PRE- AND POSTSYNAPTIC EFFECTS OF TEMPERATURE AT THE CYTONEURAL JUNCTION OF THE ISOLATED FROG LABYRINTH.

Resting mEPSPs were intracellularly recorded from the posterior nerve of frogs (Rana esculenta), which had been stored at room temperature (20-22° C), in the presence of TTX (10-6 M) to abolish action potentials. For each preparation continuous recordings were obtained from single units during a 10° C change of the bath temperature imposed by means of a Peltier device with a feedback controller. The range explored was from 8° to 30° C. mEPSP waveform was estimated at the beginning and at the end of the temperature step while mEPSP rate was checked also during the temperature change. In units where mEPSP rate was high (>100/s) single events extensively overlap preventing any direct evaluation of their numbers; therefore the estimate of their mean rate and size was obtained by means of two alternative noise analysis procedures in which the mEPSP waveform was calculated from the autoregressive (minimum phase) fit to the autocorrelation of the summated potentials. If mEPSP rate was less than 300/s the event waveform was used to build a Wiener filter to deconvolve the signal. Tests of Wiener filtering performance on simulated records indicated that this procedure gave unreliable results when mEPSP rate was higher than 300/s; under these conditions, mEPSP rate and amplitude were thus determined by means of an alternative statistical method based on the measure of variance and skewness of membrane potential fluctuations. mEPSP rate increased on increasing temperature and vice-versa, the Q10 ranging from 1.11 to 2.66. Statistical analysis in low- or high-frequency units (alter Wiener-filtering) performed before and at the end of the temperature change showed that: 1) mEPSP peak amplitude distributions were continuous, unimodal and well fitted by lognormal functions; 2) mEPSPS time interval distributions were always monoexponential. Peak amplitude, however, proved to decrease on increasing temperature leading to lognormal distributions markedly shifted to the left with respect to those obtained before the temperature change. Average mEPSP rate increased with increased temperature, this relation being approximately exponential. In the fibres where isolated events could be detected, the elementary waveform was evaluated by fitting the average of 20-50 unitary events with a modified gamma function. From this fit the following parameters were evaluated: 1) shape; 2) duration; 3) maximum amplitude; 4) time-to-peak. Not only peak amplitude, but also time-to-peak and event duration proved to decrease on increasing temperature. Results demonstrate a limited dependence of transmitter release on temperature, which does not affect the basic mechanism of secretion. More consistent are the postsynaptic effects, as suggested by the modifications in the event properties.