Brain Research, 85 (1975) 261-266
© Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands
261
S U B C O R T I C A L A F F E R E N T S TO T H E F R O N T A L LOBE IN T H E RHESUS M O N K E Y S T U D I E D BY MEANS OF R E T R O G R A D E H O R S E R A D I S H PEROXIDASE TRANSPORT*
JOB KIEVIT ANDHENRICUS G. J. M. KUYPERS Department of Anatomy, Rotterdam Medical Faculty, Erasmus University, Rotterdam (The Netherlands)
The present study tried to determine in the rhesus monkey the precise location of various subcortical cell populations which send their fibers to different parts of the precentral gyrus and of the rostrally adjoining cortical areas. For this purpose in 14 rhesus monkeys horseradish peroxidase (HRP) was injected in different parts of the frontal lobe followed by a detailed study of the location of the diencephalic neurons to which the enzyme was transported retrogradely. In 14 animals 6-30 closely spaced injections of 0.6 #1 of 10 ~ H R P (Sigma type VI) in distilled water were made in the different cortical areas indicated in Fig. la. In 5 monkeys, the injections were made unilaterally but in the other animals they were made bilaterally. In one control animal, 30 x 0.6 #1 of 0.0075 ~o saline was injected in the precentral gyrus on one side. This solution has approximately the same molarity as 1 0 ~ HRP. The animals survived the operations for 3 days. They were then deeply anesthetized with Nembutal and subsequently perfused with a solution of 6 ~ dextran in 0.9 ~ saline followed by a mixture of 2.5 ~ glutaraldehyde-0.5 9/00paraformaldehyde in 0.1 M cacodylate buffer (pH 7.2). After the brains had been removed they were saturated with a solution of 3 0 ~ sucrose in cacodylate buffer and cut in 40-#m transverse sections on a Leitz freezing microtome. The sections were incubated in a medium containing Tris buffer, 3,3'-diaminobenzidine and hydrogen peroxide, and then dehydrated and covered. They were studied microscopically under dark-field and bright-field illumination. Some sections were lightly counter-stained with cresyl violet. The location of the neurons to which the enzyme was transported retrogradely was charted with the aid of an X - Y plotter. In the animals with H R P injected in the frontal lobe, retrogradely labeled neurons, containing H R P reaction granules in the cell body and proximal parts of the dendrites, were present in the thalamus as well as in the substantia innominata and the hypothalamus (Fig. 2a). The latter areas will be referred to as the basal forebrain areas. In some cases, labeled neurons were also present in the claustrum and in 7 animals in which the lower brain stem was studied, labeled neurons were also found in the locus coeruleus (Fig. 2b). In the 4 animals in which the enzyme was injected in one * A preliminary report on some of these findings is being published in Science.
262
19
c
Fig. 1. a: the cortical areas containing HRP reaction product after injection of the enzyme in the cortex of the 17 animals used for this study, b: the various cortical strips, c: reconstruction of a horizontal plane through the thalamus with the location of the longitudinally oriented bands of thalamic neurons from which the respective cortical strips receive afferents. SP, sulcus principalis; SA, sulcus arcuatus; SC, sulcus centralis; SIP, sulcus intraparietalis; SL, sulcus lunatus. frontal lobe, the H R P positive neurons in the thalamus were present exclusively ipsilaterally while in the basal forebrain areas and the locus coeruleus some such cells were also present contralaterally. In view of the unilateral distribution of the labeled neurons in the thalamus in these cases, the enzyme was injected on both sides in further studies. With respect to the distribution of the H R P positive neurons in the specific thalamic nuclei the following findings were obtained. After injection of the enzyme in the face area of the precentral gyrus, retrogradely labeled neurons were found in the V.P.M. (nucleus ventralis posterior medialis of Olszewsky 9) in the ventromedial parts of the V.L.c. (nucleus ventralis lateralis pars caudalis, Olszewsky 9) and of the V.I.M. (nucleus ventralis intermedius - - which corresponds to the nucleus ventralis posterior lateralis pars oralis of Olszewsky 9) and in the ventral part of the V.L. (nucleus ventralis lateralis). After injection of the enzyme in the precentral hand area (Woolsey et al. 1~) the retrogradely labeled cells were located more laterally in V.L.c., V.I.M., and V.L. After injection in the precentral foot area the labeled cells were located in a lateral band immediately adjoining the reticular nucleus, which band extended from the rostral part of the V.P.L.c. (nucleus ventralis posterior lateralis pars caudalis) through the V.L.c. and the V.I.M. into the V.L. When injections were made in the caudal part of the precentral gyrus in between the hand and foot areas, labeled neurons were also present in the V.L.c., the V.I.M. and the V.L. These neurons were located in between the two populations of neurons that were labeled retrogradely from the precentral hand and foot areas respectively. These findings indicate that the
263
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~ . ~ CAUD
< a
Zfz b
Fig. 2. a: the distribution of HRP positive neurons in the basal forebrain areas, i.e. the substantia innominata, especially the nucleus basalis, the medullary laminae of the globus pallidus, and the hypothalamus - - in the case of a large injection in the precentral gyrus, b: the distribution of retrogradely labeled neurons in the nucleus locus coeruleus of the same animal. caudal parts of the precentral gyrus receive fibers from the V.L.c., the ventrolateral half of the V.I.M. and ventrolateral one-third of the V.L. The most lateral portion of this precentral area appears to receive fibers mainly from the most medially located neurons in these nuclei, while progressively more medially located parts of this area receive fibers from progressively more laterally located neurons. When the enzyme was injected in the rostral part of the precentral corticospinal area - - corresponding to the trunk area of Woolsey e t a l . 15 - - H R P positive neurons were also present in V.L.c., V.I.M., and V.L. nuclei. These neurons were located in a band dorsally and medially adjoining the areas containing labeled neurons after injection of the enzyme in the caudal part of the precentral gyrus. However, the former population o f H R P positive neurons continued further rostrally in the ventral nuclei of the thalamus than the latter and also occupied the rostral part of the V.L. nucleus as well as the extreme caudolateral part of the V.A. nucleus (nucleus ventralis anterior of Olszewskyg). When the enzyme was injected further rostrally in the frontal lobe, i . e . in the areas above and below the arcuate sulcus, H R P positive neurons were present in the ventral nuclei but also in the nucleus medialis dorsalis (M.D.). This population of labeled neurons also formed a band which ran from caudomedial to rostrolateral. This band occupied caudally the dorsomedial portion o f the V.I.M. and the lateral part of the M.D. More rostrally it occupied the medial part of V.L. and the lateral half of V.A. When the enzyme was injected further rostrally, in the areas above and below the arcuate sulcus, another band of H R P positive neurons appeared which also ran from caudomedial to rostrolateral. This band occupied at caudal levels the medial part of the M.D. nucleus and at more rostral levels the lateral part of the M.D.
264 nucleus and the medial and intermediate parts of the V.A. nucleus. In one case H R P was injected in the arcuate gyrus. In this case the H R P positive neurons were found in roughly the same areas as occupied by retrogradely labeled neurons after injection above and below the arcuate sulcus. These findings supplement earlier findings1,2.4,14 and strongly suggest that transverse strips of frontal cortex oriented parallel to the central sulcus receive fibers from longitudinal cell bands in the thalamus which are oriented from caudomedial to rostrolateral and are located in that portion of the thalamus which encompasses the V.L.c., V.I.M., V. L., V.A., and M. D. nuclei. The most caudal transverse strip of frontal cortex, which adjoins the central sulcus, receives fibers from the most caudal cell population which is located in the caudolateral part of the V.L.c., V.I.M. and V.L. area of the thalamus. The progressively more rostrally located transverse strips of frontal cortex receive fibers from progressively more rostromedially located bands of thalamic cells. Thus, the very rostrally located strip, encompassing the rostral parts of the areas above and below the arcuate sulcus, receives fibers from a band of thalamic cells occupying the caudomedial part of the M.D., the rostrolateral part of the M.D. and the rostromedial half of the V.A. The findings further indicate that at least in the most caudal strip of frontal cortex, i.e. the caudal part of the precentral gyrus, a mediolateral organization also exists in that progressively more laterally located parts of this strip of cortex receive afferents from progressively more medially located cell groups in the corresponding portion of the ventral thalamus. With respect to these findings it is of interest to note that the terminal distribution area of the brachium conjunctivum in the thalamus extends from V.L.c: through the V.I.M. and V.L. into the most lateral part of the V.A. nucleus 5,7. The rostral border of this terminal distribution area, therefore, seems to be oriented in a caudomedial to rostrolateral direction similar to that of the longitudinal bands of retrogradely labeled thalamic neurons. The same seems to hold true for the rostral border of the terminal distribution area of the fastigial fibers in the thalamus:'. Following the H R P injections in the frontal cortex in these cases some retrogradely labeled neurons were also found in the nonspecific thalamic nuclei. After injection in the precentral gyrus such neurons were especially found in the nucleus centralis lateralis and the most caudal portion of the centrum medianum. These findings are in keeping with the recent findings of Jones and Leavitt 4. In the cases with injections of the enzyme in the frontal cortex, some H R P positive neurons were present further in the basal forebrain areas, mainly ipsilaterally. At the level immediately rostral to the anterior commissure a concentration of labeled neurons was present at the foot of the septum. Some of these neurons were located in the area of the nucleus of the diagonal band of Broca s,l° but others were located more laterally. At the level of the optic chiasm this population of H R P positive neurons shifted laterally through the substantia innominata where the labeled neurons were present in an area corresponding approximately to the nucleus basalis 8,9. Caudal to the optic chiasm the labeled neurons were located in the caudal part of the nucleus basalis immediately ventral to the globus pallidus. Some labeled cells were also present more dorsally in the internal and external medullary laminae of the globus pallidus
265 which reportedly s contain cells of the nucleus basalis. Labeled neurons were found also in the lateral hypothalamus medial to the optic tract, in the area around the fornix and in the dorsomedial part of the hypothalamus. In some of the cases with injections in the areas above and below the arcuate sulcus and in the precentral gyrus, the lower brain stem was also studied. In these cases H R P positive neurons were found in the locus coeruleus, bilaterally 11,13. In 3 other animals injections were made in the postcentral, parietal and occipital areas. This was done in order to determine whether the enzyme was also transported retrogradely from these parts of the cerebral cortex to the basal forebrain areas. Following injections in the postcentral and parietal areas, H R P positive neurons were present in the basal forebrain areas and were distributed in roughly the same fashion as after injections in the precentral gyrus. After injection of the enzyme in occipital lobe, however, the labeled neurons were distributed differently and were confined to the caudal parts of the nucleus basalisS,L Only a few cells were found in the hypothalamus in this case. In these 3 cases the H R P positive neurons in the specific thalamic nuclei were distributed as follows. After injection of the enzyme in the postcentral gyrus the labeled neurons were present mainly in the nucleus ventralis posterior lateralis pars caudalis (V.P.L.c.). After injections in the parietal lobules retrogradely labeled neurons were concentrated in the nucleus lateralis posterior (L.P.) but were also present in the rostral part of the nucleus paracentralis (P.C.N.). In the case in which the enzyme was injected in the occipital lobe, retrogradely labeled neurons were abundant in the lateral geniculate body and were found also in the lateral and inferior parts of the pulvinar. The fact that in 17 monkeys injections of H R P in the frontal, postcentral and parietal cortical areas consistently resulted in retrograde transport of the enzyme to neurons in the nucleus basalis of the substantia innominata and in the hypothalamus indicates that neurons in the basal forebrain areas also send axons to the neocortex of these lobes. These direct fiber connections from the basal forebrain areas to the cortex of the frontal and parietal lobes resemble the ascending monoaminergic pathways11,13 which also lead directly from cell groups in the brain stem to the cortex, as confirmed by the present findings with respect to the locus coeruleus. The neurons in the substantia innominata and lateral hypothalamus, to which H R P injected in the cortex was transported retrogradely, appear to be located at the cross roads of limbic brain stem pathways 8. The activity of the limbic system has been shown to be related to motivational and emotional states and stimulation of the limbic basal forebrain areas has been shown to elicit behavioral sleep, food intake and sexual activityS,6,1L The present findings make it likely that the behavioral phenomena which can be elicited by stimulation of limbic basal forebrain areas may be brought about not only by way of their descending pathways to the brain stem 8, but also by way of the direct connections to the cerebral cortex demonstrated in the present study. The authors thank Dr. S. Jacobson and Mr. J. Trojanowski for their help in preparing some of the animals and Dr. D. A. Hopkins for reading the manuscript. This study was supported in part by Grant 13-31-12 of the F U N G O (Dutch
266 Organization for Fundamental mental Government
R e s e a r c h in M e d i c i n e ) a n d by a D u t c h I n t e r d e p a r t -
Grant.
1 AKERT, K., Comparative anatomy of the frontal cortex and thalamofrontal connections. In J. M. WARREN AND K. AKERT (Eds.), The Frontal Granular Cortex and Behavior, McGraw-Hill, New York, 1964, pp. 372-394. 2 CHow, K. L., AND PRIBRAM, K. H., Cortical projection of the thalamic ventrolateral nuclear group in monkeys, J. eomp. Neurol., 104 (1956) 57-75. 3 HESS, W. R., Das Zwischenhirn, Benno Schwabe and Co., Basel, 1949. 4 JONES, E. G., AND LEAVITT, R. Y., Retrograde axonal transport and the demonstration of nonspecific projections to the cerebral cortex and striatum from thalamic intralaminar nuclei in the rat, cat and monkey, J. comp. Neurol., 154 (1974) 349-378. 5 KIEVlT, J., AND KUYPERS, H. G. J. M., Fastigial cerebetlar projections to the ventrolateral nucleus of the thalamus and the organization of the descending pathways. In T. FRIGYESI,E. RINVlK AND M. D. YAHR (Eds.), Corticothalamic Projections and Sensorimotor Activities, Raven Press, New York, 1972, pp. 91-114. 6 MACLEAN, P. D., The hypothalamus and emotional behavior. In W. HAYMAKER,E. ANDERSON AND W. J. H. NAUTA (Eds.), The Hypothalamus, Thomas, Springfield, II1., 1969, pp. 659-678. 7 MEHLER, W. R., VERNIER, V. G,, AND NAUTA, W. J. H., Efferent projections from dentate and interpositus nuclei in primates, Anat. Rec., 130 (1958) 430-431. 8 NAUTA, W. J. H., AND HAYMAKER,W., Hypothalamic nuclei and fiber connections. In W. HAYMAKER, E. ANDERSONAND W. J. H. NAUTA (Eds.), The Hypothalamus, Thomas, Springfield, 111., 1969, pp. 136-209. 90LSZEWSKY, J., The Thalamus of the Macaca mulatta, S. Karger, New York, 1952. 10 PAPEZ, J. W., AND ARONSON, L. R., Thalamic nuclei of the Pithecus (Macacus) rhesus, Arch. Neurol. Psychiat. (Chic.), 32 (1934) 1-26. 11 PICKEL,V. M., SEGAL, M., AND BLOOM, F. E., A radioautographic study of the efferent pathways of the nucleus locus coeruleus, J. comp. Neurol., 155 (1974) 15-42. 12 ROBXNSON,B. W., AND MISI-IKIN,M., Alimentary responses to forebrain stimulation in monkeys, Exp. Brain Res., 4 (1968), 330-366. 13 UNGERSTEDT, U., Stereotaxic mapping of monoamine pathways in the rat brain, Acta physiol. scand., Suppl. 367 (1971), 1-44. 14 WALKER,A. E., The Primate Thalamus, University of Chicago Press, Chicago, I11., 1938. 15 WOOLSEY,C. N., SETTLAGE,P. H., MEYER, D. R., SPENCER,W., HAMUY,T. P., AND TRAVIS,A. M., Patterns of localization in precentral and 'supplementary' motor areas and their relation to the concept of a premotor area, Res. Publ. Ass. nerv. ment. Dis., 30 (1950) 238-264.
© Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands
261
S U B C O R T I C A L A F F E R E N T S TO T H E F R O N T A L LOBE IN T H E RHESUS M O N K E Y S T U D I E D BY MEANS OF R E T R O G R A D E H O R S E R A D I S H PEROXIDASE TRANSPORT*
JOB KIEVIT ANDHENRICUS G. J. M. KUYPERS Department of Anatomy, Rotterdam Medical Faculty, Erasmus University, Rotterdam (The Netherlands)
The present study tried to determine in the rhesus monkey the precise location of various subcortical cell populations which send their fibers to different parts of the precentral gyrus and of the rostrally adjoining cortical areas. For this purpose in 14 rhesus monkeys horseradish peroxidase (HRP) was injected in different parts of the frontal lobe followed by a detailed study of the location of the diencephalic neurons to which the enzyme was transported retrogradely. In 14 animals 6-30 closely spaced injections of 0.6 #1 of 10 ~ H R P (Sigma type VI) in distilled water were made in the different cortical areas indicated in Fig. la. In 5 monkeys, the injections were made unilaterally but in the other animals they were made bilaterally. In one control animal, 30 x 0.6 #1 of 0.0075 ~o saline was injected in the precentral gyrus on one side. This solution has approximately the same molarity as 1 0 ~ HRP. The animals survived the operations for 3 days. They were then deeply anesthetized with Nembutal and subsequently perfused with a solution of 6 ~ dextran in 0.9 ~ saline followed by a mixture of 2.5 ~ glutaraldehyde-0.5 9/00paraformaldehyde in 0.1 M cacodylate buffer (pH 7.2). After the brains had been removed they were saturated with a solution of 3 0 ~ sucrose in cacodylate buffer and cut in 40-#m transverse sections on a Leitz freezing microtome. The sections were incubated in a medium containing Tris buffer, 3,3'-diaminobenzidine and hydrogen peroxide, and then dehydrated and covered. They were studied microscopically under dark-field and bright-field illumination. Some sections were lightly counter-stained with cresyl violet. The location of the neurons to which the enzyme was transported retrogradely was charted with the aid of an X - Y plotter. In the animals with H R P injected in the frontal lobe, retrogradely labeled neurons, containing H R P reaction granules in the cell body and proximal parts of the dendrites, were present in the thalamus as well as in the substantia innominata and the hypothalamus (Fig. 2a). The latter areas will be referred to as the basal forebrain areas. In some cases, labeled neurons were also present in the claustrum and in 7 animals in which the lower brain stem was studied, labeled neurons were also found in the locus coeruleus (Fig. 2b). In the 4 animals in which the enzyme was injected in one * A preliminary report on some of these findings is being published in Science.
262
19
c
Fig. 1. a: the cortical areas containing HRP reaction product after injection of the enzyme in the cortex of the 17 animals used for this study, b: the various cortical strips, c: reconstruction of a horizontal plane through the thalamus with the location of the longitudinally oriented bands of thalamic neurons from which the respective cortical strips receive afferents. SP, sulcus principalis; SA, sulcus arcuatus; SC, sulcus centralis; SIP, sulcus intraparietalis; SL, sulcus lunatus. frontal lobe, the H R P positive neurons in the thalamus were present exclusively ipsilaterally while in the basal forebrain areas and the locus coeruleus some such cells were also present contralaterally. In view of the unilateral distribution of the labeled neurons in the thalamus in these cases, the enzyme was injected on both sides in further studies. With respect to the distribution of the H R P positive neurons in the specific thalamic nuclei the following findings were obtained. After injection of the enzyme in the face area of the precentral gyrus, retrogradely labeled neurons were found in the V.P.M. (nucleus ventralis posterior medialis of Olszewsky 9) in the ventromedial parts of the V.L.c. (nucleus ventralis lateralis pars caudalis, Olszewsky 9) and of the V.I.M. (nucleus ventralis intermedius - - which corresponds to the nucleus ventralis posterior lateralis pars oralis of Olszewsky 9) and in the ventral part of the V.L. (nucleus ventralis lateralis). After injection of the enzyme in the precentral hand area (Woolsey et al. 1~) the retrogradely labeled cells were located more laterally in V.L.c., V.I.M., and V.L. After injection in the precentral foot area the labeled cells were located in a lateral band immediately adjoining the reticular nucleus, which band extended from the rostral part of the V.P.L.c. (nucleus ventralis posterior lateralis pars caudalis) through the V.L.c. and the V.I.M. into the V.L. When injections were made in the caudal part of the precentral gyrus in between the hand and foot areas, labeled neurons were also present in the V.L.c., the V.I.M. and the V.L. These neurons were located in between the two populations of neurons that were labeled retrogradely from the precentral hand and foot areas respectively. These findings indicate that the
263
PL"--'~'~
LOC COERULEUS
~ . ~ CAUD
< a
Zfz b
Fig. 2. a: the distribution of HRP positive neurons in the basal forebrain areas, i.e. the substantia innominata, especially the nucleus basalis, the medullary laminae of the globus pallidus, and the hypothalamus - - in the case of a large injection in the precentral gyrus, b: the distribution of retrogradely labeled neurons in the nucleus locus coeruleus of the same animal. caudal parts of the precentral gyrus receive fibers from the V.L.c., the ventrolateral half of the V.I.M. and ventrolateral one-third of the V.L. The most lateral portion of this precentral area appears to receive fibers mainly from the most medially located neurons in these nuclei, while progressively more medially located parts of this area receive fibers from progressively more laterally located neurons. When the enzyme was injected in the rostral part of the precentral corticospinal area - - corresponding to the trunk area of Woolsey e t a l . 15 - - H R P positive neurons were also present in V.L.c., V.I.M., and V.L. nuclei. These neurons were located in a band dorsally and medially adjoining the areas containing labeled neurons after injection of the enzyme in the caudal part of the precentral gyrus. However, the former population o f H R P positive neurons continued further rostrally in the ventral nuclei of the thalamus than the latter and also occupied the rostral part of the V.L. nucleus as well as the extreme caudolateral part of the V.A. nucleus (nucleus ventralis anterior of Olszewskyg). When the enzyme was injected further rostrally in the frontal lobe, i . e . in the areas above and below the arcuate sulcus, H R P positive neurons were present in the ventral nuclei but also in the nucleus medialis dorsalis (M.D.). This population of labeled neurons also formed a band which ran from caudomedial to rostrolateral. This band occupied caudally the dorsomedial portion o f the V.I.M. and the lateral part of the M.D. More rostrally it occupied the medial part of V.L. and the lateral half of V.A. When the enzyme was injected further rostrally, in the areas above and below the arcuate sulcus, another band of H R P positive neurons appeared which also ran from caudomedial to rostrolateral. This band occupied at caudal levels the medial part of the M.D. nucleus and at more rostral levels the lateral part of the M.D.
264 nucleus and the medial and intermediate parts of the V.A. nucleus. In one case H R P was injected in the arcuate gyrus. In this case the H R P positive neurons were found in roughly the same areas as occupied by retrogradely labeled neurons after injection above and below the arcuate sulcus. These findings supplement earlier findings1,2.4,14 and strongly suggest that transverse strips of frontal cortex oriented parallel to the central sulcus receive fibers from longitudinal cell bands in the thalamus which are oriented from caudomedial to rostrolateral and are located in that portion of the thalamus which encompasses the V.L.c., V.I.M., V. L., V.A., and M. D. nuclei. The most caudal transverse strip of frontal cortex, which adjoins the central sulcus, receives fibers from the most caudal cell population which is located in the caudolateral part of the V.L.c., V.I.M. and V.L. area of the thalamus. The progressively more rostrally located transverse strips of frontal cortex receive fibers from progressively more rostromedially located bands of thalamic cells. Thus, the very rostrally located strip, encompassing the rostral parts of the areas above and below the arcuate sulcus, receives fibers from a band of thalamic cells occupying the caudomedial part of the M.D., the rostrolateral part of the M.D. and the rostromedial half of the V.A. The findings further indicate that at least in the most caudal strip of frontal cortex, i.e. the caudal part of the precentral gyrus, a mediolateral organization also exists in that progressively more laterally located parts of this strip of cortex receive afferents from progressively more medially located cell groups in the corresponding portion of the ventral thalamus. With respect to these findings it is of interest to note that the terminal distribution area of the brachium conjunctivum in the thalamus extends from V.L.c: through the V.I.M. and V.L. into the most lateral part of the V.A. nucleus 5,7. The rostral border of this terminal distribution area, therefore, seems to be oriented in a caudomedial to rostrolateral direction similar to that of the longitudinal bands of retrogradely labeled thalamic neurons. The same seems to hold true for the rostral border of the terminal distribution area of the fastigial fibers in the thalamus:'. Following the H R P injections in the frontal cortex in these cases some retrogradely labeled neurons were also found in the nonspecific thalamic nuclei. After injection in the precentral gyrus such neurons were especially found in the nucleus centralis lateralis and the most caudal portion of the centrum medianum. These findings are in keeping with the recent findings of Jones and Leavitt 4. In the cases with injections of the enzyme in the frontal cortex, some H R P positive neurons were present further in the basal forebrain areas, mainly ipsilaterally. At the level immediately rostral to the anterior commissure a concentration of labeled neurons was present at the foot of the septum. Some of these neurons were located in the area of the nucleus of the diagonal band of Broca s,l° but others were located more laterally. At the level of the optic chiasm this population of H R P positive neurons shifted laterally through the substantia innominata where the labeled neurons were present in an area corresponding approximately to the nucleus basalis 8,9. Caudal to the optic chiasm the labeled neurons were located in the caudal part of the nucleus basalis immediately ventral to the globus pallidus. Some labeled cells were also present more dorsally in the internal and external medullary laminae of the globus pallidus
265 which reportedly s contain cells of the nucleus basalis. Labeled neurons were found also in the lateral hypothalamus medial to the optic tract, in the area around the fornix and in the dorsomedial part of the hypothalamus. In some of the cases with injections in the areas above and below the arcuate sulcus and in the precentral gyrus, the lower brain stem was also studied. In these cases H R P positive neurons were found in the locus coeruleus, bilaterally 11,13. In 3 other animals injections were made in the postcentral, parietal and occipital areas. This was done in order to determine whether the enzyme was also transported retrogradely from these parts of the cerebral cortex to the basal forebrain areas. Following injections in the postcentral and parietal areas, H R P positive neurons were present in the basal forebrain areas and were distributed in roughly the same fashion as after injections in the precentral gyrus. After injection of the enzyme in occipital lobe, however, the labeled neurons were distributed differently and were confined to the caudal parts of the nucleus basalisS,L Only a few cells were found in the hypothalamus in this case. In these 3 cases the H R P positive neurons in the specific thalamic nuclei were distributed as follows. After injection of the enzyme in the postcentral gyrus the labeled neurons were present mainly in the nucleus ventralis posterior lateralis pars caudalis (V.P.L.c.). After injections in the parietal lobules retrogradely labeled neurons were concentrated in the nucleus lateralis posterior (L.P.) but were also present in the rostral part of the nucleus paracentralis (P.C.N.). In the case in which the enzyme was injected in the occipital lobe, retrogradely labeled neurons were abundant in the lateral geniculate body and were found also in the lateral and inferior parts of the pulvinar. The fact that in 17 monkeys injections of H R P in the frontal, postcentral and parietal cortical areas consistently resulted in retrograde transport of the enzyme to neurons in the nucleus basalis of the substantia innominata and in the hypothalamus indicates that neurons in the basal forebrain areas also send axons to the neocortex of these lobes. These direct fiber connections from the basal forebrain areas to the cortex of the frontal and parietal lobes resemble the ascending monoaminergic pathways11,13 which also lead directly from cell groups in the brain stem to the cortex, as confirmed by the present findings with respect to the locus coeruleus. The neurons in the substantia innominata and lateral hypothalamus, to which H R P injected in the cortex was transported retrogradely, appear to be located at the cross roads of limbic brain stem pathways 8. The activity of the limbic system has been shown to be related to motivational and emotional states and stimulation of the limbic basal forebrain areas has been shown to elicit behavioral sleep, food intake and sexual activityS,6,1L The present findings make it likely that the behavioral phenomena which can be elicited by stimulation of limbic basal forebrain areas may be brought about not only by way of their descending pathways to the brain stem 8, but also by way of the direct connections to the cerebral cortex demonstrated in the present study. The authors thank Dr. S. Jacobson and Mr. J. Trojanowski for their help in preparing some of the animals and Dr. D. A. Hopkins for reading the manuscript. This study was supported in part by Grant 13-31-12 of the F U N G O (Dutch
266 Organization for Fundamental mental Government
R e s e a r c h in M e d i c i n e ) a n d by a D u t c h I n t e r d e p a r t -
Grant.
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