How are NDMA receptors of the central vestibular neurons activated?

Synaptic excitation of central vestibular neurons of the frog was investigated. EPSPs were recorded in vitro after electrical stimulation of either the ipsi N.VIII or the contra N.VIII/nuc. Using a Mg⁺⁺ free saline, the eEPSP was reported to be mediated via NMDA receptors. In the presence of 1 mM Mg⁺⁺ the initial part of the eEPSP remained unaffected during bath application D-APV (25 µM). However, D-APV blocked reversibly the later component of the cEPSP. During wash out of Mg⁺⁺ this later component increased and spontaneous and EPSP-triggered jumps (1–2 mV) in membrane potential were observed. Similar jumps were evoked by application of NMDA.

The D-APV sensitive component of the cEPSP could be due to a direct synaptic activation of NMDA receptors. However, the occurrence of jumps in in drug free solution suggests a rather continuous activation of NMDA receptors. Since the NMDA-ionophore exhibits a range of negative slope conductance the shape of the evoked cEPSP could indirectly depend on the degree of postsynaptic NMDA receptor activation.