- Email: [email protected]
Laminar origin of efferent projections from auditory cortex in the golden Syrian hamster
Brain Research, 114 (1976) 497-500
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
497
Laminar origin of efferent projections from auditory cortex in the golden Syrian hamster*
RICHARD J. RAVIZZA, ROGER B. STRAW** and PHILLIP D. LONG Department of Psychology, The Pennsylvania State University, University Park, Pa. 16802 (U.S.A.)
(Accepted June 1lth, 1976)
Traditionally, anatomical studies of the central auditory system have focused on the afferent pathway from cochlea to auditory cortex; consequently, relatively few investigations have examined the organization of descending auditory pathways. In recent years the development of the Nauta and Fink-Heimer stains has greatly accelerated the study of the descending auditory system (e.g., ref. 1). One part of this inquiry has focused on the organization of efferent projections from auditory cortex to other parts of the brain. For example, it is now known that neurons originating in auditory cortex project to a wide variety of brain structures including medial geniculate nucleus, inferior and superior colliculi, basal ganglia, parabrachial region, pontine nuclei and contralateral auditory cortex (e.g., refs. 2, 6, 9, 11). Although an understanding of at least the extent of these efferent projections is beginning to emerge, other questions such as the distribution of their cell bodies within the various layers of auditory cortex cannot be readily answered with anterograde procedures. Fortunately, the recent application of H R P retrograde transport techniques to the study of neurological pathways by the LaVails 7 now permits the extension of this analysis to a detailed description of the laminar origin of these efferent projections within auditory cortex itself. In recent months we have been using the H R P technique to determine the laminar origin of the cortical efferent projections in the hamster. Our present purpose is to describe the layers of auditory cortex in which those neurons projecting to inferior colliculus, medial geniculate and contralateral auditory cortex are located. To date, we have made individual injections of H R P into these structures in 47 hamsters. In the present communication we wish to limit our discussion to 3 cases which typify the laminar distribution of cortically labeled cells following injection of H R P into inferior colliculus, medial geniculate or contralateral auditory cortex. In the present cases 0.02-0.05 #1 of H R P (Type VI, Sigma Chemical Company) in a 50 ~ concentration were injected unilaterally into either the ipsilateral inferior * This research was supported by NINCDS Grant 11554, ** N1MH predoctoral fellow MH 05386,
498 colliculus or medial geniculate, or the contralateral auditory cortex. Surgical procedures were employed which permitted direct injection of H R P into inferior colliculus as well as contralateral auditory cortex. The medial geniculate injections involved approaches t h r o u g h either ipsilateral or contralateral cortex. In either case the distribution o f labeled neurons in auditory cortex was remarkably similar. Further, injections limited to cortical areas usually traversed in our medial geniculate penetrations revealed virtually no labeled neurons in auditory cortex. A l t h o u g h the present data involve uniform survival periods of 24 h, we have found similar results at survival times ranging f r o m 12 to 96 h for each injection site. All animals were perfused with 0.05 %~ p a r a f o r m a l d e h y d e and 2.5 °/o glutaraldehyde in sodium cacodylate buffer (pH 7.2). In each case the brain was removed, placed in
/
/
Fig. l. The left column shows i~ection sites of 3 cases which typify our chief findings. Thecenter column contains a representative cortical section from each case. A composite created by superimposing the 3 cortical sections is shown on the right. In all cortical sections the appropriate position of layer 4 is indicated by the fine solid line. Note that the laminar origin of cortical efferents is highly organized with projections to inferior colliculus originating in the upper part of layer 5, those to medial geniculate in layer 6 and finally those to contralateral auditory cortex in layers 2, 3, 4 and 5,
499 the fixative for 2 h and then stored in cacodylate buffer (pH 7.2) for 15 h. Frozen sections were cut at 40 # m and were developed in 3,3'-diaminobenzidine tetrahydrochloride and hydrogen peroxide. Every fifth section was mounted and lightly stained with cresyl violet acetate. The relationship between labeled neurons and cortical laminar architectonics was determined by the following prodecure. First each section was drawn with the aid of a microprojector to establish cortical lamina. Then, on a second drawing of the same section each labeled cell was microscopically located under × 400 dark-field observation and then marked with the aid of an X - Y plotter coupled to the stage of the microscope. In this way the two independent drawings could simply be superimposed to determine the relationship between the position of the cortical lamina and the distribution of labeled neurons. The results of a typical large injection in the inferior colliculus are shown in the upper row of Fig. 1. In this case a large number of labeled pyramidal cells are evident in the upper portions of layer 5 over a wide area of auditory cortex. Although many neurons were labeled by this injection, other pyramidal cells in this same part of layer 5 showed no evidence of label even under × 400 dark-field observation. In contrast, injections of the medial geniculate consistently revealed labeled neurons predominantly in layer 6 of auditory cortex (Fig. 1, middle row) with only very occasional neurons labeled in more superficial cortical layers. Further, unlike the occurrence of numerous unlabeled cells in layer 5, followir~g inferior colliculus injections, the great majority of neurons in layer 6 was labeled. Finally, our cases with contralateral auditory cortex injections resulted in labeled neurons within layers 2, 3, 4 and 5 of the auditory cortex (Fig. 1, lower row). Detailed analysis of these cases revealed two facts about the distribution of labeled neurons f~llowing injections of the contralateral auditory cortex. First, we noted a consistent tendency for more labeled neurons to appear in the lower than in the upper portion of layer 5. Second, the labeled neurons located within layer 4 were invariably pyramidal cells scattered among somewhat smaller neurons, which were presumably granular cells. When the above results are combined with what is already known abeut the laminar origin of cortical efferents in other sensory systems, several similarities emerge. First, our finding of auditory cortex projections to tectum originating within layer 5 closely parallels available evidence from other experiments. For example, projections to rectum originating within layer 5 have already been described for monkey and cat visual cortex3, s and tree shrew auditory cortex ~. Second, the fact that hamster auditory cortical-thalamic projections originate within layer 6 corresponds to earlier reports of cortical-thalamic projections originating in either layers 5 or 6. That is, published reports indicate that neurons from layer 6 of visual cortex project to the dorsal lateral geniculate nucleus in both squirreP 0 and monkey s and that layer 5 neurons project to monkey inferior pulvinar s. Similarly, WisO 2 reports projections to the ventral posterior nucleus originating within layer 5 of SI and layer 6 of SI! in the rat. Finally, our observation of neurons in layers 2, 3, 4 and 5 projecting to contralateral cortex parallels earlier reports describing commissural connections originating within these
500 s a m e w i d e s p r e a d cortical layers in r a t a n d m o u s e somatic, visual o r a u d i t o r y c o r t e x 4,1'~, 13 T a k e n together, available evidence indicates a c o n s i d e r a b l e degree o f specificity in the l a m i n a r origin o f cortical efferents. Indeed, available evidence is consistent with the idea t h a t cortical-tectal p r o j e c t i o n s originate in layer 5, c o r t i c a l - t h a l a m i c p r o j e c t i o n s in either layers 5 or 6, a n d c o m m i s s u r a l c o n n e c t i o n s in layers 2, 3, 4 a n d 5. U n f o r t u n a t e l y , because these studies t y p i c a l l y e x a m i n e no m o r e t h a n the p r o j e c t i o n s f r o m a single cortical region for a n y given species, the question o f the l a m i n a r origin o f cortical p r o j e c t i o n s from different areas o f sensory c o r t e x for any single species remains unanswered.
We would like to express our thanks to Gloria Bathurst for typing this manuscript and to Jorene Rath for her help in preparing the illustration.
1 Desmedt, S. E., Physiological studies of the efferent recurrent auditory system. In W. D. Koid¢l and W. D. Neff (Eds.), Handbook o f Sensory Physiology: Auditory System, Vol. V/2, Springer, New York, 1975, pp. 219-246. 2 Diamond, I. T., Jones, E. G. and Poweil, T. P. S., The projection of the auditory cortex upon the diencephalon and brain stem in the cat, Brain Research, 15 (1969) 305-340. 3 Gilbert, L. D., and Kelly, J. P., The projections of cells in different layers of the cat's visual cortex, J. comp. Neurot., 163 (1975) 81"106. 4 Jacobsen, S. and Trojanowski, J. Q., The cells of origin of the corpus callosum in rat, cat and rhesus monkey, Brain Research, 74 (1974) 149"155. 5 Jones, D. R., Casseday, J. H. and Diamond, I. T., Further study of parallel auditory pathways in the tre¢ shrew, Tupaia glis, Anat. Rec., 184 (1976) 438-439. 6 Kemp, J, M. and Powelt, T: P. S., The cortico-striate projection in the monkey, Brain, 93 (1970) 525-546. 7 LaVail, J. H. and LaVail, M. M., The retrograde intraaxonal transport of horseradish peroxidase in the chick visual system: a light and electron microscopic study, J. comp. Neurol., 157 (1974) 303-357. 8 Lund, J. S., Lund, R. D., Hendrickson, A. E., Bunt, A. H. and Fuchs, A. F., The origin of efferent pathways from the primary visual cortex, area 17, of the macaque monkey as shown by retrograde transport of horseradish pcroxidase, J. comp. Neurol., 164 (1975) 287-304. 90tellin, V. A., Projections of the auditory cortex to the neostriatum, Neurosci. Behav. Physiol., 5 (1972) 267-274. 10 Robson, J. A. and Hall, W. C., Connections of layer VI in striate cortex of the grey squirrel (Sciurus carolinensis), Brain Research, 93 (1975) 133-139. 11 Webster, K. E., The cortico-striatal projection in the cat, J. Anat. (Lend.), 99 (1965) 329-337. 12 Wise, S P., The laminar organization ofcartain afferent and efferent fiber systems in the rat somato, sensory cortex, Brain Research, 90 (1975) 139-142. 13 Yorke, C. H. and Caviness, V. S., Interhemispheric neocortical connections of the corpus callosum in the normal mouse: a study based on anterograde and retrograde methods, J. comp. Neurol., 164 (1975) 233-246.
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
497
Laminar origin of efferent projections from auditory cortex in the golden Syrian hamster*
RICHARD J. RAVIZZA, ROGER B. STRAW** and PHILLIP D. LONG Department of Psychology, The Pennsylvania State University, University Park, Pa. 16802 (U.S.A.)
(Accepted June 1lth, 1976)
Traditionally, anatomical studies of the central auditory system have focused on the afferent pathway from cochlea to auditory cortex; consequently, relatively few investigations have examined the organization of descending auditory pathways. In recent years the development of the Nauta and Fink-Heimer stains has greatly accelerated the study of the descending auditory system (e.g., ref. 1). One part of this inquiry has focused on the organization of efferent projections from auditory cortex to other parts of the brain. For example, it is now known that neurons originating in auditory cortex project to a wide variety of brain structures including medial geniculate nucleus, inferior and superior colliculi, basal ganglia, parabrachial region, pontine nuclei and contralateral auditory cortex (e.g., refs. 2, 6, 9, 11). Although an understanding of at least the extent of these efferent projections is beginning to emerge, other questions such as the distribution of their cell bodies within the various layers of auditory cortex cannot be readily answered with anterograde procedures. Fortunately, the recent application of H R P retrograde transport techniques to the study of neurological pathways by the LaVails 7 now permits the extension of this analysis to a detailed description of the laminar origin of these efferent projections within auditory cortex itself. In recent months we have been using the H R P technique to determine the laminar origin of the cortical efferent projections in the hamster. Our present purpose is to describe the layers of auditory cortex in which those neurons projecting to inferior colliculus, medial geniculate and contralateral auditory cortex are located. To date, we have made individual injections of H R P into these structures in 47 hamsters. In the present communication we wish to limit our discussion to 3 cases which typify the laminar distribution of cortically labeled cells following injection of H R P into inferior colliculus, medial geniculate or contralateral auditory cortex. In the present cases 0.02-0.05 #1 of H R P (Type VI, Sigma Chemical Company) in a 50 ~ concentration were injected unilaterally into either the ipsilateral inferior * This research was supported by NINCDS Grant 11554, ** N1MH predoctoral fellow MH 05386,
498 colliculus or medial geniculate, or the contralateral auditory cortex. Surgical procedures were employed which permitted direct injection of H R P into inferior colliculus as well as contralateral auditory cortex. The medial geniculate injections involved approaches t h r o u g h either ipsilateral or contralateral cortex. In either case the distribution o f labeled neurons in auditory cortex was remarkably similar. Further, injections limited to cortical areas usually traversed in our medial geniculate penetrations revealed virtually no labeled neurons in auditory cortex. A l t h o u g h the present data involve uniform survival periods of 24 h, we have found similar results at survival times ranging f r o m 12 to 96 h for each injection site. All animals were perfused with 0.05 %~ p a r a f o r m a l d e h y d e and 2.5 °/o glutaraldehyde in sodium cacodylate buffer (pH 7.2). In each case the brain was removed, placed in
/
/
Fig. l. The left column shows i~ection sites of 3 cases which typify our chief findings. Thecenter column contains a representative cortical section from each case. A composite created by superimposing the 3 cortical sections is shown on the right. In all cortical sections the appropriate position of layer 4 is indicated by the fine solid line. Note that the laminar origin of cortical efferents is highly organized with projections to inferior colliculus originating in the upper part of layer 5, those to medial geniculate in layer 6 and finally those to contralateral auditory cortex in layers 2, 3, 4 and 5,
499 the fixative for 2 h and then stored in cacodylate buffer (pH 7.2) for 15 h. Frozen sections were cut at 40 # m and were developed in 3,3'-diaminobenzidine tetrahydrochloride and hydrogen peroxide. Every fifth section was mounted and lightly stained with cresyl violet acetate. The relationship between labeled neurons and cortical laminar architectonics was determined by the following prodecure. First each section was drawn with the aid of a microprojector to establish cortical lamina. Then, on a second drawing of the same section each labeled cell was microscopically located under × 400 dark-field observation and then marked with the aid of an X - Y plotter coupled to the stage of the microscope. In this way the two independent drawings could simply be superimposed to determine the relationship between the position of the cortical lamina and the distribution of labeled neurons. The results of a typical large injection in the inferior colliculus are shown in the upper row of Fig. 1. In this case a large number of labeled pyramidal cells are evident in the upper portions of layer 5 over a wide area of auditory cortex. Although many neurons were labeled by this injection, other pyramidal cells in this same part of layer 5 showed no evidence of label even under × 400 dark-field observation. In contrast, injections of the medial geniculate consistently revealed labeled neurons predominantly in layer 6 of auditory cortex (Fig. 1, middle row) with only very occasional neurons labeled in more superficial cortical layers. Further, unlike the occurrence of numerous unlabeled cells in layer 5, followir~g inferior colliculus injections, the great majority of neurons in layer 6 was labeled. Finally, our cases with contralateral auditory cortex injections resulted in labeled neurons within layers 2, 3, 4 and 5 of the auditory cortex (Fig. 1, lower row). Detailed analysis of these cases revealed two facts about the distribution of labeled neurons f~llowing injections of the contralateral auditory cortex. First, we noted a consistent tendency for more labeled neurons to appear in the lower than in the upper portion of layer 5. Second, the labeled neurons located within layer 4 were invariably pyramidal cells scattered among somewhat smaller neurons, which were presumably granular cells. When the above results are combined with what is already known abeut the laminar origin of cortical efferents in other sensory systems, several similarities emerge. First, our finding of auditory cortex projections to tectum originating within layer 5 closely parallels available evidence from other experiments. For example, projections to rectum originating within layer 5 have already been described for monkey and cat visual cortex3, s and tree shrew auditory cortex ~. Second, the fact that hamster auditory cortical-thalamic projections originate within layer 6 corresponds to earlier reports of cortical-thalamic projections originating in either layers 5 or 6. That is, published reports indicate that neurons from layer 6 of visual cortex project to the dorsal lateral geniculate nucleus in both squirreP 0 and monkey s and that layer 5 neurons project to monkey inferior pulvinar s. Similarly, WisO 2 reports projections to the ventral posterior nucleus originating within layer 5 of SI and layer 6 of SI! in the rat. Finally, our observation of neurons in layers 2, 3, 4 and 5 projecting to contralateral cortex parallels earlier reports describing commissural connections originating within these
500 s a m e w i d e s p r e a d cortical layers in r a t a n d m o u s e somatic, visual o r a u d i t o r y c o r t e x 4,1'~, 13 T a k e n together, available evidence indicates a c o n s i d e r a b l e degree o f specificity in the l a m i n a r origin o f cortical efferents. Indeed, available evidence is consistent with the idea t h a t cortical-tectal p r o j e c t i o n s originate in layer 5, c o r t i c a l - t h a l a m i c p r o j e c t i o n s in either layers 5 or 6, a n d c o m m i s s u r a l c o n n e c t i o n s in layers 2, 3, 4 a n d 5. U n f o r t u n a t e l y , because these studies t y p i c a l l y e x a m i n e no m o r e t h a n the p r o j e c t i o n s f r o m a single cortical region for a n y given species, the question o f the l a m i n a r origin o f cortical p r o j e c t i o n s from different areas o f sensory c o r t e x for any single species remains unanswered.
We would like to express our thanks to Gloria Bathurst for typing this manuscript and to Jorene Rath for her help in preparing the illustration.
1 Desmedt, S. E., Physiological studies of the efferent recurrent auditory system. In W. D. Koid¢l and W. D. Neff (Eds.), Handbook o f Sensory Physiology: Auditory System, Vol. V/2, Springer, New York, 1975, pp. 219-246. 2 Diamond, I. T., Jones, E. G. and Poweil, T. P. S., The projection of the auditory cortex upon the diencephalon and brain stem in the cat, Brain Research, 15 (1969) 305-340. 3 Gilbert, L. D., and Kelly, J. P., The projections of cells in different layers of the cat's visual cortex, J. comp. Neurot., 163 (1975) 81"106. 4 Jacobsen, S. and Trojanowski, J. Q., The cells of origin of the corpus callosum in rat, cat and rhesus monkey, Brain Research, 74 (1974) 149"155. 5 Jones, D. R., Casseday, J. H. and Diamond, I. T., Further study of parallel auditory pathways in the tre¢ shrew, Tupaia glis, Anat. Rec., 184 (1976) 438-439. 6 Kemp, J, M. and Powelt, T: P. S., The cortico-striate projection in the monkey, Brain, 93 (1970) 525-546. 7 LaVail, J. H. and LaVail, M. M., The retrograde intraaxonal transport of horseradish peroxidase in the chick visual system: a light and electron microscopic study, J. comp. Neurol., 157 (1974) 303-357. 8 Lund, J. S., Lund, R. D., Hendrickson, A. E., Bunt, A. H. and Fuchs, A. F., The origin of efferent pathways from the primary visual cortex, area 17, of the macaque monkey as shown by retrograde transport of horseradish pcroxidase, J. comp. Neurol., 164 (1975) 287-304. 90tellin, V. A., Projections of the auditory cortex to the neostriatum, Neurosci. Behav. Physiol., 5 (1972) 267-274. 10 Robson, J. A. and Hall, W. C., Connections of layer VI in striate cortex of the grey squirrel (Sciurus carolinensis), Brain Research, 93 (1975) 133-139. 11 Webster, K. E., The cortico-striatal projection in the cat, J. Anat. (Lend.), 99 (1965) 329-337. 12 Wise, S P., The laminar organization ofcartain afferent and efferent fiber systems in the rat somato, sensory cortex, Brain Research, 90 (1975) 139-142. 13 Yorke, C. H. and Caviness, V. S., Interhemispheric neocortical connections of the corpus callosum in the normal mouse: a study based on anterograde and retrograde methods, J. comp. Neurol., 164 (1975) 233-246.