Direct block of SK2 and SK3 current by the sigma agonist 1,3-di-(2-tolyl)guanidine

Sigma receptors are widely distributed in the central nervous system where they modulate neurotransmitter release, receptor function, ionic channel activity and calcium homeostasis. Two subtypes of sigma receptors have been identified (sigma-1 and sigma-2) with different pharmacological profiles, anatomical distribution and physiological functions. 1,3-Di-(2-tolyl)guanidine (DTG) is a sigma-1 and sigma-2 agonist which is widely used to probe the function of these receptors. It has recently been shown that sigma-1 receptor activation reduces the opening of SK channels in the hippocampus. We have observed that DTG (100 µM) reduces the apamin-sensitive afterhyperpolarization (AHP) of dopaminergic neurons within a slice preparation by ~60%, an effect not observed with other sigma agonists. In addition, neither the selective sigma-1 antagonist BD 1047 (30 µM) nor haloperidol (1 µM) blocked the effect of DTG, which suggested that the inhibition of the AHP might result from a direct block of the underlying SK channels.
Whole-cell recordings were made from HEK293 cells transiently transfected with rSK2 or hSK3 cDNA in symmetrical K+ conditions with currents activated by a [Cai] of 1 µM. Expressed SK2 and SK3 channels displayed a classical pharmacology, being blocked by apamin with mean IC50’s of 100 pM and 4 nM, respectively. In contrast, both channel subtypes were blocked with equal sensitivity by N-methyl-laudanosine (NML). DTG inhibited both SK2 and SK3 currents with the same potency (IC50’s were ~30 µM). A mutation that rendered both SK2 and SK3 insensitive to apamin and NML produced a current that was still sensitive to DTG. This direct block of SK channels may be important to consider in relation to the pharmacological effects of this compound.