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.     

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)     

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.     

Nigral and pallidal inputs to functionally segregated thalamostriatal neurons in the centromedian/parafascicular intralaminar nuclear complex in monkey

In primates, thalamostriatal projections from the centromedian (CM) and parafascicular (Pf) nuclei are strong and organized according to a strict pattern of functional connectivity with various regions of the striatal complex. In turn, the CM/Pf complex receives a substantial innervation from the internal globus pallidus (GPi). In this study, we demonstrate that the substantia nigra pars reticulata (SNr) also provides a massive input to Pf in monkeys. These pallidothalamic and nigrothalamic projections provide routes whereby information can flow in functional loops between the basal ganglia and the intralaminar nuclear group. To understand better the anatomical organization and the degree of functional specificity of these loops, we combined retrograde and anterograde labeling methods from functionally defined regions of the striatum and GPi/SNr to determine the relationships between thalamostriatal neurons and basal ganglia afferents. Together with previous studies, our data suggest the existence of tightly connected functional circuits between the basal ganglia and the CM/Pf in primates: 1) A "sensorimotor" circuit links together the medial two-thirds of CM, the postcommissural putamen, and the ventrolateral part of the caudal GPi; 2) a "limbic" circuit involves the rostral one-third of Pf, the ventral striatum, and the rostromedial pole of GPi; and 3) an "associative"circuit exists between the caudal two-thirds of Pf, the caudate nucleus, and the SNr. An additional "associative" circuit that involves the caudate-receiving territory of GPi (dorsal one-third), the dorsolateral Pf (Pfdl), and the precommissural putamen was also disclosed. In conclusion, findings of this study provide additional evidence for the high degree of functional specificity of the thalamostriatal system through which CM/Pf may provide attention-specific sensory information important for conditional responses to the primate striatum.     

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. J. Comp. Neurol. 386:601–612, 1997. © 1997 Wiley-Liss, Inc.     

An EM autoradiographic study of the hypothalamo-hippocampal projection

Previous studies have shown that in many different mammals there is a small but distinct projection from the supramammillary region in the caudal hypothalamus to the junctional region between the regio superior and regio inferior of the hippocampus. We have analyzed the mode of termination of this hypothalamo-hippocampal projection in the rat by electron microscopic (EM) autoradiography following injections of [3H]proline into the caudal hypothalamus. The projection is confined to the regio inferior where it is centered over the subicular end of field CA3, but also spans the adjoining region, field CA2. In our material the highest densities of labeling have been seen over the deeper part of the pyramidal cell layer and in the adjacent stratum oriens but, in addition, above background levels of labeling have been found superficial to the pyramidal cell layer in the stratum lucidum and the deeper part of the stratum radiatum. Most of the labeled synapses appear to be on the perikarya and primary dendrites of the hippocampal pyramidal cells, but some axo-spinous contacts have also been seen. All the labeled boutons contained clear, spheroidal synaptic vesicles and made asymmetric, Type I, contacts with their targets.     

Distribution of external cuneate nucleus afferents to the cerebellum: II. Topographical distribution and zonal pattern--an experimental study with radioactive tracers in the cat

Small injections of tritiated leucine and the autoradiographic method were used to demonstrate efferents from restricted portions of the external cuneate nucleus (NCE) to the cerebellum. Sites of injection were analyzed by reference to the distribution of primary muscle afferents in NCE. On transverse sections, the silver deposits form longitudinal bands that, in certain regions, are packed together and label the entire surface of the granular layer; in other parts, they are separated by empty longitudinal bands. The longitudinal deposits are not continuous in the rostrocaudal direction. On the basis of the distribution of the longitudinal bands, 14 zones have been described for lobules II-VI, and 6 zones were recognized in lobules I, VIII, and the paramedian lobule. Afferents from NCE are distributed topographically. Regions of the nucleus receiving axial and neck muscles project mainly to vermal regions of lobules I-III, and to parts of lobules VIII and IX. Regions receiving afferents from forelimb muscles send their fibers preferentially to the vermian region of lobule V, to paravermian regions of lobules IV-VI, to parts of lobules VIII and IX, and to the paramedian lobule. These distributions in several respects are in agreement with the somatotopical maps of the cerebellum. However, other features support a "mosaic" arrangement: efferents from a region of NCE are distributed over several distinct sites of the cortex and efferents from different parts of the nucleus also converge to neighboring cortical regions.     

Axonal projections from the lateral and medial superior olive to the inferior colliculus of the cat: A study using electron microscopic autoradiography

The superior olivary complex is the first site in the central auditory system where binaural interactions occur. The output of these nuclei is direct to the central nucleus of the inferior colliculus, where binaural inputs synapse with monaural afferents such as those from the cochlear nuclei. Despite the importance of the olivary pathways for binaural information processing, little is known about their synaptic organization ir the colliculus. The present study investigates the structure of the projections from the lateral and medial superior olivary nuclei to the inferior colliculus at the electron microscopic level. Stereotaxic placement and electrophysi ological responses to binaural sounds were used to locate the superior olive. Anterograde axonal transport of 3H-leucine was combined with light and electron microscopic autoradiography to reveal the location and morphology of the olivary axonal endings. The results show that the superior olivary complex contributes different patterns of synaptic input to the central nucleus of the inferior colliculus. Each projection from the superior olivary complex to the colliculus differs in the number and combinations of endings. Axonal endings from the ipsilateral medial superior olive were exclusively the round (R) type that contain round synaptic vesicles and make asymmetrical synaptic junctions. This morpholo is usually associated with excitatory synapses and neurotransmitters such as glutamate. Endings from medial superior olive terminate densely in the central nucleus. The projection from the contralateral lateral superior olive also terminates primarily as R endings. This projection also includes small numbers of pleomorphic (PL) endings that contain pleomorphic synaptic vesicles and usually make symmetrical synaptic junctions. The PL morpholo is associated with inhibitory synapses and transmitters such as gamma-aminobutyric acid and glycine. All endings from the contralateral lateral superior olive terminate much less densely than endings from the medial olive. In contrast, the projection from the ipsilateral lateral superior olive contributes both R and PL endings in roughly equal proportions. These ipsilateral afferents are heterogeneous in density and can terminate in lower or higher concentrations than endings from the contralateral side. These data show that the superior olive is a major contributor to the synaptic organization of the centr nucleus of the inferior colliculus. The ipsilateral projections of the medial and lateral superior olive may produce higher concentrations of R endings than other inputs to the central nucleus. Such endings may participate in excitatory synapses. The highest concentra tions of PL endings come from the ipsilateral lateral superior olive. In combination with inputs from the contralateral dorsal nucleus of the lateral lemniscus, PL endings from the superior olive may participate in many inhibitory synapses found in the central nucleus. These different patterns of synaptic input from the superior olivary complex will influence how binaural information is transmitted to the inferior colliculus. © 1995 Wiley-Liss, Inc.     

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.     

Immunocytochemical study of enkephalin-like cell bodies in the thalamus of the cat

Using an indirect immunoperoxidase technique, the localization of enkephalin-like cell bodies in the thalamus of the cat was carried out. Enkephalin-like cell bodies are widely distributed in the cat thalamus. However, immunoreactive cells may be regrouped in 4 clusters which do not exactly correlate with the anatomical subdivisions of the thalamus. One is located in the dorsocaudal aspect of the thalamus, another in the midline area, and the others are formed by the nuclei geniculatum mediale and laterale.     

An autoradiographic study on the terminal distribution of cerebellothalamic fibers in the cat

Distribution of cerebellothalamic fibers was studied in the cat by the anterograde tracing method. The vast majority of cerebellothalamic fibers were distributed contralaterally. The fastigial fibers arose mostly from the caudal half of the nucleus. These ended moderately in the VM and the most ventromedial regions of the VA-VL complex, and sparsely in the ventral portions of the CL, Pc and NCM; a few also ended in the CM and ZI. The dentate fibers ended moderately in the median and rostrodorsal VA-VL regions, and sparsely in the VM, CL, NCM, CM, LP and MD. The existence of the dentatopulvinar fibers was also confirmed. The posterior interpositus fibers ended heavily in the central VA-VL regions, moderately in the subparafascicular nucleus and ZI, and sparsely in the CM and the ventral lateral geniculate nucleus. The anterior interpositus fibers ended mainly in the ventrolateral VA-VL regions, and additionally in the CL and CM. The ipsilateral cerebellothalamic fibers arose mainly from the fastigial nucleus, and additionally from the dentate nucleus; those arising from the interpositus nuclei were very sparse.     

Prefrontal projections to the mediodorsal nucleus of the thalamus in the rhesus monkey.

The corticothalamic projections to the prefrontal cortex have been shown to be topographically organized. However, the underlying basis for this topography as it relates to the organization of the different architectonically defined areas of the prefrontal cortex has not been systematically studied. In the present investigation we have reassessed the thalamic projections from the different architectonic areas of the prefrontal cortex by using the technique of autoradiography in the rhesus monkey. The results show that the prefronto-mediodorsal projections are organized according to the architectonic differentiation of the prefrontal cortices. Thus architectonically less differentiated medial and orbital prefrontal regions project to the medial sector of the mediodorsal nucleus, the magnocellular subdivision. In contrast, highly differentiated prefrontal area 8 projects to the most lateral sector of the mediodorsal nucleus, the multiformis subdivision. Lateral prefrontal areas with intermediate architectonic features project to the central parvocellular sector of the mediodorsal nucleus. Additionally, these projections also reveal a dorsoventral topography. Thus areas in the medial and dorsolateral cortices project to the dorsal part of the mediodorsal nucleus. In contrast, areas in orbital and ventrolateral cortices project to the ventral part of the mediodorsal nucleus. The topographic organization of the corticothalamic connections described in this study corresponds to the progressive elaboration and differentiation of the architectonic features of the different prefrontal areas. This successive and dichotomous organization of prefrontothalamic connections may provide the basis for the observed differential functions of the prefrontal cortex and the mediodorsal nucleus.     

Fine structure of the spinothalamic projections to the central lateral nucleus of the rat thalamus

The fine structure of labelled spinothalamic terminals in the central lateral nucleus has been studied in the rat following injection of wheat germ agglutinin-horseradish peroxidase into the spinal cord. Myelinated axons gave rise to the labelled terminals, which were large profiles which contained round vesicles, numerous mitochondria, and formed asymmetrical contacts with large dendrites or dendritic protrusions. These profiles are similar to those described in other somatosensory thalamic nuclei, and in many other nuclei of the thalamus.     

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)     

Projection systems and terminal localization of dorsal column afferents: an autoradiographic and horseradish peroxidase study in the rat

Projection systems from the gracile nucleus and the cuneate nuclear complex to their terminal sites in the mesencephalon, diencephalon, and cerebellum were examined by means of anterograde autoradiography and retrograde horseradish peroxidase methods. Three projection systems emerge from the dorsal column nuclei, decussate via internal arcuate fibers, and form the contralateral medial lemniscus (ML). At the obex, some fibers split off the ML and course dorsolaterally, forming an ascending lateral system which fits the "lemniscal adjunct channel" (LAC) concept of Graybiel ('72). The ML continues rostrally as the "main lemniscal line channel" (MLLC). At the inferior colliculus, some LAC fibers terminate in the pontine nuclei, parabrachial, dorsal reticular nuclei, and the external and ventral medial part of the central nucleus of the inferior colliculus. More rostrally at the level of the superior colliculus, terminal fields are found in the medial nucleus of the medial geniculate body, the suprageniculate, pretectal, and mesencephalic reticular nuclei, marking the end of the LAC. In the diencephalon, gracile fibers leave the MLLC and form a crescentlike terminal field along the extreme lateral border of the ventral posterior lateral nucleus (VPL) of the thalamus. Cuneate MLLC fibers terminate in a bandlike formation in the VPL medial to the gracile termination. The third fiber system, the cuneocerebellar projection, emerges from the cuneate, the external cuneate nuclei, and the "cellular bridge" and immediately enters the ipsilateral inferior cerebellar peduncle. Upon entering the cerebellum, the major fiber component remains ipsilateral and terminates as vertical bands in vermal and paravermal lobules, and lobules I through IVa. The posterior cerebellar lobe contains terminal bands in lobules VII-IX, the copula pyramidis, and the paramedian lobule. It is concluded that the dorsolateral fiber system conforms to Graybiel's LAC. It is more divergent and probably less modality specific, whereas the medial lemniscal system conforms to the MLLC, which is said to be modality specific, less divergent, and locked to specific sensory-motor response characteristics. The topography of cerebellar terminal bands indicates that there is sensory-motor representation from all parts of the body to all parts of the cerebellum, at least in the rat.     

Connections between the thalamus and the somatosensory areas of the anterior ectosylvian gyrus in the cat

The thalamocortical and corticothalamic connections of the second somatic sensory area (SII) and adjacent cortical areas in the cat were studied with anterograde and retrograde tracers. Injections consisted of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) or a mixture of equal parts of tritiated leucine and proline. The cortical regions to be injected were electrophysiologically studied with microelectrodes to determine the localization of the selected components of the body representation in SII. The distribution of recording points was correlated in each case with the extent of the injection mass in the cortex. Distributions of retrograde and anterograde labeling in the thalamus were reconstructed from serial coronal sections. The results from cases with injections of tracers exclusively confined to separate parts of the body map in SII indicated a fairly precise topographical organization of projections from the ventrobasal complex (VB) to SII. The labeled cells and fibers were located within a series of lamella-like rods that curved throughout the dorsoventral and rostrocaudal axis of VB. The position and extent of these lamellae shifted from medial and ventral, in the medial subdivision of ventral posterior lateral nucleus (VPLm) for radial forelimb digit zones of SII, to dorsal, Posterior, and lateral, in the lateral subdivision of ventral posterior lateral nucleus (VPLl) for proximal leg and trunk regions in SII. For every injected area in SII the densest clustering of labeled cells and fibers was usually more posteriorly represented in VB. The distribution in these dense zones of labeling often extended through the central core of VB. SII projecting neurons were also consistently noted in the extreme rostral portion of the medial subdivision of the posterior nuclei (Pom) that lies dorsal to VB. Corticothalamic and thalamocortical connections for SII Were entirely reciprocal. Injections of tracers into cortical areas surrounding SII labeled other parts of the posterior complex but failed to label any part of VB except when the injection mass also diffused into SII. Injections into the somatic sensory cortex located lateral to SII, within the lips and depth of the upper bank of the anterior ectosylvian sulcus (AES), heavily labeled the central and posterior portions of Pom. Substantial labeling was noted in the lateral (Pol) and intermediate (Poi) divisions of Po only when the injections involved some part of the auditory area that occupies the most posterior part of the AEG and both banks of the immediately adjoining AES. The magnocellular nucleus of the medial geniculate (MGmc) was labeled only when some part of the auditory cortex was injected. The suprageniculate nucleus (SG) was labeled from the insula and lower bank of the AES. These results indicated that medial (rostral and caudal Pom) and lateral components (Poi, Pol, MGmc) of the Posterior complex have separate cortical projection zones to somatic sensory and auditory cortical regions, respectively. SIV and the lateral extent of area 5a located in the medial bank of the anterior suprasylvian sulcus sent projections to the deep layers of the supe- rior colliculus and the ventrolateral periaqueductal gray. No cortico-tectal projections were seen from SII.     

Dual olfactory representation in the rat thalamus: an anatomical and electrophysiological study

A combination of electrophysiological and anatomical techniques was used to determine the sites of termination of olfactory projections to the thalamus and the distribution of the cells of origin of these projections within the olfactory cortex. Following electrical stimulation of the olfactory bulb, short-latency unit responses were recorded not only in the central segment of the mediodorsal thalamic nucleus but also in the ventral and anterior parts of the submedial thalamic nucleus. Responses were not obtained in the ventral or lateral parts of the mediodorsal nucleus, in the dorsal part of the submedial nucleus, or in the intralaminar nuclei between the mediodorsal and submedial nuclei. The cells of origin of the projection were identified by making injections of horseradish peroxidase conjugated to wheat germ agglutinin (HRP WGA) into the thalamus and examining the olfactory cortex for retrogradely labeled cells. Following injections into the mediodorsal nucleus, labeled cells were found in the polymorphic cell zone deep to the olfactory tubercle, in the ventral endopiriform nucleus deep to the piriform cortex, and in an equivalent position deep to the periamygdaloid and lateral entorhinal cortices. After injections into the submedial nucleus, a smaller number of labeled cells were found in similar locations, except that they were restricted to the rostral olfactory cortical areas and were not found deep to the lateral part of the piriform cortex. Retrogradely labeled cells and anterogradely labeled axons were also found in the lateral orbital and ventral agranular insular areas of the prefrontal cortex with injections into the mediodorsal nucleus, and in the ventrolateral orbital area with injections into the submedial nucleus. Anterograde tracing experiments, using the autoradiographic method, have confirmed these results. Injections of 3H-leucine deep to the junction between the anterior piriform cortex and the olfactory tubercle label axons in both the central segment of the mediodorsal nucleus and the ventral part of the submedial nucleus, while injections deep to the posterior piriform cortex label axons in the mediodorsal nucleus only. Within the mediodorsal nucleus, the projection also appears to be organized so that fibers which arise more rostrally terminate ventrolaterally in the central segment, while fibers which arise more caudally terminate more dorsomedially. These results indicate that there is a substantial and possibly dual thalamocortical mechanism available for processing of olfactory stimuli.     

Distribution of nicotinic receptors in human thalamus as visualized by3H- nicotine and3H-acetylcholine receptor autoradiography

Nicotinic cholinergic receptors in human thalamus were measured using (–)³H-nicotine (20 nM) and³H-acetylcholine (³H-ACh) (20 nM) as radioligands. The specific binding for³H-nicotine to homogenates of thalamus was 51.6±8.3 pmol/g protein and for³H-ACh 18.6±1.9pmol/g protein. Receptor autoradiography indicated a high labelling of both³H-Nicotine and³H-ACh in the antero-ventral nucleus of thalamus and dorso-medial nucleus of thalamus, while the labelling was lower in the postero-lateral nucleus of thalamus and in the postero-lateral ventral nucleus of thalamus. Quantitative measurement of the³H-nicotine autoradiograms showed highest labelling in the anteroventral nucleus of thalamus (17.34±0.76pmol/g tissue). This study indicates a heterogenous distribution of high-affinity nicotinic receptors in the human thalamus.     

Ultrastructure of ChAT-immunoreactive synaptic terminals in the thalamic reticular nucleus of the rat

The thalamic reticular nucleus has been shown to receive cholinergic innervation from both the nucleus basalis of Meynert in the forebrain and the pedunculopontine and laterodorsal tegmental nuclei in the brainstem (Steriade et al.: Brain Res. 408:372-376, '87; Levey et al.: Neurosci. Lett. 74:7-13, '87). Relatively dense populations of choline acetyltransferase-(ChAT) immunoreactive axons and terminallike varicosities have been shown to be distributed throughout this nucleus (Levey et al.: J. Comp. Neurol. 257:317-332, '87). In this study, the ultrastructure of ChAT-immunoreactive axons and of their synaptic terminals in the reticular nucleus was examined in the electron microscope. All ChAT-immunoreactive axonal profiles in the reticular nucleus were presynaptic; the postsynaptic elements were exclusively dendritic profiles; and no axo-axonic or axosomatic contacts from labelled axons were observed. Most ChAT-immunoreactive synaptic contacts were made by profiles less than 0.25 micron in minor diameter. Single ChAT-immunoreactive axons made synaptic contact with several dendritic profiles as the axons were followed through serial sections. These results suggest that the cholinergic innervation of the reticular nucleus will modulate the function of reticular neurons by synapsing onto the dendrites of its neurons without direct effect on the corticothalamic and thalamocortical terminals which also innervate the reticular nucleus.     

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.