QUANTAL NATURE OF SYNAPTIC TRANSMISSION AT THE CYTONEURAL JUNCTION IN THE 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 statistical analysis of time intervals and peak amplitudes. In single units EPSPs display highly variable size, so it is not clear whether they are 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 measured directly. Peak amplitude histograms were continuous, unimodal and well fitted by log normal distributions. Time-interval histograms were well described by single exponentials. 3. At high EPSP rates (either at rest or during experimental treatments), where single events overlapped extensively, peak amplitude histograms were skewed markedly towards high values. Under these conditions, the EPSP waveform was estimated by autoregressive fit to the autocorrelation of the recorded signal. The fit was used to build a Wiener filter, for sharpening the original signal, before computing time-interval and peak amplitude histograms. This yielded consistent log normal peak amplitude distributions with no 'excess' skewness, similar to those obtained with low resting rates. 4. After sharpening by the Wiener filter, shoulders or small second peaks in amplitude distributions were observed only at the highest EPSP rates (> 300 s(-1)). The number of 'multiquantal' events was reduced by Wiener filtering, and was in general consistent with the expectation that more than one independent event occurred within the duration of the single event. This suggests that the events are uniquantal, random and independent, i.e. miniature EPSPs (mEPSPs). 5. In general, peak amplitude distributions obtained with modified external Ca2+ concentration ([Ca2+](o)) and/or during mechanical stimulation or under efferent activation were not significantly altered with respect to those obtained in the same units at rest. Time-interval histograms were generally mono-exponential at rest as well as during mechanical or efferent stimulation, and irrespective of [Ca2+](o). Resting mEPSP rate was slightly increased by elevated [Ca2+](o) and reduced by low [Ca2+](o). The increase in mEPSP rate produced by mechanical excitation was depressed by both high and low [Ca2+](o), whereas both conditions enhanced mechanical inhibition. Efferent inhibition was little affected. High [Ca2+](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 resting mEPSP rate was observed in about 50% of units. ATP effect was absent in all fibres where efferent stimulation produced inhibition and present in all fibres under facilitatory efferent control. In these fibres, efferent facilitation, measured after the effect of ATP had vanished, was reduced with respect to facilitation in control solution. The effects of ATP were mimicked by its analogue adenosine-5'-O-3-thiotriphosphate (ATP-gamma-S).