The research program is directed toward studying the biochemistry and physiology of secretion at the nerve terminal with particular emphasis on intracellular ion transients, modulation secretion, and mechanism of exocytotic secretion. The neurohypophysial nerve terminals and the giant synapse served as experimental models. Experiments include studies on secretion of vasopressin and oxytocin from isolated neurosecretosomes, and measurement of calcium ion transients in the nerve terminals. Depolarizations causes calcium increase in the nerve terminal cytoplasm via at least two different types of voltage-activated calcium channels, and calcium remains elevated as long as depolarization exists. Secretion, on the other hand, undergoes inactivation despite the elevated cytoplasmic calcium channels. Calcium ions entering the terminal via specific calcium channels are more efficacious in initiating secretion. Microinjection into the squid giant synapse of analogues of guanine nucleotides demonstrated that GTP hydrolysis and GDP/GTP exchanges were necessary events in transmitter release. Measurements of calcium in the pre-synaptic terminal argued against the possibility that these GTP analogues were influencing the activity of ion channels and suggested that the effects were due to perturbation of reactions that are triggered by calcium. Electron microscopy showed that these GTP analogues had affected vesicle recycling at the terminal, consistent with their established effects on intracellular membrane trafficking. The most likely synaptic target of these analogues is rab3A, a synaptic vesicle protein. A cDNA encoding a portion of squid rab3A was isolated by RT/PCR and used to screen a cDNA library prepared from squid brain. Candidate clones containing the entire coding sequence of rab3A are being analyzed.
