Neurophysiological study on salt appetite in rats.

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Salt appetite as well as thirst plays an important role on homeostatic control of body fluid to maintain isosmosis and isovolemia in animals. The purpose of this study is to elucidate possible neuronal mechanisms which induce salt appetite in rats. The attention has been directed to the roles of peripheral hepatoportal osmoreception and the area postrema, one of the circumventricular organs.1. Hepatoportal osmoreception There exists an osmoreceptor mechanism in the hepatoportal region. It has been clarified that afferent impulses from the hepatoportal osmoreceptors are projected within the caudal portion of the nucleus tractus solitarius (NTS) dominantly on left hand side and they are modulated and integrated inside this nucleus. Three different pathways have been confirmed from the NTS to the hypothalamus : (1) A direct projection pathway, (2) A pathway via the parabrachial nucleus, both pathways may participate in salt appetite or thirst. (3) A pathway via the caudal ventrolateral medulla, which may play a role on regulation of ADH secretion.2. Sodium (or osmo-) responsive neurons within the area postrema. The area postrema (AP) is one of the circumventricular organs and lacks the blood brain barrier. Evidence has been established that sodium (or osmo-) responsive neurons exist within the Ap and hindbrain osmoreceptor mechanism may be involved in this structure in addition to the well known chemoreceptor trigger function for vomitting. It suggests that the Ap osmoreception may contribute to the homeostatic control of body fluid due to functional linkage between the AP and the NTS where the peripheral osmotic afferent signal are projected, and transmitted to the hypothalamus.