the PVT throughout its rostrocaudal extent (Fig. 1a–e). A small to moderate number of labelled cells was also identified in the mediodorsal thalamic nucleus, principally in its rostral portion
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Fig. 2. Photomicrographs of retrogradely-labelled cells in the PVT from injections of FR and FG into the accumbens core (a, c) and infralimbic cortex (b, d), respectively. Open arrows in a and b indicate the midline points on the ventricular surface. Boxed areas in a and b are shown under high power magnification in c and d, respectively. Asterisks in c and d indicate dually-labelled cells. Data from case ACC-16. Scale bar=100 µm (a, b); 50 µm (c, d).
(Fig. 1b). The paratenial (Fig. 1a, b) and central medial (Fig. 1a–d) thalamic nuclei were lightly labelled. Ventral constituents of the midline thalamus, i.e. reuniens and rhomboid nuclei (Figs 1b–d,
3b, d, 4b), as well as the medial part of the parafascicular nucleus were also labelled (Figs 1e, 4d). Retrogradely-labelled cells were sparse in the ventral tier and in other thalamic nuclei (not illustrated).
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Fig. 3. Photomicrographs of retrogradely-labelled cells in the reuniens thalamic nucleus from injections of FR and FG into the accumbens core (a, c) and infralimbic cortex (b, d), respectively. Boxed areas in a and b are shown under high power magnification in c and d, respectively. Arrows in c and d indicate dually-labelled cells. Data from case ACC-16. Scale bar=100 µm (a, b); 50 µm (c, d).
Dually-labelled neurons. The overlap of cortical afferents and striatal afferents was significantly observed in the dorsal part of the midline nuclei. A large number of dually-labelled neurons were located throughout the rostrocaudal extent of the PVT, with a mediodorsal prevalence (Figs 1, 2). Almost 50% of FR-positive and/or FG-positive neurons were dually labelled (Fig. 9). Dually-labelled neurons were also found in other midline nuclei, including, reuniens (Figs 1b–d, 3), rhomboid (Figs 1c, d, 4a, b), and medial part of the parafascicular thalamic nuclei (Figs 1e, 4c, d). Dually-labelled cells were sparse in the paratenial (Fig. 1a, b), intermediodorsal (Fig. 1c, d), and central medial thalamic nuclei (Fig. 1b–d) Study II: Combined injections into the medial part of the ventral striatum and medial prefrontal cortical area In this series of studies, cases ACC-13 and 14 received injections of FR and FG into the medial
part of the ventral striatum and medial prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 5 (ACC-13). The FR deposit covered most of the lateral part of the shell of the nucleus accumbens and its adjacent regions, including the ventral pallidum, and does not extend into the lateral septum or the core subdivision of the nucleus accumbens. The injection deposit of FG involved infralimbic and prelimbic cortices just as in Study I, resulting in almost identical distribution patterns of labelled cell bodies. The ACC-14 revealed similar injection sites to the ACC-13, resulting in almost identical patterns of distribution of single- and/or double-labelled cell bodies. FluoroRed-labelled neurons. Injections of FR into the shell subdivision of the nucleus accumbens resulted in heavy labelling of thalamic structures, exclusively on the side ipsilateral to the injection site. The labelled neurons were concentrated in PVT,
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Fig. 4.
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including the rostroventral extension (Fig. 5a–e). In the PVT, they skewed medially and were less abundant in the lateral border zone, transitional with the mediodorsal nucleus (Fig. 5b–d). Cells labelled from the accumbens shell were also seen in other midline nuclei, i.e. the paratenial (Fig. 5a, b) and intermediodorsal thalamic nuclei (Fig. 5c, d). In sharp contrast to the cases with injection into accumbens core, retrogradely-labelled neurons were sparse in central medial, rhomboid, reuniens nuclei, and medial portion of the parafascicular nucleus. Dually-labelled neurons. A moderate to small number of dually-labelled neurons were identified in the PVT throughout its rostrocaudal extent, including its rostroventral extension (Fig. 5a–e). The ratio of dually-labelled neurons in the PVT was 27.6% (Fig. 9). Dually-labelled neurons were also present in the paratenial nucleus, especially its rostral part (Fig. 5a, b). The intermediodorsal, rhomboid nuclei and medial part of the parafascicular nucleus contains few dually-labelled cells. Study III: Combined injections into the lateral part of the ventral striatum and the lateral prefrontal cortical area In this study, cases ACC-04 and 06 received injections of FR and FG into the lateral part of the nucleus accumbens and the lateral prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 6 (ACC-04). The FR deposit was restricted in the lateral part of the core of the nucleus accumbens with involvement of the lateral part of the anterior limb of the anterior commissure, just as in Study I, and resulted in almost identical patterns of retrograde labelling. The FG deposit in case ACC-04 was centred on the agranular insular area with spread into the adjacent cortical area including the dysgranular insular cortex, and the diffusion sphere infiltrated virtually all cortical laminae of the agranular insular cortex. FluoroGold-labelled neurons. Cell bodies labelled from the agranular insular cortex were principally distributed in the midline and paramedian areas of the thalamus, exclusively on the side ipsilateral to the injection site. A few labelled cells was identified in the PVT (Fig. 6a–d), paratenial (Fig. 6a), intermediodorsal (Fig. 6c), central medial (Fig. 6b, c) and the reuniens nuclei (Fig. 6b, c) and the medial part of the parafascicular thalamic nucleus (Fig. 6d). The medial subdivision of the mediodorsal thalamic nucleus bordering the PVT and intermediodorsal nucleus
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were also backfilled (Fig. 6b, c). Neurons were also labelled in the ventral tier nuclei (not illustrated). Dually-labelled neurons. In these cases, most of cortical and accumbal afferents were spatially segregated. A small number of dually-labelled neurons were detected in the PVT (Fig. 6), where less than 10% of FR-positive and/or FG-positive cell bodies were doubly labelled (Fig. 9). Dually-labelled cells were scattered also in other midline/intralaminar thalamic nuclei, i.e. the paratenial (Fig. 6b), intermediodorsal (Fig. 6c), rhomboid (Fig. 6b), reuniens (Fig. 6b, c), central medial (Fig. 6b, c) and parafascicular thalamic nuclei (Fig. 6d). Study IV: Combined injections into the medial part of the ventral striatum and the lateral prefrontal cortical area Two cases (ACC-09 and 11) received injections of FR and FG into the medial part of the ventral striatum and the lateral prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 7. The FR deposit involved the medial part of the accumbens shell and the olfactory tubercle, just as in Study II and resulted in almost identical patterns of distribution of labelled cell bodies. The FG deposit involved the agranular and dysgranular insular areas, and the diffusion sphere infiltrated all cortical laminae and the distribution patterns of FG-labelled neurons were comparable to those observed in Study III. Dually-labelled neurons. In these cases, most of cortical and accumbal afferents were spatially segregated and only a few neurons were doubly labelled in the midline, mainly in the PVT including its rostroventral extension (Figs 7a–d, 8). In the PVT, less than 10% of FR-positive and/or FG-positive cell bodies were doubly labelled (Fig. 9). In the paratenial, reuniens, central medial nuclei and the medial part of the parafascicular nucleus, dually-labelled cells were rarely found. DISCUSSION
The present study indicates that a substantial number of single midline thalamic neurons send projection fibres to both the prefrontal cortical areas and the nucleus accumbens. A majority of such branching projections was identified as arising from the PVT and projecting to the medial wall of the prefrontal cortex, including the infra- and prelimbic areas, and the core subdivision of the nucleus accumbens (as revealed in the Study I).
Fig. 4. Photomicrographs of retrogradely-labelled cells in the rhomboid thalamic nucleus from injections of FR and FG into the accumbens core (a) and infralimbic cortex (b), respectively, and those in the medial part of the parafascicular thalamic nucleus from injection of FR and FG into the accumbens core (c) and infralimbic cortex (d), respectively. Arrows indicate dually-labelled cells. Data from case ACC-16. Scale bar=50 µm.
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Fig. 5. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-13. The tracer FG was injected into the medial prefrontal cortex (open arrow) and the FR into the shell subdivision of the nucleus accumbens (solid arrow). Five representative transverse sections are arranged rostocaudally in a–e. FG-labelled cell indicated by dots; FR-labelled cells by open circles and dually-labelled cells by solid circles.
Methodological considerations Recently, Shinonaga et al.,62 by employing a similar retrograde double-labelling technique, have shown that projections from the midline thalamic
nuclei, including the PVT, to the prefrontal cortex and the nucleus accumbens take their origin from separate neuronal populations. This was discovered during the attempt to investigate single basal amygdaloid nucleus neurons sending bifurcating axons to
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Fig. 6. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-04. The tracer FG was injected into the lateral prefrontal cortex (open arrow) and the FR into the core subdivision of the nucleus accumbens (solid arrow). Four representative transverse sections are arranged rostocaudally in a–d. FG-labelled cell indicated by dots; FR-labelled cells by open circles and dually-labelled cells by solid circles.
the same two targets. The discrepancy between the result of Shinonaga et al.62 and that of ours is likely due to subtle differences in the centre of injection sites and may be relevant to the recent observation that
projection fibres originating in the midline thalamus and those originating in the basolateral amygdala show different patterns of distribution in similar regions in the nucleus accumbens and the prefrontal
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Fig. 7. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-09. The tracer FG was injected into the lateral prefrontal cortex (open arrow) and the FR into the shell subdivision of the nucleus accumbens (solid arrow). Four representative transverse sections are arranged rostocaudally in a–d. FG-labelled cell indicated by dots; FR-labelled cells by open circles and dually-labelled cells by solid circles.
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Fig. 8. Photomicrographs of retrogradely-labelled cells in the PVT from injections of FR and FG into the accumbens shell (a, c) and insular cortex (b, d), respectively. Open arrows in a and b indicate the midline points on the ventricular surface. Boxed areas in a and b are shown under high power magnification in c and d, respectively. Arrows in c and d indicate dually-labelled cells. Data from case ACC-11. Scale bar in b=100 µm (a, b) and in d=50 µm (c, d).
cortices.71,72 For example, the fibres originating in the basolateral amygdaloid nucleus were observed predominantly within the cell clusters in the accumbal shell which are avoided by fibres from the PVT.71
The tracer deposits in our study precisely overlapped the areas which receive heavy projection fibres from the midline thalamus.4,5 Besides, we made coronal knife cuts just rostral to the accumbal injection sites
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Fig. 9. A bar chart showing the ratios of the numbers of cell bodies single-labelled with FR, those single-labelled with FG, and double-labelled cell bodies within the PVT, obtained from representative sections of each study.
to exclude the possibility of artificial double-labelling caused by accumbal injection (via uptake by fibres passing through the nucleus accumbens en route to the medial prefrontal cortex38,71) as well as by cortical injection (via axon terminal uptake). Functional considerations The midline thalamus, including the PVT, receives diverse afferent inputs conveyed by exteroceptive and visceral relay nuclei13,42,44–47,60 and thus is thought to lie in a pivotal position between the sensorium and the cerebral cortex. Although the precise function ascribed to the midline nuclei is still unclear, several lines of evidence suggest that these nuclei are involved in non-discriminative aspects of information, in contrast to the specific or discriminative thalamic relay nuclei concerned with relaying epicritical sense.2,11,52,73 The evidence of branching projections identified in the present study may represent a potential link between the cerebral cortex and basal ganglia, and thus indicates that non-discriminative or affective information conveyed to the prefrontal cortex might influence the ventral striatal (accumbal) functions through the same neurons. The thalamo-
cortical projection fibres arising from the PVT terminate predominantly in the layers I, V and VI of the prefrontal cortical areas,5 which are closely related to the ventral striatum through corticostriatal projections.17 Thus, the PVT is suggested to lie in a position to influence the ventral striatum indirectly as well via cortical neurons (cortically-relayed indirect projections). Recent anatomical study demonstrated that thalamostriatal fibres originating in the PVT and afferents from the prefrontal cortex exhibit highly organized patterns of distribution within the nucleus accumbens.71 In the core, for example, both sets of fibres overlap in the small regions (i.e. patches), which are known to project to the pars compacta of substantia nigra,7 and thus direct thalamo-accumbal fibres and cortically-relayed indirect fibres possibly impose hierarchical controls over somatic motor function with an intimate relation with the substantia nigra dopaminergic neurons. The present results indicate that PVT may act as an interface of the autonomic–limbic and the somatic motor function. On the other hand, in the accumbens shell, the cortical fibres are most prominent in the circumscribed regions with strong calbindin immunoreactivity which are avoided by fibres arising in the PVT;71 the PVT neurons likely have a different influence upon the accumbens shell based on the mode of projections, i.e. direct projections versus cortically-relayed indirect projections. The shell of the accumbens is established as a region related with the autonomic and limbic motor control. The projections of PVT neurons to the prefrontal cortex and the shell of the accumbens might be related to this function. A large body of evidence suggests the importance of the nucleus accumbens in behaviour, in which mesolimbic dopamine system play a pivotal role (see Ref. 51 for review). Examples include the observation that injections of dopamine or its agonists into the nucleus accumbens enhanced locomotor activity in rats.14,54,55,56 Dense projections from the PVT to dopamine-rich areas in the nucleus accumbens (see, e.g., Ref. 70) indicates the possibility of the role of PVT in modulating the behavioural function ascribed to the nucleus accumbens. Perhaps related is evidence that enhancement of the activity of the PVT neurons induces defense behaviour27 as well as increased dopamine utilization in the nucleus accumbens.33 Electrical stimulation of the medial prefrontal cortex, primarily the prelimbic and infralimbic areas resulted in cardiovascular and gastromotor effects.9,10,22,29,39 Structurally corroborative are anatomical findings that infralimbic efferents innervate a number of autonomic-related sites, including several hypothalamic nuclei, the bed nucleus of the stria terminalis, the periaqueductal gray matter, the parabrachial nuclei, the nucleus of the solitary tract, the dorsal motor vagal nucleus, the ventrolateral medulla, and also the intermediolateral column of the spinal cord.3,28,39,40,45,58,65–67 Autonomic changes
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associated with lesions or stimulation of the midline thalamus, including the PVT,8,57 might be related to the neural circuits relayed by the medial prefrontal cortex. CONCLUSIONS
Neurons in the midline thalamic nuclei, including PVT, give off projection fibres to both the nucleus accumbens and the medial prefrontal cortex. It is conceivable that these branched afferents likely
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couple behavioural and autonomic components of emotional expression and may contribute to some autonomic changes observed during affective behaviour (see, e.g., Ref. 34 for review).
Acknowledgements—The authors thank Mrs Mie Taguchi for her technical assistance. The gift of FluoroRed from Tombow Co., Ltd is gratefully acknowledged. This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 08780720 and 08680810) from the Japan Ministry of Education, Science, Sports and Culture.
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Neuroscience Vol. 86, No. 2, pp. 635–649, 1998 Copyright 1998 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0306–4522/98 $19.00+0.00 S0306-4522(98)00062-1
SINGLE MIDLINE THALAMIC NEURONS PROJECTING TO BOTH THE VENTRAL STRIATUM AND THE PREFRONTAL CORTEX IN THE RAT K. OTAKE* and Y. NAKAMURA Department of Anatomy, Faculty of Medicine, Tokyo Medical and Dental University, Yushima 1-5-45, Bunkyo-ku, Tokyo 113-8519, Japan Abstract––The midline thalamic nuclei have been known to send projection fibres to the ventral striatum and the autonomic/limbic-associated areas of the prefrontal cortex. In the present study, we sought to determine whether or not single midline thalamic neurons project both to the ventral striatum and to the cerebral cortical areas. Experiments were performed on chloral hydrate-anaesthetized male Sprague– Dawley rats; two fluorescent retrograde tracers were centred on the medial or lateral part of the nucleus accumbens—the major part of the ventral striatum—and the medial or lateral prefrontal viscerolimbic cortex. Our retrograde double-labelling study revealed that a subset of midline thalamic neurons send projection fibres to both the nucleus accumbens and the cerebral cortex. Such neurons projecting to both targets were principally identified in the paraventricular thalamic nucleus. The majority of the duallylabelled neurons in the paraventricular thalamic nucleus projected to the lateral part of the nucleus accumbens and the medial wall of the prefrontal cortex. Dually-labelled neurons were additionally found in other midline nuclei, including the paratenial, intermediodorsal, rhomboid, and reuniens nuclei, as well as in the medial part of the parafascicular thalamic nucleus. Dually-projecting neurons identified in the present study may represent a potential link between the limbic striatum and the viscerolimbic-associated cortex, thus suggesting that non-discriminative information relayed to the prefrontal cortex might exert an influence through the same neurons on the nucleus accumbens implicated in affective behaviour. 1998 IBRO. Published by Elsevier Science Ltd. Key words: paraventricular thalamic nucleus, branching projection, retrograde double labelling, nucleus accumbens, infralimbic cortex, affective behaviour.
The ventral striatum, comprising the nucleus accumbens, the medium-cell parts of the olfactory tubercle and the fundus striati,24 has been associated with the limbic system due to its dense afferents from limbic-related structures including the amygdala, the hippocampal formation and the limbic-affiliated areas of the cerebral cortex.20,35,64 The ventral striatum receives large projections also from the thalamus, primarily from the intralaminar and midline nuclei,4,19,30,41,53 as does the dorsal striatum.23,36,49,61,68 The nucleus accumbens, a major sector of the ventral striatum implicated in various types of behaviour (see Ref. 51, for review), has been shown to exhibit a highly compartmental organization; it is now widely accepted that this nucleus is subdivided into a ventromedial part (referred to as the ‘‘shell’’ together with the olfactory tubercle) and a dorsolateral part (referred to as ‘‘core’’) in the rat, based on afferent/efferent relationships and/or differences in immunoreactivity for a variety of substances.6,7,21,25,26,69,74–76
*To whom correspondence should be addressed. Abbreviations: FG, FluoroGold; FR, FluoroRed; PVT, paraventricular thalamic nucleus. 635
The midline thalamic nuclei innervate the specific regions of the prefrontal cortex, which largely overlap the cortical areas sending projection fibres to the thalamic targets in the ventral striatum, and thus constitute the ‘‘parallelly arranged’’ basal gangliathalamocortical circuits.4,5,18,37,53 A critical question arising here is whether or not single midline thalamic neurons sending projection fibres to the ventral striatum also project directly to the cerebral cortex so as to influence the prefrontal cortical system at both the cortical and striatal levels. The possibility of such projections was investigated as for the thalamic neurons sending axons to both the dorsal striatum and the cortex, and the thalamostriatal projection fibres were shown to be, at least, partly derived from branching axons of the thalamocortical fibres.12,31,36,59,63 On the other hand, the existence of thalamic neurons sending projection fibres to both the ventral striatum and the cerebral cortex was denied in a recent study.61 The present study was therefore designed to re-evaluate the topographical organization of midline thalamic neurons which project to the nucleus accumbens and the prefrontal cortical areas with special emphasis on branching projections to these targets. Of particular interest is the possibility of branching projections from the
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paraventricular thalamic nucleus (PVT), a dorsal midline thalamic constituent which was demonstrated as a major source of thalamic input to the nucleus accumbens.4 This midline nucleus has long been considered as epithalamic due to the experimental data suggesting the lack of cortical projections, as well as from a developmental point of view.1,32 Recent anterograde transport studies verified the PVT as clearly sending substantial projection fibres to the medially-located infralimbic as well as to laterallay-located agranular insular areas of the prefrontal cerebral cortex.5,38 Although the PVT reportedly send projection fibres to both the core and shell of the nucleus accumbens, previous works suggested that the projections to these two accumbal subdivisions might originate from separate populations of the PVT neurons.4,16,53 Therefore, the present study also focused on the topographical organization of the core-projecting populations and shell-projecting populations of the PVT neurons. A preliminary report of this study was presented in abstract form.43 EXPERIMENTAL PROCEDURES
Data were obtained from eight adult male Sprague– Dawley rats (Nippon Bio-Supp. Center, Tokyo, Japan) weighing 200–250 g. Rats were anaesthetized with chloral hydrate (400 mg/kg, i.p.) and then mounted on a stereotaxic apparatus. The brain was exposed through small burr holes, and injections of retrograde tracers were made at coordinates derived from the atlas of Paxinos and Watson.50 Each rat received two kinds of fluorescent retrograde tracers, FluoroRed15 (FR, generously provided by Tombow Co., Ltd., Tokyo, Japan) and FluoroGold (FG, Fluorochrome, Englewood, CO, U.S.A.) in the following structures unilaterally on the same side: FR was injected into the shell or core subdivision of the nucleus accumbens, and FG into the infralimbic or agranular insular area of the prefrontal cortex. Since the thalamocortical fibres arising in the PVT appear to reach their target area in the medial prefrontal cortex, in part, through the dorsal part of the shell of the nucleus accumbens,38,71 coronal knife cuts (1 mm wide) were made just rostral to the shell of the nucleus accumbens before each injection.37,48 Injections of tracers were then made by pressure with glass micropipettes filled with an approximate volume of 50 nl of a 2% aqueous solution of FG and an approximate volume of 100 nl of undiluted FR.
The micropipettes were left in situ for 5–10 min following each injection. After a survival period of five to seven days, the rats received an overdose of chloral hydrate and were perfused transcardially with heparinized saline, followed by 4% paraformaldehyde in 0.1 M phosphate buffer. The brains were removed from the skull, postfixed for 2–3 h in the same fixative, and immersed in 0.1 M phosphate buffer containing 10% sucrose overnight at 4C. The brains were sectioned transversely at 40 µm on a freezing microtome, mounted on gelatin-coated slides, and air dried. The sections were examined with an epifluorescence microscope (Nikon FX) and photographed on T-MAX 400 film (Kodak). RESULTS
Study I: Combined injections into the lateral part of the ventral striatum and the medial prefrontal cortical area Two rats (ACC-15 and 16) received injections of FR and FG into the lateral part of the ventral striatum and the medial prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 1 (ACC-15). The FR deposit involved the lateral part of the core of the nucleus accumbens and the lateral part of the anterior limb of the anterior commissure and did not spread into adjacent part of the caudate–putamen, which was nevertheless penetrated by micropipette tracks. The FG deposit was centred on the infralimbic area, where virtually all cortical laminae were involved. The adjacent cortical areas, including the prelimbic area and dorsal peduncular cortex, are also involved to a variable degree, but the tracer did not spread into the nucleus accumbens. The ACC-16 revealed similar injection sites to the ACC-15, resulting in almost identical patterns of distribution of single- and/or double-labelled cell bodies. FluoroRed-labelled neurons. Neurons labelled retrogradely with FR were distributed in the midline and medial portions of the thalamus, exclusively on the side ipsilateral to the injection site. Many retrogradely-labelled neurons were found in the PVT throughout its rostrocaudal extent including its rostroventral extension (Figs 1a–e, 2a,c). A small
Abbreviations used in the figures ac Acb AcbC AcbSh AI CeM CL CP DI f fr IAM IL IMD LHb MD
anterior commissure nucleus accumbens nucleus accumbens, core nucleus accumbens, shell agranular insular cortex central medial thalamic nucleus central lateral thalamic nucleus caudate–putamen dysgranular insular cortex fornix fasciculus retroflexus interanteromedial thalamic nucleus infralimbic cortex intermediodorsal thalamic nucleus lateral habenular nucleus mediodorsal thalamic nucleus
MHb mt PC PFl PFm Pir PL PT PVT rf Re Rh sm SPF V3
medial habenular nucleus mammillothalamic tract paracentral thalamic nucleus parafascicular thalamic nucleus, lateral part parafascicular thalamic nucleus, medial part piriform cortex prelimbic cortex paratenial thalamic nucleus paraventricular thalamic nucleus fasciculus retroflexus reuniens thalamic nucleus rhomboid thalamic nucleus stria medullaris subparafascicular nucleus third ventricle
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Fig. 1. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-15. The tracer FG was injected into the medial prefrontal cortex (open arrow) and the FR into the core subdivision of the nucleus accumbens (solid arrow). Five representative transverse sections are arranged rostocaudally in a–e. Cells single-labelled with FG are indicated by dots; cells single-labelled with FR by open circles and dually-labelled cells by solid circles.
to moderate number of labelled neurons were found in other midline/intralaminar nuclei, including the paratenial (Fig. 1a, b), the central medial (Fig. 1b–d), intermediodorsal (Fig. 1c,d), reuniens (Figs 1b–d, 3a,c) and rhomboid thalamic nuclei (Fig. 1b–d and Fig. 4a). The medial part of the parafascicular nucleus were extensively labelled (Figs 1e, 4c). Retrogradely-labelled neurons were also found in mediodorsal nucleus (Fig. 1b–d).
FluoroGold-labelled neurons. Neurons retrogradely labelled from the infralimbic area were distributed in the midline and medial portions of the thalamus, exclusively on the side ipsilateral to the injection site. A major source of infralimbic afferents was mapped to the PVT throughout its rostrocaudal extent (Fig. 1a–e). A small to moderate number of labelled cells was also identified in the mediodorsal thalamic nucleus, principally in its rostral portion
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Fig. 2. Photomicrographs of retrogradely-labelled cells in the PVT from injections of FR and FG into the accumbens core (a, c) and infralimbic cortex (b, d), respectively. Open arrows in a and b indicate the midline points on the ventricular surface. Boxed areas in a and b are shown under high power magnification in c and d, respectively. Asterisks in c and d indicate dually-labelled cells. Data from case ACC-16. Scale bar=100 µm (a, b); 50 µm (c, d).
(Fig. 1b). The paratenial (Fig. 1a, b) and central medial (Fig. 1a–d) thalamic nuclei were lightly labelled. Ventral constituents of the midline thalamus, i.e. reuniens and rhomboid nuclei (Figs 1b–d,
3b, d, 4b), as well as the medial part of the parafascicular nucleus were also labelled (Figs 1e, 4d). Retrogradely-labelled cells were sparse in the ventral tier and in other thalamic nuclei (not illustrated).
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Fig. 3. Photomicrographs of retrogradely-labelled cells in the reuniens thalamic nucleus from injections of FR and FG into the accumbens core (a, c) and infralimbic cortex (b, d), respectively. Boxed areas in a and b are shown under high power magnification in c and d, respectively. Arrows in c and d indicate dually-labelled cells. Data from case ACC-16. Scale bar=100 µm (a, b); 50 µm (c, d).
Dually-labelled neurons. The overlap of cortical afferents and striatal afferents was significantly observed in the dorsal part of the midline nuclei. A large number of dually-labelled neurons were located throughout the rostrocaudal extent of the PVT, with a mediodorsal prevalence (Figs 1, 2). Almost 50% of FR-positive and/or FG-positive neurons were dually labelled (Fig. 9). Dually-labelled neurons were also found in other midline nuclei, including, reuniens (Figs 1b–d, 3), rhomboid (Figs 1c, d, 4a, b), and medial part of the parafascicular thalamic nuclei (Figs 1e, 4c, d). Dually-labelled cells were sparse in the paratenial (Fig. 1a, b), intermediodorsal (Fig. 1c, d), and central medial thalamic nuclei (Fig. 1b–d) Study II: Combined injections into the medial part of the ventral striatum and medial prefrontal cortical area In this series of studies, cases ACC-13 and 14 received injections of FR and FG into the medial
part of the ventral striatum and medial prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 5 (ACC-13). The FR deposit covered most of the lateral part of the shell of the nucleus accumbens and its adjacent regions, including the ventral pallidum, and does not extend into the lateral septum or the core subdivision of the nucleus accumbens. The injection deposit of FG involved infralimbic and prelimbic cortices just as in Study I, resulting in almost identical distribution patterns of labelled cell bodies. The ACC-14 revealed similar injection sites to the ACC-13, resulting in almost identical patterns of distribution of single- and/or double-labelled cell bodies. FluoroRed-labelled neurons. Injections of FR into the shell subdivision of the nucleus accumbens resulted in heavy labelling of thalamic structures, exclusively on the side ipsilateral to the injection site. The labelled neurons were concentrated in PVT,
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Fig. 4.
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including the rostroventral extension (Fig. 5a–e). In the PVT, they skewed medially and were less abundant in the lateral border zone, transitional with the mediodorsal nucleus (Fig. 5b–d). Cells labelled from the accumbens shell were also seen in other midline nuclei, i.e. the paratenial (Fig. 5a, b) and intermediodorsal thalamic nuclei (Fig. 5c, d). In sharp contrast to the cases with injection into accumbens core, retrogradely-labelled neurons were sparse in central medial, rhomboid, reuniens nuclei, and medial portion of the parafascicular nucleus. Dually-labelled neurons. A moderate to small number of dually-labelled neurons were identified in the PVT throughout its rostrocaudal extent, including its rostroventral extension (Fig. 5a–e). The ratio of dually-labelled neurons in the PVT was 27.6% (Fig. 9). Dually-labelled neurons were also present in the paratenial nucleus, especially its rostral part (Fig. 5a, b). The intermediodorsal, rhomboid nuclei and medial part of the parafascicular nucleus contains few dually-labelled cells. Study III: Combined injections into the lateral part of the ventral striatum and the lateral prefrontal cortical area In this study, cases ACC-04 and 06 received injections of FR and FG into the lateral part of the nucleus accumbens and the lateral prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 6 (ACC-04). The FR deposit was restricted in the lateral part of the core of the nucleus accumbens with involvement of the lateral part of the anterior limb of the anterior commissure, just as in Study I, and resulted in almost identical patterns of retrograde labelling. The FG deposit in case ACC-04 was centred on the agranular insular area with spread into the adjacent cortical area including the dysgranular insular cortex, and the diffusion sphere infiltrated virtually all cortical laminae of the agranular insular cortex. FluoroGold-labelled neurons. Cell bodies labelled from the agranular insular cortex were principally distributed in the midline and paramedian areas of the thalamus, exclusively on the side ipsilateral to the injection site. A few labelled cells was identified in the PVT (Fig. 6a–d), paratenial (Fig. 6a), intermediodorsal (Fig. 6c), central medial (Fig. 6b, c) and the reuniens nuclei (Fig. 6b, c) and the medial part of the parafascicular thalamic nucleus (Fig. 6d). The medial subdivision of the mediodorsal thalamic nucleus bordering the PVT and intermediodorsal nucleus
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were also backfilled (Fig. 6b, c). Neurons were also labelled in the ventral tier nuclei (not illustrated). Dually-labelled neurons. In these cases, most of cortical and accumbal afferents were spatially segregated. A small number of dually-labelled neurons were detected in the PVT (Fig. 6), where less than 10% of FR-positive and/or FG-positive cell bodies were doubly labelled (Fig. 9). Dually-labelled cells were scattered also in other midline/intralaminar thalamic nuclei, i.e. the paratenial (Fig. 6b), intermediodorsal (Fig. 6c), rhomboid (Fig. 6b), reuniens (Fig. 6b, c), central medial (Fig. 6b, c) and parafascicular thalamic nuclei (Fig. 6d). Study IV: Combined injections into the medial part of the ventral striatum and the lateral prefrontal cortical area Two cases (ACC-09 and 11) received injections of FR and FG into the medial part of the ventral striatum and the lateral prefrontal cortical area, respectively. Representative injection sites are shown in Fig. 7. The FR deposit involved the medial part of the accumbens shell and the olfactory tubercle, just as in Study II and resulted in almost identical patterns of distribution of labelled cell bodies. The FG deposit involved the agranular and dysgranular insular areas, and the diffusion sphere infiltrated all cortical laminae and the distribution patterns of FG-labelled neurons were comparable to those observed in Study III. Dually-labelled neurons. In these cases, most of cortical and accumbal afferents were spatially segregated and only a few neurons were doubly labelled in the midline, mainly in the PVT including its rostroventral extension (Figs 7a–d, 8). In the PVT, less than 10% of FR-positive and/or FG-positive cell bodies were doubly labelled (Fig. 9). In the paratenial, reuniens, central medial nuclei and the medial part of the parafascicular nucleus, dually-labelled cells were rarely found. DISCUSSION
The present study indicates that a substantial number of single midline thalamic neurons send projection fibres to both the prefrontal cortical areas and the nucleus accumbens. A majority of such branching projections was identified as arising from the PVT and projecting to the medial wall of the prefrontal cortex, including the infra- and prelimbic areas, and the core subdivision of the nucleus accumbens (as revealed in the Study I).
Fig. 4. Photomicrographs of retrogradely-labelled cells in the rhomboid thalamic nucleus from injections of FR and FG into the accumbens core (a) and infralimbic cortex (b), respectively, and those in the medial part of the parafascicular thalamic nucleus from injection of FR and FG into the accumbens core (c) and infralimbic cortex (d), respectively. Arrows indicate dually-labelled cells. Data from case ACC-16. Scale bar=50 µm.
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Fig. 5. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-13. The tracer FG was injected into the medial prefrontal cortex (open arrow) and the FR into the shell subdivision of the nucleus accumbens (solid arrow). Five representative transverse sections are arranged rostocaudally in a–e. FG-labelled cell indicated by dots; FR-labelled cells by open circles and dually-labelled cells by solid circles.
Methodological considerations Recently, Shinonaga et al.,62 by employing a similar retrograde double-labelling technique, have shown that projections from the midline thalamic
nuclei, including the PVT, to the prefrontal cortex and the nucleus accumbens take their origin from separate neuronal populations. This was discovered during the attempt to investigate single basal amygdaloid nucleus neurons sending bifurcating axons to
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Fig. 6. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-04. The tracer FG was injected into the lateral prefrontal cortex (open arrow) and the FR into the core subdivision of the nucleus accumbens (solid arrow). Four representative transverse sections are arranged rostocaudally in a–d. FG-labelled cell indicated by dots; FR-labelled cells by open circles and dually-labelled cells by solid circles.
the same two targets. The discrepancy between the result of Shinonaga et al.62 and that of ours is likely due to subtle differences in the centre of injection sites and may be relevant to the recent observation that
projection fibres originating in the midline thalamus and those originating in the basolateral amygdala show different patterns of distribution in similar regions in the nucleus accumbens and the prefrontal
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Fig. 7. Drawings of injection sites and distribution of retrogradely-labelled cells in the thalamic midline in the case ACC-09. The tracer FG was injected into the lateral prefrontal cortex (open arrow) and the FR into the shell subdivision of the nucleus accumbens (solid arrow). Four representative transverse sections are arranged rostocaudally in a–d. FG-labelled cell indicated by dots; FR-labelled cells by open circles and dually-labelled cells by solid circles.
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Fig. 8. Photomicrographs of retrogradely-labelled cells in the PVT from injections of FR and FG into the accumbens shell (a, c) and insular cortex (b, d), respectively. Open arrows in a and b indicate the midline points on the ventricular surface. Boxed areas in a and b are shown under high power magnification in c and d, respectively. Arrows in c and d indicate dually-labelled cells. Data from case ACC-11. Scale bar in b=100 µm (a, b) and in d=50 µm (c, d).
cortices.71,72 For example, the fibres originating in the basolateral amygdaloid nucleus were observed predominantly within the cell clusters in the accumbal shell which are avoided by fibres from the PVT.71
The tracer deposits in our study precisely overlapped the areas which receive heavy projection fibres from the midline thalamus.4,5 Besides, we made coronal knife cuts just rostral to the accumbal injection sites
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Fig. 9. A bar chart showing the ratios of the numbers of cell bodies single-labelled with FR, those single-labelled with FG, and double-labelled cell bodies within the PVT, obtained from representative sections of each study.
to exclude the possibility of artificial double-labelling caused by accumbal injection (via uptake by fibres passing through the nucleus accumbens en route to the medial prefrontal cortex38,71) as well as by cortical injection (via axon terminal uptake). Functional considerations The midline thalamus, including the PVT, receives diverse afferent inputs conveyed by exteroceptive and visceral relay nuclei13,42,44–47,60 and thus is thought to lie in a pivotal position between the sensorium and the cerebral cortex. Although the precise function ascribed to the midline nuclei is still unclear, several lines of evidence suggest that these nuclei are involved in non-discriminative aspects of information, in contrast to the specific or discriminative thalamic relay nuclei concerned with relaying epicritical sense.2,11,52,73 The evidence of branching projections identified in the present study may represent a potential link between the cerebral cortex and basal ganglia, and thus indicates that non-discriminative or affective information conveyed to the prefrontal cortex might influence the ventral striatal (accumbal) functions through the same neurons. The thalamo-
cortical projection fibres arising from the PVT terminate predominantly in the layers I, V and VI of the prefrontal cortical areas,5 which are closely related to the ventral striatum through corticostriatal projections.17 Thus, the PVT is suggested to lie in a position to influence the ventral striatum indirectly as well via cortical neurons (cortically-relayed indirect projections). Recent anatomical study demonstrated that thalamostriatal fibres originating in the PVT and afferents from the prefrontal cortex exhibit highly organized patterns of distribution within the nucleus accumbens.71 In the core, for example, both sets of fibres overlap in the small regions (i.e. patches), which are known to project to the pars compacta of substantia nigra,7 and thus direct thalamo-accumbal fibres and cortically-relayed indirect fibres possibly impose hierarchical controls over somatic motor function with an intimate relation with the substantia nigra dopaminergic neurons. The present results indicate that PVT may act as an interface of the autonomic–limbic and the somatic motor function. On the other hand, in the accumbens shell, the cortical fibres are most prominent in the circumscribed regions with strong calbindin immunoreactivity which are avoided by fibres arising in the PVT;71 the PVT neurons likely have a different influence upon the accumbens shell based on the mode of projections, i.e. direct projections versus cortically-relayed indirect projections. The shell of the accumbens is established as a region related with the autonomic and limbic motor control. The projections of PVT neurons to the prefrontal cortex and the shell of the accumbens might be related to this function. A large body of evidence suggests the importance of the nucleus accumbens in behaviour, in which mesolimbic dopamine system play a pivotal role (see Ref. 51 for review). Examples include the observation that injections of dopamine or its agonists into the nucleus accumbens enhanced locomotor activity in rats.14,54,55,56 Dense projections from the PVT to dopamine-rich areas in the nucleus accumbens (see, e.g., Ref. 70) indicates the possibility of the role of PVT in modulating the behavioural function ascribed to the nucleus accumbens. Perhaps related is evidence that enhancement of the activity of the PVT neurons induces defense behaviour27 as well as increased dopamine utilization in the nucleus accumbens.33 Electrical stimulation of the medial prefrontal cortex, primarily the prelimbic and infralimbic areas resulted in cardiovascular and gastromotor effects.9,10,22,29,39 Structurally corroborative are anatomical findings that infralimbic efferents innervate a number of autonomic-related sites, including several hypothalamic nuclei, the bed nucleus of the stria terminalis, the periaqueductal gray matter, the parabrachial nuclei, the nucleus of the solitary tract, the dorsal motor vagal nucleus, the ventrolateral medulla, and also the intermediolateral column of the spinal cord.3,28,39,40,45,58,65–67 Autonomic changes
Branching projections from midline thalamus
associated with lesions or stimulation of the midline thalamus, including the PVT,8,57 might be related to the neural circuits relayed by the medial prefrontal cortex. CONCLUSIONS
Neurons in the midline thalamic nuclei, including PVT, give off projection fibres to both the nucleus accumbens and the medial prefrontal cortex. It is conceivable that these branched afferents likely
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couple behavioural and autonomic components of emotional expression and may contribute to some autonomic changes observed during affective behaviour (see, e.g., Ref. 34 for review).
Acknowledgements—The authors thank Mrs Mie Taguchi for her technical assistance. The gift of FluoroRed from Tombow Co., Ltd is gratefully acknowledged. This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 08780720 and 08680810) from the Japan Ministry of Education, Science, Sports and Culture.
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