Mechanisms of neuroendocrine cell excitability.

Oxytocin (OT) and vasopressin (VP), two neuronally synthesized nonapeptides, are made in the hypothalamic paraventricular and supraoptic nuclei of mammals and released into their blood, eventually to have profound hormonal actions on peripheral tissues. In the rat both OT and VP neurons fire slowly and irregularly under conditions of low demand for peptide release, but natural or artificial depolarizing stimuli result in differential patterns of activity: either regular continuous firing, strongly associated with OT cells, or phasic bursting, characteristic of VP neurons. Recently published findings offer an explanation for the dominant presence of certain Ca(2+)-dependent membrane potentials that typically lead to phasic firing in VP neurons. Mechanisms of excitability involved in the differential activities of the two cell types, as well as of the same cell type under different physiological conditions, include such factors as Ca2+ binding proteins, voltage- and ligand-gated ion channels, release of Ca2+ from internal stores and gap junctional conductances. The evidence for these factors is reviewed here.