Abstract
BACKGROUND: Modulation of γ-aminobutyric acid type A receptors (GABAARs) by general anesthetics may contribute to their ability to produce amnesia. Receptors containing α5 subunits, which mediate tonic and slow synaptic inhibition, are co-localized with β3 and γ2 subunits in dendritic layers of the hippocampus and are sensitive to low (amnestic) concentrations of anesthetics. Because α5 and β3 subunits influence performance in hippocampus-dependent learning tasks in the presence and absence of general anesthetics, and the experimental inhaled drug 1,2-dichlorohexafluorocyclobutane (F6) impairs hippocampus-dependent learning, we hypothesized that F6 would modulate receptors that incorporate α5 and β3 subunits. We hypothesized further that the β3(N265M) mutation, which controls receptor modulation by general anesthetics, would similarly influence modulation by F6. METHODS: Using whole-cell electrophysiologic recording techniques, we tested the effects of F6 at concentrations ranging from 4 to 16 μM on receptors expressed in human embryonic kidney 293 cells. We measured drug modulation of wild-type α5β3 and α5β3γ2L GABAARs and receptors harboring the β3(N265M) mutation. We also tested the effects of F6 on α1β2γ2L receptors, which were reported previously to be insensitive to this drug when expressed in Xenopus oocytes. RESULTS: F6 enhanced the responses of wild-type α5β3γ2L but not α1β2γ2L receptors to low concentrations of GABA in a concentration-dependent manner. Receptors that incorporated the mutant β3(N265M) subunit were insensitive to F6. When applied together with a high concentration of GABA, F6 blocked currents through α5β3 but not α5β3γ2L receptors. F6 did not alter deactivation of α5β3γ2L receptors after brief high- concentration pulses of GABA. CONCLUSIONS: The nonimmobilizer F6 modulates GABAARs in a manner that depends on subunit composition and mode of receptor activation by GABA, supporting a possible role for α5-containing receptors in suppression of learning and memory by F6. Furthermore, common structural requirements indicate that similar molecular mechanisms may be responsible for the enhancing effects of F6 and conventional general anesthetics.