Midazolam at Low Nanomolar Concentrations Affects Long-term Potentiation and Synaptic Transmission Predominantly via the α₁-γ-Aminobutyric Acid Type A Receptor Subunit in Mice

Background: Midazolam amplifies synaptic inhibition via different γ-aminobutyric acid type A (GABA_A) receptor subtypes defined by the presence of α₁-, α₂-, α₃-, or α₅-subunits in the channel complex. Midazolam blocks long-term potentiation and produces postoperative amnesia. The aims of this study were to identify the GABA_Areceptor subtypes targeted by midazolam responsible for affecting CA1 long-term potentiation and synaptic inhibition in neocortical neurons. Methods: The effects of midazolam on hippocampal CA1 long-term potentiation were studied in acutely prepared brain slices of male and female mice. Positive allosteric modulation on GABA_Areceptor-mediated miniature inhibitory postsynaptic currents was investigated in organotypic slice cultures of the mouse neocortex. In both experiments, wild-type mice and GABA_Areceptor knock-in mouse lines were compared in which α₁-, α₅-, α_1/2/3-, α_1/3/5- and α_2/3/5-GABA_Areceptor subtypes had been rendered benzodiazepine-insensitive. Results: Midazolam (10 nM) completely blocked long-term potentiation (mean ± SD, midazolam, 98 ± 11%, n = 14/8 slices/mice vs. control 156 ± 19%, n = 20/12; P < 0.001). Experiments in slices of α₁-, α₅-, α_1/2/3-, α_1/3/5-, and α_2/3/5-knock-in mice revealed a dominant role for the α₁-GABA_Areceptor subtype in the long-term potentiation suppressing effect. In slices from wild-type mice, midazolam increased (mean ± SD) charge transfer of miniature synaptic events concentration-dependently (50 nM: 172 ± 71% [n = 10/6] vs. 500 nM: 236 ± 54% [n = 6/6]; P = 0.041). In α_2/3/5-knock-in mice, charge transfer of miniature synaptic events did not further enhance when applying 500 nM midazolam (50 nM: 171 ± 62% [n = 8/6] vs. 500 nM: 175 ± 62% [n = 6/6]; P = 0.454), indicating two different binding affinities for midazolam to α_2/3/5- and α₁-subunits. Conclusions: These results demonstrate a predominant role of α₁-GABA_Areceptors in the actions of midazolam at low nanomolar concentrations. At higher concentrations, midazolam also enhances other GABA_Areceptor subtypes. α₁-GABA_Areceptors may already contribute at sedative doses to the phenomenon of postoperative amnesia that has been reported after midazolam administration.