In our present study, we used sodium orthovanadate (SOV) to establish the role of BDNF and oxidative stress in depression. Moreover there is an inverse relationship between them, reflecting a possible mechanism of interaction between oxidative stress and neurotrophin dysfunction (Zhang, Chen et al. 2015). Corticosterone, a stress hormone, is an indicator of the anxiety and depressive- like behavior in an individual. Stress elevates corticosterone levels by activating the HPA axis, resulting in neuronal atrophy arising due to decreased brain levels of neurotrophins like BDNF. Corticosterone decreases BDNF mRNA expression gradually, resulting in diminished levels of BDNF protein translation (Liu, Walther et al. 2005). BDNF is one of the growth factors that trigger neuronal survival after BDNF–TrkB signaling. Impairment in the neurotrophins, mainly BDNF, leads to depressive-like behavior, increased hippocampal dendritic atrophy, cell death, and reduced LTP. Impaired BDNF expression has also been reported in MDD. A study has revealed that cortisol levels get elevated, and serotonin levels get declined on the induction of chronic stress via serotonin reuptake, further resulting in the depressive-like behavior (Sheikh, Hayden et al. 2010). Chronic stress causes HPA axis dysregulation, and many studies have reported a decrease in the proliferation and survival of hippocampal neurons when the HPA axis is dysregulated (Gomez, Calderón-Rivera et al. 2020). Morover chronic stress induced HPA axis dyfunctioning results in the production of proinflammatory cytokines (Belleau, Treadway et al. 2019). Neuroinflammation leads to oxidative stress and both of them together generate a vicious cycle resulting in reduced hippocampal neurogenesis. Therefore, considering this CUMS paradigm, which is a well-validated model of depression produced by the set of stressors in rodents (Schildkraut 1965), the present study was used to elucidate the impaired role of BDNF levels in CUMS induced depressive like behavior and its amelioration with SOV.
CUMS significantly downregulates the levels of BDNF and CREB, resulting in a depressive-like behavior (Levi-Montalcini 1950). Increased corticosterone levels and decreased levels of BDNF after CUMS in our results are in line with the above findings. However, our study also revealed that the treatment with SOV significantly decreased the plasma corticosterone levels. The effect produced by SOV per-se on corticosterone levels is consistent with previous findings where vanadium compound attenuated corticosterone levels in rats (Katayama, Yamada et al. 2010). According to a prior study, the chronic FLX treatment in CUMS exposed rats also normalized the corticosterone levels (Karege, Vaudan et al. 2005) as reported in our study. ELISA was performed to predict the effect of FLX treatment on BDNF. It was observed that fluoxetine significantly increased the levels of BDNF in both hippocampus and cortex may be via triggering transcription regulator, i.e., CREB (Chen, Dowlatshahi et al. 2001).
Body weight indicates the pathogenesis of the disease, and it was found that CUMS rats demonstrated a significant decrease in weight compared to the control group, thus depicting one of the core signs of a depressive-like behavior (Babiec, Guglietta et al. 2014). In our study, treatment with SOV produced no significant gain in body weight compared to CUMS group. However, SOV per-se has made a significant decrease in weight gain as compared to the control group. The results support the previous finding in which vanadium-fed dams had lower food intakes and weight gains than controls during pregnancy (Ferré, Baler et al. 2009). Vanadium compound-induced weight loss could also be attributed to the reduction in neuropeptide Y synthesis (NPY), which is responsible for the stimulation of appetite (Ayoub and Pfleger 2010). At the same time, significant weight gain was observed in the fluoxetine and control groups. In the former case, the antidepressant tends to increase body weight (Trifilieff, Rives et al. 2011), appetite, and a study conducted on humans also support our results regarding the elevation of body weight by fluoxetine as serotonin is responsible for appetite (Hern, Baig et al. 2010).
Increased immobility in tail suspension test (TST), regarded as a condition of 'failure to adapt to stress' (Hasbi, Fan et al. 2009) was produced after CUMS induction. Significant association exist between decreased immobility and the potency of antidepressants in tail suspension tests (Gomes, Ferreira et al. 2013).
CUMS model shows a declined sensitivity to reward, termed as the anhedonic state (Hutchinson, Chou et al. 2009), one of the core symptoms of depression. A study confirmed that mice exposed to CUMS consume less sucrose fluid (Fukuchi 2020), supporting our findings. An open field test was conducted to analyze the locomotor activity and explore the novel environment (Wills 1965). Many studies depict the decline in the number of line crossing activity (OFT) in CUMS group, reflecting a depressive-like behaviors (Fukuchi 2020). Our study also showed a similar finding reflecting the effect of CUMS in the induction of a depressive-like behaviors. However, both the SOV doses in the present study improved the core features of depression in rodents like anhedonia, despair behavior, and hypo-locomotion. SOV per-se resulted in decreased sucrose consumption that could result from its roles in improving leptin and insulin signaling, which play an essential role in regulating energy balance through food-associated reward control (Zhang, Chen et al. 2015).
Hippocampal oxidative stress is induced due to low BDNF after chronic unpredictable mild stress. Numerous studies have highlighted that stress significantly decreases BDNF mRNA expression in the frontal cortex and hippocampus (Kim, Lee et al. 2019), thus indicating a link between low BDNF and oxidative stress. A study found that BDNF downregulates ethanol-induced cellular oxidative stress and apoptosis in developing hypothalamic neuronal cells (Frühauf-Perez, Temp et al. 2018) thus affirming antioxidant-like activities of BDNF (Mehrpouya, Nahavandi et al. 2015). In the present study, we analyzed MDA levels (an indicator of lipid peroxidation), NO, GSH, and SOD in the hippocampal and cortical tissues of mice exposed to CUMS. CUMS exposure resulted in the generation of oxidative stress and nitrosative stress in both the cortex and hippocampus. However, chronic SOV treatment ameliorated oxidative and nitrosative stress in the brain, emphasizing the role of BDNF in mediating antioxidant effects (Bibring 1953). SOV significantly increased SOD levels after CUMS exposure and the findings are consistent with previous study indicating neuroprotective effect of SOV. MDD is associated with lipid peroxidation and decreased antioxidative enzyme activities resulting in reactive oxygen species (ROS) generation (Schiller, Minkel et al. 2013; Kim, Lee et al. 2019). Neuronal degeneration is a significant consequence of ROS generation.
In vitro studies suggested that ROS act in a neurotoxic as well as in a neuroprotective manner, which is enhanced by TrkB (Harlow, Newcomb et al. 1986). However, ROS has a significant role in psychiatric disease due to the vulnerability of the central nervous system to oxidative stress and CUMS results in induction of ROS expression by Akt pathway modulation (Mulinari 2012). In addition to the above finding, a study conducted on cisplatin-induced ROS suggested that BDNF attenuates ROS generation, resulting in the decline in ROS levels (Carroll 1971). Indeed, a specific pathway has not been evolved fully to support the link between ROS and TrkB.
Meanwhile, a study conducted on cell lines revealed that vanadate compound per-se could generate ROS resulting in the decreased levels of SOD via MAPK pathway activation (Fawcett and Barkin 1997). And our study also implies that SOV per-se decreased SOD and increased nitrite levels per-se while it does not affect MDA levels. Whereas it produced a significant antioxidant effect in CUMS exposed rats supported by previously conducted study (Kim, Lee et al. 2019).
Our study revealed that CUMS exposure increased nitrite levels, whereas SOV (per-se) also increased nitrite levels via the Akt pathway's activation (Aid, Kazantseva et al. 2007) while SOV at a dose of 10 mg/kg significantly decreased its level. CUMS activates microglia, which gets activated to produce inflammatory cytokines. These inflammatory markers are the leading cause for the production of nitrites in the brain whereas protein tyrosine phosphatase 1B (PTP1B), a member of the protein tyrosine phosphatases (PTPs) family, positively regulates neuroinflammation by causing dephosphorylation of proteins at tyrosine residues. SOV, a PTP inhibitor reduces this feature induced by CUMS, resulting in a decline in nitrite levels than the CUMS group (Seifer, Feng et al. 2006).
A report suggests that PTP1B down-regulates the neuronal BDNF-TrkB pathway through the dephosphorylation of TrkB, whereas PTP1B inhibition boosts BDNF signaling (Zhang, Chen et al. 2015). It is well postulated that SOV, directly acting at tyrosine residues of TrkB, preserves its signaling and also recovers tyrosine kinase activity of TrkB by upregulating m-BDNF (Akhtar, Bishnoi et al. 2020).