Pallidal afferent territory of the Macaca mulatta thalamus: Neuronal and synaptic organization of the VAdc

Ventral anterior thalamic nucleus pars densicellularis (VAdc) as delineated earlier (Ilinsky and Kultas-Ilinsky [1987] J. Comp. Neurol. 262:331–364) was analyzed by using qualitative and quantitative neuroanatomical techniques. Projection neurons (PN), retrogradely labeled with wheat germ agglutinin conjugated horseradish peroxidase from the cortex, were small to medium in size (mean area, 312 μm²) with numerous primary dendrites displaying a tufted branching pattern. Local circuit neurons (LCN), immunoreactive for gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase, were small (mean area, 110 μm²), and gave off few dendrites. Two subpopulations of GABA positive boutons (F1 type) were distinguished: large (mean area, 2.6 μm²) terminals with symmetric synapses containing few pleomorphic vesicles and numerous mitochondria densely covered proximal PN sites; smaller F1 boutons with a slightly different morphology contacted mostly distal PN dendrites. Two subpopulations of terminals containing round vesicles and forming asymmetric synapses were distinguished by bouton size (mean areas, 0.4 μm² and 1.6 μm², respectively). These targeted mainly distal PN dendrites, but some synapsed proximally next to large F1 boutons. On distal dendrites, representatives of both types were labeled from the cortex. The density of boutons with symmetric and asymmetric synapses (the number of boutons per 100 μm of PN membrane length) was 3.3:0.2 on primary, 2.5:1.2 on secondary, and 0.8:12 on distal dendrites. The numerical density of synapses formed by presynaptic LCN dendrites on all PN levels was 20 to 40 times less than that of axon terminals at the same sites. Afferent input to LCN from boutons of all types, including that from 50% of labeled cortical boutons, mainly targeted distal dendrites. Overall, the findings suggest that PN in VAdc receive massive inhibitory input proximally intermingled with some presumably excitatory input, and that LCN contribution to PN inhibition is modest. J. Comp. Neurol. 386:573–600, 1997. © 1997 Wiley-Liss, Inc.     

Projections from the lateral and interposed cerebellar nuclei to the thalamus of the rat: A light and electron microscopic study using single and double anterograde labelling

The lateral and interposed cerebellar nuclei may have different functions in the control of movement. Efferent fibres from both nuclei project predominantly to areas of the thalamus, which in turn project to the motor cortex. In this study, single and double anterograde-tracing techniques have been used to examine and compare the pathways from the lateral and interposed nuclei to the thalamus in the rat by using both light and electron microscopy to look for evidence of organisational or structural features that may underlie the proposed functional differences between these nuclei. Terminals from the lateral nucleus were found to be located most medially in the thalamus, predominantly in the ventral lateral nucleus and the rostral pole of the posterior nuclear group. Terminals from the posterior interposed nucleus were located slightly rostral and lateral to those from the lateral nucleus, mainly around the border between the ventral lateral nucleus and the ventral posterior medial nucleus. Terminals from the anterior interposed nucleus were located slightly rostral and lateral to those from the posterior interposed nucleus, predominantly in the rostral pole of the ventral posterior lateral nucleus. Terminals from the lateral and interposed nuclei were also found in double anterograde-tracing experiments to be nonoverlapping in the regions between these main areas of termination. The structure of terminals from the lateral and interposed nuclei, however, as well as their synaptic relationship with thalamic neurones, were found to be similar. The terminals are large and form synapses with proximal dendrites of thalamic neurones. They contained round vesicles and formed multiple synaptic contacts with dendritic shafts, as well as dendritic spines. The findings indicate that information from the lateral and interposed nuclei is processed in separate regions of the thalamus but that the mode of synaptic transfer to thalamic neurones is likely to be similar for the two projections. © 1994 Wiley-Liss, Inc.     

GABA neurons in the superficial layers of the rat dorsal cochlear nucleus: light and electron microscopic immunocytochemistry

This article is an application of light and electron microscopic immunocytochemistry to the study of the neuronal circuit of the superficial layers in the rat dorsal cochlear nucleus (DCN). An antiserum against the intrinsic marker glutamate decarboxylase (GAD) is used to identify and map axon terminals and neurons that use gamma aminobutyric acid (GABA) as a neurotransmitter. It is demonstrated that layers 1 and 2 of the DCN contain a very high density of GABAergic boutons, matched only by the granule cell domains of the ventral cochlear nucleus, especially the superficial granule cell domain. These two layers also contain much higher concentrations of GABAergic cell bodies than all other magnocellular regions of the cochlear nuclear complex. Cartwheel and stellate neurons, and probably also Golgi cells, previously characterized in Golgi and electron microscopic investigations, appear immunostained and, therefore, are presumably inhibitory. The synaptic relations between parallel fibers, the axons of granule cells, and cartwheel and stellate neurons are confirmed. The present study also supports the conclusion that stellate cells are coupled to one another by gap junctions. Also scattered in layer 1 are large, GABAergic neurons that occur with irregular frequency and presumably represent displaced Purkinje cells, previously identified with a Purkinje-cell-specific marker. Granule neurons and pyramidal neurons remain unstained, even after topical injection of colchicine, which enhances immunostaining of the other glutamate-decarboxylase-positive cells, and therefore must use transmitters different from GABA. The possible analogies between the spiny cartwheel and the aspiny stellate cells of the DCN and the cerebellar Purkinje and stellate/basket cells are discussed in the light of data from Golgi, electron microscopy, and transmitter imunocytochemistry.     

Nucleus reticularis thalami connections with the mediodorsal thalamic nucleus: A light and electron microscopic study in the monkey

Wheat germ agglutinin conjugated horseradish peroxidese (WGA-HRP) and biotinylated dextran amine (BDA) were used as tracers to study nucleus reticularis (NRT) connections with the mediodorsal nucleus (MD). Injections of WGA-HRP in the MO resulted in retrograde labeling of cells in the anteromedial segment of the NRT, the so-called rostral NRT pole. Injections of WGA-HRP and BDA in this NRT region resulted in dense anterograde labeling in the MD. Labeled NRT fibers gave off several collaterals to different MD regions ending with terminal plexuses of thin varicose fibers. In the neuropil, the varicosities were distributed at random, and no tendency to form pericellular baskets was noted. Postembedding immunocytochemistry for GABA was performed on the tissue containing anterograde WGA-HRP label for identification of NRT boutons under electron microscope. The double-labeled boutons were of small to medium size, contained a large number of pleomorphic vesicles, few mitochondria, and formed multiple symmetric synaptic contacts. The number of contacts established by one bouton ranged from 1 to 4 with an average of 1.8 per bouton. About 60% of these boutons made synapses on distal dendrites of GABAergic local circuit neurons; 33% of synaptic contacts were on distal dendrites of thalamocortical neurons, and the rest on their proximal dendrites and soma. NRT boutons were also found in serial synapses and triads. The results demonstrate that the NRT input to the MD is organized so that a single fiber innervates different MD regions and its terminals form numerous synaptic contacts mostly on the distal dendrites of a large number of local circuit neurons and projection neurons.     

Fine structure of nigral and pallidal afferents in the thalamus: an EM autoradiography study in the cat

Ultrastructural characteristics and distribution of nigral and pallidal axon terminals on thalamic neurons were studied after injections of tritiated leucine into substantia nigra and entopeduncular nucleus respectively. Adult cats received 0.1–0.2-μl injections of 2, 3, 4, 5, ³H-leucine in a concentration 60 μCi/μl and were allowed to survive for 4–5 days. The brain tissue was processed for electron (EM) and light microscopic (LM) autoradiography. EM samples were obtained from the ventral medial and ventral anterior thalamic nuclei. Ultrastructural features of labelled nigral and pallidal boutons were analyzed both qualitatively and quantitatively. Ultrastructural characteristics of nigral and pallidal boutons appeared similar. Their length along postsynaptic membrane ranged from 0.8 to 10 μm, with average length of apposition around 2 μm. Both types of bouton contained small clear vesicles of extremely variable shape and formed symmetrical type contacts. Mean diameter of synaptic vesicles profiles (n = 500) was 32.5 nm and 33.3 nm in nigral and pallidal terminal respectively, and mean vesicle profile areas were 846 nm² in nigral terminals and 878m² in pallidal. Both parameters showed normal distribution in percentage distribution histograms. The mean ratios of longest and shortest diameters was 1.6 for synaptic vesicles in both types of boutons. Thus, no significant differences in morphological parameters of nigral and pallidal axon terminals were discovered except that pallidal afferents often formed “en passant”-type synapses while nigral did not. However, this feature alone is not sufficient for distinction between the two types of termi-nals in unlabelled tissue. Analysis of distribution of synaptic sites showed that only pallidal bou-tons formed axosomatic synapses on thalamocortical projection neurons (TCPN), which comprised 21% of total number of pallidal terminals studied. On primary dendritic trunks of TCPN the proportion of nigral boutons was larger (28.8%) as compared to pallidal (19%). The percentage of both types of bouton contacting secondary TCPN dendrites was similar (36% pallidal, 30.6% nigral), while the proportion of nigral terminals on tertiary TCPN den-drites was larger (23.6% versus 13%). Both afferents revealed a tendency to synapse preferentially at the branching points of TCPN dendrites with sev-eral boutons often found along the perimeter of the branching site. Small but equal proportions (8%) of both types of axon terminal were found to synapse on vesicle-containing dendrites of local circuit neurons. Nigral boutons were also found in complex synaptic arrangements in glomeruli. It is concluded that the organizations of pallidal and nigral afferent in-puts in the thalamus are rather similar. Both occupy strategic positions which would allow them to exert strong influence on the firing pattern of TCPN.     

The differentiation of the olfactory placode in Xenopus laevis: a light and electron microscope study

Corticothalamic projections from postcruciate area 4, located on the rostral part of the posterior sigmoid gyms, were traced with the autora-diographic technique in the dog. Injections of tritiated amino acids were made into the lateral and medial parts of area 4 in regions corresponding to the forelimb and hindlimb areas of the primary motor cortex, respectively. In cases with injections placed in the lateral part of areas, dense accu-mulations of label were present in the lateral part of the ventral anterior nucleus (VA), the central part of the ventral lateral nucleus (VL), the ventral half of the ventral posterior inferior nucleus (VPI), the caudal part of the central lateral nucleus (CL), and the centrum medianum (CM). Lighter label was also present in the lateral part of the cytoarchitectonically distinct VL region bordering the ventrobasal complex (VB), as well as in the ventro-lateral part of the mediodorsal nucleus (MD), and in the lateral posterior nucleus (LP). In one case in which the injection site involved an adjacent part of area 3a, label was also seen ventrally in the medial division of the posterior nuclear group (POm). However, no detectable differences in VL, MD, or intralaminar labeling patterns were noted between this case and the four other cases with injections confined to the lateral part of area 4. In two cases with injections restricted to the medial part of area 4, dense label was present in the lateralmost part of VL, the ventral part of VPI, the caudal part of CL, and CM. Lighter label was also present in the VL region bordering the dorsolateral edge of VB and in LP. An additional case in which the injection also involved the rostral border of area 3a showed a similar pattern cf thalamic labeling. Projections from both the lateral and medial parts of area 4 were also noted in the subthalamic nucleus, zona incerta, and nucleus of Darkschewitsch. These results suggest that Corticothalamic projections from postcruciate area 4 to VL are organized topographically such that projections from the lateral part of area 4 project centrally within VL while those from the medial part of area 4 project more laterally. Both parts of area 4 also project top-ographically to a cytoarchitectonically distinct region of VL located im-mediately adjacent to VB, In contrast, the projections to the intralaminar nuclei do not appear to be topographically organized. The data from cases involving spread of the injection into area 3a suggest that projection pat-terns from area 3a to ventral, intralaminar, and medial thalamic nuclei are similar to those from area 4. However, it appears that at least the lateral part of area 3a also projects to POm.     

Immunoelectron microscopic study of glutamate inputs from the retrosplenial granular cortex to identified thalamocortical projection neurons in the anterior thalamus of the rat

We have carried out an ultrastructural study to determine the characteristics and distribution of glutamate-containing constituents of the anterodorsal (AD) and anteroventral (AV) thalamic nuclei in adult rats. We used a polyclonal antibody to glutamate and a postembedding immunogold detection method in animals in which the neurons of AD/AV projecting to the cortex had been retrogradely labelled and the terminals of corticothalamic afferents anterogradely labelled by injection of cholera toxin-horseradish peroxidase (HRP) into the retrosplenial granular cortex. The heaviest immunogold labelling was over axon terminals 0.42 to 2.2 microm in diameter containing round synaptic vesicles and establishing Gray type 1 (asymmetric) synaptic contact (type 1 terminals) on HRP-labelled or non-labelled dendrites. Mean gold particle densities over such terminals were 3-4 times higher than the densities over the dendrites to which they were presynaptic and 5-6 times higher than over terminals establishing Gray type 2 (symmetric) synaptic contacts (type 2 terminals). Gold particle densities over neuronal cell bodies and dendrites and over a subpopulation of myelinated axons were intermediate between the densities over type 1 and type 2 terminals. In adjacent serial sections immunoreacted for gamma aminobutyric acid, type 2 terminals were heavily immunolabelled whereas type 1 terminals and other profiles with moderate gold particle densities after glutamate immunoreaction displayed very low labelling. A subpopulation of small type 1 axon terminals (up to 1 microm diameter) contained HRP reaction product identifying them as cortical in origin; they contacted small dendritic profiles (most <1 microm diameter) many of which also contained HRP reaction product. We conclude that terminals of the corticothalamic projection from retrosplenial granular cortex to AD/AV are glutamatergic and innervate predominantly distal dendrites of thalamocortical projection neurons.     

Cholinergic innervation of the rat hippocampus as revealed by choline acetyltransferase immunocytochemistry: a combined light and electron microscopic study

The cholinergic innervation of the rat hippocampus proper and fascia dentata was investigated by using a monoclonal antibody against choline acetyltransferase (ChAT). At the light microscopic level, thin varicose ChAT-immunoreactive fibers were observed mainly in the vicinity of the pyramidal and granular layers where they formed a fine network around proximal dendrites of pyramidal and granule cells. In addition, many ChAT-immuno-reactive fibers were found in the hilar region and in stratum oriens, radiatum, and lacunosum-moleculare of all hippocampal sectors. Electron microscopic analysis revealed ChAT immunoreactivity in thin unmyelinated varicose axons and terminals which established synaptic contacts. Asymmetric contacts of ChAT-immunoreactive terminals were found on small spines in the dendritic layers of the hippocampus proper and in the molecular layer of the fascia dentata. Symmetric synaptic contacts were formed on the cell bodies of pyramidal and granule cells. Both symmetric and asymmetric synaptic contacts occurred on dendritic shafts. The analysis of serial thin sections, which allows identification of postsynaptic elements, suggests that pyramidal cells, granule cells, and nonpyramidal neurons of the hippocampus receive a cholinergic input.     

Red nucleus of Macaca fascicularis: an electron microscopic study of its synaptic organization

The parvicellular and magnocellular divisions of the red nucleus of the old world monkey, Macaca fascicularis, were analyzed at an electron microscopic level to examine the morphology of the synaptic profiles terminating on rubral neurons and to categorize them by their individual characteristics. The parvicellular division, or anterior two-thirds of the nucleus, is composed of small (10-15 microns) and medium-size (20-30 microns) cells, which are uniformly distributed with high packing density throughout this portion of the nucleus. These cells have invaginated nuclei and are often indented by blood vessels and glial cell somata (satellite cells) that lie in close proximity. The magnocellular portion, occupying the caudal one-third of the nucleus, is composed of an additional population of large cells, ranging from 50-90 microns in diameter, which often contain prominent lipofuscin granules and are frequently indented by blood vessels. Satellite glial cells are not a prominent feature in the magnocellularis portion of the nucleus. The large cells are separated one from the other by fields of myelinated axons either coursing through the nucleus or projecting to and from the nucleus itself. Although the divisions of the nucleus in the Macaca fascicularis are spatially distinct, each possesses a morphological similarity in regard to the categories of synaptic profiles seen at the electron microscopic level. These synaptic profiles are classified as follows: large terminals containing numerous, predominantly rounded vesicles (LR), which can often be seen to form the central profile in a synaptic glomerular arrangement; terminals of similar size with predominantly rounded vesicles but with a pale axoplasmic matrix (LRP); small profiles with rounded vesicles (SR); profiles containing granular dense-cored vesicles (DCV); profiles with numerous flattened vesicles (F); profiles containing pleomorphic vesicles (PL), some of which can be interpreted as presynaptic dendrites (PSD) because they are seen to be postsynaptic and contain ribosomes; and profiles with rounded synaptic vesicles, which are associated with subsynaptic Taxi bodies (T). Most of the various synaptic profile types were found to have similar distributions on the dendritic arbors of rubral neurons in both divisions of the nucleus. However, the LRP-type terminal predominates on the cell bodies and proximal dendrites of the large neurons in magnocellularis. Unlike other regions in the nervous system, F type terminals are rarely seen to contact neuronal somata. This study provides a basis for future experimental studies of afferents to the nucleus in this species.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dorsal cochlear nucleus projections to the inferior colliculus in the cat: A light and electron microscopic study

In the Present report retrograde and anterograde labeling techniques are used to study the Projections of the dorsal cochlear nucleus (DCN) to the inferior colliculus in the cat. Horseradish peroxidase (HRP) or wheat germ agglutinin (WGA-HRP) injections into the inferior colliculus produce large numbers of labeled neurons in the DCN on the opposite side. Labeled cells with projections to the colliculus are identified as fusiform and giant cells and are organized into rostrocaudal bands. The axons of these DCN neurons are labeled by anterograde transport of ³H-leucine and/or proline and studied in light and electron microscopic autoradiographs. Axons from the DCN terminate within the central nucleus of the inferior colliculus in densely labeled, rostrocaudally oriented bands. Less heavily labeled extensions of these bands are found in the deepest layer of the dorsal cortex, and light labeling is found adjacent to the bands in the central nucleus and in the ventrolateral nucleus. Cells in the dorsomedial DCN project to the most ventromedial part of the central nucleus while progressively more ventrolateral cells in the DCN project to more dorsolateral parts of the central nucleus. This present evidence suggests that the DCN sends afferents to only two of the four subdivisions of the central nucleus. Within these regions, the axons from the DCN form terminal boutons or boutons de passage characterized by medium-sized, round synaptic vesicles. The labeled endings nearly always make asymmetric synaptic contacts on the dendrites of disc-shaped and stellate cells in the central nucleus. A few axosomatic contacts are found on one particular cell type, possibly the stellate variety. The results support the hypothesis that each subdivision of the central nucleus receives afferents from a different set of cell types in the auditory nuclei of the lower brainstem. The banding patterns of the efferent cells in the cochlear nucleus and the axons within the central nucleus suggest that these inputs are congruent to the fibrodendritic layers of the central nucleus and may contribute to tonotopic organization in the central nucleus. Finally, the results suggest that each of the two major classes of cells in the central nucleus receives different patterns of inputs from the DCN. These morphological differences could contribute to different electrophysiological responses to the sound stimuli by these cells.     

Sagittal cytoarchitectonic maps of the Macaca mulatta thalamus with a revised nomenclature of the motor-related nuclei validated by observations on their connectivity

Cytoarchitectonic atlas plates of the Macaca mulatta thalamus are presented in the sagittal plane of section with a revised nomenclature of the motor thalamic region. The proposed changes in nomenclature are based on the analysis of topographical relationships between nigral, pallidal, and cerebellar projections to the thalamus studied in 13 rhesus monkeys with the use of autoradiography technique. Mapping of the projection zones of these motor-related systems in serial sagittal sections revealed that they are completely segregated with each honoring cytoarchitectonic boundaries of specific nuclear subdivisions. The available data on thalamic connectivity together with the results of the present study allowed us to divide the primate "motor" thalamus into two major territories: (1) the ventral anterior region (VA) and (2) the ventral lateral region (VL). Although the designation of these two areas of the motor thalamus is the same as the classic one, the nuclear subdivisions that compose them differ significantly from those described in previous classifications. As is delineated in the maps, VA represents the basal ganglia territory of the motor thalamus where nigral projections coincide with its magnocellular part (VAmc), and pallidal projections occupy densicellular (VAdc) and parvicellular (VApc) subdivisions. VAdc corresponds closely to VLo of Olszewski; however, we prefer the new term in order to avoid possible conceptual confusions with the ventral lateral region (VL), which does not receive basal ganglia projections. The VL region is characterized as a distinct cytoarchitectonic entity of the motor thalamus that receives cerebellar projections and includes area X, VPLo, VLc, and VLps of Olszewski. The ventral medial region (VM in the present study or VLm in Olszewski terminology) is usually considered together with the basal ganglia territory on a common connectional basis. However, we did not obtain convincing data to support this view, since evidence of terminal labeling was observed only in (or around) fiber bundles passing through the nucleus with other areas free of label. Rather, in this study VM was treated as an intermediate zone between the subthalamus and motor thalamus where fiber bundles from basal ganglia and cerebellum are organized in a topographical manner before reaching their destinations in the VA and VL regions, respectively. Other major thalamic regions represented in the maps were delineated purely on cytoarchitectonic grounds and their traditional nomenclature was maintained.(ABSTRACT TRUNCATED AT 400 WORDS)     

A light and electron microscopic study of the rat superior cervical ganglion cells by intracellular HRP-labeling

Horseradish peroxidase (HRP) was injected intracellularly into single neurons of the isolated rat superior cervical ganglia. Intracellular iontophoresis of HRP did not seem to damage the sympathetic neurons or to affect synaptic transmission. Under the light microscope, 9 of the 27 HRP-labeled sympathetic neurons exhibited varicosites in their dendrites, but not in their axons; the varicose dendrites came into close contacts with adjacent non-labeled neurons. With the electron microscope, the varicose dendrites of 3 separate neurons lightly stained with HRP, morphological features of synapses could be identified at the contact site: clusters of vesicles in the varicose dendrites, intercellular space of about 20 nm separating the apposed membranes, and an intermediate density on the postjunctional membrane. These findings suggest at the ultrastructural level the occurrence of dendro-dendritic and dendro-somatic synapses in mammalian sympathetic ganglia.     

Quantitative and qualitative characteristics of dentate and fastigial afferents identified by electron microscopic autoradiography in the cat thalamus

Dentato- and fastigiothalamic afferents were identified in the VM and medial VA and VL using electron microscopic (EM)-autoradiography. Synaptic vesicles in labeled dentate and fastigial boutons differed significantly in both their size and shape, which allows these two types of terminals to be distinguished in the normal neuropil. Differences in the mode of terminations of cerebellar afferents upon the neurons in the thalamic nuclei studied are also discussed.     

Projections from the cerebellar interposed and dorsal column nuclei to the thalamus in the rat: A double anterograde labelling study

It is generally agreed that cerebellar and lemniscal pathways project to largely separate areas of the thalamus and influence different functional areas of the cerebral cortex. Cerebellar afferents arise from neurones in the deep cerebellar nuclei and terminate in the ventral lateral group of thalamic nuclei or the “motor thalamus,” whereas lemniscal afferents arise from the dorsal column nuclei and terminate in the adjacent ventral posterior group of thalamic nuclei or “sensory thalamus.” However, it remains unclear whether or not these pathways converge onto thalamic neurones in the border zone between motor and sensory thalamus. The aim of this study was to compare directly the locations of cerebellar interposed and dorsal column nuclei terminals in the rat thalamus by using a double anterograde labelling technique. Microinjections of dextran-tetramethylrhodamine and dextran-fluorescein were made into the interposed and dorsal column nuclei, and labelled terminals in the thalamus were examined in the same sections. The labelled cerebellar and lemniscal terminals were located in separate areas throughout most of the ventral lateral and ventral posterior lateral nuclei, and there was only a limited region around the rostral border between these nuclei where the two groups of terminals came in close proximity to each other. In this common projection zone, however, cerebellar and lemniscal terminals seldom intermingled, and they mostly occupied separate, discreet areas. The results show that cerebellar and lemniscal fibres do indeed project to the border zone between the sensory and cerebellar thalamic nuclei, but they show practically no overlap in this region and are likely to influence separate thalamic neurones. © 1996 Wiley-Liss, Inc.     

Thalamic afferents to cytoarchitectonic subdivisions of area 6 on the anterior sigmoid gyrus of the dog: a retrograde and anterograde tracing study

The cytoarchitecture and thalamic afferents of cortical area 6 located on the anterior sigmoid gyrus were mapped and analyzed in the dog by means of cytoarchitectonic, horseradish peroxidase (HRP), and autoradiographic methods. Cytoarchitectonically, area 6 consists of medial and lateral subdivisions that correspond, respectively, to areas 6a alpha and 6a beta in the cat. In the dog, area 6a alpha is characterized by a wide layer III, the merging of borders between layers III and V, the presence of small-to-medium-size pyramidal cells in layer V, and a pallisade arrangement of cells in layer VI. Area 6a beta appears more stratified, with a relatively acellular layer present between layers V and VI and the presence of large pyramidal cells in layer V. Neither area 6a alpha nor 6a beta contains a layer IV. Data obtained from injections of HRP into areas 6a alpha or 6a beta revealed that labeled thalamic neurons were distributed in a longitudinal band extending from the rostral part of the ventral anterior nucelus (VA) through the caudal part of the mediodorsal nucleus (MD). Labeled cells were observed in the ventral lateral and ventral medial thalamic nuclei as well as in several of the intralaminar nuclei including the central lateral, central medial, parafascicular, and centrum medianum nuclei. A few labeled cells were also located in the suprageniculate nucleus. The densest thalamic labeling was present in VA and MD following injections into area 6a alpha. Equivalent or even larger injections into area 6a beta resulted in much less thalamic labeling. The band of labeled cells also extended into the hypothalamus, zona incerta, amygdala, claustrum, periaqueductal gray of the midbrain, and the nucleus of Darkschewitsch. Results from autoradiographic experiments showed that area 6 subdivisions receive a loosely organized topographic input from VA. Injections of tritiated amino acids were made into selected regions of VA and into the caudal part of MD, areas in which the largest numbers of HRP-labeled cells were located. Area 6a alpha receives afferents primarily from the rostromedial part of VA and the caudal part of MD while area 6a beta receives its principal input from the caudal and lateral parts of VA with minimal input from MD. Axons originating from VA terminate in both layers I and III of area 6 while those originating from the caudal part of MD terminate only in layer III.(ABSTRACT TRUNCATED AT 400 WORDS)     

Distribution of cerebellothalamic and nigrothalamic projections in the dog: A double anterograde tracing study

The distribution of nigrothalamic and cerebellothalamic projections was investigated in the dog by a double labeling strategy combining the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and tritiated amino acids. Following tritiated amino acid injections into the substantia nigra pars reticulata (SNr) and WGA-HRP injections into the contralateral cerebellar nuclei, we found that the nigrothalamic and cerebellothalamic afferents distribute to three main targets: the central portion of the ventral anterior nucleus (VA) and the ventral lateral nucleus (VL), the internal medullary lamina (IML) region, which includes the paralaminar VA, the mediodorsal nucleus (MD) and the central lateral nucleus (CL), and finally the ventromedial nucleus (VM). We observed three distribution patterns of labeled fibers: (a) Dense single label was observed in the central portion of VA following the SNr injections and in VL following the cerebellar nuclei injections. (b) A complementary pattern consisting of alternating foci of nigral and cerebellar label was found in the IML region. This pattern was also observed in the caudal intralaminar nuclei where cerebellar label predominated in the centrum medianum (CM), while the parafascicular nucleus (Pf) primarily contained nigral label. (c) An overlapping pattern of autoradiographic and WGA-HRP label was found in the lateral half of the VM. Overall, the distribution of nigrothalamic and cerebellothalamic projections was widespread throughout much of rostrocaudal thalamus. However, the pattern of projections varied along a continuum from lateral to medial thalamus. In lateral thalamus, nigral and cerebellar projections distributed to separate nuclei while in medial thalamus, the projection pattern changed to focal and complementary in the IML and overlapping in VM. Taken together, these thalamic projections may constitute crucial links in different functional channels involved in alerting and orienting mechanisms associated with motor behavior. Our findings also suggest that the organization of motor thalamic afferents in the dog shares similarities with the segregated and parallel circuitry characteristic of primates as well as with the overlapping and converging circuits of rodents and other carnivores.     

Thalamic projections of the superior colliculus in the rhesus monkey, Macaca mulatta. A light and electron microscopic study.

The projections of the superior colliculus to the thalamus have been studied in the monkey, Macaca mulatta, with anterograde degeneration techniques. The superior colliculus has been shown to project to the inferior nucleus of the pulvinar in a topographical manner with the lower visual field represented dorsomedially and the upper field ventrolaterally. The peripheral zone is located along the medial border and the fovea at the dorsolateral angle adjacent to the lateral geniculate nucleus. The superior colliculus also sends a dense projection to the ipsilateral intralaminar complex, i.e., to the parafascicular, central lateral and paracentral nuclei, and a lesser projection to the same contralateral nuclei. Degenerating tectal fibers were also found in the lateral geniculate nuclei. Four types of vesicle containing profiles were observed in the inferior pulvinar and paracentral nucleus. The large RL and small RS terminals contain round vesicles of uniform size and form asymmetric contacts mainly with large and small dendrites respectively. The F terminal contains a mixture of small round and flat vesicles. It forms symmetric contacts with dendrites and cell somata. The P profile is very pale and contains a relatively sparse population of vesicles showing a great variation in size. It forms symmetric contacts with medium to large dendrites. It is frequently found postsynaptic to the other types, especially the RL terminal, and is regularly seen as the intermediate element of serial and triadic synaptic arrangements. The experimental electron microscopic study has shown that many fibers from the superior colliculus terminate as RL profiles, undergoing direct dense degeneration, in both the inferior pulvinar and the paracentral nucleus. Others probably end as smaller RS terminals.     

Synaptic termination of afferents from the ventrolateral nucleus of the thalamus in the cat motor cortex. A light and electron microscopy study

The synaptic termination in the cat motor cortex of afferents from the ventrolateral nucleus of the thalamus (VL) has been studied with experimental light and electron microscopic methods. The distribution of normal synapses on motor cortex pyramidal, stellate, and Betz cells was also examined. Synapses in the motor cortex can be classified into two general types. The first and most prominent type contains flat vesicles, lacks a compact postsynaptic density, and corresponds to Colonnier's ('68) symmetrical synapse. Stellate neurons receive synapses of both types on their cell bodies and proximal dendritic shafts, while pyramidal cells have only symmetrical synapses at these sites. The dendritic spines of both stellate and pyramidal cells are contacted by predominantly asymmetrical synapses. Betz cells, like smaller pyramidal neurons, receive only symmetrical synapses on their cell bodies. The proximal portions of the Betz cells apical dendrites, however, receive both asymmetrical and symmetrical synapses. Following VL lesions, degenerating synapses were mainly found in three cortical layers: the upper third of layer I (18%), layer III (66%), and layer VI (13%). Degenerating synapses were not seen in the lower two-thirds of layer I or in layer II, and were only rarely seen in layer V (3%). Ninety-one percent of the VL synapses were found on spines and 8% on stellate-type dendritic shafts. Stellate cell bodies rarely received VL synapses (1%) and none occurred on pyramidal or Betz cell bodies and their proximal dendrites. A VL synapse within layer III was found on two dendritic spines of a Betz cell apical dendrite. Thus, part of the VL input to layer III synapses on the processes of both motor cortex output neurons (Betz cells in layer V) and cortical interneurons (stellate cells in layer III).     

Neuronal and synaptic composition of the mediodorsal thalamic nucleus in the rat: a light and electron microscopic Golgi study.

The distribution and dendritic domain of neurons in each segment of the mediodorsal thalamic nucleus (MD) have been studied in the rat with the Golgi technique. In addition, a combined Golgi method-electron microscopic (Golgi-EM) study was undertaken to determine the distribution of morphologically distinct synapse types along the dendrites of individual identified neurons in MD. All the subdivisions or "segments" of MD (medial, central, lateral) contained both stellate and fusiform cells. The dendritic domain of both types of cells was predominantly restricted to the same segment of MD that contained the cell body of the neuron. Typical stellate neurons were found near the center of each segment, with radiating dendrites that extended to but not across the boundaries of the segment. Fusiform cells were usually located close to the segmental or nuclear boundaries and tended to have dendrites oriented parallel to those borders; again, the dendrites tended not to extend across borders between segments or at the outer edge of MD. In the medial segment of MD many fusiform cells had especially bipolar dendritic configurations, generally with a dorsoventral orientation. Because no small neurons were identified that might correspond to thalamic interneurons, all the impregnated cells in MD are presumed to be thalamocortical projection neurons. These results indicate that cells and their major dendrites are confined to a single segment of MD, with little dendritic overlap across segmental or nuclear borders. The segments of MD may therefore be considered to be relatively independent subnuclei. The distribution of the four types of synapses previously identified in MD (Kuroda and Price, J. Comp. Neurol., 303:513-533, 1991) was determined along several identified dendrites studied with the Golgi-EM method. Primary dendrites were contacted mostly by large axon terminals, including both large, round vesicle (LR) terminals and large, pleomorphic vesicle (LP) terminals, as well as a few small to medium sized terminals with pleomorphic vesicles (SMP). No small terminals with round vesicles (SR terminals) were observed to make synapses with primary dendrites. Secondary and tertiary dendrites received synapses from all types of axon terminals. Higher order dendrites were contracted predominantly by SR boutons, but they also carried some LR and SMP terminals. In addition, SMP boutons were often found to form symmetric contacts with cell somata.     

The ventral pallidal projection to the mediodorsal thalamus: a study with fluorescent retrograde tracers and immunohistofluorescence

We have examined rat basal forebrain projections to the mediodorsal thalamic nucleus (MD) by making injections of retrogradely transported fluorescent tracers into the MD. Additionally, in some animals, we also stained sections for glutamate decarboxylase (GAD) by the indirect fluorescent antibody technique. Our results demonstrate that the following basal forebrain areas project to the MD: lateral orbital cortex, agranular insular cortex superficial to claustrum, primary olfactory cortex, diagonal band nuclei, ventral pallidum, and amygdala. A large number of labeled cells are present in the olfactory tubercle, and these cells are almost without exception located in dense GAD-positive ventral pallidal areas rather than in striatal regions of the tubercle. This ventral pallidal projection to the MD strengthens the concept of a ventral striatal-pallidal system in parallel to the classic striatal-pallidal system which projects to the ventral thalamus. These results are also discussed in relationship to the olfactory system.