Selective Modulation of α5 GABA_A Receptors Exacerbates Aberrant Inhibition at Key Hippocampal Neuronal Circuits in...

Selective negative allosteric modulators (NAMs), targeting α5 subunit-containing GABA_A receptors (GABA_ARs) as potential therapeutic targets for disorders associated with cognitive deficits, including Alzheimer's disease (AD), continually fail clinical trials. We investigated whether this was due to the change in the expression of α5 GABA_ARs, consequently altering synaptic function during AD pathogenesis. Using medicinal chemistry and computational modeling, we developed aqueous soluble hybrids of 6,6-dimethyl-3-(2-hydroxyethyl) thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophene-4(5H)-one, that demonstrated selective binding and high negative allosteric modulation, specifically for the α5 GABA_AR subtypes in constructed HEK293 stable cell-lines. Using a knock-in mouse model of AD (APP ^NL-F/NL-F), which expresses a mutant form of human amyloid-β (Aβ), we performed immunofluorescence studies combined with electrophysiological whole-cell recordings to investigate the effects of our key molecule, α5-SOP002 in the hippocampal CA1 region. In aged APP ^NL-F/NL-F mice, selective preservation of α5 GABA_ARs was observed in, calretinin- (CR), cholecystokinin- (CCK), somatostatin- (SST) expressing interneurons, and pyramidal cells. Previously, we reported that CR dis-inhibitory interneurons, specialized in regulating other interneurons displayed abnormally high levels of synaptic inhibition in the APP ^NL-F/NL-F mouse model, here we show that this excessive inhibition was "normalized" to control values with bath-applied α5-SOP002 (1 microM). However, α5-SOP002, further impaired inhibition onto CCK and pyramidal cells that were already largely compromised by exhibiting a deficit of inhibition in the AD model. In summary, using a multi-disciplinary approach, we show that exposure to α5 GABA_AR NAMs may further compromise aberrant synapses in AD. We, therefore, suggest that the α5 GABA_AR is not a suitable therapeutic target for the treatment of AD or other cognitive deficits due to the widespread neuronal-networks that use α5 GABA_ARs.