Asiatic acid, an active substance of Centella asiatica, presynaptically depresses glutamate release in the rat hippocampus.

ABSTRACT

Inhibiting glutamate release can reduce neuronal excitability and is recognized as a key mechanism of anti-epileptic drugs. In this study, by using isolated nerve terminal (synaptosome) and slice preparations, we investigated the effect of asiatic acid, a triterpene isolated from Centella asiatica with antiepileptic activity, on glutamate release in the hippocampus of rats. In hippocampal synaptosomes, application of asiatic acid resulted in a concentration-dependent inhibition of 4-aminopyridine-evoked glutamate release. This inhibitory action was dependent on extracellular calcium, blocked by inhibiting the vesicular transporter, but was unaffected by inhibiting the glutamate transporter. In addition, asiatic acid decreased the 4-aminopyridine-induced increase in the intraterminal calcium and failed to alter the synaptosomal potential. Furthermore, the asiatic acid-mediated release inhibition was significantly suppressed by the N- and P/Q-type calcium channel inhibitor ω-conotoxin MVIIC or protein kinase C inhibitor GF109203X. Western blotting data in synaptosomes also revealed that asiatic acid reduced 4-aminopyridine-induced phosphorylation of protein kinase C. In hippocampal slices, asiatic acid decreased the frequencies of spontaneous excitatory postsynaptic currents without changing their amplitudes and glutamate-activated currents in CA3 pyramidal neurons. We also observed that asiatic acid significantly suppressed 4-aminopyridine-induced burst firing. These data suggest that, in rat hippocampal nerve terminals, asiatic acid attenuates the calcium influx via N- and P/Q-type calcium channels, subsequently suppressing protein kinase C activity and decreasing glutamate release.