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Metabolic and structural correlates of the vibrissae representation in the thalamus of the adult rat
Neuroscience Letters, 60 (1985) 319-324
319
Elsevier Scientific Publishers Ireland Ltd.
NSL 03554
METABOLIC AND STRUCTURAL CORRELATES OF THE VIBRISSAE REPRESENTATION IN THE THALAMUS OF THE ADULT RAT
P.W. LAND* and D.J. SIMONS
Center for Neuroscience and Departments of Anatomy and Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261 ( U.S.A. J (Received April 16th, 1985; Revised version received July 1st, 1985; Accepted July 2nd, 1985)
Key words." barreloid - adult rat - cytochrome oxidase - thalamus
Cytochrome oxidase (CO) histochemistry was used to examine patterns of metabolic activity in the ventral posteromedial nucleus of the adult rat thalamus. In sections cut in an oblique horizontal plane, CO staining reveals distinct patches of heightened activity arranged in a fashion remniscent of the pattern o f vibrissae on the contralateral face and which corresponds to the known somatotopic organization of the nucleus. The CO-reactive zones coincide with oval cylinders of thalamic neurons that appear to be anatomically linked with corresponding barrels in the contralateral somatosensory cortex.
In adult rats and mice there is a precise one-to-one correspondence between the mystacial vibrissae and consistent, readily identifiable clusters of neurons, called barrels, in layer IV of the contralateral somatosensory cortex (SmI) [14, 16]. The cortical barrels are organized in a pattern of rows and arcs that is isomorphic to the distribution of vibrissae on the face. Similarly arranged groupings of cells and/or afferent fibers have been observed in the brainstem trigeminal nuclei of both species [2, 5] and in the ventral posteromedial (VPM) thalamic nucleus of neonatal and adult mice [11, 17]. These latter structures have been termed 'barreloids' [11]. Using a histochemical stain for the mitochondrial enzyme succinic dehydrogenase (SDH) Belford and Killackey [2] showed that VPM of 4- and 6-day-old rats contains patches of dense reactivity that are arranged in a fashion reminiscent of the vibrissae pattern on the face. Ivy and Killackey [3] described a comparable segmentation of retrogradely labeled thalamic neurons following large horseradish peroxidase (HRP) injections into SmI of rats between 3 and 10 days postnatal. These investigators reported that both the SDH- and the HRP-revealed patterns become obscure by postnatal day 13-15 and are not visible in adult animals. In both studies, however, specimens were examined only in standard section planes. We recently have shown that the histochemical reaction for another mitochondrial enzyme, cytochrome oxidase (CO) is especially useful in revealing patterns of neuro*Author for correspondence and reprint requests at: Department of Anatomy & Cell Biology, University of Pittsburgh, School of Medicine, Pittsburgh, PA 15261, U.S.A. 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
321)
.6.
Fig. 1. Pattern of cytochrome oxidase (CO) activity in the rat ventrobasal thalamus. A and B show, respectively, a CO-reacted 80-#m section and a camera lucida reconstruction of two adjacent sections through the ventral posteromedial nucleus (VPM) of an adult rat. The dorsolateral half of VPM contains a curvilinear array of dense CO-reactive patches. As can be seen more clearly in the reconstruction in B, thesc patches form a pattern which closely resembles the spatial organization of the mystacial vibrissae on the contralateral face pad. Proceeding dorsal to ventral on the face whisker rows are labeled A through E. Within each row whiskers are denoted 1 through 4-7, proceeding caudal to rostral; barreloids associated with lower order arcs are closest to the external medullary lamina, i.e. location of letters A-E. Orientation: dorsotateral, up; rostral, left. LGNd, dorsal lateral geniculate nucleus; PO, thalamic posterior complex: Ret., reticular nucleus; VPL, ventral posterolateral nucleus; so, barreloid corresponding to orbital sinus hair. Bar in B = 3 0 0 # m for A and B. C and D show barreloids C1~C3 from A at higher magnification, In C the thionine counterstain has been suppressed by the use of a dark blue filter, and the CO-reactive zones are seen separated by narrow, unreactive fiber plexuses. D shows the corresponding Nissl pattern. Note that each CO-positive zone contains a ring of thalamic neurons forming its outer margin and surrounding a relatively cell-sparse hollow. B a r = 100/~m.
321
nal organization within the SmI barrel cortex that are not readily apparent in Nisslstained material [4]. In the present study we examined CO stained specimens of the adult rat thalamus to determine whether a morphologic correlate of the vibrissae representation could be visualized there as in the cortex. Sixteen adult rats (42-98 days of age) were perfused transcardially with 2~o paraformaldehyde-l.5~o glutaraldehyde in 0.1 M phosphate buffer, pH 7.3, and sections through their thalami were reacted for CO according to the method of Wong-Riley [15], mounted onto subbed slides and counterstained with 0.1~o thionine. Although several planes of section were used, we Ibund that the vibrissae representation was most clearly revealed in oblique horizontal sections which approached the VPM from its dorsomedial surface (see also ref. 17). Fig. 1A, B shows, respectively, a CO-reacted 80-#m section and a camera lucida reconstruction of two adjacent sections through VPM from a specimen prepared in this manner. VPM is notable for its dense reactivity relative to other thalamic nuclei. CO staining within the nucleus is not uniform, however, but is segregated into a series of oval patches of heightened activity that are separated from one another by narrow zones where the activity is considerably lower. The latter appear to consist of fiber plexuses as seen in myelin stains. The regions of heightened activity in VPM are spatially distributed in a pattern that resembles the arrangement of the mystacial vibrissae on the contralateral face and that corresponds to the physiologically defined somatotopic organization of the nucleus [12, 13]. Thus, CO barreloids representing dorsal rows of whiskers (e.g. row A; see Fig. 1) are located caudally in VPM while those for ventral whisker rows (e.g. row E) are located rostrally. Rows of barreloids are oriented dorsolaterally to ventromedially. Barreloids representing the large caudal whiskers (e.g. arc 1) are located near the external medullary lamina. Rows C-E curve to run in a rostrocaudal direction so that higher order arcs within these rows (e.g. arcs 5-10) are represented ventromedial to arc 4 in rows A and B. Fig. 1C, D shows higher magnification photomicrographs of the C 1-C3 barreloids from Fig. IA. In panel C, the thionine stain has been suppressed and the CO staining enhanced by use of a dark blue filter. In panel D both the CO and the corresponding Nissl pattern are shown, and it can be seen that each CO-reactive zone coincides with a ring of thalamic neurons surrounding a relatively cell-sparse center. These rings~of neurons form the walls of oval cylinders (100-250 pm wide) that extend through much of the thickness of VPM. In contrast to the SmI cortical barrels [4], thalamic neurons forming the sides of individual barreloids are generally contained within the CO-positive zone. The CO reactivity of these cell bodies is roughly equivalent to that of the surrounding neuropil and to that of cell bodies within the central aspect of each barreloid. The anatomical relationship between individual barreloids and their corresponding cortical barrels was verified in one animal that received an iontophoretic deposition of H R P into the B1 barrel. Tangential sections through the SmI and oblique horizontal sections through the ipsilateral thalamus were reacted for CO and/or for H R P according to the method of Adams [1]. A section through layer IV of the cortex is shown in Fig. 2A, where it can be seen that the H R P injection site is confined to
322
(3
Fig. 2. Barreloid-barrel relations. A: photomicrograph of a tangential section through layer IV of the SmI cortex reacted for CO and H RP showing the pattern of CO-reactive barrel centers and an HRP deposition in the BI barrel. Rows of barrels are labeled by letters A E placed in arc 1. BI is unlabeled to show the full extent of the HRP deposition. :~-6, barrels associated with vibrissae that straddle the whisker rows at their caudal end. Bar = 300 ~m. B: photomicrograph showing retrogradely labeled neurons in VPM resulting from the HRP deposition shown in A. Two labeled neurons (arrows) are seen in the dorsolateral side of the B1 barreloid. EML, external medullary lamina; ~, 13, barreloids corresponding to vibrissae straddling row B. B a r - 5 0 / ~ m . C: reconstruction of 10 serial 40-,um HRP-reacted sections through the VPM nucleus of the specimen shown in A and B. The locations of the 15 retrogradely labeled thalamic neurons (dots) have been plotted onto the reconstruction. Note that all but 3 of the labeled cells are found within the side of the B1 barreloid closest to the straddler barreloids. Orientation: dorsolateral, up; rostral, left. Bar - 200 l~m.
323 the posterolateral aspect of the B1 barrel (i.e. near its border with ~ and fl). Fig. 2B shows a 40-/~m HRP-reacted section through the VPM ipsilateral to the cortical injection. Note the two H RP-labeled neurons (arrows) in the side of the B l barreloid, the boundaries of which can be discerned by reference to the whorled pattern of neuronal somata. Most labeled neurons appeared to have dendrites that radiated toward the center of their parent barreloid. A total of 15 labeled cells was found in a complete series of sections through the VPM; 12 of these were located in the B 1 barreloid, two were in B2 and one was in ~. The locations of these cells are depicted graphically in Fig. 2C, which is a camera lucida reconstruction made from l0 serial H R P and C O / H R P reacted sections. Interestingly, labeled cells in B l tended to occupy the dorsolateral half of the barreloid (i.e. nearer ct and fl). These results demonstrate the existence of distinct cytoarchitectonic and metabolic subdivisions of the adult rat thalamus that appear to correspond to individual vibrissae on the contralateral face. The spatial organization of the barreloids is consistent with the somatotopic representation of the vibrissae defined physiologically [12, 13] and anatomically [8]. Thalamic barreloids undoubtedly form the anatomical substrate linking barrel-like arrays in the trigeminal nuclear complex with their structural and functional counterparts (i.e. barrels) in the SmI cortex. There is an important difference, however, between the nature of the CO staining described here for the thalamus and that reported previously for barrels in the SmI cortex [4]. In the latter, large highly reactive neuronal somata often were observed within the CO-rich barrel centers; this heterogeneous staining of barrel neurons probably reflects morphological and functional differences among cells in the barrels [9, 10]. By contrast, thalamic neurons as a population are more homogeneous in their CO staining and do not appear more reactive than the surrounding neuropil. This is in accord with recent studies indicating that the rat ventrobasal complex (VB) is composed exclusively of thalamocortical relay cells which are homogeneous with respect to their overall dendritic morphology [6]. Increases in the complexity of the thalamic neuropil has been invoked to account for the apparent obscuring of segmentation within VB during early postnatal development [2, 3]. In our experience, however, the section plane is critically important for visualizing barreloids in adult rats using CO or Nissl stains; this undoubtedly is the case for other histological methods as well. In retrospect, we have found it possible to occasionally identify individual rows or arcs of barreloids in adult brains sectioned transversely or horizontally. Taken together, our findings and those of Ivy and Killackey [3] suggest that the ventrobasal complex undergoes a rotation as the rat matures so that, for example, barreloid A1 shifts dorsally and caudally with respect to the major axes of the brain (see also ref. 7). The present findings underscore the precise anatomical and functional organization of all stations of the vibrissa-bartel system. They also provide a valuable context for detailed anatomical studies of thalamocorticai relations in the rodent somatosensory system. We thank Maria Antonich for typing the manuscript and Melinda Rau for technical assistance. Supported by N I H Grant NS19950.
324 1 Adams, J.C., Heavy metal intensification of DAB-based HRP reaction product, J. Histochem. Cytochem.. 29 (1981) 775. 2 Belford, G.R. and Killackey, H.P., The sensitive period in the development of the trigeminal system of the neonatal rat, J. Comp. Neurol., 193 (1980) 335-350. 3 Ivy, G.O. and Killackey, H.P., Ephemeral cellular segmentation in the thalamus of the neonatal rat, Dev. Brain Res., 2 ( 1981) 1 17. 4 Land, P.W. and Simons, D.J., Cytochrome oxidase staining in the rat SmI barrel cortex, J. Comp. Neurol., 238 (1985) 225 235. 5 Ma, P.M. and Woolsey, T.A., Cytoarchitectonic correlates of the vibrissae in the medullary trigeminat complex of the mouse, Brain Res., 306 (1984)374-379. 6 Peschanksi, M., Lee, C.L. and Ralston, H.J., lII, The structural organization of the ventrobasal complex of the rat as revealed by the analysis of physiologically characterized neurons injected intracellularly with horseradish peroxidase, Brain Res., 297 (1984) 63 74. 7 Rakic, P., Genesis of the dorsal lateral geniculate nucleus in the rhesus monkey: site and time of origin, kinetics of proliferation, routes of migration and pattern of distribution of neurons, J. Comp. Neurol., (1977) 23-52. 8 Sapporta, S. and Kruger, U, The organization of thalamocortical relay neurons in the rat ventrobasal complex studied by the retrograde transport of horseradish peroxidase, J. Comp, Neurol., 174 (1977) 187 208. 9 Simons, D.J., Response properties of vibrissa units in the rat SI somatosensory neocortex, J. Neurophysiol., 41 (1978) 798-820. 10 Simons, D.J., and Woolsey, T.A., Morphology of Golgi-Cox-impregnated barrel neurons in rat Sml cortex, J. Comp. Neurol., 230 (1984) 119-132. 11 Van der Loos, H., Barreloids in the mouse somatosensory thalamus, Neurosci. Lett., 2 (1976) 1 6. 12 Verley, R. and Onnen, 1., Somatotopic organization of the tactile thalamus in normal adult and developing mice and in adult mice dewhiskered since birth, Exp. Neurol., 72 (1981) 462-474. 13 Waite, P.M.E., Somatotopic organization of vibrissal responses in the ventro-basal complex of the rat thalamus, J. Physiol. (Lond.), 228 (1973) 527-540. 14 Welker, C., Microelectrode delineation of the fine grain somatotopic organization of SmI cerebral neocortex in albino rat, Brain Res., 26 ( 1971 ) 259 275. 15 Wong-Riley, M.T.T., Changes in the visual system of monocularly sutured or enucleated cats demonstrable with cytochrome oxidase histochemistry, Brain Res., 171 (1979) 11- 28. 16 Woolsey, T.A. and Van der Loos, H., The structural organization of layer IV in the somatosensory region ($1) of the mouse cerebral cortex, Brain Res., 17 (1970) 205-242. 17 Woolsey, T.A., Anderson, J.R., Wann, J.R. and Stanfield, B.B., Effects of early vibrissae damage on neurons in the ventrobasal (VB) thalamus of the mouse, J. Comp. Neurol., 184 (1979) 363-380.
319
Elsevier Scientific Publishers Ireland Ltd.
NSL 03554
METABOLIC AND STRUCTURAL CORRELATES OF THE VIBRISSAE REPRESENTATION IN THE THALAMUS OF THE ADULT RAT
P.W. LAND* and D.J. SIMONS
Center for Neuroscience and Departments of Anatomy and Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261 ( U.S.A. J (Received April 16th, 1985; Revised version received July 1st, 1985; Accepted July 2nd, 1985)
Key words." barreloid - adult rat - cytochrome oxidase - thalamus
Cytochrome oxidase (CO) histochemistry was used to examine patterns of metabolic activity in the ventral posteromedial nucleus of the adult rat thalamus. In sections cut in an oblique horizontal plane, CO staining reveals distinct patches of heightened activity arranged in a fashion remniscent of the pattern o f vibrissae on the contralateral face and which corresponds to the known somatotopic organization of the nucleus. The CO-reactive zones coincide with oval cylinders of thalamic neurons that appear to be anatomically linked with corresponding barrels in the contralateral somatosensory cortex.
In adult rats and mice there is a precise one-to-one correspondence between the mystacial vibrissae and consistent, readily identifiable clusters of neurons, called barrels, in layer IV of the contralateral somatosensory cortex (SmI) [14, 16]. The cortical barrels are organized in a pattern of rows and arcs that is isomorphic to the distribution of vibrissae on the face. Similarly arranged groupings of cells and/or afferent fibers have been observed in the brainstem trigeminal nuclei of both species [2, 5] and in the ventral posteromedial (VPM) thalamic nucleus of neonatal and adult mice [11, 17]. These latter structures have been termed 'barreloids' [11]. Using a histochemical stain for the mitochondrial enzyme succinic dehydrogenase (SDH) Belford and Killackey [2] showed that VPM of 4- and 6-day-old rats contains patches of dense reactivity that are arranged in a fashion reminiscent of the vibrissae pattern on the face. Ivy and Killackey [3] described a comparable segmentation of retrogradely labeled thalamic neurons following large horseradish peroxidase (HRP) injections into SmI of rats between 3 and 10 days postnatal. These investigators reported that both the SDH- and the HRP-revealed patterns become obscure by postnatal day 13-15 and are not visible in adult animals. In both studies, however, specimens were examined only in standard section planes. We recently have shown that the histochemical reaction for another mitochondrial enzyme, cytochrome oxidase (CO) is especially useful in revealing patterns of neuro*Author for correspondence and reprint requests at: Department of Anatomy & Cell Biology, University of Pittsburgh, School of Medicine, Pittsburgh, PA 15261, U.S.A. 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
321)
.6.
Fig. 1. Pattern of cytochrome oxidase (CO) activity in the rat ventrobasal thalamus. A and B show, respectively, a CO-reacted 80-#m section and a camera lucida reconstruction of two adjacent sections through the ventral posteromedial nucleus (VPM) of an adult rat. The dorsolateral half of VPM contains a curvilinear array of dense CO-reactive patches. As can be seen more clearly in the reconstruction in B, thesc patches form a pattern which closely resembles the spatial organization of the mystacial vibrissae on the contralateral face pad. Proceeding dorsal to ventral on the face whisker rows are labeled A through E. Within each row whiskers are denoted 1 through 4-7, proceeding caudal to rostral; barreloids associated with lower order arcs are closest to the external medullary lamina, i.e. location of letters A-E. Orientation: dorsotateral, up; rostral, left. LGNd, dorsal lateral geniculate nucleus; PO, thalamic posterior complex: Ret., reticular nucleus; VPL, ventral posterolateral nucleus; so, barreloid corresponding to orbital sinus hair. Bar in B = 3 0 0 # m for A and B. C and D show barreloids C1~C3 from A at higher magnification, In C the thionine counterstain has been suppressed by the use of a dark blue filter, and the CO-reactive zones are seen separated by narrow, unreactive fiber plexuses. D shows the corresponding Nissl pattern. Note that each CO-positive zone contains a ring of thalamic neurons forming its outer margin and surrounding a relatively cell-sparse hollow. B a r = 100/~m.
321
nal organization within the SmI barrel cortex that are not readily apparent in Nisslstained material [4]. In the present study we examined CO stained specimens of the adult rat thalamus to determine whether a morphologic correlate of the vibrissae representation could be visualized there as in the cortex. Sixteen adult rats (42-98 days of age) were perfused transcardially with 2~o paraformaldehyde-l.5~o glutaraldehyde in 0.1 M phosphate buffer, pH 7.3, and sections through their thalami were reacted for CO according to the method of Wong-Riley [15], mounted onto subbed slides and counterstained with 0.1~o thionine. Although several planes of section were used, we Ibund that the vibrissae representation was most clearly revealed in oblique horizontal sections which approached the VPM from its dorsomedial surface (see also ref. 17). Fig. 1A, B shows, respectively, a CO-reacted 80-#m section and a camera lucida reconstruction of two adjacent sections through VPM from a specimen prepared in this manner. VPM is notable for its dense reactivity relative to other thalamic nuclei. CO staining within the nucleus is not uniform, however, but is segregated into a series of oval patches of heightened activity that are separated from one another by narrow zones where the activity is considerably lower. The latter appear to consist of fiber plexuses as seen in myelin stains. The regions of heightened activity in VPM are spatially distributed in a pattern that resembles the arrangement of the mystacial vibrissae on the contralateral face and that corresponds to the physiologically defined somatotopic organization of the nucleus [12, 13]. Thus, CO barreloids representing dorsal rows of whiskers (e.g. row A; see Fig. 1) are located caudally in VPM while those for ventral whisker rows (e.g. row E) are located rostrally. Rows of barreloids are oriented dorsolaterally to ventromedially. Barreloids representing the large caudal whiskers (e.g. arc 1) are located near the external medullary lamina. Rows C-E curve to run in a rostrocaudal direction so that higher order arcs within these rows (e.g. arcs 5-10) are represented ventromedial to arc 4 in rows A and B. Fig. 1C, D shows higher magnification photomicrographs of the C 1-C3 barreloids from Fig. IA. In panel C, the thionine stain has been suppressed and the CO staining enhanced by use of a dark blue filter. In panel D both the CO and the corresponding Nissl pattern are shown, and it can be seen that each CO-reactive zone coincides with a ring of thalamic neurons surrounding a relatively cell-sparse center. These rings~of neurons form the walls of oval cylinders (100-250 pm wide) that extend through much of the thickness of VPM. In contrast to the SmI cortical barrels [4], thalamic neurons forming the sides of individual barreloids are generally contained within the CO-positive zone. The CO reactivity of these cell bodies is roughly equivalent to that of the surrounding neuropil and to that of cell bodies within the central aspect of each barreloid. The anatomical relationship between individual barreloids and their corresponding cortical barrels was verified in one animal that received an iontophoretic deposition of H R P into the B1 barrel. Tangential sections through the SmI and oblique horizontal sections through the ipsilateral thalamus were reacted for CO and/or for H R P according to the method of Adams [1]. A section through layer IV of the cortex is shown in Fig. 2A, where it can be seen that the H R P injection site is confined to
322
(3
Fig. 2. Barreloid-barrel relations. A: photomicrograph of a tangential section through layer IV of the SmI cortex reacted for CO and H RP showing the pattern of CO-reactive barrel centers and an HRP deposition in the BI barrel. Rows of barrels are labeled by letters A E placed in arc 1. BI is unlabeled to show the full extent of the HRP deposition. :~-6, barrels associated with vibrissae that straddle the whisker rows at their caudal end. Bar = 300 ~m. B: photomicrograph showing retrogradely labeled neurons in VPM resulting from the HRP deposition shown in A. Two labeled neurons (arrows) are seen in the dorsolateral side of the B1 barreloid. EML, external medullary lamina; ~, 13, barreloids corresponding to vibrissae straddling row B. B a r - 5 0 / ~ m . C: reconstruction of 10 serial 40-,um HRP-reacted sections through the VPM nucleus of the specimen shown in A and B. The locations of the 15 retrogradely labeled thalamic neurons (dots) have been plotted onto the reconstruction. Note that all but 3 of the labeled cells are found within the side of the B1 barreloid closest to the straddler barreloids. Orientation: dorsolateral, up; rostral, left. Bar - 200 l~m.
323 the posterolateral aspect of the B1 barrel (i.e. near its border with ~ and fl). Fig. 2B shows a 40-/~m HRP-reacted section through the VPM ipsilateral to the cortical injection. Note the two H RP-labeled neurons (arrows) in the side of the B l barreloid, the boundaries of which can be discerned by reference to the whorled pattern of neuronal somata. Most labeled neurons appeared to have dendrites that radiated toward the center of their parent barreloid. A total of 15 labeled cells was found in a complete series of sections through the VPM; 12 of these were located in the B 1 barreloid, two were in B2 and one was in ~. The locations of these cells are depicted graphically in Fig. 2C, which is a camera lucida reconstruction made from l0 serial H R P and C O / H R P reacted sections. Interestingly, labeled cells in B l tended to occupy the dorsolateral half of the barreloid (i.e. nearer ct and fl). These results demonstrate the existence of distinct cytoarchitectonic and metabolic subdivisions of the adult rat thalamus that appear to correspond to individual vibrissae on the contralateral face. The spatial organization of the barreloids is consistent with the somatotopic representation of the vibrissae defined physiologically [12, 13] and anatomically [8]. Thalamic barreloids undoubtedly form the anatomical substrate linking barrel-like arrays in the trigeminal nuclear complex with their structural and functional counterparts (i.e. barrels) in the SmI cortex. There is an important difference, however, between the nature of the CO staining described here for the thalamus and that reported previously for barrels in the SmI cortex [4]. In the latter, large highly reactive neuronal somata often were observed within the CO-rich barrel centers; this heterogeneous staining of barrel neurons probably reflects morphological and functional differences among cells in the barrels [9, 10]. By contrast, thalamic neurons as a population are more homogeneous in their CO staining and do not appear more reactive than the surrounding neuropil. This is in accord with recent studies indicating that the rat ventrobasal complex (VB) is composed exclusively of thalamocortical relay cells which are homogeneous with respect to their overall dendritic morphology [6]. Increases in the complexity of the thalamic neuropil has been invoked to account for the apparent obscuring of segmentation within VB during early postnatal development [2, 3]. In our experience, however, the section plane is critically important for visualizing barreloids in adult rats using CO or Nissl stains; this undoubtedly is the case for other histological methods as well. In retrospect, we have found it possible to occasionally identify individual rows or arcs of barreloids in adult brains sectioned transversely or horizontally. Taken together, our findings and those of Ivy and Killackey [3] suggest that the ventrobasal complex undergoes a rotation as the rat matures so that, for example, barreloid A1 shifts dorsally and caudally with respect to the major axes of the brain (see also ref. 7). The present findings underscore the precise anatomical and functional organization of all stations of the vibrissa-bartel system. They also provide a valuable context for detailed anatomical studies of thalamocorticai relations in the rodent somatosensory system. We thank Maria Antonich for typing the manuscript and Melinda Rau for technical assistance. Supported by N I H Grant NS19950.
324 1 Adams, J.C., Heavy metal intensification of DAB-based HRP reaction product, J. Histochem. Cytochem.. 29 (1981) 775. 2 Belford, G.R. and Killackey, H.P., The sensitive period in the development of the trigeminal system of the neonatal rat, J. Comp. Neurol., 193 (1980) 335-350. 3 Ivy, G.O. and Killackey, H.P., Ephemeral cellular segmentation in the thalamus of the neonatal rat, Dev. Brain Res., 2 ( 1981) 1 17. 4 Land, P.W. and Simons, D.J., Cytochrome oxidase staining in the rat SmI barrel cortex, J. Comp. Neurol., 238 (1985) 225 235. 5 Ma, P.M. and Woolsey, T.A., Cytoarchitectonic correlates of the vibrissae in the medullary trigeminat complex of the mouse, Brain Res., 306 (1984)374-379. 6 Peschanksi, M., Lee, C.L. and Ralston, H.J., lII, The structural organization of the ventrobasal complex of the rat as revealed by the analysis of physiologically characterized neurons injected intracellularly with horseradish peroxidase, Brain Res., 297 (1984) 63 74. 7 Rakic, P., Genesis of the dorsal lateral geniculate nucleus in the rhesus monkey: site and time of origin, kinetics of proliferation, routes of migration and pattern of distribution of neurons, J. Comp. Neurol., (1977) 23-52. 8 Sapporta, S. and Kruger, U, The organization of thalamocortical relay neurons in the rat ventrobasal complex studied by the retrograde transport of horseradish peroxidase, J. Comp, Neurol., 174 (1977) 187 208. 9 Simons, D.J., Response properties of vibrissa units in the rat SI somatosensory neocortex, J. Neurophysiol., 41 (1978) 798-820. 10 Simons, D.J., and Woolsey, T.A., Morphology of Golgi-Cox-impregnated barrel neurons in rat Sml cortex, J. Comp. Neurol., 230 (1984) 119-132. 11 Van der Loos, H., Barreloids in the mouse somatosensory thalamus, Neurosci. Lett., 2 (1976) 1 6. 12 Verley, R. and Onnen, 1., Somatotopic organization of the tactile thalamus in normal adult and developing mice and in adult mice dewhiskered since birth, Exp. Neurol., 72 (1981) 462-474. 13 Waite, P.M.E., Somatotopic organization of vibrissal responses in the ventro-basal complex of the rat thalamus, J. Physiol. (Lond.), 228 (1973) 527-540. 14 Welker, C., Microelectrode delineation of the fine grain somatotopic organization of SmI cerebral neocortex in albino rat, Brain Res., 26 ( 1971 ) 259 275. 15 Wong-Riley, M.T.T., Changes in the visual system of monocularly sutured or enucleated cats demonstrable with cytochrome oxidase histochemistry, Brain Res., 171 (1979) 11- 28. 16 Woolsey, T.A. and Van der Loos, H., The structural organization of layer IV in the somatosensory region ($1) of the mouse cerebral cortex, Brain Res., 17 (1970) 205-242. 17 Woolsey, T.A., Anderson, J.R., Wann, J.R. and Stanfield, B.B., Effects of early vibrissae damage on neurons in the ventrobasal (VB) thalamus of the mouse, J. Comp. Neurol., 184 (1979) 363-380.