Pavlovian influences over food and drug intake.

Consuming food and taking drugs share several important characteristics. In particular, each causes changes in important physiological parameters that are constantly being monitored and regulated by the brain. As examples, blood glucose increases after meals; and body temperature decreases after ethanol is taken. Such changes elicit neurally-mediated homeostatic responses that serve to reduce the magnitude and duration of the perturbation. It is argued that when an individual can accurately anticipate pending meals or drugs, it can make appropriate responses to minimize or totally neutralize the meal/drug-elicited perturbations. This phenomenon, which is the basis for meal and drug tolerance, relies upon Pavlovian conditioning. Literature is reviewed which documents the role of conditioning processes in the development of tolerance. The argument is made that conditioned responses enable individuals to derive necessary or desirable aspects of food and drugs while minimizing some of their negative effects. In a final section, drug tolerance is discussed as a natural consequence of evolution-derived, meal-related learning processes, with associated negative consequences.