Studies of porcine and human sperm suggesting a role for a sperm glycine receptor/Cl- channel in the zona pellucida-initiated acrosome reaction

Abstract

It is known that Cl is required for the zona pellucida (zona)- and progesterone-initiated mammalian sperm acrosome reaction (AR). Recent evidence indicates the involvement of a unique progesterone receptor resembling a GABA(A) receptor/Cl channel in the latter event. In the study reported here, we investigated whether the in vitro porcine sperm AR initiated by heat-solubilized porcine zonae involves a different receptor/Cl channel than does the progesterone-initiated AR. Capacitated porcine sperm failed to undergo the zona- and progesterone initiated AR when suspended in a medium deficient in Cl. We then employed 1) strychnine, an antagonist of neuronal glycine receptor/Cl channels at nanomolar concentrations and of GABA(A) receptor/Cl channels at micromolar concentrations and 2) (+)- bicuculline, an antagonist of neuronal GABA(A) receptor/Cl channels that inhibits glycine receptor/Cl channels only at higher concentrations. We report here that the zona-initiated AR was inhibited by 50 nM strychnine, but the progesterone-initiated AR required 1 μM for inhibition. Moreover, 1 μM (+)-bicuculline inhibited the progesterone-initiated AR, but the zona initiated AR required 50 μM for inhibition. In addition, glycine initiated the porcine sperm AR, and that event was inhibited by strychnine and (+)- bicuculline with the same order of potency observed for the zona-initiated AR. Finally, glycine initiated the AR in human sperm, and that initiation was inhibited by 50 nM strychnine. This work provides the first evidence suggesting that a glycine receptor/Cl- channel is involved in the zona- initiated mammalian sperm AR.