Endomorphins are two endogenous opioid peptides. Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are tetrapeptides with the highest known affinity and specificity for the μ opioid receptor. Endomorphin-1 is located 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 behaviors1. It is assumed that endomorphins are the cleavage products of a larger precursor, but this polypeptide or protein has not yet been identified. Perikarya expressing EM2-like immunoreactivity were present in the posterior hypothalamus, whereas those expressing EM1-like immunoreactivity were present in both the posterior hypothalamus and the nucleus of the solitary tract (NTS). EM1-like immunoreactivity was more widely and densely distributed throughout the brain than was EM2-like immunoreactivity, whereas EM2-like immunoreactivity was more prevalent in the spinal cord than was EM1-like immunoreactivity. endomorphins participate in modulating nociceptive and autonomic nervous system processes and responsiveness to stress.
References:
1. Greco, MA; Fuller, PM; Jhou, TC; Martin-Schild, S; Zadina, JE; Hu, Z; Shiromani, P; Lu, J (2008). "Opioidergic projections to sleep-active neurons in the ventrolateral preoptic nucleus". Brain Research 1245: 96–107.