Inhibition of dehydration induced drinking in rats by reduction of CSF Na concentration

Male rats were dehydrated for 22 h and then given 4 h intracerebroventricular (i.c.v.) infusions which commenced 2 h prior to the beginning of a 2-h fluid access period. I.c.v. infusion of iso-osmotic 0.27 M mannitol-CSF more than halved the amount of water normally drunk by dehydrated rats during the fluid access period. Whilst i.c.v. infusion of 0.7 M mannitol-CSF did not alter the amount of water drunk during the fluid access period. Presumably both infusates reduce CSF [Na] but only 0.7 M mannitol elevates CSF osmolality. The evidence is consistent with the involvement of both sodium and osmoreceptors in thirst in the rat. A reduction of CSF [Na] will inhibit dehydration induced water drinking provided the osmotic pressure of the CSF is not greatly elevated. In addition evidence is provided to show that a contrived reduction of CSF [Na] alone is not a sufficient physiological trigger to initiate salt appetite in rats.