Neuroprotective Effects Of Genistein On The Dopaminergic Neurons In The Substantia Nigra Against The Animal Models Of Parkinson's Disease And Its Mechanism

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the declining levels of dopamine (DA) in the striatum. Clinical studies have demonstrated a greater incidence and prevalence of PD in men than in women. Even though the cause of this disease is not very clear, the clinical evidence implies a role of sex hormones, especially estrogen, in the incidence of PD. Although data suggest beneficial effects of estrogen in PD and the other senile diseases, many women turn to phtoestrogens as an alternative to hormone replacement therapy because of the undesirable side effects. Genistein, a natural isoflavone phytoestrogen present in soybean, have a chemical structure similar to steroidal estrogens and have long been recognized to have estrogen-like activities. However little is known about the neuroprotective effects and the possible mechanism of genistein on the dopaminergic neurons against the animal models of PD. Object: To study the neuroprotective effects of genistein on the dopaminergic neurons against the animal models of PD and its mechanism. Methods: C57BL6 mice were ovariectomized and treated with genistein (i.p) or estradiol benzoate (i.p) for 3 days prior to and throughout the MPTP injection. Immunohistochemistry was used to assess the number of tyrosine hydroxylase-immunoreactive (TH-IR) neurons and Bcl-2 protein expression in the substantia nigra pars compacta (SNpc); Reverse transcription-polymerase chain reaction (RT-PCR) was used to determine the gene expressions of TH, dopamine transporter (DAT) and Bcl-2 in the SN. The contents of DA and its metabolites dihydroxy-phenylacetic acid (DOPAC) and homovanillic (HVA) in the striatum were determined by using high performance liquid chromatography-electrical chemical detection (HPLC-ECD). Wistar female rats were ovariectomized and treated with vehicle, genistein or 17-βestradiol intracerebroventricularly for 4 weeks after unilateral injection of 6-OHDA into the medial forebrain bundle (MFB). Immuohistochemistry was used to detect the TH-IR neurons and Bcl-2 protein expression. Perls iron staining was used to determine the changes of iron contents in SN. Results: 1. The numbers of TH-IR neurons and the TH and DAT mRNA expression decreased in the SN of MPTP treated mice (P<0.01, compared with that of control group). Pre-treatment with genistein and estrogen reversed the above changes (P<0.01). 2. DA and its metabolites DOPAC, HVA in the MPTP group decreased compared with the control group (P<0.01). These effects could be partly restored by genistein and estrogen treatment (P<0.01). 3. Compared with the control group, the Bcl-2 protein and mRNA expression decreased in the SN of MPTP treated mice (P<0.01). Pre-treatment with genistein and estrogen reversed the above changes (P<0.01). 4. The numbers of TH-IR neurons decreased in the SNpc of the injured side of 6-OHDA induced PD model (P<0.01, compared with that of control group). Pre-treatment with genistein and estrogen reversed the above changes (P<0.01). 5. Compared with the control group, the iron staining increased in the SN of the injured side of 6-OHDA induced PD mode] (P<0.01). Pre-treatment with genistein and estrogen reversed the above changes(P<0.01). 6. The protein expression of Bcl-2 decreased in the SN of the injured side of 6-OHDA induced PD model (P<0.01, compared with that of control group). Pre-treatment with genistein and estrogen reversed the above changes (P<0.01). Conclusion: These results demonstrated that genistein has neuroprotective effects on dopaminergic neurons against MPTP and 6-OHDA toxicities. The mechanisms underlying genistein mediated neuroprotection maybe due to its estrogen-like activities such as anti-apoptotic and antioxidation properties. These results provide experimental evidence for the clinical application of genistein for prevention and treatment of PD.