Excitatory and Inhibitory Vestibular Pathways to the Extraocular Motor Nuclei in Goldfish

Abstract

Electrophysiological, ultrastructural, and immunohistochemical techniques were utilized to describe the excitatory and inhibitory vestibular innervation of extraocular motor nuclei in the goldfish. In antidromically activated oculomotor motoneurons, electrical stimulation of the intact contralateral vestibular nerve produced short-latency, variable amplitude electrotonic excitatory postsynaptic potentials (EPSPs) at 0.5-0.7 ms followed by chemical EPSPs at 1.0-1.3 ms. Stimulation of the ipsilateral vestibular nerve produced small amplitude membrane hyperpolarizations at a latency of 1.3-1.7 ms in which equilibrium potentials were slightly more negative than resting potentials. The inhibitory postsynaptic potentials (IPSPs) reversed with large amplitudes after the injection of chloride ions suggesting a proximal soma-dendritic location of terminals exhibiting high efficacy inhibitory synaptic conductances. In antidromically identified abducens motoneurons and putative internuclear neurons, electrical stimulation of the contralateral vestibular nerve produced large-amplitude, short-latency electrotonic EPSPs at 0.5 ms followed by chemical depolarizations at 1.2-1.3 ms. Stimulation of the ipsilateral vestibular nerve evoked IPSPs at 1.4 ms that were reversed after injection of current and/or chloride ions. gamma-Aminobutyric acid (GABA) antibodies labeled inhibitory neurons in vestibular subdivisions with axons projecting into the ipsilateral medial longitudinal fasciculus (MLF). Putative GABAergic terminals surrounded oculomotor, but not abducens, motoneurons retrogradely labeled with horseradish peroxidase. Hence the spatial distribution of GABAergic neurons and terminals appears highly similar in the vestibuloocular system of goldfish and mammals. Electron microscopy of motoneurons in the oculomotor and abducens nucleus showed axosomatic and axodendritic synaptic endings containing spheroidal synaptic vesicles establishing chemical, presumed excitatory, synaptic contacts with asymmetric pre- and/or postsynaptic membrane specializations. The majority of contacts with spheroidal vesicles displayed gap junctions in which the chemical and electrotonic synapses were either en face to dissimilar or adjacent to one another on the same soma/dendritic profiles. Another separate set of axosomatic synaptic endings, presumed to be inhibitory, contained pleiomorphic synaptic vesicles with symmetric pre- and/or postsynaptic membrane specializations that never included gap junctions. Excitatory and inhibitory synaptic contacts appeared equal in number but were more sparsely distributed along the soma-dendritic profiles of oculomotor as compared with abducens motoneurons. Collectively these data provide evidence for both disynaptic vestibular inhibition and excitation in all subdivisions of the extraocular motor nuclei suggesting the basic vestibulooculomotor blueprint to be conserved among vertebrates. We propose that unique vestibular neurons, transmitters, pathways, and synaptic arborizations are homologous structural traits that have been essentially preserved throughout vertebrate phylogeny by a shared developmental plan.