Evidence for N-methyl-D-aspartic acid receptor-mediated modulation of the commissural input to central vestibular neurons of the frog.

We have investigated the role of N-methyl-D-aspartate (NMDA) receptors in the excitatory synaptic transmission to central vestibular neurons in the isolated superfused brainstem of the frog. In superfusate containing 1 mM Mg2+ field potentials in the vestibular nuclei evoked by electrical stimulation of either the ipsi- or the contralateral VIIIth nerve were not affected by bath-applied D-2-amino-5-phosphonovaleric acid (D-APV, 25-50 microM), a selective NMDA antagonist. In a low Mg2+ solution postsynaptic field potential components were larger than control but still unaffected by D-APV. Ipsi- and contralaterally evoked excitatory postsynaptic potentials (EPSPs) differed in their shape parameters as well as in their pharmacological sensitivity. Ipsilaterally evoked EPSPs were not affected by D-APV and has a rise time that was faster than that of contralaterally evoked EPSPs. The peak amplitude of hte latter was reduced by D-APV (25-50 microM) to about 65% of the control value in the presence of 1 mM Mg2+. During bath application of NMDA (100 microM) an increased input resistance and repetitive de- and hyperpolarizing membrane potential shifts were observed. Similar events were observed during a reduction of the Mg2+ concentration. Bath application of NMDA (0.1-1 microM) resulted in an enhanced size of the recorded EPSPs. Dendritic and somatic EPSPs were simulated on a computer with the assumption of a constant NMDA receptor activation and a pulse-like non-NMDA receptor activation. The results of these simulations are consistent with the hypothesis that the efficacy of non-NMDA-mediated vestibular commissural synaptic transmission is modulated through tonically activated NMDA receptors.