The mammalian sperm acrosome reaction (AR) is essential to fertilization and is believed to be initiated in vivo by ZP3, a glycoprotein component of the egg zona pellucida (ZP). Recently, we reported the results of antagonist studies suggesting that a nicotinic acetylcholine receptor (nAChR) containing an α7 subunit (α7nAChR) plays a role in the human sperm AR initiated by recombinant human ZP3 or by acetylcholine (ACh). Here, we show that ACh can initiate the mouse sperm AR and that antagonists of the nAChR inhibit the AR initiated by ACh or by ZP obtained from ovarian oocytes (isolated heat-solubilized mouse ZP). Preincubation with three antagonists of the nAChR, α-bungarotoxin (100 nM), α-conotoxin IMI (100 nM), and methyllycaconitine (100 nM), significantly blocked AR initiation by ACh or by isolated heat-solubilized mouse ZP (P ≤ 0.002). Because the only nAChR subunit known to bind all three antagonists is the α7, an α7nAChR appears to be involved in the mouse sperm AR initiated by mouse ZP or by ACh. The nAChR antagonists did not inhibit the AR initiated by calcium ionophore A23187, suggesting that the role of α7nAChR is upstream from Ca2 influx. Pertussis toxin (PTX, 100 ng/ml) did not inhibit the AR initiated by ACh, suggesting that the α7nAChR might be a candidate for the PTX-insensitive, poorly selective cation channel shown previously to play a role in ZP-initiated mouse sperm AR. These studies with mouse sperm and ovary-derived ZP strongly support our previous conclusion that activation of an α7nAChR is important to the mammalian AR initiated by the egg ZP.
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