CONTEXT: Endogenous opioid peptides signal through , and µ-opioid receptors. Some of these peptides such as endorphins and enkephalins are present in the male reproductive tract, but the presence of the corresponding receptors in human sperm cells has not yet been reported.

OBJECTIVE: To study the expression and localization of , and µ-opioid receptors on human spermatozoa and the implication in sperm motility.

METHODS: The expression of receptors was studied by RT-PCR, Western blot and immunofluorescence techniques. We evaluated the effects of activation of each opioid receptor by specific agonist and antagonist.

RESULTS: Human spermatozoa express , and µ-opioid receptors. These receptors were located in different parts of the head, in the middle region and in the tail of the sperm. Progressive motility of spermatozoa, an important parameter to evaluate male fertility, was found to be significantly reduced after incubation with the µ-receptor agonist morphine, whereas this effect was antagonized in the presence of the corresponding antagonist, naloxone. The receptor antagonist naltrindole significantly reduced progressive motility immediately after its addition. However, the receptor agonist DPDPE had no significant effect. Finally, neither the receptor agonist U50488 nor its antagonist nor-binaltorphimine significantly affected the progressive motility of human spermatozoa.

CONCLUSION: We report for first time the presence of functional -, - and µ-opioid receptors in human sperm membranes. These findings are indicative of a role for the opioid system in the regulation of sperm physiology.