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Evidence for interconnections between the two segments of the globus pallidus in primates: a PHA-L anterograde tracing study
Brain Research, 533 (1990) 171-175 Elsevier
171
BRES 24381
Evidence for interconnections between the two segments of the globus pallidus in primates: a PHA-L anterograde tracing study L.-N. Hazrati 1, A. Parent 1, S. Mitchell 2 and S.N. Haber 3 ~Centre de Recherche en Neurobiologie, Universit~ Laval et H~pital de l' Enfant-Jdsus, Quebec (Canada), 2Research Service 151 VA Medical Center and Department of Physiology, SUNY Health Science Center, Syracuse, NY (U. S.A.) and ~Department of Neurobiology and Anatomy, University of Rochester School of Medicine, Rochester, IVY (U.S.A.)
(Accepted 7 August 1990) Key words: Basal ganglia; Pallidum; Pallidal connection; Squirrel monkey; Saimiri sciureus; Rhesus monkey; Macaca mulatta; Phaseolus vulgaris leucoagglutinin
Small injections of the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) in the external segment of the pallidum (GPe) in the squirrel monkey (Saimiri sciureus) and in the rhesus monkey (Macaca mulatta) led to anterograde labeling of fibers in the internal segment of the pallidum (GPi). These fibers formed numerous large varicosities reminiscent of terminal boutons, which closely surrounded GPi ceU bodies and primary dendrites. Conversely, PHA-L injections in the GPi of squirrel monkeys produced anterograde fiber labeling in the GPe. However, in contrast to fibers in GPi, those in GPe did not make prominent pericellular contacts. Instead, they displayed a rather linear course, had long intervaricose segments, and appeared to contact several GPe neurons along their course by close appositions on cell bodies and primary dendrites. These results suggest the existence of a reciprocal connection between the two pallidal segments, which may play a crucial role in the functional organization of the basal ganglia in primates. The globus pallidus occupies a strategic position in the basal ganglia circuitry and our knowledge of its connections in primates derived from numerous studies undertaken with various neuroanatomical techniques. For instance, the pioneering investigation of Nauta and Mehler 8 with anterograde degeneration methods has revealed much of the efferent connections of the globus pallidus. These findings were confirmed and further extended by axonal transport studies with tritiated amino acids as anterograde tracers 1,6. Moreover, retrograde tracer experiments have demonstrated that the external (GPe) and the internal (GPi) segment of the globus pallidus are largely innervated by separate afferent systems3,9. Hence, despite the fact that all pallidal neurons display the same morphological and neurotransmitter phenotype, data on neuronal connections clearly indicate that the GPe and the GPi should be considered as distinct entities. We recently undertook a comparative study of the efferent projections of the GPe and the GPi in primates with the highly sensitive anterograde tracer Phaseolus vulgarisieucoagglutinin (PHA-L) and noted evidence for interconnections between the two pallidal segments. Two adult squirrel monkeys (Saimiri sciureus) and one adult rhesus monkey (Macaca mulatta) were used in the present study. The squirrel monkeys received injections
of PHA-L (2.5% solution, Vector Labs, Burlingame, CA) in the GPe on one side and the GPi on the other side, whereas the rhesus monkey received a unilateral injection of the same tracer in the GPe only. The P H A - L was iontophoretically injected through glass micropipettes (tip diameter 25-30/~m) with a 7-10 microampere positive current delivered in 7 second pulses every 14 seconds over a 20-30 min period by using a constant current generator (Midgar Electronics, Canton, MA). The stereotaxic coordinates were chosen according to the atlas of Emmers and Akert 2 for the squirrel monkey. The injection in the rhesus monkey was made with the help of stereotaxic coordinates and electrophysiological identification of neuronal activity in the GPe. After a survival period of 10-12 days, the animals were deeply anesthetized with an overdose of pentobarbital and perfused transcardially with a phosphate buffer solution (0.1 M, pH 7.4) containing 1% sucrose, followed consecutively by a fixative containing 4% paraformaldehyde and 0.1% glutaraldehyde in phosphate buffer (0.1 M, pH 7.4) and a 4% paraformaldehyde solution in phosphate buffer (0.1 M, pH 7.4) at 4 °C. The brains were then removed from the skull and sectioned in the transverse plane with a freezing microtome at 40/~m for the squirrel monkeys and 50/~m for the rhesus monkey. Complete series of
Correspondence.. L.-N. Hazrati, Centre de recherche en neurobiologie, Hrpital de I'Enfant-Jrsus, 1401, 18e Rue, Quebec, Qua., Canada G1J 1Z4.
0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
172 brain sections were processed for P H A - L immunohistochemistry as described in detail elsewhere TM, whereas alternate sections were stained with cresyl violet to clearly delineated the various forebrain structures. The sections were examined under both light- and darkfield illumination and the location of PHA-L-positive profiles was mapped out with the help of a camera lucida. In the two squirrel monkeys the P H A - L injections were located in the dorsomedial half of the GPe (Fig. 1A) and gave rise to anterograde fiber labeling in the ipsilateral GPi. Numerous anterogradely labeled fibers were seen to arise from the injection loci and traversed the internal medullary lamina to invade the dorsolateral half of the GPi. The labeled fibers terminating directly within the GPi consisted of short, thin, and non-varicose axons scattered among GPi neurons whose cell bodies displayed a light nonspecific staining (Fig. 1B). When examined at high magnifications, these smooth fibers appeared poorly branched and arborized in a pericellular, basket-like pattern around somata and proximal dendrites of GPi neurons (Fig. 1C-I). Typically, one perikaryon was innervated by a single axon displaying numerous large axonal varicosities reminiscent of terminal boutons (Fig. 1F). Also worth noting was the presence of labeled fibers displaying close pericellular contacts with cell bodies of the substantia nigra pars reticulata after GPe injections (not shown). A multitude of anterogradely labeled fibers were also seen to leave the injection site in the GPe en route to the subthalamic nucleus and the substantia nigra. These fibers, in contrast to those terminating directly wihtin the GPi, did not traverse the GPi but accumulated into the internal medullary lamina and coursed through the ventral portion of the internal capsule to reach the subthalamic nucleus and the substantia nigra. Since the labeled fibers disclosed within the GPi did not seem to leave this structure and were poorly ramified, it was not possible to specify if these fibers were collaterals of those reaching the subthalamic nucleus and/or the substantia nigra. However, collateralization may occur close to the GPe cell bodies, where the analysis was rendered difficult due to the presence of the tracer and the abundance of
labeled neuronal profiles. The use of the retrograde fluorescent double-labeling technique would obviously be useful to determine the degree of axonal collateralization of the GPe neurons. In the same squirrel monkeys, the P H A - L injections on the other side of the brain were confined to the medial portion of the GPi (Fig. 1J). Numerous anterogradely labeled fibers were seen to leave the injection site in the GPi, coursed through the internal medullary lamina to terminate in the dorsomedial half of the GPe. These fibers displayed long intervaricose segments proximally (Fig. 1K), and branched rather frequently distally (Fig. 1L) making contact along their course with several GPe neurons (Fig. 1M). This pattern of terminal arborization suggests that the single GPi axons arborizing within GPe can influence several GPe cells in a rather diffuse manner by comparison to the more prominent, cell-to-cell type of relationship displayed by GPe axons in the GPi. In the rhesus monkey the injection site was small and confined to the central portion of the GPe (Fig. 2A). Anterogradely labeled fibers were seen to leave the injection site and travel in bundles medially into the GPi. Although most axons remained within fiber bundles on their way to the subthalamic nucleus, a number of fibers did split off and appeared to terminate within the lateral portion of the GPi, the medial portion remaining relatively free of fiber labeling. As these bundles crossed through the GPi, some smaller bundles of fibers broke off from the main group and coursed in various directions. Frequently small diameter fibers, which looked like single axons, departed from these smaller bundles (Fig. 2B). As these thin fibers leave the main group they often bifurcated, became varicose, and appeared to contact either cell bodies or dendrites of GPi neurons (Fig. 2C,D). Often several of these thin varicose fibers were closely intertwined and looked as if they were ensheathing a non-stained central core structure (Fig. 2C). The varicose fibers that appeared to wrap a central, unstained core, were similar albeit less densely entwined than those observed with peptide immunohistochemistry in the globus pallidus and referred to as woolly fibers 5. Similar fibers were observed in the globus pallidus after
Fig. 1. Photomicrographs illustrating some features of the pallidal labeling observed in one squirrel monkey after PHA-L injection in the external (A-I) and internal (J-M) segment of the globus pallidus. The maximal extent of the PHA-L injection sites in GPe and in GPi are illustrated in A and J, respectively. B offers a slightly higher magnification of the dorsolateral sector of the GPi where numerous, short, and smooth labeled fibers (open arrows) descending from the injection site can be seen. The filled arrows point to some of the GPi cells or primary dendrites that are illustrated at much higher magnifications in C-I. C-H show examples of axonal varicosities (filled arrows) making close pericellular contacts with cell bodies displaying a weak nonspecific staining. H depicts a smooth and linear GPe fiber (filled arrows) whose terminal portion displays numerous large varicosities closely surrounding one pole of a GPi cell, whereas I shows some thick fibers of the woolly type (filled arrows) near two GPi cell bodies which received pericellular contacts. K-M illustrate some features of the fiber labeling observed after GPe injection (J). K shows a typical labeled fiber as it emerges from the injection site in GPi, whereas L depicts the more distal portion of the same fiber in GPe. M displays some short varicose fibers (open arrows) close to an unstained cell body (filled arrow) in the GPe. GPe, external segment of the globus pallidus; GPi, internal segment of the globus pallidus; IC, internal capsule; PUT, putamen. Bar = 450 ~m (A,J), 20 pm (B), 10/tm (C-H), and 15 pm (I-M).
173 PHA-L injections into the striatum 4. Based on these tracing studies as well as the peptidergic staining, these characteristic profiles likely represent afferent fibers
wrapping the long, thick dendrites of GPi neurons. This suggests a direct input from the GPe to the GPi in the rhesus monkey.
174
*m b
B
C
175
Fig. 2. Photomicrographs illustrating some features of the pallidal labeling observed in one rhesus monkey after PHA-L injection in the external segment of the globus pallidus. A shows the small PHA-L injection site (filled arrow) at its maximal extent in the central portion of the GPe. B depicts small bundles or single smooth fibers (open arrows) as well as some varicose axons (closed arrows) present in the lateral half of the GPi. C shows varicose fibers entwining a central unstained core in the GPi (filled arrows), whereas part of a smooth fiber is visible in the lower portion of the figure (open arrow). D illustrates part of a thin and varicose fiber in the GPi as seen after GPe injection. GPe, external segment of the globus paUidus; GPi, internal segment of the giobus pallidus; IC, internal capsule; PUT, putamen. Bar = 1700/~m (A), 20 #m (B), 11.1/~m (C), and 8.3 #m (D).
The present investigation has thus provided the first evidence for the existence of direct and reciprocal connections between the G P e and the GPi in a primate, namely the squirrel m o n k e y . T h e results o b t a i n e d in the rhesus m o n k e y further support the existence of a projection from the G P e to GPi in primates, but the projection from the GPi to G P e remains to be investigated in the macaque. These findings are in a g r e e m e n t with a recent preliminary r e p o r t in which stimulation of the globus pallidus in the rat (homologue of the primate G P e ) was said to p r o d u c e a powerful inhibition of neurons of the e n t o p e d u n c u l a r nucleus (homologue of the p r i m a t e GPi) and of the substantia nigra 7. In the same r e p o r t fibers of the globus pallidus anterogradely l a b e l e d with P H A - L were described as forming large symmetrical synapses with s o m a t a and primary dendrites of e n t o p e d u n c u l a r and nigral neurons 7 (see also ref. 10). Since virtually all pallidal neurons display y-aminobutyric acid ( G A B A ) immunoreactivity in primates 12, it is very
1 DeVito, J.L. and Anderson, M.E., An autoradiographic study of efferent connections of the globus pallidus in Macaca mulana, Exp. Brain Res,, 46 (1982) 107-117. 2 Emmers, E. and Akert, K., A Stereotaxic Atlas of the Brain of the Squirrel Monkey (Saimiri sciureus), The University of Wisconsin Press, Madison, 1963. 3 Gim6nez-Amaya, J.M. and Graybiel, A.M., Compartmental origins of the striatopallidal projection in the primate, Neuroscience, 34 (1990) 111-I26. 4 Haber, S.N., Lynd, E., Klein, C. and Groenewegen, H.J., Topographic organization of the ventral striatal efferent projections in the rhesus monkey: an anterograde tracing study, J. Comp. NeuroL, 293 (1990) 282-288. 5 Haber, S.N. and Nauta, W.J.H., Ramification of the globus pallidus in the cat as demonstrated by patterns of immunohistochemistry, Neuroscience, 9 (1983) 245-260. 6 Kim, R., Nakano, K., Jayaraman, A. and Carpenter, M.B., Projections of the globus pallidus and adjacent structures: an autoradiographic study in the monkey, J. Comp. Neurol., 169 (1976) 263-290.
likely that the G P e / G P i interconnection described above is a reciprocal inhibitory G A B A e r g i c system. A comparison of the patterns of innervation o b s e r v e d in the G P e and the GPi of the squirrel m o n k e y suggests that G P e neurons may exert a strong inhibitory action u p o n GPi cells, whereas the GPi to G P e p r o j e c t i o n may act as a m o r e diffuse f e e d b a c k inhibitory mechanism. Electron microscopic studies are obviously n e e d e d in primates to reveal m o r e details about the types of contacts m a d e by this short pallidopallidal system, which could play an i m p o r t a n t role in the functional organization of the basal ganglia.
The authors thank Carole Harvey and Lisette Bertrand for technical assistance and Suzanne Bilodeau for typing the manuscript. This research was supported by Grant MT-5781 of the Medical Research Council (MRC) of Canada to A.P. and by NIMH Grant 50MH40381 to S.N.H. The financial support of the FRSQ and FCAR is also acknowledged. L.-N.H. was the recipient of a Studentship from the FCAR.
7 Kitai, S.T. and Kita, H., Dual Striatonigral Inhibitory Actions, Proc. Int. Conf. Neural Mech. Disord. Mov., Manchester, U.K., 1988, p. 21. 8 Nauta, W.J.H. and Mehler, W.R., Projections of the lentiform nucleus in the monkey, Brain Research, 1 (1966) 3-42. 9 Parent, A., Smith, Y., Filion, M. and Dumas, J., Distinct afferents to internal and external pallidal segments in the squirrel monkey, Neurosci. Left., 96 (1989) 140-144. 10 Smith, Y. and Bolam, J.P., Neurons of the substantia nigra reticulata receive a dense GABA-containing input from the globus pallidus, Brain Research, 493 (1989) 160--167. 11 Smith, Y., Hazrati, L,-N. and Parent, A., Efferent projections of the subthalamic nucleus in the squirrel monkey as studied by the PHA-L anterograde tracing method, J. Comp. NeuroL, 294 (1990) 306-323. 12 Smith, Y., Parent, A., S6gu~la, P. and Descarries, L., Distribution of GABA-immunoreactive neurons in the basal ganglia of the squirrel monkey (Saimiri sciureus), J. Comp. NeuroL, 259 (1987) 50-65.
171
BRES 24381
Evidence for interconnections between the two segments of the globus pallidus in primates: a PHA-L anterograde tracing study L.-N. Hazrati 1, A. Parent 1, S. Mitchell 2 and S.N. Haber 3 ~Centre de Recherche en Neurobiologie, Universit~ Laval et H~pital de l' Enfant-Jdsus, Quebec (Canada), 2Research Service 151 VA Medical Center and Department of Physiology, SUNY Health Science Center, Syracuse, NY (U. S.A.) and ~Department of Neurobiology and Anatomy, University of Rochester School of Medicine, Rochester, IVY (U.S.A.)
(Accepted 7 August 1990) Key words: Basal ganglia; Pallidum; Pallidal connection; Squirrel monkey; Saimiri sciureus; Rhesus monkey; Macaca mulatta; Phaseolus vulgaris leucoagglutinin
Small injections of the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) in the external segment of the pallidum (GPe) in the squirrel monkey (Saimiri sciureus) and in the rhesus monkey (Macaca mulatta) led to anterograde labeling of fibers in the internal segment of the pallidum (GPi). These fibers formed numerous large varicosities reminiscent of terminal boutons, which closely surrounded GPi ceU bodies and primary dendrites. Conversely, PHA-L injections in the GPi of squirrel monkeys produced anterograde fiber labeling in the GPe. However, in contrast to fibers in GPi, those in GPe did not make prominent pericellular contacts. Instead, they displayed a rather linear course, had long intervaricose segments, and appeared to contact several GPe neurons along their course by close appositions on cell bodies and primary dendrites. These results suggest the existence of a reciprocal connection between the two pallidal segments, which may play a crucial role in the functional organization of the basal ganglia in primates. The globus pallidus occupies a strategic position in the basal ganglia circuitry and our knowledge of its connections in primates derived from numerous studies undertaken with various neuroanatomical techniques. For instance, the pioneering investigation of Nauta and Mehler 8 with anterograde degeneration methods has revealed much of the efferent connections of the globus pallidus. These findings were confirmed and further extended by axonal transport studies with tritiated amino acids as anterograde tracers 1,6. Moreover, retrograde tracer experiments have demonstrated that the external (GPe) and the internal (GPi) segment of the globus pallidus are largely innervated by separate afferent systems3,9. Hence, despite the fact that all pallidal neurons display the same morphological and neurotransmitter phenotype, data on neuronal connections clearly indicate that the GPe and the GPi should be considered as distinct entities. We recently undertook a comparative study of the efferent projections of the GPe and the GPi in primates with the highly sensitive anterograde tracer Phaseolus vulgarisieucoagglutinin (PHA-L) and noted evidence for interconnections between the two pallidal segments. Two adult squirrel monkeys (Saimiri sciureus) and one adult rhesus monkey (Macaca mulatta) were used in the present study. The squirrel monkeys received injections
of PHA-L (2.5% solution, Vector Labs, Burlingame, CA) in the GPe on one side and the GPi on the other side, whereas the rhesus monkey received a unilateral injection of the same tracer in the GPe only. The P H A - L was iontophoretically injected through glass micropipettes (tip diameter 25-30/~m) with a 7-10 microampere positive current delivered in 7 second pulses every 14 seconds over a 20-30 min period by using a constant current generator (Midgar Electronics, Canton, MA). The stereotaxic coordinates were chosen according to the atlas of Emmers and Akert 2 for the squirrel monkey. The injection in the rhesus monkey was made with the help of stereotaxic coordinates and electrophysiological identification of neuronal activity in the GPe. After a survival period of 10-12 days, the animals were deeply anesthetized with an overdose of pentobarbital and perfused transcardially with a phosphate buffer solution (0.1 M, pH 7.4) containing 1% sucrose, followed consecutively by a fixative containing 4% paraformaldehyde and 0.1% glutaraldehyde in phosphate buffer (0.1 M, pH 7.4) and a 4% paraformaldehyde solution in phosphate buffer (0.1 M, pH 7.4) at 4 °C. The brains were then removed from the skull and sectioned in the transverse plane with a freezing microtome at 40/~m for the squirrel monkeys and 50/~m for the rhesus monkey. Complete series of
Correspondence.. L.-N. Hazrati, Centre de recherche en neurobiologie, Hrpital de I'Enfant-Jrsus, 1401, 18e Rue, Quebec, Qua., Canada G1J 1Z4.
0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
172 brain sections were processed for P H A - L immunohistochemistry as described in detail elsewhere TM, whereas alternate sections were stained with cresyl violet to clearly delineated the various forebrain structures. The sections were examined under both light- and darkfield illumination and the location of PHA-L-positive profiles was mapped out with the help of a camera lucida. In the two squirrel monkeys the P H A - L injections were located in the dorsomedial half of the GPe (Fig. 1A) and gave rise to anterograde fiber labeling in the ipsilateral GPi. Numerous anterogradely labeled fibers were seen to arise from the injection loci and traversed the internal medullary lamina to invade the dorsolateral half of the GPi. The labeled fibers terminating directly within the GPi consisted of short, thin, and non-varicose axons scattered among GPi neurons whose cell bodies displayed a light nonspecific staining (Fig. 1B). When examined at high magnifications, these smooth fibers appeared poorly branched and arborized in a pericellular, basket-like pattern around somata and proximal dendrites of GPi neurons (Fig. 1C-I). Typically, one perikaryon was innervated by a single axon displaying numerous large axonal varicosities reminiscent of terminal boutons (Fig. 1F). Also worth noting was the presence of labeled fibers displaying close pericellular contacts with cell bodies of the substantia nigra pars reticulata after GPe injections (not shown). A multitude of anterogradely labeled fibers were also seen to leave the injection site in the GPe en route to the subthalamic nucleus and the substantia nigra. These fibers, in contrast to those terminating directly wihtin the GPi, did not traverse the GPi but accumulated into the internal medullary lamina and coursed through the ventral portion of the internal capsule to reach the subthalamic nucleus and the substantia nigra. Since the labeled fibers disclosed within the GPi did not seem to leave this structure and were poorly ramified, it was not possible to specify if these fibers were collaterals of those reaching the subthalamic nucleus and/or the substantia nigra. However, collateralization may occur close to the GPe cell bodies, where the analysis was rendered difficult due to the presence of the tracer and the abundance of
labeled neuronal profiles. The use of the retrograde fluorescent double-labeling technique would obviously be useful to determine the degree of axonal collateralization of the GPe neurons. In the same squirrel monkeys, the P H A - L injections on the other side of the brain were confined to the medial portion of the GPi (Fig. 1J). Numerous anterogradely labeled fibers were seen to leave the injection site in the GPi, coursed through the internal medullary lamina to terminate in the dorsomedial half of the GPe. These fibers displayed long intervaricose segments proximally (Fig. 1K), and branched rather frequently distally (Fig. 1L) making contact along their course with several GPe neurons (Fig. 1M). This pattern of terminal arborization suggests that the single GPi axons arborizing within GPe can influence several GPe cells in a rather diffuse manner by comparison to the more prominent, cell-to-cell type of relationship displayed by GPe axons in the GPi. In the rhesus monkey the injection site was small and confined to the central portion of the GPe (Fig. 2A). Anterogradely labeled fibers were seen to leave the injection site and travel in bundles medially into the GPi. Although most axons remained within fiber bundles on their way to the subthalamic nucleus, a number of fibers did split off and appeared to terminate within the lateral portion of the GPi, the medial portion remaining relatively free of fiber labeling. As these bundles crossed through the GPi, some smaller bundles of fibers broke off from the main group and coursed in various directions. Frequently small diameter fibers, which looked like single axons, departed from these smaller bundles (Fig. 2B). As these thin fibers leave the main group they often bifurcated, became varicose, and appeared to contact either cell bodies or dendrites of GPi neurons (Fig. 2C,D). Often several of these thin varicose fibers were closely intertwined and looked as if they were ensheathing a non-stained central core structure (Fig. 2C). The varicose fibers that appeared to wrap a central, unstained core, were similar albeit less densely entwined than those observed with peptide immunohistochemistry in the globus pallidus and referred to as woolly fibers 5. Similar fibers were observed in the globus pallidus after
Fig. 1. Photomicrographs illustrating some features of the pallidal labeling observed in one squirrel monkey after PHA-L injection in the external (A-I) and internal (J-M) segment of the globus pallidus. The maximal extent of the PHA-L injection sites in GPe and in GPi are illustrated in A and J, respectively. B offers a slightly higher magnification of the dorsolateral sector of the GPi where numerous, short, and smooth labeled fibers (open arrows) descending from the injection site can be seen. The filled arrows point to some of the GPi cells or primary dendrites that are illustrated at much higher magnifications in C-I. C-H show examples of axonal varicosities (filled arrows) making close pericellular contacts with cell bodies displaying a weak nonspecific staining. H depicts a smooth and linear GPe fiber (filled arrows) whose terminal portion displays numerous large varicosities closely surrounding one pole of a GPi cell, whereas I shows some thick fibers of the woolly type (filled arrows) near two GPi cell bodies which received pericellular contacts. K-M illustrate some features of the fiber labeling observed after GPe injection (J). K shows a typical labeled fiber as it emerges from the injection site in GPi, whereas L depicts the more distal portion of the same fiber in GPe. M displays some short varicose fibers (open arrows) close to an unstained cell body (filled arrow) in the GPe. GPe, external segment of the globus pallidus; GPi, internal segment of the globus pallidus; IC, internal capsule; PUT, putamen. Bar = 450 ~m (A,J), 20 pm (B), 10/tm (C-H), and 15 pm (I-M).
173 PHA-L injections into the striatum 4. Based on these tracing studies as well as the peptidergic staining, these characteristic profiles likely represent afferent fibers
wrapping the long, thick dendrites of GPi neurons. This suggests a direct input from the GPe to the GPi in the rhesus monkey.
174
*m b
B
C
175
Fig. 2. Photomicrographs illustrating some features of the pallidal labeling observed in one rhesus monkey after PHA-L injection in the external segment of the globus pallidus. A shows the small PHA-L injection site (filled arrow) at its maximal extent in the central portion of the GPe. B depicts small bundles or single smooth fibers (open arrows) as well as some varicose axons (closed arrows) present in the lateral half of the GPi. C shows varicose fibers entwining a central unstained core in the GPi (filled arrows), whereas part of a smooth fiber is visible in the lower portion of the figure (open arrow). D illustrates part of a thin and varicose fiber in the GPi as seen after GPe injection. GPe, external segment of the globus paUidus; GPi, internal segment of the giobus pallidus; IC, internal capsule; PUT, putamen. Bar = 1700/~m (A), 20 #m (B), 11.1/~m (C), and 8.3 #m (D).
The present investigation has thus provided the first evidence for the existence of direct and reciprocal connections between the G P e and the GPi in a primate, namely the squirrel m o n k e y . T h e results o b t a i n e d in the rhesus m o n k e y further support the existence of a projection from the G P e to GPi in primates, but the projection from the GPi to G P e remains to be investigated in the macaque. These findings are in a g r e e m e n t with a recent preliminary r e p o r t in which stimulation of the globus pallidus in the rat (homologue of the primate G P e ) was said to p r o d u c e a powerful inhibition of neurons of the e n t o p e d u n c u l a r nucleus (homologue of the p r i m a t e GPi) and of the substantia nigra 7. In the same r e p o r t fibers of the globus pallidus anterogradely l a b e l e d with P H A - L were described as forming large symmetrical synapses with s o m a t a and primary dendrites of e n t o p e d u n c u l a r and nigral neurons 7 (see also ref. 10). Since virtually all pallidal neurons display y-aminobutyric acid ( G A B A ) immunoreactivity in primates 12, it is very
1 DeVito, J.L. and Anderson, M.E., An autoradiographic study of efferent connections of the globus pallidus in Macaca mulana, Exp. Brain Res,, 46 (1982) 107-117. 2 Emmers, E. and Akert, K., A Stereotaxic Atlas of the Brain of the Squirrel Monkey (Saimiri sciureus), The University of Wisconsin Press, Madison, 1963. 3 Gim6nez-Amaya, J.M. and Graybiel, A.M., Compartmental origins of the striatopallidal projection in the primate, Neuroscience, 34 (1990) 111-I26. 4 Haber, S.N., Lynd, E., Klein, C. and Groenewegen, H.J., Topographic organization of the ventral striatal efferent projections in the rhesus monkey: an anterograde tracing study, J. Comp. NeuroL, 293 (1990) 282-288. 5 Haber, S.N. and Nauta, W.J.H., Ramification of the globus pallidus in the cat as demonstrated by patterns of immunohistochemistry, Neuroscience, 9 (1983) 245-260. 6 Kim, R., Nakano, K., Jayaraman, A. and Carpenter, M.B., Projections of the globus pallidus and adjacent structures: an autoradiographic study in the monkey, J. Comp. Neurol., 169 (1976) 263-290.
likely that the G P e / G P i interconnection described above is a reciprocal inhibitory G A B A e r g i c system. A comparison of the patterns of innervation o b s e r v e d in the G P e and the GPi of the squirrel m o n k e y suggests that G P e neurons may exert a strong inhibitory action u p o n GPi cells, whereas the GPi to G P e p r o j e c t i o n may act as a m o r e diffuse f e e d b a c k inhibitory mechanism. Electron microscopic studies are obviously n e e d e d in primates to reveal m o r e details about the types of contacts m a d e by this short pallidopallidal system, which could play an i m p o r t a n t role in the functional organization of the basal ganglia.
The authors thank Carole Harvey and Lisette Bertrand for technical assistance and Suzanne Bilodeau for typing the manuscript. This research was supported by Grant MT-5781 of the Medical Research Council (MRC) of Canada to A.P. and by NIMH Grant 50MH40381 to S.N.H. The financial support of the FRSQ and FCAR is also acknowledged. L.-N.H. was the recipient of a Studentship from the FCAR.
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