Benzodiazepines
are clinically relevant drugs that bind to GABAA neurotransmitter
receptors at the α+/γ2–
interfaces and thereby enhance GABA-induced chloride ion flux leading
to neuronal hyperpolarization. However, the structural basis of benzodiazepine
interactions with their high-affinity site at GABAA receptors
is controversially debated in the literature, and in silico studies led to discrepant binding mode hypotheses. In this study,
computational docking of diazepam into α+/γ2– homology
models suggested that a chiral methyl group, which is known to promote
preferred binding to α5-containing GABAA receptors
(position 3 of the seven-membered diazepine ring), could possibly
provide experimental evidence that supports or contradicts the proposed
binding modes. Thus, we investigated three pairs of R and S isomers of structurally different chemotypes,
namely, diazepam, imidazobenzodiazepine, and triazolam derivatives.
We used radioligand displacement studies as well as two-electrode
voltage clamp electrophysiology in α1β3γ2-, α2β3γ2-,
α3β3γ2-, and α5β3γ2-containing
GABAA receptors to determine the ligand binding and functional
activity of the three chemotypes. Interestingly, both imidazobenzodiazepine
isomers displayed comparable binding affinities, while for the other
two chemotypes, a discrepancy in binding affinities of the different
isomers was observed. Specifically, the R isomers
displayed a loss of binding, whereas the S isomers
remained active. These findings are in accordance with the results
of our in silico studies suggesting the usage of
a different binding mode of imidazobenzodiazepines compared to those
of the other two tested chemotypes. Hence, we conclude that different
chemically related benzodiazepine ligands interact via distinct binding
modes rather than by using a common binding mode.
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