The alkaloid 6-benzoylheteratisine inhibits voltage-gated Na+ channels in rat brain synaptosomes

The effects of the Aconitum alkaloid 6-benzoylheteratisine on the aconitine-, veratridine-, oubain- and KCl-induced alterations in free synaptosomal Na + ([Na+]i) and Ca2+ ([Ca2+]i) and the release of endogenous glutamate from rat cerebrocortical synaptosomes were investigated. [Na+]i and [Ca2+]i were fluorometrically determined employing SBFI and Fura-2 as the Na+ and Ca2+ sensitive dyes, respectively. Glutamate was detected by a continuous enzyme-linked fluorometric assay. The study revealed a concentration-dependent inhibitory effect of 6-benzoylheteratisine on aconitine-induced increases in [Na+]i, [Ca2+]i and the release of glutamate. The IC50 values were 4.1 microM (Na+), 4.8 microM (Ca2+) and 4.8 microM (glutamate release). Application of 100 microM 6-benzoylheteratisine after stimulation with 5 microM veratridine also reduced the induced [Na+]i and [Ca2+]i with half-lives of 72.1 and 44.7 s, respectively. Furthermore, 100 microM 6-benzoylheteratisine reduced the ouabain-induced Na+ influx to the same extent as the Na+ channel inhibitor tetrodotoxin, which points to an inhibition of non-activated Na+ channels by 6-benzoylheteratisine. Additionally, 100 microM 6-benzoylheteratisine failed to affect the release of glutamate and the increase in [Ca2+]i induced by 30 mM KCl, indicating that voltage-gated Ca2+ channels were not affected by 6-benzoylheteratisine. The data suggest an inhibitory effect of 6-benzoylheteratisine on voltage-gated Na+ channels as the only target, whereas mechanisms of Na+ and Ca2+ homoeostasis and pathways of glutamate release seem not to be affected by the drug.