The α-conotoxins are selective inhibitors of nicotinic acetylcholine receptors (nAChRs). An α-conotoxin, PnIA, isolated from Conus pennaceus has been demonstrated to be an inhibitor of native nAChRs in dissociated neurones and recombinant nAChRs, with a poor selectivity between receptor subtypes. Substitution of a leucine for alanine at position 10 renders the toxin a selective inhibitor of the α7 nAChR subtype (Hogg et al. 1999; Luo et al. 1999). It is assumed that PnIA and [A10L]PnIA inhibit the α7 nAChR by stabilising the receptor in a non-conducting desensitised or resting state. To determine which state these compounds stabilise and which amino acid residues are involved, the effects of PnIA, [A10L]PnIA and alanine scan mutants of [A10L]PnIA were investigated in homomeric chick α7 nAChRs and α7 receptors with the L247T mutation expressed in Xenopus oocytes. The α7L247T mutation causes the desensitised state of the receptor to become conducting (Bertrand et al. 1997). Use of the desensitized open α7L247T mutant allows us to probe the toxin effects on an otherwise electrophysiologically silent state of the receptor.

PnIA completely inhibited the ACh-evoked current in wild-type α7 receptors and α7L247T receptors with IC₅₀ values of 349 and 194 nM, respectively. [A10L]PnIA and the [A10L]PnIA alanine mutants inhibited ACh-evoked currents in wild-type α7 nAChRs with an IC₅₀ of 168 nM. In contrast, when co-applied with ACh, [A10L]PnIA and the alanine mutants potentiated the ACh-induced responses at α7L247T receptors and in addition [A10L]PnIA activated a current at α7L247T receptors in the absence of ACh. This illustrates that PnIA and [A10L]PnIA inhibit α7 receptors by stabilising different states of the receptor. PnIA stabilises a state which is non-conducting in both the wild-type and αL247T receptors that might correspond to the resting state, whereas [A10L]PnIA is supposed to stabilise the desensitised state which is closed in the wild-type and conducting in the αL247T receptor. These data illustrate that the presence of the longer aliphatic side chain at position 10 is sufficient to change the selectivity of the toxin for different states of the receptor.