MEPSP PROPERTIES AT THE POSTERIOR CANAL CYTO-NEURAL JUNCTION

To study the transmitter release mechanism at the canal cyto-neural junction, low frequency (<100/s) non overlapping mEPSPs were recorded from single afferent fibres in the isolated frog labyrinth. The mEPSPs were digitized and the peak values of a large number of events were computed from recordings performed: 1) at rest; 2) during inhibitory acceleratory steps (2-8 deg/s2 ); 3) under electrical activation of the posterior canal inhibitory efferent system. All these conditions were tested: 1) in control solution; 2) high extracellular Ca2+ (9mM); 3) Ca-free EGTA (5mM) solution. For each unit single and averaged potentials (mean of 25-100 events) in control solution were compared to those observed in at least one of the test conditions. The analytical reconstruction of the mEPSP waveform indicated that a gamma function provides good fits both to single and averaged events. Similar values of the exponential factor in the gamma function (range 1.14-2.22) were obtained from control and test experiments in the same unit, thus indicating that no systematic modifications in cable properties had ensued. Moreover, despite the scatter in mEPSP time-to-peak (range 0.66-1.17 ms) and peak amplitudes (range 0.96-2.69 mV), no significant differences were observed between control and test conditions. Previous experiments have shown that peak mEPSP amplitudes are well fitted by similar lognormal distribution functions ín control and test experíments, thus suggesting that the quantum size of the elementary event did not change. This analysis of mEPSP characteristics confirms that the basic mechanism of quantal release is independent of: 1) external calcium levels; 2) modifications in the hair cell conductance during the action of the efferent transmitter; 3) decrease in the afferent transmitter release by inhibitory cupula deflection.