Oxytocin reduces intravenous heroin self-administration in heroin-tolerant rats

Intravenous self-administration of heroin was studied in experimentally naive rats, as compared to this behavior in animals rendered tolerant to heroin by multiple injections. The tolerant rats also exhibited mild signs of spontaneous and naloxone-precipitated heroin withdrawal. Self-administration behavior developed earlier in the tolerant rats. In heroin-naive rats, oxytocin treatment did not influence the acquisition of heroin self-administration behavior. In the tolerant rats, on the other hand, oxytocin decreased the acquisition of heroin self-administration. When maintenance of heroin self-administration was studied in the tolerant rats, graded doses of oxytocin (0.05, 0.5 and 5 micrograms s.c.) decreased heroin intake. This finding, which is in agreement with previous data indicating that oxytocin attenuates the development of tolerance to and dependence on narcotic analgesics, suggests that the neuropeptide reduced the reinforcing efficacy of heroin in the tolerant organism.