Direct and indirect effects of muscimol on medial vestibular nucleus neurones in guinea-pig brainstem slices.

Inhibitory amino acids are considered as major transmitters in the vestibular system. Using intracellular recordings in slices, we applied gamma-aminobutyric acid (GABA) and muscimol (a specific agonist of the GABAA receptor) to the two main types of medial vestibular nucleus neurones (A and B MVNn). In either a high Mg2+/low Ca2+ solution, or a solution containing tetrodotoxin, all MVNn were hyperpolarized by GABA and muscimol. This indicates that both types of MVNn are endowed with postsynaptic, hyperpolarising GABAA receptors. In a normal medium, about half of A and B MVNn were, in contrast, depolarised by GABA and muscimol, whereas the remaining cells were hyperpolarised. These results could be due to a modulation by GABA and muscimol of a tonic GABA release in the slice. Such a release was, indeed, suggested by results showing the depolarising effect of either tetrodotoxin (TTX) or bicuculline, when applied alone. The cells that were depolarised by GABA or muscimol in control conditions were always hyperpolarised in the presence of TTX. Our data therefore suggest that GABA acting at GABAA receptors in the medial vestibular nucleus can play a role either through a postsynaptic hyperpolarising action or indirectly by inhibiting a tonic GABA release, probably resulting from the spontaneous activity of local inhibitory interneurones. A GABAergic regulation of these interneurones could be important in processes of vestibular habituation and/or adaptation.