Endomorphin 1 (trifluoroacetate salt) (Synonyms: Tyr-Pro-Trp-Phe-NH2)

Endomorphin 1 is an endogenous neuropeptide and μ-opioid receptor agonist (Ki = 0.36 nM) that has analgesic, positive reinforcing, and gastrointestinal properties. It is found in the brain and, to a lesser extent, in the spinal cord. It is selective for μ- over δ- and κ-opioid receptors (Ki = 1,506 and 5,428 nM, respectively). Endomorphin 1 also binds to mouse brain membranes and rat recombinant μ-opioid receptors expressed in CHO cells (Kis = 0.94 and 0.66 nM, respectively). It inhibits cAMP accumulation induced by forskolin in CHO cells expressing rat recombinant μ-opioid receptors and in SH-SY5Y human neuroblastoma cells (IC50s = 9.33 and 19.1 nM, respectively). In mice, endomorphin 1 increases analgesia in a radiant heat tail-flick assay following intracerebroventricular or intrathecal administration (ED50s = 4.75 and 0.726 μg, respectively) and induces conditioned place preference when administered intracerebroventricularly at a dose of 10 μg. Endomorphin 1 (12 μg, i.c.v.) also decreases gastrointestinal transit time in mice.