The neural and hormonal mechanisms underlying the expression of sodium appetite during sodium depletion have been and still are being extensively investigated. In contrast, little is known of what influences the observed variations in the spontaneous salt intake by the need-free individual. In the course of studying salt appetite, I observed that daily intake of 0.3 M NaCl was dramatically and persistently elevated from a baseline of 5 to an asymptote of 35 ml/day for months after a single episode of sodium depletion. Further studies have determined that the increased salt intake is not specific to the method of depletion, nor is it secondary to excessive renal loss of sodium, nor is it dependent on the ingestion of saline at the time of the sodium depletion. Preliminary data show that an episode of sodium depletion at 12 but not at 6 days postnatal is effective in increasing the daily need-free salt intake in adulthood. This is the first report of an early treatment that reliably produces a large increase in the adult intake of salt. These data strongly suggest that variations in individual salt intake may be in part explained by different histories of sodium need. This proposal seeks to understand the neural and hormonal mechanisms which are responsible for the induction and maintenance of the persistent increases in need-free salt intake. Using a combination of behavioral and physiological techniques, the following questions are asked: 1) Do naturally-occurring episodes of sodium depletion produce persistent increases in need-free salt intake?; 2) How does the degree of sodium depletion affect the need-free intake of salt and are successive sodium depletions additive?; 3) Is the increased salt intake specific to the taste of sodium and how is the preference aversion function for saline affected?; 4) What is the contribution of the hormones angiotensin II, aldosterone and vasopressin to the induction and maintenance of the persistently increased salt intake in the pup and in the adult rat?; and 5) What is the minimum age at which persistent changes in salt intake can be affected by sodium depletion? By understanding how sodium metabolism history affects the need-free salt intake of rats, we may gain insight into the factors underlying the excessive sodium intake of humans. Salt intake has been linked to hypertension and other pathologies; thus it is important to understand the mechanisms which contribute to the voluntary need-free intake of salt.
