Differential effect of naloxone on food and self-stimulation rewarded acquisition of a behavioral response pattern

The involvement of endogenous opioids in self-stimulation reward was investigated by repeated administration of the opioid antagonist naloxone to rats during acquisition of a behavioral response pattern that was rewarded with electrical (self-)stimulation of the ventral tegmental area. A control experiment was performed using food deprived rats in which a comparable response pattern was rewarded with food pellets. The response patterns consisted of gradually decreasing amounts of reward per response, which could be reset to maximal reward by another response. It was found that naloxone disrupted the acquisition of the stimulation rewarded response pattern, while it did not influence the food rewarded behavior. It is suggested that endorphin systems are actively involved in the acquisition of self-stimulation reward procedures, and that this involvement may be specific for self-stimulation reward.