Autocrine regulation of human sperm motility by the met-enkephalin opioid peptide

Abstract

Objective: To verify the presence of protein precursor pro-enkephalin (PENK) and met-enkephalin in human spermatozoa and to characterize the effects of exogenous and endogenous enkephalins on sperm motility.

Design: We carried out expression assays for met-enkephalin and its protein precursor PENK by reverse transcription-polymerase chain reaction (RT-PCR), Western blot, and immunofluorescence techniques in sperm cells and motility analysis after incubation of semen samples with met-enkephlin enzyme inhibitors and the opioid receptor antagonist naloxone. Met-enkephalin secretion was analyzed by flow cytometry.

Setting: Assisted reproduction unit and academic research laboratory.

Patient(s): Semen from 50 normozoospermic healthy human donors.

Intervention(s): Spermatozoa isolated from semen on discontinuous Percoll gradient (40%-80%) followed by a swim-up was used for all techniques.

Main outcome measure(s): Immunoblotting blots, indirect immunofluorescence antibody assays, RT-PCR blots, flow cytometry plots, and percentage of motile sperm.

Result(s): We found by RT-PCR and immunofluorescence that met-enkephalin and its protein precursor PENK are present in the head of human sperm cells. Endogenous met-enkephalin increased sperm motility, whereas the addition of exogenous met-enkephalin had a biphasic effect on motility, likely due to the activation of distinct receptor subtypes.

Conclusion(s): We provide evidence for a new role of met-enkephalin as an endogenous mediator of sperm motility. This autocrine regulation of sperm function by the opioid system represents a new mechanism of regulation of male factor fertility and could be useful as an emerging target for male contraception.