LACK OF INVOLVEMENT OF DELTA-OPIOID RECEPTORS IN MEDIATING THE REWARDING EFFECTS OF COCAINE

The non-selective opioid antagonist naltrexone and the partial agonist buprenorphine have been reported to reduce cocaine self-administration (SA) and relapse in both humans and rhesus monkeys. Data suggesting an involvement of delta-opioid receptors in modulating the conditioned rewarding effects of cocaine were also recently presented. In view of such findings, the present SA and place conditioning studies were conducted to examine the influence of the selective delta-opioid receptor antagonist naltrindole upon the rewarding effects of cocaine. Sprague-Dawley rats were trained to self-administer cocaine (1.0 mg/kg per infusion) on an FR2 schedule of reinforcement, Dose-response and antagonist testing commenced once stable rates of cocaine SA were achieved. For antagonist testing, rats received naltrindole (0.03-10.0 mg/kg, IP) 30 min prior to the start of 2-h SA sessions. SA behavior in response to cocaine delivery (0.25 and 1.0 mg/kg per infusion) was then determined. Naltrindole in doses of 0.03-3.0 mg/kg did not alter the number of cocaine infusions taken by the rats. A higher dose of naltrindole (10.0 mg/kg), which markedly depressed locomotor activity, resulted in a 16% reduction of cocaine (0.25 mg/kg per infusion) SA behavior. When SA sessions were terminated and naltrindole (1.0 mg/kg) was administered repeatedly for 3 days, no alterations in the re-acquisition of cocaine SA were seen. Place conditioning studies also failed to find an effect of naltrindole (0.1-3.0 mg/kg) on cocaine (10 mg/kg) - induced conditioned place preferences. Naltrindole, by itself, did not induce significant place conditioning. These data fail to indicate a role of delta-opioid receptors in modulating either the positive reinforcing or conditioned rewarding effects of cocaine. Furthermore, they suggest that the therapeutic actions of naloxone, naltrexone and buprenorphine on cocaine SA behavior may not result from the specific blockade of delta-opioid receptors.