Valerenic acid potentiates and inhibits GABA A receptors: Molecular mechanism and subunit specificity

Abstract

Valerian is a commonly used herbal medicinal product for the treatment of anxiety and insomnia. Here we report the stimulation of chloride currents through GABA A receptors (I GABA) by valerenic acid (VA), a constituent of Valerian. To analyse the molecular basis of VA action, we expressed GABA A receptors with 13 different subunit compositions in Xenopus oocytes and measured I GABA using the two-microelectrode voltage-clamp technique. We report a subtype-dependent stimulation of I GABA by VA. Only channels incorporating β 2 or β 3 subunits were stimulated by VA. Replacing β 2/3 by β 1 drastically reduced the sensitivity of the resulting GABA A channels. The stimulatory effect of VA on α 1β 2 receptors was substantially reduced by the point mutation β 2N265S (known to inhibit loreclezole action). Mutating the corresponding residue of β 1 (β 1S290N) induced VA sensitivity in α 1β 1S290N comparable to α 1β 2 receptors. Modulation of I GABA was not significantly dependent on incorporation of α 1, α 2, α 3 or α 5 subunits. VA displayed a significantly lower efficiency on channels incorporating α 4 subunits. I GABA modulation by VA was not γ subunit dependent and not inhibited by flumazenil (1 μM). VA shifted the GABA concentration–effect curve towards lower GABA concentrations and elicited substantial currents through GABA A channels at ≥30 μM. At higher concentrations (≥100 μM), VA and acetoxy-VA inhibit I GABA. A possible open channel block mechanism is discussed. In summary, VA was identified as a subunit specific allosteric modulator of GABA A receptors that is likely to interact with the loreclezole binding pocket.