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Involvement of central mu and delta opioid receptors in mediating the reinforcing effects of beta-endorphin in the rat.

An unbiased place preference conditioning procedure was used to identify the central opioid receptor types through which the endogenous opioid peptide, beta-endorphin, acts to exert its reinforcing effects in rats in vivo. The intracerebroventricular administration of beta-endorphin, and selective mu (DAGO) or delta (DPDPE) opioid receptor agonists produced marked preferences for the drug-associated place. Intracerebroventricular pretreatment with the selective mu antagonist, CTOP, eliminated the place preference produced by DAGO but not that produced by DPDPE. Pretreatment with the selective delta antagonist, ICI 174,864, abolished the place preference induced by DPDPE. It did not modify the effect of DAGO. In contrast, pretreatment with either ICI 174,864 or CTOP abolished the effects of beta-endorphin. These data demonstrate that both mu and delta receptors are involved in mediating the reinforcing effect of beta-endorphin and indicate that the activation of both receptor types is required for the expression of the motivational effects of beta-endorphin. Further they suggest that beta-endorphin produces its motivational effects via an interaction with an opioid receptor complex composed of both mu and delta receptors.