The ultrastructure of GABA-immunoreactive vestibular commissural neurons related to velocity storage in the monkey.

The purpose of the present study was to visualize the synaptic interactions of GABAergic neurons involved in the mediation of velocity storage. In the previous report, ultrastructural studies of degenerating neurons were conducted following midline section of rostral medullary commissural fibers with subsequent behavioral testing. The midline lesion caused functionally discrete damage to the velocity storage component, but not to the direct pathway, of the angular vestibulo-ocular reflex, and the degenerating neurons were interpreted as potential participants in the velocity storage network. We concluded that at least some of the commissural axons mediating velocity storage originate from clusters of neurons in the lateral crescents of the rostral medial vestibular nucleus. In the present report, immunocytochemical evidence is presented that many vestibular commissural neurons, putatively involved in mediating velocity storage, are GABAergic. These cells have large nuclei, small round or narrow tubular mitochondria, occasional cisterns and vacuoles, but few other organelles. Their axons are thinly-myelinated, and terminate in boutons containing mitochondria of similar ultrastructural appearance and a moderate density of round/pleomorphic synaptic vesicles. Such terminals often form axoaxonic synapses, and less frequently axodendritic contacts, with non-GABAergic elements. On the basis of the present results, we conclude that a portion of the commissural neurons of the velocity storage pathway is GABAergic. The observation of GABAergic axoaxonic synapses in this pathway is interpreted as a structural basis for presynaptic inhibition of medial vestibular nucleus circuits by velocity storage-related commissural neurons. Conversely, substantial ultrastructural evidence for postsynaptic inhibition of non-GABAergic commissural cells argues for a dual role for GABAergic terminals mediating velocity storage: presynaptic inhibition of non-GABAergic vestibular cells by GABAergic velocity storage commissural axons, and postsynaptic inhibition of non-GABAergic velocity storage cells by GABAergic axons. Both pre- and postsynaptic inhibitory arrangements could provide the morphologic basis for disinhibitory activation of the velocity storage network within local neuronal circuits.