Somatosensory Systems, PainMaturation of endomorphin-2 in the dorsal horn of the medulla and spinal cord of the ratBarr, Gordon A.1,3; Zadina, James E.2Author Information 1Department of Psychology, Hunter College, City University of New York and New York State Psychiatric Institute, 695 Park Avenue, New York, NY 10021 2VA Medical Center and Tulane University School of Medicine, New Orleans, LA 70146, USA 3Corresponding Author: Gordon A. Barr ACKNOWLEDGEMENTS: We acknowledge the expert histochemical assistance of Jianxin Cheng and Shaoning Wang and the advice of S. Martin-Schild. Supported by NS36130, DA00660, DA00325, DA11655, Paralyzed Veteran's Association 1865 and the VA. Received 18 August 1999; accepted 6 October 1999 NeuroReport: December 16th, 1999 - Volume 10 - Issue 18 - p 3857-3860 Buy 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. © 1999 Lippincott Williams & Wilkins, Inc.