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H. Qian, X. Song, A. Xie, H. Ripps; Cyclothiazide Is a Subunit-Specific Inhibitor on GABAc Receptors. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5797. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Cyclothiazide (CTZ) is commonly used to block desensitization of AMPA-type glutamate receptors, but its action on inhibitory neurotransmitter receptors has not been fully explored. This study examined the mechanism of the inhibitory action of cyclothiazide on GABAC receptors formed by homomeric ρ subunits.
To study the effects of cyclothiazide on homomeric GABA ρ receptors, GABA-elicited currents were recorded with two-electrode voltage clamp from oocytes and with whole-cell patch clamp from HEK cells. A rapid perfusion system was used to deliver GABA and CTZ to the preparation.
We found that inhibition by CTZ of homomeric GABA ρ receptors was subunit specific. CTZ had no effect on homomeric ρ1 receptors, but reduced the GABA-elicited current on homomeric ρ2 receptors with an IC50 of about 12 µM. The differential sensitivity of CTZ on the two GABAC receptor subtypes is mediated by a single amino acid located on the second transmembrane domain. Mutating the residue at this position from serine to proline on the ρ2 subunit eliminated CTZ sensitivity, whereas switching proline to serine on the ρ1 subunit made the receptor CTZ sensitive. CTZ inhibition on ρ2 receptors was due mainly to a non-competitive mechanism that reduced the maximum response elicited by GABA, and was independent of membrane voltage. In addition, there was a prominent re-bound of membrane current when co-application of GABA and CTZ was terminated. Pre-incubating the cells with CTZ slowed the kinetics of GABA activation, indicating that CTZ could also bind to the GABAC receptor in the absence of any agonist. However, the time constant of CTZ binding to the receptor in the absence of GABA (4.7 seconds) was more than 10 times slower than its binding to the receptor in the presence of GABA (0.4 seconds).
CTZ selectively inhibits the GABAC receptor formed by GABA ρ2 subunits. The properties of CTZ inhibition are consistent with its action as a channel blocker on ρ2 GABAC receptors.
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