[3H]linopirdine binding to rat brain membranes is not relevant for M-channel interaction

Linopirdine was developed as a cognitive enhancing molecule and demonstrated to specifically block the potassium current generated by the brain specific KCNQ2-KCNQ3 proteins (M-channel). In this study we investigated the relevance of [3H]linopirdine binding in rat brain extracts to the interaction with the M-channel proteins. Our results confirm the presence of a high affinity site for [3H]linopirdine in rat brain tissues (KD = 10 nM) but we also identified a high affinity binding site for [3H]linopirdine in rat liver tissues (KD =  9 nM). Competition experiments showed that [3H]linopirdine is displaced by unlabelled linopirdine with comparable affinities from its binding sites on rat brain and rat liver membranes. [3H]linopirdine was completely displaced by a set of cytochrome P450 (CYP450) ligands suggesting that [3H]linopirdine binding to rat brain and liver membranes is linked to CYP450 interaction. The testing of CYP450 ligands on the M-channel activity, using a Rb+ efflux assay on cells expressing the KCNQ2-KCNQ3 proteins, demonstrated that [3H]linopirdine binding results cannot be correlated to M-channel inhibition. The results obtained in this study demonstrate that [3H]linopirdine binding to rat brain and rat liver membranes is representative for CYP450 interaction and not relevant for the binding to the M-channel proteins.