Mechanisms of potentiation of the mammalian GABAA receptor by the marine cembranoid eupalmerin acetate

Background and purpose: Eupalmerin acetate (EPA) is a marine diterpene compound isolated from the gorgonian octocorals Eunicea succinea and Eunicea mammosa. The compound has been previously shown to modulate muscle-type and neuronal nicotinic acetylcholine receptors, which are inhibited in the presence of low micromolar concentrations of EPA. In this study, we examined the effect of EPA on another transmitter-gated ion channel, the GABA(A) receptor. Experimental approach: Whole-cell and single-channel recordings were made from HEK 293 cells transiently expressing rat wild-type and mutant alpha 1 beta 2 gamma 2L GABA(A) receptors. Key results: Our findings demonstrate that, at micromolar concentrations, EPA potentiates the rat alpha 1 beta 2 gamma 2L GABA(A) receptor. The analysis of single-channel currents recorded in the presence of EPA showed that the kinetic mode of action of EPA is similar to that of neuroactive steroids. Mutations to residues alpha 1Q241 and alpha 1N407/Y410, previously shown to affect receptor modulation by neurosteroids, also diminished potentiation by EPA. Exposure to a steroid antagonist, (3 alpha, 5 alpha)-17-phenylandrost- 16-en-3-ol, reduced potentiation by EPA. Additionally, exposure to EPA led to potentiation of GABA(A) receptors activated by very high concentrations (1-10 mu M) of allopregnanolone. In tadpole behavioural assays, EPA caused loss of righting reflex and loss of swimming reflex. Conclusions and implications: We conclude that EPA either interacts with the putative neurosteroid binding site on the GABA(A) receptor or shares with neurosteroids the key transduction elements involved in channel potentiation by steroids. The results indicate that cembranoids represent a novel class of GABA(A) receptor modulators.