Figure 1

Control of male reproductive function by the opioid system at multiple levels. (1) At the level of the CNS, EOPs inhibit the secretion of GnRH, thereby suppressing the release of LH from the pituitary. (2) At the testes level, EOPs are synthesized mainly in Leydig cells after LH stimulation, and they exert an inhibitory effect on Sertoli cells. In particular, EOPs can regulate the levels of testosterone (T) indirectly, inhibiting the production of ABP that is stimulated by FSH in Sertoli cells. (3) Genes encoding opioid peptide precursors are differentially expressed in germ cells, and somatic cells of the testes and their transcripts are not efficiently translated in spermatogenic germ cells. (4) In spegrmatozoa, the opioid system regulates sperm motility in a distinct manner by the activation of distinct receptors.