Prenatal exposure to morphine enhances cocaine and heroin self-administration in drug-naive rats.

Drugs of abuse putatively exert their rewarding actions by activating specific mechanisms in the brain. Sensitivity of these mechanisms to stimulation by drugs may be a factor in the development of drug dependence. As endogenous opioid systems may be involved in drug reward, manipulation of the functional state of opioid systems may affect this development. In the present study male rats exposed to morphine or placebo during their foetal period were tested for the development of intravenous self-administration of either heroin, cocaine or saline. Heroin and cocaine represent two fundamentally different classes of abused drugs. Prenatal exposure to morphine enhanced rates of heroin and of cocaine, but not of saline self-administration. The data indicate that this was not simply the result of increased motor activity. Given the fact that the unit-doses of heroin and cocaine were threshold doses for the development of intravenous drug self-administration behaviour, it is concluded that the reinforcing efficacy of both drugs is enhanced by prenatal morphine treatment, and that such treatment possibly facilitates development of drug dependence in general.