Chronic intermittent heroin produces locomotor sensitization and long-lasting enhancement of conditioned reinforcement

► Repeated injections of heroin in rats produces sensitization to its locomotor-stimulant effects. ► Repeated injections of heroin in rats enhances responding reinforced by a food-associated conditioned stimulus. ► The enhancement of appetitive learning and motivational processes observed with chronic heroin lasts into a protracted withdrawal period, indicating that it is relatively long-lasting.