GABAA α5 subunit-containing receptors do not contribute to reversal of inflammatory-induced spinal sensitization as indicated by the unique selectivity profile of the GABAA receptor allosteric modulator NS16085

GABA(A) receptor positive allosteric modulators (PAMs) mediate robust analgesia in animal models of pathological pain. Restoration of diminished spinal GABA(A)-alpha 2 and -alpha 3 subunit-containing receptor function is a principal contributor to this analgesia, albeit involvement of GABA(A)-alpha 5-receptors has not been excluded. Thus, we compared NS11394 and TPA023 (PAMs with selectivity/efficacy at GABA(A)-alpha 2/ alpha 3/alpha 5 receptors) with TP003 (a reportedly GABA(A)-alpha 3 selective PAM) against spinal sensitization. However, in-house electrophysiology studies designed to confirm the selectivity of TPA023 and TP003 for human GABA(A) receptors did not corroborate published data, with TP003 displaying considerable GABA(A)-alpha 5 receptor efficacy. Therefore, we identified a novel PAM, NS16085, which possesses negligible efficacy at GABA(A)-alpha 5 receptors, but with GABA(A)-alpha 2/alpha 3 efficacy equivalent to NS11394. At the GABA(A)-alpha 1 receptor the compound gives low level of negative modulation further separating it from the other compounds. Rat pups with carrageenan-induced hindpaw inflammatory hyperalgesia were used to make ex vivo spinal dorsal root-evoked ventral root recordings. Some spontaneous activity and large numbers of spikes to repetitive stimulation of dorsal roots at C-fibre intensity, indicative of wind-up and sensitization were observed. Equimolar concentrations of NS11394, TP003 and NS16085 all attenuated wind-up to a similar degree; TPA023 was clearly less effective. In adult rats, NS16085 (3-30 mg/kg, p.o.) dose-dependently reduced formalin-induced hindpaw flinching with efficacy comparable to NS11394. Thus, potentiation of GABA(A)-alpha 2 and-alpha 3 receptors is sufficient to depress spinal sensitization and mediate analgesia after inflammatory injury. Positive modulation at GABA(A)-alpha 5-receptors is apparently dispensable for this process, an important consideration given the role of this receptor subtype in cognitive function. (C) 2014 Elsevier Inc. All rights reserved.