Dissociation of analgesic and rewarding effects of endomorphin-1 in rats

The mu -receptor is the primary mediator of the effects of morphine and the endogenous opiates, endomorphin-1 and endomorphin-2. Here we demonstrate a dissociation of the analgesic and rewarding effects of endomorphin-1 in ra ts. Tail-flick results revealed that endomorphin-1 produced significant ana lgesic effects within 10-min after injection. However, it failed to show re ward properties in the standard 45-min conditioned place preference (CPP) p aradigm or in an abbreviated 10-min pairing which paralleled the time frame of the tail-flick findings. Morphine induced both analgesia and reward. En domorphin-1 therefore is the first mu opiate shown to produce potent analge sia in the absence of reward behavior, and thus may have significant clinic al potential. (C) 2000 Elsevier Science inc. AII rights reserved.