Smith_1983_Exp.Neurol_80_520

To study variable activation of synaptic release sites during repetitive stimulation, quantal release was measured at two synaptic sites innervated by the same presynaptic axon on the opener muscle of the crayfish walking leg using focal extracellular recording techniques. Binomial statistics describing this release were then calculated. Values of m, the average amount of quanta released in response to a presynaptic action potential, measured at one site were sometimes found to exhibit stationarity while those at the other site on the same fiber were not. This nonstationarity was attributed to conduction failures in the terminal arborization. To minimize bias in the estimates of the parameters n and p, the data were screened carefully for nonuniformity of the release probabilities and nonstationarity. Immediately from the onset of stimulation of frequencies from 5 to 30 Hz, 65% of the sites exhibited nonuniformity. After 1 min of repetitive stimulation, this was reduced to 10%. Using uniform and stationary data, the parameters n and p were estimated during facilitation and depression. Facilitation was associated with an increase only in n at lower frequencies (5 to 10 Hz) of repetitive stimulation and in p, and occasionally n, at the higher frequencies (10 to 30 Hz). Depression was associated primarily with decreases in n. The results indicate frequency-dependent variability in the activation of synaptic release sites.