Iptakalim inhibits nicotinic acetylcholine receptor-mediated currents in dopamine neurons acutely dissociated from rat substantia nigra pars compacta

Iptakalim hydrochloride, a novel cardiovascular ATP-sensitive K(+) (K(ATP)) channel opener, has shown remarkable antihypertensive and neuroprotective effects in a variety of studies using in vivo and in vitro preparations. We recently found that iptakalim blocked human alpha4-containing nicotinic acetylcholine receptors (nAChRs) heterologously expressed in the human SH-EP1 cell line. In the present study, we examined the effects of iptakalim on several neurotransmitter-induced current responses in single DA neurons freshly dissociated from rat substantia nigra pars compacta (SNc), using perforated patch-clamp recordings combined with a U-tube rapid drug application. In identified DA neurons under voltage-clamp configuration, glutamate-, NMDA-, and GABA-induced currents were insensitive to co-application with iptakalim (100 microM), while whole-cell currents induced by ACh (1 mM+1 microM atropine) or an alpha4beta2 nicotinic acetylcholine receptors relatively selective agonist, RJR-2403 (300 microM), were eliminated by iptakalim. Iptakalim inhibited RJR-2403-induced current in a concentration-dependent manner, and reduced maximal RJR-2403-induced currents at the highest agonist concentration, suggesting a non-competitive block. In current-clamp mode, iptakalim failed to affect resting membrane potential and spontaneous action potential firing, but abolished RJR-2403-induced neuronal firing acceleration. Together, these results indicate that in dissociated SNc DA neurons, alpha4-containing nAChRs, rather than ionotropic glutamate receptors, GABA(A) receptors or perhaps K-ATP channels are the sensitive targets to mediate iptakalim's pharmacological roles.