The incidence of DD is increasing globally in recent years, which has been a worldwide public health problem. Safe and effective anti-depression drug development is always required. Recent studies have shown that ethnomedicine is a great treasure for the development of effective drugs treating depression [27]. In particular, some formulas of CTM have been reported to be useful to improve DD [28]. To our best known, here we first report the effect of TSD on depression.
In this current study, depression-like symptoms were induced with CUMS that is a modeling method well imitating the occurrence and pathogenesis of human depression [29]. After the modeling within rats, results of behavioral experiments exhibit that such a modeling method is reliable. Also, those rats treated with TSD emerge significant increases of time of immobility in the behavioral experiments. The improvements of depression-like symptoms in rats are to some extent even better than the effect of fluoxetine according to our data. A further investigation based on metabolomic techniques is conducted to provide a better understanding of aberrant metabolic variations associated with CUMS and of the pharmacological effect of TSD on DD. Dysregulations of two urea cycle intermediates, urea, and ornithine, are observed to be primary differential factors for the discrimination between rats with and without TSD gavage, and between the blank group and the model group. The increases of these two metabolites in the serum of model group rats indicate enhanced arginase degradation, which is found to be ameliorated after the intervention of TSD administration. The effect is further demonstrated by the results of serum Arg I, which shows that the variations of serum arginase I are in line with those of urea and ornithine. It is documented previously that the increase of arginase in the blood is associated with the occurrence of depression [30]. Interestingly, the level of arginase I is also regarded to be predicable for liver and heart injury [31]. Correspondingly, TSD was widely used for the treatment of heart diseases and was documented to be effective for improving liver injury [32, 33]. Therefore, the restoration of arginase after the dosing of TSD may also indicate reversals of organ injuries caused by the CUMS modeling. In addition, the up-regulation of arginase is also related to immune defection and aberrant responses to inflammation [34]. The reversal of arginase disorder within TSD-treated rats may be partly owing to the anti-inflammatory effect of TSD, which is documented in another study [35].
General declines of AAs (including glycine, alanine, proline, phenylalanine, glutamate, lysine, and tyrosine) in the serum are remarkable features of CUMS. After treated with TSD, levels of most AAs recover in the serum of rats. First, decreases in AAs suggest a malnutrition situation of CUMS rats, which is reported previously [36]. Such a symptom is evidenced by the weight losses (Fig. 1a) in the model group. The impaired anabolic metabolism can be partly reflected by the lower levels of energetic metabolites such as citrate and lactate in CUMS rats, compared to other groups. Notably, common discriminants between M vs. B and M vs. TSD-treated like alanine and glutamate are AAs involved in the anaplerotic reactions from AA to the TCA cycle, which is the core metabolic pathway of energy supply. Second, ameliorations in the defective Tyr/Phe pathway are also found in the TSD-treated rats which is in accordance with the study by Liu et al [37]. Finally, glycine as well as glutamate which can be converted into GABA are associated with neurotransmitters. Thus, besides the diminished secretion of dopamine and 5-HT, depressive appearances of CUMS rats can be strongly attributed to the inhibited transmitter synthesis.
Other common discriminant metabolites between M vs. B and M vs. TSD include 3-AIB, pseudouridine, and 3-HB. 3-AIB is known as a critical synthetic precursor of AAs, its variation pattern is in line with most serum AAs in rats. Peseudouridine was reported recently as a potential biomarker of post-stroke depression [38]. 3-HB is an end-product of beta-oxidation in fatty acids. Decrease 3-HB in the CUMS rats is consistent with the diminished serum fatty acids such as propanoic acid in comparison with the blank controls, and with lower levels of arachidic acid found in the comparison between CUMS and non-treated rats.
Our data also suggest that the dramatic dysregulation of Arg I in depressive rats is involved in the inhibited BDNF/CREB signaling and is reversible after the intervention of TSD. Evidence support that BDNF as well as its membranal receptor in the brain, TrkB, in the hippocampus is implicated in both the pathogenesis of depression and antidepressant actions [39]. Several studies have shown that medications targeting the BDNF-TrkB-CREB axis improve depression-like symptoms in animal models [40]. Interestingly, a putative compound hydroxysafflor A issued from safflower is reported to up-regulate BDNF expression [41]. Similar results are available in another recent work showing the effect of safflower yellow, a drug extracted from safflower, on the activation of BDNF/TrkB/ERK signaling [42]. Alternatively, ethanol extract of Radix Rehmanniae praeparata also helps to enhance BDNF levels in the serum of CUMS rats [43]. Other potential compounds contained in TSD involving depression therapy encompass albiflorin from Radix Paeoniae alba [44], ferulic acid from both Rhizoma Chuanxiong and Angelica Sinensis [45], and ligustrazine from Rhizoma Chuanxiong. Remarkably, ferulic acid can not only increase BNDF expression but also reduce oxidative stress induced by chronic corticosterone stimulation in mice [46]. Given that Arg I is a pivotal marker of NO production, with hints that the effect of TSD on depression may be due to the anti-inflammatory effect. Ligustrazine was also reported to attenuate inflammation and oxidative stress in rats recently [47]. Furthermore, our findings support that the effect of TSD on DD is linked to the regulation of serotonin secretion. Emerging data show close interactions between BDNF and serotonin. Some studies suggested that BDNF was the target of serotonin [48]. However, the reverse regulation on serotonin receptor 5-HT1A by BDNF expression was also reported by Homberg et al. [49]. Together, our data imply that the actions of TSD on BDNF and serotonin are interdependent.
Our work does have some limitations. First, it remains unclear that what is the most effective composition or compound in TSD improving CUMS-induced depressive symptoms. Second, even though obvious metabolic differences and key discriminants are revealed between CUMS rats and TSD-treated ones, the variation patterns of the discriminants are not dose-dependent according to our data. This issue is supposed to be ascribed to the imitated sample size. Further study will be an investigation of antidepressant effects of the six component medications of TSD within a larger number of samples.
To summarize, our findings display that the TCM formula TSD can be a useful complementary therapy for depression management. The underlying mechanism behind the efficacy of TSD on DD should be multi-targeted, which can be related to the enhancement of neural transmitters such as glycine and serotonin, as well as to the improvement of disorders on the BNDF-CREB-Arg I axis.