Apelin attenuates depressive-like behavior and neuroinflammation in rats co-treated with chronic stress and lipopolysaccharide

Highlights

• Apelin-13 reverses the depressive phenotype in rats co-treated with chronic stress and LPS.

• Apelin-13 decreases the activation of glial cells in a stress model

• Apelin-13 decreases the expression of inflammatory mediators in the hippocamspus of stressed rats.

• Apelin-13 inhibits inflammatory signaling pathways in the hippocamspus of stressed rats.

Abstract

Several experimental studies have proved that activation of neuroinflammation pathways may contribute to the development of depression, a neuropsychiatric disorder disease. Our previous studies have shown the antidepressant properties of apelin, but the mechanism was unkown. This study was performed to verify whether the antidepressant effect of apelin was related to its anti-inflammation effect in the central nervous system. To achieve our aim, we selected the co-treatment of chronic stress and LPS to induced an inflammatory process in rats. The effect of this co-treatment was evaluated through the expression of inflammatory markers and glial cell activation. LPS injection co-treated with unpredictable chronic mild stress resulted in the activation of microglial cell and astrocyte, expression of inflammatory markers and depressive behaviors. Treatment with apelin significantly attenuates the deleterious effects in these rats. Our results showed that apelin improved depressive phenotype and decreased the activation of glial cells in stress co-treatment group. The down-regulations of p-NF-κB and p-IKKβ suggested that the effects are possibly mediated by inhibition of the NF-κB-mediated inflammatory response. These findings speculated that intracerebroventricular injection of apelin could be a therapeutic approach for the treatment of depression, and the antidepressant function of apelin may closely associated with its alleviation in neuroinflammation.

Introduction

Depression is a severe chronic disorder with high morbidity and mortality. The main symptoms of depression include feelings of sadness, loneliness and guilt, lack of enthusiasm, sleep disorders, and self-contempt (Miller et al., 2014; Ceretta et al., 2012a, Ceretta et al., 2012b, Ceretta et al., 2012c). Depression leads to poor quality of life and great social and financial burdens (Miret et al., 2013). Although antidepressant drugs and electroconvulsive therapy may play an important role in the clinic, their effects are still limited (Nestler et al., 1989). For example, almost two-thirds of patients with depression are insensitive to that therapy (Nestler et al., 1989). Therefore, we must determine the pathogenesis of depression and seek more effective antidepressant drugs as well as therapies with less side effects and better therapeutic effects than the current treatment options.

Preclinical and clinical evidence suggests that neuroinflammation is critically implicated in the pathological process of depression (Raison and Miller, 2015; Young et al., 2014; Haapakoski et al., 2015; Strawbridge et al., 2015). Animal research has shown that microglia and astrocytes, the immune cells in the central nervous system (CNS), are potential mediators of inflammatory alterations in depression (Romina et al., 2013). Activation of microglia and astrocytes in response to inflammatory stimuli is able to increase the release of proinflammatory cytokines such as TNF-α, IL-1β and IL-6 (Diz-Chaves et al., 2012; Zhao et al., 2014a). NF-κB has been known to act as essential transcription factors for the expression of inflammatory mediators, such as iNOS, COX-2, IL-1β and TNF-α (Lawrence and Fong, 2010), which lead to depressive-like behavior (Biesmans et al., 2013; Yuan et al., 2015). In addition, NF-κB signaling may play a critical role in activation of glia cells (Kim et al., 2014), indicating that the activation of NF-κB signaling precedes microglial and astroglial activation. These cytokines impact the neurotransmitters in the hippocampus and neuronal apoptosis in the brain and trigger the activation of microglia, astrocytes and inflammatory processes, all of which are potential mechanisms of depression (Smith, 1991; Dowlati et al., 2010; Kim et al., 2007). In clinical research, patients with inflammatory disorders often exhibit depressive symptoms (Ceretta et al., 2012a, Ceretta et al., 2012b, Ceretta et al., 2012c). This relationship is bidirectional, as patients with depressive disorder display increased serum levels of IL-1β, IL-6, IL-8, and TNF-α (Howren et al., 2009). One of the antidepressant effects of fluoxetine is associated with a decrease in the expression of inflammatory cytokines in the CNS (Orio et al., 2010), and cyclooxygenase inhibitors have been shown to decrease the expression of inflammatory cytokines in the periphery to ameliorate depressive symptoms in depressed patients (Na et al., 2014; Köhler et al., 2014). Therefore, anti-inflammatory drugs may become the new antidepressant drugs.

Stress is an important cause of mental illnesses such as depression. Almost all psychiatric patients have experienced stressful events (Kessler et al., 2010). Stress is known to contribute to the development of depression. Antidepressant drugs such as selective serotonin reuptake inhibitors and serotonin and norepinephrine reuptake inhibitors reduce microglial activation and inhibit inflammatory responses (Ohgi et al., 2013; Tynan et al., 2012). In animal research, stress has been shown to lead to a series of depressive behaviors indicated by weight loss, adrenal weight gain, an increase in immobility time in the forced swimming test and a change in relative sucrose intake in the sucrose preference test (Gibson-Smith et al., 2015; Forbes et al., 1996; Clark et al., 2016; Sakhaee et al., 2017; O'Keane et al., 2012). In addition, stress sensitizes hippocampal neurons to a proinflammatory stimulus (Espinosa-Oliva et al., 2011a, Espinosa-Oliva et al., 2011b; de Pablos et al., 2006), accompanied by microglia and astrocyte activation and inflammatory cytokine expression (Shih et al., 2015; Lawrence and Fong, 2010).

Apelin is a newly identified type of adipocyte factor. Apelin was first isolated from the stomach extract of cattle, and its precursor protein has 77 amino acid residues (Tatemoto et al., 1998). This preproapelin can be proteolyzed by angiotensin-converting enzyme 2 and cleaved into four active peptides including apelin-12, apelin-13, apelin-17 and apelin-36. Apelin-13 is one of the most biologically active peptides (Hosoya et al., 2000). Apelin peptide and its receptor APJ are widely distributed in the cardiovascular system, lung tissue, lymphocytes, kidney and pancreas, as well as the hypothalamus, hippocampus, limbic structures, midbrain, caudate nucleus and other brain regions in the central nervous system, in which it plays important physiological functions (O'Carroll et al., 2013; O'Carroll et al., 2003; Reaux et al., 2001). Apelin also plays an important role in physiological responses to homeostatic perturbations, including cardiovascular disease control, water balance, HPA axis regulation and metabolic homeostasis (O'Carroll et al., 2013). In vitro and in vivo, apelin has been reported to exert an anti-neuroinflammatory effect in myocardial infarction (Xin et al., 2015), suppress anxiety (Telegdy and Jászberényi, 2014), modulate learning (Telegdy et al., 2013; Han et al., 2014) and memory and exert a neuroprotective function (Yang et al., 2015).

Our previous studies demonstrated that apelin-13 exerts an antidepressant effect, as shown by the reversal of despair behavior in the forced swim (FS) test and escape failures in the learned helplessness (LH) test (Li et al., 2016). We compared the antidepressant effect of apelin-13 and imipramine, a positive control, certified the idea that apelin-13 has an antidepressant effect. Apelin-13 also has a protective effect against corticosterone-induced apoptosis in PC12 cells (Zou et al., 2016). However, the effect of apelin on unpredictable chronic mild stress (UCMS) and lipopolysaccharide (LPS) co-treatment in depression and whether the effect is associated with inflammation inhibition are still unclear. Our previous studies mainly focused on acute stress, while this study examined chronic stress. Studies have found that patients with inflammatory diseases who have undergone stressful events have a high risk of depression. Therefore we examined the effect of apelin on rats exposed to chronic stress and lipopolysaccharide co-treatment to determine whether apelin can exert an antidepressant effect on this model and the possible mechanism.