QUANTAL NATURE OF SYNAPTIC TRANSMISSION AT THE CYTONEURAL JUNCTION INTHE FROG LABYRINTH

1. The mechanism of transmitter release at the cytoneural junction of the frog posterior canal was investigated by recording intracellularly subthreshold postsynaptic potentials (EPSPs), and performing a statis tical analysis of time intervals and peak amplitudes. In single units EPSPs display highly variable size, so it is not clear whether they ar e generated by the release of single quanta of transmitter and whether large ones represent giant events, multiquantal events, or the random summation of independent unitary events. 2. In units with low resting EPSP rates, peak amplitudes and time intervals between EPSPs were mea sured directly. Peak amplitude histograms were continuous, unimodal an d well fitted by log normal distributions. Time-interval histograms we re well described by single exponentials. 3. At high EPSP rates (eithe r at rest or during experimental treatments), where single events over lapped extensively, peak amplitude histograms were skewed markedly tow ards high values. Under these conditions, the EPSP waveform was estima ted by autoregressive fit to the autocorrelation of the recorded signa l. The fit was used to build a Wiener filter, for sharpening the origi nal signal, before computing time-interval and peak amplitude histogra ms. This yielded consistent log normal peak amplitude distributions wi th no 'excess' skewness, similar to those obtained with low resting ra tes. 4. After sharpening by the Wiener filter, shoulders or small seco nd peaks in amplitude distributions were observed only at the highest EPSP rates (> 300 s(-1)). The number of 'multiquantal' events was redu ced by Wiener filtering, and was in general consistent with the expect ation that more than one independent event occurred within the duratio n of the single event. This suggests that the events are uniquantal, r andom and independent, i.e. miniature EPSPs (mEPSPs).5. In general, pe ak amplitude distributions obtained with modified external Ca2+ concen tration ([Ca2+](o)) and/or during mechanical stimulation or under effe rent activation were not significantly altered with respect to those o btained in the same units at rest. Time-interval histograms were gener ally mono-exponential at rest as well as during mechanical or efferent stimulation, and irrespective of [Ca2+](o). Resting mEPSP rate was sl ightly increased by elevated [Ca2+](o) and reduced by low [Ca2+](o). T he increase in mEPSP rate produced by mechanical excitation was depres sed by both high and low [Ca2+](o), whereas both conditions enhanced m echanical inhibition. Efferent inhibition was little affected. High [C a2+](o) hastened adaptation during efferent facilitation. Low [Ca2+](o ) reduced peak response during facilitation, but suppressed its waning .6. In the presence of ATP a consistent though transient increase in r esting mEPSP rate was observed in about 50% of units. ATP effect was a bsent in all fibres where efferent stimulation produced inhibition and present in all fibres under facilitatory efferent control. In these f ibres, efferent facilitation, measured after the effect of ATP had van ished, was reduced with respect to facilitation in control solution. T he effects of ATP were mimicked by its analogue adenosine-5'-O-3-thiot riphosphate (ATP-gamma-S).