EXTRINSIC MODULATION OF SPIKE AFTERPOTENTIALS IN RAT HYPOTHALAMONEUROHYPOPHYSEAL NEURONS

1. Magnocellular neurosecretory cells (MNCs) in the rat hypothalamus a dopt a phasic pattern of spike discharge under conditions demanding en hanced vasopressin release, such as during dehydration or hemorrhage, The emergence of phasic firing minimizes the occurrence of secretory f atigue from the axon terminals of MNCs, thereby maximizing vasopressin release from the neurohypophysis, 2. Intracellular and whole-cell rec ordings from hypothalamic slices or explants in vitro have shown that phasic firing is supported by the presence of a plateau potential whic h arises from the summation of spike depolarizing afterpotentials (DAP s), Modulatory actions of neurotransmitters on the amplitude of the DA P, therefore, represent possible mechanisms by which the expression of phasic firing may be regulated in vivo. 3. Here we review the basis f or phasic firing in MNCs of the rat supraoptic nucleus and present rec ent findings concerning the direct and indirect mechanisms through whi ch selected neurotransmitters have been found to regulate the amplitud e of DAPs.