One of the essential steps in mammalian fertilization is the acrosome reaction (AR), a modified exocytotic event in the sperm head that occurs upon contact with the glycoprotein matrix of the zona pellucida (ZP) surrounding the oocyte. Acetylcholine (ACh) at concentrations of 10–250 μM and nicotine at 10–250 nM significantly initiate the AR of capacitated human sperm. Preincubation with three antagonists of the nicotinic acetylcholine receptor (nAChR), α-bungarotoxin (α-BTX, 100 nM), α-conotoxin IMI (α-CTX IMI, 250 nM and 25 nM), and methyllycaconitine (MLA, 100 nM and 10 nM), significantly blocked AR initiation by ACh. α-BTX is an anatagonist of several nAChRs, including the α7 nAChR, and α-CTX IMI and MLA are highly specific antagonists of α7 subunit-containing AChRs. The sperm nAChR plays a role in the AR initiated in vitro by a purified recombinant human ZP protein (rhZP3). Previously, rhZP3 was able to stimulate the AR by mechanisms similar to those seen with native ZP. Preincubation of human sperm with α-BTX (from 10 μM to 100 nM), α-CTX IMI (250 and 100 nM), or MLA (100 nM and 10 nM) caused a significant inhibition in the rhZP3-initated AR. The inhibition of the ACh-initiated and rhZP3-initiated AR by these nAChR antagonists strongly suggests the involvement of an α7 subunit-containing nAChR in the AR initiated by both ligands. AR initiation by progesterone was not inhibited by MLA or α-BTX, suggesting that this particularnAChR is not involved in the AR initiated by that ligand. In vitro results show for the first time that ACh can initiate the human sperm AR and strongly suggest that a human sperm α7 subunit-containing nAChR plays a role in the rhZP3-initiated AR. This nAChR ligand-gated ion channel may be important to the signal transduction events of ZP-initiated AR in vivo.
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