Molecular mechanisms of interaction between the neuroprotective substance riluzole and GABAA-receptors

The antiepileptic drug riluzole is used as a therapeutic agent in amyotrophic lateral sclerosis due to its neuroprotective effects. Besides presynaptic inhibition of GABAergic and preferentially glutamatergic transmission, it also potentiates postsynaptic GABA A -receptor function. We investigated the postsynaptic effects of riluzole on GABA A -receptor channels by use of the patch-clamp technique. Recombinant α 1 β 2 γ 2s and α 1 β 2 GABA A receptors were expressed in HEK 293 cells by transient transfection. Pulses of GABA were applied in combination with different concentrations of riluzole to whole cell or outside-out patches with either α 1 β 2 γ 2s or α 1 β 2 GABA A -receptor channels. Co-application of riluzole led to a slight decrease of absolute peak current amplitudes and steady-state currents in prolonged presence of GABA at saturating concentrations. In the presence of riluzole, enhancement of current amplitudes was observed with lower concentrations of GABA at α 1 β 2 γ 2s receptors and to a lower extent also at α 1 β 2 receptors. Thus, the potentiating effect of riluzole was shown to be not abolished in the absence of the γ2s-subunit. A further prominent effect of riluzole was a highly significant acceleration of the time course of current decay, most probably pointing to an open-channel block-like mechanism of action. As both receptor subtypes were affected similarly by the block, it could be concluded that the respective binding sites should be assumed within a region of high sequence homology like it is given for the channel-lining M2 domain of GABA A -receptor subunits. In conclusion, three different molecular mechanisms of interaction of the neuroprotective compound riluzole were observed at two different subtypes of GABA A receptor channels. The results further point to the impact of the inhibitory as well as the excitatory synaptic activity as a pharmacological target to counteract chronic excitotoxicity and reveal molecular mechanisms of action of the only one neuroprotective drug in current clinical use in patients suffering from amyotrophic lateral sclerosis.