Corticostriate projections from area 6 in the raccoon

Summary Corticostriate projections from area 6 in the raccoon were studied using the autoradiographic tracing method. Following injections of tritiated amino acids into two different cytoarchitectonic subdivisions of area 6, widespread and dense anterograde label was found in both the ipsilateral and contralateral caudate nucleus and putamen. The densest label was located adjacent to the internal capsule in the lateral part of the head of the caudate nucleus. This bilateral projection pattern from area 6 to the caudate nucleus and putamen is consistent with the hypothesis that the neostriatum maintains a close anatomical and functional relationship with area 6.     

Topographic segregation of corticostriatal projections from posterior parietal subdivisions in the macaque monkey.

The distribution of corticostriatal projections from areas 7m, 7a, 7b and 7ip of the posterior parietal cortex was studied in rhesus monkeys using horseradish peroxidase conjugated with wheat-germ agglutinin as an anterograde tracer. All parietal subdivisions project bilaterally over a broad anteroposterior expanse of the caudate nucleus and putamen; however, the zones of densest terminal labeling varied for each parietal subdivision. Thus, area 7m projects preferentially to dorsal and dorsolateral portions of the head and anterior part of the body of the caudate nucleus. The main striatal target of area 7a is also in the head and anterior portion of the body of the caudate nucleus, but at dorsal and dorsomedial zones. The preferential target region of area 7ip in the striatum is in the posterior two-thirds of the body of the caudate nucleus, where the labeled terminals spare only the medial border. In contrast to the other parietal subdivisions, 7b projects preferentially to the putamen. In this nucleus, the location of labeling after 7b injections appears to correspond to the zones containing the representations of the distal forelimb and head. Each parietal subdivision projects to a rather extended anteroposterior domain in the contralateral neostriatum, the projection zones being always less extensive than in the ipsilateral side, but with a similar topographic distribution. Because we have shown previously that each parietal subdivision is part of a distinct distributed corticocortical network, the neostriatal territories innervated by each subdivision can be correlated with the corresponding network, thus providing insight into the functional specializations of the striatum.     

The corticostriatal and corticotectal projections of the feline lateral suprasylvian cortex demonstrated with anterograde biocytin and retrograde fluorescent techniques.

The relationship between the visual cortex and the striatum (ST) of the cat is poorly understood. The present experiments were an attempt to determine if regions along the lateral suprasylvian cortex (LS), known to send dense visual projections to the superior colliculus (SC), also project to the striatum and, if so, to determine whether corticostriatal and corticotectal axons arise from the same neurons. Injections of the anterograde tracer, biocytin, into the posterior portion of the lateral suprasylvian cortex resulted in dense label in both ST and SC. In ST, labeled fibers and terminals were found predominantly in the caudal part of the head of the ipsilateral caudate nucleus and the caudal portion of the ipsilateral putamen. These injections also resulted in label in the superficial and deep laminae of SC. After paired injections of retrogradely transported fluorescent dyes (dextran tetramethylrhodamine and dextran fluorescein) into ST and SC, numerous labeled LS neurons were observed in layer V and modest numbers in layer III: the corticostriatal neurons were found in layers III and V whereas corticotectal neurons were seen only in layer V. Although labeled neurons from each injection were intermingled in layer V, very few of them were double-labeled. These data suggest that while ST and SC receive substantial visual inputs from the same cortical area, the nature of the information they receive may be quite different.     

The cingulate bridge between allocortex,isocortex and thalamus

The Fink-Heimer silver impregnation and the autoradiographic methods were used to study the fiber projections of the cingulate cortex in the squirrel monkey. It was found that this cortex provides inputs to the striatum, thalamus and several areas of isocortex. Evidence was found for a number of fiber projections (1) Fibers from the anterior limbic area were traced to the central part of the head of the caudate nucleus, putamen, septum, dorsomedial nucleus of the thalamus, anterior hypothalamus and lateral basal nucleus of the amygdala. (2) Projections from the cingulate area were traced to the lateral part of the head of the caudate nucleus, putamen, and to the centromedian, anterior, lateral dorsal, and lateral ventral thalamic nuclei and to medial nuclei of the base of the pons. (3) There were projections from the retrosplenial area of the anterior, lateral dorsal, dorsomedial, and posterior thalamic nuclei and lateral nuclei of the pons. These results indicate that most of the cingulate gyrus is an intermediate structure between the thalamus and overlying cortex. The anterior limbic area forms a bridge between the thalamus and other areas of the cingulate gyrus and the frontal cortex. (4) The retrosplenial area and the posterior part of the cingulate area bridge the adjacent visual sensory association cortex and pelvic areas of the sensory motor cortex, respectively. These areas of the cingulate gyrus project directly to the striatum as well as to the thalamus, structurally providing limbic system input to subcortical motor structures.     

Efferent connections of area 20 in the cat: HRP-WGA and autoradiographic studies

Summary Following injections of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) and tritiated leucine into area 20 of the cat, terminal labeling was observed in visual areas 19, 21, the splenial visual area, the lateral suprasylvian area as well as in premotor, association and limbic related cerebral cortical regions. Labeled terminals in the subcortex were distributed in the caudate nucleus, the claustrum, the putamen, the anterior ventral nucleus, the intralaminar nuclei, the caudal division of the intermediate lateral nucleus, the lateralis posterior-pulvinar complex, the parvocellular C laminae of the dorsal lateral geniculate nucleus and the ventral lateral geniculate nucleus. In HRP-WGA preparations, retrogradely labeled somata were observed in these regions with the exception of certain subcortical structures. The projections are discussed with respect to the possible role area 20 plays in the cortical control of pupillary constriction.     

Topographical organization of the projections from the cerebral cortex to the head of the caudate nucleus. A horseradish peroxidase study in the cat

The topographical organization of the projections from the cerebral cortex to the head of the caudate nucleus was studied in the cat using the horseradish peroxidase method. Various amounts of horseradish peroxidase were injected into several sites of the head portion of the caudate nucleus at about the frontal level where its cross section was widest. Injections of small amounts of horseradish peroxidase retrogradely labeled neurons in rather limited cortical areas bilaterally, showing the localized organization of the projections. Neurons in the lateral portions of the ventral bank of the cruciate sulcus and in the dorsal bank (areas 4 gamma and 4 delta) were labeled after horseradish peroxidase injections into the dorsolateral part of the head of the caudate nucleus. Neurons in the intermediate portions of the ventral bank (areas 6 a delta and 6 infra fundum) were strongly labeled after dorsolateral or ventrointermediate injections, and neurons in the medial portion (area 6a beta), after dorsomedial, dorsointermediate, ventrointermediate or central injections. These findings indicate that areas 4 gamma and 4 delta project to the dorsolateral part of the caudate nucleus, areas 6a delta and 6 infra fundum to the lateral half, and area 6a beta to a more medial portion. Other findings revealed that the gyrus proreus projects to the medial part of the caudate nucleus and the anterior cingulate gyrus to the dorsal region.     

The cingulate bridge between allocortex, isocortex and thalamus.

The Fink-Heimer silver impregnation and the autoradiographic methods were used to study the fiber projections of the cingulate cortex in the squirrel monkey. It was found that this cortex provides inputs to the straitum, thalamus and several areas of isocortex. Evidence was found for a number of fiber projections (1) Fibers from the anterior limbic area were traced to the central part of the head of the caudate nucleus, putamen, septum, dorsomedial nucleus of the thalamus, anterior hypothalamus and lateral basal nucleus of the amygdala. (2) Projections from the cingulate area were traced to the lateral part of the head of the caudate nucleus, putamen, and to the centromedian, anterior, lateral dorsal, and lateral ventral thalamic nuclei and to medial nuclei of the base of the pons. (3) There were porjections from the retrosplenial area of the anterior, lateral dorsal, dorsomedial, and posterior thalamic nuclei and lateral nuclei of the pons. These results indicate that most of the cingulate gyrus is an intermediate structure between the thalamus and overlying cortex. The anterior limbic area forms a bridge between the thalamus and other areas of the cingulate gyrus and the frontal cortex. (4) the retrosplenial area and the posterior part of the cingulate area bridge the adjacent visual snesory association cortex and pelvic areas of the snesory motor cortex, respectively. These areas of the cingulate gyrus project directly to the striatum as well as to the thalamus, structurally providing limbic system input to subcortical motor structures.     

Topographic projections from the basal ganglia to the nucleus tegmenti pedunculopontinus pars compacta of the cat with special reference to pallidal projections

Summary Projections from the basal ganglia to the nucleus tegmenti pedunculopontinus pars compacta (TPC) were studied by using anterograde and retrograde tracing techniques with horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) in the cat. Following WGA-HRP injections into the medial TPC area, a substantial number of retrogradely labeled cells were seen in the entopeduncular nucleus (EP) and medial half of the substantia nigra pars reticulata (SNr), whereas following WGA-HRP injections into the lateral TPC area, labeled cells were marked in the caudal half of the globus pallidus (GP) and lateral half of the SNr. To confirm the retrograde tracing study, WGA-HRP was injected into the EP or the caudal GP, and anterograde labeling was observed in the TPC areas. Terminal labeling was located in the medail TPC area in the EP injection case, while terminal labeling was observed in the lateral TPC area in the caudal GP injection case. Projections from the striatum to the pallidal complex (the EP and the caudal GP) were also studied autoradiographically by injecting amino acids into various parts of the caudate nucleus and the putamen. Terminal labeling was distributed over the whole extent of the EP and the rostral GP following injections into the rostral striatum (the head of the caudate nucleus or the rostral part of the putamen), while terminal labeling was distributed over the caudal GP following injections into the caudal striatum (the body of the caudate nucleus or the caudal part of the putamen). From these findings, we conclude that there exists a medio-lateral topography in the projection from the basal ganglia to the TPC: The EP receives afferent projections from the rostral striatum and projects to the medial TPC area, whereas the caudal GP receives projections from the caudal striatum and sends fibers to the lateral TPC area.Abbreviations BC brachium conjunctivum - CD caudate nucleus - CP cerebral peduncle - DBC decussation of the brachium conjunctivum - EP entopeduncular nucleus - GP globus pallidus - IC internal capsule - ICo inferior colliculus - LH lateral habenular nucleus - ML medial lemniscus - PN pontine nuclei - PUT putamen - SCo superior colliculus - SI substantia innominata - SN substantia nigra - SNc substantia nigra pars compacta - SNr substantia nigra pars reticulata - STN subthalamic nucleus - TH thalamus - TPC nucleus tegmenti pedunculopontinus pars compacta     

Auditory corticocortical interconnections in the cat: evidence for parallel and hierarchical arrangement of the auditory cortical areas

Summary The origin and laminar arrangement of the homolateral and callosal projections to the anterior (AAF), primary (AI), posterior (PAF) and secondary (AII) auditory cortical areas were studied in the cat by means of electrophysiological recording and WGA-HRP tracing techniques. The transcallosal projections to AAF, AI, PAF and AII were principally homotypic since the major source of input was their corresponding area in the contralateral cortex. Heterotypic transcallosal projections to AAF and AI were seen, originating from the contralateral AI and AAF, respectively. PAF received heterotypic commissural projections from the opposite ventroposterior auditory cortical field (VPAF). Heterotypic callosal inputs to AII were rare, originating from AAF and AI. The neurons of origin of the transcallosal connections were located mainly in layers II and III (70–92%), and less frequently in deep layers (V and VI, 8–30%). Single unit recordings provided evidence that both homotypic and heterotypic transcallosal projections connect corresponding frequency regions of the two hemispheres. The regional distribution of the anterogradely labeled terminals indicated that the homotypic and heterotypic auditory transcallosal projections are reciprocal. The present data suggest that the transcallosal auditory interconnections are segregated in 3 major parallel components (AAF-AI, PAF-VPAF and AII), maintaining a segregation between parallel functional channels already established for the thalamocortical auditory interconnections. For the intrahemispheric connections, the analysis of the retrograde tracing data revealed that AAF and AI receive projections from the homolateral cortical areas PAF, VPAF and AII, whose neurons of origin were located mainly in their deep (V and VI) cortical layers. The reciprocal interconnections between the homolateral AAF and AI did not show a preferential laminar arrangement since the neurons of origin were distributed almost evenly in both superficial (II and III) and deep (V and VI) cortical layers. On the contrary, PAF received inputs from the homolateral cortical fields AAF, AI, AII and VPAF, originating predominantly from their superficial (II and III) layers. The homolateral projections reaching AII originated mainly from the superficial layers of AAF and AI, but from the deep layers of VPAF and PAF. The laminar distribution of anterogradely labeled terminal fields, when they were dense enough for a confident identification, was systematically related to the laminar arrangement of neurons of origin of the reciprocal projection: a projection originating from deep layers was associated with a reciprocal projection terminating mainly in layer IV, whereas a projection originating from superficial layers was associated with a reciprocal projection terminating predominantly outside layer IV. This laminar distribution indicates that the homolateral auditory cortical interconnections have a feed-forward/feed-back organization, corresponding to a hierarchical arrangement of the auditory cortical areas, according to criteria previously established in the visual system of primates. The principal auditory cortical areas could be ranked into 4 distinct hierarchical levels. The tonotopically organized areas AAF and AI represent the lowest level. The second level corresponds to the non-tonotopically organized area AII. Higher, the tonotopically organized areas VPAF and PAF occupy the third and fourth hierarchical levels, respectively.Abbreviations AAF anterior auditory cortical area - AI primary auditory cortical area - AII secondary auditory cortical area - BF best frequency - C cerebral cortex - CA caudate nucleus - CL claustrum - D dorsal nucleus of the dorsal division of the MGB - ea anterior ectosylvian sulcus - ep posterior ectosylviansulcus - IC internal capsule - LGN lateral geniculate nucleus - LV pars lateralis of the ventral division of the MGB - LVe lateral ventricule - M pars magnocellularis of the medial division of the MGB - MGB medial geniculate body - MGBv ventral division of the MGB - OT optic tract - OV pars ovoidea of the ventral division of the MGB - PAF posterior auditory cortical area - PH parahippocampal cortex - PO lateral part of the posterior group of thalamic nuclei - PU putamen - RE reticular complex of thalamus - rs rhinal sulcus - SG suprageniculate nucleus of the dorsal division of the MGB - ss suprasylvian sulcus - TMB tetrametylbenzidine - VBX ventrobasal complex - VLa ventrolateral complex - VL ventro-lateral nucleus of the ventral division of the MGB - WGA-HRP wheat germ agglutinin conjugated to horse-radish peroxidase - WM white matter - VPAF ventro-posterior auditory cortical area     

A comparison of the behavioural effects of embryonic nigral grafts in the caudate nucleus and in the putamen of marmosets with unilateral 6-OHDA lesions

The behaviour of marmosets with unilateral 6-hydroxydopamine lesions of the nigrostriatal bundle and grafts of embryonic mesencephalon in either the caudate nucleus or the putamen was compared with that of lesion-alone and unoperated controls. The grafts comprised injections of cell suspensions prepared from marmoset ventral mesencephalon (i.e. allografts) targeted at four sites either entirely within the caudate nucleus or entirely within the putamen. Behavioural tests, including measures of amphetamine-induced rotation, neglect and use of each arm to retrieve food from inside tubes, were given before and after the 6-hydroxydopamine lesion and at regular intervals for 6 months after transplantation surgery. Grafts in the caudate nucleus reduced the ipsilateral rotation induced by amphetamine, whereas grafts in the putamen did not. Despite the absence of an effect on rotation, the putamen grafts were effective in reducing lesion-induced deficits on the task in which the marmosets were required to reach into tubes. In this latter task, the caudate grafts were also effective when the monkeys were given a free choice of which hand to use. However, when constrained to use the hand contralateral to the lesion and graft, the performance of the marmosets with caudate grafts was not significantly improved compared with that of lesion-alone controls. Neither the grafts in the caudate nucleus nor the grafts in the putamen abolished the contralateral somatosensory neglect induced by the lesion, although there was a trend for the marmosets with putamen grafts to contact the label on the contralateral side more quickly than those with caudate grafts or the lesion-alone controls. These results demonstrate that the location of embryonic nigral grafts within the primate striatum influences the profile of functional recovery.     

The organization of the efferent projections of the pedunculopontine tegmental nucleus to the striatum of the dog brain

Using the method of retrograde axonal transport, the detailed study was performed to demonstrate the efferent projections of the individual substructures of the pedunculopontine tegmental midbrain nucleus to the functionally different segments of the striatal structures in the dog. The compact and diffuse parts of this nucleus were found to project to the segments of the putamen, caudate nucleus and nucleus accumbens which belong to both limbic and motor systems. The medial region of the diffuse part adjacent to the decussation of superior cerebellar peduncles (presumably midbrain extrapyramidal area), projected only to the dorsal segments of the caudate nucleus and the putamen belonging to motor system.     

Neocortical connections in fetal cats

The major extrinsic projections to and from visual and auditory areas of cerebral cortex were examined in fetal cats between 46 and 60 days of gestation (E46-E60) using axonal transport of horseradish peroxidase either alone or in combination with tritiated proline. Projections to visual cortex from the dorsal lateral geniculate nucleus and lateral-posterior/pulvinar complex exist by E46, and those from the contralateral hemisphere, claustrum, putamen, and central lateral nucleus of the thalamus are present by E54-E56. In addition, cells in the medial geniculate nucleus project to auditory cortex by E55. At E54-E56 efferent cortical projections reach the contralateral hemisphere, claustrum, putamen, lateral-posterior/pulvinar complex and reticular nucleus of the thalamus. Cells in visual cortex also project to the dorsal and ventral lateral geniculate nuclei, pretectum, superior colliculus and pontine nuclei, and cells in auditory cortex project to the medial geniculate nucleus. Except for interhemispheric projections, all pathways demonstrated are ipsilateral, and projections linking cerebral cortex with claustrum, dorsal lateral geniculate nucleus and lateral-posterior/pulvinar complex are reciprocal. The reciprocal projections formed with the dorsal lateral geniculate nucleus, lateral-posterior/pulvinar complex and the claustrum show a greater degree of topological organization compared to the projections formed with the contralateral hemisphere and superior colliculus, which show little or no topological order. Therefore, the results of the present study show that the major extrinsic projections of the cat's visual and auditory cortical areas with subcortical structures are present by the eighth week of gestation, and that the origins and terminations of many of these projections are arranged topologically.     

Projections from the visual cortex to the contralateral claustrum of the cat revealed by an anterograde axonal transport method

Contralateral projections from visual areas 17, 18, 19 and the Clare-Bishop area of the cerebral cortex to the claustrum have been investigated in the cat using intracortical injections of [3H]proline. Radioactive material was found in a dorsocaudal region of the contralateral claustrum. This region was homotopic with respect to that found for the ipsilateral projection from visual cortex. The contralateral connection is assumed to be a monosynaptic pathway. The pattern by which the corticofugal fibres terminate in the claustrum is quite similar to the one described for the opposite hemisphere [6].     

A comparison of magnification functions in area 19 and the lateral suprasylvian visual area in the cat

A retinotopic map can be described by a magnification function that relates magnification factor to visual field eccentricity. Magnification factor for primary visual cortex (VI) in both the cat and the macaque monkey is directly proportional to retinal ganglion cell density. However, among those extrastriate areas for which a magnification function has been described, this is often not the case. Deviations from the pattern established in V1 are of considerable interest because they may provide insight into an extrastriate area's role in visual processing. The present study explored the magnification function for the lateral suprasylvian area (LS) in the cat. Because of its complex retinotopic organization, magnification was calculated indirectly using the known magnification function for area 19. Small tracer injections were made in area 17, and the extent of anterograde label in LS and in area 19 was measured. Using the ratio of cortical area labeled in LS to that in area 19, and the known magnification factor for area 19 at the corresponding retinotopic location, we were able to calculate magnification factor for LS. We found that the magnification function for LS differed substantially from that for area 19: central visual field was expanded, and peripheral field compressed in LS compared with area 19. Additionally, we found that the lower vertical meridian's representation was compressed relative to that of the horizontal meridian. We also examined receptive field size in areas 17, 19, and LS and found that, for all three areas, receptive field size was inversely proportional to magnification factor.     

Brain stem projections from cortical area 18 in the albino rat

Efferent brain stem projections from area 18 of the albino rat cortex were traced by autoradiography. Since results could have been compromised by spread of injected label to neighboring areas 17 and 29, studies of projections from these areas were used as controls. A hitherto unreported projection from area 18 to the thalamus in the region of its anterior nuclei was found, terminating in a roughly circular area including parts of the anteromedial, ventral, ventromedial and rhomboid nuclei. At this same level, a very light projection was seen in the contralateral anteromedial nucleus. More caudally the projection pattern was similar to that of area 17, with the lateral, lateral posterior, posterior and pretectal nuclei as targets. However, area 18 showed no projections to either division of lateral geniculate nucleus, projections which are commonly seen following injections in area 17. More caudally, the area 18 projection to the superior colliculus was confined to its four deepest laminae, a projection identical to that from areas 29c and 29b. Area 18 also resembled the cingulate cortex in that it sent a small projection to the dorsolateral central gray, although the routes taken to this region were slightly different. It was decided that the projections from this visual association area were, for the most part, unique to that area, although some of them resembled those from either the cingulate cortex or the primary visual area.     

Topographical organization of the cortical afferent connections to the cortex of the anterior ectosylvian sulcus in the cat

Summary The cortical afferents to the cortex of the anterior ectosylvian sulcus (SEsA) were studied in the cat, using the retrograde axonal transport of horseradish peroxidase technique. Following injections of the enzyme in the cortex of both banks, fundus and both ends (postero-dorsal and anteroventral) of the anterior ectosylvian sulcus, retrograde labeling was found in: the primary, secondary, and tertiary somatosensory areas (SI, SII and SIII); the motor and premotor cortices; the primary, secondary, anterior and suprasylvian fringe auditory areas; the lateral suprasylvian (LS) area, area 20 and posterior suprasylvian visual area; the insular cortex and cortex of posterior half of the sulcus sylvius; in area 36 of the perirhinal cortex; and in the medial bank of the presylvian sulcus in the prefrontal cortex. Moreover, these connections are topographically organized. Considering the topographical distribution of the cortical afferents, three sectors may be distinguished in the cortex of the SEsA. 1) The cortex of the rostral two-thirds of the dorsal bank. This sector receives cortical projections from areas SI, SII and SIII, and from the motor cortex. It also receives projections from the anterolateral subdivision of LS, and area 36. 2) The cortex of the posterior third of the dorsal bank and of the posterodorsal end. It receives cortical afferents principally from the primary, secondary and anterior auditory areas, from SI, SII and fourth somatosensory area, from the anterolateral subdivision of LS, vestibular cortex and area 36. 3) The cortex of the ventral bank and fundus. This sulcal sector receives abundant connections from visual areas (LS, 20, posterior suprasylvian, 21 and 19), principally from the lateral posterior and dorsal subdivisions of LS. It also receives abundant connections from the granular insular cortex, caudal part of the cortex of the sylvian sulcus and suprasylvian fringe. Less abundant cortical afferents were found to arise in area 36, second auditory area and prefrontal cortex. The abundant sensory input of different modalities which appears to converge in the cortex of the anterior ectosylvian sulcus, and the consistent projection from this cortex to the deep layers of the superior colliculus, make this cortical region well suited to play a role in the control of the orientation movements of the eyes and head toward different sensory stimuli.Supported by FISSS grants 521/81 and 1250/84     

Auditory cortical field projections to the basal ganglia of the cat

Projections to the basal ganglia from four auditory cortical fields in the cat were studied by combining microelectrode-mapping of the neurons' best frequencies with autoradiographic and histochemical tract-tracing techniques. Each auditory field is a source of projections to the homolateral basal ganglia. The distribution of labeling within the basal ganglia is related to the cortical field in which the injection site is located. The dorsal portion of the putamen and adjacent caudate nucleus are connected with cortical fields situated anteriorly and dorsally, while the ventral portion of the putamen and adjacent lateral amygdaloid nucleus are related to auditory fields situated posteriorly and ventrally. Injections of two different tracers into different best-frequency loci of one cortical field provided evidence that low best-frequency neurons project medially within the basal ganglia while high best-frequency neurons project more laterally.We concluded that there was a basic similarity among patterns of terminations in the basal ganglia from axons that originate in different auditory cortical fields. When the source of a projection was confined to a restricted portion of an auditory cortical field, labeling appeared as dense patches of silver grains separated from each other by areas of less dense labeling. Often, these patches were distributed within a sheet of tissue, elongated both dorsoventrally and anteroposteriorly. Loci having the same best-frequency representation, but situated in different auditory cortical fields, project upon overlapping but not coextensive portions of a single sheet of tissue. Thus the projections from geographically distant cortical loci possessing similar best-frequency representations are notably distinguished on a topographic basis. By comparison, two adjacent sheets of tissue were labeled when two injections were made into the low best-frequency and high best-frequency representations of the same auditory field. Doubleinjection, double-tracer experiments revealed that adjacent sheets of tissue received projections from different best-frequency loci. These observations suggested a degree of tonotopic organization to this projection system which was equipoise to the evidence obtained for a topographic organization.     

Projections from fetal neocortical transplants placed in the frontal neocortex of newborn rats. A Phaseolus vulgaris-leucoagglutinin tracing study

Summary Fetal rat neocortex grafted into lesion cavities made in the newborn rat neocortex can exchange multiple axonal connections with the host brain. Most previous studies demonstrating efferent transplant-tohost brain connections have used fluorescent retrograde tracers injected into the host brain (Castro et al. 1985, 1987; Floeter and Jones 1984; O'Leary and Stanfield 1989). Other studies have used anterograde axonal tracing with either tritium-labelled amino acids impregnating the transplant and its efferents (Floeter and Jones 1985) or horseradish peroxidase injected into the transplants (Chang et al. 1984, 1986). In the present study we used the anterograde axonal tracer Phaseolus vulgaris — leucoagglutinin (PHA-L) to examine in detail the course and termination of the efferent neocortical graft fibers. Twenty-six newborn rats had the right frontal cortex forepaw area removed by vacuum aspiration, while anesthetized by hypothermia. A piece of fetal frontal cortex 14–16 embryonic days old (E14–16) was immediately thereafter placed in the lesion, and the recipient rats allowed to survive for 5–7 months. At this time the rats were reoperated under sodium pentobarbital (Nembutal) anesthesia and the transplants iontophoretically injected with PHA-L. Two weeks later the animals were again anesthetized, perfused, and processed for PHA-L immunocytochemistry and routine histology. Analysis of acetylcholinesterase- (AChE) and Nissl-stained sections showed graft survival in 19 of the 26 animals used in this study. When these 19 brains were processed for PHA-L immunocytochemistry, 5 of them were found with certainty to have the PHA-L injection confined to the transplant. Based on these cases PHA-L-reactive fibers arising from labelled transplant neurons were traced into the ipsilateral host neocortex adjacent to the transplant and found to project through the subcortical white matter to the ipsilateral parietal neocortical area 1, and claustrum. Callosal fibers were traced to the contralateral frontal neocortical forelimb and parietal areas. Transplant fibers were also observed to descend through the caudate putamen in the dispersed fiber bundles of the internal capsule to distribute as terminal branches and varicose fibers within the mesencephalic periaqueductal gray, red nucleus, deep mesencephalic nucleus, and intermediate gray of the superior colliculus, as well as in the pontine gray. Similar fibers and terminations were present in the caudate putamen, the reticular, ventrobasal, centrolateral, posterior, and parafascicular thalamic nuclei. On the side contralateral to the transplant, fewer fibers were observed in the caudate putamen, the ventrobasal, centrolateral, and posterior thalamic nuclei, as well as more caudally in the deep mesencephalic nucleus and the intermediate gray of the superior colliculus. Our findings demonstrate that homotopic grafts of fetal rat frontal neocortex can project to the developing host brain in a manner which for most projections corresponds to the normal rat neocortical parietal area 1–2 and forelimb area. The density of these transplant-to-host projections is, however, less than in the normal rat corticofugal pathways.     

Ascending projections of presumed dopamine-containing neurons in the ventral tegmentum of the rat as demonstrated by horseradish peroxidase

The projections of presumed dopamine-containing neurons in the zona compacta of the substantia nigra and the ventral tegmental area were examined by stereotaxic injections of horseradish peroxidase into diverse cortical and subcortical regions which are known to include dopamine-containing terminals. Neurons in the lateral half of the substantia nigra pars compacta were labelled after injections into the caudolateral aspect of the caudate-putamen, while neurons in the medial part of the substantia nigra pars compacta and lateral aspect of the ventral tegmental area projected to the anteromedial portion of the caudate putamen. Injections of horseradish peroxidase into the amygdala resulted in the appearance of reactive neurons in the anterior portion of the ventral tegmental area, but the more caudally located entorhinal cortex received projections from the posterior half of the ventral tegmental area. Injections of horseradish peroxidase into the frontal cortex, anterior to the genu, produced scattered labelled cells in the rostral half of the ventral tegmental area, whereas more posterior injections into the cingulate cortex resulted in the appearance of reactive cells which were confined to the medial one-quarter of the substantia nigra pars compacta. The near-midline structure, the lateral septum, was innervated by neurons with cell bodies primarily in the medial half of the ventral tegmental area. Injections of horseradish peroxidase into the nucleus accumbens, which contains very high levels of dopamine, resulted in the appearance of many labelled neurons throughout the ventral tegmental area and some reactive neurons in the medial part of the substantia nigra pars compacta. A few labelled cells were also occasionally observed in the contralateral ventral tegmental area after accumbens injections.These results suggest that although there is considerable overlap, and that the same subdivisions within the substantia nigra pars compacta and the ventral tegmental area appear to innervate diverse regions of the forebrain, there also exists a general topographical organization with respect to the projections of these neurons.Injections of horseradish peroxidase into some of the forebrain regions also resulted in the appearance of reactive cells in mesencephalic nuclei not known to contain dopaminergic perikarya. For example, labelled cells were observed in the supramamillary nucleus after injections into the frontal cortex, entorhinal cortex, accumbens and lateral septum. Injections into the amygdala produced reactive cells in the suprageniculate nucleus, the peripeduncular nucleus, and the magnocellular nucleus of the medial geniculate. These latter results are discussed with reference to the possibility that such pathways may mediate the responsiveness of cells in the amygdala to a wide range of sensory stimuli.     

Projections from inferior temporal cortex to prefrontal cortex via the uncinate fascicle in rhesus monkeys

Summary In five rhesus monkeys (Macaca, mulatta) we used anterograde and retrograde tracing techniques to investigate the projection from the inferior temporal cortex (area TE) to the prefrontal cortex as well as the course of the projecting fibers. The results showed that TE projects to both the inferior convexity and orbital surface of prefrontal cortex and that these projections course almost exclusively via the uncinate fascicle. Transection of the uncinate fascicle deprives the prefrontal cortex of virtually all input from TE, but leaves intact inputs from prestriate and parietal visual areas as well as the amygdala. Such transection also leaves intact many projections from TE to targets other than the prefrontal cortex, including the amygdala, ventral putamen, tail of the caudate nucleus, and pulvinar.