Brain Research, 98 (1975) 177-182 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
177
Confirmation of the location of spinothalamic neurons in the cat and monkey by the retrograde transport of horseradish peroxidase
DANIEL L. TREVINO AND EARL CARSTENS Department of Physiology and the Neurobiology Program, University of North Carolina School of Medicine, Chapel Hill, N.C. 27514 (U.S.A.)
(Accepted July 1st, 1975)
Recently the cells of origin of the spinothalamic tract in cats and rhesus monkeys have been identified by stimulating the caudal diencephalon and searching for the back-fired neurons in the spinal cord with a microelectrode 1,2,4,17-19. In the lumbar cord of the cat the antidromically activated neurons were found primarily in the contralateral laminae VIII and medial VII of Rexed 15 (see Fig. 5 in ref. 19). In the lumbar cord of the monkey the neurons were found primarily in contralateral laminae I, IV and V with the rest in laminae VI, VII and VIII (see Fig. 7 in ref. 18). Signal averaging was used by one laboratory18,19 to increase the probability of detecting the antidromically fired units located some distance away from the tip of the recording microelectrode. This approach has been criticized by other workers as possibly leading to a misinterpretation of some unitary potentials which may actually be orthodromically driven by the thalamic stimulating electrode 1. The recently introduced method of using the retrograde transport of horseradish peroxidase (HRP) to trace anatomical pathways in the central nervous system 9-~4 appears to be a more suitable way to identify cells of origin of the spinothalamic tract because: (1) a large injection of H R P into the thalamus of one animal will label many if not most of the spinothalamic neurons throughout the entire spinal cord, so only a few animals are needed for a complete study; (2) the HRP-labeled neurons in the spinal cord are thalamic projecting cells and they have terminals and/or axons in the immediate vicinity of the injection siteT,10,12,13 and (3) the somata of labeled neurons are filled with H R P reaction product, revealing not only their precise locations but also their sizes and shapes. We therefore used this technique to determine again the locations of spinothalamic tract cells in cats and monkeys and to compare the results with the locations as determined by the antidromic activation technique. Using sterile procedures, injections of 0.5-3.0/~1 of a 30 ~ solution of H R P (in water or 0.970 saline) were made unilaterally into several regions of the caudal diencephalon/rostral mesencephalon of 5 cats and 2 rhesus monkeys anesthetized with pentobarbital. The H R P was delivered by means of I-3 penetrations of a 1 or 10/~1 Hamilton microsyringe needle which was placed stereotaxically through a trephined hole in the calvarium. For each penetration, 0.5-1.0 #1 of H R P were slowly injected
178 manually over 20-45 min. After 2-3 days survival the animals were reanesthetized with pentobarbital and fixed by transcardiac perfusion with 0.85 °/,, saline followed by 0.5~,~, paraformaldehyde-2.5~o glutaraldehyde in 0. 1M phosphate buffer at pH 7.2. The brains and spinal cord were then removed, cut into blocks and postfixed in the same fixative for at least 4 h at 4 °C, then transferred to 0.1 M phosphate buffer (pH 7.2) containing 5 ~, (w/v) sucrose and stored overnight at 4 °C. Frozen sections (40 #m) were cut from the thalamus and from selected segments throughout the spinal cord. Sections were collected in 0.1 M phosphate buffer, pH 7.6, and every third section was transferred to an incubation medium containing 0.05 ~,~ 3,3-diaminobenzidine tetrahydrochloride and 0.0l ~ HzO.~ in Tris buffer 6. The sections were incubated in this medium for 30 min at room temperature and then transferred through 2 changes of distilled water and mounted on slides with alcoholic gelatin. Some slides were left uncounterstained, but most of them were counterstained with 0.1 )'~ cresylecht violet. Sections of the spinal cord were scanned microscopically at low and high power using both dark- and bright-field illumination. Neuron somata containing granular reaction productT,11,13 were plotted on a drawing of the section and all the drawings from one segment were compiled to obtain the distribution pattern of the cells. Sections through the injection site were examined both microscopically and by projection to determine the region of highest concentration of HRP. Fig. 1A and B are dark-field photomicrographs of neuronal somata in the spinal cord which contained HRP-positive granules. Their appearance is not different from that of retrogradely labeled neurons in the brain as described in detail by other workersg, 11-13. Both intensely labeled and lightly labeled cells were seen throughout the spinal cord, but no attempt was made to quantitate the ratio of the two types. JonesT, 9 has suggested that the intensity of retrograde labeling is related to the density of the terminals available for taking up the H R P at the injection site. Fig. 2B illustrates the extent of the injection site at the level of the syringe needle penetrations in one cat. The brain stem structures which were totally or partially involved by the H R P injection in this case include: Ret. M, MG, MGmc, PO, SG, LP, Pul, LG, VPL, VPM, VL, Ret.T., CL, CM, MD, Hb, Pc, CeM (forabbreviations see p. 181). Fig. 2A is a composite of the locations of the HRP-labeled somas in the L7 segment of this cat. The greatest density of labeled somata were seen in medial lamina VII and dorsal lamina VIII contralateral to the injection site. Many labeled somata were also present in lamina I, but only a few were seen in the rest of the dorsal horn. lpsilateral to the injection almost all the labeled somata were similarly in medial laminae VII and dorsal lamina VIII. Comparable results were obtained in another cat with a similar large injection of H R P in the caudal diencephalon, which encroached more caudally into the midbrain. However, in this case, relatively more cells were found in the dorsal horn (lamina V) and scattered in the dorsolateral funiculus on the side contralateral to the injection while ipsilaterally, cells were seen in lamina I and in the dorsolateral funiculus as well as in laminae VII and VIII. In a third cat with an injection which was somewhat more rostral than in the first cat the yield of labeled cells in the spinal cord was not as great but their distribution was nearly identical to that shown in Fig. 2A. In a fourth cat with an injection which involved the lateral half of the thalamus at the
179
Fig. 1. A and B: dark-field photomicrographs of HRP-labeled neurons in the lumbar spinal cord of the monkey. These cells are located in the lateral half of lamina V, contralateral to the injection site (see Fig. 3A). The 100 p m calibration bar applies to both A and B.
2A
B
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Figs. 2A and 3A. Composite diagrams of the locations of HRP-labeled neurons plotted from every third section through the L7 segments. 2A: cat. 3A: monkey. The lamination scheme in 2A is after Rexed la and is also adapted to the monkey's gray matter in 3A for convenience. Figs. 2B and 3B. Drawings of the extent of spread of 30 ~ H R P delivered from parallel (mediolateral) penetrations of the syringe needle, 2 mm apart. See text for tile structures which were involved rostrally and caudally from these levels. 2B: cat, 3.0/~I, 3 penetrations. 3B: monkey, 2.0/~1, 2 penetrations.
180 level of the rostral ventrobasal complex, including VL and the zona incerta, all the labeled cells in the L7 segment were in lamina I except for one cell located in lamina V. In the last cat a small injection was made which involved VPL almost exclusively. Although labeled cells were distributed throughout the contralateral dorsal column nuclei in this cat, none were found in the spinal cord. The results in this latter case seem to support recent degeneration studies showing that the spinal cord does not project to VPL in the cat3, s. Fig. 3B illustrates the extent of the injection site at the level of the needle penetrations in one rhesus monkey. The brain stem structures which were totally or partially invaded by the H R P injection include: PT, Li, MD, SG, MG, MGmc, Pul, LG, Ret.T., CM, VPM, VPI, VPL, VL, CL, Pc, LD, ZI, Put, [C. Fig. 3A is a composite of the locations of the H R P labeled somata in the L7 segment of this monkey. Although there is as yet no study confirming a cytoarchitectonic lamination of the gray matter in the macaque spinal cord equivalent to the cat, it is used here for convenience in describing the locations of labeled somata. Contralateral to the injection site the greatest proportion of the cells were located at the 'neck" of the dorsal horn (lateral laminae V and IV). The rest of the cells were located in laminae I, VII and VIII and a few embedded among lateral funicular fibers immediately adjacent to the dorsal horn. Only a few cells were seen ipsilateral to the injection site. In another monkey with a much smaller injection into the same thalamic region the yield of retrogradely labeled cells was very low, but those which were present were distributed among the same areas shown in Fig. 3A. O f special interest is how the location of these HRP-labeled somata compare with the previously reported locations determined by antidromic activation ~s,19. The results obtained by the two methods are in remarkably close agreement. We therefore conclude that the use of signal averaging in the previous studies did not result in the reporting of spurious locations of antidromically excited units. One exception to the otherwise close agreement is the identification of HRP-labeled cells in laminae I and in the dorsolateral funiculus in the cat which were missed with the antidromic stimulation method. This omission would be an expected deficiency of the antidromic method since not all spinothalamic neurons could be expected to be fired by the antidromic stimulus. Likewise, the probability of finding antidromically excited spinothalamic neurons which were located in an area where only a few such cells exist in one segment, such as the interstitial portion of the dorsolateral funiculus~ is very low. There is an obvious species difference between the locations of spinothalamic neurons in the lumbar enlargements of cats and monkeys (Figs. 2A and 3A). This difference was noted when the locations of spinothalamic units were determined antidromically is. Examination of the locations of HRP-labeled neurons in the cervical enlargements in cats and monkeys 16 (Trevino and Carstens, in preparation) reveals much fewer cells in laminae VII and Villi than in the lumbar enlargements in agreemerit with Dilly et al. 4 who found spinothalamic units (antidromically) only in the dorsal horn of the cervical spinal cord. Therefore the high concentration of spinothalamic neurons in these deeper laminae in cats is unique to the lumbar segments. The relationship between location and functional properties of spinothalamic
181 n e u r o n s in the m o n k e y has been described 17,2°. Units which could be activated by m a n i p u l a t i o n of deep structures, e.g. b e n d i n g joints, squeezing muscles, included a group located in l a m i n a e VII a n d VIII. However, the spinothalamic units identified in the cat could n o t be successfully excited by n a t u r a l stimulation 19 a n d consequently their functional properties have n o t yet been determined. Cells in this same region project to the b u l b a r reticular f o r m a t i o n a n d their response to peripheral stimulation have been described 2,5. I n cats some spinoreticular units were excited by cutaneous mechanical stimuli, most r e s p o n d i n g best to noxious intensities2,5 while some were excited by strong pressure over deep structures such as joints, muscles, etcP. It is possible that the thalamic projecting units m a y show similar responses. We are currently p u r s u i n g a r e - e x a m i n a t i o n of these units in cats with the intent of determining their f u n c t i o n a l properties. We wish to t h a n k Dr. Aldo R u s t i o n i for his critical reading of this manuscript, Dr. E. R. Perl for generously providing the use of laboratory facilities a n d e q u i p m e n t a n d Ms.'s Carol Metz, Sharyn Sawick a n d Gail Burd for their expert technical assistance. This work was supported by N S l l 1 3 2 , M H l l l 0 7 , a n d a General Research S u p p o r t grant, all from the U S P H S , by the U N C University Research Council a n d by The Alfred P. Sloan F o u n d a t i o n . ABBREVIATIONS USED IN THE FIGURES AND 1N THE TEXT CeM -CG = CL -CM -CP = Hb : IC -LD -LG -Li -LP -MD -MG : MGmc : Pc =
nucleus centralis medialis central gray nucleus centralis lateralis nucleus centrum medianum cerebral peduncle habenular nuclei internal capsule nucleus lateralis dorsalis lateral geniculate nucleus nucleus limitans nucleus lateralis posterior nucleus medialis dorsalis medial geniculate nucleus magnocellular division of MG nucleus paracentralis
PO = PT = Pul Put = R = Ret.M.= Ret.T. = SC = SG = VL = VPI = VPL = VPM = Z1 =
posterior group pretectal nuclei pulvinar putamen nucleus ruber mesencephalic reticular formation nucleus reticularis thalami superior colliculus nucleus suprageniculatus nucleus ventralis [ateralis nucleus ventralis posterior inferior nucleus ventralis posterior lateralis nucleus ventralis posterior medialis zona incerta
1 ALBE-FESSARD,D., LEVANTE,A., AND LAMOUR, Y., Origin of spino-thalamic tract in monkeys, Brain Research, 65 (1974) 503-509. 2 ALBE-FESSARD,D., LEVANTE,A., AND LAMOUR,Y., Origin of spinothalamic and spinoreticular pathways in cats and monkeys. In J. J. BONICA(Ed.), International Symposium on Pain, Advances in Neurology, VoL 4, Raven Press, New York, 1974, pp. 157-166. 3 BOWIE,J., The termination of the spinothalamic tract in the cat. An experimental study with silver impregnation methods, Exp. Brain Res., 12 (1971) 331-353. 4 DILLY,P. N., WALL,P. D., AND WEBSTER,K. E., Cells of origin of the spinothalamic tract in the cat and rat, Exp. Neurok, 21 (1968) 550-562. 5 FIELDS,H. L., WAGNER,G. W., ANDANDERSON,S. D., Some properties of spinal neurons projecting to the medial brain stem formation, Exp. Neurol., 47 (1975) 118-134.
182 6 GRAHAM,R. C., JR., AND KARNOVSKY,M. J., The early stages of absorption of injected horseradish peroxidase in the proximal tubules of mouse kidney: ultrastructural cytochemistry by a new technique, J. Histochem. Cytochem., 14 (1966) 291 302. 7 JONES, E. G., Possible determinants of the degree of retrograde neuronal labeling with horseradish peroxidase, Brain Research, 85 (1975) 249-253. 8 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) 395432. 9 JONES, E. G., AND LEAVITT, R. Y., Demonstration of thalamocortical connectivity in the cat somatosensory system by retrograde axonal transport of horseradish peroxidase, Brain Research, 63 (1973) 414-418. 10 KUYPERS, H. G. J. M., KIEVIT, J., AND GROEN-KLEVANT, A. C., Retrograde axonal transport of horseradish peroxidase in rat's forebrain, Brain Research, 67 (1974) 211-218. 11 LAVAIL, J. H., AND LAVAIL, M. M., Retrograde axonal transport in the central nervous system, Science, 176 (1972) 1416-1417. 12 LAVAIL, J. H., WINSTON, K. R., AND TISH, A., A method based on retrograde axonal transport of protein for identification of cell bodies of origin of axons terminating within the CNS, Brain Research, 58 (1973) 470-477. 13 NAUTA, H. J. W., PRINZ, M. B., AND LASEK, R. J., Afferents to the rat caudoputamen studied with horseradish peroxidase. An evaluation of a retrograde neuroanatomical research method, Bra#l Research, 67 (1974) 219-238. 14 RALSTON, H. J., Ili, AND SHARP, P. V., The identification of thalamocortical relay ceils in the adult cat by means of retrograde axonal transport of horseradish peroxidase, Brain Research, 62 (1973) 273-278. 15 REXED, B., A cytoarchitectonic atlas of the spinal cord in the cat, J. comp. Nenrol., 100 (1954) 297-379. 16 TREVINO, D. L., ANO CARSrENS, E., Locat ion of the cells of origin of the spinothalamic tract in the cat and rhesus monkey as determined by the retrograde transport of horseradish peroxidase, Anat. Rec., 181 (1975)494-495. 17 TREVINO, D. L., COULTER, J. D., MAUNZ, R. A., AND WILLIS, W. D., Location and functional properties of spinothalamic cells in the monkey. In J. J. BON1CA(Ed.), International Symposium on Pain, Advances in Neurology, Vol. 4, Raven Press, New York, 1974, pp. 167-170. 18 TREVINO, D. L., COULTER, J. D., AND WrLLlS, W. D., Location of cells of origin of spinothalamic tract in lumbar enlargement of the monkey, J. Neurophysiol., 36 (1973) 750-761. 19 TREVINO, D. L., MAUNZ, R. A., BRYAN, R. N., AND WILLIS, W. D., Location of cells of origin of the spinothalamic tract in the lumbar enlargement of cat, Exp. Neurol., 34 (1972) 64-77. 20 WILLIS, W. D., TRI~VINO, D. L., COULTER,J. D., AND MAUNZ, R. A., Responses of primate spinothalamic tract neurons to natural stimulation of hindlimb, J. Neurophysiol., 37 (1974) 358-372.
177
Confirmation of the location of spinothalamic neurons in the cat and monkey by the retrograde transport of horseradish peroxidase
DANIEL L. TREVINO AND EARL CARSTENS Department of Physiology and the Neurobiology Program, University of North Carolina School of Medicine, Chapel Hill, N.C. 27514 (U.S.A.)
(Accepted July 1st, 1975)
Recently the cells of origin of the spinothalamic tract in cats and rhesus monkeys have been identified by stimulating the caudal diencephalon and searching for the back-fired neurons in the spinal cord with a microelectrode 1,2,4,17-19. In the lumbar cord of the cat the antidromically activated neurons were found primarily in the contralateral laminae VIII and medial VII of Rexed 15 (see Fig. 5 in ref. 19). In the lumbar cord of the monkey the neurons were found primarily in contralateral laminae I, IV and V with the rest in laminae VI, VII and VIII (see Fig. 7 in ref. 18). Signal averaging was used by one laboratory18,19 to increase the probability of detecting the antidromically fired units located some distance away from the tip of the recording microelectrode. This approach has been criticized by other workers as possibly leading to a misinterpretation of some unitary potentials which may actually be orthodromically driven by the thalamic stimulating electrode 1. The recently introduced method of using the retrograde transport of horseradish peroxidase (HRP) to trace anatomical pathways in the central nervous system 9-~4 appears to be a more suitable way to identify cells of origin of the spinothalamic tract because: (1) a large injection of H R P into the thalamus of one animal will label many if not most of the spinothalamic neurons throughout the entire spinal cord, so only a few animals are needed for a complete study; (2) the HRP-labeled neurons in the spinal cord are thalamic projecting cells and they have terminals and/or axons in the immediate vicinity of the injection siteT,10,12,13 and (3) the somata of labeled neurons are filled with H R P reaction product, revealing not only their precise locations but also their sizes and shapes. We therefore used this technique to determine again the locations of spinothalamic tract cells in cats and monkeys and to compare the results with the locations as determined by the antidromic activation technique. Using sterile procedures, injections of 0.5-3.0/~1 of a 30 ~ solution of H R P (in water or 0.970 saline) were made unilaterally into several regions of the caudal diencephalon/rostral mesencephalon of 5 cats and 2 rhesus monkeys anesthetized with pentobarbital. The H R P was delivered by means of I-3 penetrations of a 1 or 10/~1 Hamilton microsyringe needle which was placed stereotaxically through a trephined hole in the calvarium. For each penetration, 0.5-1.0 #1 of H R P were slowly injected
178 manually over 20-45 min. After 2-3 days survival the animals were reanesthetized with pentobarbital and fixed by transcardiac perfusion with 0.85 °/,, saline followed by 0.5~,~, paraformaldehyde-2.5~o glutaraldehyde in 0. 1M phosphate buffer at pH 7.2. The brains and spinal cord were then removed, cut into blocks and postfixed in the same fixative for at least 4 h at 4 °C, then transferred to 0.1 M phosphate buffer (pH 7.2) containing 5 ~, (w/v) sucrose and stored overnight at 4 °C. Frozen sections (40 #m) were cut from the thalamus and from selected segments throughout the spinal cord. Sections were collected in 0.1 M phosphate buffer, pH 7.6, and every third section was transferred to an incubation medium containing 0.05 ~,~ 3,3-diaminobenzidine tetrahydrochloride and 0.0l ~ HzO.~ in Tris buffer 6. The sections were incubated in this medium for 30 min at room temperature and then transferred through 2 changes of distilled water and mounted on slides with alcoholic gelatin. Some slides were left uncounterstained, but most of them were counterstained with 0.1 )'~ cresylecht violet. Sections of the spinal cord were scanned microscopically at low and high power using both dark- and bright-field illumination. Neuron somata containing granular reaction productT,11,13 were plotted on a drawing of the section and all the drawings from one segment were compiled to obtain the distribution pattern of the cells. Sections through the injection site were examined both microscopically and by projection to determine the region of highest concentration of HRP. Fig. 1A and B are dark-field photomicrographs of neuronal somata in the spinal cord which contained HRP-positive granules. Their appearance is not different from that of retrogradely labeled neurons in the brain as described in detail by other workersg, 11-13. Both intensely labeled and lightly labeled cells were seen throughout the spinal cord, but no attempt was made to quantitate the ratio of the two types. JonesT, 9 has suggested that the intensity of retrograde labeling is related to the density of the terminals available for taking up the H R P at the injection site. Fig. 2B illustrates the extent of the injection site at the level of the syringe needle penetrations in one cat. The brain stem structures which were totally or partially involved by the H R P injection in this case include: Ret. M, MG, MGmc, PO, SG, LP, Pul, LG, VPL, VPM, VL, Ret.T., CL, CM, MD, Hb, Pc, CeM (forabbreviations see p. 181). Fig. 2A is a composite of the locations of the HRP-labeled somas in the L7 segment of this cat. The greatest density of labeled somata were seen in medial lamina VII and dorsal lamina VIII contralateral to the injection site. Many labeled somata were also present in lamina I, but only a few were seen in the rest of the dorsal horn. lpsilateral to the injection almost all the labeled somata were similarly in medial laminae VII and dorsal lamina VIII. Comparable results were obtained in another cat with a similar large injection of H R P in the caudal diencephalon, which encroached more caudally into the midbrain. However, in this case, relatively more cells were found in the dorsal horn (lamina V) and scattered in the dorsolateral funiculus on the side contralateral to the injection while ipsilaterally, cells were seen in lamina I and in the dorsolateral funiculus as well as in laminae VII and VIII. In a third cat with an injection which was somewhat more rostral than in the first cat the yield of labeled cells in the spinal cord was not as great but their distribution was nearly identical to that shown in Fig. 2A. In a fourth cat with an injection which involved the lateral half of the thalamus at the
179
Fig. 1. A and B: dark-field photomicrographs of HRP-labeled neurons in the lumbar spinal cord of the monkey. These cells are located in the lateral half of lamina V, contralateral to the injection site (see Fig. 3A). The 100 p m calibration bar applies to both A and B.
2A
B
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Figs. 2A and 3A. Composite diagrams of the locations of HRP-labeled neurons plotted from every third section through the L7 segments. 2A: cat. 3A: monkey. The lamination scheme in 2A is after Rexed la and is also adapted to the monkey's gray matter in 3A for convenience. Figs. 2B and 3B. Drawings of the extent of spread of 30 ~ H R P delivered from parallel (mediolateral) penetrations of the syringe needle, 2 mm apart. See text for tile structures which were involved rostrally and caudally from these levels. 2B: cat, 3.0/~I, 3 penetrations. 3B: monkey, 2.0/~1, 2 penetrations.
180 level of the rostral ventrobasal complex, including VL and the zona incerta, all the labeled cells in the L7 segment were in lamina I except for one cell located in lamina V. In the last cat a small injection was made which involved VPL almost exclusively. Although labeled cells were distributed throughout the contralateral dorsal column nuclei in this cat, none were found in the spinal cord. The results in this latter case seem to support recent degeneration studies showing that the spinal cord does not project to VPL in the cat3, s. Fig. 3B illustrates the extent of the injection site at the level of the needle penetrations in one rhesus monkey. The brain stem structures which were totally or partially invaded by the H R P injection include: PT, Li, MD, SG, MG, MGmc, Pul, LG, Ret.T., CM, VPM, VPI, VPL, VL, CL, Pc, LD, ZI, Put, [C. Fig. 3A is a composite of the locations of the H R P labeled somata in the L7 segment of this monkey. Although there is as yet no study confirming a cytoarchitectonic lamination of the gray matter in the macaque spinal cord equivalent to the cat, it is used here for convenience in describing the locations of labeled somata. Contralateral to the injection site the greatest proportion of the cells were located at the 'neck" of the dorsal horn (lateral laminae V and IV). The rest of the cells were located in laminae I, VII and VIII and a few embedded among lateral funicular fibers immediately adjacent to the dorsal horn. Only a few cells were seen ipsilateral to the injection site. In another monkey with a much smaller injection into the same thalamic region the yield of retrogradely labeled cells was very low, but those which were present were distributed among the same areas shown in Fig. 3A. O f special interest is how the location of these HRP-labeled somata compare with the previously reported locations determined by antidromic activation ~s,19. The results obtained by the two methods are in remarkably close agreement. We therefore conclude that the use of signal averaging in the previous studies did not result in the reporting of spurious locations of antidromically excited units. One exception to the otherwise close agreement is the identification of HRP-labeled cells in laminae I and in the dorsolateral funiculus in the cat which were missed with the antidromic stimulation method. This omission would be an expected deficiency of the antidromic method since not all spinothalamic neurons could be expected to be fired by the antidromic stimulus. Likewise, the probability of finding antidromically excited spinothalamic neurons which were located in an area where only a few such cells exist in one segment, such as the interstitial portion of the dorsolateral funiculus~ is very low. There is an obvious species difference between the locations of spinothalamic neurons in the lumbar enlargements of cats and monkeys (Figs. 2A and 3A). This difference was noted when the locations of spinothalamic units were determined antidromically is. Examination of the locations of HRP-labeled neurons in the cervical enlargements in cats and monkeys 16 (Trevino and Carstens, in preparation) reveals much fewer cells in laminae VII and Villi than in the lumbar enlargements in agreemerit with Dilly et al. 4 who found spinothalamic units (antidromically) only in the dorsal horn of the cervical spinal cord. Therefore the high concentration of spinothalamic neurons in these deeper laminae in cats is unique to the lumbar segments. The relationship between location and functional properties of spinothalamic
181 n e u r o n s in the m o n k e y has been described 17,2°. Units which could be activated by m a n i p u l a t i o n of deep structures, e.g. b e n d i n g joints, squeezing muscles, included a group located in l a m i n a e VII a n d VIII. However, the spinothalamic units identified in the cat could n o t be successfully excited by n a t u r a l stimulation 19 a n d consequently their functional properties have n o t yet been determined. Cells in this same region project to the b u l b a r reticular f o r m a t i o n a n d their response to peripheral stimulation have been described 2,5. I n cats some spinoreticular units were excited by cutaneous mechanical stimuli, most r e s p o n d i n g best to noxious intensities2,5 while some were excited by strong pressure over deep structures such as joints, muscles, etcP. It is possible that the thalamic projecting units m a y show similar responses. We are currently p u r s u i n g a r e - e x a m i n a t i o n of these units in cats with the intent of determining their f u n c t i o n a l properties. We wish to t h a n k Dr. Aldo R u s t i o n i for his critical reading of this manuscript, Dr. E. R. Perl for generously providing the use of laboratory facilities a n d e q u i p m e n t a n d Ms.'s Carol Metz, Sharyn Sawick a n d Gail Burd for their expert technical assistance. This work was supported by N S l l 1 3 2 , M H l l l 0 7 , a n d a General Research S u p p o r t grant, all from the U S P H S , by the U N C University Research Council a n d by The Alfred P. Sloan F o u n d a t i o n . ABBREVIATIONS USED IN THE FIGURES AND 1N THE TEXT CeM -CG = CL -CM -CP = Hb : IC -LD -LG -Li -LP -MD -MG : MGmc : Pc =
nucleus centralis medialis central gray nucleus centralis lateralis nucleus centrum medianum cerebral peduncle habenular nuclei internal capsule nucleus lateralis dorsalis lateral geniculate nucleus nucleus limitans nucleus lateralis posterior nucleus medialis dorsalis medial geniculate nucleus magnocellular division of MG nucleus paracentralis
PO = PT = Pul Put = R = Ret.M.= Ret.T. = SC = SG = VL = VPI = VPL = VPM = Z1 =
posterior group pretectal nuclei pulvinar putamen nucleus ruber mesencephalic reticular formation nucleus reticularis thalami superior colliculus nucleus suprageniculatus nucleus ventralis [ateralis nucleus ventralis posterior inferior nucleus ventralis posterior lateralis nucleus ventralis posterior medialis zona incerta
1 ALBE-FESSARD,D., LEVANTE,A., AND LAMOUR, Y., Origin of spino-thalamic tract in monkeys, Brain Research, 65 (1974) 503-509. 2 ALBE-FESSARD,D., LEVANTE,A., AND LAMOUR,Y., Origin of spinothalamic and spinoreticular pathways in cats and monkeys. In J. J. BONICA(Ed.), International Symposium on Pain, Advances in Neurology, VoL 4, Raven Press, New York, 1974, pp. 157-166. 3 BOWIE,J., The termination of the spinothalamic tract in the cat. An experimental study with silver impregnation methods, Exp. Brain Res., 12 (1971) 331-353. 4 DILLY,P. N., WALL,P. D., AND WEBSTER,K. E., Cells of origin of the spinothalamic tract in the cat and rat, Exp. Neurok, 21 (1968) 550-562. 5 FIELDS,H. L., WAGNER,G. W., ANDANDERSON,S. D., Some properties of spinal neurons projecting to the medial brain stem formation, Exp. Neurol., 47 (1975) 118-134.
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