Postnatal development of phase-locked high-fidelity synaptic transmission in the medial nucleus of the trapezoid body of the rat.

Synaptic transmission in the medial nucleus of the trapezoid body of rats was analyzed in postnatal days 4-13 (P4-P13) by applying the whole-cell patch-recording technique to brain slices. In P4-P6 animals, evoked EPSCs fluctuated extensively in amplitude and occurred in marked asynchrony, followed by spontaneous EPSCs. With development of animals, the evoked EPSCs increased in amplitude, and the rise time became faster. In addition, the synaptic transmission became phase-locked. The coefficient of variation (CV) of EPSC amplitude decreased with development (0.32 +/- 0.03 for P4-P5 and 0. 05 +/- 0.01 for P9-P11). The amplitude of miniature EPSCs did not change throughout the postnatal days investigated (-30.2 +/- 0.3 pA at -70 mV). The CV was dependent on extracellular Ca2+ concentration ([Ca2+]o) and was reduced with the increase of [Ca2+]o, and this [Ca2+]o dependence was shifted toward lower [Ca2+]o with development. Direct patch recording of the presynaptic terminals demonstrated an increase in Ca2+ currents during these postnatal days. The phase-locked high-fidelity transmission in this synapse is achieved with development likely through the increase of Ca2+ currents and Ca2+ sensitivity of transmitter release mechanisms in the presynaptic terminal.