Modulation of recombinant small-conductance Ca(2+)-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-(triflu oromethyl)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 Ca(2+) 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 Ca(2+)-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 Ca(2+)-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 Ca(2+). NS 1619 did not activate rSK2 channel currents; it inhibited rSK2 channel currents activated by the other three test compounds or by high intracellular Ca(2+). 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.

Block of rat brain recombinant SK channels by tricyclic antidepressants and related compounds.

SK channels are small conductance, Ca(2+)-activated K(+) channels that underlie neuronal slow afterhyperpolarization and mediate spike frequency adaptation. Using the patch clamp technique, we tested the effects of eight clinically relevant psychoactive compounds structurally related to the tricyclic antidepressants, on SK2 subtype channels cloned from rat brain and functionally expressed in the human embryonic kidney cell line, HEK293. Amitriptyline, carbamazepine, chlorpromazine, cyproheptadine, imipramine, tacrine and trifluperazine blocked SK2 channel currents with micromolar affinity. The block was reversible and concentration-dependent. The potency differed according to chemical structure. In contrast, the cognitive enhancer linopirdine was ineffective at blocking these channels. Our results point to a distinct pharmacological profile for SK channels.

Patch-clamp analysis of anesthetic interactions with recombinant SK2 subtype neuronal calcium-activated potassium channels.

UNLABELLED: Small conductance calcium-activated potassium channels (SK) mediate spike frequency adaptation and underlie the slow afterhyperpolarization in central neurons. We tested the actions of several anesthetics on the SK2 subtype of recombinant SK channels, cloned from rat brain and functionally expressed in a mammalian cell line. Butanol, ethanol, ketamine, lidocaine, and methohexital blocked recombinant SK2 channel currents, measured in the whole-cell patch clamp recording mode. The block was reversible, dose-dependent, and of variable efficacy. The inhaled anesthetics chloroform, desflurane, enflurane, halothane, isoflurane, and sevoflurane produced little or no block when applied at 1 minimum alveolar anesthetic concentration; varying degrees of modulation were observed at very large concentrations (10 minimum alveolar concentration). The extent of block by inhaled anesthetics did not appear to depend on concentration or membrane voltage. IMPLICATIONS: We describe differential effects of anesthetics on cloned small conductance calcium-activated potassium channels from brain that may play a role in generating the effects or side effects of anesthetics.