The ontogeny of sodium appetite in the rat: A behavioral, physiological, and pharmacological analysis

Sodium intake is vital for mammals in their maintenance of body fluid homeostasis. Adult rats will avidly ingest sodium in response to body sodium loss to rectify their sodium deficit state. This sodium appetite may be elicited by the actions of either central angiotensin II (AII) alone, as in the adrenalectomized rat, or by the synergistic actions in the brain of AII and aldosterone (ALDO), as occurs naturally in the sodium depleted rat. The following experiments examined (1) the ontogenetic calendar for emergence of sodium depletion and adrenalectomy-induced sodium appetite in the rat, (2) circulating plasma levels of AII and ALDO through-out the suckling period of rat pups during their normal hydrational state and in response to sodium depletion and adrenalectomy, (3) effects of selective angiotensin II Type 1 (AT$\sb1)$ and Type 2 (AT$\sb2)$ receptor antagonists on the sodium appetite of preweanling rats, and (4) levels of brain AII receptors in normal, sodium depleted, and adrenalectomized rats at various ages. Sodium depletion and adrenalectomy-induced sodium appetite emerges at 11 and 12 days of age, respectively, and appears selective for NaCl in that intakes of water, milk or CaCl$\sb2$ are not different from control rats. Measurable levels of plasma AII and ALDO are present at birth, and in response to the two treatments, levels of AII and ALDO (sodium depletion) or AII alone (adrenalectomy) were elevated as early as 3 days of age, well before the behavioral emergence of sodium appetite. Direct interference with brain AT$\sb1$ and AT$\sb2$ receptors in 12 day old rats showed that occupancy of cerebral AT$\sb1,$ and not AT$\sb2$ receptors, was important in expression of both sodium depletion and adrenalectomy-induced sodium appetites. Direct measurement of AT$\sb1$ and AT$\sb2$ receptors in selected brain regions of rat pups taken at ages prior to or after the emergence of sodium appetite revealed very little change in binding activity following sodium depletion or adrenalectomy. Together, these experiments show that the sodium appetite of the sodium depleted and adrenalectomized rat emerges at 11-12 days of age and thereafter it appears indistinguishable from that of the adult rat