Brain Research, 113 (1976) 589-596 (() Elsevier Scientific Publishing Company, Amsterdam
589 Printed in The Netherlands
Autoradiographic tracing of nucleus accumbens efferents in the rat
L. C. A. CONRAD and D. W. PFAFF Rockefeller University, New York, N.Y. lO021 (U.S.A.)
(Accepted May 24th, 1976)
The nucleus accumbens has been refractory to classification as either a nuclear group of the septum, or a component of the striatum. Evidence supporting its grouping with the striatium has come primarily from developmental studies3,10,21 and cytoarchitectonic observations~,8, TM. However, on the basis of behavioral studies ~4 and anatomical connections4,12, as, some authors have considered the nucleus accumbens more closely allied to the septal nuclei. In this study, we have used tritiated amino acid autoradiography to trace the efferent connections of the nucleus accumbens in adult albino rats. For comparison, [ZH]amino acid injections were also placed medial to the nucleus accumbens, in the medial septal nucleus. A complete description and discussion of the methods can be found in Conrad and Pfaff 1. Briefly, in the experiments described here, 10-50 nl injections of tritiated proline or leucine (concentration 200 #Ci/#l) in physiological saline were made over a 1 h period, into the nucleus accumbens (n = 5) or into the septum (n -- 5). The animals were sacrificed 48 h later, and their brains were processed according to the usual autoradiographic methods; the autoradiograms were allowed 30 days exposure time. Scanning and charting of the autoradiograms were done with a microprojector and both light and dark field microscopy. In Fig. 1, the efferent projections traced from an injection of [ZH]proline into the nucleus accumbens are illustrated. The region of labeled cells appeared to be restricted to the posterior half of the medial nucleus accumbens. Anterior to the injection site, labeled fibers extended (Fig. IA) to the anterior septum, hippocampal rudiment, medial anterior frontal cortex and the anterior tip of the caudatoputamen. The lateral septal nucleus received a distinct projection from the nucleus accumbens (Fig. 1B, C). No labeled fibers were seen entering the fornix. Descending labeled nucleus accumbens axons streamed ventrally and posteriorly from the injection site in many small bundles, curving laterally on the dorsal edge of the diagonal band (Fig. 1B). In the dorsal half of the lateral preoptic area, the small bundles coalesced into a single large group beneath the anterior commissure (Fig. 1C). At levels including the decussation of the anterior commissure, some labeled nucleus accumbens axons from this bundle turned sharply dorsally into the medial globus paltidus (Fig. I D). Hugging the medial edge of this nucleus, some of these nucleus
590
Fig. 1. Charts of the distribution of labeled projections from an injection site in the nucleus accumbens (case 28, transverse section). Large dots represents labeled fibers; small dots represent fields of individual grains. The region of labeled cell bodies (the injection site) is indicated in solid black. Abbreviations used in this and the following figure: a, nucleus accumbens; AC, anterior commissure; am, medial nucleus of the amygdala; ce, entorhinal cortex; CG, central gray; CP, cerebral peduncle; cp, caudatoputamen; DB, diagonal band of Broca; din, dorsomedial nucleus of the hypothalamus; F, fornix; FR, fasciculus retroftexus; gp, globus pallidus; ha, anterior hypothalamus; hi, hippocampus ; hi, lateral hypothalamus; hp, posterior hypothalamus; ip, interpeduncular nucleus; lh, lateral habenula; md, mediodorsal nucleus of the thalamus; MFB, medial forebrain bundle; mh, medial habenula; ML, medial lemniscus; tomb, medial nucleus of the mammillary bodies; mpo, medial preoptic area; MT, mammillothalamic tract; pv, paraventricular nucleus of the hypothalamus; pvt, periventricular nucleus of the thalamus; OC, optic chiasm; olt, olfactory tubercle; OT, optic tract; r, red nucleus; RF, reticular formation; s, subiculum; sf, septofimbrial nucleus; sl, lateral septal nucleus; sla, anterior lateral septum; sm, medial septal nucleus; sma, anterior medial septum; SN, substantia nigra; st, bed nucleus of the stria terminalis; vm, ventromedial nucleus of the hypotbalamus ; VTA, ventral tegmental area of Tsai.
591
Fig. 2. Distribution of labeled projections observed fi'om an injection site in medial septal nucleus (case 31, transverse section). Symbols for charting are the same as in Fig. 1. accumbens axons turned posteriorly, while others spread further dorsally (Fig. 1D) and rostrally (Fig. 1C) in a projection to a medial and anterior zone of the globus pallidus (Fig. 3). The caudate nucleus and the putamen were unlabeled at this level. Many nucleus accumbens axons did not turn into the globus pallidus but continued their descent near the dorsal border of the medial forebrain bundle (MFB) (Fig. 1D). Both medial and lateral preoptic areas were diffusely labeled. No projections appeared laterally in the substantia innominata or olfactory tubercle (Fig. 1D). No labeled axons were observed entering the stria medullaris or stria terminalis, and no projections to the habenula or amygdala were noted. In the medial region of the anterior hypothalamus (Fig. I E), light scattered labeling was seen, just dorsal and ventral to the paraventricular nucleus. A few labeled
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Fig. 3. Photomicrographs of projections from nucleus accumbens to medial globus pallidus (At and medial substantia nigra, pars reticulata (B) (transverse section, . 343). Medial is to the left; dorsal is up. In A, a small part of the bed nucleus of the stria terminalis is visible just medial to the globus pallidus, at the left of the photograph. Note that the bed nucleus of the stria terminalis is relatively free of label. In B, fiber bundles of the cerebral peduncle are visible at the bottom of the photograph, ventral to the substantia nigra. Photomicrographs are from sections stained with Luxol fast blue for myelinated fibers, and with cresyl violet for cell bodies.
593 nucleus accumbens axons spread dorsally in a light projection to the parataenial and periventricular nuclei of the thalamus (Fig. 1E). Most labeled nucleus accumbens axons at anterior hypothalamic levels, however, remained within the MFB (Fig. 1E). These nucleus accumbens axons in the MFB were joined laterally by the labeled fibers which had turned posteriorly from the globus pallidus projection (Fig. 1D) to descend with the medial fiber bundles of the internal capsule (Fig. IE). At more caudal hypothalamic levels, nucleus accumbens axons in the cerebral peduncle were continuous medially with labeled axons in the MFB (Fig. IF, G). More medially, the anterior hypothalamus, dorsomedial hypothalamus and anteromedial zona incerta received projections (Fig. IF). No label was noted in the ventromedial, arcuate, premammillary or mammillary nuclei. Light scattered projections to posterior hypothalamus, supramammillary regions and medial anterior central grey were observed, however. Labeled nucleus accumbens fibers remaining in the MFB through posterior hypothalamic levels (Fig. 1G) became diffusely scattered in the ventral tegmental area of Tsai (Fig. 1G, H), or spread laterally into the substantia nigra. In the substantia nigra, nucleus accumbens projections were distributed medially, in the pars compacta and in the dorsal and medial pars reticulata (Figs. 1H and 3). A few labeled nucleus accumbens axons continued dorsally and laterally through the substantia nigra to reach the ventrolateral tegmental reticular formation (Fig. 1H). No labeled fibers were observed posterior to this level. The projections from the nucleus accumbens contrasted markedly with those traced from injection sites placed at similar anterior-posterior and dorsal-ventral levels, but medially, in the medial septal nucleus (Fig. 2B). Such injections did not label cells in the nucleus accumbens. Rostral to the medial septal nucleus, the anterior septum, hippocampal rudiment and medial tip of the olfactory tubercle were labeled (Fig. 2A). Labeled medial septal axons streamed dorsally into the fornix, and ventrally into the diagonal band of Broca (Fig. 2B, C). Labeled medial septal axons running dorsally and posteriorly in the fornix resulted in labeling of the septofimbrial nucleus (Fig. 2C, D): the lateral septum was only lightly and diffusely labeled (Fig. 2C). Medial septal axons in the fornix maintained a dorsal position in this tract as they coursed posteriorly toward the hippocampus (Fig. 2E). Within the hippocampus, diffuse labeling was distributed in all cell fields, including the subiculum (Fig. 2F, G, H). At more posterior levels, however, the ventral hippocampus appeared more heavily labeled than the dorsal regions (Fig. 2G). Streaming ventrally and laterally from the injection site in the diagonal band of Broca (Fig. 2B, C), labeled medial septal axons collected in the MFB (Fig. 2D) and turned caudally. Throughout their descending trajectory in the diagonal band and the MFB, most medial septal axons remained ventral to the position assumed by most descending nucleus accumbens axons (Figs. I B-E, 2B-E). From the medial septum, diffuse projections to medial and lateral preoptic areas and light projections to the bed nucleus of the stria terminalis were noted (Fig. 2C, D). At the preoptic-anterior hypothalamic border, some labeled medial septal
594 fibers spread dorsally into the parataenial and anterior mediodorsal nuclei or" the thalamus, with some axons continuing caudally into the medial and lateral habenular nuclei (Fig. 2E, F). From the small medial septal injection site illustrated, no labeled fibers could be traced into the stria medullaris, but larger septal injections did label fibers in this tract. In no medial septal case we re labeled axons seen in t he stria te rminalis. Labeled medial septal axons descending through the hypothalamus in the M FB remained in a ventrally coursing bundle (Fig. 2E, F). A few labeled axons spread laterally to give a very light projection to the medial amygdala; others turned ventromedially to form a small, discrete clump at the lateral edge of the median eminence (Fig. 2E, F). The periventricular region of the hypothalamus, the dorsal half or" the anterior hypothalamus and the dorsomedial hypothalamus received projections tFig. 2E, F), but the ventromedial nucleus was not labeled. More posteriorly, light labeling of the arcuate, dorsal premammillary and posterior hypothalamic nuclei and of the very anterior central gray (Fig. 2G) was noted. A projection to the medial mammillary nucleus was observed, along with diffuse labeling of the supramammillary region and ventral tegmentat area (Fig. 2G, H). No projections were seen more posteriorly, and no labeling was observed in midbrain reticular formation or raphe nuclei. We conclude that patterns of projections from the medial septal nucleus and nucleus accumbens are overlapping, but different. Both medial septal and nucleus accumbens send descending axons through the M FB, but medial septal fibers occupy a position in the bundle ventral to those from nucleus accumbens. Nucleus accumbens axons in the MFB ultimately spread laterally into the substantia nigra and reticular formation, while those from the septum terminate largely in the medial mammillary nuclei. Septal, but not nucleus accumbens, axons reach the arcuate region of the hypothalamus. The medial septum projects to the hippocampus, a connection never seen following nucleus accumbens isotope placements. From nucleus accumbens, but not from the medial septum, projections can be traced to the medial globus pallidus and to or through the entopeduncular nucleus. Within the thalamus, medial septal fibers are distributed to mediodorsal and habenular nuclei, while nucleus accumbens axons are distributed in the periventricular nuclei. Both nucleus accumbens and medial septum send fibers into parataeniat thalamus and also project to the anterior septum, preoptic area, anterior and dorsomedial hypothalamus, supramammillary regions, anterior central gray and ventral tegmental area of Tsai. Some of the efferents we traced from nucleus accumbens have been previously reported. Nucleus accumbens axons in the medial forebrain bundle have been observed by a number of other workers 7,~:'.~s,e~ as have nucleus accumbens eflbrents to the septumJS, ~s The nucleus accumbens has been shown to also receive afferents from t he septum aS,~s,l'9, and through the MFB 6 and fornix r),ls ~2°. In two recent reports using autoradiographic techniques is,z1, nucleus accumbens projections to globus pallidus and to substantia nigra were seen. Swanson and Cowan zl have suggested that a topographic organization of nucleus accumbens efferents may exist: our observation of projections from medial nucleus accumbens to medial globus pallidus and medial substantia nigra would support this suggestion. Swanson and Cowan ~1 also observed, as we did, nucleus accumbens efferents running through the entopeduncular nucleus and lateral hypothalamus.
595 M a n y o f the efferents o b s e r v e d f r o m the m e d i a l septal nucleus have also been r e p o r t e d previously, in studies using lesion a n d d e g e n e r a t i o n techniquesg,16,19,2°. D u e to difficulties in distinguishing p r o j e c t i o n s o f h i p p o c a m p a l fibers-of-passage f r o m those arising in the septal nuclei, some p r o j e c t i o n s we have d e m o n s t r a t e d to originate in the medial s e p t u m (e.g., the p r o j e c t i o n s to ventral t e g m e n t a l area, medial m a m m i l l a r y nuclei, m e d i o d o r s a l t h a l a m u s a n d p a r a t a e n i a l t h a l a m u s ) have previously been attributed to the h i p p o c a m p u s . The p r o j e c t i o n we observed f r o m the medial septum to the ventral h i p p o c a m p u s has also been d e m o n s t r a t e d by o t h e r w o r k e r s (Siegel, p e r s o n a l communication). F r o m o u r work, it a p p e a r s t h a t the nucleus a c c u m b e n s resembles the septum in its projections to some regions o f the h y p o t h a l a m u s , to p a r a t a e n i a l t h a l a m u s a n d to ventral t e g m e n t a l a r e a a n d central gray, b u t is also similar to the s t r i a t u m in projecting to globus pallidus, e n t o p e d u n c u l a r nucleus a n d s u b s t a n t i a nigra 1~-,~2. F u n c t i o n a l l y , the nucleus a c c u m b e n s a p p e a r s m o r e like the s e p t u m : lesions of the nucleus a c c u m b e n s p r o d u c e b e h a v i o r a l deficits m o r e similar to those seen after septal lesions t h a n those o b s e r v e d following d a m a g e to the striatum11,13,14. H o w e v e r , the p a t t e r n o f neurogenesis a n d d e v e l o p m e n t o f the nucleus a c c u m b e n s resembles t h a t o f the s t r i a t u m 3,2'. Thus, a l t h o u g h s e p a r a t e cases can be m a d e for classification o f nucleus a c c u m b e n s with s e p t u m or striatum, the nucleus a c c u m b e n s m a y m o r e logically be considered interm e d i a t e between these two structures.
1 Conrad, L. C. A. and Pfaff, D. W., Efferents from medial basal forebrain and hypothalamus in the rat. I. An autoradiographic study of the medial preoptic area, (1976) in press. 2 Cowan, W. M. and Powell, T. P. S., The projections of the midline and intralaminar nuclei of the thalamus of the rabbit, J. Neurol. Neurosurg. Psychiat., 18 (1955) 266-279. 3 Creps, E. S., Time of neuron origin in preoptic and septal areas of the mouse: an autoradiographic study, J. comp. Neurol., 157 (1974) 161-243. 4 DeFrance, J. F. and Yoshihara, H., Fimbria input to the nucleus accumbens septi, Brain Research, 90 (1975) 159 163. 5 Fox, C. A., Certain basal telencephalic centers in the cat, J. comp. Neurol., 72 (1940) 1-62. 6 Guillery, R. W., Degeneration in the hypothalamic connexions of the albino rat, J. Anat. (Lond.), 91 (1957) 91 114. 7 Gurdjian, E., The diencephalon of the albino rat, J. comp. Neurol., 43 (1927) 1 114. 8 Gurdjian, E., The corpus striatum of the rat, J. eomp. Neurol., 45 (1928) 249-281. 9 Johnson, T. N., An experimental study of the fornix and hypothalamo-tegmental tracts in the cat, J. eomp. Neurol., 125 (1965) 29-40. 10 Kappers, C. V. A., Huber, G. C. and Crosby, E. C., The Comparative Anatomy of the Nervous System of Vertebrates including Man, Vol. 2, MacMillan, New York, 1936. 11 Kirby, R. J. and Kimble, D. P., Avoidance and escape behavior following striatal lesions in the rat, Exp. Neurol., 20 (1968) 215 227. 12 Lauer, E. W., The nuclear pattern and fiber connections of certain basal telencephalic centers in the macaque, J. comp. Neurol., 82 (1945) 215-246. 13 Lints, C. E. and Harvey, J. A., Altered sensitivity to footschock and decreased brain content of serotonin following brain lesions in the rat, J. comp. physiol. P.s3,chol., 67 (1969) 23 31. 14 Lorens, S. A., Sorensen, J. P. and Harvey, J. A., Lesions in the nucleus septi of the rat: behavioral and neurochemical effects, J. comp. physiol. Psychol., 73 (1970) 284~290. 15 Mizuno, N., Clemente, C. D. and Sauerland, E. K., Fiber projections from rostral basal forebrain in the cat, Exp. Neurol., 25 (1969) 220-237. 16 Nauta, W. J. H., Hippocampal projections and related neural pathways to the mid-brain in the cat, Brain, 81 (1958) 319 340.
596 17 Nauta, W. J. H. and Mehler, W. R., Projections of the lentiform nucleus in the monkey, tCram Research, 1 (1966)3~42. 18 Powell, E. W. and Leman, R. B., Connections of the accumbens nucleus in the squirrel monkey. In Proc. Soc. Neurosci. St. Louis, Mo., 1974. 19 Raisman, G., The connexions of the septum, Brain, 89 (1966) 317-349. 20 Siegel, A. and Tassoni, J. P., Differential efferent projections of the lateral and medial septal nuclei to the hippocampus in the cat, Brain Behav. EvoL, 4 (1971) 201-219. 21 Swanson, L. W. and Cowan, W. M., A note on the connections and development of the nucleus accumbens, Brain Research, 92 (1975) 324 330. 22 Voneida, T. J., An experimental study of the course and destination of fibers arising in the head of the caudate nucleus in the cat and monkey, J. comp. Neurol., 115 (1960) 75-87.
589 Printed in The Netherlands
Autoradiographic tracing of nucleus accumbens efferents in the rat
L. C. A. CONRAD and D. W. PFAFF Rockefeller University, New York, N.Y. lO021 (U.S.A.)
(Accepted May 24th, 1976)
The nucleus accumbens has been refractory to classification as either a nuclear group of the septum, or a component of the striatum. Evidence supporting its grouping with the striatium has come primarily from developmental studies3,10,21 and cytoarchitectonic observations~,8, TM. However, on the basis of behavioral studies ~4 and anatomical connections4,12, as, some authors have considered the nucleus accumbens more closely allied to the septal nuclei. In this study, we have used tritiated amino acid autoradiography to trace the efferent connections of the nucleus accumbens in adult albino rats. For comparison, [ZH]amino acid injections were also placed medial to the nucleus accumbens, in the medial septal nucleus. A complete description and discussion of the methods can be found in Conrad and Pfaff 1. Briefly, in the experiments described here, 10-50 nl injections of tritiated proline or leucine (concentration 200 #Ci/#l) in physiological saline were made over a 1 h period, into the nucleus accumbens (n = 5) or into the septum (n -- 5). The animals were sacrificed 48 h later, and their brains were processed according to the usual autoradiographic methods; the autoradiograms were allowed 30 days exposure time. Scanning and charting of the autoradiograms were done with a microprojector and both light and dark field microscopy. In Fig. 1, the efferent projections traced from an injection of [ZH]proline into the nucleus accumbens are illustrated. The region of labeled cells appeared to be restricted to the posterior half of the medial nucleus accumbens. Anterior to the injection site, labeled fibers extended (Fig. IA) to the anterior septum, hippocampal rudiment, medial anterior frontal cortex and the anterior tip of the caudatoputamen. The lateral septal nucleus received a distinct projection from the nucleus accumbens (Fig. 1B, C). No labeled fibers were seen entering the fornix. Descending labeled nucleus accumbens axons streamed ventrally and posteriorly from the injection site in many small bundles, curving laterally on the dorsal edge of the diagonal band (Fig. 1B). In the dorsal half of the lateral preoptic area, the small bundles coalesced into a single large group beneath the anterior commissure (Fig. 1C). At levels including the decussation of the anterior commissure, some labeled nucleus accumbens axons from this bundle turned sharply dorsally into the medial globus paltidus (Fig. I D). Hugging the medial edge of this nucleus, some of these nucleus
590
Fig. 1. Charts of the distribution of labeled projections from an injection site in the nucleus accumbens (case 28, transverse section). Large dots represents labeled fibers; small dots represent fields of individual grains. The region of labeled cell bodies (the injection site) is indicated in solid black. Abbreviations used in this and the following figure: a, nucleus accumbens; AC, anterior commissure; am, medial nucleus of the amygdala; ce, entorhinal cortex; CG, central gray; CP, cerebral peduncle; cp, caudatoputamen; DB, diagonal band of Broca; din, dorsomedial nucleus of the hypothalamus; F, fornix; FR, fasciculus retroftexus; gp, globus pallidus; ha, anterior hypothalamus; hi, hippocampus ; hi, lateral hypothalamus; hp, posterior hypothalamus; ip, interpeduncular nucleus; lh, lateral habenula; md, mediodorsal nucleus of the thalamus; MFB, medial forebrain bundle; mh, medial habenula; ML, medial lemniscus; tomb, medial nucleus of the mammillary bodies; mpo, medial preoptic area; MT, mammillothalamic tract; pv, paraventricular nucleus of the hypothalamus; pvt, periventricular nucleus of the thalamus; OC, optic chiasm; olt, olfactory tubercle; OT, optic tract; r, red nucleus; RF, reticular formation; s, subiculum; sf, septofimbrial nucleus; sl, lateral septal nucleus; sla, anterior lateral septum; sm, medial septal nucleus; sma, anterior medial septum; SN, substantia nigra; st, bed nucleus of the stria terminalis; vm, ventromedial nucleus of the hypotbalamus ; VTA, ventral tegmental area of Tsai.
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Fig. 2. Distribution of labeled projections observed fi'om an injection site in medial septal nucleus (case 31, transverse section). Symbols for charting are the same as in Fig. 1. accumbens axons turned posteriorly, while others spread further dorsally (Fig. 1D) and rostrally (Fig. 1C) in a projection to a medial and anterior zone of the globus pallidus (Fig. 3). The caudate nucleus and the putamen were unlabeled at this level. Many nucleus accumbens axons did not turn into the globus pallidus but continued their descent near the dorsal border of the medial forebrain bundle (MFB) (Fig. 1D). Both medial and lateral preoptic areas were diffusely labeled. No projections appeared laterally in the substantia innominata or olfactory tubercle (Fig. 1D). No labeled axons were observed entering the stria medullaris or stria terminalis, and no projections to the habenula or amygdala were noted. In the medial region of the anterior hypothalamus (Fig. I E), light scattered labeling was seen, just dorsal and ventral to the paraventricular nucleus. A few labeled
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Fig. 3. Photomicrographs of projections from nucleus accumbens to medial globus pallidus (At and medial substantia nigra, pars reticulata (B) (transverse section, . 343). Medial is to the left; dorsal is up. In A, a small part of the bed nucleus of the stria terminalis is visible just medial to the globus pallidus, at the left of the photograph. Note that the bed nucleus of the stria terminalis is relatively free of label. In B, fiber bundles of the cerebral peduncle are visible at the bottom of the photograph, ventral to the substantia nigra. Photomicrographs are from sections stained with Luxol fast blue for myelinated fibers, and with cresyl violet for cell bodies.
593 nucleus accumbens axons spread dorsally in a light projection to the parataenial and periventricular nuclei of the thalamus (Fig. 1E). Most labeled nucleus accumbens axons at anterior hypothalamic levels, however, remained within the MFB (Fig. 1E). These nucleus accumbens axons in the MFB were joined laterally by the labeled fibers which had turned posteriorly from the globus pallidus projection (Fig. 1D) to descend with the medial fiber bundles of the internal capsule (Fig. IE). At more caudal hypothalamic levels, nucleus accumbens axons in the cerebral peduncle were continuous medially with labeled axons in the MFB (Fig. IF, G). More medially, the anterior hypothalamus, dorsomedial hypothalamus and anteromedial zona incerta received projections (Fig. IF). No label was noted in the ventromedial, arcuate, premammillary or mammillary nuclei. Light scattered projections to posterior hypothalamus, supramammillary regions and medial anterior central grey were observed, however. Labeled nucleus accumbens fibers remaining in the MFB through posterior hypothalamic levels (Fig. 1G) became diffusely scattered in the ventral tegmental area of Tsai (Fig. 1G, H), or spread laterally into the substantia nigra. In the substantia nigra, nucleus accumbens projections were distributed medially, in the pars compacta and in the dorsal and medial pars reticulata (Figs. 1H and 3). A few labeled nucleus accumbens axons continued dorsally and laterally through the substantia nigra to reach the ventrolateral tegmental reticular formation (Fig. 1H). No labeled fibers were observed posterior to this level. The projections from the nucleus accumbens contrasted markedly with those traced from injection sites placed at similar anterior-posterior and dorsal-ventral levels, but medially, in the medial septal nucleus (Fig. 2B). Such injections did not label cells in the nucleus accumbens. Rostral to the medial septal nucleus, the anterior septum, hippocampal rudiment and medial tip of the olfactory tubercle were labeled (Fig. 2A). Labeled medial septal axons streamed dorsally into the fornix, and ventrally into the diagonal band of Broca (Fig. 2B, C). Labeled medial septal axons running dorsally and posteriorly in the fornix resulted in labeling of the septofimbrial nucleus (Fig. 2C, D): the lateral septum was only lightly and diffusely labeled (Fig. 2C). Medial septal axons in the fornix maintained a dorsal position in this tract as they coursed posteriorly toward the hippocampus (Fig. 2E). Within the hippocampus, diffuse labeling was distributed in all cell fields, including the subiculum (Fig. 2F, G, H). At more posterior levels, however, the ventral hippocampus appeared more heavily labeled than the dorsal regions (Fig. 2G). Streaming ventrally and laterally from the injection site in the diagonal band of Broca (Fig. 2B, C), labeled medial septal axons collected in the MFB (Fig. 2D) and turned caudally. Throughout their descending trajectory in the diagonal band and the MFB, most medial septal axons remained ventral to the position assumed by most descending nucleus accumbens axons (Figs. I B-E, 2B-E). From the medial septum, diffuse projections to medial and lateral preoptic areas and light projections to the bed nucleus of the stria terminalis were noted (Fig. 2C, D). At the preoptic-anterior hypothalamic border, some labeled medial septal
594 fibers spread dorsally into the parataenial and anterior mediodorsal nuclei or" the thalamus, with some axons continuing caudally into the medial and lateral habenular nuclei (Fig. 2E, F). From the small medial septal injection site illustrated, no labeled fibers could be traced into the stria medullaris, but larger septal injections did label fibers in this tract. In no medial septal case we re labeled axons seen in t he stria te rminalis. Labeled medial septal axons descending through the hypothalamus in the M FB remained in a ventrally coursing bundle (Fig. 2E, F). A few labeled axons spread laterally to give a very light projection to the medial amygdala; others turned ventromedially to form a small, discrete clump at the lateral edge of the median eminence (Fig. 2E, F). The periventricular region of the hypothalamus, the dorsal half or" the anterior hypothalamus and the dorsomedial hypothalamus received projections tFig. 2E, F), but the ventromedial nucleus was not labeled. More posteriorly, light labeling of the arcuate, dorsal premammillary and posterior hypothalamic nuclei and of the very anterior central gray (Fig. 2G) was noted. A projection to the medial mammillary nucleus was observed, along with diffuse labeling of the supramammillary region and ventral tegmentat area (Fig. 2G, H). No projections were seen more posteriorly, and no labeling was observed in midbrain reticular formation or raphe nuclei. We conclude that patterns of projections from the medial septal nucleus and nucleus accumbens are overlapping, but different. Both medial septal and nucleus accumbens send descending axons through the M FB, but medial septal fibers occupy a position in the bundle ventral to those from nucleus accumbens. Nucleus accumbens axons in the MFB ultimately spread laterally into the substantia nigra and reticular formation, while those from the septum terminate largely in the medial mammillary nuclei. Septal, but not nucleus accumbens, axons reach the arcuate region of the hypothalamus. The medial septum projects to the hippocampus, a connection never seen following nucleus accumbens isotope placements. From nucleus accumbens, but not from the medial septum, projections can be traced to the medial globus pallidus and to or through the entopeduncular nucleus. Within the thalamus, medial septal fibers are distributed to mediodorsal and habenular nuclei, while nucleus accumbens axons are distributed in the periventricular nuclei. Both nucleus accumbens and medial septum send fibers into parataeniat thalamus and also project to the anterior septum, preoptic area, anterior and dorsomedial hypothalamus, supramammillary regions, anterior central gray and ventral tegmental area of Tsai. Some of the efferents we traced from nucleus accumbens have been previously reported. Nucleus accumbens axons in the medial forebrain bundle have been observed by a number of other workers 7,~:'.~s,e~ as have nucleus accumbens eflbrents to the septumJS, ~s The nucleus accumbens has been shown to also receive afferents from t he septum aS,~s,l'9, and through the MFB 6 and fornix r),ls ~2°. In two recent reports using autoradiographic techniques is,z1, nucleus accumbens projections to globus pallidus and to substantia nigra were seen. Swanson and Cowan zl have suggested that a topographic organization of nucleus accumbens efferents may exist: our observation of projections from medial nucleus accumbens to medial globus pallidus and medial substantia nigra would support this suggestion. Swanson and Cowan ~1 also observed, as we did, nucleus accumbens efferents running through the entopeduncular nucleus and lateral hypothalamus.
595 M a n y o f the efferents o b s e r v e d f r o m the m e d i a l septal nucleus have also been r e p o r t e d previously, in studies using lesion a n d d e g e n e r a t i o n techniquesg,16,19,2°. D u e to difficulties in distinguishing p r o j e c t i o n s o f h i p p o c a m p a l fibers-of-passage f r o m those arising in the septal nuclei, some p r o j e c t i o n s we have d e m o n s t r a t e d to originate in the medial s e p t u m (e.g., the p r o j e c t i o n s to ventral t e g m e n t a l area, medial m a m m i l l a r y nuclei, m e d i o d o r s a l t h a l a m u s a n d p a r a t a e n i a l t h a l a m u s ) have previously been attributed to the h i p p o c a m p u s . The p r o j e c t i o n we observed f r o m the medial septum to the ventral h i p p o c a m p u s has also been d e m o n s t r a t e d by o t h e r w o r k e r s (Siegel, p e r s o n a l communication). F r o m o u r work, it a p p e a r s t h a t the nucleus a c c u m b e n s resembles the septum in its projections to some regions o f the h y p o t h a l a m u s , to p a r a t a e n i a l t h a l a m u s a n d to ventral t e g m e n t a l a r e a a n d central gray, b u t is also similar to the s t r i a t u m in projecting to globus pallidus, e n t o p e d u n c u l a r nucleus a n d s u b s t a n t i a nigra 1~-,~2. F u n c t i o n a l l y , the nucleus a c c u m b e n s a p p e a r s m o r e like the s e p t u m : lesions of the nucleus a c c u m b e n s p r o d u c e b e h a v i o r a l deficits m o r e similar to those seen after septal lesions t h a n those o b s e r v e d following d a m a g e to the striatum11,13,14. H o w e v e r , the p a t t e r n o f neurogenesis a n d d e v e l o p m e n t o f the nucleus a c c u m b e n s resembles t h a t o f the s t r i a t u m 3,2'. Thus, a l t h o u g h s e p a r a t e cases can be m a d e for classification o f nucleus a c c u m b e n s with s e p t u m or striatum, the nucleus a c c u m b e n s m a y m o r e logically be considered interm e d i a t e between these two structures.
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