The binaural auditory pathway: excitatory amino acid receptors mediate dual timecourse excitatory postsynaptic currents in the rat medial nucleus of the trapezoid body.

We show here that synaptic transmission to the medial nucleus of the trapezoid body (MNTB) is mediated principally by excitatory amino acid receptors and has two components. A fast excitatory postsynaptic current (EPSC) is mediated by non-NMDA receptors and a slow EPSC is mediated by NMDA receptors. Each neuron receives a large synaptic input (calyx of Held) which produces an EPSC with a mean peak conductance of 37 nS. The somatic location of this synapse gives good resolution of the EPSC timecourse with the fast EPSC decaying with a time constant of 1.1 ms (at 25 degrees C). The slow EPSC exhibits a double exponential decay with time constants of 41 ms and 106 ms and is voltage dependent in the presence of extracellular magnesium. Other smaller EPSCS mediated by NMDA and non-NMDA receptors, and a strychnine-sensitive synaptic current, are also present. Although the intrinsic membrane properties of MNTB neurons (Forsythe & Barnes-Davies (Proc. R. Soc. Lond. B 251, 143 (1993)), preceding paper) promote high-fidelity transmission, we show that voltage-dependent modulation of synaptic transmission can occur. Given the specialization of the calyx of Held, it seems that the NMDA-receptor ion channel complex is not primarily serving to potentiate a subthreshold input, but may be involved in the development and maintenance of this exuberant somatic synapse.