Morphiceptin is a tetrapeptide (Tyr-Pro-Phe-Pro-NH<sub>2</sub>) that is a selective ü-opioid receptor agonist. It is derived from ò-casomorphin and has over 1,000 times selectivity for ü- over ô-opioid receptors. When injected intracerebroventricularly (into the ventricular system of the brain), morphiceptin had an analgesic ED<sub>50</sub> of 1.7 nmol per animal. The analgesic effects of morphiceptin were reversed by naloxone, meaning that the analgesic effect is mediated by the ü-opioid receptor.
Morphiceptin is the (1S,2S,3S,4S)-form whereas deproceptin is the (1S,2S,3S,4R)-form [84799-23-5].