Some well-kept hypothalamic secrets disclosed

The magnocellular neuropeptidergic cells of the supraoptic and paraventricular nuclei comprise much of what is known as the hypothalamoneurohypophyseal system and is involved in several functions, including body fluid balance, parturition, and lactation. In vivo experiments have not produced a clear understanding of some of the crucial features associated with the functioning of this system. In particular, questions relating to the osmosensitivity of magnocellular neurons and the mechanisms(s) by which their characteristic firing patterns are generated have not been answered by using the older approaches. Electrophysiological studies with brain slices present direct evidence for osmosensitivity, and perhaps even osmoreceptivity, of magnocellular neurons. Other evidence is reviewed indicating that the phasic bursting patterns of activity associated with vasopressin-releasing neurons: 1) occur in the absence of patterned chemical synaptic input, 2) are probably influenced by localized changes in extracellular K+ concentrations, 3) may be modulated by electrotonic conduction across gap junctions connecting magnocellular neurons, and 4) are likely to be generated by endogenous membrane currents.