Background

Midazolam amplifies synaptic inhibition via different γ-aminobutyric acid type A (GABAA) 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 GABAA receptor 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 GABAA receptor–mediated miniature inhibitory postsynaptic currents was investigated in organotypic slice cultures of the mouse neocortex. In both experiments, wild-type mice and GABAA receptor knock-in mouse lines were compared in which α1-, α5-, α1/2/3-, α1/3/5- and α2/3/5-GABAA receptor 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-, α1/2/3-, α1/3/5-, and α2/3/5–knock-in mice revealed a dominant role for the α1-GABAA receptor 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 α2/3/5–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 α2/3/5- and α1-subunits.

Conclusions

These results demonstrate a predominant role of α1-GABAA receptors in the actions of midazolam at low nanomolar concentrations. At higher concentrations, midazolam also enhances other GABAA receptor subtypes. α1-GABAA receptors may already contribute at sedative doses to the phenomenon of postoperative amnesia that has been reported after midazolam administration.

Editor’s Perspective
What We Already Know about This Topic
  • Administration of the benzodiazepine midazolam induces anterograde amnesia via γ-aminobutyric acid type A (GABAA) receptor–dependent mechanisms

  • Midazolam blocks hippocampal long-term potentiation, a cellular correlate for learning and memory

  • The specific GABAA receptor subunits mediating the amnestic actions of midazolam are incompletely understood

What This Article Tells Us That Is New
  • Using a combination of γ-aminobutyric acid type A (GABAA) α-receptor subunit knock-in mice revealed that low concentrations (10 nM) of midazolam blocked long-term potentiation in the hippocampal slice preparation predominantly via α1-GABAA receptors

  • Electrophysiologic recordings in neocortical slice cultures imply a dominant role for the α1 subtype in governing inhibitory postsynaptic current kinetics at nanomolar concentrations of midazolam

  • These observations suggest that, at low concentrations, midazolam enhances synaptic transmission of GABAA receptors via targeting α1 subtypes and provides mechanistic explanation for the drug’s sedative and amnestic action

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