Selective modulation of different GABAA receptor isoforms by diazepam and etomidate in hippocampal neurons

Diazepam modulation of native γ2-containing GABAA (γGABAA) receptors increases channel conductance by facilitating protein interactions involving the γ2-subunit amphipathic (MA) region, which is found in the cytoplasmic loop between transmembrane domains 3 and 4 (Everitt et al., 2009). However, many drugs, predicted to act on different GABAA receptor subtypes, increase channel conductance leading us to hypothesize that conductance variation in GABAA receptors may be a general property, mediated by protein interactions involving the cytoplasmic MA stretch of amino acids. In this study we have tested this hypothesis by potentiating extrasynaptic GABAA currents with etomidate and examining the ability of peptides mimicking either the γ2- or δ-subunit MA region to affect conductance. In inside-out hippocampal patches from newborn rats the general anesthetic etomidate potentiated GABA currents, producing either an increase in open probability and single-channel conductance or an increase in open probability, as described previously (Seymour et al., 2009). In patches displaying high conductance channels application of a δ(392–422) MA peptide, but not a scrambled version or the equivalent γ2(381–403) MA peptide, reduced the potentiating effects of etomidate, significantly reducing single-channel conductance. In contrast, when GABA currents were potentiated by the γ2-specific drug diazepam the δ MA peptide had no effect. These data reveal that diazepam and etomidate potentiate different extrasynaptic GABAA receptor subtypes but both drugs modulate conductance similarly. One interpretation of the data is that these drugs elicit potentiation through protein interactions and that the MA peptides compete with these interactions to disrupt this process.

► Ion conductance, through neuronal GABAA channels is increased by the general anesthetic etomidate.
► Increased conductance is attenuated by a peptide mimicking the cytoplasmic amphipathic region of the δ- but not the γ-subunit.
► The specificity of these peptides suggests that they act by competing with native protein interactions.
► Such interactions mediate conductance variation in GABAA channels.
► Drugs may elicit potentiation through protein interactions in GABAA receptors.