QUANTAL NEUROTRANSMITTER SECRETION RATE EXHIBITS FRACTAL BEHAVIOR

The rats of exocytic events from both neurons and nonneuronal cells ex hibits fluctuations consistent with fractal (self-similar) behavior in time, as evidenced by a number of statistical measures. We explicitly demonstrate this for neurotransmitter Secretion at Xenopus neuromuscu lar junctions and for rat hippocampal synapses in culture; the exocyto sis of exogenously supplied neurotransmitter from cultured Xenopus myo cytes and from rat fibroblasts behaves similarly, The magnitude of the fluctuations of the rate of exocytic events about the mean decreases slowly as the rate is computed over longer and longer time periods, th e periodogram decreases in power-law manner with frequency, and the Al lan factor (relative variance of the number of exocytic events) increa ses as a power-law function of the counting time. These features are h allmarks of self-similar behavior. Their description requires models t hat exhibit long-range correlation(memory] in event occurrences, Mle h ave developed a physiologically plausible model that accords with ail of the statistical measures that we have examined, The appearance of f ractal behavior at synapses, as well as in systems comprising collecti ons of synapses, indicates that such behavior is ubiquitous in neural signaling.