Ultrastructure of ascending cholinergic terminals in the anteroventral thalamic nucleus of the rat: a comparison with the mammillothalamic terminals

In this study, to identify the ultrastructure and distribution of ascending cholinergic afferent terminals in the anteroventral thalamic nucleus, we used an anti-vesicular acetylcholine transporter antibody as marker of cholinergic afferents, and characterized the immunoreactive terminals at the ultrastructural level. We then compared the distribution pattern of the cholinergic terminals and that of the mammillothalamic terminals identified by anterograde transport of a tracer injected into the mammillary body. The cholinergic terminals were small, and formed both symmetrical and asymmetrical synaptic contacts throughout the dendritic arborizations, particularly in the distal region. This distribution pattern differed from that of mammillothalamic terminals, that were of LR (large terminal containing round synaptic vesicles) type and were preferentially distributed in the proximal region of dendrites. We also found relatively numerous cholinergic terminals making contact directly with immunonegative excitatory terminals, both LR and SR (small terminal containing round vesicles) terminals, without clear postsynaptic specialization. A few cholinergic terminals even seemed to form a synaptic complex with the LR or SR terminals. These findings suggest that the ascending cholinergic afferents in the anteroventral thalamic nucleus can effectively modulate excitatory inputs from both the mammillothalamic and corticothalamic terminals, in close vicinity to a synaptic site.     

Distribution of the piriform cortical terminals to cells in the central segment of the mediodorsal thalamic nucleus of the rat.

A Golgi electron microscopic study was undertaken to investigate the distribution of terminals from the piriform cortex that synapse on identified dendrites of neurons in the central segment of the mediodorsal thalamic nucleus of the rat. The piriform cortical terminals were identified as degenerating terminals following lesions in the cortex. They consisted of two types, i.e., large (LR type) and small (SR type) presynaptic terminals, both of which had round synaptic vesicles and formed asymmetric synaptic contacts. SR boutons terminated preferentially onto distal dendrites and never synapsed on primary dendrites. LR terminals synapsed preferentially on proximal dendrites, but were also found on more distal dendritic segments.     

Synaptic organization of projections from basal forebrain structures to the mediodorsal thalamic nucleus of the rat.

The synaptic organization of the mediodorsal thalamic nucleus (MD) in the rat was studied with the electron microscope, and correlated with the termination of afferent fibers labeled with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Presynaptic axon terminals were classified into four categories in MD on the basis of the size, synaptic vesicle morphology, and synaptic membrane specializations: 1) small axon terminals with round synaptic vesicles (SR), which made asymmetrical synaptic contacts predominantly with small dendritic shafts; 2) large axon terminals with round vesicles (LR), which established asymmetrical synaptic junctions mainly with large dendritic shafts; 3) small to medium axon terminals with pleomorphic vesicles (SMP), which formed symmetrical synaptic contacts with somata and small-diameter dendrites; 4) large axon terminals with pleomorphic vesicles (LP), which made symmetrical synaptic contacts with large dendritic shafts. Synaptic glomeruli were also identified in MD that contained either LR or LP terminals as the central presynaptic components. No presynaptic dendrites were identified. In order to identify terminals arising from different sources, injections of WGA-HRP were made into cortical and subcortical structures known to project to MD, including the prefrontal cortex, piriform cortex, amygdala, ventral pallidum and thalamic reticular nucleus. Axons from the amygdala formed LR terminals, while those from the prefrontal and insular cortex ended exclusively in SR terminals. Fibers labeled from the piriform cortex formed both LR and SR endings. Based on their morphology, all of these are presumed to be excitatory. In contrast, the axons from the ventral pallidum ended as LP terminals, and those from the thalamic reticular nucleus formed SMP terminals. Both are presumed to be inhibitory. At least some terminals from these sources have also been identified as GABAergic, based on double labeling with anterogradely transported WGA-HRP and glutamic acid decarboxylase (GAD) immunocytochemistry.     

Ultrastructure and synaptic organization of axon terminals from brainstem structures to the mediodorsal thalamic nucleus of the rat.

The ultrastructural characteristics and synaptic organization of afferent terminals from the brainstem to the mediodorsal thalamic nucleus (MD) of the rat have been studied with the electron microscope, by means of anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Labeled fibers were seen predominantly in the lateral portion of MD after the injections of WGA-HRP into the substantia nigra pars reticulata (SNr), the superior colliculus (SC), and the dorsal tegmental region (DT). The boutons arising from the SC were relatively small (less than 1.5 microns in diameter), formed asymmetric synaptic contacts with small dendrites and dendritic spines, and contained round synaptic vesicles. The axon terminals from the DT were mostly large boutons (2-4.5 microns) with asymmetric synaptic specializations and round vesicles. These boutons and their postsynaptic targets formed synaptic glomeruli that were entirely or partially ensheathed by glial lamellae. The ultrastructural features are almost identical to those of boutons in the medial and central segments of MD that were previously shown to originate from the basal amygdaloid nucleus and the piriform cortex. The boutons from the SNr had a wide range in size, but the majority were medium-sized to large (1.5-4 microns). The nigral boutons established symmetric synaptic contacts with dendritic shafts and occasionally with somata, and contained pleomorphic vesicles. However, like the DT terminals, they participated in glomerular formations. The nigral terminals closely resemble previously described terminals in the medial part of MD from the ventral pallidum, except that the nigral terminals formed en passant and axosomatic synapses as well as axodendritic synapses. A combined immunohistochemistry and WGA-HRP tracing study revealed that the nigral inputs were immunoreactive for glutamic acid decarboxylase and the axon terminals from the DT were immunoreactive for choline acetyltransferase. In a separate study, the colliculothalamic fibers have been shown to take up and transport the transmitter specific tracer [3H]-D-aspartate, and are therefore putatively glutamatergic and/or aspartatergic. Taken together with this, the present results suggest that the collicular afferents are excitatory and glutamatergic and/or aspartatergic, that the inputs from the DT are also excitatory and cholinergic, while the nigral inputs are inhibitory and GABAergic.     

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)     

Synaptic organization of monosynaptic connections from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

Synaptological characteristics of synapses between axonal boutons of the trigeminal mesencephalic nucleus (Vme) neurons and the hypoglossal nucleus (XII) motoneurons (MNs) were studied using biotinylated dextran amine (BDA) anterograde labeling combined with horseradish peroxidase (HRP) retrograde transport in the rat. BDA was initially iontophoresed into Vme unilaterally and 7 days later HRP was injected into the anterior two-thirds of the ipsilateral tongue. After histochemical reactions, BDA anterogradely labeled boutons were seen to appose closely to somata and dendrites of HRP retrogradely labeled MNs in XII by light microscopy. A total of 212 BDA-labeled Vme boutons were examined ultrastructurally, which had an average diameter of 1.3 +/- 0.4 microm and contain small clear spherical vesicles. Eighty-eight percent of Vme boutons (187/212) synapsed on dendrites of HRP-labeled XII MNs. Twenty-five Vme boutons (25/212, 12%) made synapses with somata of HRP-labeled XII MNs. Thirty-five percent (74/212) of BDA-labeled Vme boutons were also contacted by unlabeled P-type terminals. Presynaptic P-type terminals contained spherical (47%, 35/74), pleomorphic (43%, 32/74), and flattened (10%, 7/74) synaptic vesicles. Thus, P-type terminals (as a presynaptic element), BDA-labeled Vme boutons, and XII MNs constitute axoaxodendritic and axoaxosomatic synaptic triads. There are four types of synaptic microcircuits in XII neuropil: synaptic convergence, synaptic divergence, presynaptic inhibition synaptic circuits, and feedforward regulation circuits. This detailed ultrastructure examination of the synaptic organization between Vme neurons and XII MNs provides insights into the synaptic mechanisms of the trigeminal proprioceptive afferents involved in the jaw-tongue reflex and coordination during oral motor behaviors.     

Ultrastructure of supraspinal dorsal root projections in the toads. I. The obex region

The ultrastructure of the dorsal column nucleus (DCN) has been investigated at the level of the obex region in normal and experimental toads. Large 'isolated' neurons (greater than 20 micrometer) and clusters of small neurons (less than 20 micrometer) have been identified in this region. Synaptic profiles have been classified into three types: large 'en passant' LR boutons, containing round synaptic vesicles and neurofilaments, small R boutons with round vesicles and F boutons with pleomorphic vesicles. The axon terminals exhibited synaptic contacts with cell somata, with dendrites of varying calibers and with other axons. The terminals involved in the axo-axonic contact were the F boutons which were presynaptic to the LR boutons, thus representing the morphological basis for presynaptic inhibition. Transection of the second dorsal root was performed in order to identify the terminals of the primary afferents to the DCN, after different survival periods (16 h--50 days). Only the LR boutons underwent degeneration, thus representing the central endings of the primary dorsal root afferents. The functional significance of these findings was discussed.     

Dual mode of corticothalamic synaptic termination in the mediodorsal nucleus of the rhesus monkey

Electron microscopic autoradiography (EM-ARG) was used to assess the synaptic organization of corticothalamic terminals in the parvicellular division of the mediodorsal thalamic nucleus. Examination of the synaptic organization in unreacted tissue revealed several distinct synaptic types distributed among glomerular and nonglomerular regions of the neuropil. Within glomeruli, three presynaptic terminal classes were found. The majority of profiles (as many as eight to ten per glomerulus) were presynaptic dendrites (PSDs) forming symmetric synaptic contacts with a central dendrite, and occasionally with other PSDs. One or two large terminals densely packed with round vesicles (LR terminals) were also present in each glomerulus. This terminal class made multiple asymmetric contacts with the central dendrite, as well as with many PSDs within the glomerulus. Finally, small terminals with round vesicles (SR terminals) formed asymmetric synaptic junctions with PSDs in some glomeruli. PSDs and SR terminals were also found in the extraglomerular neuropil, although in different proportions than in the glomeruli. In the extraglomerular neuropil SR terminals were the most abundant terminal class and these terminals made synaptic contacts with dendrites of all sizes. PSDs were seen in considerably smaller numbers than in the glomeruli. Finally, the extraglomerular neuropil contained a moderate number of small to medium terminals that formed symmetric synaptic junctions (SF terminals) with cell bodies and dendrites of all sizes. Synaptic profiles related to corticothalamic inputs were identified by injecting the prefrontal cortex of two rhesus monkeys with 3H-leucine and -proline and analyzing the distribution and morphology of radiolabeled terminals. Quantitative analysis of the density of silver grains over different tissue compartments revealed a positive labeling index for two terminal classes: SR and LR terminals. Labeled SR terminals were concentrated in the extraglomerular neuropil and labeled LR terminals were found within glomeruli where they formed synaptic contact with the central dendrite, as well as with presynaptic dendrites of the glomerulus. In contrast to many other thalamic nuclei, cortical input to the mediodorsal nucleus arrives via two distinctive synaptic pathways, one terminating extraglomerularly and the other terminating within the synaptic glomeruli. The dual mode of corticothalamic terminations in the mediodorsal nucleus suggests a more potent and possibly different role for cortical input in the regulation of neuronal activity in this association nucleus than in sensory nuclei of the thalamus.     

Synaptic organization of the nucleus gracilis of the cat. Experimental identification of dorsal root fibers and cortical afferents

The synaptic organization throughout the nucleus gracilis has been investigated in unoperated cats. Axon terminals of variable size can establish synaptic contacts with neuronal somata, dendritic processes, initial segment of axons or with other axon terminals at “complex synaptic arrangements.” Large boutons with rounded vesicles are regularly associated with smaller boutons containing flattened vesicles; the latter type of bouton forms frequently a double synapse being presynaptic to the large bouton and to the element postsynaptic to this (“complex synaptic arrangements”). Medium-sized to small axon terminals of the “isolated” type contain primarily either rounded or flattened vesicles. These boutons are surrounded by a thin glial process which also wraps the postsynaptic element, mostly represented by a small dendritic profile. The “isolated” type of bouton seems to be more abundant in the rostral than in the caudal part of the nucleus. In all unoperated control animals altered axons and axon terminals are present. They are enlarged and display hyperplasia and dilatation of tubular profiles of smooth endoplasmic reticulum as well as proliferation of microtubules and various aspects of mitochondrial degeneration. In cats sacrificed 48 hours after section of lumbo-sacral dorsal roots a high number of “dark” boutons are observed in various stages of degeneration. These terminals are identifiable with the large boutons containing rounded vesicles and postsynaptic to the smaller boutons with flattened vesicles. The morphology of dorsal root terminals in the nucleus gracilis is discussed in relation to that of primary afferent terminals in other central structures and to the functional aspects of axo-axonic contacts. The sensori-motor cortex was removed in another series of animals which were sacrificed after one to four days. As a consequence of such lesions cortical fiber terminals in the nucleus gracilis may undergo either the “dark” or the “light” type of degeneration. These terminals are of smaller size than those of primary afferents, they usually synapse on dendritic profiles of small diameter, are not involved in axo-axonic contacts and seem to contain rounded vesicles. Therefore they can be identified with at least some of the small and medium-sized boutons of the “isolated” type.     

Ultrastructural features of synapse from dorsal parvocellular reticular formation neurons to hypoglossal motoneurons of the rat

The dorsal parvocellular reticular formation (PCRt) receives projection of the trigeminal mesencephalic nucleus neurons. It contains the dorsal group of interneurons that integrate and coordinate activity of the oral motor nuclei. Ultrastructural features of synaptic connection from the dorsal PCRt neurons to the motoneurons of the hypoglossal nucleus (XII) were examined at both the light and electron microscopic levels in rats. Biotinylated dextran amine (BDA) was initially iontophoresed into the dorsal part of PCRt unilaterally. Seven days later horseradish peroxidase (HRP) was injected into the body of the tongue. After histochemical reaction for visualization of HRP and BDA, the BDA-labeled fibers and terminals were seen distributing bilaterally in XII with ipsilateral predominance. BDA-labeled terminals were closely apposed upon HRP retrogradely labeled somata and dendrites of the XII motoneurons. A total of 1408 BDA-labeled boutons were examined ultrastructurally, which had mean size of 1.22+/-0.37 microm in diameter. Five hundred-ninety three of these boutons in both the ipsilateral (n=401) and contralateral (n=192) XII were seen to synapse on both the dendrites and somata of HRP-labeled motoneurons. The vast majorities of synapses were axodendritic (98%, 580/593), while 2% of them were axosomatic. Of the 1408 BDA-labeled boutons, 69.6% of them were S-type boutons containing small clear and spherical synaptic vesicles and 30.4% of them were PF-type boutons containing pleomorphic and flattened synaptic vesicles. Approximately 64% of synapses between BDA-labeled boutons and HRP-labeled motoneurons were asymmetric, and 33% of synapses were symmetric. No axoaxodendritic or axoaxosomatic synaptic triad was observed. The present study illustrated the anatomical pathway and synaptological characteristics of neuronal connection between the dorsal PCRt premotor neurons and the XII motoneurons. Its functional significance in coordinating activity of XII motoneurons during oral motor behaviors has been discussed.     

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.     

Electron microscopic observation of synaptic connections of jaw-muscle spindle and periodontal afferent terminals in the trigeminal motor and supratrigeminal nuclei in the cat

Previous studies indicate that the trigeminal motor nucleus (Vmo) and supratrigeminal nucleus (Vsup) receive direct projections from muscle spindle (MS) and periodontal ligament (PL) afferents. The aim of the present study is to examine the ultrastructural characteristics of the two kinds of afferent in both nuclei using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Our observations are based on complete or near-complete reconstructions of 288 MS (six fibers) and 69 PL (eight fibers) afferent boutons in Vmo, and of 93 MS (four fibers) and 188 PL (four fibers) afferent boutons in Vsup. All the labeled boutons contained spherical synaptic vesicles and were presynaptic to neuronal elements, and some were postsynaptic to axon terminals containing pleomorphic, synaptic vesicles (P-endings). In Vmo neuropil, MS afferent boutons were distributed widely from soma to distal dendrites, but PL afferent boutons predominated on distal dendrites. Most MS afferent boutons (87%) formed synaptic specialization(s) with one postsynaptic target while some (13%) contacting two or three dendritic profiles; PL afferents had a higher number of boutons (43%) contacting two or more dendritic profiles. A small but significant number of MS afferent boutons (12%) received contacts from P-endings, but PL afferent boutons (36%) received three times as many contacts from P-endings as MS afferents. In Vsup neuropil, most MS (72%) and PL (87%) afferent boutons formed two contacts presynaptic to one dendrite and postsynaptic to one P-ending, and their participation in synaptic triads was much more frequent than in Vmo neuropil. The present study indicates that MS and PL afferent terminals have a distinct characteristic in synaptic arrangements in Vmo and Vsup and provides evidence that the synaptic organization of primary afferents differs between the neuropils containing motoneurons and their interneurons. © 1996 Wiley-Liss, Inc.     

Light and electron microscopic observations of a direct projection from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

A direct projection from rat mesencephalic trigeminal nucleus (Vme) neurons to the hypoglossal nucleus (XII) motoneurons was studied using a double labeling method of anterogradely biotinylated dextran amine (BDA) tracing combined with retrogradely horseradish peroxidase (HRP) transport at both light and electron microscopic levels. BDA was iontophoresed unilaterally into the caudal Vme, and 7 days later HRP was injected into the ipsilateral tongue to label hypoglossal motoneurons. The BDA-labeled fibers were seen descended along Probst' tract and were traced to the caudal medulla. In this course, the fibers gave off axon collaterals bearing varicosities in the trigeminal motor nucleus (Vmo), the parvicellular reticular formation (PCRt), the dorsomedial portions of the subnuclei of oralis (Vodm) and interpolaris (Vidm) and in the XII ipsilaterally. The labeling of terminals was most dense in the PCRt at the levels of caudal pons and rostral medulla, which displayed a "dumbbell-shaped" form in the transverse planes. In the XII, labeled terminals were distributed mainly in the dorsal compartment of the nucleus. One hundred sixty-eight appositions made by BDA-labeled terminals on HRP-labeled motoneurons were seen in the dorsal compartment (71%) and in the lateral subcompartment (24%) of the ventral XII. Under electron microscopy BDA-labeled boutons containing clear, spherical synaptic vesicles were found to form synaptic contacts with the somata and dendrites of hypoglossal motoneurons with asymmetric specializations. The present study provides new evidence that the trigeminal proprioceptive afferent neurons terminate in the XII and make synaptic contacts with their motoneurons.     

Morphology of physiologically identified slowly adapting lung stretch receptor afferents stained with intra-axonal horseradish peroxidase in the nucleus of the tractus solitarius of the cat. II. An ultrastructural analysis

The nucleus of the tractus solitarius is a site for termination of primary afferents originating from a variety of visceral receptors. The localization of bouton terminals of slowly adapting lung stretch (SAR) afferent fibers originating from the tracheobronchial tree have been described in the companion paper (Kalia and Richter, '85). The most conspicuous finding regarding the location of SAR terminals is that they are concentrated within specific subnuclear groups of the nucleus of the tractus solitarius (nTS) and are distributed widely in the rostrocaudal plane of the medulla oblongata. These light microscopic features have provided us with valuable information with regard to the organization of visceral afferents in the central nervous system. The synaptic profiles formed by the 476 bouton terminals of these HRP-labeled afferents have been described in this paper in serial thin sections. All of the bouton terminals examined under the electron microscope were found to contain round synaptic vesicles. Synaptic boutons (1.0-3.0 microns in diameter) were usually of the en passant variety and made contact with different structures depending upon the subnucleus which was examined. In the ventral (v) and the ventrolateral (vl) subnuclei of the nTS, asymmetrical (type I) synaptic contacts containing round, clear synaptic vesicles of 35-50 microns in diameter were found and these contacts were made with (1) the soma of cell bodies located in that subnucleus; (2) spiny dendrites in that nucleus; (3) vesicle-containing axon terminals that were presynaptic to the HRP-labeled bouton terminal; and (4) vesicle-containing dendrites in which the HRP profile was presynaptically located. The terminal axon remained myelinated till the last 1 micron before the bouton terminal was formed. There was no distinct, unmyelinated portion of the terminal axon. The synaptic bouton received axon-axonal synapses from unlabeled bouton terminals containing round, clear vesicles. This is the first report of the localization of these afferent fibers as well as of the regional variations in the ultrastructure of boutons of physiologically identified terminals. It appears likely that the lung stretch afferent fibers, by having axon-axonal as well as axon-somatic contact in the ventral, ventrolateral, and intermediate subnuclei of the nTS, can interact in a variety of different ways in this region. The significance of these features in relation to the precise influence of respiratory afferents on central respiratory mechanisms needs to be evaluated further.     

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)     

Dopamine-immunoreactivity in the rat mesencephalic trigeminal nucleus: an ultrastructural analysis

The ultrastructure and distribution of dopaminergic boutons within the rat mesencephalic trigeminal (Me5) nucleus was examined with the use of electronmicroscopic immunocytochemistry. A total of 5102 boutons, comprising axosomatic and axodendritic synaptic terminals as well as non-synaptic boutons (or varicosities), located in the ventrocaudal portion of Me5 was analysed. Approximately 20% of these boutons were dopamine-immunoreactive. Morphological analysis showed that the dopaminergic synaptic terminals, axodendritic as well as axosomatic, were exclusively of the S- and G-bouton type; they contained, respectively, small spherical vesicles or small pleomorphic vesicles in combination with large granular dense-cored vesicles. All dopaminergic varicosities in the Me5 were of the G-bouton type. Quantitative analysis revealed that most of the dopaminergic synaptic terminals in the Me5 nucleus contacted dendrites, while only a minority (12%) contacted Me5 somata. This dopaminergic somatic input comprised about half (52%) of the total axosomatic input on Me5 neurons. The present results and previous findings with respect to the prominent serotonergic component of the axosomatic input to Me5 neurons indicate that dopamine and serotonin account for most of the axosomatic input in the ventrocaudal part of the Me5 nucleus. In fact, the present results seem to support previous observations regarding the existence of a population of afferent neurons in which dopamine and serotonin are colocalized.     

Ultrastructural analyses of afferent terminals in the subthalamic nucleus of the cat with a combined degeneration and horseradish peroxidase tracing method

The synaptic organization of the feline subthalamic nucleus (STN) was studied electron microscopically. Following horseradish peroxidase (HRP) injections into the globus pallidus (GP) and electrolytic lesions of the nucleus tegmenti pedunculopontinus pars compacta (TCP) in the same cat, both degenerating and HRP-labeled terminals were found in the STN with abundant retrogradely HRP-labeled neurons. Degenerating terminals of TPC origin were medium-sized and characterized by asymmetric synaptic contacts. They synapsed widely on the STN neuronal surface, including the somata, dendrites of varying dimensions, dendritic spines and vesicle-containing processes. They formed 25.1%, 65.1%, 4.7%, and 4.7%, respectively, of all TPC efferent terminals. Some of the postsynaptic components were labeled with HRP. Occasionally both degenerating terminals and HRP-labeled terminals were in synaptic contact with the same HRP-labeled neuron: therefore, afferents of TPC and GP converge on the same STN projection neuron. In order to discover the origin of these HRP-labeled terminals, a mixed solution containing HRP and kainic acid was injected into the GP. Numerous degenerating terminals were observed to synapse with HRP-labeled STN neurons, but no HRP-labeled terminal was observed. These degenerating terminals were similar in appearance to the above-mentioned HRP-labeled terminals. They were characterized by their relatively large size, predominantly symmetric synapses, and preferential distribution on the somata and large or medium-sized dendrites. They formed 39.6%, 20.1%, and 31.1%, respectively, of all GP efferent terminals. Therefore, it became clear that both the HRP-labeled terminals of the first experiment and the degenerating terminals of the second experiment originated from the GP. Following surgical ablations of the primary sensorimotor cortex (Cx), some axon terminals in the STN showed degeneration. These degenerating terminals were small and formed asymmetric synapses mainly with dendritic spines, small dendrites and vesicle-containing processes. They formed 48.0%, 28.0%, and 12.0%, respectively, of all Cx efferent terminals. These electron microscopic investigations reveal the convergence of TPC and GP afferents and that STN projection neurons relay the TPC and pallidal inputs directly to the GP.     

Axonal projections and synapses from the supratrigeminal region to hypoglossal motoneurons in the rat

Neural circuits from the supratrigeminal region (Vsup) to the hypoglossal motor nucleus were studied in rats using anterograde and retrograde neuroanatomical tracing methodologies. Iontophoretic injection of 10% biotinylated dextran amine (BDA) unilaterally into the Vsup anterogradely labeled axons and axon terminals bilaterally in the hypoglossal nucleus (XII) as well as other regions of the brainstem. In the ipsilateral XII, the highest density of BDA labeling was found in the dorsal compartment and the ventromedial subcompartment of the ventral compartment, where BDA labeling formed a dense, patchy distribution. Microinjection of 20% horseradish peroxidase (HRP) ipsilaterally or bilaterally into the tongue resulted in retrograde labeling of XII motoneurons confined to the dorsal and ventral compartments of the hypoglossal motor nucleus. Under light microscopical examination, BDA-labeled terminals were observed closely apposing the somata and primary dendrites of HRP-labeled hypoglossal motoneurons. Two hundred and sixty-five of these BDA-labeled terminals were examined at the ultrastructural level. One hundred and twelve BDA-labeled axon terminals were observed synapsing with either the somata (39%, 44/112) or the large or medium-size dendrites (61%, 68/112) of retrogradely labeled hypoglossal motoneurons. Axon terminals containing spherical vesicles (S-type) formed asymmetric synapses with HRP-labeled hypoglossal motoneuron dendrites. In contrast to this, F_F-type axon terminals, containing flattened vesicles, formed symmetric synapses with both the somata and dendrites of HRP-labeled hypoglossal motoneurons with a preponderance of the contacts on their somata. Axon terminals containing pleomorphic vesicles (F_P-type) were noted forming both symmetric and asymmetric synapses with HRP-labeled hypoglossal motoneuron somata and dendrites. The present study provides anatomical evidence of neuronal projections and synaptic connections from the supratrigeminal region to hypoglossal motoneurons. These data suggest that the supratrigeminal region, as one of the premotor neuronal pools of the hypoglossal nucleus, may coordinate and modulate the activity of tongue muscles during oral motor behaviors.     

GABA and glycine-like immunoreactivity at axoaxonic synapses on 1a muscle afferent terminals in the spinal cord of the rat

The object of this study was to analyze the synaptic interactions of identified muscle spindle afferent axon terminals in the spinal cord of the rat. Group 1a muscle afferents supplying the gastrocnemius muscle were impaled with microelectrodes in the dorsal white matter of the spinal cord and stained by intracellular injection with Neurobiotin. Postembedding immunogold techniques were used to reveal GABA- and glycine-like immunoreactivity in boutons presynaptic to afferent terminals in the ventral horn and the deep layers of the dorsal horn. Serial-section reconstruction was used to reveal the distribution of synaptic contacts of different types on the afferent terminals. The majority of afferent boutons received axoaxonic and made axodendritic or axosomatic synaptic contacts. In the ventral horn, 91% of boutons presynaptic to the afferent terminals were immunoreactive for GABA alone and 9% were immunoreactive for both GABA and glycine. The mean number of axo-axonic contacts received per terminal was 2.7, and the mean number of synaptic contacts at which the terminal was the presynaptic element was 1.4. In the deep layers of the dorsal horn, 58% of boutons presynaptic to afferent terminals were immunoreactive for GABA alone, 31% were immunoreactive for GABA and glycine, and 11% for glycine alone. The mean number of axoaxonic contacts received per afferent terminal in this region was 1.6 and the mean number of synaptic contacts at which the terminal was the presynaptic element was 0.86. This clearly establishes the principle that activity in 1a afferents is modulated by several neurochemically distinct populations of presynaptic neuron.     

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.