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