Differential levels of brain amino acids in rat models presenting learned helplessness or non-learned helplessness

Natural products for the treatment of stress-induced depression: Pharmacology, mechanism and traditional use

ETHNOPHARMACOLOGICAL RELEVANCE

Depression, one of the most common psychiatric disorders, is the fourth leading cause of long-term disability worldwide. A series of causes triggered depression, including psychological stress and conflict, as well as biological derangement, among which stress has a pivotal role in the development of depression. Traditional herbal medicine has been used for the treatment of various disorders including depression for a long history with multi-targets, multi-levels and multi-ways, attracting great attention from scholars. Recently, natural products have been commercialized as antidepressants which have become increasingly popular in the world health drug markets. Major research contributions in ethnopharmacology have generated and updated vast amount of data associated with natural products in antidepressant-like activity.

AIMS OF THE REVIEW

This review aims to briefly discuss the pathological mechanism, animal models of stress-induced depression, traditional use of herbal medicines and especially recapitulate the natural products with antidepressant activity and their pharmacological functions and mechanism of action, which may contribute to a better understanding of potential therapeutic effects of natural products and the development of promising drugs with high efficacy and low toxicity for the treatment of stress-induced depression.

MATERIALS AND METHODS

The contents of this review were sourced from electronic databases including PubMed, Sci Finder, Web of Science, Science Direct, Elsevier, Google Scholar, Chinese Knowledge On frastructure (CNKI), Wan Fang, Chinese Scientific and Technological Periodical Database (VIP) and Chinese Biomedical Database (CBM). Additional information was collected from Yao Zhi website (https://db.yaozh.com/). Data were obtained from April 1992 to June 2021. Only English language was applied to the search. The search terms were 'stress-induced depression', 'pathological mechanism' in the title and 'stress', 'depression', 'animal model' and 'natural products' in the whole text.

RESULTS

Stress-induced depression is related to the monoaminergic system, hypothalamic-pituitary-adrenal (HPA) axis, neuronal plasticity and a series of inflammatory factors. Four main types of animal models of stress-induced depression were represented. Fifty-eight bioactive phytochemical compounds, fifty-six herb medicines and five formulas from traditional Chinese medicine were highlighted, which exert antidepressant effects by inhibiting monoamine oxidase (MAO) reaction, alleviating dysfunction of the HPA axis and nerve injury, and possessing anti-inflammatory activities.

CONCLUSIONS

Natural products provide a large number of compounds with antidepressant-like effects, and their therapeutic impacts has been highlighted for a long time. This review summarized the pathological mechanism and animal models of stress-induced depression, and the natural products with antidepressant activity in particular, which will shed light on the action mechanism and clinical potential of these compounds. Natural products also have been a vital and promising source for future antidepressant drug discovery.

Structural and biochemical imaging reveals systemic LPS-induced changes in the rat brain

Despite mounting evidence for the role of inflammation in Major Depressive Disorder (MDD), in vivo preclinical investigations of inflammation-induced negative affect using whole brain imaging modalities are scarce, precluding a valid model within which to evaluate pharmacological interventions. Here we used an E. coli lipopolysaccharide (LPS)-based model of inflammation-induced depressive signs in rats to explore brain changes using multimodal neuroimaging methods. During the acute phase of the LPS response (2 h post injection), prior to the emergence of a task-quantifiable depressive phenotype, striatal glutamine levels and splenial, retrosplenial, and peri-callosal hippocampal cortex volumes were greater than at baseline. LPS-induced depressive behaviors observed at 24 h, however, occurred concurrently with lower than control levels of striatal glutamine and a reversibility of volume expansion (i.e., shrinkage of splenial, retrosplenial, and peri-callosal hippocampal cortex to baseline volumes). In both striatum and hippocampus at 24 h, mRNA expression in LPS relative to control animals demonstrated alterations in enzymes and transporters regulating glutamine homeostasis. Collectively, the observed behavioral, in vivo structural and metabolic, and mRNA expression alterations suggest a critical role for astrocytic regulation of inflammation-induced depressive behaviors.

The alterations of glutamate transporter 1 and glutamine synthetase in the rat brain of a learned helplessness model of depression

BACKGROUND

Although glutamate transmission via astrocytes has been proposed to contribute to the pathophysiology of depression, the precise mechanisms are unknown. Herein, we investigated the levels of glutamate transporter-1 (GLT-1) and glutamine synthetase (GS) of astrocytes in learned helplessness (LH) rats (an animal model of depression) and non-LH rats (an animal model of resilience).

METHODS

We administered inescapable mild electric shock to rats and then discriminated the LH and non-LH rats by a post-shock test. Almost 55% of the rats acquired LH. We then measured the expressions of GLT-1 and GS in several brain regions of LH and non-LH rats by Western blot analysis.

RESULTS

The levels of GLT-1 and GS in the CA-1, CA-3, dentate gyrus (DG), medial prefrontal cortex (mPF), and nucleus accumbens (NAc) of the LH group were significantly higher than those of the control group. The GS levels in the amygdala of the LH rats were significantly decreased compared to the controls. There were significant differences in GLT-1 and GS levels between the non-LH and LH rats in the CA-1 and CA-3.

CONCLUSIONS

These results suggest that the LH rats experienced up-regulations of GLT-1 and GS in the CA-1, CA-3, DG, mPF, and NAc and a down-regulation of GS in the amygdala. It is possible that the effects of the GLT-1 and GS levels on astrocytes in the CA-1 and CA-3 are critical for the differentiation of resilience from vulnerability.

Monoaminergic balances predict non-depression-like phenotype in Learned Helplessness Paradigm

Monoamine neuronal system abnormality is hypothesized to be the neurochemical pathology in depression, as it is supported by the efficacy of conventional antidepressants. The learned helplessness paradigm generates depression-like (LH) and non-depression-like (non-LH) behavioral models. Examination of the neurochemical states accompanying such distinct behavioral phenotypes can facilitate investigations of the mechanisms underlying resilience and the search for new strategies for depression prevention and therapy. Here, we measured the levels of monoamines, including noradrenaline (NA), serotonin (5-HT), and dopamine (DA), and their metabolites in the medial prefrontal cortex (mPFC), orbitofrontal cortex (OFC), hippocampus, nucleus accumbens (NAc), amygdala, and striatum in LH, non-LH, and non-manipulated (naïve) rats. Compared with LH rats, non-LH rats showed lower 3-methoxy-4-hydroxyphenylglycol (MHPG) levels and NA turnovers in the amygdala and higher 5-HT levels in the NAc. Compared with naïve rats, non-LH rats showed increased DA and homovanillic acid (HVA) levels in the amygdala and increased 5-hydroxyindoleacetic acid (5-HIAA) levels in the hippocampus and NAc, whereas LH rats exhibited increased HVA levels and DA turnovers in the hippocampus, decreased 5-HIAA levels in the mPFC, increased DA turnovers in the OFC, and decreased DA turnovers in the amygdala. Comparison between LH and non-LH suggest that suppressed amygdaloid NA activity and elevated 5-HT activity in the NAc are related to stress resilience. Changes that occurred in LH or non-LH rats when compared with those in naïve rats suggest that suppressed DA activity in the hippocampus and OFC; elevated DA activity in the amygdala; and facilitated 5-HT activity in the hippocampus, mPFC, and NAc are phenomena related to the expression of a non-depression-like phenotype.

Curculigoside facilitates fear extinction and prevents depression-like behaviors in a mouse learned helplessness model through increasing hippocampal BDNF

Curculigoside (CUR) is the main active component of traditional Chinese medicine Curculigoorchioides Gaertn (Xianmao in Chinese), which exhibits a variety of pharmacological activities. In this study we investigated the effects of CUR on fear extinction and related depression-like behaviors in mice. In fear conditioning task, we found that administration of CUR (1.6, 8, 40 mg·kg-1·d-1, ip, for 7 days) did not affect memory consolidation, but CUR at higher doses (8, 40 mg·kg-1·d-1) significantly facilitated fear extinction, especially on D3 and D4. Moreover, CUR administration significantly ameliorated the fear conditioning-induced depression-like behaviors, likely through promoting fear extinction. We showed that CUR increased the expression of brain-derived neurotrophic factor (BDNF) and phosphorylation of tropomyosin receptor kinase B (TrkB) in the hippocampus, and activated protein kinase B (Akt)-mammalian target of the rapamycin (mTOR) signaling pathway. Administration of the selective TrkB agonist 7,8-dihydroxyflavone (7,8-DHF, 5 mg·kg-1·d-1, ip) also facilitated fear extinction, ameliorated depression-like behaviors. We established a mouse learned helplessness (LH) model to evaluate the antidepressant activity of CUR. The spatial memory was assessed in Morris water maze. We showed that LH-induced depression-like behaviors, including prolonged immobility times in forced swim and tail suspension tests as well as spatial memory impairments; LH also downregulated BDNF expression and the Akt-mTOR signaling pathway in the hippocampus. Administration of CUR (1.6, 8, 40 mg·kg-1·d-1, ip, for 14 days) or 7,8-DHF (5 mg·kg-1·d-1, ip, for 3 days) prevented LH-induced depression-like behaviors and promoted BDNF expression and the Akt-mTOR signaling pathway. In conclusion, CUR can accelerate the fear memory extinction and ameliorate depression-like behaviors in mice via promoting BDNF expression and activating the Akt-mTOR signaling pathway in the hippocampus.

Abnormal composition of gut microbiota is associated with resilience versus susceptibility to inescapable electric stress

Increasing evidence indicates that abnormalities in the composition of gut microbiota might play a role in stress-related disorders. In the learned helplessness (LH) paradigm, ~60-70% rats are susceptible to LH in the face of inescapable electric stress. The role of gut microbiota in susceptibility in the LH paradigm is unknown. In this study, male rats were exposed to inescapable electric stress under the LH paradigm. The compositions of gut microbiota and short-chain fatty acids were assessed in fecal samples from control rats, non-LH (resilient) rats, and LH (susceptible) rats. Members of the order Lactobacillales were present at significantly higher levels in the susceptible rats than in control and resilient rats. At the family level, the number of Lactobacillaceae in the susceptible rats was significantly higher than in control and resilient rats. At the genus level, the numbers of Lactobacillus, Clostridium cluster III, and Anaerofustis in susceptible rats were significantly higher than in control and resilient rats. Levels of acetic acid and propionic acid in the feces of susceptible rats were lower than in those of control and resilient rats; however, the levels of lactic acid in the susceptible rats were higher than those of control and resilient rats. There was a positive correlation between lactic acid and Lactobacillus levels among these three groups. These findings suggest that abnormal composition of the gut microbiota, including organisms such as Lactobacillus, contributes to susceptibility versus resilience to LH in rats subjected to inescapable electric foot shock. Therefore, it appears likely that brain-gut axis plays a role in stress susceptibility in the LH paradigm.

Resilience Dysregulation in Major Depressive Disorder: Focus on Glutamatergic Imbalance and Microglial Activation

BACKGROUND

Many studies have been shown an important role of glutamatergic system as well microglial activation in the pathophysiology of major depressive disorder (MDD). In humans most resistant to the development of psychiatric disorders, including MDD, are observed a greater degree of resilience resulting from stress. Less resilience is associated with neuroendocrine and neuroinflammatory markers, as well as with glutamatergic system dysregulation. Thus, this review we highlighted findings from literature identifying the function of glutamatergic system, microglial activation and inflammation in resilience.

METHODS

We conducted a review of computerized databases from 1970 to 2017.

RESULTS

There is an association between microglial activation and glutamatergic system activation with stress vulnerability and resilience.

CONCLUSIONS

Glutamate neurotransmission, including neurotransmitter synthesis, signalling, and glutamate receptor functions and expression all seem to be involved with both stress vulnerability and resilience. Moreover, inflammation and microglial activation mediate individual differences in resilience and the risk of stress-induced MDD.

Testing the excitation/inhibition imbalance hypothesis in a mouse model of the autism spectrum disorder: in vivo neurospectroscopy and molecular evidence for regional phenotypes

Background

Excitation/inhibition (E/I) imbalance remains a widely discussed hypothesis in autism spectrum disorders (ASD). The presence of such an imbalance may potentially define a therapeutic target for the treatment of cognitive disabilities related to this pathology. Consequently, the study of monogenic disorders related to autism, such as neurofibromatosis type 1 (NF1), represents a promising approach to isolate mechanisms underlying ASD-related cognitive disabilities. However, the NF1 mouse model showed increased γ-aminobutyric acid (GABA) neurotransmission, whereas the human disease showed reduced cortical GABA levels. It is therefore important to clarify whether the E/I imbalance hypothesis holds true. We hypothesize that E/I may depend on distinct pre- and postsynaptic push-pull mechanisms that might be are region-dependent.

Methods

In current study, we assessed two critical components of E/I regulation: the concentration of neurotransmitters and levels of GABA(A) receptors. Measurements were performed across the hippocampi, striatum, and prefrontal cortices by combined in vivo magnetic resonance spectroscopy (MRS) and molecular approaches in this ASD-related animal model, the Nf1^+/− mouse.

Results

Cortical and striatal GABA/glutamate ratios were increased. At the postsynaptic level, very high receptor GABA(A) receptor expression was found in hippocampus, disproportionately to the small reduction in GABA levels. Gabaergic tone (either by receptor levels change or GABA/glutamate ratios) seemed therefore to be enhanced in all regions, although by a different mechanism.

Conclusions

Our data provides support for the hypothesis of E/I imbalance in NF1 while showing that pre- and postsynaptic changes are region-specific. All these findings are consistent with our previous physiological evidence of increased inhibitory tone. Such heterogeneity suggests that therapeutic approaches to address neurochemical imbalance in ASD may need to focus on targets where convergent physiological mechanisms can be found.

Bifidobacterium in the gut microbiota confer resilience to chronic social defeat stress in mice

Accumulating evidence suggests that abnormalities in the composition of the gut microbiota may play a role in the pathogenesis of depression. Although approximately 30% mice are resilient to chronic social defeat stress (CSDS), the role of gut microbiota in this stress resilience is unknown. In this study, male C57BL/6 mice were exposed to a different CD1 aggressor mouse for 10 min on 10 consecutive days. A social interaction test was applied to distinguish between resilient and susceptible mice. Using 16S rRNA analysis, we examined the composition of gut microbiota in feces from control, resilient, and susceptible mice. The marked appearance of Bifidobacterium was detected in the resilient mice, whereas in the control and susceptible mice, Bifidobacterium were below the detection limit. Oral intake of Bifidobacterium significantly increased the number of resilient mice after CSDS compared with vehicle-treated mice. These findings suggest that Bifidobacterium may confer resilience to CSDS. Therefore, supplementation of Bifidobacterium may prevent the onset of depression from stress in humans. In addition, supplementation of Bifidobacterium may prevent or minimize relapse from remission induced by inflammation and/or stress in depressed patients.

Metabolomics identifies perturbations in amino acid metabolism in the prefrontal cortex of the learned helplessness rat model of depression

Major depressive disorder is a serious psychiatric condition associated with high rates of suicide and is a leading cause of health burden worldwide. However, the underlying molecular mechanisms of major depression are still essentially unclear. In our study, a non-targeted gas chromatography-mass spectrometry-based metabolomics approach was used to investigate metabolic changes in the prefrontal cortex of the learned helplessness (LH) rat model of depression. Body-weight measurements and behavioral tests including the active escape test, sucrose preference test, forced swimming test, elevated plus-maze and open field test were used to assess changes in the behavioral spectrum after inescapable footshock stress. Rats in the stress group exhibited significant learned helpless and depression-like behaviors, while without any significant change in anxiety-like behaviors. Using multivariate and univariate statistical analysis, a total of 18 differential metabolites were identified after the footshock stress protocol. Ingenuity Pathways Analysis and MetaboAnalyst were applied for predicted pathways and biological functions analysis. "Amino Acid Metabolism, Molecule Transport, Small Molecule Biochemistry" was the most significantly altered network in the LH model. Amino acid metabolism, particularly glutamate metabolism, cysteine and methionine metabolism, arginine and proline metabolism, was significantly perturbed in the prefrontal cortex of LH rats.

Alterations in brain-derived neurotrophic factor (BDNF) and its precursor proBDNF in the brain regions of a learned helplessness rat model and the antidepressant effects of a TrkB agonist and antagonist

Role of brain-derived neurotrophic factor (BDNF)-TrkB signaling in a learned helplessness (LH) model of depression was investigated. LH rats showed a reduction of BDNF in the medial prefrontal cortex (mPFC), CA3, and dentate gyrus (DG) of the hippocampus, whereas LH rats showed an increase in BDNF in the nucleus accumbens (NAc). Furthermore, levels of proBDNF, a BDNF precursor, were higher in the mPFC, but lower in the NAc, of LH rats. A single bilateral infusion of a TrkB agonist 7,8-DHF, but not a TrkB antagonist ANA-12, into the infralimbic (IL) of mPFC, DG, and CA3, but not the prelimbic (PrL) of mPFC, exerted antidepressant effects in LH rats. In contrast, a single bilateral infusion of ANA-12, but not 7,8-DHF, into the core and shell of NAc exerted antidepressant-like effects in LH rats, with more potent effects observed for the NAc core than for NAc shell. Interestingly, a single administration of 7,8-DHF (10mg/kg, i.p.) significantly improved a decreased phosphorylation of TrkB in the mPFC, CA3, and DG of LH rats. Additionally, ANA-12 (0.5mg/kg, i.p.) significantly improved an increased phosphorylation of TrkB in the NAc of LH rats. In conclusion, these results suggest that LH causes depression-like behavior by altering BDNF in the brain regions, and that proBDNF-BDNF processing and transport may be altered in the mPFC-NAc circuit of LH rats. Therefore, TrkB agonists might exert antidepressant effects by stimulating TrkB in the IL, CA3, and DG, while TrkB antagonists might exert antidepressant effects by blocking TrkB in the NAc.

Peripheral interleukin-6 promotes resilience versus susceptibility to inescapable electric stress

OBJECTIVE

Accumulating evidences suggest that pro-inflammatory cytokines such as interleukin-6 (IL-6) play a role in the pathophysiology of depression. In the learned helplessness (LH) paradigm, ~35% rats are resilient to inescapable stress.

METHODS

Levels of IL-6 in the serum and medial prefrontal cortex (mPFC) of LH rats (susceptible) and non-LH rats (resilience) were measured using enzyme-linked immunosorbent assay and western blot analysis, respectively.

RESULTS

Serum levels of IL-6 in the LH rats were significantly higher than those of control and non-LH rats. In contrast, tissue levels of IL-6 in the mPFC were not different among three groups.

CONCLUSION

The results suggest that peripheral IL-6 may contribute to resilience versus susceptibility to inescapable stress.

R-ketamine: a rapid-onset and sustained antidepressant without psychotomimetic side effects

Although the efficacy of racemate ketamine, a rapid onset and sustained antidepressant, for patients with treatment-resistant depression was a serendipitous finding, clinical use of ketamine is limited, due to psychotomimetic side effects and abuse liability. Behavioral and side-effect evaluation tests were applied to compare the two stereoisomers of ketamine. To elucidate their potential therapeutic mechanisms, we examined the effects of these stereoisomers on brain-derived neurotrophic factor (BDNF)-TrkB signaling, and synaptogenesis in selected brain regions. In the social defeat stress and learned helplessness models of depression, R-ketamine showed a greater potency and longer-lasting antidepressant effect than S-ketamine (esketamine). Furthermore, R-ketamine induced a more potent beneficial effect on decreased dendritic spine density, BDNF-TrkB signaling and synaptogenesis in the prefrontal cortex (PFC), CA3 and dentate gyrus (DG) of the hippocampus from depressed mice compared with S-ketamine. However, neither stereoisomer affected these alterations in the nucleus accumbens of depressed mice. In behavioral tests for side effects, S-ketamine, but not R-ketamine, precipitated behavioral abnormalities, such as hyperlocomotion, prepulse inhibition deficits and rewarding effects. In addition, a single dose of S-ketamine, but not R-ketamine, caused a loss of parvalbumin (PV)-positive cells in the prelimbic region of the medial PFC and DG. These findings suggest that, unlike S-ketamine, R-ketamine can elicit a sustained antidepressant effect, mediated by increased BDNF-TrkB signaling and synaptogenesis in the PFC, DG and CA3. R-ketamine appears to be a potent, long-lasting and safe antidepressant, relative to S-ketamine, as R-ketamine appears to be free of psychotomimetic side effects and abuse liability.

Regional differences in brain-derived neurotrophic factor levels and dendritic spine density confer resilience to inescapable stress

BACKGROUND

In the learned helplessness (LH) paradigm, approximately 35% of rats are resilient to inescapable stress.

METHODS

The roles of brain-derived neurotrophic factor (BDNF) and dendritic spine density in the brain regions of LH (susceptible) and non-LH rats (resilient) were examined. Western blot analysis and Golgi staining were performed.

RESULTS

BDNF levels in the medial prefrontal cortex, CA3, and dentate gyrus (DG) were significantly lower in the LH group than in the control and non-LH groups, whereas BDNF levels in the nucleus accumbens (NAc) in the LH group but not the non-LH group were significantly higher than those in the control group. Furthermore, spine density in the prelimbic cortex, CA3, and DG was significantly lower in the LH group than in the control and non-LH groups, although spine density in the NAc was significantly higher in the LH group than in the control and non-LH groups.

CONCLUSIONS

The results suggest that regional differences in BDNF levels and spine density in rat brain may contribute to resilience to inescapable stress.