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A projection to the striatum from the medial subdivision of the posterior group of the thalamus in the cat
Brain Research, 300 (1984)351-356 Elsevier
351
BRE 20189
Short Communications
A projection to the striatum from the medial subdivision of the posterior group of the thalamus in the cat ROBERT M. BECKSTEAD
Department of Anatomy, Universityof Virginia, School of Medicine, Charlottesville, VA 22908 (U.S.A.) (Accepted January 17th, 1984)
Key words: posterior thalamic group - - caudate nucleus- - putamen - - wheat germ agglutinin- horseradish peroxidase --autoradiography
Injections of wheat germ agglutinin-conjugatedhorseradish peroxidase in the lateral part of the caudate nucleus or the putamen of the cat result in retrograde thalamic cell-labeling in the rostral extension of the medial subdivision of the posterior group (POM). Autoradiography after [3H]aminoacid injection of POM reveals a dense and discontinuous distribution of axons in the lateral half of the caudate and putamen concentrated at their middle rostrocaudal levels. This newlydiscovered thalamostriatal projection of POM may account for somatosensoryactivityobserved in striatal cells. The telencephalic projections of the posterior group of thalamic nuclei have proven difficult to study because of their deep-seated position and obscure borders. For this reason, the efferent connections of the posterior group are not well established. The evidence to date, mainly from the cat brain, suggests that more caudal parts project to cortical association areas between the somatic and auditory sensory areaslS-t7,20,37. Recently, however, as part of a study of thalamostriatal projections in the cat, a newly discovered projection was reported from the medial subdivision of the posterior group (POM) to the striatum 1. Because POM has been implicated in the processing of ascending somatosensory information in genera110,12,28,38 and possibly nociception in particularS,28, it may represent an important thalamic relay of such sensory input to the striatum. It is valuable, therefore, to confirm the existence of this projection and to examine the areal distribution of POM axons in the caudate nucleus and putamen. Observations were made in a total of 12 cats in which small deposits of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) were
placed in various small zones of the caudate nucleus and putamen, or deposits of tritiated leucine and proline were placed in various thalamic nuclei. All tracer injections were made stereotaxically by pressure through a glass micropipette glued to a 0.5/~l Hamilton syringe in cats anesthetized with 1% halothane. For W G A - H R P cases, the cats survived 2 days and were killed by transcardial perfusion with sequentially, 0.1 M phosphate buffer (pH 7.2) containing 0.5% paraformaldehyde (briefly), the same buffer with 3% glutaraldehyde and 5% sucrose (3000 ml over 2 h), and finally cold (4 °C) buffer containing 10% sucrose (3000 ml over 20 min). The brains were removed, blocked, stored in 20% sucrose overnight, and cut frozen at 40 g m thickness. Every third section was collected on a gelatin-coated slide and processed according to a modification2 of the Mesulam tetramethylbenzidine method 25. For autoradiography, the tracer was a one-to-one mixture of [3H]leucine and proline (New England Nuclear) in saline (40 gCi/gl). After 7 days survival, the cats were perfused with 10% formalin and processed for autoradiography9 using Kodak NTB-3 emulsion and a 16-week expo-
Correspondence: R. M. Beckstead, Dept. Anatomy, Box 439, Univ. of Virginia Medical School, Charlottesville, VA 22908, U.S.A. 0006-8993/84/$03.00 © 1984Elsevier Science Publishers B.V.
352
Fig. 1. An example of a WGA-HRP deposit in the lateral part of the body of the caudate nucleus is shown in A. In rostrocaudal order, parts B to D show the location of HRP-positive neurons (black dots) charted onto photographs of selected frontal sections through
353 sure time. The present delineation of POM is taken from the description of Rinvik 29 and Berman and Jones 3. As expected from the results of earlier studies, small, localized deposits of W G A - H R P in the caudate nucleus and putamen label thalamic cells in all of the intralaminar nuclei, in some midline nuclei, and in some nuclei of the lateral thalamic massl, 35,36. Neurons are labeled in POM, however, only in cases where the tracer deposits encroach upon lateral portions of the caudate nucleus (Fig. 1D) or middle levels of the putamen. The labeled cells in POM are located predominantly in the rostral extension of the nucleus where it is situated between the ventrobasal complex ventrally and lateral posterior complex dorsally (Fig. 1A-C, E). In all cases in which POM cells are labeled, there are also several HRP-positive cells in the medially adjacent centromedian nucleus (CM). The cells in POM usually are separated from those in CM by a thin region free of cell-labeling (Fig. 1B, C). In no case of striatal W G A - H R P deposit are any labeled cells present in the suprageniculate nucleus caudally adjacent to POM or in any other nucleus of the posterior group. In order to confirm the existence of a POM-striatal projection and clarify its intrastriatal axonal distribution, a [3H]proline-leucine deposit was placed in POM, centered in its rostral extension where the retrogradely labeled cells had been most commonly concentrated (Fig. 2). The isotope deposit encroaches somewhat on cells in the ventral part of the lateral posterior nucleus (LP), but is well removed from the striatal-projecting cells of CM and the central lateral nucleus. Thus, the axon-labeling in the striatum can be confidently attributed to the cells of POM that have incorporated the [3H]amino acids. Labeled axons leave the injection site rostrolaterally to enter the internal capsule from which several pass dorsomedially into the caudate nucleus and ventrally into the putamen. In the caudate, the labeled fibers distribute densely in the lateral part of the head and body. Rostrally,
Fig. 2. An autoradiogram of a frontal section through the thalamus showing the central part of a deposit of [3H]proline-leucine in the thalamus (6.3 x). this distribution is sparse and even, but at the levels of the caudal head and rostral body of the caudate, the distribution is at its densest and discontinuous (Fig. 3A, B). This discontinuity can be appreciated in the photograph of Fig. 3A which clearly shows variably sized and irregularly shaped zones of high grain density interspersed between equally irregular zones of negligible grain density. The caudate labeling gradually diminishes at progressively further caudal levels (Fig. 3B), and none is present in the tail of the caudate. In the putamen, the labeled axons distribute most densely at middle rostrocaudal levels. At the rostral end of this distribution, the labeled fibers are most abundant dorsally in the putamen (Fig. 3C). At progressively more caudal levels, the labeled axons distribute throughout the dorsoventral extent of the putamen, but are restricted to only the lateral half of the nucleus along its border with the external capsule
posterior levels of the thalamus. Some HRP-labeled cells in POM are shown in the dark-field photomicrograph (125 x) of part E which corresponds to the area indicated by the rectangle in C. The large blood vessel (arrow) in C can be seen at the bottom of E. Abbreviations: CD, caudate nucleus; CL, central lateral nucleus; Cla, claustrum; CM, centromedian nucleus; FR, retroflex bundle; GP, globus pallidus; H, habenular complex; IC, internal capsule; LG, lateral geniculate nucleus; LP, lateral posterior complex; MD, mediodorsal nucleus; POL, lateral division of the posterior group; POM, medial division of the posterior group; PU, putamen; ST, stria terminalis; VP, ventral posterior complex.
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Fig. 3. Dark-field photomicrographs of representative autoradiograms of frontal sections through (A) the caudal head of, and (B) the body of the caudate nucleus, and (C and D) the middle rostrocaudal levels of the putamen (10 x ). Note the irregular discontinuities of the silver grains especially in A.
355 (Fig. 3D). The most rostral and caudal ends of the putamen are devoid of significant axon-labeling. The only other axon-labeling to result from this [3H]amino acid deposit in POM occurs in the cerebral cortex in a pattern consistent with earlier descriptions of thalamocortical projections of the posterior grouptS,17. No projection was observed from POM to the amygdaloid complex, although such a projection has been reported for other parts of the posterior group 15. Isotope deposits in LP immediately dorsal to POM and those in more caudal subnuclei of the posterior group (suprageniculate and limitans nuclei) do not produce axon-labeling in any part of the caudate nucleus or putamen. The term 'posterior group' was first applied to the cat thalamus by Rose and Woolsey 34 and since has been widely accepted (see ref. 3 for review). The first detailed description of this region in the cat is that of Poggio and Mountcastle 28, who included a ventral part of LP with the posterior group. Although earlier physiological studies showed that the posterior group as a whole is polysensoryT,35, 38, it has since been resolved into more modality specific subnuclei. Thus, a lateral subdivision (POL) has been identified on the basis of ascending auditory input from the inferior colliculus 27 and descending input from auditory areas of the cerebral cortex 14. The remaining medial part, POM, is more clearly associated with the somatosensory system. It receives ascending axons of the spinothalamic tract6A9,23,25, and via the medial lemniscus, from the dorsal column nucleiS, 19 and the lateral cervical nucleus4, 24. Perhaps expectedly, POM also appears to be reciprocally connected with the somatosensory areas (SI, SII, Sill) of the cortex 13,17,21, 25,30-32,37.
POM corresponds to what earlier authors had referred to as the ventral portion of LP (ref. 33) or as a transition zone between the ventrobasal nucleus and CM 18. It is important to point out that although there is a clear-cut boundary between POM and CM, the dorsal border of POM with LP is not distinct. While the present observation that POM projects to the striatum sets it apart from the remainder of the posterior group, this criterion does not necessarily dis1 Beckstead, R. M., The thalamostriatal projection in the cat, J. comp. Neurol., in press.
2 Beckstead, R. M. and Frankfurter, A., A direct projection from the retina to the intermediate gray layer of the superi-
tinguish POM from LP, since it has been shown that cells in the dorsal part of LP also contribute to the thalamostriatal projection1,35, 36. However, the axons from LP appear to project to a more medial territory of the head of the caudate nucleus and not at all to the putamen, whereas the POM axons distribute exclusively in lateral and more caudal regions of both the caudate and putamen. Furthermore, the cells in LP labeled by intrastriatal enzyme deposits are invariably located dorsally in the nucleus, rostral to and remote from the border zone with POM. The separation of POM from the more dorsal parts of LP seems justified on the basis of its distinct afferent fiber connections as well: LP does not share in either the ascending or cortical somatosensory associations of POM. That the posterior group of nuclei is particularly involved with nociception is a long-standing notion that has been received favorablyS,2s. It is now clear, however, that the individual components of the posterior group may differ markedly from one another in their major sensorial associations. Curry and his coworkers 10-t2 have challenged the notion that POM is a 'pain centre' and have shown further that POM is dominated by somesthetic impulses. Neurons of the striatum are well-known to be responsive to sensory stimulation, including somesthetic impulses (for review see ref. 22). There is little evidence that the sensory responses of striatal cells reflect the projections from the cerebral cortex. It seems likely, therefore, that the thalamostriatal projection, at least in part, may be responsible for the sensory responsiveness observed in striatal cells. In particular, the present data suggest that POM represents a major provenance of somatosensory input to special territories of the caudate nucleus and putamen. Elucidation of the effects of this projection on the operational fabric of the striatum awaits exploration. This work was supported by N I H Grant NS17827. I thank K. Kersey for technical assistance and M. Staton for typing.
or colliculus demonstrated by anterograde transport of horseradish peroxidase in monkey, cat and rat, Exp. Brain. Res., 52 (1983) 261-268. 3 Berman, A. L. and Jones, E. G., The Thalamus and Basal
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Telencephalon of the Cat. A Cytoarchitectonic Atlas with Stereotaxic Coordinates, Univ. of Wisconsin Press, Madison, 1982. Boivie, J., The termination of the cervicothalamic tract in the cat: an experimental study with silver impregnation methods, Brain Research, 19 (1970) 333-360. Boivie, J., The termination in the thalamus and the zona incerta of fibers from the dorsal column nuclei (DCN) in the cat: an experimental study with silver impregnation methods, Brain Research, 28 (1971) 459-490. Boivie, J., The termination of the spinothalamic tract in the cat: an experimental study with silver impregnation methods, Exp. Brain. Res., 12 (1971) 331-353. Calma, I., Observation on the activity of the posterior group of nuclei of the thalamus, J. Physiol. (Lond.), 172 (1964) 47. Casey, K. L., Unit analysis of nociceptive mechanisms in the thalamus of the awake squirrel monkey, J. Neurophysiol., 29 (1966) 727-750. Cowan, W. M., Gottlieb, D. I., Hendrickson, A. E., Price, J. L. and Woolsey, T. A., The autoradiographic demonstration of axonal connections in the central nervous system, Brain Research, 37 (1972) 21-51. Curry, M. J., The extoreceptive properties of neurons in the somatic part of the posterior group (PO), Brain Research, 44 (1972) 439-462. Curry, M. J., The effects of stimulating the somatic sensory cortex on single neurons in the posterior group (PO) of the cat, Brain Research, 44 (1972) 463~,81. Curry, M. J. and Gordon, G., The spinal input to the posterior group in the cat: an electrophysiological investigation, Brain Research, 44 (1972) 417-437. DeVito, J. L., Thalamic projection of the anterior ectosylvian gyrus (somatic area II) in the cat, J. comp. Neurol., 131 (1967) 67-78. Diamond, I. T., Jones, E. G. and Powell, T. P. S., The projection of the auditory cortex upon the diencephalon and brain stem in the cat, Brain Research, 15 (1969) 305-340. Graybiel, A. M., The thalamo-cortical projection of the socalled posterior nuclear group: a study with anterograde degeneration methods in the cat, Brain Research, 49 (1973) 22%244. Heath, C. J., Distribution of axonal degeneration following lesions of the posterior group of thalamic nuclei in the cat, Brain Research, 21 (1970) 354-357. Heath, C. J. and Jones, E. G., An experimental study of ascending connections from the posterior group of thalamic nuclei in the cat, J. cornp. Neurol., 141 (1971) 397-426. Ingram, W. R., Hannett, F. I. and Ranson, S. W., The topography of the nuclei of the diencephalon of the cat, J. comp. Neurol., 55 (1932) 333-394. Jones, E. G. and Burton, H., Cytoarchitecture and somatic sensory connectivity of thalamic nuclei other than the ventrobasal complex in the cat, J. comp. Neurol., 154 (1974) 395-432. Jones, E. G. and Leavitt, R. Y., Retrograde axonal transport and the demonstration of non-specific projections to the cerebral cortex and striatum from tbalamic intralaminar nuclei in the rat, cat and monkey, J. comp. Neurol., 154 (1974) 34%378. Jones, E. G. and Powell, T. P. S., The projection of the somatic sensory cortex upon the thalamus in the cat, Brain Research, 10 (1968) 36%391.
22 Krauthamer, G. M.. Sensory functions of the neostriatum. In I. Divac and R. G. E. Oberg (Eds.), The Neostriatum, Pergamon, New York, 1979, pp. 263-289. 23 Mantyh, P. W., The terminations of the spinothalamic tract in the cat, Neurosci. Lett., 38 (1983) 11%124. 24 Mash, D. C. and Berkley, K. J., Projections from the spinal cord and the gracile, cuneate, trigeminal and lateral cervical nuclei to the posterior group of nuclei in the thalamus of the cat, Neurosci. Abstr., 3 (1977) 486. 25 Mehler, W. R., Some observations on secondary ascending afferent systems in the central nervous system. In R. S. Knighton and P. R. Dumke (Eds.), Pain, Little and Brown, Boston, 1966, pp. 11-32. 26 Mesulam, M.-M., Tetramethyl benzidine for horseradish peroxidase neurohistochemistry: a non-carcinogenic blue reaction-product with superior sensitivity for visualizing neural afferents and efferents, J. Histochem. Cytochem., 26 (1978) 106-117. 27 Moore, R. Y. and Goldberg, J. M., Ascending projections of the inferior colliculus in the cat, J. comp. Neurol., 121 (1963) 109-135. 28 Poggio, G. F. and Mountcastle, V. B., A study of the functional contributions of the lemniscal and spinothalamic systems to somatic sensibility: central nervous mechanisms in pain, Bull. Johns Hopk. Hosp., 106 (1960) 226-316. 29 Rinvik, E., A re-evaluation of the cytoarchitecture of the ventral nuclear complex of the cat's thalamus on the basis of corticothalamic connections, Brain Research, 8 (1968) 237-254. 30 Rinvik, E., The corticothalamic projection from the gyrus proreus and the medial wall of the rostral hemisphere in the cat: an experimental study with silver impregnation methods, Exp. Brain. Res., 5 (1968) 12%152. 31 Rinvik, E., The corticothalamic projection from the pericruciate and coronal gyri in the cat: an experimental study with silver-impregnation methods, Brain Research, 10 (1968) 7%119. 32 Rinvik, E., The corticothalamic projection from the second somatosensory cortical area in the cat: an experimental study with silver impregnation methods, Exp. Brain. Res., 5 (1968) 153-172. 33 Rioch, D. McK., Studies on the diencephalon of carnivora: Part I. The nuclear configuration of the thalamus, epithalamus, and hypothalamus of the dog and cat, J. comp. Neurol., 49 (1929) 1-119.. 34 Rose, J. E. and Woolsey, C. N., Cortical connections and functional organization of the thalamic auditory system of the cat. In H. F. Harlow and C. N. Woolsey (Eds.), Biological and Biochemical Bases of Behavior, Univ. of Wisconsin Press, Madison, 1958, pp. 127-150. 35 Royce, G. J., Cells of origin of subcortical afferents to the caudate nucleus: a horseradish peroxidase study in the cat, Brain Research, 153 (1978) 465-475. 36 Sato, M., Itoh, K. and Mizuno, N., Distribution of thalamocaudate neurons in the cat as demonstrated by horseradish peroxidase, Exp. Brain. Res., 34 (1979) 143-153. 37 Tanji, D. G., Wise, S. P., Dykes, R. W. and Jones, E. G., Cytoarchitecture and thalamic connectivity of the third somatic area of the cat cerebral cortex, J. Neurophysiol., 41 (1977) 268-284. 38 Whitlock, D. G. and Perl, E. R., Afferent projections through ventrolateral funiculi to thalamus of cat, J. Neurophysiol., 22 (1959) 133-148.
351
BRE 20189
Short Communications
A projection to the striatum from the medial subdivision of the posterior group of the thalamus in the cat ROBERT M. BECKSTEAD
Department of Anatomy, Universityof Virginia, School of Medicine, Charlottesville, VA 22908 (U.S.A.) (Accepted January 17th, 1984)
Key words: posterior thalamic group - - caudate nucleus- - putamen - - wheat germ agglutinin- horseradish peroxidase --autoradiography
Injections of wheat germ agglutinin-conjugatedhorseradish peroxidase in the lateral part of the caudate nucleus or the putamen of the cat result in retrograde thalamic cell-labeling in the rostral extension of the medial subdivision of the posterior group (POM). Autoradiography after [3H]aminoacid injection of POM reveals a dense and discontinuous distribution of axons in the lateral half of the caudate and putamen concentrated at their middle rostrocaudal levels. This newlydiscovered thalamostriatal projection of POM may account for somatosensoryactivityobserved in striatal cells. The telencephalic projections of the posterior group of thalamic nuclei have proven difficult to study because of their deep-seated position and obscure borders. For this reason, the efferent connections of the posterior group are not well established. The evidence to date, mainly from the cat brain, suggests that more caudal parts project to cortical association areas between the somatic and auditory sensory areaslS-t7,20,37. Recently, however, as part of a study of thalamostriatal projections in the cat, a newly discovered projection was reported from the medial subdivision of the posterior group (POM) to the striatum 1. Because POM has been implicated in the processing of ascending somatosensory information in genera110,12,28,38 and possibly nociception in particularS,28, it may represent an important thalamic relay of such sensory input to the striatum. It is valuable, therefore, to confirm the existence of this projection and to examine the areal distribution of POM axons in the caudate nucleus and putamen. Observations were made in a total of 12 cats in which small deposits of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) were
placed in various small zones of the caudate nucleus and putamen, or deposits of tritiated leucine and proline were placed in various thalamic nuclei. All tracer injections were made stereotaxically by pressure through a glass micropipette glued to a 0.5/~l Hamilton syringe in cats anesthetized with 1% halothane. For W G A - H R P cases, the cats survived 2 days and were killed by transcardial perfusion with sequentially, 0.1 M phosphate buffer (pH 7.2) containing 0.5% paraformaldehyde (briefly), the same buffer with 3% glutaraldehyde and 5% sucrose (3000 ml over 2 h), and finally cold (4 °C) buffer containing 10% sucrose (3000 ml over 20 min). The brains were removed, blocked, stored in 20% sucrose overnight, and cut frozen at 40 g m thickness. Every third section was collected on a gelatin-coated slide and processed according to a modification2 of the Mesulam tetramethylbenzidine method 25. For autoradiography, the tracer was a one-to-one mixture of [3H]leucine and proline (New England Nuclear) in saline (40 gCi/gl). After 7 days survival, the cats were perfused with 10% formalin and processed for autoradiography9 using Kodak NTB-3 emulsion and a 16-week expo-
Correspondence: R. M. Beckstead, Dept. Anatomy, Box 439, Univ. of Virginia Medical School, Charlottesville, VA 22908, U.S.A. 0006-8993/84/$03.00 © 1984Elsevier Science Publishers B.V.
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Fig. 1. An example of a WGA-HRP deposit in the lateral part of the body of the caudate nucleus is shown in A. In rostrocaudal order, parts B to D show the location of HRP-positive neurons (black dots) charted onto photographs of selected frontal sections through
353 sure time. The present delineation of POM is taken from the description of Rinvik 29 and Berman and Jones 3. As expected from the results of earlier studies, small, localized deposits of W G A - H R P in the caudate nucleus and putamen label thalamic cells in all of the intralaminar nuclei, in some midline nuclei, and in some nuclei of the lateral thalamic massl, 35,36. Neurons are labeled in POM, however, only in cases where the tracer deposits encroach upon lateral portions of the caudate nucleus (Fig. 1D) or middle levels of the putamen. The labeled cells in POM are located predominantly in the rostral extension of the nucleus where it is situated between the ventrobasal complex ventrally and lateral posterior complex dorsally (Fig. 1A-C, E). In all cases in which POM cells are labeled, there are also several HRP-positive cells in the medially adjacent centromedian nucleus (CM). The cells in POM usually are separated from those in CM by a thin region free of cell-labeling (Fig. 1B, C). In no case of striatal W G A - H R P deposit are any labeled cells present in the suprageniculate nucleus caudally adjacent to POM or in any other nucleus of the posterior group. In order to confirm the existence of a POM-striatal projection and clarify its intrastriatal axonal distribution, a [3H]proline-leucine deposit was placed in POM, centered in its rostral extension where the retrogradely labeled cells had been most commonly concentrated (Fig. 2). The isotope deposit encroaches somewhat on cells in the ventral part of the lateral posterior nucleus (LP), but is well removed from the striatal-projecting cells of CM and the central lateral nucleus. Thus, the axon-labeling in the striatum can be confidently attributed to the cells of POM that have incorporated the [3H]amino acids. Labeled axons leave the injection site rostrolaterally to enter the internal capsule from which several pass dorsomedially into the caudate nucleus and ventrally into the putamen. In the caudate, the labeled fibers distribute densely in the lateral part of the head and body. Rostrally,
Fig. 2. An autoradiogram of a frontal section through the thalamus showing the central part of a deposit of [3H]proline-leucine in the thalamus (6.3 x). this distribution is sparse and even, but at the levels of the caudal head and rostral body of the caudate, the distribution is at its densest and discontinuous (Fig. 3A, B). This discontinuity can be appreciated in the photograph of Fig. 3A which clearly shows variably sized and irregularly shaped zones of high grain density interspersed between equally irregular zones of negligible grain density. The caudate labeling gradually diminishes at progressively further caudal levels (Fig. 3B), and none is present in the tail of the caudate. In the putamen, the labeled axons distribute most densely at middle rostrocaudal levels. At the rostral end of this distribution, the labeled fibers are most abundant dorsally in the putamen (Fig. 3C). At progressively more caudal levels, the labeled axons distribute throughout the dorsoventral extent of the putamen, but are restricted to only the lateral half of the nucleus along its border with the external capsule
posterior levels of the thalamus. Some HRP-labeled cells in POM are shown in the dark-field photomicrograph (125 x) of part E which corresponds to the area indicated by the rectangle in C. The large blood vessel (arrow) in C can be seen at the bottom of E. Abbreviations: CD, caudate nucleus; CL, central lateral nucleus; Cla, claustrum; CM, centromedian nucleus; FR, retroflex bundle; GP, globus pallidus; H, habenular complex; IC, internal capsule; LG, lateral geniculate nucleus; LP, lateral posterior complex; MD, mediodorsal nucleus; POL, lateral division of the posterior group; POM, medial division of the posterior group; PU, putamen; ST, stria terminalis; VP, ventral posterior complex.
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Fig. 3. Dark-field photomicrographs of representative autoradiograms of frontal sections through (A) the caudal head of, and (B) the body of the caudate nucleus, and (C and D) the middle rostrocaudal levels of the putamen (10 x ). Note the irregular discontinuities of the silver grains especially in A.
355 (Fig. 3D). The most rostral and caudal ends of the putamen are devoid of significant axon-labeling. The only other axon-labeling to result from this [3H]amino acid deposit in POM occurs in the cerebral cortex in a pattern consistent with earlier descriptions of thalamocortical projections of the posterior grouptS,17. No projection was observed from POM to the amygdaloid complex, although such a projection has been reported for other parts of the posterior group 15. Isotope deposits in LP immediately dorsal to POM and those in more caudal subnuclei of the posterior group (suprageniculate and limitans nuclei) do not produce axon-labeling in any part of the caudate nucleus or putamen. The term 'posterior group' was first applied to the cat thalamus by Rose and Woolsey 34 and since has been widely accepted (see ref. 3 for review). The first detailed description of this region in the cat is that of Poggio and Mountcastle 28, who included a ventral part of LP with the posterior group. Although earlier physiological studies showed that the posterior group as a whole is polysensoryT,35, 38, it has since been resolved into more modality specific subnuclei. Thus, a lateral subdivision (POL) has been identified on the basis of ascending auditory input from the inferior colliculus 27 and descending input from auditory areas of the cerebral cortex 14. The remaining medial part, POM, is more clearly associated with the somatosensory system. It receives ascending axons of the spinothalamic tract6A9,23,25, and via the medial lemniscus, from the dorsal column nucleiS, 19 and the lateral cervical nucleus4, 24. Perhaps expectedly, POM also appears to be reciprocally connected with the somatosensory areas (SI, SII, Sill) of the cortex 13,17,21, 25,30-32,37.
POM corresponds to what earlier authors had referred to as the ventral portion of LP (ref. 33) or as a transition zone between the ventrobasal nucleus and CM 18. It is important to point out that although there is a clear-cut boundary between POM and CM, the dorsal border of POM with LP is not distinct. While the present observation that POM projects to the striatum sets it apart from the remainder of the posterior group, this criterion does not necessarily dis1 Beckstead, R. M., The thalamostriatal projection in the cat, J. comp. Neurol., in press.
2 Beckstead, R. M. and Frankfurter, A., A direct projection from the retina to the intermediate gray layer of the superi-
tinguish POM from LP, since it has been shown that cells in the dorsal part of LP also contribute to the thalamostriatal projection1,35, 36. However, the axons from LP appear to project to a more medial territory of the head of the caudate nucleus and not at all to the putamen, whereas the POM axons distribute exclusively in lateral and more caudal regions of both the caudate and putamen. Furthermore, the cells in LP labeled by intrastriatal enzyme deposits are invariably located dorsally in the nucleus, rostral to and remote from the border zone with POM. The separation of POM from the more dorsal parts of LP seems justified on the basis of its distinct afferent fiber connections as well: LP does not share in either the ascending or cortical somatosensory associations of POM. That the posterior group of nuclei is particularly involved with nociception is a long-standing notion that has been received favorablyS,2s. It is now clear, however, that the individual components of the posterior group may differ markedly from one another in their major sensorial associations. Curry and his coworkers 10-t2 have challenged the notion that POM is a 'pain centre' and have shown further that POM is dominated by somesthetic impulses. Neurons of the striatum are well-known to be responsive to sensory stimulation, including somesthetic impulses (for review see ref. 22). There is little evidence that the sensory responses of striatal cells reflect the projections from the cerebral cortex. It seems likely, therefore, that the thalamostriatal projection, at least in part, may be responsible for the sensory responsiveness observed in striatal cells. In particular, the present data suggest that POM represents a major provenance of somatosensory input to special territories of the caudate nucleus and putamen. Elucidation of the effects of this projection on the operational fabric of the striatum awaits exploration. This work was supported by N I H Grant NS17827. I thank K. Kersey for technical assistance and M. Staton for typing.
or colliculus demonstrated by anterograde transport of horseradish peroxidase in monkey, cat and rat, Exp. Brain. Res., 52 (1983) 261-268. 3 Berman, A. L. and Jones, E. G., The Thalamus and Basal
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