Localisation of two classes of acetylcholine receptor-like molecules in sperms of different animal species.

The distribution of different classes of acetylcholine (ACh) receptor-like molecules in sperms of different invertebrate and vertebrate species is described. ACh receptor molecules belong to one of two classes: muscarinic receptors (mAChRs), associated with signal transduction mechanisms in the inner domain of the cell, and nicotinic receptors (nAChRs), capable of opening Na+ channels when activated by the ligand. Molecules immunologically related to mAChRs and to ACh can be identified by specific antibodies, and revealed by immunofluorescent or immunogold staining; the nicotinic receptor-like molecules are localised as curare-sensitive affinity sites for alpha-bungarotoxin. In all species studied, both classes of receptors were found, with a similar distribution. Muscarinic-like molecules were found mainly in the sperm head regions of most species; such a localisation may be correlated to a function in sperm-egg interaction, for instance in the regulation of the block to polyspermy. Nicotinic-like molecules are present mainly in the tail and in the post-acrosomal region of most animals, thus confirming their function in the regulation of sperm propulsion, but are also present at the acrosomal region of most species. The distribution patterns of the different classes of molecules indicate that both may be involved in sperm-egg interactions, in addition to their known function in the regulation of sperm propulsion.