Nicotinic Acetylcholine Receptors Containing the a7-Like Subunit Mediate Contractions of Muscles Responsible for Space Positioning of the Snail, Helix pomatia L. Tentacle

Three recently discovered tentacle muscles are crucial to perform patterned movements of upper tentacles of the terrestrial snail, Helix pomatia. The muscles receive central and peripheral excitatory cholinergic innervation lacking inhibitory innervation. Here, we investigate the pharmacology of acetylcholine (ACh) responses in muscles to determine the properties of the ACh receptor (AChR), the functional availability of which was assessed using isotonic contraction measurement. Using broad spectrum of nicotinic and muscarinic ligands, we provide the evidence that contractions in the muscles are attributable to the activation of nAChRs that contain the a7-like subunit. Contractions could be evoked by nicotine, carbachol, succinylchloride, TMA, the selective a7-nAChR agonist choline chloride, 3-Bromocytisine and PNU-282987, and blocked by nAChR selective antagonists such as mytolon, hexamethonium, succinylchloride, d-tubocurarine, hemicholinium, DMDA (decamethonium), methyllycaconitine, a-Bungarotoxin (aBgTx) and a-Conotoxin IMI. The specific muscarinic agonist oxotremorine and arecoline failed to elicit contractions. Based on these pharmacological properties we conclude that the Na+ and Ca2+ permeable AChRs of the flexor muscle are nicotinic receptors that contain the a7-like subunit. Immunodetection experiments confirmed the presence of a7- or a7-like AChRs in muscle cells, and a4-AChRs in nerves innervating the muscle. These results support the conclusion that the slowly desensitizing aBgTx-sensitive responses obtained from flexor muscles are produced by activation of a7- like AChRs. This is the first demonstration of postsynapti