Antinociceptive Effects of a Novel α2/α3-Subtype Selective GABAA Receptor Positive Allosteric Modulator

Pain remains a challenging clinical condition and spinal GABAA receptors are crucial modulators of pain processing. alpha 2/alpha 3-subtype GABAA receptors mediate the analgesic actions of benzodiazepines. Positive allosteric modulators (PAMs) at alpha 2/alpha 3-subtype GABAA receptors may have analgesic potential. Here we report a new selective alpha 2/alpha 3-subtype GABAA receptor PAM in in vitro and in vivo pain assays. KRM-II-81 demonstrated similar efficacy at alpha l/alpha 2/alpha 3 GABAA receptors and negligible efficacy at alpha 4/alpha 5/alpha 6 GABAA receptors, with alpha 2 and alpha 3-subtypes being 17 and 28-fold more potent than alpha 1 subtypes in HEK-293T cells expressing GABAA receptors with different alpha subunits. In contrast, KRM-1I-18B showed significant efficacy at alpha 1/alpha 2/alpha 3/alpha 5 subtypes, with similar potency at alpha 1/alpha 2/alpha 3 subtypes. Both PAMs and morphine dose-dependently decreased 0.6% acetic acid- and 0.32% lactic acid-induced writhing. The effects of both PAMs were reversed by the benzodiazepine receptor antagonist flumazenil, confirming their action at the benzodiazepine binding site of GABAA receptors. Both PAMS and morphine all dose-dependently reversed 0.32% lactic acid (but not 0.6% acetic acid) induced suppression of nesting behavior. Acetaminophen, but not the PAMs, reversed acid-depressed locomotor activity. Combined, these findings suggest that KRM-H-81 is a selective alpha 2/alpha 3 subtype GABAA PAM with significant antinociceptive effects in chemical stimulation-induced pain in mice.