Benzodiazepine modulation of partial agonist efficacy and spontaneously active GABA_A receptors supports an allosteric model of modulation

摘要

1. Benzodiazepines (BZDs) have been used extensively for more than 40 years because of their high therapeutic index and low toxicity. Although BZDs are understood to act primarily as allosteric modulators of GABA_A receptors, the mechanism of modulation is not well understood. 2. The applicability of an allosteric model with two binding sites for γ-aminobutyric acid (GABA) and one for a BZD-like modulator was investigated. 3. This model predicts that BZDs should enhance the efficacy of partial agonists. 4. Consistent with this prediction, diazepam increased the efficacy of the GABA_A receptor partial agonist kojic amine in chick spinal cord neurons. 5. To further test the validity of the model, the effects of diazepam, flurazepam, and zolpidem were examined using wild-type and spontaneously active mutant α1(L263S)β3γ2 GABA_A receptors expressed in HEK-293 cells. 6. In agreement with the predictions of the allosteric model, all three modulators acted as direct agonists for the spontaneously active receptors. 7. The results indicate that BZD-like modulators enhance the amplitude of the GABA response by stabilizing the open channel active state relative to the inactive state by less than 1 kcal, which is similar to the energy of stabilization conferred by a single hydrogen bond.