Motivational properties of opioids: evidence that an activation of σ-receptors mediates reinforcement processes

Abstract

The role of central σ-opioid receptors in the mediation of opioid reinforcement and endogenous reward processes was examined using a non-biased place-preference conditioning procedure. Intracerebroventricular (i.c.v.) administration of the selective σ-receptor agonists, [d-Pen², d-Pen⁵]-enkephalin (DPDPE, 10.0–25.0 μg) produced a significant preference for the drug-associated place and a similar effect was observed following i.c.v. injections of morphine (10.0 μg). Administration of the σ-receptor antagonist, ICI 174,864, at doses (1.0–5.0 μg, i.c.v.) which had no aversive effects when tested alone, abolished the reinforcing properties of DPDPE. Such treatment did not, however, modify the effect of morphine. These findings demonstrate the involvement σ- as well as μ-receptors in the motivational properties of opioids and suggest that the activation of either receptor type is sufficient for the elicitation of appetively reinforcing effects.