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A Nicotinic Acetylcholine Receptor Is Involved in the Acrosome Reaction of Human Sperm Initiated by Recombinant Human ZP3¹

^a Department of Cell Biology and Human Anatomy, School of Medicine, University of California,Davis, California 95616-8643

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), -bungarotoxin (-BTX, 100 nM), -conotoxin IMI (-CTX IMI, 250 nM and 25 nM), and methyllycaconitine (MLA, 100 nM and 10 nM), significantly blocked AR initiation by ACh. -BTX is an anatagonist of several nAChRs, including the 7 nAChR, and -CTX IMI and MLA are highly specific antagonists of 7 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 -BTX (from 10 µM to 100 nM), -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 7 subunit-containing nAChR in the AR initiated by both ligands. AR initiation by progesterone was not inhibited by MLA or -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 7 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.

First decision: 4 March 2002.

¹ This work was supported by NIH grants HD-33368 and HD-23098 to S.M. C.B. is a Lalor Foundation Fellow.

² Correspondence: Stanley Meizel, Department of Cell Biology and Human Anatomy, School of Medicine, University of California, One Shields Ave., Davis, CA 95616-8643. FAX: 530 752 8520;smeizel{at}ucdavis.edu

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