Thirst and salt appetite induced by hypovolemia in rats: analysis of drinking behavior

A detailed description of the increased intake of water and 0.5 M NaCl solution by rats after colloid-induced hypovolemia was obtained by measuring drinking activity every 6 s for 23 h. After an initial phase of largely single-bout water drinking that reflected hypovolemic thirst, there was a marked increase in saline drinking in multiple-bout episodes. This salt appetite developed while rats were volume depleted but persisted for hours even after the plasma volume deficits were repaired. Their drinking episodes then seemed to reflect osmoregulation, since cumulative intakes of water and saline were in appropriate proportions to produce a near-isotonic NaCl solution. Remarkably, rats concocted a 1% NaCl solution within 29% of the drinking episodes, by alternating intakes of water and saline every 30-90 s. This alternation was too rapid to allow significant absorption of ingested fluids from the intestines and changes in plasma osmolality, and thereby to permit central osmoreceptors to influence ongoing consumption. Instead, we propose that, in these episodes, rats are guided by gustatory receptors to obtain the desired NaCl in a palatable solution.