1) Opioid peptide 
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 specificity for the μ-opioid receptor. Endomorphin-1 is found in the nucleus of the solitary tract, the periventricular hypothalamus, and the d...