Ultrastructural identification and localization of climbing fiber terminals in the fastigial nucleus of the cat

Following injections of 3H-leucine and 35S-methionine in the caudal half of the medial accessory olive, labeled climbing fibers were found contralateral to the injection site in the sagittal A-zone of the cerebellar vermis and in the fastigial nucleus. Labeling in the fastigial nucleus was analyzed with ultrastructural autoradiography. Labeled boutons of climbing fibers were found in the neuropil but never on somata. They contain spherical vesicles and occasionally some dense core vesicles in an electron-lucent matrix. The terminals of climbing fiber collaterals in the fastigial nucleus resemble climbing fiber terminals in the molecular layer with respect to their internal ultrastructural characteristics.     

GABAergic elements in the neuronal circuits of the monkey neostriatum: a light and electron microscopic immunocytochemical study

An antibody raised in rabbits against a GABA-BSA conjugate was used together with the PAP technique to label elements in the neostriatum of three Old World monkeys. Light microscopy revealed numerous immunoreactive medium-size neurons of various staining intensities, some of which had indented nuclei, as well as an occasional large cell. The neuropil showed a plexus of fine processes with frequent puncta. Ultrastructurally, the medium-size GABA-positive neurons were of two types: one with smooth nuclei and scanty cytoplasm, similar to spiny I cells, the other with invaginated nuclear envelopes and more abundant perikaryon, resembling the aspiny type. Correspondingly, labeled dendrites were either spiny or varicose. Some stained axons were myelinated, and the boutons had either large and ovoid, or small and pleomorphic vesicles. All of these boutons formed symmetric synapses, the former type with GABA-positive dendritic shafts but also with unlabeled dendrites; the latter type usually with GABA-negative elements, either dendrites, dendritic spines, or somata. Synapses were also observed between unreactive boutons and immunostained dendrites. Terminals with densely packed, small round vesicles that established asymmetric synapses with spines were always GABA-negative. Glial elements were consistently unlabeled, save for some astroglial endfeet. These findings provide positive evidence for the existence of two classes of GABAergic striatal neurons corresponding to a long-axoned spiny I type and an aspiny interneuron. Furthermore, the simultaneous labeling of GABA-immunoreactive presynaptic and postsynaptic profiles offers possible morphologic bases for the various kinds of intrastriatal inhibitory processes, including the feedforward, feedback, and "autaptic" types.     

Fine structure of the optic fiber termination layer in the tectum of the teleost Rutilus: a stereological and morphometric study

The stratum fibrosum et griseum superficiale (SFGS) of the Rutilus optic tectum, which receives a massive fiber projection from the contralateral retina, was studied by electron microscopy. The qualitative and quantitative analysis of the laterodorsal (LD) portion of the stratum involved both a stereological examination of the different elements and a morphometric study of the various profiles containing synaptic vesicles (PCSVs). The relative volume of each element in the LD SFGS was as follows: myelinated and unmyelinated axons, 6.6%; PCSVs, 38%; dendrites without vesicles, spines, and cell bodies, 41.7%; glia, 10.5%. With the fixation employed, 35% of PCSVs showed spheroidal synaptic vesicles. These profiles could be subdivided into three types: (1) S1 (23.5%) represented optic terminals, since they degenerated after retinal ablation or were labeled after intraocular injection of HRP or [3H] proline. Three subgroups of S1 were identified: S1m--profiles containing clear mitochondria;S1c--profiles that were contiguous with S1m and lacked mitochondria;S1i--isolated profiles without mitochondria. (2) S2 (9.3%) were characterized mainly by their dark mitochondria. (3) S3 (2.2%) corresponded to small nonvisual terminals that were isolated and lacked mitochondria. The PCSVs with pleiomorphic synaptic vesicles (65%) were subdivided into three groups: P1 (38%), P2 (19%), and P3 (8%). P1 and P2 were axonal in nature; P2 could be distinguished from P1 by a greater density of synaptic vesicles. P3 was of dendritic origin. Analysis of synaptic patterns revealed a small number of serial synapses. The presynaptic elements were optic boutons, whereas the intermediate profiles were dendrites with synaptic vesicles (P3). Results are compared with ultrastructural data obtained in the superficial tectal layers of other teleosts and other vertebrate groups.     

An ultrastructural study of serially sectioned renshaw cells. III. Quantitative distribution of synaptic boutons

The quantitative distribution of synaptic boutons on 17 presumed Renshaw cells has been studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellular HRP-stained triceps surae α-motoneuorons and were located in lamina VII, ventromedially to the main motor nuclei.In each of the presumed Renshaw cells, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S+F-type boutons were estimated on the cell body and in two dendritic compartments. The F/S percentage synaptic covering ratio was also calculated. The previously demonstrated differences within the present group of neurons, with respect to the site of axonal origin, were not accompanied by any corresponding differences in the quantitative distribution of synaptic boutons. However, it is suggested that the presumed Renshaw cells may possibly fall into two categories with respect to the F/S percentage synaptic covering ratio. The results are discussed in relation to previous studies on the neuronal architecture and synaptic types on the same presumed Renshaw cells, as well as in relation to earlier observations on the quantitative distribution of boutons on central neurons, particularly spinal α-motoneurons.     

The subnuclear organization of the rat interpeduncular nucleus: a light and electron microscopic study

The subnuclear organization of rat interpeduncular nucleus (IPN) has been examined by light microscopy following staining with Nissl and Holmes methods, 3H-leucine autoradiography, acetylcholinesterase (AChE), and cytochrome oxidase histochemistry on plastic sections stained with toluidine blue, and by electron microscopy. Three unpaired and four paired subnuclei are recognized. The rostral subnucleus is heavily stained for AChE, which clearly delineates its borders. It is distinguished ultrastructurally by two types of synapses on dendrites, and two on perikarya. Of the former, one type is formed by presynaptic processes which contain spherical and dense-cored vesicles and make asymmetrical contacts. Dense-cored vesicles are observed in many of the postsynaptic dendrites. A second type has presynaptic processes containing small, pleomorphic vesicles which make symmetrical contacts. Synapses on perikarya are found in the rostral, central, intermediate, lateral, and interstitial subnuclei. The dorsal subnucleus is continuous with the serotonin-containing B8 cells. The central subnucleus is distinguished by longitudinally oriented medial habenular axons separating palisades of cell bodies. These axons, which also traverse the intermediate subnuclei, form en passant S synapses with small dendrites of the central subnucleus. The intermediate subnuclei react faintly for AChE and intensely for cytochrome oxidase. They contain crest synapses formed by two habenular afferents, one from each medial habenula, which contact a narrow dendritic process en passant. The lateral subnuclei react intensely for AChE and have ultrastructural features similar to the rostral subnuclei. The interstitial subnuclei lie within each fasciculus retroflexus as it enters IPN. The small dorsal lateral subnuclei are evident by light microscopy.     

Changes in size and shape of synaptic connections after visual training: An ultrastructural approach of synaptic plasticity

The effect of visual training³⁰ on the synapses in the visual cortex of rabbits was quantitatively investigated. The number, the size and the fine structural organization of synaptic grids were analyzed in 0.5 μm E-PTA sections. The size of the pre- and postsynaptic terminals, the length of the apposition zones, the number of synaptic vesicles per terminal, the dimensions of the synaptic cleft and postsynaptic density and the frequency of mitochondria and spine apparatuses were studied in conventional OsO₄ sections.As compared to control animals, the synaptic population of visually trained rabbits exhibits 3 important changes:

1. (1) the number of complex grids has significantly increased; (perforated synapses according to refs. 6 and 24),

2. (2) the size of the synaptic grids has decreased. As is deduced from careful electron microscopic observations, this decrease in size is the result of a focalization of large grids.

3. (3) a significant increase in thickness of the postsynaptic density and a decrease in synaptic cleft is observed. The changes are most pronounced in deep layers (L IV and V) and are almost absent in superficial layers (L I).

On account of recent insights into the mechanism of synaptic transmission and of the intrinsic relation between the fine structural components of the synaptic junction and the process of transmitter release, the observed changes are interpreted as an expression of enhanced synaptic efficacy. It is further postulated that modification of synaptic efficacy must be considered as an important factor in the adaptation of the nervous system to changing environmental conditions and for the consolidation of memory traces.     

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.     

Participation of Golgi neuron processes in the cerebellar glomeruli: An electron microscope study

Summary Synaptic relations, within the cerebellar isles, of Golgi II neuron axons and dendrites have been studied in the cat. Golgi axon endings can be identified with some probability in the outer (cortical) zone of the cerebellar glomeruli in normal material. They can well be recognized in the chronically isolated folium in which mossy fibers have completely degenerated. The Golgi axons are very thin preterminal fibers with small enlargements containing synaptic vesicles and contacting the preterminal intraglomerular parts of the granule cell dendrites as well as their terminal spheroid protrusions. The spheroid protrusions of the granule dendrites are the main postsynaptic loci of the granule neuron having their main synapse with the mossy fiber — generally of central position in the glomerulus — and additional synapses, more often on their outer surface, with the Golgi axons. No significant difference is seen between the two contacts, from which one is known to be excitatory (mossy) and the other inhibitory (Golgi ax.). The Golgi cell has also descending dendrites, known from light microscopy to be engaged in the cerebellar isles. By tracing these dendrites from the cell bodies and using their characteristic short finger-like processes as a criterion for their identification, the synapses between mossy endings and Golgi dendrites could be identified under the EM. They are broad contacts between a dendrite passing along one side of the mossy ending, with several synaptic attachment plaques and with small dendritic processes protruding into invaginations of the mossy ending. — The cerebellar glomerulus is thus a complex synaptic apparatus with two different axonal elements (mossy and Golgi endings) and often two dendritic elements (granule and Golgi dendrites) involved. — The possible functional significance of the Golgi cell is discussed in the light of these findings and the new discoveries by Eccles et al. (1964b, 1966) on its inhibitory nature.     

Ultrastructural observations on beaded α-motoneuron dendrites

Beaded dendrites of 1α-motoneurons intracellularly labelled with horseradish peroxidase (HRP) were studied ultrastructurally in eight adult cats. For comparison, adjacent unlabelled beaded dendrites of unknown origin were also included in the study. Electron microscopy revealed no signs of degeneration or poor fixation according to common criteria. With the exception of the HRP-reaction product no difference in structure was observed between labelled and unlabelled beaded dendrites. Both the beads and their interconnecting segments were postsynaptic to boutons of normal appearance containing spherical (S-type boutons) or flattened vesicles (F-type boutons). The values for synaptic covering and synaptic packing density of the beaded dendritic regions, which usually were located in the periphery of the dendritic trees, were clearly lower than values obtained previously for cell bodies and proximal dendrites of a-motoneurons.     

Direct synaptic projections to esophageal motoneurons in the nucleus ambiguus from the nucleus of the solitary tract of the rat

Neurons of the nucleus of the solitary tract (NTS) serve as interneurons in swallowing. We investigated the synaptology of the terminals of these neurons and whether they project directly to the esophageal motoneurons in the compact formation of the nucleus ambiguus (AmC). Following wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the NTS, many anterogradely labeled axodendritic terminals were found in the neuropil of the AmC. The majority of labeled axodendritic terminals (89%) contained round vesicles and made asymmetric synaptic contacts (Gray's type I), but a few (11%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II). More than half of the labeled terminals contacted intermediate dendrites (1-2 μm diameter). There were no retrogradely labeled medium-sized motoneurons, but there were many retrogradely labeled small neurons having anterogradely labeled axosomatic terminals. A combined retrograde and anterograde transport technique was developed to verify the direct projection from the NTS to the esophageal motoneurons. After the esophageal motoneurons were retrogradely labeled by cholera toxin subunit B conjugated HRP, the injection of WGA-HRP into the NTS permitted ultrastructural recognition of anterogradely labeled axosomatic terminals contacting directly labeled esophageal motoneurons. Serial sections showed that less than 20% of the axosomatic terminals were labeled in the esophageal motoneurons. They were mostly Gray's type I, but a few were Gray's type II. In the small neurons, more than 30% of axosomatic terminals were labeled, which were exclusively Gray's type I. These results indicate that NTS neurons project directly not only to the esophageal motoneurons, but also to the small neurons which have bidirectional connections with the NTS. J. Comp. Neurol. 381:18-30, 1997. © 1997 Wiley-Liss, Inc.