Role of mu, but not delta or kappa, opioid receptors in context-induced reinstatement of oxycodone seeking.

Relapse to non-medical use of prescription opioids often occurs after exposure to places previously associated with drug use. Here, we describe a rat model of context-induced reinstatement of oxycodone seeking after repeated cycles of drug self-administration and extinction-induced abstinence. We also determined the role of mu, delta and kappa opioid receptors (MOR, DOR, KOR) in this reinstatement. We trained rats to self-administer oxycodone for 6 h/day in context A; lever pressing was paired with a discrete cue. Next, we extinguished the lever pressing in the presence of the discrete cue in context B and then tested the rats for reinstatement of oxycodone seeking in both contexts. We retrained rats to self-administer oxycodone in context A, re-extinguished their lever pressing in context B and retested them for reinstatement in both contexts. Prior to testing, we injected the rats with vehicle or antagonists of MOR (naltrexone; 0.5 or 1.0 mg/kg), DOR (naltrindole; 7.5 or 15 mg/kg) or KOR (LY2456302; 5 or 10 mg/kg). We also tested the effect of naltrexone, naltrindole and LY2456302 on oxycodone self-administration under fixed-ratio-1 (FR1) and progressive ratio (PR) reinforcement schedules. We observed context-induced reinstatement of oxycodone seeking after repeated cycles of drug self-administration and extinction. Naltrexone, but not naltrindole or LY2456302, injections decreased this reinstatement. Additionally, naltrexone increased oxycodone self-administration under the FR1 schedule and decreased oxycodone self-administration under the PR schedule; naltrindole and LY2456302 were ineffective. Results demonstrate a critical role of MOR, but not DOR or KOR, in context-induced reinstatement of oxycodone seeking and oxycodone self-administration.