Maturation of endomorphin-2 in the dorsal horn of the medulla and spinal cord of the rat

THE endomorphins are potent and selective endogenous agonists at the mu opi oid receptor. We describe here postnatal ontogeny of endomorphin-2 like imm unoreactivity (EM2-LI) in the dorsal horn of the rat medulla and spinal cor d. EM2-LI is dense in the superficial lamina of the dorsal horn of the adol escent and adult rat, with fibers also present in skin and dorsal root gang lia. No staining was noted at 3 days of age or younger. Faint and limited s taining was noted by 7 days of age. The density and distribution of the imm unoreactivity increased with age, reaching an adult-like distribution by 21 days of age. Stress-induced responses mediated by endogenous opioids occur late in development and may be related to the late appearance of endomorph in-2. (C) 1999 Lippincott Williams & Wilkins.