INVOLVEMENT OF CENTRAL MU-OPIOID 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, β-endorphin, acts to exert its reinforcing effects in rats in vivo. The intracerebroventricular administration of β-endorphin, and selective μ (DAGO) or δ (DPDPE) opioid receptor agonists produced marked preferences for the drug-associated place. Intracerebroventricular pretreatment with the selective μ antagonist, CTOP, eliminated the place preference produced by DAGO but not that produced by DPDPE. Pretreatment with the selective δ 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 β-endorphin. These data demonstrate that both μ and δ receptors are involved in mediating the reinforcing effect of β-endorphin and indicate that the activation of both receptor types is required for the expression of the motivational effects of β-endorphin. Further they suggest that β-endorphin produces its motivational effects via an interaction with an opioid receptor complex composed of both μ and δ receptors. © 1990.