Background and Purpose: Cognitive decline is a major symptom in Alzheimer’s disease (AD), which is closely associated with synaptic excitatory-inhibitory imbalance. Here, we investigated whether astrocytic mechanisms involving the astrocyte-specific GABA transporter 3/4 (GAT3/4) play a role in altering the synaptic balance in AD and whether these mechanisms correlate with presynaptic cannabinoid type-1 receptors (CB1-Rs). Experimental approach: Using the APPNL-F/NL-F knock-in mouse model of AD, aged-matched to wild-type mice, we performed in vitro electrophysiological whole-cell recordings combined with immunohistochemistry in the CA1 and dentate gyrus (DG) regions of the hippocampus. Comparative neuroanatomy experiments were also performed in post-mortem brain tissue from human AD patients, age-matched to healthy controls. Results: We observed a higher expression of GABA content and GAT3/4 co-localised with reactive astrocytes, which enhanced tonic inhibition in the CA1, and DG of APPNL-F/NL-F mice compared to the age-matched wild-type animals. Blocking GAT3/4 - associated tonic inhibition in APPNL-F/NL-F mice resulted in an enhanced frequency of synaptic excitation, suggesting a presynaptic mechanism. These data also correlated with an up-regulation of CB1-Rs in astrocytes and cholecystokinin (CCK)-containing interneurons, which also enhanced tonic inhibition in the AD model, but did not affect GAT3/4 -associated tonic inhibition. The neuroanatomical results were mirrored in post-mortem tissue of AD patients. Conclusions: Our data suggest that reactive astrocytes lead to augmented tonic inhibition in the hippocampus, which probably plays an important presynaptic compensatory role in attempting to restore AD-associated neuronal hyperactivity. Therefore, reducing tonic inhibition through GAT3/4 may not be a good therapeutic strategy for AD.

Background and Purpose: Selective negative allosteric modulators (NAMs), targeting α5 subunit-containing GABAA receptors (GABAARs) 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 alteration of synaptic mechanisms associated with α5 GABAARs in AD. Experimental approach: Using medicinal chemistry and computational modelling, we developed aqueous soluble hybrids of 6,6-dimethyl-3-(2-hydroxyethyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one, that demonstrated selective binding and high negative allosteric modulation, specifically for the α5 GABAAR subtype in constructed HEK293 stable cell-lines. Using a knock-in mouse model of AD (APPNL-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. Key Results: In aged APPNL-F/NL-F mice, a selective preservation of α5 GABAARs was observed in: dis-inhibitory, calretinin- (CR), cholecystokinin- (CCK), somatostatin- (SST) expressing interneurons, and pyramidal cells. Synaptic inhibition recorded from CR interneurons in APPNL-F/NL-F mice, was abnormally excessive, but was “normalised” with bath-applied α5-SOP002 (1 μM). 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. Conclusions and Implications: Using a multi-disciplinary approach, we show that exposure to α5 GABAAR NAMs may further compromise aberrant synapses in AD. We therefore suggest that the α5 GABAAR is not a suitable therapeutic target for the treatment of AD or other cognitive deficits due to the widespread neuronal-networks that use α5 GABAARs.