Miniature EPSPs were recorded from single fibres of the posterior nerve (in the presence of TTX 10-6 M) in normal Ringer (250 mOsm/l) as well as in solutions where NaCl was either reduced or increased to a final osmolarity of 105-420 mOsm/l, respectively. Altematively, the control Ringer was made hypertonic by adding variable amounts of sodium gluconate so as to obtain a final osmolarity in the 380-550 mOsm range. Hypotonic solutions decreased, white hypertonic solutions increased mEPSP frequency. When mEPSP rate is high (>100/s) single events extensively overlap so that noise analysis had to be exploited. The mEPSP waveform was obtained from the autoregressive (minimum phase) fit to the autocorrelation of the summated potentials and used to construct a Wiener filter to deconvolve the signal. Filtering eliminates the overlap among potentials and yields isotated sharpened events whose location indicates the mEPSP occurrence and whose size is proportionat to the original event amplitude. mEPSP rate was determined in control records and a few minutes after application of the test solution. Statistical analyses show that: 1) mEPSP rate increased linearly with osmolarity; 2) either in control or in test solutions, size histograms of synaptic potentials were well described by single lognormal functions and time interval distributions proved to be monoexponential; 3) event peak amplitudes were slightly decreased by switching to hypo- or hypertonic solutions; 4) the relationship between peak amplitude and time integral of the event was linear and the event equivalent duration (area under the event/peak amplitude) did not change markedly in the solutions tested. Results suggest that the basic mechanism of transmitter release remains asynchronous and uniquantal irrespective of the osmotic treatments, which act presynaptically to affect the rate, but not the waveform, of the mEPSPs
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