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The organization of the catecholamine innervation of somatosensory cortex: the barrel field of the mouse
Brain Research, 153 (1978) 577-584 '~ Elsevier/North-Holland Biomedical Press
577
Short Communications "['he organization of the catecholamine innervation of somatosensory cortex: the barrel field of the mouse
H. G. W. LIDOV, F. L. RICE and M. E. MOLLIVER Department of Cell Biology and Anatomy, Department of Neurology, The Johns Hopkins University School of Medicine, BMthnare, Md. 21205 (U.S.A.)
(Accepted April 27th, 1978)
The cerebral neocortex receives a widespread, direct, noradrenergic (NA) projection from the nucleus locus coeruleus4, e°. Formaldehyde-induced fluorescence reveals catecholamine (CA) axon terminals located primarily in layer I of cortex 4,8, whereas more sensitive techniques have recently demonstrated a dense NA innervation of deep as well as superficial cortical layers3,10,11,14. Precise determination of the distribution of CA axons in specific layers requires direct correlation with cytoarchitectonic features of cortex. However, the layers are not usually discernible with fluorescence microscopy and simultaneous Nissl counterstaining is not compatible with CA histofluorescence. Moreover, fields of fluorescent axons observed at high magnification are difficult to localize in sections stained subsequently for identification of cell layers. In previous studies the NA innervation of layer IV in lateral neocortex has been a topic of particular controversy: a sparse innervation by NA axons has been described by some authors 8,a°, while Morrison et al. 14 describe an abundance of D B H * immunoreactive axons in that layer. In immature rat neocortex, Lidov et al. H found numerous CA axons at the lower border of the cortical plate but criteria for the precise delineation of layer IV were lacking. As part of an effort to characterize monoamine projections to cortex, we have undertaken a study of the CA innervation of the barrels, conspicuous cytoarchitectural markers of layer IV in rodent somatosensory cortex 2~. In 8-day-old mice the barrels have attained their adult configuration 18 and in neocortex from rats at the same age, biochemical and histofluorescence markers for CA innervation approach adult levels2, 9. Thirty-five Swiss-Webster mice at this age were perfused with a glyoxylic acid solution 11 and the brains rapidly removed. In most cases, sections were cut with a Vibratome at a setting of 60/~m and p-ocessed for histofluorescenceH; a few brains were freeze-dried, embedded in paraffin, sectioned at 10/~m and then processed. The sections were taken either coronally or tangentially through the barrel field; in the latter case the brains were oriented with the aid of a specially designed guillotine iv. Following fluorescence * DBH: dopamine-fi-hydroxylase, the terminal enzyme in norepinephrine synthesis is a specific marker for NA neurons.
mmm
~J
579
Fig. 2. Tangential section of barrel field. Upper left contains larger posterior barrels (PMBSF) with rows indicated in accordance with ref. 21. Smaller anterior barrels occupy the center of the figure. Note absence of fluorescence in walls, and preponderence of axons in hollows.
microscopy, coverslips were removed and the sections were counterstained with toluidine blue. Sections were examined with a Leitz fluorescence microscope equipped with the Ploem incident illuminator (filters: AL405 and TK410 dichroic mirror; K470 barrier), an H B O 200 mercury lamp and 16 x and 25 × Zeiss immersion objectives. With this system o f illumination, both the catecholamine fibers and the barrels are visible simultaneously (Figs. 1-4). Barrel hollows emit a diffuse, green background fluorescence that is absent in the intervening walls. Septa, when seen, also have background fluorescence. That the barrel hollows are fluorescent and the walls non-fluorescent was confirmed by inspection at high magnification (625 × ) where cell nuclei can be discerned as discrete dark dots and barrel structure identified; additional confirmation was obtained by alternating between fluorescence and dark-field illumination. The background fluorescence was seen only in layer IV (also extending beyond the barrel field) but catecholamine fibers were densely distributed to layers 1, V and VI, as well as IV.
Fig. 1. Coronal section through SI cortex in the barrel region. White arrows indicate barrel walls in layer IV. Note relative paucity of fibers immediately above and below layer IV and conspicuous fluorescent axons in barrel hollows. Figs. 1-4 are fluorescence photomicrographs of unstained Vibratome sections.
Fig. 3. Details o f barrels in coronal sections. A : white arrows indicate barrel walls. Note dense tangles o f fluorescent fibers in hollows {black arrows). B: detail from A. Note tibers ramify ~tg extensively in hollows a n d sending branches over lhe top m a r g i n s o f barrel ,,vails (empty arro~a~
Fig. 4. Details of barrels in tangential sections. A and B: anterior barrels, C and D : posterior barrels. Note fibers running along walls at the perimeter of hollows (white arrows) as well as conspicuous nests of axons within hollows.
582 Thus, although the fluorescent substance is unidentified, we suspect that this background is a form of specific autofluorescence and does not result from diffusion of catecholamine. Spanning the cortex there is a rich network of varicose, fluorescent fibers that emit the blue-green color typical of catecholamine-derived fluorophore. In coronal sections, CA fibers form two distinct plexuses (Fig. l). Co-extensive with layer 1 is a superficial plexus comprised of fibers with a prevailing tangential orientation, A deep plexus extends from the upper limits of the barrels down to the white matter, i.e. coextensive with layers IV, V and VI, and is comprised of fibers criss-crossing in many different directions. In layers II and III, between the deep and superficial plexuses, there are relatively few CA fibers and their prevailing orientation is radial. The morphology of the CA fibers is not homogeneous across the cortex; in layer IV some axons are of larger caliber and more brilliant than those in other layers. In the upper part of layer V the density of CA fibers is markedly less than in eitherlayer IV, above, or deep layer V, below. The NA fibers in the barrel field are distributed in a discontinuous pattern that is in register with the barrels. Deep to the barrels, the catecholamine fibers have no apparent preferential distribution or orientation. As seen in both coronal (Figs. l and 3) and tangential (Figs. 2 and 4) sections, there is a highly ramified tangle of CA fibers in each barrel hollow and fibers frequently radiate within the hollows in a sunburstqike pattern (Fig. 4B). It is not uncommon to observe a tangentially oriented fiber running across a hollow and giving rise to collaterals on route: frequently one such fiber traverses more than one barrel, sending out ramifications in each hollow. Those fibers which cross from one barrel to another rarely branch within the walls. Other fibers are seen running adjacent to the wall at the perimeter of the hollowl CA fibers coursing within a wall, i.e. around the circumference of the barrel, are uncommon. The identity of the specific catecholamine responsible for the fluorescence of fibers in the barrel field has not been established. While histofluorescence alone does not distinguish between noradrenergic and dopaminergic fibers, the location of the barrel field, distant from those cortical areas innervated by dopaminergic fibers 1,12, suggests that the CA axons we have observed are noradrenergic. Previous studies of rat cortex have shown abundant catecholamine fibers in layers I, V, and VI and possibly in layer IV 3,10,11,14. In this study, taking advantage of the barrels as unequivocal, in situ markers of layer IV 25 that are visible with fluorescence microscopy, we found that layer IV, like the deeper cortical layers and layer I, contains a highly ramified plexus of tortuous CA axons that is suggestive of a terminal field. Ultrastructural studies to confirm that these axons do form a terminal field are in progress. A prior ultrastructural study demonstrated that in layer IV of immature rat cortex there is an extraordinarily high proportion (70~o) of synapses that are presumably monoaminergic 2,1z. The barrels provide a morphologic landmark by which the same Piece of cortex can be consistently located in a variety of preparations, hence it is possible to integrate several types of neurohistologic data derived from the same organizational unit of cortical circuitry. The components of the barrel field are segregated into two compartments: the cell-dense walls and the cell-poor hollows. The walls contain, in addition
583 to perikarya, bundles of apical dendrites 23, axons of passage 23, cortico-cortical axon terminals and their postsynaptic elements 15,16, as well as arterioles (unpublished observation). The hollows contain terminal arbors of the thalamo-cortical afferents and stellate cell dendrites on which they synapse 7,16,19/)4. Superimposed on this arrangement the distribution of the CA fibers suggests that they innervate the neuronal elements within the hollows, whereas their influence on the constituents of the walls is probably minimal. The overlap of the terminals of catecholamine and of thalamo-cortical afterents raises the possibility that both may be involved in the same local circuits and may even synapse upon the same dendrites. Similarly, an overlap between CA fibers and thalamocortical terminals 26,28 also occurs in layer I and deep V. In contrast, layers II, III, and superficial V receive primarily cortico-cortical inputs 26,27 and are sparsely innervated by CA and thalamo-cortical afferents. The close spatial association between locus coeruleus and thalamic afferents in at least 3 cortical laminae, indicates that an intimate functional relation between these two may be a general phenomenon in the neocortex. While in layer IV both types of afferents may actually converge on a common postsynaptic element, the heterogeneity of elements outside the barrel hollows makes extending this relationship to other layers hazardous. The presence of nests of CA fibers in the barrel hollows and some tangential fibers crossing between barrels suggests two plausible distributions of the CA system to the barrels. Each barrel may receive its own discrete NA afferents with some overlap, in a manner similar to the thalamic projection 6,21,22. This pattern of input would be consistent with a somatotopic organization of the NA system contrary to the present evidence from retrograde transport studies 3,5. Alternatively, the nests of NA fibers in individual barrels may be local elaborations of a continuous tangential projection traversing layer IV. This distribution would be a substrate for a tangential input, suggested by Morrison et al. 14, cutting across the predominantly radial organization of the cortex. This study was supported by U.S.P.H.S. Research Grants NS-08153 and NS10920 to M . E . M . H . G . W . L . is supported by Training Grant GM-7309 and F.L.R. by N.I.H. Grant RR-5338 and Fellowship NS-05790.
I Berger, B., Thierry, A. M., Tassin, J. P. and Moyne, M. A., Dopaminergic innervation of rat prefrontal cortex: a fluorescence histochemical study, Brain Research, 106 (1976) 133-145. 2 Coyle, J. T. and Molliver, M. E., Major innervation of newborn rat cortex by monoaminergic neurons, Science, 196 (1977) 444-447. 3 Freedman, R., Foote, S. L. and Bloom, F. E., Histochemical characterization of a neocortical projection of the nucleus locus coeruleus in the squirrel monkey, J. comp. NeuroL, 164 (1975) 209 232. 4 Fuxe, K., Hamberger, B. and H6kfelt, T., Distribution of noradrenaline nerve terminals in cortical areas of the rat, Brain Research, 8 (1968) 125-131. 5 Gatter, K. C. and Powell, T. P. S., The projection of the locus coeruleus upon the neocortex in the macaque monkey, Neuroscience, 2 (1977) 441~445. 6 Killackey, H. P., Belford, G., Ryugo, R. and Ryugo, D. K., Alterations in thalamocortical projections to the rat somatosensory cortex followingneonatal vibrissae removal. Neurosci. Abstr., I (1975) 125.
584 7 Killackey, H. P. and Leshin, S., The organization of specific thalamocortical projections to the posteromedial barrel subfield of the rat somatic sensory cortex, Brain Research, 86 (1975) 469-472. 8 Lapierre, Y., Beaudet, A., Demianczuk, N. and Descarries, L., Noradrenergic axon terminals in the cerebral cortex of rat. I!. quantitative data revealed by light and electron microscope radioautography of the frontal cortex, Brain Research, 63 (1973) 175 182. 9 Levitt, P. and Moore, R. Y., Development of the noradrenergic innervation of the neocortex in the rat, Neurosci. Abstr. 3 (1977). 10 Levitt, P. and Moore, R. Y , Noradrenaline neuron innervation of the neocortex in the rat, Brain Research, t39 (1978) 219-231. 1 I Lidov, H. G. W., Molliver, M. E. and Zecevic, N. R., Characterization of the monoaminergic innervation of developing rat neocortex: a histofluorescence analysis, J. comp. NeuroL, in press. 12 Lindvall, O., Bj6rklund, A., Moore, R. Y. and Stenevi, U., Mesencephalic dopamine neurons projecting to neocortex, Brain Research, 81 (1974) 325-331. 13 Molliver, M. E. and Kristt, D. A., The fine structural demonstration of monoaminergic synapses in immature rat neocortex, Neurosci. Lett., 1 (1975) 305 310. 14 Morrison, J. H., Gzanna, R., Coyle, J. T. and Molliver, M. E., The distribution and orientation of noradrenergic fibers in neocortex of the rat: an immunofluorescence study, J. comp. Neurol., (1978) in press. 15 Perentes, E., Analyse du Cortex Somatosensoriel (PMBSF; de la Souris; Ultrastructure des Bords et des Septa, Doctoral Thesis, institut d'Anatomie Universit6 de Lausanne, Lausanne, Switzerland, 1977. 16 Petrovicky, P, and Druga, R., Peculiarities of the cytoarchitectonics and some afferent systems o1 the parietal cortex, Folia morph., 20 (1972) 161-162. 17 Rice, F. L. and Anders, D . , A small guillotine to prepare brains for sectioning in defined, odd planes, Neurosci. Lett., 6 (1977) 157-163. 18 Rice, F. L. and Van der Loos, H., Development of the barrels and barrel field in the somatosensory cortex of the mouse, J. comp. Neurol., 171 (1977) 545-560. 19 Steffen, H., Golgi-stained barrel-neurons in the somatosensory region of the mouse cerebral cortex, Neurosci. Lett., 2 (1976) 57-59. 20 Ungerstedt, U., Stereotaxic mapping of the monoamine pathways in the rat brain, ,4cta physiol. scand., Suppl. 367 (1971) 1-48. 2l VanderLoos, H. and Woolsey, T.A.,Somatosensorycortex:structural alterations following early injury to sense organs, Science, 179 (1973) 395-398. 22 Welker, C., Microelectrode delineation of fine grain somatotopic organization of Sml cerebral neocortex in albino rat, Brain Research, 26 (1971) 259-275. 23 White, E. L., Ultrastructure and synaptic contacts in barrels of mouse SI cortex, Brain Research, 105 (1976) 229-251. 24 Woolsey, T. A., Dierker, M. L. and Wann, D. F., Mouse Sml cortex : qualitative and quantitative classification ofgolgi-impregnated barrel neurons, Proc. nat. Acad. Sei. (Wash.), 72 (1975) 21652169. 25 Woolsey, T. A. and Van der Loos, H., The structural organization of layer 1V in the somatosensory region (S1) of mouse cerebral cortex. The description of a cortical field composed of discrete cytoarchitectonic units, Brain Research, 17 (1970) 205 242. 26 Wise, S. P., The laminar organization of certain afferent and efferent fiber systems in the rat somatosensory cortex, Brain Research, 90 (1975) 139-142. 27 Wise, S. P. and Jones, E. G., The organization and postnatal development of the commissural projection of the somatic sensory cortex of the rat, J. comp. Neurol., 168 (1976) 313.-343. 28 Wise, S. P. and Jones, E. G., Developmental studies of thalamocortical and commissural connections in the rat somatic sensory cortex, J. comp. Neurol., 178 (1978) 187-208.
577
Short Communications "['he organization of the catecholamine innervation of somatosensory cortex: the barrel field of the mouse
H. G. W. LIDOV, F. L. RICE and M. E. MOLLIVER Department of Cell Biology and Anatomy, Department of Neurology, The Johns Hopkins University School of Medicine, BMthnare, Md. 21205 (U.S.A.)
(Accepted April 27th, 1978)
The cerebral neocortex receives a widespread, direct, noradrenergic (NA) projection from the nucleus locus coeruleus4, e°. Formaldehyde-induced fluorescence reveals catecholamine (CA) axon terminals located primarily in layer I of cortex 4,8, whereas more sensitive techniques have recently demonstrated a dense NA innervation of deep as well as superficial cortical layers3,10,11,14. Precise determination of the distribution of CA axons in specific layers requires direct correlation with cytoarchitectonic features of cortex. However, the layers are not usually discernible with fluorescence microscopy and simultaneous Nissl counterstaining is not compatible with CA histofluorescence. Moreover, fields of fluorescent axons observed at high magnification are difficult to localize in sections stained subsequently for identification of cell layers. In previous studies the NA innervation of layer IV in lateral neocortex has been a topic of particular controversy: a sparse innervation by NA axons has been described by some authors 8,a°, while Morrison et al. 14 describe an abundance of D B H * immunoreactive axons in that layer. In immature rat neocortex, Lidov et al. H found numerous CA axons at the lower border of the cortical plate but criteria for the precise delineation of layer IV were lacking. As part of an effort to characterize monoamine projections to cortex, we have undertaken a study of the CA innervation of the barrels, conspicuous cytoarchitectural markers of layer IV in rodent somatosensory cortex 2~. In 8-day-old mice the barrels have attained their adult configuration 18 and in neocortex from rats at the same age, biochemical and histofluorescence markers for CA innervation approach adult levels2, 9. Thirty-five Swiss-Webster mice at this age were perfused with a glyoxylic acid solution 11 and the brains rapidly removed. In most cases, sections were cut with a Vibratome at a setting of 60/~m and p-ocessed for histofluorescenceH; a few brains were freeze-dried, embedded in paraffin, sectioned at 10/~m and then processed. The sections were taken either coronally or tangentially through the barrel field; in the latter case the brains were oriented with the aid of a specially designed guillotine iv. Following fluorescence * DBH: dopamine-fi-hydroxylase, the terminal enzyme in norepinephrine synthesis is a specific marker for NA neurons.
mmm
~J
579
Fig. 2. Tangential section of barrel field. Upper left contains larger posterior barrels (PMBSF) with rows indicated in accordance with ref. 21. Smaller anterior barrels occupy the center of the figure. Note absence of fluorescence in walls, and preponderence of axons in hollows.
microscopy, coverslips were removed and the sections were counterstained with toluidine blue. Sections were examined with a Leitz fluorescence microscope equipped with the Ploem incident illuminator (filters: AL405 and TK410 dichroic mirror; K470 barrier), an H B O 200 mercury lamp and 16 x and 25 × Zeiss immersion objectives. With this system o f illumination, both the catecholamine fibers and the barrels are visible simultaneously (Figs. 1-4). Barrel hollows emit a diffuse, green background fluorescence that is absent in the intervening walls. Septa, when seen, also have background fluorescence. That the barrel hollows are fluorescent and the walls non-fluorescent was confirmed by inspection at high magnification (625 × ) where cell nuclei can be discerned as discrete dark dots and barrel structure identified; additional confirmation was obtained by alternating between fluorescence and dark-field illumination. The background fluorescence was seen only in layer IV (also extending beyond the barrel field) but catecholamine fibers were densely distributed to layers 1, V and VI, as well as IV.
Fig. 1. Coronal section through SI cortex in the barrel region. White arrows indicate barrel walls in layer IV. Note relative paucity of fibers immediately above and below layer IV and conspicuous fluorescent axons in barrel hollows. Figs. 1-4 are fluorescence photomicrographs of unstained Vibratome sections.
Fig. 3. Details o f barrels in coronal sections. A : white arrows indicate barrel walls. Note dense tangles o f fluorescent fibers in hollows {black arrows). B: detail from A. Note tibers ramify ~tg extensively in hollows a n d sending branches over lhe top m a r g i n s o f barrel ,,vails (empty arro~a~
Fig. 4. Details of barrels in tangential sections. A and B: anterior barrels, C and D : posterior barrels. Note fibers running along walls at the perimeter of hollows (white arrows) as well as conspicuous nests of axons within hollows.
582 Thus, although the fluorescent substance is unidentified, we suspect that this background is a form of specific autofluorescence and does not result from diffusion of catecholamine. Spanning the cortex there is a rich network of varicose, fluorescent fibers that emit the blue-green color typical of catecholamine-derived fluorophore. In coronal sections, CA fibers form two distinct plexuses (Fig. l). Co-extensive with layer 1 is a superficial plexus comprised of fibers with a prevailing tangential orientation, A deep plexus extends from the upper limits of the barrels down to the white matter, i.e. coextensive with layers IV, V and VI, and is comprised of fibers criss-crossing in many different directions. In layers II and III, between the deep and superficial plexuses, there are relatively few CA fibers and their prevailing orientation is radial. The morphology of the CA fibers is not homogeneous across the cortex; in layer IV some axons are of larger caliber and more brilliant than those in other layers. In the upper part of layer V the density of CA fibers is markedly less than in eitherlayer IV, above, or deep layer V, below. The NA fibers in the barrel field are distributed in a discontinuous pattern that is in register with the barrels. Deep to the barrels, the catecholamine fibers have no apparent preferential distribution or orientation. As seen in both coronal (Figs. l and 3) and tangential (Figs. 2 and 4) sections, there is a highly ramified tangle of CA fibers in each barrel hollow and fibers frequently radiate within the hollows in a sunburstqike pattern (Fig. 4B). It is not uncommon to observe a tangentially oriented fiber running across a hollow and giving rise to collaterals on route: frequently one such fiber traverses more than one barrel, sending out ramifications in each hollow. Those fibers which cross from one barrel to another rarely branch within the walls. Other fibers are seen running adjacent to the wall at the perimeter of the hollowl CA fibers coursing within a wall, i.e. around the circumference of the barrel, are uncommon. The identity of the specific catecholamine responsible for the fluorescence of fibers in the barrel field has not been established. While histofluorescence alone does not distinguish between noradrenergic and dopaminergic fibers, the location of the barrel field, distant from those cortical areas innervated by dopaminergic fibers 1,12, suggests that the CA axons we have observed are noradrenergic. Previous studies of rat cortex have shown abundant catecholamine fibers in layers I, V, and VI and possibly in layer IV 3,10,11,14. In this study, taking advantage of the barrels as unequivocal, in situ markers of layer IV 25 that are visible with fluorescence microscopy, we found that layer IV, like the deeper cortical layers and layer I, contains a highly ramified plexus of tortuous CA axons that is suggestive of a terminal field. Ultrastructural studies to confirm that these axons do form a terminal field are in progress. A prior ultrastructural study demonstrated that in layer IV of immature rat cortex there is an extraordinarily high proportion (70~o) of synapses that are presumably monoaminergic 2,1z. The barrels provide a morphologic landmark by which the same Piece of cortex can be consistently located in a variety of preparations, hence it is possible to integrate several types of neurohistologic data derived from the same organizational unit of cortical circuitry. The components of the barrel field are segregated into two compartments: the cell-dense walls and the cell-poor hollows. The walls contain, in addition
583 to perikarya, bundles of apical dendrites 23, axons of passage 23, cortico-cortical axon terminals and their postsynaptic elements 15,16, as well as arterioles (unpublished observation). The hollows contain terminal arbors of the thalamo-cortical afferents and stellate cell dendrites on which they synapse 7,16,19/)4. Superimposed on this arrangement the distribution of the CA fibers suggests that they innervate the neuronal elements within the hollows, whereas their influence on the constituents of the walls is probably minimal. The overlap of the terminals of catecholamine and of thalamo-cortical afterents raises the possibility that both may be involved in the same local circuits and may even synapse upon the same dendrites. Similarly, an overlap between CA fibers and thalamocortical terminals 26,28 also occurs in layer I and deep V. In contrast, layers II, III, and superficial V receive primarily cortico-cortical inputs 26,27 and are sparsely innervated by CA and thalamo-cortical afferents. The close spatial association between locus coeruleus and thalamic afferents in at least 3 cortical laminae, indicates that an intimate functional relation between these two may be a general phenomenon in the neocortex. While in layer IV both types of afferents may actually converge on a common postsynaptic element, the heterogeneity of elements outside the barrel hollows makes extending this relationship to other layers hazardous. The presence of nests of CA fibers in the barrel hollows and some tangential fibers crossing between barrels suggests two plausible distributions of the CA system to the barrels. Each barrel may receive its own discrete NA afferents with some overlap, in a manner similar to the thalamic projection 6,21,22. This pattern of input would be consistent with a somatotopic organization of the NA system contrary to the present evidence from retrograde transport studies 3,5. Alternatively, the nests of NA fibers in individual barrels may be local elaborations of a continuous tangential projection traversing layer IV. This distribution would be a substrate for a tangential input, suggested by Morrison et al. 14, cutting across the predominantly radial organization of the cortex. This study was supported by U.S.P.H.S. Research Grants NS-08153 and NS10920 to M . E . M . H . G . W . L . is supported by Training Grant GM-7309 and F.L.R. by N.I.H. Grant RR-5338 and Fellowship NS-05790.
I Berger, B., Thierry, A. M., Tassin, J. P. and Moyne, M. A., Dopaminergic innervation of rat prefrontal cortex: a fluorescence histochemical study, Brain Research, 106 (1976) 133-145. 2 Coyle, J. T. and Molliver, M. E., Major innervation of newborn rat cortex by monoaminergic neurons, Science, 196 (1977) 444-447. 3 Freedman, R., Foote, S. L. and Bloom, F. E., Histochemical characterization of a neocortical projection of the nucleus locus coeruleus in the squirrel monkey, J. comp. NeuroL, 164 (1975) 209 232. 4 Fuxe, K., Hamberger, B. and H6kfelt, T., Distribution of noradrenaline nerve terminals in cortical areas of the rat, Brain Research, 8 (1968) 125-131. 5 Gatter, K. C. and Powell, T. P. S., The projection of the locus coeruleus upon the neocortex in the macaque monkey, Neuroscience, 2 (1977) 441~445. 6 Killackey, H. P., Belford, G., Ryugo, R. and Ryugo, D. K., Alterations in thalamocortical projections to the rat somatosensory cortex followingneonatal vibrissae removal. Neurosci. Abstr., I (1975) 125.
584 7 Killackey, H. P. and Leshin, S., The organization of specific thalamocortical projections to the posteromedial barrel subfield of the rat somatic sensory cortex, Brain Research, 86 (1975) 469-472. 8 Lapierre, Y., Beaudet, A., Demianczuk, N. and Descarries, L., Noradrenergic axon terminals in the cerebral cortex of rat. I!. quantitative data revealed by light and electron microscope radioautography of the frontal cortex, Brain Research, 63 (1973) 175 182. 9 Levitt, P. and Moore, R. Y., Development of the noradrenergic innervation of the neocortex in the rat, Neurosci. Abstr. 3 (1977). 10 Levitt, P. and Moore, R. Y , Noradrenaline neuron innervation of the neocortex in the rat, Brain Research, t39 (1978) 219-231. 1 I Lidov, H. G. W., Molliver, M. E. and Zecevic, N. R., Characterization of the monoaminergic innervation of developing rat neocortex: a histofluorescence analysis, J. comp. NeuroL, in press. 12 Lindvall, O., Bj6rklund, A., Moore, R. Y. and Stenevi, U., Mesencephalic dopamine neurons projecting to neocortex, Brain Research, 81 (1974) 325-331. 13 Molliver, M. E. and Kristt, D. A., The fine structural demonstration of monoaminergic synapses in immature rat neocortex, Neurosci. Lett., 1 (1975) 305 310. 14 Morrison, J. H., Gzanna, R., Coyle, J. T. and Molliver, M. E., The distribution and orientation of noradrenergic fibers in neocortex of the rat: an immunofluorescence study, J. comp. Neurol., (1978) in press. 15 Perentes, E., Analyse du Cortex Somatosensoriel (PMBSF; de la Souris; Ultrastructure des Bords et des Septa, Doctoral Thesis, institut d'Anatomie Universit6 de Lausanne, Lausanne, Switzerland, 1977. 16 Petrovicky, P, and Druga, R., Peculiarities of the cytoarchitectonics and some afferent systems o1 the parietal cortex, Folia morph., 20 (1972) 161-162. 17 Rice, F. L. and Anders, D . , A small guillotine to prepare brains for sectioning in defined, odd planes, Neurosci. Lett., 6 (1977) 157-163. 18 Rice, F. L. and Van der Loos, H., Development of the barrels and barrel field in the somatosensory cortex of the mouse, J. comp. Neurol., 171 (1977) 545-560. 19 Steffen, H., Golgi-stained barrel-neurons in the somatosensory region of the mouse cerebral cortex, Neurosci. Lett., 2 (1976) 57-59. 20 Ungerstedt, U., Stereotaxic mapping of the monoamine pathways in the rat brain, ,4cta physiol. scand., Suppl. 367 (1971) 1-48. 2l VanderLoos, H. and Woolsey, T.A.,Somatosensorycortex:structural alterations following early injury to sense organs, Science, 179 (1973) 395-398. 22 Welker, C., Microelectrode delineation of fine grain somatotopic organization of Sml cerebral neocortex in albino rat, Brain Research, 26 (1971) 259-275. 23 White, E. L., Ultrastructure and synaptic contacts in barrels of mouse SI cortex, Brain Research, 105 (1976) 229-251. 24 Woolsey, T. A., Dierker, M. L. and Wann, D. F., Mouse Sml cortex : qualitative and quantitative classification ofgolgi-impregnated barrel neurons, Proc. nat. Acad. Sei. (Wash.), 72 (1975) 21652169. 25 Woolsey, T. A. and Van der Loos, H., The structural organization of layer 1V in the somatosensory region (S1) of mouse cerebral cortex. The description of a cortical field composed of discrete cytoarchitectonic units, Brain Research, 17 (1970) 205 242. 26 Wise, S. P., The laminar organization of certain afferent and efferent fiber systems in the rat somatosensory cortex, Brain Research, 90 (1975) 139-142. 27 Wise, S. P. and Jones, E. G., The organization and postnatal development of the commissural projection of the somatic sensory cortex of the rat, J. comp. Neurol., 168 (1976) 313.-343. 28 Wise, S. P. and Jones, E. G., Developmental studies of thalamocortical and commissural connections in the rat somatic sensory cortex, J. comp. Neurol., 178 (1978) 187-208.