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

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Abstract
THE endomorphins are potent and selective endogenous agonists at the mu opioid receptor. We describe here the postnatal ontogeny of endomorphin-2 like immunoreactivity (EM2-LI) in the dorsal horn of the rat medulla and spinal cord. EM2-LI is dense in the superficial lamina of the dorsal horn of the adolescent and adult rat, with fibers also present in skin and dorsal root ganglia. No staining was noted at 3 days of age or younger. Faint and limited staining was noted by 7 days of age. The density and distribution of the immuno-reactivity 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 endomorphin-2.