Anticipation of Pharmacological and Nonpharmacological Events: Classical Conditioning and Addictive Behavior

Typically, pharmacological phenomena such as tolerance, sensitization, and dependence have been viewed as resulting from the operation of feedback mechanisms: pharmacologically disturbed homeostatic functioning is countered by compensatory responses that restore physiological equilibrium. We summarize the results of research indicating that feedforward mechanisms (i.e., regulatory responses made in anticipation of a drug) also importantly contribute to drug effects. Such feedforward mechanisms operate on the basis of Pavlovian conditioning principles. We also discuss the role of such physiological feedforward mechanisms in areas that are not primarily pharmacological: immunology, exercise physiology, and stress.