Glial cell line-derived neurotrophic factor expression and promoter methylation in a rat model of alcohol addiction

Introduction:

Alcohol dependency is one of the most disabling diseases worldwide. Glial cell line-derived neurotrophic factor (Gdnf) shows promising results concerning the inhibition of ethanol consumption in a rodent model. Recent research emphasizes the importance of epigenetic mechanisms in the regulation of neurotrophic factors, such as brain-derived neurotrophic factor (Bdnf) in alcohol dependence. Epigenetic regulation of neurotrophic factors, such as the nerve growth factor (Ngf) and Bdnf are suggested to contribute to alcohol dependence. Little is known concerning the epigenetic regulation of Gdnf and alcohol consumption, withdrawal, and relapse. Our study aimed to analyze the epigenetic regulation of Gdnf during ethanol consumption and withdrawal in a rat model of alcohol addiction.

Methods:

32 male and female Wistar rats underwent an intermittent-access to 20% alcohol in a 2-bottle choice drinking procedure. Whole blood was collected, and the nucleus accumbens (NAc), as well as the ventral tegmental area (VTA), was dissected immediately after the last 24 hours of an alcohol-drinking session (Alcohol group), 24 hours after withdrawal from alcohol (withdrawal group), or without previous alcohol exposure (control group). Gdnf mRNA levels were measured using quantitative real-time polymerase chain reaction. Bisulfite-conversion of DNA and capillary sequencing was used to determine methylation levels of the Gdnf core promoter and the negative regulatory element, as well as both fragments together (whole fragment).

Results:

Gdnf mRNA expression levels in the VTA were significantly lower in the withdrawal group compared to controls (p < 0.05). In the NAc, Gdnf mRNA expression was significantly higher in the withdrawal group (p < 0.05). The withdrawal group was significantly less methylated in the whole Gdnf promoter fragment in the VTA (p < 0.001) and the NAc (p < 0.01) compared to controls which was driven by hypomethylation of the negative regulatory element (NRE).

Conclusion:

Changes in the cerebral Gdnf expression correspond to alterations in DNA methylation of the Gdnf promoter in a rodent model. Especially in the withdrawal group, we found less methylation in the NRE which was associated with a lower mRNA expression in the VTA. In the NAc on the other hand a higher mRNA expression was found although the methylation in the NRE was significantly lower compared to controls. Alcohol intake seems to alter methylation levels in the VTA and NAc differently. Further research is necessary to confirm the findings, that methylation of specific parts of the promoter region of Gdnf could lead to opposite effects on Gdnf expression. This could be considerably interesting for persons undergoing alcohol withdrawal, since decreased serum levels of Gdnf have been reported to be associated with alcohol-dependency in humans. Further research is necessary to confirm our findings and differences also in peripheral tissues like blood since Gdnf could be a promising approach to identify persons at risk for relapse and could be a target for pharmacological interventions.