The Ontogeny of Endomorphin‐1‐ and Endomorphin‐2‐like Immunoreactivity in Rat Brain and Spinal Cord

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Abstract
Endomorphin‐1 and endomorphin‐2 are recently described peptides with high affinity and specificity for the μ opioid receptor. They are believed to be the endogenous ligands for that receptor. We describe the maturation of the endomorphins in brain and spinal cord using recently characterized antibodies to each. Endomorphin‐1‐like immunoreactivity was examined in brain, focusing on the periaqueductal gray of the midbrain and the diagonal band of Broca; endomorphin‐2‐like immunoreactivity is reported for the medulla and spinal cord. In these regions, and in all other regions studied but not described in this paper, the endomorphins were not seen at birth or at 3 days of age. Staining was present in 7‐day‐old and older animals. At these early ages, the pattern and density of staining are not fully developed, but appear complete by 21 days of age. The results suggest that both endomorphin‐1 and endomorphin‐2 develop relatively late compared to other opioid peptides.