Dual axonal terminations from the retrosplenial and visual association cortices in the laterodorsal thalamic nucleus of the rat

Light and electron microscopic tracing studies were conducted to assess the synaptic organization in the laterodorsal thalamic nucleus (LD) of the rat and the laminar origins of corticothalamic terminals from the retrosplenial and visual association cortices to LD. A survey of the general ultrastructure of LD revealed at least three types of presynaptic terminals identified on the basis of size, synaptic vesicle morphology, and synaptic membrane specializations: (1) small axon terminals with round synaptic vesicles (SR), which accounted for the majority of terminal profiles and made asymmetric synaptic contacts predominantly with small dendritic shafts and spines; (2) large axon terminals with round synaptic vesicles (LR), which formed asymmetric synaptic contacts mainly with large dendritic shafts; and (3) small to medium-size axon terminals with pleomorphic synaptic vesicles (SMP), which symmetrically synapsed with a wide range of postsynaptic structures from cell bodies to small dendrites. Synaptic glomeruli were identified, whereas no presynaptic dendrites were found. To characterize and identify corticothalamic terminals arising from the retrosplenial and visual association cortices that project to LD, wheat germ agglutinin conjugated to horseradish peroxidase (WGA–HRP) was injected into these cortices. Axons anterogradely labeled with WGA–HRP ended in both SR and LR terminals. On the other hand, dextran-tetramethylrhodamine injected into LD as a retrograde fluorescent tracer labeled large pyramidal cells of layer V as well as small round or multiform cells of layer VI in the retrosplenial and visual association cortices. These findings provide the possibility that corticothalamic terminations from cortical neurons in layer V end as LR terminals, while those from neurons in layer VI end as SR boutons.     

Synaptic organization of the nucleus raphe dorsalis of the cat

Summary This electron microscopic study describes the different types of synaptic terminals found in the nucleus raphe dorsalis of the adult cat. Serial section analysis was used extensively to confirm the nature of the synaptic contact established by the various classes of terminals.Five different classes of terminals are identified according to the shape and packing density of the synaptic vesicles and type of contact they establish. The most common class (RDI-type) contains densely packed, round, agranular synaptic vesicles and establishes asymmetrical synaptic contacts. A second class (RDII-type) also contains spherical synaptic vesicles, but establishes symmetrical synaptic contacts with dendrites of all sizes. Most of the terminals in these two classes contain a few dense-cored synaptic vesicles, but a small sub-group contains many dense-cored vesicles. A third, less frequent, class (RSI-type) contains sparsely packed spherical synaptic vesicles and the majority of these terminals have asymmetrical contacts. A fourth terminal class contains pleomorphic synaptic vesicles (P-type), contacts dendrites of all sizes, and usually establishes symmetrical synaptic contacts. Finally, boutons thought to be the vesicle-filled excrescences of dendrites (postsynaptic dendrites) are found and in some cases the dendritic origin of these profiles was confirmed by serial sectioning. Such boutons containing pleomorphic vesicles are presynaptic to other such dendrites as well as conventional dendrites, and are postsynapticto the other terminal types described.Somata within the nucleus exhibit somatic spines but receive few synaptic contacts. Most axo-somatic terminals contain either round or pleomorphic vesicles and have postsynaptic thickenings intermediate to the symmetric and asymmetric types.     

Fine structural survey of tyrosine hydroxylase immunoreactive terminals in the myenteric ganglion of the rat duodenum

We have examined whether the noradrenergic neurons have direct synaptic projections to the myenteric ganglion neurons of the duodenum and the ultrastructure of their terminals by using immunogold–silver labeling for tyrosine hydroxylase. In the neuropil of the myenteric ganglia, about half of the axon terminals contained round clear vesicles and the rest of them contained pleomorphic clear vesicles. The sizes of axon terminals contacting the dendrites as a whole were 1.62 ± 0.07 μm. All axon terminals formed asymmetric synaptic contacts with dendrites or somata. Immunohistochemical study revealed that the tyrosine hydroxylase-immunoreactive nerve terminals were distributed throughout the ganglia and contained exclusively pleomorphic clear synaptic vesicles (about 20–80 nm long). The tyrosine hydroxylase-immunoreactive terminals were generally large (1.99 ± 0.07 μm). A considerable number of the tyrosine hydroxylase-immunoreactive terminals made asymmetric synaptic contacts with small dendrites, spines or somata of the myenteric ganglion neurons. Serial ultrathin sections through the myenteric neurons revealed that about 16% of the total number of axosomatic terminals showed tyrosine hydroxylase immunoreactivity. These results indicated that the myenteric ganglion neurons of the duodenum receive direct synaptic projection of sympathetic noradrenergic neurons and that their terminals contain pleomorphic vesicles and form asymmetric synaptic contacts.     

Leucine-enkephalin-like immunoreactive afferent fibers to pudendal motoneurons in the cat

Leucine-enkephalin-like immunoreactive (ENK-LI) afferent fibers to the feline homologue of the Onuf's nucleus (pudendal motoneurons) originate mainly from lamina X of the sacral cord. They make synaptic contacts chiefly upon dendrites of pudendal motoneurons; the synaptic terminals most often contain pleomorphic synaptic vesicles and occasionally round synaptic vesicles. Large-cored vesicles in the axon terminals within the Onuf's nucleus often showed ENK-LI.     

Fine structural survey of tyrosine hydroxylase immunoreactive terminals in the myenteric ganglion of the rat duodenum

We have examined whether the noradrenergic neurons have direct synaptic projections to the myenteric ganglion neurons of the duodenum and the ultrastructure of their terminals by using immunogold–silver labeling for tyrosine hydroxylase. In the neuropil of the myenteric ganglia, about half of the axon terminals contained round clear vesicles and the rest of them contained pleomorphic clear vesicles. The sizes of axon terminals contacting the dendrites as a whole were 1.62 ± 0.07 μm. All axon terminals formed asymmetric synaptic contacts with dendrites or somata. Immunohistochemical study revealed that the tyrosine hydroxylase-immunoreactive nerve terminals were distributed throughout the ganglia and contained exclusively pleomorphic clear synaptic vesicles (about 20–80 nm long). The tyrosine hydroxylase-immunoreactive terminals were generally large (1.99 ± 0.07 μm). A considerable number of the tyrosine hydroxylase-immunoreactive terminals made asymmetric synaptic contacts with small dendrites, spines or somata of the myenteric ganglion neurons. Serial ultrathin sections through the myenteric neurons revealed that about 16% of the total number of axosomatic terminals showed tyrosine hydroxylase immunoreactivity. These results indicated that the myenteric ganglion neurons of the duodenum receive direct synaptic projection of sympathetic noradrenergic neurons and that their terminals contain pleomorphic vesicles and form asymmetric synaptic contacts.     

Ultrastructure of somatostatin-immunoreactive nerve terminals in laminae I and II of the rat trigeminal subnucleus caudalis.

The morphology and distribution of somatostatin-immunoreactive synaptic boutons was studied in the rat trigeminal subnucleus caudalis using pre-embedding electron microscopic techniques. Immunoreactive terminals were found in lamina I and throughout lamina II but were more concentrated in outer lamina II. All immunoreactive terminals contained many round or pleomorphic agranular small synaptic vesicles and some large dense-cored vesicles. Lamina I terminals were all simple dome-shaped and relatively small. They established one asymmetric or slightly asymmetric synapse over a dendritic spine or a small, medium or large dendritic shaft. The large dendrites are probably derived from Waldeyer neurons. Many lamina II immunoreactive terminals were also simple dome-shaped terminals and established asymmetric synaptic contacts with one postsynaptic structure, usually a dendritic spine or a small to medium-sized dendritic shaft. However, other lamina II immunoreactive terminals were larger and displayed more complex morphology and synaptology. Complex immunoreactive terminals had scalloped or smooth contours and made synaptic contacts with more than one postsynaptic profile. In outer lamina II they sometimes constituted the central terminals of typical glomerular synaptic complexes. We conclude that many of the immunoreactive simple terminals probably originate from intrinsic somatostatin-immunoreactive interneurons while some of the more complex ones and the central glomerular terminals are likely to originate from primary afferents. These results are consistent with our accompanying light microscopic study (Alvarez and Priestley, Neuroscience 38, 343-357, 1990) which indicates that somatostatin-immunoreactive primary afferents project preferentially to outer lamina II while the lamina I somatostatin-immunoreactive plexus is likely to originate largely from laminae I and II interneurons. In addition somatostatin-immunoreactive cell bodies were found in lamina II. The heaviest immunoreactivity in these cells was in the Golgi apparatus. Also some vesicles containing dendrites were immunostained, and these were most abundant in inner lamina II. Thus, in trigeminal subnucleus caudalis, somatostatin may be derived from primary afferent synaptic boutons, interneuron synaptic boutons and interneuron dendrites. However, each of these sites probably makes a proportionately different contribution to the total amount of somatostatin released in each lamina or sublamina.     

The synaptic organization of terminals traced from individual labeled retino-geniculate axons in the cat

Individual retino-geniculate axons in the dorsal lateral geniculate nucleus of the cat were filled with horseradish peroxidase and studied with both the light and electron microscope. A procedure was followed which allowed us to identify the size, shape and arrangement of particular terminal swellings by light microscopy and then to study their patterns of synaptic contacts with the electron microscope. Many of the labeled terminals in laminae A and Al have the same fine structural features as retino-geniculate terminals that have been described previously. They are large, contain round synaptic vesicles and pale mitochondria, and are the central processes in glomeruli where they form asymmetric contacts with dendrites and terminals containing pleomorphic synaptic vesicles. Other terminals have the same cytological features but are quite small and are not the central processes in glomeruli. Some of these small terminals form simple axo-dendritec contacts while others participate in very large glomeruli containing several terminals from a single retino-geniculate axon. These different patterns of synaptic contacts made by different terminals can be found on branches of a single axon and correspond to the variations in terminal arbors described in the preceding paper (MASON & ROBSON, 1978).     

Ultrastructural identification of synaptic terminals from cortical axons and from collateral axons of geniculo-cortical relay cells in the perigeniculate nucleus of the cat

Summary Electron microscopic analysis of sections of the perigeniculate nucleus (PGN) of the cat processed with horseradish peroxidase (HRP) histochemistry after massive injections of this enzyme in the visual cortex showed two types of synaptic terminals labeled with HRP reaction products. One type (RLD terminals) is characterized by round synaptic vesicles, large size, dark mitochondria and asymmetrical synaptic contacts with somata and dendrites. The second type (RSD terminals) is characterized by round synaptic vesicles, small size, dark mitochondria and asymmetrical synaptic contacts with dendrites. The HRP + RSD terminals, which were also found in the dorsal lateral geniculate nucleus (LGN), are interpreted as terminals of cortical origin both in the PGN and LGN, since previous studies have identified cortical terminals as being of RSD type in the LGN and in other thalamic nuclei. The HRP + RLD terminals are interpreted as synaptic terminals of collaterals axons of geniculo-cortical relay cells in the PGN labeled by retrograde transport of HRP from the cortex. In addition, in semithin and ultrathin sections somata in the PGN were never found labeled with HRP products indicating the absence of a PGN projection to the visual cortex.     

Ultrastructure of the central subnucleus of the nucleus tractus solitarii and the esophageal afferent terminals in the rat

The central subnucleus of the nucleus tractus solitarii (ceNTS) receives afferent projections from the esophageal wall and projects to the nucleus ambiguus, thus serving as a relay nucleus for peristalsis of the esophagus. Here we examine the synaptic organization of the ceNTS, and its esophageal afferents by using transganglionic anterograde transport of cholera toxin-conjugated horseradish peroxidase (CT-HRP). When CT-HRP was injected into the subdiaphragmatic esophagus, many anterogradely labeled terminals were found only in the ceNTS. The ceNTS was composed of round or oval-shaped, small neurons (14.7x8.7 micro m) containing sparse organelles and an irregularly shaped nucleus. The average number of axosomatic terminals was only 1.3 per section cut through the nucleolus. Most of them (92%) contained round vesicles and formed asymmetric synaptic contacts (Gray's type I), and a few (8%) contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray's type II). All anterogradely labeled terminals contacted dendrites but not the neuronal somata. The labeled terminals were large (2.55+/-0.07 micro m) and exclusively Gray's type I. More than half of them (60%) contacted small dendrites (less than 1 micro m in diameter), and contained dense-cored vesicles. More than 40% of the labeled terminals contacted two to four dendrites, thus forming a synaptic glomerulus. Sometimes a labeled terminal that contacted an unlabeled terminal by an adherent junction was found within the glomerulus. The large terminals and these complex synaptic relations appeared to characterize the esophageal afferent projections in the ceNTS.     

Direct synaptic projections to the myenteric ganglion of the rat subdiaphragmatic esophagus from the dorsal motor nucleus of the vagus

We have examined the ultrastructure of the myenteric ganglion of the subdiaphragmatic esophagus and determined whether the ganglion neurons receive direct projections from the dorsal motor nucleus of the vagus (DMV) using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) as an anterograde tracer. The neurons (22.2 microm x 13.3 microm) of myenteric ganglion in the esophagus contained dark cytoplasm having many free ribosomes, mitochondria, and an oval nucleus, and received only a few axon terminals contacting somata. All axon terminals formed asymmetric synaptic contacts with dendrites or somata. Approximately 85% of the axon terminals contacting dendrites and about 50% of the axon terminals contacting somata contained pleomorphic vesicles, while the rest contained round synaptic vesicles. When WGA-HRP was injected into the DMV, anterogradely labeled fibers and terminals were found in the myenteric ganglia. The WGA-HRP labeled terminals were large (1.97 microm) and contained round clear vesicles and small granular vesicles. These labeled terminals contacted exclusively the small dendrites, but not the somata. These results suggest that the DMV neurons project directly to the myenteric ganglion neurons and regulate the esophageal muscles via the ganglion neurons.     

大鼠延髓后角神经降压肽(NT)的亚细胞定位和胞吐释放

神经降压肽(NT)广泛分布于哺乳动物的中枢神经系统,具有明显的镇痛作用,为了探索其镇痛机理的形态学基础,本文应用电镜免疫组化技术,对大鼠延髓后角NT的超微结构和胞吐释放进行了研究。超微结构显示延髓后角浅层NT轴突终末形态多样,大小不一,含有圆形或多形性清亮小泡及数量不等的大颗粒小泡,它们主要与未标记的树突形成轴-树突触,其突触后成分有的还含有少量清亮小泡。NT免疫反应阳性树突可分为两类:一类主要含微管;另一类主要含大颗粒小泡,有的尚可见少量清亮小泡。这两类NT树突可成为未标记的含圆形小泡终末、多形性小泡终末以及突触小球中央轴突终末的突触后成分,提示后角浅层NT神经元可接受不同种类轴突终末(包括一级伤害性传入纤维)的传入(?)动,然后可能再通过一个抑制性中间神经元,抑制痛觉的传递。本文还观察到有少量NT终末内的大颗粒小泡靠近突触活性区处,而更多见它们沿非突触部位轴膜分布,并与其融合,形成胞吐。本文认为NT既可在突触活性区处又可能在非突触部位释放。     

Ultrastructure and synaptic relations of neural elements containing glutamic acid decarboxylase (GAD) in the perigeniculate nucleus of the cat

Summary The perigeniculate nucleus of the cat (PGN) was examined at light and electron microscopic levels after immunocytochemical labeling for the gamma-aminobutyric acid (GABA) synthesizing enzyme, glutamic acid decarboxylase (GAD). In light microscopic sections, virtually all perikarya were found to be labeled (GAD+), as well as proximal dendrites, fibres and punctiform elements. Cells in the thalamic reticular nucleus (TRN) dorsal to PGN were also labeled. Ultrastructural analysis of PGN showed immunoreactivity in all somata, in dendrites and in the following vesicle containing profiles: 1.) F1 terminals, which are characterized by large size, dark mitochondria, and pleomorphic vesicles. These terminals form symmetrical synaptic contacts with somata, somatic spines and with dendrites of GAD+ PGN cells. 2.) F2 terminals, which are smaller than F1 terminals, contain also pleomorphic vesicles and frequently make serial synapses of the symmetric type with other F2 terminals. Presumably, F1 terminals are formed by collaterals of PGN-cell axons and F2 terminals by vesicle containing dendrites of PGN cells. Terminals devoid of immunoreactivity included: 1.) RLD terminals characterized by large size, round vesicles, dark mitochondria, and by asymmetric synaptic contacts with somata, especially with somatic spines, and with dendrites of GAD+ perigeniculate neurons; 2.) RSD terminals, characterized by small size, round vesicles and dark mitochondria, which make asymmetric synapses with GAD+ dendrites of medium and small size; 3.) Multivesicular (MV) terminals with variably shaped vesicles including dense core vesicles synapsing on GAD+ dendrites. There are reasons to believe that RSD terminals belong to corticofugal axons and RLD terminals to collateral axons of LGN relay cells. The origin of MV terminals remains to be determined. The GABAergic nature of the PGN cells conforms with the presumed function of these cells as mediators of inhibition of LGN relay cells. The complex synaptic relations observed between GAD+ elements in the PGN would allow for reciprocal inhibition between perigeniculate cells.Supported in part by NIH grants EY02877 to V.M. Montero and HD 03352 to the Waisman Center     

Glycine-containing terminals in the rat dorsal vagal complex

The present study examined the distribution of glycine and glycine-receptors in the dorsal vagal complex using pre-embedding immunocytochemistry. Glycine-immunoreactive terminals were present in moderate densities in the medial, intermediate, interstitial, commissural and ventrolateral subnuclei of the nucleus tractus solitarii. The dorsolateral nucleus tractus solitarii and the dorsal vagal motor nucleus contained only very few, scattered glycine-containing terminals. Glycine terminals appeared to be concentrated in regions of the dorsal vagal complex receiving primary vagal afferents, though previous studies have suggested that glycine is not present in these afferents. A conspicuously high concentration of glycine terminals was observed in the medial nucleus tractus solitarii where a population of cholinergic neurons has been identified previously. Ultrastructurally glycine immunoreactivity was principally associated with terminals containing flattened, pleomorphic vesicles and forming symmetrical synaptic contacts, mostly with dendrites. Glycine receptor immunoreactivity was present throughout the dorsal vagal complex with little evidence of subnuclear localization. With electron-microscopic examination, glycine receptor immunoreactivity was associated with dendritic membranes and was associated presynaptically with terminals containing flattened pleomorphic vesicles.

Overall, the present data provide evidence consistent with a neurotransmitter role for glycine in the dorsal vagal complex. The presence of glycine in regions of the dorsal vagal complex receiving vagal afferents suggests a prominent role for this neurotransmitter in autonomic regulation.     

Fine distribution of substance P-like immunoreactivity in the dorsal nucleus of the vagus nerve in cats

The ultrastructure of substance P (SP)-immunoreactive elements in the cat dorsal motor nucleus of the vagus nerve was examined using pre- and post-embedding immunocytochemical procedures. Substance P-like immunoreactivity was observed in axon terminals and axon fibres which were mostly unmyelinated. Quantitative data showed that at least 16% of axon terminals contained SP. Their mean diameter was larger than that of their non-immunoreactive counterparts. Most (83%) SP-containing terminals were seen to contact dendrites but some were observed adjoining soma or entirely embedded in the cytoplasm of vagal neurons (4.5%). Only 0.5% were observed to contact soma of internuerons. A few immunoreactive axon terminals (4%) were observed in contact with non-immunoreactive axon terminals. Round agranular vesicles and numerous dense core vesicles were visible in most SP-containing axon terminals (84.6%). The immunogold procedure showed the preferential subcellular location of SP to be dense core vesicles. In 32.4% of cases, SP-containing terminals were involved in synaptic contacts that were generally of the asymmetrical Gray type 1 and mainly apposed dendrites. The theoretical total of synaptic contacts was 74.5% and this suggests the existence of weak non-synaptic SP innervation involving approximately 25% of SP-containing axon terminals. No axo-axonic synapses were observed in the dorsal vagal nucleus. These results support the hypothesis that SP found in the dorsal vagal nucleus originates partly from vagal afferents and is involved in direct modulation of visceral functions mediated by vagal preganglionic neurons.     

Glutamatergic components of the retrosplenial granular cortex in the rat

The ultrastructural characteristics, distribution and synaptic relationships of identified, glutamate-enriched thalamocortical axon terminals and cell bodies in the retrosplenial granular cortex of adult rats is described and compared with GABA-containing terminals and cell bodies, using postembedding immunogold immunohistochemistry and transmission electron microscopy in animals with injections of cholera toxin- horseradish peroxidase (CT-HRP) into the anterior thalamic nuclei. Anterogradely labelled terminals, identified by semi-crystalline deposits of HRP reaction product, were approximately 1 microm in diameter, contained round, clear synaptic vesicles, and established asymmetric (Gray type I) synaptic contacts with dendritic spines and small dendrites, some containing HRP reaction product, identifying them as dendrites of corticothalamic projection neurons. The highest densities of immunogold particles following glutamate immunostaining were found over such axon terminals and over similar axon terminals devoid of HRP reaction product. In serial sections immunoreacted for GABA, these axon terminals were unlabelled, whereas other axon terminals, establishing symmetric (Gray type II) synapses were heavily labelled. Cell bodies of putative pyramidal neurons, containing retrograde HRP label, were numerous in layers V-VI; some were also present in layers I-III. Most were overlain by high densities of gold particles in glutamate but not in GABA immunoreacted sections. These findings provide evidence that the terminals of projection neurons make synaptic contact with dendrites and dendritic spines in the ipsilateral retrosplenial granular cortex and that their targets include the dendrites of presumptive glutamatergic corticothalamic projection neurons.     

Fine structural survey of the intermediate subnucleus of the nucleus tractus solitarii and its glossopharyngeal afferent terminals

The intermediate subnucleus of the nucleus tractus solitarii (imNTS) receives somatosensory inputs from the soft palate and pharynx, and projects onto the nucleus ambiguus, thus serving as a relay nucleus for swallowing. The ultrastructure and synaptology of the rat imNTS, and its glossopharyngeal afferent terminals, have been examined with cholera toxin-conjugated horseradish peroxidase (CT-HRP) as an anterograde tracer. The imNTS contained oval or ellipsoid-shaped, small to medium-sized neurons (18.2×11.4 μm) with little cytoplasm, few cell organelles and an irregularly shaped nucleus. The cytoplasm often contained one or two nucleolus-like stigmoid bodies. The average number of axosomatic terminals was 1.8 per profile. About 83% of them contained round vesicles and formed asymmetric synaptic contacts (Gray’s type I), while about 17% contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray’s type II). The neuropil contained small or large axodendritic terminals, and about 92% of them were Gray’s type I. When CT-HRP was injected into the nodose ganglion, many labeled terminals were found in the imNTS. All anterogradely labeled terminals contacted dendrites but not somata. The labeled terminals were usually large (2.69±0.09 μm) and exclusively of Gray’s type I. They often contacted more than two dendrites, were covered with glial processes, and formed synaptic glomeruli. A small unlabeled terminal occasionally made an asymmetric synaptic contact with a large labeled terminal. The large glossopharyngeal afferent terminals and the neurons containing stigmoid bodies characterized the imNTS neurons that received pharyngeal afferents.     

大鼠孤束核内5－羟色胺能轴突终末的突触联系

用顺行演变与免疫组织化学结合的双标技术，电镜下观察大鼠孤束核内５－羟色胺样免疫反应（５－ＨＴ－ＬＩ）轴突终末的突触联系，特别是与迷走神经初级传入（溃变）终末的关系。发现：１．５－ＨＴ－ＬＩ轴突终末和迷走神经初级传入终末终止于同一个树突形成轴－树突触；２．５－ＨＴ－ＬＩ轴突终末与树突形成轴－树突触；３．５－ＨＴ－ＬＩ轴突终末与迷走神经初级传入终末或非标记的轴突终未形成轴一轴相贴（ａｘｏ－ａｘｏｎｉｃ－ｃｏｎｔａｃｔ）。以上结果提示，孤束核内的５－ＨＴ能神经成分可能通过突触后、突触前机制调控经迷走神经传入的信息。     

Glutamatergic components of the retrosplenial granular cortex in the rat

The ultrastructural characteristics, distribution and synaptic relationships of identified, glutamate-enriched thalamocortical axon terminals and cell bodies in the retrosplenial granular cortex of adult rats is described and compared with GABA-containing terminals and cell bodies, using postembedding immunogold immunohistochemistry and transmission electron microscopy in animals with injections of cholera toxin- horseradish peroxidase (CT-HRP) into the anterior thalamic nuclei. Anterogradely labelled terminals, identified by semi-crystalline deposits of HRP reaction product, were approximately 1 m in diameter, contained round, clear synaptic vesicles, and established asymmetric (Gray type I) synaptic contacts with dendritic spines and small dendrites, some containing HRP reaction product, identifying them as dendrites of corticothalamic projection neurons. The highest densities of immunogold particles following glutamate immunostaining were found over such axon terminals and over similar axon terminals devoid of HRP reaction product. In serial sections immunoreacted for GABA, these axon terminals were unlabelled, whereas other axon terminals, establishing symmetric (Gray type II) synapses were heavily labelled. Cell bodies of putative pyramidal neurons, containing retrograde HRP label, were numerous in layers V–VI; some were also present in layers I–III. Most were overlain by high densities of gold particles in glutamate but not in GABA immunoreacted sections. These findings provide evidence that the terminals of projection neurons make synaptic contact with dendrites and dendritic spines in the ipsilateral retrosplenial granular cortex and that their targets include the dendrites of presumptive glutamatergic corticothalamic projection neurons.     

Ultrastructural features of non-commissural GABAergic neurons in the medial vestibular nucleus of the monkey.

The ultrastructural characteristics of non-degenerating GABAergic neurons in rostrolateral medial vestibular nucleus were identified in monkeys following midline transection of vestibular commissural fibers. In the previous papers, we reported that most degenerated cells and terminals in this tissue were located in rostrolateral medial vestibular nucleus, and that many of these neurons were GABA-immunoreactive. In the present study, we examined the ultrastructural features of the remaining neuronal elements in this medial vestibular nucleus region, in order to identify and characterize the GABAergic cells that are not directly involved in the vestibular commissural pathway related to the velocity storage mechanism. Such cells are primarily small, with centrally-placed nuclei. Axosomatic synapses are concentrated on polar regions of the somata. The proximal dendrites of GABAergic cells are surrounded by boutons, although distal dendrites receive only occasional synaptic contacts. Two types of non-degenerated GABAergic boutons are distinguished. Type A terminals are large, with very densely-packed spherical synaptic vesicles and clusters of large, irregularly-shaped mitochondria with wide matrix spaces. Such boutons form symmetric synapses, primarily with small GABAergic and non-GABAergic dendrites. Type B terminals are smaller and contain a moderate density of round/pleomorphic vesicles, numerous small round or tubular mitochondria, cisterns and vacuoles. These boutons serve both pre- and postsynaptic roles in symmetric contacts with non-GABAergic axon terminals. On the basis of ultrastructural observations of immunostained tissue, we conclude that at least two types of GABAergic neurons are present in the rostrolateral portion of the monkey medial vestibular nucleus: neurons related to the velocity storage pathway, and a class of vestibular interneurons. A multiplicity of GABAergic bouton types are also observed, and categorized on the basis of subcellular morphology. We hypothesize that "Type A" boutons correspond to Purkinje cell afferents in rostrolateral medial vestibular nucleus, "Type B" terminals represent the axons of GABAergic medial vestibular nucleus interneurons, and "Type C" boutons take origin from vestibular commissural neurons of the velocity storage pathway.