A homeostatic model of Pavlovian conditioning: tolerance to scopolamine-induced adipsia

Several experiments support a model proposing that tolerance to scopolamine-induced adipsia involves an interplay between processes of homeostatic regulation and Pavlovian conditioning. In Experiment 1a, rats given access to water while under the influence of scopolamine developed adipsic tolerance, whereas rats denied access to water in the drug state did not. In Experiment 1b, rats displayed adipsic tolerance only when scopolamine was administered with cues associated with previous drug injections. In Experiment 1c, adipsic-tolerant rats showed a polydipsic response to an injection of phenobarbital (which unconditionally augments water consumption) relative to nontolerant animals with the same pharmacological history. Experiment 2 assessed the effect of deprivation level on the Pavlovian extinction of adipsic tolerance. Rats satiated with water during extinction showed a loss of adipsic tolerance, whereas water-deprived rats did not. The present model is discussed in relation to (a) tolerance in other response systems such as morphine analgesia, (b) theories of extinction for nonpharmacological Pavlovian conditioning, and (c) homeostatic regulation and incentive motivation.