Elsevier

Neuroscience

Volume 93, Issue 1, June 1999, Pages 183-193
Neuroscience

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

https://doi.org/10.1016/S0306-4522(99)00140-2Get rights and content

Abstract

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.

Section snippets

Experimental animals

Brainstem tissue from one rhesus (M. mulatta; M9316) and two cynomolgus (M. fasicularis; M1190 and M1194) monkeys was used to characterize the normal ultrastructure and neurotransmitter localization in the monkey MVN. In addition, one cynomolgus (M9210) and two rhesus (M502 and M613) monkeys had midline brainstem lesions of vestibular commissural fibers. Oculomotor and vestibular tests were conducted before and after the rostral medullary transections, in order to assess the integrity of the

GABAergic cell bodies

At the ultrastructural level, normal GABAergic cells in rostrolateral MVN of the lesioned animals showed remarkably consistent features. Such neurons were small, 10–15 μm in diameter, with centrally-placed nuclei (Fig. 1A). The nuclear envelope was often slightly irregular, although frank indentations were not typically observed. The cytoplasm was full of small round or tubular mitochondria with wide matrix spaces. Cisterns and vacuoles were very common, but other organelles such as lysosomes

Discussion

The ultrastructural characteristics of non-degenerating GABAergic neurons and terminals in rostrolateral MVN have been identified in monkeys following midline transection of vestibular commissural fibers. This lesion causes functionally-discrete damage to the indirect aVOR pathway, but leaves all other vestibular and oculomotor functions intact.24., 47., 48. In the previous studies, we reported that most degenerated cells and terminals are located rostrolaterally in MVN, and share common

Conclusions

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

Acknowledgements

Supported by National Institutes of Health research grants DC01705 (G.R.H.) and DC02451 (G.R.H.) from the National Institute on Deafness and Other Communication Disorders, NS00294 (B.C.) from the National Institute of Neurological Diseases and Stroke, EY11812 (B.C.) and EY01867 (B.C.) from the National Eye Institute, and NASA research grant NAG2-946 (G.R.H.). The authors wish to thank Ms Rosemary Lang and Mr Victor Rodriguez for their invaluable assistance with various aspects of this work.

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