Antidepressant-like effect of endogenous SO2 on depression caused by chronic unpredictable mild stress

Sulfur dioxide (SO2) is a toxic gas with harmful effects on various organs. However, recent studies have confirmed the protective effect of SO2 on ischemic heart disease, atherosclerosis, and lung infections. Therefore, the present study was designed to investigate the effect of endogenous SO2 on depression. The chronic unpredictable mild stress (CUMS) model was performed to cause depression. Depression-like behaviors in animals were determined using an open-field test, forced swimming test, and sucrose consumption. Animal spatial learning and memory were also assessed using the Morris water maze. Besides, the oxidative status of the hippocampus and serum corticosterone level were evaluated. A reduction in the tendency to consume sucrose, mobility, and curiosity, as well as learning and memory disorders were observed in CUMS animals. Depressed animals treated with SO2 revealed a significant improvement in behavioral and cognitive functions. SO2 also reduced neuronal damage and lipid peroxidation of the hippocampus and serum corticosterone level in the CUMS group. Various shreds of evidence support a mutual relationship between inflammation and depression; also, growing studies show the role of oxidative stress in the pathogenesis of mood-related disorders such as depression. This study indicated that increased hippocampal malondialdehyde (MDA) and serum corticosterone levels can be due to the existence of oxidative stress and possible activation of inflammatory processes. SO2 donors diminished MDA and corticosterone levels in depressed animals. According to the study results, SO2 may be able to reduce tissue damage and eventually behavioral disorders caused by depression by lowering oxidative stress and inflammation.


Introduction
Depression is a complex, multifactorial, and often chronic mental disorder and is considered one of the most widespread psychiatric disorders in the world. The World Health Organization (WHO) has reported an 11% prevalence of this disorder worldwide (Kendler and Gardner 2010). This disease disrupts the patient's life by causing behavioral and physical problems. Even though copious studies explain how depression develops, there is still no clear concept defining the causes and mechanisms of the onset and progression of depressive disorders (Sullivan et al. 2000). One of the main findings of depression is the reduction of monoamine and acetylcholine neurotransmitters in the central nervous system (CNS) (Seki et al. 2018;Dulawa and Janowsky 2019). So, a wide range of antidepressants aims to increase the level of these transmitters (Aricioglu et al. 2019). Regardless, the monoamine hypothesis is not enough to explain the pathogenesis of depression, and other studies are required to explore other possible mechanisms involved in depression.
Growing studies have indicated a mutual relationship between inflammation and depression. Clinical studies have also shown increased levels of proinflammatory cytokines in the blood of depressed individuals, which can be a reason for their resistance to antidepressant drugs (Strawbridge et al. 2015;Aricioglu et al. 2019). Studies have shown that exposure to chronic stress leads to psychological disorders such as anxiety and depression, which are associated with increased hypothalamic-pituitary-adrenal responses and systemic inflammation (Strawbridge et al. 2015;Shahidpour et al. 2021). As well as the influence of inflammatory factors in the pathogenesis of psycho-neurological disorders, the role of oxidative stress was also investigated and confirmed in numerous studies (Hassan et al. 2014). The presence and activity of oxidative stress in the central nervous system are predominant compared to other organs due to its high oxygen consumption and relatively limited antioxidant reserve capabilities. CNS also contains rich sources of unsaturated fatty acids which are favorite targets for oxidative stress (Andersen 2004). Furthermore, the decline of enzymatic and non-enzymatic components of antioxidants which leads to the imbalance of oxidants was observed in various neurological disorders such as depression, Alzheimer's, autism, and Parkinson's (Hassan et al. 2014). A direct association has been established between increased oxidative stress in peripheral leukocytes, cerebellum, hippocampus, and cortex with the occurrence of anxiety-like behaviors (Bouayed et al. 2007;Rammal et al. 2008). Thus, using agents with antioxidant and anti-inflammatory properties can be effective in improving the prognosis of depression.
In this study, sulfur dioxide (SO 2 ) donor salts, which increase endogenous SO 2 , were used to treat depression (Lu et al. 2012). SO 2 is a toxic gas and is known as one of the most dangerous air pollutants that can have destructive effects on body tissues, especially the respiratory, cardiovascular, and nervous systems (Rall 1974). The damaging role of SO 2 is well known, but new studies have looked at the lesser-known beneficial effects of SO 2 , which is produced endogenously in the body by the metabolism of the amino acids cysteine and homocysteine in the presence of the enzyme aspartate aminotransferase. Also, SO 2 is produced from oxidation of H 2 S Zhang et al. 2018). It is involved as a gas transmitter in the body's physiological processes. Several studies have shown the anti-inflammatory and antioxidant role of SO 2 in reducing ischemic heart and brain damage, atherosclerosis, hypertension, and pulmonary inflammation (Lu et al. 2012;Li et al. 2018;Yang et al. 2018;Zare Mehrjerdi et al. 2018).
Considering the antioxidant and anti-inflammatory effects of sulfur dioxide, this study aimed at scrutinizing the possible role of sulfur dioxide in reducing injuries following unpredictable chronic mild stress.
The animals were maintained during the experimental period, except stress days, in standard conditions of 12 h/12 h circadian rhythm, temperature of 22-25 °C and free access to water and food.
This study was approved by the Ethics Committee of Yazd Shahid Sadoughi University of Medical Sciences which is accordance with the Guide for the Care and Use of Laboratory Animals from the USA National Institutes of Health with ethical code: IR.SSU.MEDICINE.REC.1399.008.
To induce the CUMS model, animals randomly exposed to one of the following stressors for 35 days: food or water deprivation for 24 h; continuous lighting; placing the animal on a 45-degree slope for 2 h; placing the animal on the shaker for 20 min with one shake per s; keep the animal in a wet cage for 24 h; swim in cold water 25 °C for 5 min; and enclose the animal in the restrainer for 1 h (Koprdova et al. 2016;Gáll et al. 2020).
Depressive-like behaviors in animals were measured during the experiment. The animals anesthetized deeply with ketamine and xylocaine at the end of the experiment. Blood sample was taken from the heart of animals to determine blood corticosterone level. After deep anesthesia and cardiac perfusion, the brains of four animals from each group were completely isolated and placed in a fixative solution for histological examination. In other animals, after complete anesthesia, the hippocampus was carefully isolated from the surrounding tissues to evaluate the activity of antioxidants and the amount of MDA.

Sucrose preference test
The sucrose preference test is based on the natural preference of rodents for sweets and is one of the standard tests to assess depression in rats. After a period of water deprivation for 24 h, each rat had free access to two bottles for 1 h: one containing 1% sucrose solution and the other containing pure water. Each bottle was weighed before and after the animal had access to water. The ratio of sucrose preference was calculated as the percentage of sucrose consumption to pure water + sucrose consumption (Koprdova et al. 2016;Gáll et al. 2020).

Forced swimming test (FST)
This test is one of the most common tests to evaluate depressive-like behaviors. The animal was placed in a cylindrical swimming tank made of Plexiglas, 50 cm high and 25 cm thick, filled to 30 cm depth with 25 °C water. This test was performed on two consecutive days. On the first day which was pretest, the animal was left in the swimming pool for 15 min. Twenty-four hours later, after leaving the animal in the tank, the animal's behavior was observed for 6 min by a camera on the roof of the room. The duration of immobility in the last 4 min of the test was assessed. The immobility of the animal's limbs and its floating was considered as immobility behavior and this time was considered as immobility time (Koprdova et al. 2016).

Open field test (OFT)
Open field test is used for behavioral studies, motor deficits, anxiety, and depression in laboratory research. The open environment is a glass compartment with dimensions of 60 × 80 × 80 cm, divided into 25 equal squares and placed in the middle of a quiet room. The animal was placed in the experimental room 1 h before the experiment. On the first day of the test, in order to acquaint the animals with the device, they were placed separately in the open field device for 10 min. Twenty-four hours later, each rat was placed in the square in the center of the open field, and the camera monitored the animal's behavior for 10 min. The number of crossing lines was considered an indicator of spontaneous movement, and the number of standing on two legs was considered an indicator of exploratory movements (Koprdova et al. 2016;Gáll et al. 2020).

Morris water maze test
In this study, the Morris water maze was used to assess the spatial learning and memory of the animals. The maze includes a circular metal pool with a diameter of 150 cm and a height of 60 cm which is divided into four hypothetical quarters and was filled with water up to a height of 40 cm. A circular platform made of colorless plastic (10 cm in diameter and 27 cm high) was placed between the center and the wall in the southeast quarter (target quarter). The maze was placed in a quiet room and geometrical extra cues were installed in all four directions of the maze to identify the spatial position of the place by the animals. Animals in the first 4 days of the test were randomly released into the maze from four directions separately after a 5-min break between each trial. Time elapsed, and the distance traveled to reach the platform in each trial was tracked by a camera at the top of the maze and recorded by video tracking software.
After 24 h, the platform came out of the maze and again, four trials were performed for each animal, and the elapsed time and distance traveled in the target quarter were tracked and recorded by video tracking software (Zare ).

Biochemical factors of the hippocampus
After deep anesthesia, the brains of six animals from each group carefully removed and the hippocampus was separated from the surrounding tissues and immediately exposed to − 80 °C, at the right time, 1 cc of PBS solution added to 100 mg of hippocampal tissue and the sample was homogenized and then centrifuged at 6000 rpm for 10 min. The supernatant was separated from suspension and the activity of enzymes and the amount of MDA was measured based on the kits protocols.

Malondialdehyde (MDA) of the hippocampus
MDA is an indicator of lipid peroxidation. The basis of MDA testing is the binding of thiobarbituric acid (TBA) to it and the formation of a pigment. This study prepared eight standard solutions with a certain concentration to evaluate the MDA following the kit protocol (Zelbio, Como, Italy). After adding 50 μl of thiobarbituric acid solution to the standard vials and samples, 1 ml of chromogen solution was added. All vials were placed at boiling temperature for 1 h, after which, the samples were incubated at 0 °C for 30 min; then, the samples were centrifuged at 0 °C at 4000 rpm for 10 min. Finally, the samples' optical density (OD) was read at 535 nm and the concentration of MDA was calculated according to the standard diagram (Zare ).

Glutathione peroxidase (GPX) and catalase (CAT) enzymes activity
Glutathione peroxidase (GPX) activity assessment was performed following Zelbio's kit. Three series of microtubules were prepared to measure GPX. The first series consisted of tissue serum samples with reagents according to the kit's protocol, and the second series consisted of blanks containing odd-water with reagents, and the third series control samples containing only reagents. All microtubules were heated at 37 °C for 5 min. Then, by adding reagent 6 to the microtubules, the samples were centrifuged and poured into microplate wells. Finally, after adding reagent 7th, the OD of the samples at 412 wavelengths was read by an ELISA reader.
To measure the catalase (CAT) activity, hippocampus homogenized samples were first added to the microplate wells. According to the kit protocol, normal saline was considered blank. One hundred microliters of assay buffer and 10 μl of H 2 O 2 reagent were added to the wells and incubated at 37 °C for 1 min. Finally, after adding chromogen and diluent solution, the OD of the samples at 405 nm was read by an ELISA reader. The following formula was used to measure catalase activity (Zare . Catalase activity (U/ml) = (ODblank − ODsample) * 271 * (1.6 *sample volume).

Serum corticosterone
Corticosterone level was measured by an ELISA kit (Zelbio, Com, Italy), in which two monoclonal antibodies were used against two separate antigen sites on the corticosterone molecule. The procedure was that 20 μl of each standard serum sample (7 concentrations of l-240 nmol) was poured into separate wells, and 200 μl of the conjugated enzyme was added to all of them incubated at room temperature for 60 min. The wells were rinsed three times with buffer solution, and 100 μl of substrate solvent was added to all wells. Then, 50 μl of stop solution was added to all plate wells, and finally, the OD of the samples was read at 450 nm, and the amount of corticosterone was calculated according to the standard diagram.

Histological studies
From each group, four animals were perfused transcardially with 10% formalin solution after deep anesthesia. The animals' brains were carefully removed and placed in a fixative solution for 48 h. Paraffin embedding was performed with a tissue processor according to the following steps: dehydration with ascending alcohols, clarification with Xylen, and impregnation with paraffin. Following the molding of samples, the coronal 5-μ sections were obtained by microtome in sections at a distance of 2.7 mm from the Bregma. The sections were in three cross-sections at a distance of 25 μm (Zare ). The sections were transferred onto an albumin-coated glass slide to explore the cell density and necrosis in the CA1 region of the hippocampus after H&E staining. To evaluate cell density, the number of healthy cells in the CA1 region of the hippocampus was counted using image J software. Small, scalloped, and swollen neurons with small nuclei are considered as necrotic cells.

Statistical data analysis
The data obtained from this study were analyzed using GraphPad Prism version 8. Data on animal weight, sucrose consumption, and spatial learning were analyzed by two-way analysis of variance followed by the Bonferroni post hoc test, and the rest of the data were analyzed by one-way analysis of variance followed by the Tukey post hoc test. Differences with P ≤ 0.05 were considered as significant level.

Animal weight
The weight of the animals was measured every week. On days 21 and 35, a significant decrease in the weight of depressed animals was observed (Fig. 1, F (15, 180) = 1.186, P = 0.286). SO 2 donors did not affect the weight of depressed animals.

Sucrose preference test
Sucrose preference was similar between groups at the first day of this test (day 26), but the CUMS group appeared a marked reduction in the sucrose preference on days 27, 28, and 29 compared with the sham group (Fig. 2a). A significant increase in the average consumption of sucrose observed in the CUMS groups treated with the both dose of SO 2 on days 27, 29 (F (9, 9) = 1.545, P = 0.2637).

Forced swimming test
In the forced swimming test, the duration of immobility in the CUMS group was significantly increased compared to the sham group (Fig. 2b). In both CUMS groups treated with SO 2 donors, a significant decrease in the mean duration of immobility was observed (Fig. 2b).

Open field test
The results showed that CUMS has affected the movement indices so that during the 6 min of this test, the number of crossings from the center to the environment and vice versa (Fig. 2c) and the number of times standing on the legs (rearing) (Fig. 2d) in the CUMS group was significantly decreased compared to the sham group. The normal levels of these open-field parameters were retrieved in the CUMS groups treated with SO 2 (Fig. 2c, d).

Morris water waze test
To determine the effect of SO 2 on spatial learning and memory, rats were subjected to the Morris water maze task. The CUMS group exhibited decrease in learning progress as increase in distance to reach the platform on days 3 and 4 (Fig. 3a) and latency in reaching the platform on day 4 (Fig. 3b). The time and distance in reaching the platform decreased in the In CUMS groups treated with SO 2 donors (Fig. 3a, b). Factor F for traveled distance was (F (9, 90) = 3.519, P = 0.0009) and for escape latency was (F (9, 90) = 2.254, P = 0.0253).
Distance and time spent in the target quarter on the last day of the water maze test was compared to evaluate spatial memory, which showed a significant decrease in the CUMS group. SO 2 donors increased the distance and time of staying in the target quadrant, which is a sign of improvement in the memory of the animals in the treatment groups (Fig. 3c, d).

MDA and CAT and GPX activity in hippocampal tissue
The level of hippocampal MDA in CUMS group increased significantly compared to the sham group (Fig. 4a). In both CUMS groups receiving SO 2 donors, MDA levels was significantly decreased (Fig. 4a). As shown in Fig. 4b, c, no significant changes were observed in glutathione and catalase activity among groups.

Serum corticosterone levels
The results of serum corticosterone measurement in CUMS group indicate a significant increase in serum corticosterone compared to the sham group (Fig. 5). The first dose of SO 2 donors reduced corticosterone levels, but second dose was ineffective (Fig. 5).

Histological studies
In histological examination, a decrease in the number of healthy cells (Fig. 6a) and an increase in the number of necrotic cells (Fig. 6b) were observed in the CA 1 subregion of hippocampus in the CUMS animals. By treating depressed animals with the first dose of SO 2 donors, the number of necrotic cells decreased (Fig. 6b).

Discussion
Depression is one of the most common diseases in the world, leading many experts to investigate it in order to identify its main symptoms, effective factors, and ways Fig. 1 Comparison of animal weights in different groups (mean ± SE, n = 10). Statistical data analysis was performed using two-way analysis of variance followed by the post hoc Bonferroni test. *P ≤ 0.05 compared to sham group and. # P ≤ 0.05 compared to the CUMS group. S1, first dose of SO 2 ; S2, second dose of SO 2 to treat this disease. Multiple studies have demonstrated that there is a close relationship between depression and stress and anxiety and those who have a background of stress and anxiety are more susceptible to depression. In addition, stress is sometimes a factor that facilitates the occurrence of depression (Antoniuk et al. 2019). One of the most common models of depression to study the behavioral and molecular effects of this disorder is the application of the chronic unpredictable mild stress (CUMS) model. In this model, animals were chronically exposed to mild and random stressors that did not conflict with their survival. This method is compatible with the causes of depression in humans.

The effect of CUMS on behavioral studies and oxidative changes in the hippocampus
The result of this study revealed that CUMS causes depression-like behaviors, impaired cognitive function, and hippocampal tissue damage in animals. Also, the serum corticosterone level of depressed animals increased.
During chronic stress, a variety of damaging factors, such as oxidative stress, can cause irreversible damage to the nervous system. Previous studies have revealed that during chronic stress, the amount of free radicals such as superoxide (O 2 − ), hydroxyl (OH − ), and hydrogen peroxide (H 2 O 2 ) increases in several regions of the brain, especially , and open field test (c, d) in the different groups (mean ± SE, n = 10). Statistical data analysis was performed using two-way analysis of variance followed by the post hoc Bonfer-roni test (a) and also using one-way analysis of variance followed by the post-hoc Tukey test (b, c, and d). *P ≤ 0.05 and **P ≤ 0.01 compared to the sham group. # P ≤ 0.05 and. ## P ≤ 0.01 compared to the CUMS group. S1, first dose of SO 2 ; S2, second dose of SO2 1 3 in the hippocampus, leading to peroxidation of cellular components, specifically membrane lipids and proteins. They also indicated that the level of antioxidant enzymes was decreased. Lipid peroxidation reduces the lifespan of neurons affecting the release of neurotransmitters. On the other hand, it is recognized as one of the main factors in the loss of cellular function during the depression. The hippocampus, which plays an important role in controlling cognitive activity and behavior, is the main target of the escalating effects of oxidative stress. Hippocampus is a brain region that may be related to the pathogenesis of depression-like symptoms. Even mild CUMS intensities lead to increased depressive and anxiety-like behaviors and decreased hippocampal neurogenesis. CUMS reduces the proliferation of neuronal stem cells (NSCs) in the hippocampus as well as the survival of newly formed neurons in the hippocampus, which is likely to contribute to hippocampal atrophy. (Şahin and Gümüşlü 2004;Bajpai et al. 2014;Vaváková et al. 2015). In addition to the prominent role of oxidative stress in the pathology of depression, the activity of the hypothalamic-pituitary-adrenal axis increases during severe depressive disorders, resulting in increased corticosterone levels in rodents and increased cortisol levels in humans Lee et al. 2015). Due to the high density of glucocorticoid receptors in the hippocampus, this area is one of the areas sensitive to increased glucocorticoids. Glucocorticoids increase the chance of dendritic atrophy and reduce synaptic connections. In the long term, even the Fig. 3 Results of spatial learning (a, b) and memory (c, d) in the different groups (mean ± SE, n = 10). Statistical data analysis was performed using one-way analysis of variance followed by the post hoc Tukey test to examine spatial learning (a, b) and two-way analysis of variance followed by the post hoc Bonferroni test for assessment spatial memory in the probe trial test (c, d). *P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001 compared to the sham group. # P ≤ 0.05, ## P ≤ 0.01, and. ### P ≤ 0.001 compared to the CUMS group. S1, first dose of SO 2 ; S2: second dose of SO 2 1 3 amount of neurotransmitters and the density and activity of their receptors will be affected. Finally, dysfunction of the hippocampus can cause changes in cognitive activity Lee et al. 2015). These findings are consistent with the findings of the present study indicating that elevated serum corticosterone may play an important role in cognitive and emotional changes following chronic stress.
In addition to the pathogenesis factors of depression investigated in this study, many inflammatory mediators, such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β), are present in depressed animal models, the same as in depressed patients (Fan et al. 2017). The administration of inflammatory mediators such as IL-1β, IL-6, TNF-α, and lipopolysaccharide can cause depressivelike behaviors in animal models (Sulakhiya et al. 2016).
Nerve growth factors, especially BDNF and NGF, are also important in the pathophysiology of depression (Jesse et al. 2015). BDNF expression, an essential neurological factor is regulated by neural activity. In depressed animal models, a decrease in BDNF was observed in the neocortex and hippocampus, which may be related to several neurological dysfunctions, including dysregulation of synaptic plasticity and neurogenesis (Grønli et al. 2006;Jesse et al. 2015). The brain's cholinergic system also plays a significant role in finding the cause of psychological disorders such as depression. The hippocampus contains many cholinergic inputs that can affect cognitive functions related to this area. Multiple recent studies have suggested that increasing the function of the cholinergic system by inhibiting cholinesterase enzyme or the use of cholinergic agonists may Fig. 4 Results of MDA level (a) and GPX and CAT activity (b, c) in the different groups (mean ± SE, n = 6). Statistical data analysis was performed using one-way analysis of variance followed by the post hoc Tukey test. **P ≤ 0.01 compared to the sham group and. # P ≤ 0.5 compared to the CUMS group. S1, first dose of SO 2 ; S2, second dose of SO 2 Fig. 5 Results of serum corticosterone measurements in different groups (mean ± SE, n = 6). Statistical data analysis was performed using one-way analysis of variance followed by the post hoc Tukey test. *P ≤ 0.05 compared to the sham group. # P ≤ 0.05 compared to the CUMS group. S1, first dose of SO 2 ; S2, second dose of SO 2 be effective in increasing depressive-like behaviors (Dulawa and Janowsky 2019).

The effects of SO 2 donors on disorders caused by CUMS
The results of the present study showed that both doses of S0 2 donors were effective on depressive-like behaviors and cognitive disorders in the CUMS animals. Also, lipid peroxidation of the hippocampus was significantly reduced in both treatment groups. Serum corticosterone levels of CUMS animals reduced only by prescribing of low dose of SO 2 donors, which needs a more detailed investigation to determine the effect of different doses of SO 2 donors on the corticosterone production pathway.
Although SO 2 donors were able to improve depressivelike behaviors in animals, they did not increase the weight of depressed animals, which requires further investigation to determine whether SO 2 donors have an effect on feeding behaviors and factors affecting appetite. According to our knowledge, no study was done on this subject. SO 2 is widely known as an air pollutant that has destructive effects on various parts of the body especially the lungs and cardiovascular system. SO 2 , as an oxidative agent, can cause lipid peroxidation and change the status of antioxidants in various organs. However, recently extensive evidence has demonstrated that sulfur dioxide has many physiological activities, particularly in the cardiovascular system. SO 2 is mostly produced endogenously following the oxidation of sulfur-containing amino acids by the aspartate aminotransferase (AAT). Disruption of the SO 2 /AAT pathway in the cardiovascular system has been associated with hypertension, atherosclerosis, the proliferation of vascular smooth muscle cells, and increased extracellular collagen, leading to vascular dysfunction (Huang et al. 2016;Wang and Wang 2018).
The results of several studies conducted in recent years show that SO 2 donors have protective effects on various disorders by reducing oxidative stress and inflammatory factors, which is consistent with the results of the present study. Studies have shown that the antioxidant function of SO 2 donors is through various mechanisms. SO 2 donors by having free electrons are able to scavenge free radicals . On the other hand, studies have shown that H 2 S as another sulfur-containing gasotransmitter is able to change the function of respiratory enzymes in the mitochondria through different mechanisms so that the Fig. 6 Neuronal density (number of healthy cells) (a) and necrotic cells (b) in the CA1 subregion of the hippocampus in the different groups (100 μm) (mean ± SE, n = 4). * (P < 0.05), ** (P < 0.01) compared to the sham group. # (P < 0.05) vs the CUMS group. Black arrows indicate healthy neurons and red arrows indicate damaged neurons (magnification × 400). (c.a) sham group; (c.b) CUMS group; (c.c) CUMS + S1; (c.d) CUMS + S2; Statistical analysis of data was performed using one-way analysis of variance followed by the Tukey post-test. S1, first dose of SO2; S2, second dose of SO 2 mitochondrial ATP production is strengthened and ultimately leads to a reduction in free radicals production Kumar and Sandhir 2020). Few studies were done on SO 2 donors. In a study that was conducted on the model of myocardial injury caused by isoproterenol, it was observed that SO 2 donors are able to reduce myocardial apoptosis associated with the mitochondria, which changes apoptosis by reducing the release of cytochrome c and closing the mitochondrial permeability transition pore (MPTP) . Inhibition of cytochrome c release from the mitochondria leads to the correct functioning of respiratory enzymes and the ultimate reduction of oxidative stress. In addition, SO 2 donors inhibit the phosphorylation of ERK 1/2 (Huang et al. 2013) and increase the activity of GRP78 which are associated with reducing oxidative stress, increasing the activity of endogenous antioxidants and changing intracellular calcium homeostasis (Yu et al. 1999;Kim and Wong 2009;Wang et al. 2011).
In a model of acute lung injury caused by oleic acid, a decrease in the amount of SO 2 in lung tissue was observed, which led to an increase in inflammatory factors such as NF-κB p65. It was found that SO 2 donors could reduce inflammatory factors. While investigating oxidative stress and apoptosis in the same model of lung damage, it was also observed that SO 2 donors reduce O − and OH − radicals generated by oleic acid and increase lung tissue's antioxidant capacity (Chen et al. 2015(Chen et al. , 2017. In a D-gal-induced aging model, SO 2 donors also reduced endothelial dysfunction by reducing oxidative stress and AT1R (Dai et al. 2018).
In two studies performed on temporary and chronic cerebral, SO 2 donors could increase the activity of antioxidants, accompanied by a decrease in lipid peroxidation. In histological studies, sulfur dioxide donors reduced the death of hippocampal pyramidal neurons in the form of apoptosis and necrosis during ischemia, which ultimately led to improving in the cognitive activities of animals (Zare Mehrjerdi et al. 2018;Ghasemi et al. 2020).
Even in a clinical study performed on children with congenital heart failure, it was found that serum SO 2 levels were inversely related to the prevalence of pulmonary hypertension in these children (Yang et al. 2014).

Conclusion
In general, the results of the present study suggest that sulfur dioxide donors might be effective in improving depressivelike behaviors and also cognitive impairment in depressed animals by reducing oxidative stress and lowering corticosterone, an indicator of stress in the blood.