Inhibition of Microglial Activation by the Herbal Flavonoid Baicalein Attenuates Inflammation-mediated Degeneration of Dopaminergic Neurons

Accumulating evidence has suggested that inflammation in the brain participates in the pathogenesis of Parkinson's disease (PD). Therefore, anti-inflammatory therapy has attracted much attention as novel interference to neurodegenerative diseases. Baicalein, a major flavonoid extracted from a traditional Chinese herb Scutellaria baicalensis Georgi (Huangqin), possesses potent anti-inflammatory and antioxidant properties. To test the potential neuroprotective effect of baicalein on dopaminergic neurons, primary midbrain neuron-glia cultures from E-14 rat embryos were used. Cultures were pretreated with baicalein for 30 min prior to stimulation with lipopolysaccharide (LPS, 10 ng/ml). LPS leads to massive activation of microglial cells revealed by OX-42 immunostaining, and produced excessive quantities of NO. Excessive elevation of superoxide level was also observed in enriched-microglia after stimulating with LPS. LPS-induced damage to dopaminergic neurons was evaluated by uptake capacity for [3H]dopamine and tyrosine hydroxylase (TH)-immunocytochemistry. Pretreatment with baicalein concentration-dependently attenuated LPS-induced decrease in [3H]dopamine uptake and loss of TH-immunoreactive (TH-ir) neurons, which the maximum protective effect was observed at the concentration of 5 microM. Post-treatment with baicalein (5 microM) was also shown to be effective even if baicalein administered up to 2 h later than LPS application. Morphological study shows that baicalein (5 microM) almost completely blocked LPS-induced activation of microglia. Excessive production of TNF(alpha) and free radicals such as NO and superoxide by LPS stimulation were also attenuated by baicalein at a concentration-dependent pattern. The present study indicates that baicalein exerts potent neuroprotective effect on LPS-induced injury of dopaminergic neurons. We hypothesize that the inhibition of LPS-induced production of NO and free radicals from microglia may underlie the mechanism of baicalein's neuroprotection.