Pathophysiology of depression: inflammation and its relation with oxidative stress and the hypothalamic-pituitary-adrenal axis

Depression is well-known to be a widespread, disabling mental disorder. Despite the many theories that have been put forth to explain the underlying pathophysiology of depression, the exact pathophysiology remains uncertain. In this review, we aim to summarize pathophysiological pathways and experimental animal models for depression, focusing mainly on inflammatory pathways. Stress is a well-known predisposing factor for depression. So, we aim to demonstrate the link between stress and inflammation in depression pathophysiology, highlighting the role of microglia activation, the release of proinflammatory cytokines, and the production of neurotoxic metabolites. We also aim to show the link between inflammation and the disturbance of serotonin, which is also known as 5-hydroxytryptamine (5-HT), and norepinephrine (NE) levels in the brain. Activated microglia produce reactive oxygen species (ROS), which further enhances the inflammatory response. Additionally, we aim to illustrate the hypothalamic-pituitary-adrenal (HPA) axis hyperactivity that occurs as a result of stressful conditions and the consequent resistance of glucocorticoid receptors (GRs), leading to the failure of glucocorticoids to suppress inflammation. We also aim to demonstrate experimental animal models of depression that are based on psychological stress, such as the maternal separation model and the social defeat stress (SDS) model, as well as reviewing the lipopolysaccharide (LPS) inflammation-based model. We also aim to briefly review the widely used chronic unpredictable mild stress (CUMS) model.


Introduction
Depression is a highly prevalent mental disorder that can greatly affect the quality of life of patients.It presents with depressed mood, sadness, loss of pleasure in activities once enjoyed, low self-esteem, feeling worthless, excessive guilt, and feeling hopeless about the future.It can also cause difficulty thinking, concentrating, or making decisions.These problems can greatly impair the capability of the patient to carry out his or her daily tasks.Accordingly, depression can cause disruptions in productivity and social interaction.In the worstcase scenario, depression can cause the patient to commit suicide [1,2].Depression can also increase the burden on society and the health care system [3,4].Depression is causing an increasing concern for global health, and it is of growing concern in low-income countries in particular.
The World Health Organization predicts that by 2030, depression will account for the majority of disease burden globally [5,6].
Studies show that the cause of depression may be a complex combination of biological and psychosocial factors [1,7].Studies of depression pathophysiology are still in progress, and till now, the exact mechanism has not been completely explained owing to the complex nature of the biological processes involved in depression.Therefore, it is difficult to achieve extensive treatment effects [2,8].The traditional theory explains depression as a decrease in central monoaminergic function [8,9].The monoamine theory in itself could not provide a complete understanding of the pathophysiology of depression.Currently, the most commonly prescribed classes of antidepressants are those that depend on the monoamine theory in their mechanism of action.However, they have their drawbacks; usually, only 50% of patients achieve significant improvement in symptoms following antidepressant medication therapy [9, 10].Therefore, there is a need to study how to prevent depressive symptoms by focusing on different possible mechanisms beyond the monoamine hypothesis.
There are numerous neuropharmacological theories other than the monoamine hypothesis to help explain the pathophysiology of depression.Some hypotheses emphasized the role of neurogenesis and neurotrophic factors, including the brain-derived neurotrophic factor, in disease pathogenesis.Other theories are also under investigation, some of which will be discussed in this review article.Stress is a well-known predisposing factor leading to depression, and there is a well-known relationship between stress and inflammation.Studies have demonstrated that inflammation is deeply involved in the pathophysiological mechanisms of depression.The production of reactive oxygen species (ROS) and the resulting oxidative stress have also been found to be involved in the pathophysiology of depression.Furthermore, hypothalamic-pituitaryadrenal (HPA) axis hyperactivity with the consequent glucocorticoid receptors (GRs) resistance is also commonly discovered in studies of depression [9,[11][12][13].The effect of inflammation and inflammatory cytokines on serotonin, which is also known as 5hydroxytryptamine (5-HT), and norepinephrine (NE), has received much attention.Several studies have demonstrated that proinflammatory cytokines can reduce 5-HT and NE levels, which in turn can contribute to the pathophysiology of depression [14,15].
Different types of animal models of depression are available for use in laboratory investigations to explore the mechanisms that are involved in depression [16,17].In laboratories, models of depression that depend on psychological stress to induce depression in animals are probably the most widely used animal models for studying depression.For example, the maternal separation model is the most commonly used animal model that is based on early life stress.On the other hand, the social defeat stress (SDS) model is the most commonly used model based on social stress.Stress is known to have the ability to initiate the pathophysiology of depression.Stress is wellknown to be an important predisposing factor leading to depression.The purpose of this review article is to provide an overview of the role of inflammation in the pathophysiology of depression as a response to stress, highlighting the role of microglial activation and the production of proinflammatory cytokines, ROS, and neurotoxic compounds in the inflammatory pathway.We also aim to provide an overview of the roles of the HPA axis and oxidative stress in depression pathophysiology and their relation to inflammation.In this review article, we also show the effect of inflammation on 5-HT and NE.We also aim to review animal models that depend on psychological stress for the induction of depression.This stress can either be early life stress, which is used for the induction of depression in the maternal separation model, or adulthood stress, which is used for the induction of depression in the SDS model.One of the most widely used models for depression that depends on applying minor stressors, the CUMS model, is also reviewed here.Also, an LPS-based model is illustrated here to show the role inflammation plays in depressive disorder.

The Role of Inflammation in Depression
Studies have hypothesized that there is a strong link between inflammation and the pathophysiology of depression.

Kynurenine Pathway
Currently, it is well known that proinflammatory cytokines, IL-1β and TNFα, can induce indoleamine 2,3-dioxygenase (IDO) and tryptophan dioxygenase (TDO) pathways.IDO is broadly distributed in the periphery and the microglial cells.TDO is mostly found in the liver.IDO and TDO enzymes metabolize tryptophan to kynurenine.Kynurenine is further metabolized to 3-OHK and QA, both of which are known to be neurotoxins [37, 48-50].
In the brain, IDO is predominantly found in the microglia.This enzyme is the initial and ratelimiting enzyme of the kynurenine pathway.Proinflammatory cytokines that are elevated in the pathophysiology of depression activate the IDO pathway in the microglia to metabolize tryptophan to kynurenine

The Role of Oxidative Stress in Depression
The high ROS levels and inflammation have been linked to the pathophysiology of depression.

The HPA Axis
Virtually all stressors can induce HPA axis and inflammatory changes.Chronic stress and HPA-axis hyperactivity are frequently proposed as primary players in the development of depression [28,59].Early life stress can cause increased reactivity of the HPA axis, which is related to the development of depression.This is in accordance with findings from the maternal separation model.Patients with depression exhibit reduced HPA negative feedback due to GRs resistance [17, 18, 39, 44, 60].

Physiology of the HPA Axis
The importance of the role that the HPA axis plays in stress response has long been recognized.Stressors of all sorts, physical and psychological, can stimulate the hypothalamus to release corticotropin-releasing hormone (CRH).

Pathological Response of the HPA Axis to Stress
In depression patients, the HPA axis is found to be overactive under stressful conditions.Elevated cortisol production and insufficient inhibition of the HPA axis by GR regulatory feedback are common features of depression [11,40].Long-term exposure to stress leads to HPA axis hyperactivity and increased levels of cortisol.The persistent hypercortisolism due to stress in depressed patients leads to GRs resistance.This resistance decreases the glucocorticoid-mediated feedback inhibition of HPA, causing persistent HPA axis hyperactivity in depressed patients [9,11,40,58,64].
All physical and psychological stressors are accompanied by immune activation and proinflammatory cytokines production.Glucocorticoids are known to be antiinflammatory hormones.They exhibit their antiinflammatory effect by suppressing the production of proinflammatory cytokines.It would have been expected that the persistent hypercortisolism associated with stress would result in the suppression of the immune system in patients with depression.However, GRs resistance applies not just to their regulatory role in the HPA axis but also to other GRs functions.In this case, GRs may also lose their ability to perform anti-inflammatory activities, allowing inflammation to occur [12, 13].
In the case of depressed patients, persistent hypercortisolemia with subsequent repeated stimulation of GRs results in resistance of GRs of immune cells, such as macrophages.The GRs resistance leads to a decrease in the inhibitory effect of glucocorticoids on inflammatory responses, causing unrestrained inflammation and increased release of proinflammatory cytokines, adding to the pathophysiology of depression

Maternal Separation Model
Early-life stress and a history of childhood stress have been identified by clinical studies as two of the most potent risk factors for depression.Depression patients who were exposed to stressful conditions in childhood show clinical features such as early age of onset, severe depressive symptoms, and reduced efficacy of common antidepressants.Children who have endured family violence, parental absence, or parenting characterized by rejection are at an increased risk of developing depression compared to children without those early life stresses [2, 39, 65].It is still unclear exactly how early-life stressful experiences lead to depression in terms of pathophysiology.

Mechanisms Involved in the Maternal Separation Model
In animals exposed to maternal separation, the pathophysiology of depression was found to involve the HPA axis and inflammation modulation.There is mounting evidence that inflammation plays an important role in response to early-life stress and the development of depression.Individuals who have experienced maternal separation usually have elevated proinflammatory cytokines levels and are more likely to develop depression [18, 39].The maternal separation model exhibits depressionlike behavior, which is linked to increased proinflammatory cytokines such as TNF-α and IL-1β in the brain and exhibits greater microglial activation and increased IDO activation and production of 3-OHK and QA [9, 39, 41].

The SDS Model
It is widely accepted that one of the most common key risk factors leading to the development of depression in humans is recurrent social stress [2, 16].In humans, loss of social control is associated with a high risk of depression.Humans who experience social defeat exhibit increased depression symptoms.Because the majority of stress stimuli in humans that cause psychopathological changes are social, the process of losing control is critical to an individual's psychosocial situation.
The most widely used model of social defeat is the SDS model, which is thought to be relevant to the human situation

Social Defeat of Laboratory Animals
The SDS animal model is based on the resident-intruder paradigm in promoting the pathogenesis of depression [7,68].Social conflict between members of the same species is necessary for the generation of emotional and psychological stress in the SDS model Customarily, in this model, the test intruder male mouse will be transferred into the home cage of another male, the resident, for 10 min.daily.In all settings, the intruder is investigated, attacked, and defeated by the resident [7, 17, 18, 67].
In addition to the physical stress during social encounters, the psychological stress of sensory contact can also be added, through which the intruder is housed in the same cage as the resident counterpart, separated by a wall made of transparent acrylic with small holes that would allow for the odors and vocalizations to circulate, allowing for sensory interaction and exposure to the stressful psychological signals emitted by the resident without fighting [7,17,18].This sequence is repeated for 10 days, with a novel opponent each day.SDS-induced depressive behavioral changes in the test animal are comparable to those of human depression [17, 18, 67].

Mechanisms Involved in the SDS Model
According to research, social defeat causes significant hyperactivity of the HPA axis in rodents, with a significant elevation in corticosterone levels [18, 68].Social defeat was found to elicit neurobiological changes relevant to increased brain proinflammatory cytokines, increased microglial activation, and IDO pathway activation with consequent production of 3-OHK and QA [18, 32, 42].

The LPS Injection Model
The pathophysiology of depression is heavily influenced by inflammatory activation caused by various stressors.

Conclusion
In summary, depression is one of the most common mental disorders.The most common risk factor for depression is stress.Inflammation in response to stressors appears to be strongly linked to depression.Inflammation is implicated in the pathophysiology of depression, according to both clinical and pre-clinical evidence.The relationship between inflammation and depression is still being investigated.Preclinical evidence suggests, however, that elevated proinflammatory cytokines are involved in mechanisms that cause depression.Proinflammatory cytokines such as TNF-α and IL-1β have potent effects on activating microglia and inducing the IDO pathway, with the consequent formation of neurotoxic tryptophan catabolites, 3-OHK, and QA.The inflammatory response in depression is accompanied by increased oxidative stress, which also promotes neuroinflammation.These factors, combined, play a role in cell death.Additionally, stress triggers the HPA axis to release large amounts of glucocorticoids, which leads to the development of GRs resistance, which fails glucocorticoids to inhibit the HPA axis, and the failure of glucocorticoids to inhibit inflammation.Proinflammatory cytokines were proven to have an impact on 5-HT and NE levels in numerous studies.Proinflammatory cytokines activate the IDO pathway, which catabolizes tryptophan, the main amino acid precursor of 5-HT, causing a reduction in tryptophan levels and a consequent reduction in the availability of 5-HT.Proinflammatory cytokines also increase the expression of reuptake transporters of 5-HT and NE, causing an increase in the reuptake of these neurotransmitters with a consequent decrease in their availability.Different types of animal models of depression can be used in laboratory studies to look into the mechanisms underlying depression.The most widely used animal models of depression are those that are based on psychological stress, such as the SDS and the maternal separation stress model.These models can exhibit signs of inflammation and a dysregulated HPA axis.The LPS injection model is also useful in studying inflammatory roles in depression in response to immune stressors.CUMS is one of the most well-known models for depression that depends on applying mild stressors that are unpredictable over some time.Signs of inflammation and dysregulation of the HPA axis were also found in animals exposed to this model.More studies are still needed to fully understand the pathophysiology of depression to find an effective treatment.

List of abbreviations
[24].Peripheral exposure to pathogens or injury/stress (sterile inflammation in the absence of infection) stimulates macrophages and monocytes [24, 31, 32].Stress stimulates the macrophages and monocytes to release proinflammatory cytokines such as IL-1β and TNF-α [24, 33].Cytokines are polypeptides of relatively large size, so they aren't able to freely cross the blood-brain barrier.However, limited amounts can enter the brain mainly via active transporters for specific cytokines like IL-1β and TNF-α or via areas lacking a fully functional blood-brain barrier.After crossing the bloodbrain barrier, cytokines cause the activation of microglia [4, 23, 32, 34-36].The effect of stress of peripheral origin on the release of inflammatory cytokines from stimulated macrophages, the entry of these cytokines to the brain, and microglial activation is illustrated in (Fig. 1.).

Fig. 1 .
Fig. 1.Effect of stress originating peripherally on macrophages, the release of proinflammatory cytokines, and the subsequent activation of microglia in the brain.CNS-localized injury also can cause persistent neuroinflammation through the

Fig. 2 .
Fig. 2. Roles of activated microglia in depression pathophysiology.Increased levels of proinflammatory cytokines, microglial activation, and their role in depression pathophysiology in response to stressors are supported by various animal studies using different animal models.Both psychologic and immune stressors can induce the inflammatory pathway [39, 40].Maternal

2 . 1 . 5 .
[19, 25].Microglia that are activated as a consequence of the inflammatory changes further metabolize kynurenine to 3-OHK and QA, which are neurotoxic [13, 35, 37, 38].Both 3-OHK and QA are N-methyl-D-aspartate receptor agonists that cause elevations in intracellular calcium levels and are therefore likely to induce neuronal apoptosis [37, 49-52].Thus, the IDO pathway amplifies the consequences of inflammatory states [49, 52].Induction of IDO by cytokines and its end products' neurotoxicity has been proposed as a mechanism by which inflammation causes depression [45, 48].The Effect of Inflammation on 5-HT and NE It is believed that neurotransmitters are essential to the etiology of depression.The monoamine hypothesis states that depression symptoms are caused by deficient 5-HT and NE, two monoamine neurotransmitters, in the brain [9, 53].Attention has been paid to the impact of inflammation and inflammatory cytokines on 5-HT and NE.Numerous human and laboratory animal investigations have shown that the 5-HT and NE levels are affected by proinflammatory cytokines [14, 15].Through the IDO pathway's activation, proinflammatory cytokines can influence 5-HT levels.IDO is activated by cytokines such as TNF-α and IL-1β.Relevant to 5-HT metabolism, IDO catabolizes tryptophan, which is the primary amino-acid precursor of 5-HT, into kynurenine.Cytokine-induced activation of IDO can lead to reduced tryptophan, which in turn can contribute to decreased 5-HT availability [11, 14, 15, 54, 55].Research has also shown that the cytokines TNF-α and IL-1β can raise the expression of the 5-HT and NE reuptake pumps (transporters), increasing the 5-HT and NE reuptake and lowering Oxidative stress and inflammation function in a complementary manner in the depression pathophysiology.Activated microglia are the major sources of ROS in the brain.When microglia are activated, they release large quantities of ROS, so inflammation promotes an increase in ROS levels and consequently oxidative stress [11, 46, 47].On the other hand, oxidative stress enhances proinflammatory factor production, promoting neuroinflammation [46, 47].One of the ways through which oxidative stress may contribute to the pathogenesis of depression is the enhancement of the inflammatory pathway, which is accompanied by higher cytokine levels that act as inducers on the IDO pathway, which eventually results in neurotoxic catabolites such as 3-OHK and QA [22, 47].The relation between oxidative stress and inflammation in the pathophysiology of depression is illustrated in (Fig. 3.).
CRH induces the pituitary gland to release adrenocorticotropic hormone (ACTH).Subsequently, ACTH stimulates the adrenal cortex to release glucocorticoids (cortisol in humans and corticosterone in mice and rats) [40, 47, 61].Glucocorticoids mediate the suppression of the HPA axis by interacting with GRs in the HPA axis.The glucocorticoids function as feedback inhibitors of the production of ACTH by the pituitary gland and the production of CRH by the hypothalamus [9, 62, 63].
[12, 13, 24, 58, 62].The effect of long-term stress on the HPA axis and its relation to inflammation in the pathophysiology of depression is expressed in (Fig. 4.).

Fig. 4 .
Fig. 4. Effect of long-term stress on the HPA axis and its relation to inflammation in depression pathophysiology.
One of the main risk factors for depression development is early-life adverse experiences [44, 66].Maternal separation, in particular, has been suggested to be a significant animal model for studying the pathophysiology of depression.The maternal separation model is, in fact, the most popular early-life stress model [44, 66].Maternal care is extremely important for rodents.Therefore, early maternal separation can have a biological and behavioral impact on the offspring.The most widely used procedure for maternal separation consists of a 3-hour daily separation from the second to the 12th day postpartum [18, 39].
Early psychological stress activates the HPA axis, as demonstrated by the maternal separation model [39, 65].The hyperactive HPA axis leads to increased corticosterone levels and consequent resistance of GRs [17, 18, 44, 60].
[2, 7, 67].The SDS model socially simulates the majority of human depression cases [2, 16].It has been demonstrated that the SDS model results in behavioral and physiological consequences that closely resemble those resulting from stress in humans [2, 18].

7, 17- 19].
One of the most widely used animal models for depression is the chronic unpredictable mild stress (CUMS) model.It mimics depression brought on by an inability to cope with stressful situations in daily life by subjecting the animals to a series of mild, unpredictable stressors [

18, 32, 42].
These activated microglia caused an elevation of proinflammatory cytokines in the CNS, leading to depression [39, 41].Other animal models based on psychologic stressors, such as the SDS model, also exhibited increased levels of brain proinflammatory cytokines and increased microglial activation in animals that were socially defeated [