A relationship of a salivary secretion-body fluid balance axis and drinking behavior.

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The volume of urine and sweat is regulated by a control system of the body fluid homeostasis. Because saliva is produced from blood, the volume of saliva, like that of urine and sweat, changes with the osmotic pressure and volume of body fluid. Olsson (1976) reported that i.c.v. injection of hypertonic NaCl solution reduced the parcctid salivary flow rate in conscious goats. By contrast, injection of hypertccnic glucose or glycerol solutions affected salivary secretion in the opposite way. On the other hand, Yoshimura et al. (1968) reported that the volume of salivary secretion induced by pirocarpine was increased by i.c.v. injection of hypertonic NaCl solution in dogs. Thus, there is no consensus of opinion about a central regulation of salivary secretion by hyperosmotic stimulation.It is believed that intracerebroventricularly injected hyperosmotic solutions exert their effects directly upon osmosensitive neurones in the circumventricular and hypothalamic regions. Nonselective channels are opened when cell volume shrinks during hyperosmotic stimulation. If a similar cellular mechanism is involved in salivary secretion, then any hypertonic stimulants, after central administration, may influence the salivary secretion in the same direction. The present study was designed to understand the effects of i.c.v. injection of hypertonic solutions on reflex parotid salivary secretion. For this purpose, we used rats as experimental animals because their central osmoregulation was well understood. The effects of intracerebroventricular (i.c.v.) injection of hypertonic NaCl and sucrose on reflex parotid salivary secretion were studied in conscious rats. Saliva volume and flow rate induced by eating solid diets were decreased by both the hypertonic solutions, compared with the normal saline. This suggests that central osmotic perception affects parotid salivary secretion in rats.