Synaptic depression at a synapse in Aplysia californica: analysis in terms of a material flow model of neurotransmitter.

When a pair of stimuli separated by an appropriate interval is given to the right visceropleural connective of Aplysia californica the amplitude of the second EPSP elicited in cell R15 is usually smaller than the amplitude of the first EPSP. In the present paper we show that this phenomenon, synaptic depression, can be analyzed in terms of the material flow model of neurotransmitter economics developed in our preceding publications. We specifically show how changes in the 4 model parameters; A, the available pool of transmitter; F, the fraction of the available pool released by a presynaptic action potential; M, the rate of transmitter mobilization into the available pool; and D, the rate constant of demobilization of transmitter from the available pool, all effect synaptic depression. In addition, we show how transient changes in F and M, that are observed immediately and for seconds after a stimulus, influence the time course of synaptic depression. Using this analysis we then tested our previous inferences about changes in the model parameters produced either by pharmacological manipulations or repetitive stimulation, by comparing the observed effects of these manipulations on synaptic depression with the theoretical predictions. The theoretical and experimental findings agreed, thereby strengthening both our previous conclusions of the mode of action of these manipulations and the model itself.