Mechanisms underlying the riluzole inhibition of glutamate release from rat cerebral cortex nerve terminals (synaptosomes)

We have examined the effect of riluzole, a neuroprotective agent with anticonvulsant properties, on the release of endogenous glutamate from rat cerebrocortical synaptosomes using an on-line enzyme-coupled fluorometric assay. Riluzole inhibited the calcium-dependent release of glutamate that was evoked by exposing cerebrocortical synaptosomes to the potassium channel blocker 4-aminopyridine, and this presynaptic inhibition was concentration-dependent. Riluzole did not alter either 4-aminopyridine-evoked depolarization of the synaptosomal membrane potential or ionomycin-mediated glutamate release, indicating that riluzole-mediated inhibition of glutamate release is not due to a decrease in synaptosomal excitability or a direct effect on the exocytotic machinery. Examination of the effect of riluzole on Ca2+ influx revealed that the diminution of glutamate release could be attributed to a reduction in cytosolic calcium. A possible effect of riluzole on synaptosomal calcium channels was confirmed in experiments where synaptosomes pretreated with P/Q-type calcium channel blocker omega-agatoxin IVA, which abolished the riluzole-mediated inhibition of glutamate release. In addition, pretreatment of synaptosomes with either the G(i)/G(o) protein inhibitor pertussis toxin or the GABA, receptor agonist baclofen, completely prevented the inhibitory effect of riluzole on 4-aminopyridine-evoked glutamate release. It is concluded that riluzole exerts their presynaptic inhibition, likely through a reduction in the calcium influx mediated by P/Q-type calcium channels, and thereby inhibits the release of glutamate from rat cerebrocortical nerve terminals. This release inhibition may involve a pertussis toxin-sensitive G protein signalling pathway. This finding provides further support that presynaptic calcium channel blockade concomitant with inhibition of glutamate release could be an important mechanism underlying the therapeutic actions of this drug. (C) 2004 IBRO. Published by Elsevier Ltd. All rights reserved.