Medial vestibular nucleus in the guinea-pig: NMDA-induced oscillations

We have recently shown in vivo that N-Methyl-D-Aspartate (NMDA) receptors are present in the guinea-pig vestibular complex and demonstrated that they are involved in the regulation of the resting discharge of vestibular neurones. A parallel in vitro study has identified in the guinea-pig medial vestibular nuclei (MVN) two main neuronal cell types, A and B MVNn, differing by their intrinsic membrane properties. One subtype of B MVNn was further characterized by the presence of a low threshold calcium spike (LTS). The present study investigated in vitro the responses of these different cell types to NMDA. Both A and B MVNn were depolarized by NMDA, which also induced a decrease in membrane resistance and an increase in the spontaneous firing rate. These effects could be blocked by D-AP5, a specific antagonist of NMDA receptors. Following a 10-30 mV hyperpolarization, a long-lasting oscillatory behavior could be induced in presence of NMDA. These oscillations were however restricted to the subtype of B MVNn without LTS. The NMDA-induced oscillations were tetrodotoxine-resistant, but could be eliminated by D-AP5 or by replacing sodium with choline. Functional implications of this oscillatory behavior are discussed.