Quetiapine effect on depressive-like behaviors, oxidative balance, and inflammation in serum of rats submitted to chronic stress

Major depressive disorder (MDD) etiology is still not completely understood, and many individuals resist the traditional treatments. Chronic exposure to stressful events can contribute to development and progression and be involved in biological changes underlying MDD. Among the biological mechanisms involved, inflammatory changes and oxidative balance are associated with MDD pathophysiology. Quetiapine, a second-generation antipsychotic, induces a better therapeutic response in individuals refractory to traditional treatments. The main objectives of this research were as follows: to evaluate the effect of chronic mild stress (CMS) on depressive-like behaviors, oxidative stress, and inflammation in adult rats; to evaluate the possible antidepressant, antioxidant, and anti-inflammatory effects of quetiapine. The animals were submitted to CMS protocols. At the end of the CMS, the animals were submitted to a chronic treatment for 14 days with the following drugs: quetiapine (20 mg/kg), imipramine (30 mg/kg), and escitalopram (10 mg/kg). At the end of the treatments, the animals were evaluated in the open field tests, anhedonia (splash test), and forced swimming. The animals were euthanized after the behavioral tests, and serum samples were collected. Myeloperoxidase (MPO) activity and interleukin-6 (IL-6) levels were analyzed. CMS induced an increase in depressive-like behaviors, and quetiapine significantly reduced these behaviors. MPO activity and IL-6 levels increased in the serum of animals submitted to CMS. Quetiapine significantly reduced MPO activity and IL-6 levels. These results corroborate other evidence, indicating that chronic stress is a relevant phenomenon in the etiology of depression and suggesting that quetiapine induces an antidepressant effect because it reduces oxidative and inflammatory mechanisms.


Introduction
Mental disorders are a global health problem (Almeida et al. 2012). The World Health Organization (WHO) estimates that about 450 million people suffer from some psychopathology. Mood disorders are the most prevalent, and major depressive disorder (MDD) is the most common psychiatric disorder (Rumble et al. 2015). It is estimated that MDD affects more than 264 million people worldwide, contributing to the global burden of disease and the leading cause of suicide (WHO 2020). The number of cases rises exponentially, and globally, depression increased by 18. 4% between 20054% between and 20154% between (WHO 2017. Chronic mild stress (CMS) is widely cited in the literature as one of the main predictors of developing a major depressive episode (Alexopoulos 2019;Kavoor 2020). The pathophysiology of MDD involves changes in the oxidative and inflammatory pathways (Ignácio et al. 2014;Miller and Raison 2016). Interleukin 6 (IL-6) and myeloperoxidase (MPO) are some of the molecules involved in inflammatory and oxidant processes, whose expression is increased in depressive individuals. MPO activity changes when interventions with quetiapine are performed, which may be chronic, with a reduction in activity recorded in the hippocampus, prefrontal cortex (PFC), and amygdala, or acutely, with records of reduction in the amygdala (Ignácio et al. 2017a). Analyzing the levels of interleukins in mice with rheumatoid arthritis, the anti-inflammatory action of quetiapine was also identified (Pan et al. 2018).
Studies with humans highlight that the higher expression of IL-6, MPO, and other inflammatory molecules directly influences the lower response to treatment with antidepressants (Miller and Raison 2016;Lindqvist et al. 2017). Recurrent depressive disorder was associated with increased mRNA expression for MPO (Gałecki et al. 2010). In this way, anti-inflammatory strategies can reduce depressive conditions (Kopschina Feltes et al. 2017).
Although the pathophysiology of depression is not fully understood, it is known that the action of free radicals and reactive oxygen and nitrogen species occurs. Current studies indicate that measuring oxidative products is essential to assess the efficacy and possible course of treatment for depression (Vaváková et al. 2015).
Chronic inflammation in the brain of depressive individuals is related to oxidative stress, as the imbalance of antiand pro-inflammatory substances directly interferes with the homeostasis of associated substances and systems, such as antioxidant levels and reactive oxygen and nitrogen species, culminating in a vicious cycle that contributes to the pathophysiology of MDD (Leonard 2018).
Studies with rodents highlight that second-generation antipsychotics induce antioxidant effects. The use of quetiapine, in particular, reduces the action of MPO and lipid peroxidation (Xuan et al. 2015;Ignácio et al. 2017aIgnácio et al. , 2018. In a previous study by our group, it was observed that quetiapine reduced MPO activity in the hippocampus, PFC, and amygdala (Ignácio et al. 2017a). However, the study did not use a stress protocol or assess peripheral blood MPO levels. The drug also induces the decrease of interleukins participating in the activation process of the hypothalamic-pituitary-adrenal (HPA) axis and indoleamine-2,3-dioxygenase (IDO) activity (Kao et al. 2016). In a study based on an animal schizophrenia protocol, researchers observed that chronic treatment with quetiapine reduced increased plasma levels of IL-6, among other pro-inflammatory cytokines . IL-6 is also altered by quetiapine, and rodents show a sharp drop in the labeling of interleukins after chronic treatment. These results indicate that the antipsychotic can effectively block or improve the action of microglia and their neuronal inflammatory action (Shao et al. 2015). On the other hand, a systematic review study observed evidence of anti-inflammatory function, mainly in preclinical studies, but suggests that there should be more investigations that allow a better understanding of the relevance of quetiapine in the involvement of inflammation in psychiatric disorders (Ferrari et al. 2022).
Since inflammation and oxidative stress strongly contribute to the pathophysiology of MDD and quetiapine is an atypical antipsychotic that has been shown to control these changes in other clinical conditions, it is hypothesized that quetiapine contributes positively to the control of depression mechanisms. Considering also the need for more studies that can show the role of quetiapine in oxidative balance and inflammation, in parallel with its potential antidepressant effect, this study investigated its effect on depressive-like behaviors, MPO activity, and IL-6 levels serum of adult rats submitted to CMS.

Animals
Male Wistar rats, 50 days old, were used, which were placed in 5 animals per box, with internal measurements of 41 × 34 × 16 cm, complying with minimum space standards per animal, according to the National Research Council (US) Committee for the Update of the Guide for the Care and Use of Laboratory Animals, 8th edition (2011). The environmental conditions of housing were as follows: 12-h light/dark cycle, a temperature of 23 ± 1 °C, food, and water ad libitum (except on the day when the stressor used was a deprivation of water or food). All the behavior assessments were performed during the morning period. The animals were treated according to each treatment group and placed in a separate and illuminated room according to the light cycle. The experiments were carried out in an appropriate room for each of the protocols, during the light cycle, in a room with lighting under the same conditions as the vivarium lighting. The experimental design is represented in Fig. 1.
All experimental procedures involving the use of animals were performed following the ethical principles governed by the Brazilian College of Animal Experimentation (COBEA) and after approval by the Federal University of the Fronteira Sul (UFFS) Ethics Committee on the Use of Animals (CEUA-UFFS), protocol number 23205.004257/2017-96.

Chronic mild stress (CMS)
The stress protocol followed the methodology previously described by Gamaro et al. (2003) with some adaptations (Garcia et al. 2009). The CMS was performed throughout the time of 40 days. For each day, a different stressor was used in a randomized manner. The following stressors were used: 24-h food deprivation, 24-h water deprivation, 1-3-h restraint, 1.5-2-h restraint at 4 °C, strobe light exposure for 120-210 min, and isolation (2-3 days).

Pharmacological treatment
For pharmacological treatment, quetiapine (20 mg/kg), imipramine (30 mg/kg), and escitalopram (10 mg/kg) were used. The choice of positive controls imipramine and escitalopram considered that quetiapine has a vast repertoire of mechanisms of action in monoaminergic transmission (Ignácio et al. 2018). Imipramine is a tricyclic antidepressant, which increases the availability of monoamines (noradrenaline, serotonin, and dopamine) (Zhou et al. 2007), while escitalopram is a selective serotonin reuptake inhibitor (SSRI), increasing serotonergic function (Aronson and Delgado 2004).
The dose of quetiapine was chosen after a study with a previously published dose curve (Ignácio et al. 2015). In this study, 20 mg/kg was the lowest dose evaluated and did not exert a sedative effect. The highest doses induced some sedative effects in the animals. The dose of escitalopram and imipramine was chosen, observing previous studies in our group and the scientific literature (Seo et al. 2017;Ignácio et al. 2017aIgnácio et al. , 2017b. The drugs were administered intraperitoneally once a day for 14 days. The animals were divided into 05 groups with 12 animals each, CMS + quetiapine (20 mg/kg), CMS + imipramine (30 mg/kg), CMS + escitalopram (10 mg/kg), CMS + saline, and saline control. Treatments started 1 day after the last day of the CMS protocol.

Behavior assessment
The locomotor activity of the animals was evaluated through the open field test, a 40 × 60 cm box surrounded by three wooden walls, a glass front wall, and a floor divided into 9 (nine) equal rectangles by black lines. During 5 min, it was counted how many times the animal crossed the lines and performed rearing.
The anhedonia test (splash test) was conducted in the same open field where the locomotor activity test occurred. The grooming time of 5 min was evaluated after spraying the animals' back with a 10% sucrose solution (Réus et al. 2013). The anhedonic behavior evaluated in the splash test considers that CMS reduces the grooming motivational behavior from a sucrose solution on the animal's back. The viscosity of the sucrose solution induces the animal to grooming behavior. The sweet taste of sucrose causes the animal to continue grooming as a motivational behavior. The splash test was pharmacologically validated, and the grooming behavior is considered to be reduced in parallel with some depressive-like behavior (Isingrini et al. 2010).
For the swimming test, the animals were placed individually in a cylinder with water at a temperature of 23 °C filled with enough water that the animal cannot rest its paws on the bottom. On the 13th day of pharmacological treatment, the rats were forced to swim for 15 min (pre-test session). On the 14th day, the last day of treatment, and 24 h after the 15-min pre-test, each animal was again forced to swim for 5 min. Mobility and immobility time was evaluated (Porsolt 2000). After the swimming test, the animals were immediately euthanized by decapitation, and blood samples were collected.
The data analysis was performed directly during the experiment by a human observer blinded to the treatment of the animals.

Biochemical analysis
Blood was collected and centrifuged to collect serum frozen at −80 °C. MPO analysis was performed in duplicate and was expressed in quinoneimine, with results using the following reagents: aminoantipyrine (AAP, 0.1016 g), hydrogen peroxide (H2O2, 17 ul), and phenol (0.375 g), incubated at 37 °C for 30 min. Read at 492 nm on the spectrophotometer. The correction factor multiplied the absorbance in duplicate. Data were expressed in quinoneimine µM (Kilciksiz et al. 2008).
The cytokine (IL-6) concentration was determined by the ELISA kit (Sigma-Aldrich, Brazil). All samples were in duplicate. The plate was sensitized the night before and incubated at 4 °C with a capture antibody diluted in phosphate-buffered saline (PBS). Afterward, the plate was washed with PBS 0.05% Tween 20% (Sigma, St. Louis, MO, USA). Blocking occurred with 1% bovine serum albumin and incubated for 1 h at room temperature before being washed with PBS 0.05% and 20% Tween. The detection antibody (concentration supplied by the TURER manufacturer) was diluted in PBS. The plate was incubated for 2 h at room temperature. After washing the plate, the streptavidin enzyme (DuoSet R&D Systems, Minneapolis, MN, USA) was added and incubated for 30 min. Finally, phenylenediamine (Sigma, St. Louis, MO, USA) was added to each well, and the reaction was carried out in the dark for 15 min. The reaction was stopped by stopping 1 M H2SO4 solution for each well. The absorbance was read on a plate reader at wavelengths of 492 nm (Emax, Molecular Devices, Minneapolis, MN, USA).

Statistical analysis
Results were evaluated by one-way analysis of variance (ANOVA), followed by the Tukey post hoc test. Differences between groups were considered significant for p < 0.05. The Statistica 7 program was used.

Effect of CMS and drugs on open field activity
Both CMS and pharmacological treatments did not induce significant changes in locomotor activity, evaluated through the number of crossings and surveys in the open field test (Fig. 2).

Effect of CMS and drugs on behavioral parameters evaluated in the forced swimming test
One-way ANOVA analysis showed a significant interaction between groups, with a main effect of the CMS on depressive-like behavior (F = 7.80; p < 0.0001). Tukey's test revealed that CMS significantly increased the immobility time in the forced swimming test (p < 0.001), and both quetiapine (p < 0,001) and treatments with the antidepressants imipramine (p < 0.001) and escitalopram (p < 0,001) reversed the effect of CMS (Fig. 3).

Effect of CMS and drugs on behavioral parameters evaluated in the anhedonia test
Likewise, one-way ANOVA analysis showed a significant interaction between groups, with a main effect of the CMS on anhedonic-like behavior (F = 6.39; p < 0.001) during the anhedonia test (splash test). Tukey's test revealed that CMS significantly reduced the grooming time (p < 0.001). Quetiapine (p < 0.001) and escitalopram (p < 0.01) treatments reversed the effect of CMS on the anhedonia test. In the imipramine-treated group, there was an increase in grooming time compared with salinetreated CMS, but the result did not have statistical significance (Fig. 4).

Myeloperoxidase enzyme-MPO activity
The results of the MPO enzyme activity are shown in Fig. 5. One-way ANOVA analysis showed a significant interaction between groups, with a main effect of the CMS on MPO activity (F = 4,91; p < 0.01). Tukey's test revealed that CMS significantly increased the MPO activity (p < 0.05). Treatments with imipramine (30 mg/kg), quetiapine (20 mg/kg), and escitalopram (10 mg/kg) significantly reduced the increase of MPO activity in stressed animals (p < 0.05).

Interleukin 6-IL-6 level
The results of the IL-6 are shown in Fig. 6. One-way ANOVA showed a significant interaction between groups, with a main effect of the CMS on serum IL-6 (F = 9.38; p < 0001). Tukey's test revealed that CMS significantly increased serum IL-6 (p < 0,05). Quetiapine-treated animals showed a significant reduction in IL-6 compared to stressed and saline-treated animals (p < 0.05).

Discussion
In the forced swimming test, the CMS significantly increased the immobility time of the animals in the control group. The groups treated with quetiapine, imipramine, and escitalopram had this behavior reversed. Immobility Effects of Quetiapine (20 mg/kg), imipramine (30 mg/ kg), and escitalopram (10 mg/ kg) on the motor and exploratory activity of animals tested in the open field (N = 12 each group). Imipramine (30 mg/kg) and escitalopram (10 mg/kg) were used as positive controls. Mean (± standard error of the mean) of the number of crossovers and lifts Fig. 3 Mobility assessment in the forced swimming test. Effect of CMS and chronic treatments with quetiapine (20 mg/kg), imipramine (30 mg/kg), and escitalopram (10 mg/kg) on mobility parameters in a forced swimming test (N = 12 each group). Imipramine (30 mg/kg) and escitalopram (10 mg/kg) were used as positive controls. Mean (± standard error of the mean) of time. *Different from the control group, p < 0.05; #different from the stress-saline group, p < 0.05 Fig. 4 Anhedonia test. Effects of CMS and chronic treatments with quetiapine (20 mg/ kg), imipramine (30 mg/kg) and escitalopram (10 mg/ kg) on anhedonia test parameters (N = 12 each group). Imipramine (30 mg/kg) and escitalopram (10 mg/kg) were used as positive controls. Mean (± standard error of the mean) of licking time. *Different from the control group, p < 0.05; #different from the stress-saline group, p < 0.05 . Imipramine (30 mg/kg) and escitalopram (10 mg/kg) were used as positive controls. Mean (± standard error of the mean) of enzyme activity. *Different from the control group, p < 0.05; #different from the stress-saline group, p < 0.05

Fig. 6
Interleukin 6 (IL-6) level. Effects of quetiapine (20 mg/ kg), imipramine (30 mg/kg), and escitalopram (10 mg/kg) on interleukin 6 (IL-6) levels (N = 5 each group). Imipramine (30 mg/kg) and escitalopram (10 mg/kg) were used as a positive control. Mean (± standard error of mean) of IL-6 levels. *Different from the control group, p < 0.05; #different from the Stress-Saline group, p < 0.05 1 3 is indicative of depressive-like behavior. Similar results are observed in studies evaluating quetiapine and its active metabolite, norquetiapine. Both significantly reduced the animals' immobility time (Cross et al. 2016). When administered in these situations, quetiapine reduces immobility time and increases climbing time (Ignácio et al. 2017a;Fernandes and Gupta 2019). The increase in quetiapineinduced climbing time appears to be related to an increase in noradrenergic activity, which may be related to some of the quetiapine's mechanisms of action (Ignácio et al. 2018). In this study, there was a non-significant reduction in the climbing time of stressed and untreated animals, and quetiapine promoted a non-significant increase. However, the tendency of quetiapine to increase climbing time in this study suggests a role of norepinephrine, mediated by quetiapine, in the antidepressant-like effect.
The pharmacological mechanism of quetiapine is not yet fully understood. However, it is observed that it acts in several ways, including the blockade of the noradrenaline transporter, increasing the availability of noradrenaline in the synaptic cleft, and its postsynaptic action. Furthermore, both quetiapine and its norquetiapine metabolite exert an effect on α 2 adrenergic autoreceptors in cell bodies by blocking them and, in this way, it increases neuronal noradrenergic transmission (Chernoloz et al. 2012;Ignácio et al. 2018).
The control animals, without the use of drugs, showed reduced interest in sucrose, while the animals treated with the investigated drugs showed increased interest in sucrose. The reduction in the animals' grooming time and their interest in sucrose water, observed in the research, are typically associated with anhedonia. This behavior is related to structural dysfunctions of the PFC, hippocampus, nucleus accumbens, and amygdala, components of the limbic system involved in MDD (Snyder et al. 2011;Fernandes and Gupta 2019).
CMS-induced anhedonia was reversed by quetiapine and escitalopram. Although not statistically significant, quetiapine and escitalopram induced a longer grooming time when compared to the control group. The biological mechanisms related to anhedonia and reward behaviors still need to be better understood. Some studies suggest that different mechanisms can be triggered in brain tissues according to the characteristics of compounds with therapeutic potential (Burstein et al. 2017). Some compounds may be triggering more mechanisms related to anhedonia and depression. Quetiapine has a broad spectrum of action in the CNS, acting on several neurotransmission circuits and systems, which may account for its potential as a therapeutic strategy in TRD (Ignácio et al. 2018). Quetiapine's action on reward circuits and other related regions and mechanisms may be associated with its robust effect in reducing anhedonic behavior. However, this study did not evaluate brain structures or mechanisms that may be involved with oxidative stress and inflammation markers, as evaluated in this study. Thus, these notes are speculative and require further studies.
Due to acute and chronic stress, functional and inflammatory changes are already observed in animals with depressive behavior. Furthermore, some researchers found that IL-1β and IL-6 increased in mice's periphery, hippocampus, and prefrontal cortex under chronic stress ). These results indicate that an increase in peripheral inflammation may be related to a rise in neuroinflammation. However, when stressed animals receive treatment for serotonergic regulation, interest in sucrose increases, immobile time decreases, and neuroinflammation is reduced (Fernandes and Gupta 2019). Similar to scientific literature studies, these research results are possibly associated with changes in the limbic system.
Considering the relevant role of serotonin in the antidepressant and anti-inflammatory effects is pertinent to highlight that quetiapine can increase the availability and action of serotonin in circuits involving quetiapine's antagonist action on 5-HT 2A receptors in the PFC, culminating in a reduction in glutamatergic function on glutamatergic N-methyl-D-aspartate (NMDA) receptors in the dorsal raphe nucleus (Coplan et al. 2014;Ignácio et al. 2018).
Regarding locomotor activity, both the CMS and the pharmacological treatments did not induce changes in the mobility parameters of the open field. This result indicates that the drugs did not induce a significant sedative or stimulant effect that could interfere with animal behaviors.
The results about behavior may be associated with the antiinflammatory effects performed by quetiapine. Similar results were seen with sinomenine, an extract of the Chinese medicinal herb with anti-inflammatory, immunosuppressive properties. Stressed animals treated with sinomenine showed reduced immobility time in the forced swim test, increased availability of norepinephrine, and decreased IL-6 labeling .
In addition to behavior, oxidative and immune changes are critical phenomena involved in the pathogenesis of depression. The imbalance between oxidative mediators and antioxidant defenses damages proteins, fatty acids, and deoxyribonucleic acid (DNA) (Garabadu et al. 2015). The brain is exceptionally vulnerable to oxidative stress by demanding high metabolic rates. When rodents are exposed to CMS, there is an increase in ROS and protein peroxidation in regions of the limbic system. At the same time, a reduction in antioxidant activities is also observed (Che et al. 2015). Quetiapine reduces CMS-induced lipid peroxidation in limbic regions (Han et al. 2015;Xuan et al. 2015).
Depressed individuals have an increased inflammatory state with greater expression of MPO, an enzyme in neutrophils, monocytes, and microglial cells, which induces the production of pro-inflammatory cytokines (Vaváková et al. 2015).
MPO is a heme enzyme that catalyzes the formation of reactive oxygen species, and its action occurs in the presence of H 2 O 2 . Hypochlorous acid (HClO) is the metabolite from this reaction, which can react with reactive oxygen species (ROS) and produce free radicals. Oxidative molecules precede inflammatory disorders, making it possible to reflect the activation of neutrophils and lymphocytes (Young et al. 1989).
MPO activity was evaluated in different groups. In the control group, the MPO activity was elevated, and treatment with quetiapine, imipramine, and escitalopram significantly reduced the activity. The high activity of MPO is related to an increase in other mediators of oxidative stress (Ignácio et al. 2017a). There is a positive relationship between the promoter region of the gene that encodes MPO and recurrent depression in humans, leading to the hypothesis that the greater the expression of MPO, the more likely refractoriness to treatment (Talarowska et al. 2015(Talarowska et al. , 2020. There still needs to be recorded in the literature that addresses this marking in serum. Therefore, the reversal of MPO activity identified in the serum of stressed animals is an essential protective function of the oxidative balance promoted by quetiapine and escitalopram. It is possible that the reduction in MPO activity could be related to a decrease in activity in brain regions, as observed in other studies (Ignácio et al. 2017a).
Inflammatory activity is associated with the development of MDD maintenance. Microglia are identified as protagonists in the recruitment of inflammatory cytokines especially IL-6, a mediator secreted by T cells and macrophages and a reliable marker of inflammatory action (Gold et al. 2015;Ignácio et al. 2017a).
IL-6 was significantly elevated in the serum of stressed animals. Only animals treated with quetiapine presented reduced IL-6, suggesting an anti-inflammatory function of this antipsychotic. The anti-inflammatory potential of quetiapine in arthritis (Kim et al. 2012) and neuroinflammation involved in psychiatric diseases is already reported (Shao et al. 2015).
Human models identify that depressed elderly have increased IL-6 at the peripheral level, as well as IL-1β, tumor necrosis factor-α (TNF-α), and C-reactive protein (CRP) (Ng et al. 2018). Inflammatory changes are also seen in young people under 18 years of age. Strong evidence points to two-way associations between depression and a pro-inflammatory state (Colasanto et al. 2020).
Longitudinal analysis in individuals with MDD also identified that higher levels of IL-6 predicted subsequent chronic course, especially in women. From this perspective, IL-6 can be a potential marker to identify the inflammatory profile of each patient, providing personalized medicine approaches for MDD treatment (Lamers et al. 2019).
In this research, while escitalopram showed a downward trend, imipramine increased IL-6 levels. The pro-inflammatory effect of imipramine is a surprising result, considering that other stress protocols have shown an anti-inflammatory effect (Ramirez and Sheridan 2016). Thus, further studies are needed in order to elucidate the role of imipramine in CMS-induced inflammation.
The anti-inflammatory action of quetiapine is reported in other pathologies that target inflammatory pathways. Schizophrenic subjects experienced a drop in baseline IL-6 level, and symptom reduction was more potent when treated with the antipsychotic. These results indicate a promising therapy route for patients with MDD resistant to traditional antidepressants. The quetiapine effects on animal behavior evidence their antidepressant potential (Feng et al. 2020).
It is essential to highlight that the increase in MPO is involved in the increase in peripheral and central production of inflammatory cytokines (Gałecki et al. 2010), and quetiapine significantly reduced the increase in MPO activity in regions of the limbic system of chronically stressed animals (Ignácio et al. 2017a). In this sense, the effect of quetiapine on the reduction of MPO may be related, at least in part, to the decrease in IL-6.
On the other hand, some clinical studies bring conflicting results about the role of quetiapine in inflammation. Studies by Halaris et al. did not show an effect of quetiapine or escitalopram on baseline IL-6 levels, although they did show a correlation between IL-6 levels and depression severity (Halaris et al. 2012). Another study even observed a proinflammatory effect, despite the antidepressant effect in bipolar II disorder (Fiedorowicz et al. 2019). From another angle, a study that evaluated the relationship between baseline levels of IL-6 in individuals with schizophrenia observed that an increase in baseline levels predicted a more significant therapeutic response to chronic treatment with quetiapine (Feng et al. 2020). The results in the literature highlight the importance of further studies, considering different clinical characteristics and animal protocols, as pointed out in recent studies (Ferrari et al. 2022).
In this study, both quetiapine and the antidepressants imipramine and escitalopram exerted an antidepressant-like effect. Furthermore, except for the effect of imipramine on IL-6 levels, all drugs reduced CMS-induced oxidative stress and inflammation. The results of this study corroborate evidence from the scientific literature, as the three drugs, in addition to clinical efficacy (Sanchez et al. 2014;Vos et al. 2021;Ignácio et al. 2018;Nuñez et al. 2022), exert an antidepressant-like effect in tests behavioral after chronic stress (Orsetti et al. 2007;Ding et al. 2016;Seo et al. 2019).
The scientific literature points to quetiapine as a possible therapeutic strategy in treatment-resistant depression (TRD), both in the clinic (Cantù et al. 2021;Nuñez et al. 2022) and in animals subjected to chronic stress (Wang et al. 2013).
However, the differentiated effect of quetiapine was not observed in this study, as a protocol was not used to specifically study animals that did not show a therapeutic response to treatments with antidepressants. Thus, considering the potential of quetiapine as a therapeutic strategy for TRD, and varied mechanisms of action, which seem to extrapolate the known mechanisms of the antidepressants evaluated, further studies are essential to verify the mechanisms involved in oxidative balance, inflammation, and other biological processes in peripheral blood and brain regions.

Conclusions and final considerations
This research corroborates other studies, which provide evidence that chronic stress is involved in depressive behaviors and that increased oxidative stress and inflammation are associated. In parallel, this study showed an antidepressant-like and anti-inflammatory effect of quetiapine. The antioxidant and anti-inflammatory effect of quetiapine seems to be associated, at least in part, with the antidepressant-like effect.
The oxidative stress mediator MPO results and the inflammatory cytokine IL-6 highlight the drug's promising anti-inflammatory action at the neuronal and systemic levels often observed in psychiatric disorders. These results open possibilities for more efficient diagnoses, considering that inflammatory serum levels can help classify the pathology subtypes and identify which therapy best fits each case. Anti-inflammatory therapies appear to hold great promise for treating MDD and other psychiatric conditions, as seen in quetiapine.
In previous work, it was verified that quetiapine has a relevant, beneficial effect on the oxidative balance in brain structures of the limbic system (Ignácio et al. 2017a). However, the study did not investigate the effect of stress or assess the effect of quetiapine on peripheral blood. The present study evaluated a significant beneficial effect of quetiapine on MPO and IL-6 in animals submitted to CMS. However, this work still has limitations, considering that oxidative and inflammatory stress markers were not evaluated in the limbic structure of animals submitted to CMS. Thus, future research is needed to corroborate some results from the scientific literature on the possible beneficial role of quetiapine in reducing oxidative stress and neuroinflammation underlying the antidepressant-like effect in animals subjected to chronic stress. Moreover, the concentrations of neurotransmitters like norepinephrine, serotonin, or dopamine and their metabolites should be determined in future studies in the relevant brain regions of animals subjected to CMS and treated with quetiapine and other antidepressants.
Even with all the advances, more research is still needed in the area, considering that, even though the literature has demonstrated some biological mechanisms in which quetiapine seems to act, exerting an antidepressant effect, it is evident that some quetiapine action mechanisms still need to be studied and deserve to be better elucidated. The elucidation of the biological process involved in the antidepressant-like effect of quetiapine may contribute to the discovery of adjuvant strategies and better therapeutic responses.