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

A novel role of the opioid system in the regulation of sperm function is demonstrated that shows that both exogenous and endogenous met-enkephalin can modulate sperm motility.

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Nerea Subirán, Ph.D., Luz Candenas, Ph.D., Francisco M. Pinto, Ph.D., Ekaitz Agirregoitia, Ph.D., Antonio Cejudo-Román, J.D., Jon Irazusta, Ph.D.

Volume 98, Issue 3, Pages 617-625.e3, September 2012



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


We carried out expression assays for met-enkephalin and its protein precursor PENK by RTPCR, 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.


Assisted reproduction unit and academic research laboratory.


Semen from 50 normozoospermic healthy human donors.


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

Main Outcome Measure(s):

Immunoblotting blots, indirect immunofluorescence antibody assays, RTPCR blots, flow cytometry plots, and percentage of motile sperm.


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.


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 fertility and could be useful as an emerging target for male contraception.

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Fertility and Sterility

Editorial Office, American Society for Reproductive Medicine

Fertility and Sterility® is an international journal for obstetricians, gynecologists, reproductive endocrinologists, urologists, basic scientists and others who treat and investigate problems of infertility and human reproductive disorders.