High Plasma Corticosterone Is Reduced Hippocampal Bdnf Gene Expression Mechanism

A major physiological response to stress in rats is the activation of the hypothalamic-pituitary-adrenal (HPA) axis, leading to an increase in circulating levels of corticosterone, which has profound effects on the growth, differentiation, physiology and viability of hippocampal neurons. For example, high plasma level of corticosterone results in an increased vulnerability of hippocampal cells to metabolic or excitotoxic insults, and atrophy of CA3 dendrites. A similar pattern was found for long-term potentiation (LTP): high level of corticosterone has been shown to reduce LTP in CA1 and dentate gyrus.Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family, which includes NGF, NT-3, NT-4/5 and NT-6. As a neurotrophic factor, BDNF is recognized to play an important role in the survival, differentiation, and outgrowth of selected peripheral and central neurons during development and also in adulthood. BDNF has also been show to play an important role in activity-dependent synaptic plasticity in the hippocampus, producing a lasting potentiation of synaptic efficacy in the dentate gyrus. It is possible that these effects may, in turn, enhance learning and memory processes and help reduce cognitive deficits associated with aging and neurodegenerative disease.High plasma level of corticosterone down-regulates the expression of BDNF gene in the hippocampus, leading to hippocampal damage. But the mechanisms by which corticosterone regulates BDNF expression is still unclear. It' s not yet know whether the decrease of BDNF expression is because of a decrease in BDNF mRNA transcription or changes in biosynthesis and/or post-translational processing. It is also attractive to explore the signal transduction pathways by which corticosterone regulates BDNF gene expression.This paper is composed of three parts.(1) The effect of cortiosteone on BDNF gene expression in the rat hippocampus. Using semi-quantify RT-PCR and western blot technique, we measured the changes in BDNF mRNA, different BDNF exon (from exon 1 to exon 4) mRNA and BDNF protein levels in the hippocampus after the administration of corticosterone in different doses and at dipperent time points. Rats were divided randomly into 5 groups in our experiments. Group 1: animals injected with polyethylene glycol (PEG) sc served as control. Group 2: corticosterone (1 mg/kg) was injected sc and animalswere sacrificed 3 hours after corticosterone administration. Group 3: corticosterone (1mg/kg) was given sc and animals were sacrificed 6 hours after corticosterone administration. In group 4 and 5, corticosterone of 2 mg/kg was given sc and animals were sacrificed 3 and 6 hours, respectively. The results showed that the BDNF mRNA levels in the hippocampus of all the corticosterone-treated rats (Groups 2, 3, 4, and 5) were lower than that of the control group (P<0.01); the BDNF mRNA levels of Group 2 (1mg/kg, 3h) was lower than that of Group 3 (1mg/kg, 6h)(P<0.05); the BDNF levels of Group 4 (2mg/kg, 3h) was lower than that of Group 5 (2mg/kg, 6h)(P<0.05). The BDNF exon1 and exon3 mRNA levels in the hippocampus of Group 2, 3, 4, and 5 were lower than that of the control (P<0.05 or <0.01); the same is group 4 and group 5 (P<0.01). There was no significiant difference in the BDNF exon 2 and exon 4 mRNA levels between the corticosterone-treated rats and the control rats. By using western blot, the BDNF protein levels were measured. The results showed that in the hippocampus of all 4 corticosterone-treated groups the BDNF protein levels were lower than that of the control. It is also interesting to note that the BDNF protein levels of both Group 3 and Group 4 were significiantly lower than that of group 2 (P<0.05). In conclusion, our experiments indicate that the down-regulation of BDNF gene in rat hippocampus by corticosterone is dose and time associated. BDNF mRNA levels already decreased 3 hour after corticosterone injection and began to increase 6 hours after injection, while the BDNF protein levels 6 hours after injection were lower than 3 hours. We also find that BDNF exon 1 and exon 3 mRNA levels changed parallel with BDNF mRNA level, suggesting that the change in BDNF mRNA levels after corticosterone might be due to the decrease of BDNF exon 1 and exon 3 transcription.(2) The down-regulating effect of corticosterone on BDNF gene expression in rat hippocampus after pretreatment of NMDA receptor anagonist MK801. Corticosterone (2mg/kg sc) was administered into Sprague Dawley rats and the BDNF mRNA, proBDNF and mature BDNF protein levels in the hippocampus were determined. The phosphorylation level of the transcription factor cAMP response element binding protein (CREB) was also measured. Furthermore, we intraperitoneally injected NMDA receptor antagonist MK801 (0.1mg/kg) 30min before corticosterone administration to investigate whether and how MK 801 affected the regulation of BDNF gene expression by corticosterone. The results showed that 3 hours after single sc injection of corticsterone, the expression of BDNF mRNA, proBDNF and matureBDNF protein decreased significantly (P<0.01). MK801 promoted the downregulation of BDNF gene expression in the rat hippocampus by corticosterone. It was also found that either applying corticosterone or co-applying corticosterone with MK801 downregulated the phosphoration level of CREB, the latter (corticosterone plus MK801) being more effective (P<0.05). Taken together, our results indicate that corticosterone downregulates BDNF gene expression in the rat hippocampus through CREB pathway and that blockade of NMDA receptor enhances this effect of corticosterone in reducing BDNF expression.(3) Plasma corticosterone level of rats with left descending coronary artery ligation and restrained rats and glutamate level in restrained rats hippocampus by high performance liquid chromatography (HPLC). The experiment periods include 3 day, 1 week and 3 weeks. The results showed that the plasma corticosterone levels in rats with left descending coronary artery ligation rats with restrain stress were significantly higher than that of the control. While the sham-operated rats (thorax opened without coronary artery ligation) exhibited high level of plasma corticosterone similar to the corticosterone level in operated animals, we could not come to conclusion that myocardial infarction could increase the plasma corticosterone levels. There was no significant difference in glutamate level in hippocampus between restrained rats and the control rats.In summary, the present study demonstrates that corticosterone down-regulates BDNF gene expression in rat hippocampus. This effect is dose- and time-dependent, and is possibly attributable to depression of L-type voltage-sensitive calcium channels and down-regulation of transcription factor CREB. The decrease in BDNF exon 1 and exon 3 transcription may be involved. Blockade of NMDA receptor enhances this effect of corticosterone in reducing BDNF expression.