OSMORECEPTORS, OSMORECEPTION, AND OSMOREGULATION

Mammals have evolved sophisticated behavioral and physiological respon ses to oppose changes in the osmolality of their extracellular fluid. The behavioral approach consists of regulating the intake of salt and water through changes in sodium appetite and thirst. The physiological approach comprises adjustments of renal excretion of water and sodium which are achieved through changes in the release of antidiuretic and natriuretic hormones. Individually, these osmoregulatory responses ar e controlled by ''osmoreceptors'': groups of specialized nerve cells c apable of transducing changes in external osmotic pressure into meanin gful electrical signals. Some of these sensors are located in the regi on of the hepatic portal vein, a strategic site allowing early detecti on of the osmotic impact of ingested foods and fluids. Changes in syst emic osmolality, however, are detected centrally, within regions that include the medial preoptic area, the median preoptic nucleus, the org anum vasculosum lamina terminalis (OVLT), the subfornical organ, and t he supraoptic nucleus (SON). While studies have indicated that these c entral and peripheral osmoreceptors participate in the control of osmo regulatory responses, little is known of the mechanisms by which this is achieved. One notable exception, however, consists of the osmotic c ontrol of electrical activity in SON neurons which, in the rat, contri butes to the regulation of natriuresis and diuresis through effects on the secretion of oxytocin and vasopressin. Previous studies have show n that these cells are respectively excited and inhibited by hypertoni c and hypotonic conditions. Experiments in vitro indicate that these r esponses result from both the endogenous osmosensitivity of these cell s and changes in synaptic drive. Patch-clamp analysis has revealed tha t SON neurons are respectively depolarized and hyperpolarized by incre ases and decreases in external osmolality and that these intrinsic res ponses result from changes in the activity of mechanosensitive cationi c channels. Moreover, intracellular recordings in hypothalamic explant s have shown that changes in electrical activity are associated with p roportional changes in the frequency of glutamatergic excitatory posts ynaptic potentials derived from osmosensitive OVLT neurons. Both of th ese mechanisms, therefore, may participate in the osmotic regulation o f neurohypophysial hormone release in situ. (C) 1994 Academic Press, I nc.