Opioid peptides in the female reproductive system: physiological implications

All endogenous opioid peptides derive from three precursor molecules i.e. proopiomelanocortin, proenkephalin and prodynorphin. The endogenous opioid peptides exert their biological effects through opioid receptors. Each endogenous opioid peptide exhibits higher binding affinity towards a specific type of opioid receptors. Current evidence suggests that endogenous opioid peptides play important regulatory roles in reproduction. Endogenous opioid peptides are present through the hypothalamic-pituitary-gonadal axis. The hypothalamic opioidergic mechanism represents one of the important central control systems of gonadotropin-releasing hormone and gonadotropin release. Opioids mediate the sex steroid effect exerted on gonadotropin-releasing hormone and luteinizing hormone secretion and play a crucial role in the integration of several neuroendocrine mechanisms. There is also evidence that suprahypothalamic mechanism enhances endogenous opioid inhibition of gonadotropin-releasing hormone. The genes of the endogenous opioid peptides are also expressed in peripheral reproductive tissues such as the endometrium and placenta. At least part of the endogenous opioid peptides effects may be paracrine or autocrine in nature. The possible roles of opioids in various physiological processes of the female reproductive system are also reviewed. Biomedical Reviews 1995; 4: 71-83.