[Parameters of a model of transmitter depletion in synapses of identified snail neurons]

Unitary EPSP were recorded in two identified neurons of the snail ganglia after stimulation of an identified presynaptic neuron. Low-frequency EPSP depression was found in the course of repetitive stimulation (0.1-0.5/s). Transmitter release at two synapses was analyzed in terms of the depletion model using known and modified methods. The fraction of available pool (F) released by a presynaptic impulse was not significantly different at synapses of two different target neurons. The available pool of transmitter (C) and the rate constant of demobilization (k) in the RPa3 neuron were found to be 2-3 times and mobilization (M) 10 times as high as in the LPa2 neuron. Deviations of the depletion model from the experimental data were found. A new method was developed permitting k to be determined and any change in F to be postulated. No significant correlation was found between parameters of the depletion model F and C and the binomial parameters p and n calculated on the basis of the quantal hypothesis, which shows that the depletion model and quantal hypothesis describe different synaptic mechanisms.