The muscle-type nicotinic receptor has two pharmacologically distinguishable acetylcholine binding sites at the alpha-gamma and alpha-delta subunit interfaces; alpha-conotoxins can bind them selectively. As reported, alpha-conotoxin MI has greater affinity for the site near the alpha-delta interface of the BC3HI cell receptor but. in the case of the Torpedo californica receptor, displays greater affinity for that near the interface. To further investigate ligand selectivity, we study the conotoxin MI-Torpedo marmorata receptor interaction. In this work, we show the binding of alpha-conotoxin MI to the T marmorata receptor and the influence of the antagonist alpha-Bungarotoxin and the agonist carbamylcholine on such binding; in addition, and contrasting, with the results for the Torpedo californica receptor. we identify the alpha-delta subunit interface as the high affinity binding site. This is the first work describing different characteristics of the interaction between alpha-conotoxin MI and receptors from different species of the same genus. (C) 2002 Elsevier Science (USA). All rights reserved.