ARE SOME MINIS MULTIQUANTAL

The amplitude distribution of miniature postsynaptic currents (minis) in many central neurons has a large variance and positive skew, but th e sources of this variance and skew are unresolved. Recently it has be en proposed that spontaneous Ca2+ influx into a presynaptic bouton wit h multiple release sites could cause spontaneous multiquantal minis by synchronizing release at all sites in the bouton, accounting for both the large variance and skew of the mini distribution. We tested this hypothesis by evoking minis with internally perfused, buffered Ca2+ an d the secretagogue alpha-latrotoxin, both in the absence of external C a2+. With these manipulations, the synchronized release model predicts that the mini distribution should collapse to a Gaussian distribution with a reduced coefficient of variation. Contrary to this expectation , we find that mini amplitude distributions under these conditions ret ain a large variance and positive skew and are indistinguishable from amplitude distributions of depolarization-evoked minis, strongly sugge sting that minis are uniquantal.