Saline preference and body fluid analyses in rats after intrahypothalamic injections of carbachol☆

Abstract

Abstract Two experiments in rats supported previous suggestions that drinking induced by intrahypothalamic injections of carbachol directly activates a thirst motivational system. In Experiment 1 the palatability and osmotic properties of the drinking fluids were varied by presenting different concentrations of NaCl solution in one-and two-bottle drinking tests. Drinking induced by carbachol was found to be closely related to the concentration of the drinking fluid, being augmented when a palatable dilute saline solution was available but decreasing when the unpalatable concentrated solutions were presented instead. Fluid intakes were similar in this regard to the well-known preference-aversion curves for NaCl solutions produced by rats made thirsty by water deprivation. These findings indicate that the carbachol injections, like normal thirst, produce a motivated behavior and not mere reflexive drinking. In Experiment 2 body fluid and tissue analyses revealed that there were no changes in plasma volume or osmolarity associated with the carbachol injections. These findings indicate that the drinking which follows the cholinergic stimulations are due to the direct activation of central neural structures controlling water balance and are not secondary to systemic alterations of body fluids.