Endomorphin
The endomorphins are a group of endogenous opioid peptides consisting of endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2). They are tetrapeptides with the highest known affinity and selectivity for the μ-opioid receptor. Endomorphin-1 is found in the nucleus of the solitary tract, the periventricular hypothalamus, and the dorsomedial hypothalamus, where it is found within histaminergic neurons and may regulate sedative and arousal behaviors. It is assumed that endomorphins are the cleavage products of a larger precursor, but this polypeptide or protein has not yet been identified.