Paradoxical changes in brain reward status during opioid self-administration in a novel test of the negative reinforcement hypothesis

Abstract

Background and Purpose The extra-medical use and addiction of prescription opioid analgesics is a growing health problem. To characterize how prescription opioid abuse develops, this study investigated the affective consequences of escalating prescription opioid use using intracranial self-stimulation (ICSS) reward and oxycodone intravenous self-administration (IVSA) models. Experimental Approach Male Wistar rats were given access to oxycodone IVSA (0.15 mg/kg/infusion, i.v.) in Short Access (ShA; 1 h) or Long Access (LgA; 12 h) sessions for 5 sessions/week followed by intermittent 60 h discontinuations from drug access, a novel explicit test of the negative reinforcement hypothesis. A separate group was first trained in the ICSS procedure and then in oxycodone IVSA in 11 h LgA sessions. Key Results Rats given LgA to oxycodone escalated their responding more than ShA rats, with significant increases following 60 h discontinuations. Pre-session brain reward thresholds increased with sequential daily LgA IVSA sessions, consistent with a growing negative affective state consequent to successive daily intoxication/abstinence cycles. A 1 h oxycodone IVSA interval was sufficient to normalize these elevated reward thresholds, as was, paradoxically, a 60 h weekend abstinence. The increase in ICSS thresholds was attenuated in a group administered the long-acting kappa opioid antagonist norBNI prior to IVSA training. Conclusions and Implications Changes in brain reward function during escalation of oxycodone selfadministration is driven by an interplay between kappa opioid receptor-mediated negative affective state associated with escalated oxycodone intake and dynamic restoration of brain reward status during longer periods of abstinence.