The terpenoids Myrtenol and Verbenol act on δ subunit-containing GABAA receptors and enhance tonic inhibition in dentate gyrus granule cells

Sideritis plants and their extracts have been used in traditional medicine as sedatives, anxiolytics and anticonvulsant agents. Pinenes are the most prevalent of the volatile aroma components in Siderites extracts and the pinene metabolites myrtenol and verbenol have been identified as the most potent positive allosteric modulators of synaptic GABA(A) receptors composed of alpha 1 beta 2 and alpha 1 beta 2 gamma 2 subunits. In view of their therapeutic spectrum, we wondered whether these two terpenoids would also augment tonic GABA currents mediated by extrasynaptic GABA(A) receptors containing the delta subunit. When we expressed alpha 4 beta 2 delta receptors in HEK293 cells, we found that co-application of myrtenol or verbenol enhanced whole-cell current responses to GABA by up to 100%. Consistent with their effects on heterologous alpha 1 beta 2 gamma 2 receptors, we found that myrtenol and verbenol, when co-applied with GABA via local perfusion, increased the amplitude and area of miniature inhibitory postsynaptic potentials (mIPSCs) recorded in whole-cell voltage-clamp recordings from granule cells in the dentate gyrus of mouse hippocampal brain slices. In addition, co-application of terpenoids with GABA was also able to enhance tonic GABA current, measured from the change in baseline current and current noise, compared to GABA perfusion alone. Our results suggest that myrtenol and verbenol act as positive allosteric modulators at synaptic and extrasynaptic GABA(A) receptors, thereby augmenting phasic and tonic GABAergic inhibition. Thus, our study reveals an important pharmacological and therapeutic target of bicyclic monoterpenoids. (C) 2016 Elsevier Ireland Ltd. All rights reserved.