OPRM1 and depressive disorder: As another example, IP acid also produces a pain-related depression of positively reinforced operant behavior maintained in rats by delivery of either food or a social reinforcer (brief access to another rat); however, MOR agonists are less effective to disrupt responding maintained by food than by the social reinforcer and correspondingly more effective to alleviate IP acid-induced depression of food- than social-maintained responding (19).