Spontaneous multiquantal release at synapses in guinea-pig hypogastric ganglia: evidence that release can occur in bursts.

1. A study was made of some properties of the spontaneous synaptic potentials recorded in cells of the hypogastric ganglia of guinea-pigs. 2. The distribution of the amplitudes of the spontaneous synaptic potentials arising from a single preganglionic fibre was found to be multimodal, with peaks at roughly integral multiples of a unit peak. 3. It was found that the amplitudes of the larger spontaneous potentials were consistent with them being the result of synchronous or near-synchronous release of two or more unit-sized quanta (multiquantal release). 4. The proportion of multiquantal potentials observed was found to be dependent on the extracellular calcium ion concentration. 5. When the stochastic properties of the spontaneous potentials were examined, it was found that the spontaneous release process was not random and independent but appeared to be clustered. Indeed, the probability of occurrence of a unit spontaneous synaptic potential was greatly enhanced during the 40--60 msec immediately following any given spontaneous synaptic potential. 6. When unit spontaneous potentials were excluded from the analysis, the multiquantal potentials were still found to be clustered although the clustering was less marked than that seen in the over-all process. 7. These results suggested that the multiquantal spontaneous potentials arose from the release of unit quanta in short high frequency bursts and it was found that a mathematical model incorporating such a mechanism could describe the spontaneous quantal release process.