TY - JOUR T1 - Lactone Modulation of the γ-Aminobutyric Acid A Receptor: Evidence for a Positive Modulatory Site JF - Molecular Pharmacology JO - Mol Pharmacol SP - 114 LP - 119 DO - 10.1124/mol.52.1.114 VL - 52 IS - 1 AU - Korwyn L. Williams AU - Joseph B. Tucker AU - Geoffrey White AU - David S. Weiss AU - James A. Ferrendelli AU - Douglas F. Covey AU - James E. Krause AU - Steven M. Rothman Y1 - 1997/07/01 UR - http://molpharm.aspetjournals.org/content/52/1/114.abstract N2 - The γ-aminobutyric acid-A (GABAA) receptor complex is allosterically modulated by a variety of substances, some of clinical importance. Barbiturates and neurosteroids augment GABA-currents and also directly gate the channel. A variety of γ-butyrolactone analogues also modulate GABA-induced currents, with some potentiating and others inhibiting. Because several γ-thiobutyrolactone analogues have biphasic effects on GABA currents, experiments with wild-type and picrotoxinin-insensitive GABAA receptors were performed to analyze whether some γ-thiobutyrolactones interact with two distinguishable sites on the GABAA receptor. β-Ethyl-β-methyl-γ-thiobutyrolactone inhibited GABA-induced currents at low concentrations (0.001–1 mm), but potentiated GABA-induced currents at higher concentrations (3–10 mm) in wild-type α1β2γ2-subunit containing ionophores. The related α-ethyl-α-methyl-γ-thiobutyrolactone potentiated submaximal GABA currents in wild-type receptors at both low and high concentrations (0.1–10 mm). Mutations in the second transmembrane domain of α1, β2, or γ2 conferred picrotoxinin-insensitivity onto GABAA receptor complexes. When these mutated α1, β2, or γ2 subunits were incorporated into the receptor complex, β-ethyl-β-methyl-γ-thiobutyrolactone potentiated GABA currents over the entire concentration range (0.1–10 mm). Neither the potentiating activity nor the EC50 of α-ethyl-α-methyl-γ-thiobutyrolactone changed in the mutant receptors. Further studies demonstrated that the mutations did not affect the EC50 of chlordiazepoxide or phenobarbital. These and our earlier results identify a modulatory site on the GABAA receptor distinct from that interacting with barbiturates, benzodiazepines, and steroids. Additionally, they show that the γ-butyrolactones probably interact at two different sites on the ionophore to produce opposite effects on GABA-mediated current. ER -