ONO-8590580, a Novel GABA_Aα5 Negative Allosteric Modulator Enhances Long-Term Potentiation and Improves Cognitive...

GABA_A receptors containing α5 subunits (GABA_Aα5) are highly expressed in the hippocampus and negatively involved in memory processing, as shown by the fact that GABA_Aα5-deficient mice show higher hippocampus-dependent performance than wild-type mice. Accordingly, small-molecule GABA_Aα5 negative allosteric modulators (NAMs) are known to enhance spatial learning and memory in rodents. Here we introduce a new, orally available GABA_Aα5 NAM that improves hippocampal functions. ONO-8590580 [1-(cyclopropylmethyl)-5-fluoro-4-methyl-N-[5-(1-methyl-1H-imidazol-4-yl)-2-pyridinyl]-1H-benzimidazol-6-amine] binds to the benzodiazepine binding sites on recombinant human α5-containing GABA_A receptors with a K_i of 7.9 nM, and showed functionally selective GABA_Aα5 NAM activity for GABA-induced Cl^- channel activity with a maximum 44.4% inhibition and an EC₅₀ of 1.1 nM. In rat hippocampal slices, tetanus-induced long-term potentiation of CA1 synapse response was significantly augmented in the presence of 300 nM ONO-8590580. Orally administered ONO-8590580 (1-20 mg/kg) dose-dependently occupied hippocampal GABA_Aα5 in a range of 40%-90% at 1 hour after intake. In the rat passive avoidance test, ONO-8590580 (3-20 mg/kg, by mouth) significantly prevented (+)-MK-801 hydrogen maleate (MK-801)-induced memory deficit. In addition, ONO-8590580 (20 mg/kg, p.o.) was also effective in improving the cognitive deficit induced by scopolamine and MK-801 in the rat eight-arm radial maze test with equal or greater activity than 0.5 mg/kg donepezil. No anxiogenic-like or proconvulsant effect was associated with ONO-8590580 at 20 mg/kg p.o. in the elevated plus maze test or pentylenetetrazole-induced seizure test, respectively. In sum, ONO-8590580 is a novel GABA_Aα5 NAM that enhances hippocampal memory function without an anxiogenic or proconvulsant risk.