Study On The Alternative Transcript Of BDNF Gene In The Hippocampus Of A Rat Depression Model

Major depressive disorder(MDD)is one of the common mental disorder diseases with despair, anhedonia, and suicidal tendencies. The incidence of depression and recurrence rate is very high, and shows an increasing trend in the world. WHO data show that by the year 2030, depression will be the leading high-burden disease. Therefore, the study of the pathogenesis and clinical treatment of depression will be very important. In addition to the clinical study of depression, the common way to study depression is through animal models. There are many kinds of animal models of depression, such as forced swim test(FST), learned helplessness(LH), chronic unpredictable mild stress(CUMS). However, these models cannot simulate the occurrence of depression well, and the prepare processes are complex. It will be helpful to the study of depression if we can make a more stable and convenient animal model of depression.At present, depression is mainly caused by long-term stimulation of stressful events around life, resulting in a sustained increase in glucocorticoid hormone in the body, causing the imbalance of neurotransmitters in the brain. Chronic administration of exogenous glucocorticoids can mimic the pathophysiology of depression, avoiding shortcomings of other physical or psychological stress models. In this paper, a rat model of depression was induced by chronic oral glucocorticoids, and the validity and reliability of the model was test too.Hippocampus plays an important role in the whole neural network related to emotional memory. Depression is associated with the atrophy of hippocampal neurons and reduces of the number of synapse and neurons. BDNF is a very important protein in the regeneration and repair of hippocampal neurons, and is widely distributed in the brain, especially in the hippocampus. BDNF can promote the regeneration and repair of neurons, and the change of BDNF is closely related to the occurrence of depression. When depression occurs, the transcription expression of BDNF gene in neurons and the signal pathway mediated by BDNF are changed. The BDNF protein is synthesized from a gene that has complex specific promoters, the BDNF gene contains nine 5′ non-coding exons(I–IX) linked to the common 3′ coding exon(IX) that produce different transcripts all translated into a mature BDNF protein. BDNF m RNA containing different exons expressed in different parts or changed with different environmental stimuli. Therefore, different stimuli can lead to m RNA expression that contains different exons. The stability and translation efficiency of different m RNA are not the same, and ultimately affect the level of BDNF protein in neurons.Therefore, we will study the change of the alternative transcription of BDNF m RNA and the expression of BDNF protein in hippocampal neurons in an animal model of depression caused by chronic oral glucocorticoid treatment. Objective:Out study was to explore an ideal model of depression in rats, and to provide a more convenient and efficient animal model for the study of etiology and treatment of depression. And we will observe alternative transcription of BDNF m RNA and the changes of BDNF protein in this rat model of depression. Methods:30 male SD rats(3 groups, 10 in each group), 2 rats per cage were housed under light/dark 12 h/12 h and allowed access to rodent chow and water ad libitum. Animals were allowed to acclimate to laboratory conditions for at least 5 days prior to use in experiments. Then, the rats in two depression model groups were oral administrated with CORT for 2 weeks(100ug/ml or 25ug/ml, respectively). Animals were then weaned as follows: 3 days with 50% of the original CORT concentration, 3 days with 25%. The changes of body weight were observed, and the variation of serum glucocorticoid in rats were measured weekly via ELISA. The effect of the model of depression were evaluated with forced swimming test, open field test, the elevated plus maze test and sucrose preference test. After the successful preparation of the model, five rats in each group were randomly selected to analyze the expression of BDNF in the hippocampus of rats via immunohistochemistry test. The hippocampus of the other rats were extracted and divided into CA1, CA3 and DG regions for PCR or QPCR test. Changes of the expression of BDNF m RNA transcripts were observed. Results:In the preparation of depression model, 10 rats in 100μg/ml CORT group, 10 in 25μg/ml CORT group and 10 in control group, all of the 30 rats were analyzed statistically.1. ELISA results showed that the serum glucocorticoid concentration in rats increased for 3 weeks after the administration of CORT.2. In the forced swimming test, the duration of immobility in the model group was decreased after the chronic administration of CORT, and showed a significant different when compare with before administration of CORT or control group(P < 0.05). Rats in the control group presented no significant alteration when compare before and after administration of CORT. There was no statistical difference in the normal group.3. In the open field test, CORT-treated animals presented a significant decrease in the number of standing, the total distance of the movement, and the time in the center of the field when compare with before administration of CORT or control group(P < 0.05), they also showed a significant increase in the time in the corner. Rats in the control group presented no significant alteration when compare before and after administration of CORT.4. In the elevated plus maze test, CORT-treated rats presented a significant decrease in the time in the open arm and an increase in the time in the close arm when compare with before CORT treatment or control group(P < 0.05), they also showed a significant increase in the time in the corner.5. In the sucrose preference test, sucrose preference was significantly reduced in model group when compare with before CORT treatment or control group(P < 0.05). There is no statistical difference in the control group.6. The weight gain of the model group was significantly lower than that of the control group(P < 0.05).7. The result of immunofluorescence staining showed that the expression of BDNF in the hippocampus of depression rats was decreased, especially in the DG region.8. QPCR results showed that compared with control group, the expression of BDNF m RNA IIa and III in the DG and CA1 regions was significant decreased(P < 0.05), but no change in CA3 region. The expression of BDNF m RNA VI in the DG was decreased(P < 0.05), but not in CA1 and CA3 region. The expression of BDNF m RNA VII was decreased in CA3 and DG region(P < 0.05), but not in CA1. The expression of BDNF m RNA VIII was decreased in DG region(P < 0.05), but not in CA1 and CA3 regions. Conclusion:(1) Continuous oral administration of glucocorticoids can produce an ideal rat model of depression.(2) The express of BDNF m RNA IIa,BDNF m RNA III,BDNF m RNA VI,BDNF m RNA VII,BDNF m RNA VIII transcripts in the hippocampus of a rat model of depression, especially in the DG region, were decreased.