FOREBRAIN LESIONS THAT DISRUPT WATER HOMEOSTASIS DO NOT ELIMINATE THESODIUM APPETITE OF SODIUM DEFICIENCY IN SHEEP

Brain structures located within the anterior wall of the third brain v entricle (subfornical organ, median preoptic nucleus and organum vascu losum of the lamina terminalis) are known to be involved in thirst as well as other aspects of body fluid and electrolyte balance. The prese nt studies evaluated the role of these structures in the Na appetite o f mildly or moderately Na-depleted sheep (sheep with a parotid fistula deprived of Na solution for 22 or 46 h). In addition, the role of the se structures was tested in mildly Na-depleted sheep in which the Na a ppetite was enhanced by decreasing cerebrospinal fluid and brain extra cellular fluid Na concentration (i.e., i.c.v. infusion of hypertonic s accharide solution) or was decreased by systemic infusion of hypertoni c saline. The results indicated that sheep with lesions which reduced or eliminated daily water intake or water intake in response to hypert onicity of body fluids had, in all situations tested, appropriate chan ges in Na appetite (i.e., similar to their prelesion changes). Thus, t he present experiments demonstrated that the brain areas involved in t hirst as well as other aspects of body fluid and electrolyte balance a re anatomically different from those involved in regulating Na appetit e.