OPIOIDS IN NEURAL AND NONNEURAL TISSUES

The endogenous opioid peptides (EOP) are grouped in three families, ea ch deriving from the posttranslational processing of a distinct precur sor molecule and exhibiting high affinity for a specific opioid recept or. The genes of EOPs are expressed in a wide variety of sites, includ ing many nerve, neurosecretory, and endocrine cells. In reviewing the vast literature on this subject, a few patterns begin to emerge. First , the distribution of EOPs in tissues appears to be a distinct charact eristic of each family of opioids. Second, the EOP-producing cells can be grouped into two broad categories: those expressing only one and t hose expressing multiple EOP genes. Most EOP-producing nerve and neuro secretory cells fall into the first category, that is, they express on e EOP gene, whereas most nonneural cells fall into the second category , that is, they express multiple EOP genes. Third, it appears that the re is a relationship between opioids, proliferation rate, and state of differentiation of cells, since it has been shown that (a) mitogenic factors may change the EOP profile of a cell, and that (b) opioids may inhibit the proliferation rate of normal or neoplastic cells. The phy siologic implication of these observations is briefly discussed.