Modulation of recombinant small-conductance Ca2+-activated K+ channels by the muscle relaxant chlorzoxazone and structurally related compounds

Using the patch clamp technique we investigated the effects of the centrally acting muscle relaxant chlorzoxazone and three structurally related compounds, 1-ethyl-2-benzimidazolinone (1-EBIO), zoxazolamine, and 1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one (NS 1619) on recombinant rat brain SK2 channels (rSK2 channels) expressed in HEK293 mammalian cells. SK channels are small conductance K+ channels normally activated by a rise in intracellular Ca2+ concentration; they modulate the electrical excitability in neurons and neuroendocrine cells. When applied externally, chlorzoxazone, 1-EBIO, and zoxazolamine activated rSK2 channel currents in cells dialyzed with a nominally Ca2+ free intracellular solution. The activation was reversible, reproducible, and depended on the chemical structure and concentration. The order of potency was 1-EBIO. chlorzoxazone. zoxazolamine. Activation of rSK2 channels by chlorzoxazone, 1-EBIO, and zoxazolamine declined at higher drug concentrations. Zoxazolamine, when applied in combination with chlorzoxazone or 1-EBIO, partially inhibited the rSK2 channel current responses, suggesting a partial-agonist mode of action. 1-EBIO failed to activate rSK2 channel currents when applied to excised inside-out membrane patches exposed to a Ca2+-free intracellular solution. In contrast, 1-EBIO activated rSK2 currents in a concentration-dependent manner when coapplied to the patches with a solution containing 20 nM free Ca2+. NS 1619 did not activate rSK2 channel currents; it inhibited rSK2 channel currents activated by the other three test compounds or by high intracellular Ca2+. We conclude that chlorzoxazone and its derivatives act through a common mechanism to modulate rSK2 channels, and SK channel modulation in the brain may partly underlie the clinical effects of chlorzoxazone.