In our present study, we evaluated SOV for the antidepressant-like effects by analyzing various behavioral tests, anti-oxidative enzyme activity, and BDNF levels associated with depressive-like behavior. Corticosterone, a stress hormone, is an indicator of 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 [42]. 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.
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 [43]. Moreover, chronic stress-induced HPA axis dysfunctioning results in the production of proinflammatory cytokines [21]. Neuroinflammation leads to oxidative stress and both together generate a vicious cycle resulting in reduced hippocampal neurogenesis.
CUMS paradigm, which is a well-validated model of depression produced by the set of stressors in rodents [44]. CUMS significantly increased the levels of corticosterone depicting a state of stress. However, the treatment with SOV significantly decreased 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 [45]. According to a prior study, the chronic FLX treatment in CUMS exposed rats also normalized the corticosterone levels [46] as reported in our study.
CUMS also downregulates the levels of BDNF and CREB, resulting in a depressive-like behavior [47] and so, we measured the levels of BDNF in both hippocampus and cortex. The results have demonstrated that both the doses of SOV and FLX significantly elevated the BDNF levels in CUMS rats. The effect of SOV be through It was observed that fluoxetine significantly increased the levels of BDNF in both hippocampus and cortex. A published report suggested that PTP1B down-regulates the neuronal BDNF-TrkB pathway through the dephosphorylation of TrkB, whereas PTP1B inhibition boosts BDNF signaling [48]. 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 [30]. Moreover, antidepressants like fluoxetine may be increasing BDNF levels via triggering transcription regulators, i.e., CREB [49].
CUMS has also shown significant effects on body weight where 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 [50]. 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 above peculiar effect of SOV concerning bodyweight has supported our results with the previous finding in which vanadium-fed dams had lower food intakes and weight gains than controls during pregnancy [51]. 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 [52]. 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 [53], 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 [54].
Increased immobility in the tail suspension test (TST), regarded as a condition of 'failure to adapt to stress' [55] was produced after CUMS induction. Significant associations were observed between decreased immobility and the potency of antidepressants in tail suspension tests [56].
CUMS model shows a declined sensitivity to reward, termed as the anhedonic state [57] and, is one of the core symptoms of depressive-like behavior. A study confirmed that mice exposed to CUMS consume less sucrose fluid [58], supporting our findings. An open field test was conducted to analyze the locomotor activity and explore the novel environment [38]. Many studies depict the decline in the number of line crossing activity (OFT) in CUMS group, reflecting depressive-like behavior [58]. Our study also showed a similar finding reflecting the effect of CUMS in the induction of depressive-like behavior. However, both the SOV doses in the present study improved the core features of depressive-like symptoms 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 [48].
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 [59], thus indicating a link between BDNF and oxidative stress. A study found that BDNF downregulates ethanol-induced cellular oxidative stress and apoptosis in developing hypothalamic neuronal cells [23] thus affirming antioxidant-like activities of BDNF [5]. 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 has ameliorated oxidative and nitrosative stress in the brain, emphasizing the role of BDNF in mediating antioxidant effects [60]. SOV significantly increased SOD levels after CUMS exposure and the findings are consistent with a previous study indicating the neuroprotective effect of SOV [59]. In our present study, FLX (10 mg/kg) in combination with SOV (5 mg/kg) has demonstrated a significant enhancement in the GSH and SOD levels.
MDD is associated with lipid peroxidation and decreased antioxidative enzyme activities resulting in reactive oxygen species (ROS) generation [59, 61]. 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 [62]. 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 [63]. In addition to the above finding, a study conducted on cisplatin-induced ROS suggested that BDNF attenuates ROS generation, resulting in a decline in ROS levels [64]. 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 [65]. And our study has also demonstrated 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 a previously conducted study [59].
Our study resulted that CUMS exposure increased nitrite levels, whereas SOV (per-se) also increased nitrite levels that may be via the Akt pathway activation [66] while SOV at a dose of 10 mg/kg significantly decreased its levels. However, the combination of SOV 5 mg/kg and FLX 10 mg/kg have not shown a significant effect in NO levels with CUMS group. This might be due to the activation of the Nrf2 pathway by FLX [67] and Akt pathway by SOV (Aid, Kazantseva et al. 2007), which has demonstrated a synergistic effect on NO upregulation. CUMS also activates microglia, which further regulate the production of 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 (10 mg/kg), a PTP inhibitor reduces this feature induced by CUMS, resulting in a decline in nitrite levels than the CUMS group [68].
From the above findings, we observed that the SOV has depicted an antidepressant-like effect which could be attributed to its antioxidant and BDNF increasing activity. However, the effect was found to be comparable with the standard drug fluoxetine and neither the combination of SOV with fluoxetine produced any synergistic effect.