Synaptic noise and multiquantal release at dendritic synapses.

1. The quantal nature of inhibitory synaptic noise recorded intracellularly from the lateral dendrite of the goldfish Mauthner cell was studied, using new detection and measurement procedures that eliminate operator intervention. In addition, we employed an analytical algorithm, not previously applied to this problem, which treats composite amplitude distributions as mixtures of gaussians of unknown separations and variances. 2. As in the soma of this neuron, the dendritic inhibitory noise is quantal, with the exception that in the dendrite multiple equally spaced classes may persist in the presence of tetrodotoxin (TTX), an observation that may be correlated with the finding that the inhibitory afferents at this level often contain more than one release site. The validity of the analysis was confirmed by superfusing with saline containing low calcium and high magnesium, which reduces composite histograms that are gaussian mixtures to a single class, equal in amplitude to that of the first component detected in the control. 3. These results suggest that spontaneous exocytotic events may be synchronized at adjacent active zones within single terminals and that lowering the probability of release by reducing calcium may then be a more effective method for isolating single miniature events than is TTX.