The antipsychotic drug loxapine is an opener of the sodium- activated potassium channel slack (Slo2.2)

Sodium-activated potassium (K _Na) channels have been suggested to set the resting potential, to modulate slow after-hyperpolarizations, and to control bursting behavior or spike frequency adaptation (Trends Neurosci 28:422-428, 2005). One of the genes that encodes K _Na channels is called Slack (Kcnt1, Slo2.2). Studies found that Slack channels were highly expressed in nociceptive dorsal root ganglion neurons and modulated their firing frequency (J Neurosci 30:14165-14172, 2010). Therefore, Slack channel openers are of significant interest as putative analgesic drugs. We screened the library of pharmacologically active compounds with recombinant human Slack channels expressed in Chinese hamster ovary cells, by using rubidium efflux measurements with atomic absorption spectrometry. Riluzole at 500 μM was used as a reference agonist. The antipsychotic drug loxapine and the anthelmintic drug niclosamide were both found to activate Slack channels, which was confirmed by using manual patch-clamp analyses (EC ₅₀ = 4.4 μM and EC ₅₀ = 2.9 μM, respectively). Psychotropic drugs structurally related to loxapine were also evaluated in patch-clamp experiments, but none was found to be as active as loxapine. Loxapine properties were confirmed at the singlechannel level with recombinant rat Slack channels. In dorsal root ganglion neurons, loxapine was found to behave as an opener of native K _Na channels and to increase the rheobase of action potential. This study identifies new K _Na channel pharmacological tools, which will be useful for further Slack channel investigations.