Gonadotropin-Releasing Hormone Increases Ability of the Spermatozoa to Bind to the Human Zona Pellucida

Sperm-zona pellucida binding, a crucial step in the process of fertilization, takes place in vivo in the upper portion of the fallopian tube. The presence of GnRH-like peptides in the female and the male genital tract has been described. In this work, the effect of GnRH and related peptides upon sperm-zona pellucida binding ability was studied. Sperm aliquots, capacitated for 4.5 h, were incubated for 5 min with saline (control) or 20 nM of GnRH, C-terminal (1-5) or N-terminal (5-10) fragments of GnRH, Substance P, dynorphin, bombesin, or mixed GnRH (a synthetic peptide with the same amino acids as GnRH but in different order). Sperm were also incubated with the GnRH antagonist Ac-3,4-dehydro-Pro1, -p-fluoro-D-Phe2, D-Trp3,6-LHRH, alone or before adding GnRH. The sperm were then tested using the hemizona assay. After 10 min, the number of zona-bound sperm was determined. In addition, the effect of GnRH upon the acrosome reactions, sperm movement characteristics, and sperm-zona collisions was evaluated. Sperm treated with GnRH bound in higher numbers to the zona than did control sperm (p < 0.005). The GnRH fragments, the GnRH antagonist, and related peptides did not have any effect on sperm-zona interaction; however, the GnRH antagonist totally blocked the stimulatory effect of GnRH. GnRH did not affect the percentage of acrosome-reacted sperm, pattern of sperm movement, or frequency of sperm-zona collisions. I suggest that the increased ability of the sperm to bind to the zona may be due to exposure and/or change of affinity of zona receptors on the sperm plasma membrane.