In bipolar patients, major mood episodes of either polarity result in an inflammatory response that has been convincingly shown in several studies. This is evident as an increase in the levels of proinflammatory cytokines (PIC) and C-reactive protein in the peripheral blood.22 The PIC include most importantly interleukin 1β (IL-1β), IL-6, and tumor necrosis factor-alpha (TNF-α). Treatment with mood stabilizers and resolution of acute affective exacerbations has been shown to normalize the levels of some PIC like IL-6 but not TNF-α, whereas a chronic, low-level inflammatory state persists even in euthymic patients.23 This points to the fact that immune-inflammatory dysregulation is fundamental to the pathophysiological processes in BD.24

It is suggested that unrelieved strain in BD activates the HPA axis with increased secretion of cortisol; in addition, there is stimulation of the sympatho-adrenal medullary axis with increased circulating levels of epinephrine and norepinephrine. These stress hormones acting by virtue of their membrane and cytosolic receptors affect the transcription of genes encoding cytokines via decreased inhibition of the canonical nuclear factor-kappa B (NF-κB) and the noncanonical inflammatory signal transduction pathways like activator-protein 1 (AP-1), Janus kinasesignal transducer and activator of transcription (JAK-STAT) factors, and mitogen-activated protein kinases (MAPK).25 The cells of the innate immune system (for example, monocytes, macrophages, and T-lymphocytes) secrete PIC, chemokines, and cell adhesion molecules that diffuse to the brain and trigger microglia, causing neuroinflammation and further activating the HPA axis.26 The unrelenting secretion of stress hormones leads to a constant low-grade inflammatory milieu in the body that is considered to be liable for the neuroprogression of the bipolar diathesis and also predisposes to cardiovascular and metabolic abnormalities often encountered in these patients. 27 Additionally, with repeated mood episodes and advancement of BD, there is a shift of the immune response from T-helper 2 (TH2) cells, which mainly secrete anti-inflammatory mediators like IL-10, to TH1 cells, which produce PIC (for example, TNF-α).28 Several case-control studies, mainly having cross-sectional designs, have revealed dysregulation of the immune-inflammatory response in BD. Table 1 summarizes recently published work in this area.

IL-6 is a ubiquitous inflammatory cytokine performing diverse biological actions that vary from regeneration and repair of cellular elements to augmenting the response to injury in various types of tissue damage.35 Several studies have documented an increase in peripheral circulating levels of IL-6 during acute mood episodes of either polarity.36 Because the brain is no longer considered privileged from the effects of peripheral inflammatory mediators, IL-6 gains access to this organ from the circulation. Upon stimulation, microglia and astrocytes also secrete IL-6 locally.37 However, the receptor IL-6R, which binds to this cytokine in nanomolar concentrations, is expressed by a few cell types only (including some leukocytes and hepatocytes). The complex of IL-6 and IL-6R binds with a second glycoprotein, gp 130, that subsequently dimerizes and kicks off intracellular signaling via the JAK/STAT pathways.38 Unlike IL-6R, gp 130 is expressed on all cells; however, the latter alone has no affinity for the cytokine and cells lacking IL-6R cannot respond to this mediator. Of note, a soluble form of IL-6R (sIL-6R) consisting of the extracellular portion of the receptor was detected in humans in plasma and other body fluids. sIL-6R is generated by partial proteolysis of the membrane-bound receptor by "a disintegrin and metalloprotease" (ADAM17).39 The sIL-6R interacts with IL-6 with similar binding properties as the membrane-bound IL-6R. IL-6 trans-signaling is the path by which gp 130–expressing cells, even in the absence of membrane-bound IL-6R, can be stimulated by the complex of IL-6 and sIL-6R.40 Because of this phenomenon, the inflammatory process can affect every organ system in the body, but under the steady state condition, uncontrolled inflammation is kept in check and physiological homeostasis is maintained.41

In the course of inflammation, proteolysis of the IL-6R from neutrophils by virtue of ADAM17 leads to the activation of endothelial cells which do not express IL-6R on their membranes and are therefore insensitive to the cytokine. Priming of the endothelial cells by the IL-6/sIL-6R complex leads to the secretion of the mononuclear cell attracting cytokine MCP-1. Thereby, the shedding of the IL-6R acts as a measure of preliminary injury as shown by the number of neutrophils involved, since these cells are the first to arrive at the site of damage.42 Furthermore, experimental models in mice have determined that classic IL-6 signaling has a regenerative and anti-apoptotic role during inflammation, for example, in the cecal puncture and ligation sepsis paradigm during which the animals are subjected to extreme stress. In contrast, IL-6 trans-signaling reflects the proinflammatory arm of the cytokine's biological activities.43 Since the proteolysis of IL-6R is mainly governed by ADAM17, it is highly likely that ADAM17 has a key function in inflammation-related phenomena (Fig. 2).44

There are therapeutic implications in this pattern, as specific agents can be developed that block the proinflammatory properties of IL-6 without stalling its anti-inflammatory actions.45 Many if not all neural cells are the target of IL-6 trans-signaling, and inhibition of this activity can be expected to have important salutary affects in neuropsychiatric disorders.46 A soluble fusion glycoprotein sgp 130Fc has been engineered from the extracellular portion of gp 130 that exclusively restrains IL-6 trans-signaling. Administration of this engineered protein is a viable treatment approach in major psychiatric conditions, including BD.47 Thus, current evidence provides a rationale for undertaking preclinical studies in mood disorders to determine the therapeutic prospects of IL-6 blocking strategies.

In biological terms, oxidative stress can be considered as a continuing discrepant interaction between antioxidants and prooxidants with a tilt toward the latter.48 The result of this fact is the disproportionate formation of free radicals, the reactive oxygen species (ROS). At low physiological concentrations, ROS perform important functions in the central nervous system (CNS).49 These include regulation of the destiny of neurons either through growth or programmed cell death via stimulation of the AP-1 transcription factor and the nerve growth factor pathways.50 ROS take part in critical signaling cascades such as the regulation of the membrane potential and cellular H⁺ fluxes, the execution of cardiovascular homeostasis and management of blood pressure through the angiotensin II receptor, and the control of the glutamatergic neurotransmission via the N-methyl-D-aspartate (NMDA) receptor.51 In addition, ROS are engaged in the neuroinflammatory response by means of priming of the microglia.52 Free radical actions may have a key function in fine-tuning the responses of neuronal cells to adverse events, either by promoting resiliency through stress-induced molecular cascades such as MAPK or by getting rid of the severely impaired cells by apoptosis.53

On the flip side, the buildup of ROS is found to enhance the vulnerability of brain tissue to damage and has an important part in the pathophysiology of a number of neuropsychiatric conditions. Unequivocal proof of increased brain oxidative damage has been revealed for neurodegenerative conditions like Alzheimer's and Parkinson's diseases, cerebrovascular disorders, demyelinating diseases, and severe psychiatric ailments such as schizophrenia, major depressive disorder, and BD.54 ROS cause glutamate excitotoxicity and alter mitochondrial activity. Mitochondrial dysfunction in turn causes NMDA receptor up-regulation and further increases oxidative stress, resulting in a detrimental self-sustaining and aggravating cellular process.55 Oxidative stress products such as superoxide and hydroxyl radicals have been shown to induce cortisol resistance by impairing the GR movement from the cytosol to the nucleus. HPA axis dysregulation by ROS in turn causes a proinflammatory response with increased circulating levels of PIC.56 Unrestricted ROS activity is also responsible for increased blood-brain barrier permeability through launching of matrix metalloproteinases and subsequent degradation of tight junctions, unrepressed neuroinflammation, and enhanced apoptosis of neurons.57

The assumed pathophysiological association between oxidative stress and mood disorders may be due to the fact that the nervous system is increasingly susceptible to oxidative damage for a number of reasons. First, of all the organs, the brain has the highest consumption rate of oxygen; it is roughly 2% of body weight but uses 20% of total inspired oxygen. This fact predisposes it to greater formation of ROS during the process of mitochondrial energy metabolism. Second, the brain's lipid content is very high, and lipids act as a substrate for the ROS. Third, there is the redox potential of several neurotransmitters, for instance, dopamine. Fourth, the defense systems against free radicals are relatively inefficient. Last, the brain has a high content of metal ions, for example, iron and copper, involved in redox reactions. 58 The protective arrangement against prooxidants is composed of an enzymatic and a nonenzymatic component. Glutathione peroxidase and glutathione reductase are recognized intracellular antioxidant enzymes. The former changes peroxides and hydroxyl radicals into benign moieties, with the oxidation of reduced glutathione (GSH) into the oxidized form glutathione disulfide (GSSG), and glutathione reductase regenerates GSH from GSSG. Additionally, other enzymes such as catalase (CAT) and superoxide dismutase (SOD) also take part in the cellular resistance against oxidative stress and act along with glutathione peroxidase, thus forming the principal enzymatic defense against accumulated free radicals. Further, glutathione-S-transferase and glucose-6-phosphate dehydrogenase are important in sustaining a stable provision of metabolic products like GSH and nicotinamide adenine dinucleotide-phosphate (NADPH), which are required for proper operation of the main antioxidant enzymes.59

The main ROS include the superoxide radical (O₂˙^-), hydrogen peroxide (H₂O₂), and the hydroxyl radical (OH˙). Interaction between ROS and nitric oxide results in the formation of highly reactive nitrogen species, such as the peroxynitrite ions (ONOO^-), which are very damaging to the basic structural molecules of the cells.60 Because ROS and reactive nitrogen species possess very short half-lives, these are hard to quantify. Therefore, it is necessary to search for and determine indirect pointers of oxidative stress; these include lipid, protein, and DNA peroxidation indicators. Whereas measurements of key enzymes involved in the neutralization of free radicals also serve as putative markers.61 Biosignatures of oxidative stress in body fluids include oxidized proteins like protein carbonyls and oxidized lipids such as 4-hydroxynonenol, thiobarbituric acid–reactive substances, malondialdehyde, and isoprostanes, which are isomers resulting from the oxidation of arachidonic acid. In addition, neuroprostanes are emerging as definitive markers of oxidative stress emanating from the CNS.62 The oxidation of DNA at the guanine residue site produces 8-hydroxydeoxyguanosine, which is associated with telomere shortening detectable in peripheral blood cells or fibroblasts.63 Telomere shortening is an indicator of accelerated aging and premature mortality owing to the increased incidence of cardiovascular and other comorbidities in major psychiatric conditions.64 There are reliable data from work on circulating markers that the brain's main antioxidants, namely, GSH, CAT, SOD, and GSH peroxidase, have changed values in bipolar subjects. In this vein, recent meta-analyses have shown increased biomarkers of oxidative stress.65 Foremost discoveries are enhanced lipid peroxidation, DNA injury, and increased amino acid nitration in bipolar patients in contrast with healthy subjects, with lipid peroxidation being the predominant finding.66

The sole purpose of NADPH oxidase (NOX) enzymes is the generation of ROS, and their induction is purportedly a significant cause of oxidative stress in major psychiatric disorders.67 These enzymes utilize cytoplasmic NADPH and catalyze the transfer of electrons to molecular O₂ to produce ROS. Acting along the transmembrane electron transport chain, NOX enzymes generate ROS in the extracellular space or the lumen of intracellular organelles. Seven NOX genes have thus far been discovered, and the best described isoform is NOX2.68 These enzymes are found in several tissues of the body, and the presence of NOX transcripts has been confirmed in total brain samples as well as in neurons, microglia, and astrocytes. In the presence of severe life stress, these are induced, causing increased production of ROS in the brain and thereby playing a role in the expression of the main psychiatric disorders.69 Animal experiments in rodents and primates with paradigms such as social isolation and establishment of communal hierarchy models have been very helpful in revealing the contribution of these enzymes to prooxidant injury and the resulting expression of different types of abnormal behavior. In the case of persistent severe life stress, the neurophysiologic impact of excessive ROS generation gives rise to continued excitatory, glutamatergic transmission leading to changes suggestive of alterations seen in psychotic spectrum disorders. These include down-regulation of the NMDA receptors, failure of the inhibitory trait of GABAergic interneurons, and reduction of hippocampal volume.70 In view of these animal experiments, the cause of oxidative stress has been shown to be the increased expression of NOX enzymes. If these findings are confirmed in humans, these enzymes might emerge as potential therapeutic targets.71 Fig. 3 illustrates this supposed model of disease development in major psychiatric disorders.

Chronotropic representations have added to our knowledge of the neurobiology of mood disorders, and dysfunctions in the circadian system are linked with the bipolar phenotype. The evident abnormalities include hormonal dysregulations (melatonin and cortisol fluctuations), actigraphic patterns (sleep/wake cycles), and chronotypic activity preferences (dimensional variations), which serve as circadian markers.86 These aberrations are seen not only during acute affective exacerbations but also in the euthymic periods. Most of these indicators are also found in the healthy relatives of bipolar patients, thus signifying a robust familial predisposition. Therefore, these may be considered as genetic attributes of the disorder.87 A number of circadian genes are known to be correlated with BD: several studies have shown affirmative links for CLOCK, NPAS2, ARNTL1, NR1D1, PER3, RORB, and CSNK1-epsilon. Thus, disruption of the molecular circadian clock is supposedly involved in the genetic vulnerability to BD.88The circadian hypothesis has directed the use of treatment methods such as interpersonal and social rhythms therapy and chronotherapeutics (bright light therapy, total sleep deprivation). Furthermore, this hypothesis may elucidate how mood stabilizers, particularly lithium, which inhibits the enzyme GSK3β, resynchronize the molecular clock and bring about their therapeutic effects.89

Over the years many theories have been expounded to clarify the etiology of mood disorders. The propositions include disturbance in monoamine neurotransmission, HPA axis malfunction, immune dysregulation, decreased neurogenesis, mitochondrial dysfunction, and altered neuropeptide signaling. Almost all patients with mood disorders have major interruptions in circadian rhythms and the sleep/wake cycle. No wonder that disruption in normal sleep pattern is one of the main diagnostic criteria for these conditions. Furthermore, environmental disturbances to circadian rhythms such as shift work, trans-meridian travel, and lopsided social timetables often induce or aggravate affective exacerbations.90 It has been shown in current research work that molecular clocks exist everywhere in the brain and body, where they partake in the working of nearly all physiological functions.91 The social zeitgeber premise of mood disorders assumes that stressful life events cause alterations in the sleep/wake pattern, which then affect molecular and cellular rhythms in susceptible people, inducing affective episodes. This postulate is supported by a number of clinical studies showing an unambiguous connection between the extent of rhythm disruptions and the severity of mood episodes; also, rhythms are re-established with successful treatment.92 In essence, all present therapies for mood disorders modify or even out circadian rhythms.

The principal circadian synchronizer is placed in the suprachiasmatic nucleus of the hypothalamus where it gets light input by a direct connection, the retinohypothalamic tract. The suprachiasmatic nucleus subsequently conveys signals via direct and indirect projections all over the brain. It also manages the timing of the discharge of several peptides and hormones, including melatonin, the sleep-promoting agent. Of note, the suprachiasmatic nucleus appears to synchronize the timing of rhythms in the periphery through changes in body temperature, which serve as a common signal to harmonize multiple organ systems to the light/dark cycle.93 The molecular clock hub consists of a sequence of transcriptional and translational periodic loops that lead to the recurrent expression of clock genes on a time scale of just over 24 hours. In the primary feedback loop, circadian locomotor output cycles kaput (CLOCK) and brain and muscle Arnt-like protein 1 (BMAL1) heterodimerize and bind to E-box-containing sequences in a number of genes including the three period genes (Per1, Per2, and Per3) and two cryptochrome genes (Cry1 and Cry2). Next in the sequence, PER and CRY proteins dimerize and translocate to the nucleus, where CRY proteins can directly stall the action of CLOCK and BMAL1. Further to this mechanism, the CLOCK and BMAL1 proteins control the expression of Rev-erbα and Rora (retinoic acid–related orphan nuclear receptors), which subsequently inhibit or trigger Bmal1 transcription, respectively, by acting via the Rev-Erb/ROR response element in the promoter.

There are a number of important catalysts that adjust the timing of the molecular clock through the biochemical processes of phosphorylation, sumoylation, etc. Casein kinases phosphorylate the PER, CRY, and BMAL1 proteins, affecting their functioning and nuclear translocation. Another enzyme, glycogen synthase kinase 3 beta (GSK3β), also phosphorylates the PER2 protein, promoting its movement into the nucleus.94

The action of the antidepressant agomelatine, a melatonin receptor agonist and a weak 5-HT_2C receptor antagonist, causes enhancement in monoaminergic neuronal activity, pointing to a regulatory function of melatonin in monoaminergic transmission and connecting the circadian and monoamine neurotransmitter systems.95 Circadian rhythm dysregulation results in an increase in PIC, and subsequently TNF-α, INF-γ, and IL-6 alter the sleep/wake cycle and circadian gene expression via NF-κB transduction. 96 Glucocorticoids show a robust diurnal variation in rhythm and crest in level just before wakening, when melatonin falls to undetectable levels in the peripheral blood. CRY proteins acting together with the GR in a ligand-dependent manner induce rhythmic suppression of its activity. Moreover, CLOCK directly acetylates GR, leading to indifference to cortisol in the morning and increased affinity at night when acetylation is annulled.97 The immediate control of GR function by circadian genes is allegedly of great importance in modulating the reaction to persistent stress.98

There is a very high incidence of metabolic diseases in mood disorders, whereas circadian oscillations in the liver, stomach, adipose tissue, and gut are very strong.99 The peptides that play a role in metabolism and feeding comprise ghrelin, orexin, leptin, and cholecystokinin, and these demonstrate a noteworthy circadian rhythm in activity. The association between corpulence, stress, and poor sleep signifies a vicious cycle, and mood disorders can be included in this sequence for a subgroup of patients.100

The nicotinamide cofactors NADPH/NADP+ and NADH/NAD+ have lately been recognized as indispensable associates of CLOCK, NPAS2, and SIRT1, establishing an immediate connection between the redox status of the cell and circadian rhythms.101 Besides, CLOCK/BMAL1 directly controls the expression of nicotinamide phosphoribosyltransferase in a circadian manner, determining the production of NAD+ in the cell over the daily cycle.102 Brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B (TrkB) are known to be crucial in the effects of antidepressants and mood stabilizers, and both of these have a strong circadian rhythm in expression in the hippocampus. 103 BDNF is no longer effective as a neurotrophic agent in the nonexistence of a normal diurnal rhythm of glucocorticoids, demonstrating that the daily oscillations of cortisol control the salutary activity of psychotropic drugs on neuronal growth via BDNF/TrkB signaling in the hippocampus.104 Fig. 5 gives an overview of the links between mood disorders and the circadian system.