Glutamic acid decarboxylase isoform 65 immunoreactivity in the motor thalamus of humans and monkeys: γ-aminobutyric acidergic connections and nuclear delineations.

The neurotransmitter γ-aminobutyric acid (GABA) plays an important role in the motor thalamic nuclei. This report analyzes the distribution of the GABA-producing enzyme glutamic acid decarboxylase isoform 65 (GAD65), stained with monoclonal antibody, in human and rhesus monkey thalami and compares it with staining patterns of some widely used cytoskeletal and calcium binding protein markers. GAD65 immunoreactivity distinctly labeled two systems: fibers and terminals of basal ganglia thalamic afferents and local circuit neurons, revealing fine features of GABAergic circuitry in the human thalamus. Gross distribution patterns of GAD65 were identical in human and rhesus monkey thalami. The area displaying specific staining of large-caliber beaded fibers coincided with nigro- and pallidothalamic afferent territories previously identified in monkeys with anterograde tracers. Accordingly, a similarly stained region in the human thalamus was considered basal ganglia territory. Except for cytoarchitecture, no specific markers differentiating between the nigro- and pallidothalamic projection zones within this territory were found. GAD65 staining in the cerebellar afferent territory reflected organization of its local circuit neuron network, distinguishing it from adjacent nuclei. Specific GAD65 staining pattern and negative calcium binding protein immunoreactivity identify the cerebellar afferent territory in humans. It is subdivided further into ventral and dorsal regions based on the cytoskeletal protein SMI31 staining pattern. The nuclear outlines revised according to the results are compared with those of Hassler (Schaltenbrand G and Bailey P [1959] Einfuhrung in die stereotaktishen Operationen mit einem Atlas des menschlichen Gehirns, vol 3. Stuttgart: Thieme) and discussed in light of the ongoing controversy regarding delineations of the motor thalamic nuclei in humans.

Distribution and binding parameters of GABAA receptors in the thalamic nuclei of Macaca mulatta and changes caused by lesioning in the globus pallidus and reticular thalamic nucleus.

Ascending output from the basal ganglia to the primate motor thalamus is carried by GABAergic nigro- and pallido-thalamic pathways, which interact with intrinsic thalamic GABAergic systems represented in primates by local circuit neurons and axons of the reticular thalamic nucleus. Disease-triggered pathological processes in the basal ganglia can compromise any of these pathways either directly or indirectly, yet the effects of basal ganglia lesioning on its thalamic afferent-receiving territories has not been studied in primates. Two GABA(A) receptor ligands, [(3)H]muscimol and [(3)H]flunitrazepam, were used to study the distribution and binding properties of the receptor in intact monkeys, those with kainic acid lesions in the globus pallidus, and those with ibotenic acid lesions in the reticular nucleus using quantitative autoradiographic technique on cryostat sections of fresh frozen brain tissue. In control monkeys the binding affinities for [(3)H]muscimol averaged 50 nM in the thalamic nuclei and 86 nM in the basal ganglia while the binding densities varied (maximum density of binding sites [Bmax] range of 99.4-1000.1 fmol/mg of tissue). Binding affinities and Bmax values for [(3)H]flunitrazepam averaged 2.02 nM and 81-113 fmol/mg of tissue, respectively. Addition of 100-microM GABA increased average affinity to 1.35 nM whereas Bmax values increased anywhere from 1-50% in different nuclei. Zolpidem (100 nM) decreased binding by 68-80%. Bmax values for both ligands were decreased at the two survival times in both medial and lateral globus pallidus implying involvement of both nuclei in the lesion. Statistically significant, 40% decrease (P=0.055) of Bmax for [(3)H]muscimol was observed in the ventral anterior nucleus pars densicellularis (VAdc, the main pallidal projection territory in the thalamus) 1 week after globus pallidus lesioning and a 36% decrease (P=0.017) 4 months post-lesioning. In contrast, [(3)H]flunitrazepam Bmax values in the VAdc of the same animals were increased by 23% (P=0.021) at 1 week and 28% (P=0.005) 4 months postlesion, respectively. One week after the reticular nucleus lesioning, the binding densities of [(3)H]muscimol and [(3)H]flunitrazepam were decreased in the thalamic nuclei receiving projections from the lesioned reticular nucleus sector by approximately 50% (P<0.05) and 10-33% (P<0.05), respectively. The results suggest that different GABA(A) receptor subtypes are associated with different GABAergic systems in the thalamus which react differently to deafferentation.

Thalamic terminal fields of individual axons from the ventral part of the dentate nucleus of the cerebellum in Macaca mulatta.

This study examined organization of the projection from the dentate nucleus of the cerebellum to the ventral lateral nucleus (VL) of the thalamus in Macaca mulatta. Small injections of biotinylated dextran amine were placed in the ventral parts of dentate nuclei. The distribution of all contralateral terminal fields in the thalamus was charted, and representative individual axons that terminated in the VL were traced in serial sections under the light microscope. These axons were reconstructed with all their branches and terminal fields in the thalamus. The geometry and size of the terminal fields as well as the number and distribution of boutons and neurons in them were analyzed. The terminal fields of all labeled axons were distributed widely over the VL either singly or in clusters. Two types of axons were found: simple axons formed only one terminal field and complex axons formed multiple terminal fields at a distance. Individual terminal fields were focal, had the form of flattened discs, and generated up to 200 boutons distributed between 10 and 29 nerve cell bodies. These findings suggest that a simple axon activates a small group of neurons at one site. The complex axons, in turn, influence similar size cell groups at different VL locations. The total number of boutons generated by a single complex axon was up to 300. Future studies should determine whether simple axons could be branches of complex axons that took off below the thalamus. The results reveal a complex organization of the input from the ventral dentate to the VL that only partially fits into the traditional concept of somatotopic organization of the nucleus.

Distribution and properties of GABA(B) antagonist [3H]CGP 62349 binding in the rhesus monkey thalamus and basal ganglia and the influence of lesions in the reticular thalamic nucleus.

GABA(B) receptors are believed to be associated with the efferents of the nucleus reticularis thalami, which is implicated in the regulation of activity in the thalamocortical-corticothalamic circuit and plays a role in absence seizures. Yet, the distribution of GABA(B) receptors in the thalamus has only been studied in the rat, and there is no comparable information in primates. The potent GABA(B) receptor antagonist [3H]CGP 62349 was used to study the distribution and binding properties of the receptor in control monkeys and those with small ibotenic acid lesions in the anterodorsal segment of the nucleus reticularis thalami. Eight-micrometer-thick cryostat sections of the fresh frozen brains were incubated in the presence of varying concentrations of the ligand. Autoradiographs were analysed using a quantitative image analysis technique, and binding parameters were calculated for select thalamic nuclei as well as basal ganglia structures present in the same sections. The overall number of GABA(B) binding sites in the monkey thalamus and basal ganglia was several-fold higher than previously reported values for the rat. In the thalamus, the receptors were distributed rather uniformly and the binding densities and affinities were high (Bmax range of 245.5-437.9 fmol/ mg of tissue, Kd range of 0.136-0.604 nM). In the basal ganglia, the number of binding sites and the affinities were lower (Bmax range of 51.1-244.2 fmol/mg of tissue; K(d) range of 0.416-1.394 nM), and the differences between nuclei were more pronounced, with striatum and substantia nigra pars compacta displaying the highest binding densities. Seven days post-lesion, a 20-30% decrease in Bmax values (P < 0.05) was found in the nuclei receiving input from the lesioned nucleus reticularis thalami sector (the mediodorsal nucleus and densicellular and magnocellular parts of the ventral anterior nucleus) without changes in affinity. No significant changes were detected in any other structures. The results of the lesioning experiments suggest that a portion of thalamic GABA(B) receptors is in a presynaptic location on the nucleus reticularis thalami efferents. The overall distribution pattern in the thalamus also suggests a partial association of GABA(B) receptors with corticothalamic terminals presynaptically.

Organization of projections from the anterior pole of the nucleus reticularis thalami (NRT) to subdivisions of the motor thalamus: light and electron microscopic studies in the rhesus monkey.

Projections to the motor-related thalamic nuclei from the anterior pole of the reticular thalamic nucleus (NRT) were studied after injections of biotinylated dextran amine and wheat germ agglutinin conjugated horseradish peroxidase at light and electron microscopic levels, respectively. Each injection resulted in anterograde labeling in the three subdivisions of the ventral anterior nucleus (pars parvicellularis, VApc; pars densicellularis, VAdc; and pars magnocellularis, VAmc) and in the ventral lateral nucleus (VL). NRT fibers had beaded shapes and coursed in a posterior direction giving rise to relatively diffuse terminal plexuses. The average size of the beads (0.7 microm2) and their density per 100 microm of fiber length (23.7-25.7) were similar between the nuclei studied. At the electron microscopic level, anterogradely labeled boutons displayed positive immunoreactivity for gamma-aminobutyric acid (GABA), contained pleomorphic synaptic vesicles, and formed relatively long (approximately 0.4 microm) symmetric synaptic contacts. Usually, a single terminal formed synapses on more than one postsynaptic structure. Synaptic contacts were on projection and local circuit neurons and targeted mainly their distal dendrites. In the VAmc, synapses on local circuit neurons composed 48% of the total sample, in the VAdc/VApc and in the VL the proportion was higher, 65% and 62%, respectively. The results suggest that the input from the anterior pole of the monkey reticular nucleus to the motor-related thalamic nuclei is organized differently from what is known on the organization of connections of NRT with sensory thalamic nuclei in other species in that the terminal fields of individual fibers are diffuse rather than focal and that at least 50% of synapses are established on GABAergic local circuit neurons.

High-frequency stimulation of the subthalamic nucleus suppresses experimental resting tremor in the monkey.

The effect of high-frequency stimulation of the subthalamic nucleus on parkinsonian-like resting tremor was investigated in two monkeys (Macaca fascicularis). Unilateral tremor of the arm and leg was induced by electrical coagulation of the brainstem area including the substantia nigra and the red nucleus. The tremor was only seen at rest condition with a very stable frequency of 4.46+/-0.59 Hz (mean+/-S.D.). Apomorphine (0.10-0.4 mg/kg, s.c.) completely blocked the tremor, suggesting that it was a dopaminergic-dependent symptom just like the parkinsonian tremor. When the stimulating frequency varied from 20 to 1000 Hz, both mono- and bipolar stimulation (square pulses, 0-5 mA, 0.06 ms) of the subthalamic nucleus suppressed resting tremor in a frequency-dependent manner but monopolar stimulation was more effective. These effects remained stable for more than two years. The present results suggest that the subthalamic nucleus is involved in the control and mechanism of resting tremor and that the high-frequency stimulation of the subthalamic nucleus can be used as an alternative therapy in parkinsonian patients with akinesia, rigidity and resting tremor.

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 microm2) 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 microm2), and gave off few dendrites. Two subpopulations of GABA positive boutons (F1 type) were distinguished: large (mean area, 2.6 microm2) 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 microm2 and 1.6 microm2, 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 microm 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.

Mode of termination of pallidal afferents to the thalamus: a light and electron microscopic study with anterograde tracers and immunocytochemistry in Macaca mulatta.

The mode of termination of individual pallidothalamic fibers in the densicellular subdivision of the ventral anterior thalamic nucleus (VAdc) of Macaca mulatta was analyzed with light and electron microscopy after injections of anterograde tracers in the medial globus pallidus. Three tracers were utilized: tritiated leucine, biotinylated dextran amine, and wheat germ agglutinin conjugated to horseradish peroxidase in combination with postembedding immunocytochemsitry for gamma-aminobutyric acid (GABA). Pallidothalamic fibers, upon entering the VAdc, gave off several collaterals that formed plexuses of varicose terminal branches within different cell clusters. The varicosities were aligned along somata and proximal dendrites of projection neurons providing dense input to each individual cell. At the electron microscopic level, labeled boutons displayed a predominantly flat and elongated shape. They contained a moderate number of pleomorphic synaptic vesicles and very large amounts of mitochondria, displayed symmetric synaptic contacts, and were immunoreactive for GABA. In the total sample of 128 autoradiographically labeled terminals, 64% were in synaptic contact with somata and primary dendrites of projection neurons, 14% formed synapses on proximal dendrites of undefined order, and only 7% established synaptic contacts on distal dendrites. Fifteen percent of the labeled boutons established synapses on distal dendrites of GABAergic local circuit neurons (LCN). Pallidal boutons were also found in complex synaptic arrangements: triads with three GABAergic synapses, and serial synapses with LCN dendrites that in turn established synaptic contacts on projection neuron somata or dendrites. These anatomical results suggest a dual effect of pallidal afferents to projection neurons: direct inhibition and disinhibition mediated by LCN. The findings indicate that the fine structure of pallidothalamic terminals in the monkey is similar to that described earlier in the cat. There are, however, interspecies differences in the distribution of pallidal input on postsynaptic targets and its participation in complex synaptic arrangements.

Reevaluation of synaptic relationships of cerebellar terminals in the ventral lateral nucleus of the rhesus monkey thalamus based on serial section analysis and three-dimensional reconstruction.

Terminals of cerebellar afferents (CB) to different regions of the ventral lateral nucleus (VL) of the rhesus monkey thalamus were labeled with wheat germ agglutinin-horseradish peroxidase following injections into the dentate nucleus. Synaptic relationships of 17 CB with projection neuron dendrites (PNd) and local circuit neuron dendrites (LCNd) were analyzed in serial ultrathin sections from dorsal and ventral VL regions, which are known to differ cytoarchitecturally and functionally. Three terminals were reconstructed using three-dimensional (3D) computer image analysis techniques to obtain volumetric and planar measurements. CB in the ventral VL were often flat and elongated with synaptic vesicles arranged in clusters. Each CB was engaged with one PNd and one to four LCNd. A single bouton formed 8-50 synaptic contacts, with those on PNd outnumbering the ones on LCNd 4.1:1. Only some CB in the ventral VL were engaged in complex synaptic arrangements such as triads and serial synapses. Most CB in the dorsal VL displayed a roundish shape and numerous uniformly distributed synaptic vesicles. They formed 5-25 synaptic contacts with a 3:1 ratio of contacts on PNd compared with those on LCNd. CB in the dorsal VL participated in a variety of complex synaptic arrangements. Two types of triads were found: classic with CB, PNd and LCNd, and unconventional with CB and two LCNd. CB were also involved in serial synapses with two LCNd or LCNd and another PNd, and serial sequential synapses with two LCNd and a PNd. Three glomerulus-like structures were encountered in the dorsal VL. 3D reconstruction and volumetric measurements revealed that synaptic contacts formed by CB on PNd had varying shapes and sizes (0.022-0.274 microns2). Synapses formed on LCNd were larger (0.09-0.407 microns2). The total area of all active zones of a single CB on LCNd was either equal to or about 40% smaller than that of synapses on PNd. The entire active zone area comprised 1-1.6% of the total CB surface area and did not seem to correlate with the volume. Synaptic contacts formed by associated LCNd on PNd in complex arrangements were usually small (0.021-0.044 micron2). The results suggest that: synapses formed by CB on PNd and LCNd, and synapses formed by LCNd on PNd may differ in strength; a variety of different circuits participate in the processing of cerebellar afferent information in the primate VL; and these circuits differ in functionally different VL subdivisions.

Neuronal and synaptic organization of the centromedian nucleus of the monkey thalamus: a quantitative ultrastructural study, with tract tracing and immunohistochemical observations.

The ultrastructure of the centromedian nucleus of the monkey thalamus was analysed qualitatively and quantitatively and projection neurons, local circuit neurons, and synaptic bouton populations identified. Projection neurons were mostly medium-sized, with oval-fusiform or polygonal perikarya, few primary dendrites, and frequent somatic spines; local circuit neurons were smaller. Four basic types of synaptic boutons were distinguished: (1) Small- to medium-sized boutons containing round vesicles (SR) and forming asymmetric contacts, identified as corticothalamic terminals. (2) Heterogeneous medium-sized boutons with asymmetric contacts and round vesicles, similar to the so-called large round (LR) boutons, which were in part of cortical origin. (3) Heterogeneous GAD-positive small- to medium-sized boutons, containing pleomorphic vesicles and forming symmetric contacts (F1 type), which included pallidothalamic terminals. (4) Presynaptic profiles represented by GAD-positive vesicle-containing dendrites of local circuit neurons. Complex synaptic arrangements, serial synapses and triads with LR and SR boutons engaging all parts of projection neuron dendrites and somata, were seen consistently, whereas classical glomeruli were infrequent. LR and SR boutons also established synapses on dendrites of local circuit neurons. F1 boutons established synapses on projection neuron somata, dendrites and initial axon segments. Compared to other previously studied motor-related thalamic nuclei, differences in synaptic coverage between proximal and distal projection neuron dendrites were less pronounced, and the density of synapses formed by local circuit dendrites on projection neuron dendrites was lower. Thus, compared to other thalamic nuclei, the overlap of different inputs was higher on monkey centromedian cells, and centromedian inhibitory circuits displayed a different organization.

Nucleus reticularis thalami input to the anterior thalamic nuclei in the monkey: a light and electron microscopic study.

Three anterograde tracers (PHA-L, WGA-HRP, BDA) placed in the most dorsomedial segment of the anterior part of the nucleus reticularis thalami (NRT) in the Rhesus monkey combined with postembedding immunoctochemistry with anti-GABA antibody resulted in double labeling of a distinct population of synaptic boutons in the thalamic nuclei of the anterior group as well as laterodorsal and mediodorsal nuclei. EM analysis of the labeled terminals in the anteroventral and anteromedial parts of the anterior nuclear complex showed that the NRT input is distributed diffusely and in equal proportion to the dendrites of projection and local circuit neurons.

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

Wheat germ agglutinin conjugated horseradish peroxidase (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 MD 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.

Anatomical distinctions between the two basal ganglia afferent territories in the primate motor thalamus.

In primates, the efferents of the two basal ganglia output structures, medial globus pallidus and substantia nigra pars reticularis, are completely segregated and target different thalamic regions. Despite similarities demonstrated earlier in non-primate species in the functional properties and ultrastructural features of the terminals of the two pathways, the present findings suggest significant differences between thalamic circuits associated with the two systems in primates. The data presented in this report further support the concept on functional and anatomical diversity of the subdivisions of the primate motor thalamus proposed in 1990 by Hinsky.

Synaptic reorganization in the feline ventral anterior thalamic nucleus induced by lesions in the basal ganglia.

This report presents the results of quantitative analysis of synaptic coverage of the dendrites of thalamocortical projection neurons in the adult cat ventral anterior thalamic nucleus (VA) and its changes at short (4 days)- and long-term (1 year) survival times after combined unilateral kainic acid lesions in the substantia nigra pars reticularis and entopeduncular nucleus. The study showed that deafferentation induced an increase in the appositional length of the dendrites of GABAergic local circuit neurons along projection neuron dendrites, accompanied by an increase in the number of dendrodendritic synapses. Dendritic or axonic origin of other unusual presynaptic structures at 1 year postlesion could not be established with certainty. All of them displayed some features of growth cones, such as an abundance of tubular and vesicular structures many with electron-dense contents. Projection neuron dendrites postsynaptic to these profiles contained numerous coated vesicles at the same survival time. The findings suggest that (i) the lesioning of the two basal ganglia output structures induces synaptic reorganization in the VA, (ii) this process appears to be active at 1 year after deafferentation, and (iii) all reactive systems display symmetric contacts and positive GAD immunoreactivity suggesting that homotypic remodeling is taking place. Despite the signs of remodeling, the density of presynaptic terminals with symmetric contacts on primary and secondary projection neuron dendrites in the VA remained below the control level at 1 year postlesion suggesting that an imbalance of excitatory and inhibitory inputs on thalamocortical projection neurons may persist for long periods after deafferentation.

Fine structure of the ventral lateral nucleus (VL) of the Macaca mulatta thalamus: cell types and synaptology.

Ultrastructure of the major cerebellar territory of the monkey thalamus, or VL as delineated in sagittal maps by Ilinsky and Kultas-Ilinsky (J. Comp. Neurol. 262:331-364, '87), was analyzed by using neuroanatomical tracing, immunocytochemical, and quantitative morphometric techniques. The VL nucleus contains nerve cells of two types. Multipolar neurons (PN) retrogradely labeled with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from the precentral gyrus display a tufted branching pattern of the proximal dendrites and have a range of soma areas from 200 to 1,000 microns2 (mean 535.2 microns2, SD = 159.5). Small glutamic acid decarboxylase (GAD) immunoreactive cells (LCN) exhibit sizes from 65 to 210 microns2 (mean 122.5 microns2, SD = 32.8) and remain unlabeled after cortical injections. The two cell types can be further distinguished by ultrastructural features. Unlike PN, LCN display little perikaryal cytoplasm, a small irregularly shaped nucleolus, and synaptic vesicles in proximal dendrites. The ratio of PN to LCN is 3:1. The LCN dendrites establish synaptic contacts on PN somata and all levels of dendritic arbor either singly or as a part of complex synaptic arrangements. They are also presynaptic to other LCN dendrites. Terminals known as LR type, i.e., large boutons containing round vesicles, are the most conspicuous in the neuropil. They form asymmetric contacts on somata and proximal dendrites of PN as well as on distal dendrites of LCN. The areas of these boutons range from 0.7 to 12 microns2 and the appositional length on PN dendrites ranges from 1.1 to 14 microns. All LR boutons except the largest ones become anterogradely labeled from large WGA-HRP injections in the deep cerebellar nuclei. These boutons are also encountered as part of triads and glomeruli, but very infrequently since the latter complex synaptic arrangements are rare. The most numerous axon terminals in the neuropil are the SR type, i.e., small terminals (mean area 0.42 micron2) containing round vesicles. The SR boutons become anterogradely labeled after WGA-HRP injections in the precentral gyrus. They form distinct asymmetric contacts predominantly on distal PN and LCN dendrites; however, their domain partially overlaps that of LR boutons at intermediate levels of PN dendrites. The SR boutons are components of serial synapses with LCN dendrites which, in turn, contact somata and all levels of dendritic arbors of PN. They also participate in complex arrangements that consist of sequences of LCN dendrites, serial synapses, and occasional boutons with symmetric contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structure of the magnocellular subdivision of the ventral anterior thalamic nucleus (VAmc) of Macaca mulatta: I. Cell types and synaptology.

The nucleus ventralis anterior pars magnocellularis (VAmc) is recognized only in primates and is the major recipient of the nigrothalamic projections. The neuronal and synaptic composition of this nucleus in the rhesus monkey was studied with the use of a variety of neuroanatomical techniques that included quantitative morphometry, anterograde and retrograde labeling with WGA-HRP from the prefrontal cortex, and immunocytochemistry for glutamic acid decarboxylase (GAD). Two major cell types were identified in the nucleus: thalamocortical projection neurons (PN) that were multipolar cells of various sizes, and small GAD-immunoreactive cells, apparently local circuit neurons (LCN). The approximate ratio of the two types of cells was 10:1. The major type of bouton encountered in the neuropil was of medium to large-sized (areas from 1.5 to 12 microns 2) and mostly of en passant type. These terminals formed symmetric contacts, contained moderate amounts of pleomorphic or mostly flat synaptic vesicles and large numbers of mitochondria, and displayed numerous puncta adhaerentia. All of these boutons exhibited positive GAD immunoreactivity. These boutons constituted the only synaptic population on somata and primary dendrites of PN and formed an overwhelming majority on the secondary PN dendrites. There were fewer of these axon terminals on tertiary PN and LCN dendrites. Additionally, boutons with similar features formed synapses on axon hillocks or initial axonal segments of PN, and somata or very proximal parts of primary dendrites of LCN. With the exception of the boutons in the last two locations, all of the other boutons in this group were shown to be terminals of the nigrothalamic afferents in the parallel EM autoradiographic study (Kultas-Ilinsky and Ilinsky: J. Comp Neurol. 294:479-489, '90). The second major bouton population in the VAmc was represented by small to medium-sized terminals (areas range from 0.2 to 2.0 microns 2) that formed distinct asymmetric contacts and contained large numbers of round vesicles and few or no mitochondria. These boutons were labeled anterogradely from the cortex and dominated on distal PN and LCN dendrites. Some of them were found on secondary PN dendrites where they formed synapses either directly or indirectly via LCN dendrites and dendro-dendritic contacts. The latter arrangements, i.e., serial synapses, were also formed between the cortical boutons and PN somata or tertiary dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structure of the magnocellular subdivision of the ventral anterior thalamic nucleus (VAmc) of Macaca mulatta: II. Organization of nigrothalamic afferents as revealed with EM autoradiography.

An EM-autoradiographic technique was used to identify the ultrastructural features and synaptic sites of nigral afferents to the ventral anterior nucleus pars magnocellularis (VAmc) of the rhesus monkey thalamus. The findings demonstrate that the nigral boutons are of medium-sized to large, with the majority being of the en passant type. These boutons form symmetric synaptic contacts, and contain pleomorphic or entirely flat vesicles and numerous mitochondria. The nigral input is heavily biased towards thalamocortical projection neurons (PN), whose somata and dendrites represent about 82% of the postsynaptic sites of labeled boutons. The distal dendrites of local circuit neurons (LCN) comprise 13% of the postsynaptic sites. Nigral terminals appear to represent a single extrinsic afferent input to the somata and primary dendrites of thalamocortical projection neurons. A nigral input to LCN somata was not demonstrated but the possibility could not be excluded. Although the basic ultrastructural features of nigral boutons in the monkey are similar to those described earlier in the cat (Kultas-Ilinsky et al.: J. Comp. Neurol. 216:390-405, '83), essential species differences exist in the intensity of the nigral input and its distribution on thalamic neurons.

GABA and benzodiazepine receptors in the cat motor thalamus after lesioning of nigro- and pallidothalamic pathways.

Binding parameters of [3H]muscimol ([3H]MUS) and [3H]flunitrazepam ([ 3H]FLU) were determined in the thalamic area of overlap of nigro- and pallidothalamic pathways at short- (1-10 weeks) and long-term (6-11 months) survival times after kainic acid lesioning of substantia nigra pairs reticularis (SNr) and/or entopeduncular nucleus (EPN). No statistically significant lesion-induced changes in Kd could be established in any of the lesioned groups. Bmax values for both binding sites, when corrected for nerve cell densities, revealed some changes in all but one instance (no statistically significant changes in the number of [3H]MUS binding sites were detected after SNr lesions). Significant bilateral increase in the number of [3H]MUS binding sites was found after unilateral EPN and combined EPN + SNr lesions. In the first group the changes were transient; in the second, the number of binding sites appeared to be still on the rise at 8 months postlesion. The latter increase was interpreted as resulting from plasticity type changes in GABAergic local circuit neurons in response to massive deafferentation from extrinsic inhibitory inputs. Changes in [3H]FLU binding sites were of different character and of extremely low magnitude compared to changes in [3H]MUS binding sites. Subtle, but statistically significant, ipsilateral increase in the number of [3H]FLU binding sites as a function of time postlesion was found in the SNr lesioned group. In two other lesioned groups small magnitude increase occurred bilaterally, although in the EPN lesioned group it was more pronounced on the operated side. The results are consistent with earlier suggestion that [3H]MUS and [3H]FLU binding sites in the motor thalamus appear to be associated with different types of GABAergic synapses with none of them being directly associated with the basal ganglia thalamic pathways.

Structural and connectional diversity of the primate motor thalamus: experimental light and electron microscopic studies in the rhesus monkey.

In this study we analyzed neuronal and synaptic organization of nigral and cerebellar afferent territories in the rhesus monkey motor thalamus using a variety of electron microscopic techniques. Significant qualitative and quantitative differences were revealed between the two regions with respect to features of nerve cells, their relationships to one another, and organization of afferent inputs on them. The data suggest that information arriving to the thalamus via nigral and cerebellar afferents is processed by different mechanisms.

Distribution and binding parameters of GABA and benzodiazepine receptors in the cat motor thalamus and adjacent nuclear groups.

Quantitative receptor binding autoradiography technique was utilized to study GABA and benzodiazepine receptors in the cat motor thalamus (ventral anterior, ventral medial and ventral lateral nuclei) and adjacent thalamic subdivisions. Binding parameters (Bmax and Kd) and distribution pattern of the binding sites for 3 tritiated ligands [3H]muscimol ([3H]MUS), [3H]flunitrazepam ([3H]FLU) and [3H]baclofen ([3H]BAC) were analyzed and compared using measurements from discrete and anatomically well-defined thalamic regions. There was little correlation in the regional distribution of the 3 binding sites. The concentration of [3H]BAC binding sites in thalamic nuclei of interest was very low, practically at the limit of resolution of the quantitative autoradiographic technique; whereas appreciable quantities of [3H]MUS and [3H]FLU binding sites were present in the motor and adjacent limbic nuclei of the thalamus. There was more difference between the nuclei in regard to the number of high affinity GABA receptors than benzodiazepine receptors. Moreover, the ratio of Bmax[3H]MUS/Bmax[3H]FLU varied from 2.2 to 4.4 in different thalamic regions suggesting the presence of a diverse population of GABAA and benzodiazepine receptors. The distribution pattern of the 3 binding sites was compared to the topography of GABAergic afferents of the basal ganglia origin and the frequency of GABAergic synapses formed by thalamic local circuit neurons (LCN) in the motor thalamus that were established earlier. It was concluded that in the cat motor thalamus: (1) none of the ligands studied appear to reveal the receptors associated with nigro- or pallidothalamic synapses; (2) [3H]MUS binding sites may be associated with the dendrodendritic contacts formed by LCN; and (3) the [3H]FLU binding sites are physically unrelated to [3H]MUS binding sites. The concentration of [3H]FLU and [3H]MUS binding sites in the midline nuclei and of [3H]MUS binding sites in the limbic nuclei was remarkably high. It was concluded that in addition to previously suggested limbic structures, the midline nuclei with their very high content of benzodiazepine receptors may be considered as a neuroanatomical substrate of certain forms of anxiety.