Intrinsic membrane properties of magnocellular neurosecretory neurons recorded in vitro.

Over the past 15 years, extracellular recordings from the rat supraoptic and paraventricular nuclei have revealed two populations of endocrine neurons that can be antidromically activated from neurosecretory axons in the neurohypophysis. Both the oxytocinergic and vasopressinergic populations of magnocellular neuroendocrine cells (MNCs) fire bursts of action potentials that facilitate hormone release from neurohypophyseal terminals. Moreover, both populations are osmosensitive, increasing their firing rate as osmolarity is elevated. Recently, slice and explant preparations of hypothalamus have enabled intracellular recording of these cells in normal and modified saline solutions. Spiking, bursting, and osmosensitivity can occur independently of synaptic input, enabling MNCs, for example, to transform an unpatterned depolarizing influence into the repetitive bursting pattern associated with vasopressin release. Current-clamp studies have started to characterize the repertoire of conductances across the MNC membrane that are responsible for action potential discharge, afterpotentials, bursting, and osmosensitivity. This provides a basis not only for further voltage-clamp studies, but for understanding transmitter effects that act by modulating intrinsic MNC currents.