Selective Modulation of α5 GABAA Receptors Exacerbates Aberrant Inhibition at Key Hippocampal Neuronal Circuits in APP Mouse Model of Alzheimer's Disease

Selective negative allosteric modulators (NAMs), targeting alpha 5 subunit-containing GABA(A) receptors (GABA(A)Rs) 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 alpha 5 GABA(A)Rs, 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 alpha 5 GABA(A)R 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-beta (A beta), we performed immunofluorescence studies combined with electrophysiological whole-cell recordings to investigate the effects of our key molecule, alpha 5-SOP002 in the hippocampal CA1 region. In aged APP(NL-F/NL-F) mice, selective preservation of alpha 5 GABA(A)Rs 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 alpha 5-SOP002 (1 mu M). However, alpha 5-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 alpha 5 GABA(A)R NAMs may further compromise aberrant synapses in AD. We, therefore, suggest that the alpha 5 GABA(A)R is not a suitable therapeutic target for the treatment of AD or other cognitive deficits due to the widespread neuronal-networks that use alpha 5 GABA(A)Rs.