SPONTANEOUS CALCIUM TRANSIENTS IN AUTONOMIC BOUTONS AND VARICOSITIES

Spontaneous multiquantal events are recorded at many different boutons and varicosities for which there is evidence that the receptor patch at these individual synapses is saturated by the transmitter unit. In order to reconcile these observations, a model is considered in which calcium release from a ryanodine channel within a nerve terminal can r each adjacent active zones in single synapses in sufficient concentrat ion to occasionally trigger exocytosis from adjacent zones synchronous ly, giving rise to multiquantal spontaneous events. It is shown that t he spatial and temporal distribution of calcium concentration at the a ctive zone after a spontaneous opening of a ryanodine channel can pred ict the amplitude and time course of observed calcium-activated potass ium channel currents. Similar calcium transients are sufficient to giv e rise to multiquantal events. Such events suggest a multiunit hypothe sis for secretion. (C) 1998 Academic Press.