Localization of enkephalin immunoreactivity in diverse tissues and cells of the developing and adult rat

The distribution of enkephalin, an endogenous opioid, in tissues and cells of the developing and adult rat was determined by immunocytochemistry with antibodies to met- and leu-enkephalin. Met- and leu-enkephalin were found in all developing cells investigated, with staining generally located throughout the cytoplasm; cell nuclei were not immunoreactive. In comparison to developing cells, immunoreactive analogues to met-enkephalin were usually difficult to detect in the adult. Some notable exceptions were reaction products in leukocytes in blood, lung, and cortex of thymus, fibroblasts in the skin, and seminiferous tubules. These results, in concert with earlier reports that opioid receptors are found largely in developing, but not adult, tissues, indicate that endogenous opioids are specifically involved in biological development, particularly cell proliferation and differentiation. Immunoreactivity in adult non-neural cells may be related to their development in some cases, but also could indicate other functions.