SV2A and SV2C contain a unique synaptotagmin-binding site.

SV2 (Synaptic Vesicle Protein 2) is expressed in neurons and endocrine cells where it is required for normal calcium-evoked neurosecretion. In mammals, there are three SV2 genes, denoted SV2A, B and C. SV2A interacts with synaptotagmin, the prime candidate for the calcium sensor in exocytosis. Here, we report that all isoforms of native SV2 bind synaptotagmin and that binding is inhibited by calcium, indicating that all isoforms contain a common calcium-inhibited synaptotagmin-binding site. The isolated amino termini of SV2A and SV2C supported an additional interaction with synaptotagmin, and binding at this site was stimulated by calcium. The amino-terminal binding site was mapped to the first 57 amino acids of SV2A, and removal of this domain decreased calcium-mediated inhibition of binding to synaptotagmin, suggesting that it modulates calcium's effect on the SV2-synaptotagmin interaction. Introduction of the amino terminus of SV2A or SV2C into cultured superior cervical ganglion neurons inhibited neurotransmission, whereas the amino terminus of SV2B did not. These observations implicate the SV2-synaptotagmin interaction in regulated exocytosis and suggest that SV2A and SV2C, via their additional synaptotagmin binding site, function differently than SV2B.

Behavioral Modes Arise From a Random Process in the Nudibranch Melibe.

Stochastic analysis was applied to observations of spontaneous behavior in the carnivorous mollusc Melibe leonina. Six behaviors were denned that could be easily recognized on inspection and it was found that transitions between each of these behaviors could be fully described by a first-order random process without memory of past behavioral choices. The behaviors are organized by frequency of transition into two modes, a feeding mode and a resting mode. Transitions within modes are more likely than transitions between modes, and the feeding and resting modes are linked by a preferred pair of behavioral transitions. The amount of time spent in the feeding mode is positively correlated with body size, but the average length of a feeding episode is independent of size. This suggests that body size regulates the probability of entry into feeding behavior but does not influence the basic pattern of feeding. In the presence of food the animals express nearly continuous feeding behavior, suggesting that food reduces the probability of exiting the feeding mode. This model of spontaneous behavior in Melibe is used to form hypotheses amenable to further exploration through neurophysiological experiments.