SYNAPTIC TRANSMISSION PROPERTIES OF THE CYTONEURAL JUNCTION IN THE ISOLATED FROG LABYRINTH.

Subthreshold postsynaptic potentials (EPSPs) were recorded intracellularly at the cytoneural junction of the frog posterior canal. In single units EPSPs display highly variable size, so it is not clear whether they are generated by release of single quanta of transmitter and whether large ones represent giant events, multiquantal events or the random summation of independently occurring unitary events. Therefore their quantal nature was investigated by performing a statistical analysis of time intervals and peak amplitudes. In general the distributions of EPSP peak amplitudes were well fit by lognormal functions, and intervals were exponentially distributed. As EPSP rate rose above 100/s, peak amplitude histograms became broader and brief intervals became less frequent than expected. A procedure of Wiener filtering, based on estimating the event waveform from the autocorrelation of the synaptic recording was set up to sharpen the signal. After filtering, peak amplitude histograms were much less sensitive to EPSP rate and time intervals were exponentially distributed as expected for random release. The number of "multiquantal" events, as estimated from the area outside the main peak in amplitude histograms, was reduced by Wiener filtering and was in general consistent with the expectation that more than one independent event occurred within the time interval corresponding to the duration of the single event. This suggests that the events are uni-quantal, random and independent, i.e. miniature EPSPs (mEPSPs). mEPSP rate of occurrence was modified by mechanical stimulation of the canal, by activation of the efferent inhibitory or facilitatory systems and by changing extracellular Ca2+ concentration (0-9 mM). Peak amplitude histograms were almost unaffected after Wiener filtering also under the new experimental conditions and time interval histograms remained exponentially distributed. Following bath application of ATP (4x104-10-3 M) or its analog ATP-yS (10-3 M) a consistent though transient increase in resting mEPSP rate was observed in about 50% of the units examined. ATP effect was absent in all fibres where efferent stimulation produced inhibition and it was present in all fibres under the control of the facilitatory efferent system. In these fibres, efferent facilitation, measured after the effect of ATP (or ATP-yS) had vanished, was consistently reduced with respect to facilitation evoked in control solution. Results suggest that: 1) mEPSPs are true miniature, uniquantal events; 2) "giant" events do not occur at the cytoneural junction under the conditions here investigated; 3) experimental procedures capable of markedly modulating the rate of transmitter release do not alter its basic mechanism; 4) ATP may be the efferent facilitatory transmitter at the basal pole of the hair cell