The sigma agonist 1,3-di-o-tolyl-guanidine directly blocks SK channels in dopaminergic neurons and in cell lines

Small conductance Ca2+-activated K+ (SK) channels are widely expressed in the brain and underlie medium-duration afterhyperpolarizations (mAHPs) in many types of neurons. It was recently reported that the activation of sigma-1 (σ1) receptors inhibits SK currents in rat hippocampus. Because many interactions between σ receptors and brain dopaminergic systems have been reported, we set out to examine putative effects of σ receptor ligands on the SK mediated mAHP in midbrain dopaminergic neurons. We found that 1,3-di-o-tolyl-guanidine (DTG) inhibited the mAHP in a concentration-dependent manner (∼ 60% inhibition at 100 µM), while other σ receptor agonists (carbetapentane, (+)-SKF10047 and PRE-084) had little effect. Moreover, the effect of DTG was not affected by high concentrations of the σ1 receptor antagonist BD 1047. A role for σ2 receptors could also be excluded by the lack of effect of the σ2 receptor ligand 5-bromo-tetrahydroisoquinolinylbenzamide. These results argue against a coupling of σ receptors to SK channels in dopaminergic neurons. We next hypothesized that DTG could directly block the channel. This hypothesis was tested in HEK-293 cells which were transiently transfected with rSK2 or hSK3 subunits. DTG inhibited the current flowing through both subtypes with mean IC50s ∼ 200 µM. This action was also unaffected by BD 1047. Other σ receptor ligands had little or no effect. We conclude that DTG directly blocks SK channels. This pharmacological action may be important to consider in future experimental settings.