Effects of dehydration on plasma osmolality, thirst-related behavior, and plasma and brain angiotensin concentrations in Couch's spadefoot toad, Schaphiopus couchii

Under dehydrating conditions, many terrestrial vertebrates species exhibit increases in plasma osmolality and their drinking behavior. Under some circ umstances, this behavioral change is accompanied by changes in plasma and c entral angiotensin concentrations, and it has been proposed that these chan ges in angiotensin levels induce the thirst-related behaviors. In response to dehydration, the spadefoot toad, Scaphiopus couchii, exhibits thirst-rel ated behavior in the form of cutaneous drinking. This behavior has been ter med water absorption response (WR) behavior. Spadefoot toads Live in harsh desert environments and are subject annually to dehydrating conditions that may induce thirst-related behavior. We tested the hypothesis that an incre ase in WR behavior is associated with both an increase in plasma osmolality and an increase in plasma and brain angiotensin concentrations. First, we determined the degree of dehydration that was necessary to initiate WR beha vior. Animals dehydrated to 85% of their standard bladder-empty weight via deprivation of water exhibited WR behavior more frequently than control toa ds left in home containers with water available. Next, using the same dehyd ration methods, we determined the plasma osmolality and sodium concentratio ns of dehydrated toads. Toads dehydrated to 85% standard weight also had a significant increase in plasma osmolality, but exhibited no overall change in plasma sodium concentrations, indicating that while an overall increase in plasma osmolality appears to be associated with WR behavior in S. couchi i, changes in sodium concentrations alone are not sufficient to induce the behavior. Finally, plasma and brain angiotensin concentrations mere measure d in control toads and toads dehydrated to 85% standard weight. Plasma and brain angiotensin concentrations did not increase in dehydrated toads, indi cating that dehydration-induced WR behavior that is associated with changes in plasma osmolality may not be induced by changes in endogenous angiotens in concentrations in S. couchii. J. Exp. Zool. 286:572-584, 2000. (C) 2000 Wiley-Liss, Inc.