[Quantal mediator release mechanism at the cyto-neural junction of the labyrinth in the frog].

The cytoneural junctions of the frog labyrinth posterior canal usually show different (m)EPSP resting rates (10-300/s). A statistical analysis of (m)EPSPs, recorded intracellularly from the posterior nerve, is carried out to investigate the basic mechanisms of transmitter release. In units with low resting rates (< 100/s) and during mechanical as well as efferent inhibitory stimulation, where a direct evaluation of the event parameters is possible, peak amplitude distributions are continuous, unimodal and fitted by lognormal functions, while time interval distributions between events are monoexponential. For higher resting rates, and systematically during mechanical excitation or activation of the facilitatory efferent system, single events extensively overlap. To reduce summation, a Wiener filter is built to sharp the original signal. The Wiener filter transfer function is calculated from the EPSP waveform estimated by the autoregressive fit to the autocorrelation of the original signal. This procedure yields peak size distributions well fitted by lognormal functions in all the situations tested. After filtering, time intervals are still monoexponential at rest as well as during the evoked release. Results suggest that only the rate of transmitter release, but not its basic mechanism, is altered during sustained synaptic activity. Furthermore, the Ca(2+)-free EGTA-solution slightly affects efferent inhibition and depresses facilitation. The high Ca(2+)-level enhances inhibition and depresses facilitation.