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.     

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.     

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.     

An ultrastructural study of neurotensin-like immunoreactive terminals in the mediodorsal thalamic nucleus of the rat.

Neurotensin-like immunoreactive (NTir) axon terminals in the mediodorsal nucleus of the thalamus (MD) in the adult rat were demonstrated by electron microscopic immunohistochemistry. Most NTir terminals were large (greater than 2 microns in diameter) with round synaptic vesicles and asymmetrical synaptic contacts although smaller (less than 1.5 microns in diameter) axon terminals were also labeled. Both types of terminals were found in the medial and central parts of MD with the greatest density in the medial part. These NTir boutons have similar ultrastructural features as anterogradely labeled terminals from the piriform cortex and the preoptic area, which have previously been identified as sources of NTir axons in MD. A few NTir boutons were also found in the medial part of MD with pleomorphic vesicles and symmetrical synaptic contacts.     

The GABAergic neurons and axon terminals in the lateralis medialis-suprageniculate nuclear complex of the cat: GABA-immunocytochemical and WGA-HRP studies by light and electron microscopy

In order to get more detailed information on the neural circuit of the lateralis medialis-suprageniculate nuclear (LM-Sg) complex of the cat, the GABAergic innervation of this complex was studied by GABA immunohistochemical techniques. Small immunoreactive cells were found throughout the LM-Sg complex. On the basis of their ultrastructural features, these GABAergic cells were identified as Golgi type II interneurons. The neuropil of this nucleus displayed a conspicuous granular immunoreactivity. Ultrastructurally, the immunoreactive neural profiles in the neuropil were identified as the presynaptic dendrites of interneurons, myelinated axons, or axon terminals. The GABAergic dendritic profiles, containing pleomorphic synaptic vesicles, were involved in synaptic glomeruli. Additionally, GABAergic axon terminals containing pleomorphic synaptic vesicles formed symmetric axodendritic synaptic contacts mainly in the extraglomerular neuropil. They appeared to correspond to either axon terminals from the thalamic reticular nucleus (TRN) or the axon terminals of interneurons. The projections from the TRN to the LM-Sg complex were studied by using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Following injection of WGA-HRP into the LM-Sg complex, a number of retrogradely labeled cells were observed in the TRN. The connections between the TRN and the LM-Sg complex appeared to be topographically organized, the dorsal TRN being connected mainly with the dorsomedial portion of the LM-Sg complex, and the ventral TRN being connected chiefly with the ventrolateral portion of the LM-Sg complex. Following injection of the tracer into the TRN, ultrastructural examination of anterograde labeling in the LM-Sg complex revealed that labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts mainly with small to medium-sized dendrites. The labeled terminals were not involved in synaptic glomeruli. The present results provide anatomic support for the contention that the projection cells of the LM-Sg complex may be inhibited by both the TRN axons and interneurons, probably through the mediation of GABA.     

Synaptic organization of the rat parafascicular nucleus,with special reference to its afferents from the superior colliculus and the pedunculopontine tegmental nucleus

The synaptic organization of afferents to the parafascicular nucleus (Pf) of the thalamus was studied in rats. In the Pf, three types of axon terminals were identified: the first type was a small terminal with round synaptic vesicles forming an asymmetric synapse, the second type was a large terminal with round synaptic vesicles forming an asymmetric synapse, and the third type was a terminal with pleomorphic vesicles forming a symmetric synapse. They were named SR, LR and P boutons, respectively. In order to determine the origin of these axon terminals, biotinylated dextran amine (BDA) was injected into the main afferent sources of the Pf, the superior colliculus (SC) and the pedunculopontine tegmental nucleus (PPN). Axon terminals from the SC were both SR and LR boutons which made synaptic contacts with somata and dendrites. PPN afferents were SR boutons, which made synaptic contacts with somata and smaller dendrites. Double-labeled electron microscopic studies, in which a retrograde tracer (wheat germ agglutinin conjugated to horseradish peroxidase: WGA-HRP) was injected into the striatum and an anterograde tracer (BDA) into the SC revealed that SC afferent terminals made synapses directly with Pf neurons that projected to the striatum. Another experiment was performed to find out whether two different afferents converged onto a single Pf neuron. To address this question, two different tracers were injected into the SC and PPN in a rat. Electron microscopically, both afferent terminals from the SC and PPN made synaptic contacts with the same dendrite. Our results prove that a single neuron of the rat Pf received convergent projections from two different sources.     

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and gamma-aminobutyric acidergic cortical cells in the prelimbic cortex of the rat

Although the reciprocal interconnections between the prefrontal cortex and the mediodorsal nucleus of the thalamus (MD) are well known, the involvement of inhibitory cortical interneurons in the neural circuit has not been fully defined. To address this issue, we conducted three combined neuroanatomical studies on the rat brain. First, the frequency and the spatial distribution of synapses made by reconstructed dendrites of nonpyramidal neurons were identified by impregnation of cortical cells with the Golgi method and identification of thalamocortical terminals by degeneration following thalamic lesions. Terminals from MD were found to make synaptic contacts with small dendritic shafts or spines of Golgi-impregnated nonpyramidal cells with very sparse dendritic spines. Second, a combined study that used anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry indicated that PHA-L-labeled terminals from MD made synaptic junctions with GABA-immunoreactive dendritic shafts and spines. Nonlabeled dendritic spines were found to receive both axonal inputs from MD with PHA-L labelings and from GABAergic cells. In addition, synapses were found between dendritic shafts and axon terminals that were both immunoreactive for GABA. Third, synaptic connections between corticothalamic neurons that project to MD and GABAergic terminals were investigated by using wheat germ agglutinin conjugated to horseradish peroxidase and postembedding GABA immunocytochemistry. GABAergic terminals in the prelimbic cortex made symmetrical synaptic contacts with retrogradely labeled corticothalamic neurons to MD. All of the synapses were found on cell somata and thick dendritic trunks. These results provide the first demonstration of synaptic contacts in the prelimbic cortex not only between thalamocortical terminals from MD and GABAergic interneurons but also between GABAergic terminals and corticothalamic neurons that project to MD. The anatomical findings indicate that GABAergic interneurons have a modulatory influence on excitatory reverberation between MD and the prefrontal cortex.     

Mamillary body in the rat: topography and synaptology of projections from the subicular complex, prefrontal cortex, and midbrain tegmentum

The retrograde and anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) has been used to trace afferent connections of the rat mamillary body (MB) at the light and electron microscopic levels. Injections of WGA-HRP into different parts of the MB resulted in heavy retrograde labeling in the subicular complex, medial prefrontal cortex, and dorsal and ventral tegmental nuclei. Injections of WGA-HRP into each of these brain regions, respectively, resulted in anterograde labeling with specific distributions and characteristic synaptic organizations in the MB. Projections from the rostrodorsal and caudoventral subiculum terminated in a topographically organized laminar fashion in the medial mamillary nucleus bilaterally, whereas afferent projections from the presubiculum and parasubiculum terminated only in the lateral mamillary nucleus. Labeled axon terminals which originated from the subicular complex were characterized by round vesicles and formed asymmetric synaptic junctions with small-diameter dendrites and dendritic spines in the medial and lateral mamillary nuclei. Projections from the prefrontal cortex originated mainly in the infralimbic area and to a lesser degree in the prelimbic and anterior cingulate areas. Injections of tracer into these brain regions gave rise to dense labeling of axon terminals in the medial mamillary nucleus, pars medianus, and in the anterior dorsomedial portion of the pars medialis. The labeled terminals were characterized by round vesicles and formed asymmetric synaptic junctions with small-diameter dendrites and dendritic spines. Projections from the dorsal tegmental nucleus terminated in the ipsilateral lateral mamillary nucleus, whereas afferent projections from the anterior and posterior subnuclei of the ventral tegmental nucleus terminated topographically in the medial mamillary nucleus. The ventral tegmental nucleus, pars anterior projected to the midline region of the medial nucleus and the dorsolateral and ventromedial subdivisions of the pars posterior projected to medial and lateral parts of the medial nucleus, respectively. In contrast to the synaptic morphology of subicular complex and medial prefrontal cortex axon terminals in the MB, labeled axon terminals in the MB which originated from the midbrain tegmentum were characterized by pleomorphic vesicles and formed symmetric synaptic junctions with neuronal somata and proximal dendrites as well as distal dendrites and dendritic spines.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Ultrastructure and Distribution of Corticothalamic Fiber Terminals From the Posterior Cingulate Cortex and the Presubiculum to the Anteroventral Thalamic Nucleus of the Rat

The ultrastructure of axon terminals in the anteroventral thalamic nucleus arising in the cingulate cortex and in the presubiculum was examined using the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase in rats. Anterogradely labeled axonal arborizations arising from the posterior cingulate cortex were concentrated bilaterally in the ventral part of the anteroventral nucleus. In electron micrographs these thalamic terminals arising from the posterior cingulate cortex were consistently small, contained round vesicles, and established asymmetric contacts on distal dendritic processes. In contrast, the axonal arborizations arising from the presubiculum were concentrated ipsilaterally in the dorsal part of the anteroventral nucleus and comprised two identifiable populations of terminals. The smaller terminals, which contained densely packed round vesicles, established asymmetric synaptic contacts on distal dendritic processes and resembled the posterior cingulate cortex terminals described above. The other population of the presubiculum terminals consisted of medium-sized terminals. These contained loosely packed round vesicles and established asymmetric synaptic contacts on proximal dendritic processes. These results indicate that the posterior cingulate cortex and the presubiculum project differentially upon the anteroventral thalamic nucleus. They also indicate that although the posterior cingulate cortex gives rise to only one type of corticothalamic terminal, the presubiculum gives rise to two types of corticothalamic terminals. When taken together, these data suggest that these different limbic cortical areas might subserve distinct roles in the anteroventral thalamic nucleus function.     

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and layer III pyramidal cells in the prelimbic cortex of the rat

A combined study of anterograde axonal degeneration and Golgi electron microscopic technique was designed to examine the distribution and density of axon terminals from the mediodorsal thalamic nucleus (MD) over layer III pyramidal cells in the prelimbic cortex of the rat. The reconstructive analysis of serial ultrathin sections of gold-toned apical and basal dendrites of layer III pyramidal cells showed that degenerating thalamocortical axon terminals from MD formed asymmetrical synaptic contacts predominantly with dendritic spines of the identified basal dendrites as well as apical dendrites. There was little difference in the numerical density of thalamocortical synapses from MD per unit length of both apical and basal dendrites.     

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.     

Electron microscopic evidence that axon terminals from the mediodorsal thalamic nucleus make direct synaptic contacts with callosal cells in the prelimbic cortex of the rat

A combined study of anterograde axonal degeneration and HRP retrograde labeling has shown that there exist monosynaptic connections between afferent fibers from the mediodorsal thalamic nucleus (MD) and callosal cells in the prelimbic cortex of the rat. Degenerating axon terminals from MD made asymmetrical synaptic contacts with dendritic spines from apical dendrites of layer III pyramidal cells that were retrogradely labeled with HRP after its injection into the prelimbic cortex contralateral to MD lesions.     

Direct synaptic contacts on the myenteric ganglia of the rat stomach from the dorsal motor nucleus of the vagus

The myenteric ganglia regulate not only gastric motility but also secretion, because a submucous plexus is sparsely developed in the rodent stomach. We have examined whether the neurons of the dorsal motor nucleus of the vagus (DMV) have direct synaptic contacts on the myenteric ganglia and the ultrastructure of the vagal efferent terminals by using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The myenteric ganglia of the rat were composed of four types of neurons, i.e., small, medium-sized, large, and elongated neurons. The average numbers of axosomatic terminals per profile were 2.0 on the small neurons, 3.1 on the medium-sized neurons, 1.2 on the large neurons, and 4.2 on the elongated neuron. More than half of the axosomatic terminals contained round vesicles and formed asymmetric synaptic contacts on the small, medium-sized, and large neurons. About 80% of the axosomatic terminals on the elongated neurons contained pleomorphic vesicles and formed asymmetric synaptic contacts. When WGA-HRP was injected into the DMV, many anterogradely labeled terminals were found around the myenteric neurons. The labeled terminals were large (3.16 +/- 0.10 microm) and contacted exclusively the somata. Most of them (about 90%) contained round vesicles and formed asymmetric synaptic contacts. Serial ultrathin sections revealed that almost all neurons in a ganglion received projections from the DMV. The vagal axon terminals generally contacted the medium-sized or the elongated neurons, whereas a few labeled terminals contacted the small and the large neurons. The present results indicate that the DMV projects to all types of neurons and that their axon terminals contain mostly round synaptic vesicles and form asymmetric synaptic contacts.     

Ultrastructure of the mammillotegmental projections to the ventral tegmental nucleus of Gudden in the rat.

This study examines the termination pattern of axons from the medial mammillary nucleus within the ventral tegmental nucleus of Gudden (TV) in rats by using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The neuropil of TV contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic synaptic vesicles. These types make up 55.6%, 26.1%, and 18.3%, respectively, of all normal axodendritic terminals. Injection of WGA-HRP into the medial mammillary nucleus permits ultrastructural recognition of anterogradely labeled terminals within the TV. More than 80% of the labeled terminals contain round synaptic vesicles and form asymmetric synaptic contacts, whereas about 16% contain flat synaptic vesicles with symmetric synaptic contacts. There are a few labeled terminals with pleomorphic vesicles and only a few axosomatic terminals. Almost all labeled terminals are small, having diameters of less than 1.5 microns. Compared with the distributions of normal and labeled terminals with round vesicles, there is an increase of the percentage of labeled terminals with round vesicles on the intermediate dendrites (1-2 microns diameter) and a decrease on the distal dendrites (less than 1 micron diameter). Anterogradely labeled axon terminals often contact retrogradely labeled dendrites. These results suggest that the medial mammillary neurons send mainly excitatory as well as a few inhibitory inputs to the dendrites of TV and have direct reciprocal contacts with the TV neurons.     

Fine structure of the lateral mammillary projection to the dorsal tegmental nucleus of Gudden in the rat.

The synaptic organization of projections from the lateral mammillary neurons within the dorsal tegmental nucleus of Gudden is studied in the rat with the aid of anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The dorsal tegmental nucleus consists of the pars ventralis (TDV) and the pars dorsalis (TDD). The normal neuropil of the dorsal tegmental nucleus contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic vesicles. They make up 44%, 5%, and 51%, respectively, of all axodendritic terminals in the TDV, and 62%, 1%, and 37% in the TDD. Injection of WGA-HRP into the lateral mammillary nucleus permits ultrastructural recognition of many anterograde labeled terminals within both the TDV and TDD. In the TDV, 81% of the labeled terminals contain round synaptic vesicles and make asymmetric synaptic contacts. A few of the labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts. More than 50% of the labeled terminals contact intermediate dendrites (1-2 microns diameter). In the TDD, almost all labeled terminals are small, contain round vesicles, and make asymmetric synaptic contacts. These terminals mainly contact intermediate as well as distal (less than 1 micron diameter) dendrites. There are only a few labeled terminals with pleomorphic vesicles and no terminals with flat vesicles. The termination pattern of the lateral mammillary neurons in the TDV is similar to that in the TDD. Anterograde labeled axon terminals often contact retrograde labeled dendrites in the TDV. No reciprocal connections are present in the TDD. These results suggest that the TDV and the TDD receive mainly excitatory and a few inhibitory inputs from the lateral mammillary nucleus. The TDV neurons also have direct reciprocal connections with the lateral mammillary neurons.     

The Convergence of Axon Terminals from the Mediodorsal Thalamic Nucleus and Ventral Tegmental Area on Pyramidal Cells in Layer V of the Rat Prelimbic Cortex

We investigated the ultrastructural basis of the synaptic convergence of afferent fibres from the mediodorsal thalamic nucleus (MD) and the ventral tegmental area (VTA) on the prefrontal cortical neurons of the rat by examining the synaptic relationships between thalamocortical or tegmentocortical terminals labelled with anterograde markers [lesion-induced degeneration or transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA—HRP)] and randomly selected unlabelled apical dendrites of layer V pyramidal cells in the prelimbic cortex. WGA—HRP-labelled terminals from the VTA ranged in diameter from 0.7 to 2.8 μm and established synaptic contacts with large dendritic profiles, i.e. proximal segments of apical dendritic shafts and spines from layer V pyramidal cells. Symmetrical synapses, i.e. inhibitory synapses, were more often seen than asymmetrical ones. Degenerating terminals from the MD formed asymmetrical synapses on dendritic spines or occasionally on small dendritic shafts of apical dendrites from layer V pyramidal cells, which received tegmentocortical synapses, mostly within layer III. Thalamocortical synapses were more distally distributed over common apical dendrites than tegmentocortical synapses, although some of them overlapped. The numerical density of direct synaptic inputs from the MD and VTA was low. These results suggest that fibres from the VTA exert their inhibitory effects directly on pyramidal cells in layer V via synaptic junctions with apical dendrites of these pyramidal cells, and that the tegmentocortical fibres are in an ideal anatomical position to modulate the reverberatory circuits between the MD and the prelimbic cortex.     

The fine structure of the perigeniculate nucleus in the cat

The fine structure of the cat's perigeniculate nucleus has been analyzed and compared to that of dorsal thalamic relay nuclei. Golgi preparations and electron micrographs of perigeniculate cells commonly show somatic spines. The most common presynaptic elements for these spines and for the adjacent perikaryal surfaces are relatively large axon terminals containing round synaptic vesicles and making multiple asymmetric contacts. These "RLD" terminals (so termed for their round vesicles, large average size of the terminals, and dark mitochondria) are also presynaptic to dendritic spines and shafts of proximal and secondary dendrites. Comparisons with adjacent parts of the dorsal lateral geniculate nucleus show that these RLD terminals are cytologically distinct from retinogeniculate terminals and that small numbers of RLD terminals also occur in the geniculate A laminae. Three other major classes of perigeniculate synaptic terminals, resemble major classes of terminals in the dorsal lateral geniculate nucleus. These include two types of terminal with flat or ovoid synaptic vesicles and dark mitochondria, "FD1" and "FD2" terminals, and a class of small terminal with densely clustered round vesicles and dark mitochondria, "RSD" terminals. RSD terminals, which resemble corticogeniculate axon terminals, represent the only class of perigeniculate terminal that does not contact perikarya. FD2 terminals resemble lateral geniculate presynaptic dendrites and participate in serial and triadic synaptic contacts, being both pre- and postsynaptic; however, in contrast to the arrangement characteristic of thalamic relay nuclei, these contacts do not occur within synaptic glomeruli. A fifth major class of perigeniculate presynaptic terminal has large flat or polymorphic synaptic vesicles and pale mitochondria. These "FP" terminals are seen infrequently in the lateral geniculate A laminae. Similarities between perigeniculate and lateral geniculate fine structure may relate in part to common sources of afferent input to the two nuclei.     

Serotonin-containing structures in the nucleus raphe dorsalis of the cat: an ultrastructural analysis of dendrites, presynaptic dendrites, and axon terminals

In the nucleus raphe dorsalis of the cat, an electron microscopic immunocytochemistry method was used to identify the fine structure of serotoninergic dendritic profiles and axon terminals analyzed in serial sections. Two classes of serotoninergic dendrites were distinguished in the nucleus. The first class was constituted by conventional serotonin (5-HT) dendrites that were contacted by unlabeled axon terminals containing differing populations of synaptic vesicles. The second class consisted of serotoninergic dendrites that contained vesicles in their dendritic shafts. Such 5-HT dendrites were further subdivided into two groups according to their synaptic contacts. In some 5-HT vesicle-containing dendrites, the vesicles were densely packed in small clusters and were associated with a well-defined synaptic specialization. These dendrites were classified as serotoninergic presynaptic dendrites and established synaptic contacts with unlabeled and labeled dendrites and were contacted by unlabeled axon terminals. In other 5-HT vesicle-containing dendrites, extensive serial section examination showed that the vesicles could be observed near the membrane but were never found to be associated with any synaptic membrane specialization. Serotoninergic axon terminals that were presumed to be recurrent collaterals of 5-HT neurons were present in the nucleus. Some of them were observed in synaptic contact with dendrites or dendritic protrusions whereas others did not exhibit synaptic specializations. The existence of serotoninergic dendrodendritic synaptic contacts and axon terminals suggests direct local interactions between serotoninergic neurons within the nucleus raphe dorsalis.     

Fine structure of calcitonin gene-related peptide immunoreactive synaptic contacts in the thalamus of the rat

Recent studies have shown a prominent calcitonin gene-related peptide immunoreactive (CGRP-ir) pathway extending from the external medial and external lateral para-brachial nuclei to the area surrounding and including the gustatory nuclei in the thalamus, and the cortex and amygdala. The function of the CGRP-ir pathway is not completely understood, but may be involved with the processing of both nociceptive and gustatory information in the thalamus. The purpose of this study was to characterize the nature of the CGRP-ir synaptic contacts in the gustatory nucleus. Electron microscopic examination of CGRP-ir synaptic contacts revealed two classes of CGRP-ir terminals. One class, which was large, formed asymmetric synaptic contacts on dendritic appendages, had many small, round synaptic vesicles, and heavy patches of reaction product which obscured any underlying organelles. Since similar terminals in unstained tissue contained large numbers of dense-cored vesicles, it was concluded that CGRP-ir was contained predominantly in dense-cored vesicles. A second class of CGRP-ir terminals was smaller and made either asymmetric or symmetric synaptic contacts. Both symmetric and asymmetric small terminals contained small, round synaptic vesicles and fewer patches of dense reaction product. Several of the CGRP-ir terminals making symmetric contacts also contained pleomorphic vesicles. There were very few contacts on cell bodies. There were no contacts on other CGRP-ir elements, somal or dendritic, or on axon terminals. None of the CGRP-ir terminal elements were postsynaptic to unlabeled terminals. Axons containing CGRP-ir were primarily unmyelinated, but a few myelinated axons were also seen. © 1993 Wiley-Liss, Inc.