The research program is directed towards studying the biochemistry and physiology of the nerve terminals. The neurohypophysial nerve terminals are used as the model system to investigate the importance of ion channels and receptors on initiation and modulation of neuronal secretion. These include studies on secretion of vasopressin and oxytocin from isolated intact posterior pituitaries and isolated neurosecretosomes, and ion channels on the nerve terminal membrane. A neurotoxin from dendroaspis angusticeps which specifically blocks a type of K+ channel, was found to enhance hormone secretion under very low frequency stimulation conditions suggesting that these transient K+ channels may be involved in frequency-dependent facilitation. Secretion from oxytocin terminals in intact neural lobes was found to be inhibited by the opiate kappa receptor agonist, dynorphin, released from vasopressin terminals. The preparation of neurosecretosomes has been maintained under tissue culture conditions and have been used to study the mechanism of calcium- dependent hormone secretion, its inactivation and modulation. This preparation is suitable for high resolution microscopy and patch clamp analysis of ion channels. Secretion from isolated neurosecretosomes was found to be inactivated rapidly under maintained depolarizations. This inactivation was shown to be calcium dependent, and requiring an enzymatic step. Cyclic GMP was found to markedly reduce the rate of this inactivation. Tetanus toxin in nM concentrations blocked secretion of both vasopressin and oxytocin induced by veratridine depolarization. This blockade was not reversed by exogenously added cyclic GMP or cGMP- phosphodiesterase inhibitor. Patch clamp studies on intermediate lobe cells in primary cultures revealed the presence of three different types of Ca++ channels based on their inactivation kinetics.
