An Improved Drugs Screening System Reveals that Baicalein Ameliorates the Aβ/AMPA/NMDA-Induced Depolarization of Neurons

Abstract

The presence of amyloid-β (Aβ) plaque and tau protein hyperphosphorylation in brain tissue is the pathological hallmark of Alzheimer's disease (AD). At least some Aβ neurotoxicity is caused by the presence of excess glutamate that has been induced by Aβ accumulation. Memantine is currently the only NMDA receptor inhibitor approved for treating moderate-to-severe AD patients. We utilized primary cortical neurons and DiBAC4(3), a slow-response voltage sensitive fluorescence dye, to create a novel system for screening herbal medicines that allows the identification of pure compounds able to ameliorate Aβ-induced abnormal depolarization. The intensity of DiBAC4(3) fluorescence was increased when primary neurons were stimulated by Aβ; furthermore, pre-treatment with memantine abolished this change. Using this system, we identified six crude extracts made from herbal medicines that effectively alleviated this Aβ-induced abnormal depolarization. Among these herbal medicines, one pure compound, baicalein, which was known to be present in Scutellaria baricalensis and is known to improve memory using an AD mouse model, was identified by our assay. However, the compound's molecular mechanism remained unknown. We found that baicalein, in addition to inhibiting Aβ-induced depolarization, possibly functions as an antagonist of AMPA and NMDA receptors. Taken together, we have established a system/platform to identify herbal medicines that ameliorate Aβ-induced depolarization of neurons. Equally important, baicalein is a candidate drug with great potential for the treatment of AD patients.

Human D-Amino Acid Oxidase: Structure, Function, and Regulation

“…Neurodegeneration induced by NMDAR hypoactivity was also proposed to contribute to AD and to be related to the progression of aging brain from mild cognitive impairment to AD (Olney et al, 1997 ; Wozniak et al, 1998 ). Compared to healthy individuals, the serum levels of DAAO are increased in patients affected by mild cognitive impairment and mild and severe AD (Lin et al, 2017 ), and DAAO levels correlate with the severity of cognitive deficit and with the D-serine level.…”

Therapeutic Potential of Baicalein in Alzheimer’s Disease and Parkinson’s Disease

“…Baicalein also showed neuroprotective effects on excitotoxic neuronal cell death in primary rat cortical cell cultures (Yang et al, 2014). Interestingly, an improved drugs screening system revealed that baicalein not only ameliorates the Aβ-induced depolarization of neurons, but also possibly functions as an antagonist of AMPA and NMDA receptors (Lin et al, 2017). In rat hippocampal nerve terminals (synaptosomes), baicalein inhibits depolarization-induced glutamate release, while in slice preparations whole cell patch-clamp experiments revealed that baicalein reduced the frequency of miniature excitatory postsynaptic currents without affecting their amplitude.…”

“…Baicalein has a wide safety margin and appears to be able to permeate the blood-brain barrier (BBB) (Chen et al, 2016;Liu et al, 2010;Tsai et al, 2002). Several in vitro studies have shown that baicalein is effective in preventing neurotoxicity induced by a variety of neurotoxins and stress inducers, including Aβ (Lin et al, 2017;Wang et al, 2004), 6-hydroxydopamine (6-OHDA) (Lee et al, 2005), rotenone (Li et al, 2012),H2O2 (Zhang et al, 2010), and glutamate (Lee et al, 2003), and others. In recent studies, moreover, baicalein has been shown to possess neuroprotective properties in several animal models of AD and PD (Cheng et al, 2008;Gao et al, 2015;Gu et al, 2016;Zhou et al, 2016).…”

Biochemical Properties of Human D-amino Acid Oxidase Variants and Their Potential Significance in Pathologies

“…Supporting this idea, D-Ser was shown to exert a neuroprotective effect against apoptosis, promoting neuronal survival (Esposito et al, 2012 ) and the DAAO competitive inhibitor sodium benzoate was found to ameliorate cognitive and overall function in patients with early phase AD (Lin et al, 2014 ). Furthermore, recently increased levels of DAAO have been reported in the serum of mild cognitive impairment patients, as well as mild and severe AD patients, compared to healthy individual (Lin et al, 2017 ). Worthy of note, the flavoenzyme levels increased with the severity of cognitive deficit and were significantly associated with D-Ser serum content.…”