Therapeutic drugs targeting GABA type-A chloride channel receptors (GABAARs) are used as anxiolytics, sedatives, and antiepileptics. These drugs are allosteric positive modulators of GABAARs that enhance GABA activity. As these drugs non-selectively target many GABAAR subtypes causing unwanted side effects, subunit-specific drugs are now desirable. Positive modulators selective for α5-containing GABAARs (α5GABAARs) have shown promising results in treating cognitive symptoms of schizophrenia and age-related dementia, whereas α5GABAAR negative modulators have proven worth as cognition enhancers, in post-stroke recovery and prevention of general anesthetic-induced amnesia, without causing sedation and anxiety. The aim of this project is to discover and characterize novel α5GABAAR-selective compounds.
This was accomplished by screening a library of synthetic compounds on HEK293 cells, transfected with α5β3γ2L GABAARs and an anion-quenchable yellow fluorescent protein (YFP), using a high-throughput fluorescence-based anion influx assay. Modulation on other GABAAR subtypes (α1β2γ2L, α5β2γ2L) and drug binding sites were explored via functional studies using two-electrode voltage clamp electrophysiology (TEVC). To prepare for TEVC, Xenopus borealis frogs were anesthetized with MS-222 and surgically incised to obtain the oocytes, which were digested and injected with GABAAR mRNAs.
TEVC recordings revealed that isomeric compounds RM 68, RM 69 and RM 70 are potent GABAAR modulators. These isomers, except RM 69, demonstrated biphasic modulation, selectively potentiating α5GABAARs in the nanomolar (sometimes picomolar) range, but inhibiting GABAARs non-selectively at higher micromolar concentrations. Removing mercaptoacetaldehyde from RM 68 to give RM 96 eliminated its α5GABAAR selectivity. Flumazenil managed to neutralize the potentiation of GABA responses by the 4 compounds, suggesting that the potentiating effect of these compounds was modulated via the benzodiazepine site.
RM compounds can be potential leads to design novel subtype specific drugs for α5-related disorders and to study the pharmacology of α5GABAARs.