Evaluation of the Protective Effects of Alkaloids Isolated from the Hooks and Stems of Uncaria sinensis on Glutamate-induced Neuronal Death in Cultured Cerebellar Granule Cells from Rats

We have previously shown that an aqueous extract of the hooks and stems of Uncaria sinensis (Oliv.) Havil., Uncariae Uncus Cum Ramulusis, protects against glutamate‐induced neuronal death in cultured cerebellar granule cells by inhibition of Ca2+ influx. Because it is not known which components of Uncaria sinensis are active, in this study we have evaluated, by 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl‐tetrazolium bromide (MTT) staining, the neuroprotective effects of the oxyindole alkaloids corynoxeine, rhynchophylline, isorhynchophylline and isocorynoxeine, and the indole alkaloids geissoschizine methyl ether, hirsuteine and hirsutine, isolated from the hooks and stems of Uncaria sinensis, on glutamate‐induced cell death. We also investigated the inhibitory effects of the compounds on 45Ca2+ influx in cultured rat cerebellar granule cells. Cell viability evaluated by the MTT assay was significantly increased by application of rhynchophylline (10−3M), isorhynchophylline (10−4‐10−3M), isocorynoxeine (10−4‐10−3M), hirsuteine (10−4‐3 × 10−4M) or hirsutine (10−4‐3 × 10−4M) compared with exposure to glutamate only, with the effect of isorhynchophylline being the strongest. The increased 45Ca2+ influx into cells induced by glutamate was significantly inhibited by administration of rhynchophylline (10−3M), isorhynchophylline (3 × 10−4‐10−3M), isocorynoxeine (3 × 10−4‐10−3M), geissoschizine methyl ether (10−3M), hirsuteine (3 × 10−4‐10−3M) or hirsutine (3 × 10−4‐10−3M). These results suggest that oxyindole alkaloids such as isorhynchophylline, isocorynoxeine and rhynchophylline and indole alkaloids such as hirsuteine and hirsutine are the active components of the hooks and stems of Uncaria sinensis which protect against glutamate‐induced neuronal death in cultured cerebellar granule cells by inhibition of Ca2+ influx.