Aims: 

Endogenous opioid peptides participate in the regulation of reproductive physiology at multiple sites. Enkephalins are opioid peptides present in human semen which are involved in the regulation of sperm physiology. The action of enkephalins is mediated by d- and/or m- receptors and the activation of each one of these receptors has an opposite effect on sperm motility. Therefore, the aim of this study was to study the effect of met-enkephalin on sperm motility.

Methods: 

Dose (1nM-100mM) and time (0, 0.5, 1 and 3h) dependent motility studies were measured after incubation of semen samples with met-enkephaline The motility analysis was conducted by computer-assisted sperm analysis (SCA).

Results: 

Met-enkephalin increased immediately the percentage of A motility grade spermatozoa (rapidly progressive) in a dose-dependent manner and this was followed by a dose-dependent decrease at 0.5 hours of incubation. However, after 3 hours of incubation, the percentage of grade A sperm increased again in a dose-independent manner whereas causing a parallel decrease in the percentage of grade B sperm (slowly or sluggishly progressive).

Conclusion(s): Met-enkephalin has a biphasic effect on sperm motility. This effect can modulate sperm cell motility through the female reproductive tract, mainly by the activation of different opioid receptors.

This work has been supported by a grant from the Spanish Ministry of Education and Science (BFU2006-07779) and IVI Foundaction S.L. (Contrato Instituto Sevillano de

Infertilidad-CSIC cod.20071101) N. Subirán hold a fellowship from the University of the Basque Country.