Marta Kubera

The inflammatory & neurodegenerative (I&ND) hypothesis of depression: leads for future research and new drug developments in depression

Despite extensive research, the current theories on serotonergic dysfunctions and cortisol hypersecretion do not provide sufficient explanations for the nature of depression. Rational treatments aimed at causal factors of depression are not available yet. With the currently available antidepressant drugs, which mainly target serotonin, less than two thirds of depressed patients achieve remission. There is now evidence that inflammatory and neurodegenerative (I&ND) processes play an important role in depression and that enhanced neurodegeneration in depression may–at least partly–be caused by inflammatory processes. Multiple inflammatory-cytokines, oxygen radical damage, tryptophan catabolites–and neurodegenerative biomarkers have been established in patients with depression and these findings are corroborated by animal models of depression. A number of vulnerability factors may predispose towards depression by enhancing inflammatory reactions, e.g. lower peptidase activities (dipeptidyl-peptidase IV, DPP IV), lower omega-3 polyunsaturated levels and an increased gut permeability (leaky gut). The cytokine hypothesis considers that external, e.g. psychosocial stressors, and internal stressors, e.g. organic inflammatory disorders or conditions, such as the postpartum period, may trigger depression via inflammatory processes. Most if not all antidepressants have specific anti-inflammatory effects, while restoration of decreased neurogenesis, which may be induced by inflammatory processes, may be related to the therapeutic efficacy of antidepressant treatments. Future research to disentangle the complex etiology of depression calls for a powerful paradigm shift, i.e. by means of a high throughput-high quality screening, including functional genetics and genotyping microarrays; established and novel animal and ex vivo–in vitro models for depression, such as new transgenic mouse models and endophenotype-based animal models, specific cell lines, in vivo and ex vivo electroporation, and organotypic brain slice culture models. This screening will allow to: 1) discover new I&ND biomarkers, both at the level of gene expression and the phenotype; and elucidate the underlying molecular I&ND pathways causing depression; and 2) identify new therapeutic targets in the I&ND pathways; develop new anti-I&ND drugs for these targets; select existing anti-I&ND drugs or substances that could augment the efficacy of antidepressants; and predict therapeutic response by genetic I&ND profiles.

The new '5-HT' hypothesis of depression: cell-mediated immune activation induces indoleamine 2,3-dioxygenase, which leads to lower plasma tryptophan and an increased synthesis of detrimental tryptophan catabolites (TRYCATs), both of which contribute to the onset of depression.

This paper reviews the body of evidence that not only tryptophan and consequent 5-HT depletion, but also induction of indoleamine 2,3-dioxygenase (IDO) and the detrimental effects of tryptophan catabolites (TRYCATs) play a role in the pathophysiology of depression. IDO is induced by interferon (IFN)γ, interleukin-6 and tumor necrosis factor-α, lipopolysaccharides and oxidative stress, factors that play a role in the pathophysiology of depression. TRYCATs, like kynurenine and quinolinic acid, are depressogenic and anxiogenic; activate oxidative pathways; cause mitochondrial dysfunctions; and have neuroexcitatory and neurotoxic effects that may lead to neurodegeneration. The TRYCAT pathway is also activated following induction of tryptophan 2,3-dioxygenase (TDO) by glucocorticoids, which are elevated in depression. There is evidence that activation of IDO reduces plasma tryptophan and increases TRYCAT synthesis in depressive states and that TDO activation may play a role as well. The development of depressive symptoms during IFNα-based immunotherapy is strongly associated with IDO activation, increased production of detrimental TRYCATs and lowered levels of tryptophan. Women show greater IDO activation and TRYCAT production following immune challenge than men. In the early puerperium, IDO activation and TRYCAT production are associated with the development of affective symptoms. Clinical depression is accompanied by lowered levels of neuroprotective TRYCATs or increased levels or neurotoxic TRYCATs, and lowered plasma tryptophan, which is associated with indices of immune activation and glucocorticoid hypersecretion. Lowered tryptophan and increased TRYCATs induce T cell unresponsiveness and therefore may exert a negative feedback on the primary inflammatory response in depression. It is concluded that activation of the TRYCAT pathway by IDO and TDO may be associated with the development of depressive symptoms through tryptophan depletion and the detrimental effects of TRYCATs. Therefore, the TRYCAT pathway should be a new drug target in depression. Direct inhibitors of IDO are less likely to be useful drugs than agents, such as kynurenine hydroxylase inhibitors; drugs which block the primary immune response; compounds that increase the protective effects of kynurenic acid; and specific antioxidants that target IDO activation, the immune and oxidative pathways, and 5-HT as well.

Anti-inflammatory Effects of Antidepressants Through Suppression of the Interferon-γ/interleukin-10 Production Ratio

There is some evidence that major depression-in particular, treatment-resistant depression (TRD)-is accompanied by activation of the inflammatory response system and that proinflammatory cytokines may play a role in the etiology of depression. This study was carried out to examine the effects of antidepressive agents, i.e., imipramine, venlafaxine, L-5-hydroxytryptophan, and fluoxetine on the production of interferon-γ (IFN-γ), a proinflammatory cytokine, and interleukin-10 (IL-10), a negative immunoregulatory cytokine. Diluted whole blood of fluoxetine-treated patients with TRD (mean age, 50.6 ± 3.9 years) and age-matched healthy controls (mean age, 51.6 ± 1.7 years) and younger healthy volunteers (mean age, 35.4 ± 9.6 years) was stimulated with phytohemagglutinin (1 μg/mL) and lipopolysaccharide (5 μg/mL) for 48 hours with and without incubation with the antidepressants at 10−6 M and 10−5 M. IFN-γ and IL-10 were quantified by means of enzyme-linked immunoassays. The ratio of IFN-γ to IL-10 production by immunocytes was computed because this ratio is of critical importance in determining the capacity of immunocytes to activate or inhibit monocytic and T-lymphocytic functions. All four antidepressive drugs significantly increased the production of IL-10. Fluoxetine significantly decreased the production of IFN-γ. All four antidepressants significantly reduced the IFN-γ/IL-10 ratio. There were no significant differences in the antidepressant-induced changes in IFN-γ or IL-10 between younger and older healthy volunteers and TRD patients. Tricyclic antidepressants, selective serotonin reuptake inhibitors, and serotonin-noradrenaline reuptake inhibitors, as well as the immediate precursor of serotonin, have a common, negative immunoregulatory effect by suppressing the IFN-γ/IL-10 production ratio. It is suggested that the therapeutic efficacy of antidepressants may be related to their negative immunoregulatory effects.

In animal models, psychosocial stress-induced (neuro)inflammation, apoptosis and reduced neurogenesis are associated to the onset of depression

Recently, the inflammatory and neurodegenerative (I&ND) hypothesis of depression was formulated (Maes et al., 2009), i.e. the neurodegeneration and reduced neurogenesis that characterize depression are caused by inflammation, cell-mediated immune activation and their long-term sequels. The aim of this paper is to review the body of evidence that external stressors may induce (neuro)inflammation, neurodegeneration and reduced neurogenesis; and that antidepressive treatments may impact on these pathways. The chronic mild stress (CMS) and learned helplessness (LH) models show that depression-like behaviors are accompanied by peripheral and central inflammation, neuronal cell damage, decreased neurogenesis and apoptosis in the hippocampus. External stress-induced depression-like behaviors are associated with a) increased interleukin-(IL)1β, tumor necrosis factor-α, IL-6, nuclear factor κB, cyclooxygenase-2, expression of Toll-like receptors and lipid peroxidation; b) antineurogenic effects and reduced brain-derived neurotrophic factor (BDNF) levels; and c) apoptosis with reduced levels of Bcl-2 and BAG1 (Bcl-2 associated athanogene 1), and increased levels of caspase-3. Stress-induced inflammation, e.g. increased IL-1β, but not reduced neurogenesis, is sufficient to cause depression. Antidepressants a) reduce peripheral and central inflammatory pathways by decreasing IL-1β, TNFα and IL-6 levels; b) stimulate neuronal differentiation, synaptic plasticity, axonal growth and regeneration through stimulatory effects on the expression of different neurotrophic factors, e.g. trkB, the receptor for brain-derived neurotrophic factor; and c) attenuate apoptotic pathways by activating Bcl-2 and Bcl-xl proteins, and suppressing caspase-3. It is concluded that external stressors may provoke depression-like behaviors through activation of inflammatory, oxidative, apoptotic and antineurogenic mechanisms. The clinical efficacity of antidepressants may be ascribed to their ability to reverse these different pathways.

Gender-specific behavioral and immunological alterations in an animal model of autism induced by prenatal exposure to valproic acid.

Autism is a severe behavioral disorder characterized by pervasive impairments in social interactions, deficits in verbal and non-verbal communication, and stereotyped behaviors, with a four times higher incidence in boys than in girls. The core symptoms are frequently accompanied by a spectrum of neurobehavioral and immunological derangements, including: aberrant sensitivity to sensory stimulation, anxiety, and decreased cellular immune capacity. Recently, a new potential rodent model of autism induced by prenatal exposure to valproic acid (VPA rats) has been proposed. In order to determine if gender has an influence on alterations observed in VPA rats, male and female rats have been evaluated in a battery of behavioral, immunological, and endocrinological tests. A plethora of aberrations has been found in male VPA rats: lower sensitivity to pain, increased repetitive/stereotypic-like activity, higher anxiety, decreased level of social interaction, increased basal level of corticosterone, decreased weight of the thymus, decreased splenocytes proliferative response to concanavaline A, lower IFN-gamma/IL-10 ratio, and increased production of NO by peritoneal macrophages. Female VPA rats exhibited only increased repetitive/stereotypic-like activity and decreased IFN-gamma/IL-10 ratio. Sexual dimorphism characteristics for measured parameters have been observed in both groups of animals, except social interaction in VPA rats. Our results confirm existence of similarities between the observed pattern of aberrations in VPA rats and features of disturbed behavior and immune function in autistic patients, and suggest that they are gender-specific, which is intriguing in light of disproportion in boys to girls ratio in autism.

Effects of serotonin and serotonergic agonists and antagonists on the production of tumor necrosis factor α and interleukin-6

Serotonin (5-HT) is a neurotransmitter and immune modulator. The effect of 5-HT on the production of cytokines by human macrophages and lymphocytes is poorly recognized. In the present article we examine the role of 5-HT in modulating the production of two pro-inflammatory cytokines, i.e. interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNFalpha), as well as the role of 5-HT(1A) and 5-HT(2) receptors in this process. The specific aims were to examine the effects of 5-HT, p-chlorophenylalanine (PCPA), a 5-HT depleting agent, flesinoxan, a 5-HT(1A) agonist, m-chlorophenylpiperazine (mCPP), a 5-HT(2B/2C) agonist, and ritanserin, a 5-HT(2A/2C) antagonist, on the production of the above cytokines. We found that: (1) 5-HT, 15 microg/ml, significantly decreased IL-6 and TNFalpha production; (2) pCPA, 5 microM, significantly suppressed the production of IL-6 and TNFalpha; and (3) mCPP, 2.7 microg/ml, significantly increased the production of IL-6 and TNFalpha. It is concluded that intracellular 5-HT is necessary for optimal synthesis of IL-6 and TNFalpha; 5-HT in physiological concentrations may increase IL-6 and TNFalpha production by stimulating 5-HT(2) receptors; and extracellular 5-HT concentrations above the baseline physiological levels may suppress the production of the above cytokines.

The role of zinc in neurodegenerative inflammatory pathways in depression

According to new hypothesis, depression is characterized by decreased neurogenesis and enhanced neurodegeneration which, in part, may be caused by inflammatory processes. There is much evidence indicating that depression, age-related changes often associated with impaired brain function and cognitive performances or neurodegenerative processes could be related to dysfunctions affecting the zinc ion availability. Clinical studies revealed that depression is accompanied by serum hypozincemia, which can be normalized by successful antidepressant treatment. In patients with major depression, a low zinc serum level was correlated with an increase in the activation of markers of the immune system, suggesting that this effect may result in part from a depression-related alteration in the immune-inflammatory system. Moreover, a preliminary clinical study demonstrated the benefit of zinc supplementation in antidepressant therapy in both treatment non-resistant and resistant patients. In the preclinical study, the antidepressant activity of zinc was observed in the majority of rodent tests and models of depression and revealed a causative role for zinc deficiency in the induction of depressive-like symptoms, the reduction of neurogenesis and neuronal survival or impaired learning and memory ability. This paper provides an overview of the clinical and experimental evidence that implicates the role of zinc in the pathophysiology and therapy of depression within the context of the inflammatory and neurodegenerative hypothesis of this disease.

Activation of cell-mediated immunity in depression: Association with inflammation, melancholia, clinical staging and the fatigue and somatic symptom cluster of depression

BACKGROUND Depression is characterized by activation of cell-mediated immunity (CMI), including increased neopterin levels, and increased pro-inflammatory cytokines (PICs), such as interleukin-1 (IL-1) and tumor necrosis factor-α (TNFα). These PICs may induce depressive, melancholic and chronic fatigue (CF) symptoms. METHODS We examined serum neopterin and plasma PIC levels in depressive subgroups in relation to the depressive subtypes and the melancholic and CF symptoms of depression. Participants were 85 patients with depression and in 26 normal controls. Severity of depression was assessed with the Hamilton Depression Rating Scale (HDRS) and severity of CF with the Fibromyalgia and Chronic Fatigue Syndrome (FF) Rating Scale. RESULTS Serum neopterin was significantly higher in depressed patients and in particular in those with melancholia. There were positive correlations between serum neopterin, the plasma PICs and the number of previous depressive episodes. Neopterin and TNFα were associated with melancholia, while both PICs were associated with CF. Melancholia-group membership was predicted by the HDRS and neopterin, and CF group membership by age, the FF score and serum TNFα. DISCUSSION Depression and melancholia are accompanied by CMI activation, suggesting that neopterin plays a role in their pathophysiology, e.g. through activation of oxidative and nitrosative stress and apoptosis pathways. The intertwined CMI and inflammatory responses are potentially associated with the onset of depression and with the melancholic and CF symptoms of depression. Exposure to previous depressive episodes may magnify the size of CMI and PIC responses, possibly increasing the likelihood of new depressive episodes. CMI activation and inflammation may contribute to the staging or recurrence of depression.

Effects of Serotonin and Serotonergic Agonists and Antagonists on the Production of Interferon-γ and Interleukin-10

Serotonin (5-HT) is a neurotransmitter and an immune modulator. In vitro, antidepressants with a serotonergic mode of action have, at concentrations within the therapeutical range, negative immunoregulatory effects, i.e., they increase the production rate of interleukin-10 (IL-10), a negative immunoregulatory cytokine. We have hypothesized that part of these effects may be explained by the serotonergic activities of antidepressants on immunocytes. This study was carried out to examine the effects of 5-HT, p-chlorophenylalanine (PCPA), a 5-HT depleting agent, flesinoxan (a 5-HT1A agonist), m-chlorophenylpiperazine (mCPP; a 5-HT2A/2C agonist), and ritanserin (a 5-HT2A/2C antagonist) on the production rate of interferon-γ (IFNγ), a proinflammatory cytokine, and IL-10 by whole blood stimulated with polyclonal activators. The IFNγ/IL-10 production ratio was computed, since this ratio reflects the pro- versus anti-inflammatory capacity of cultured whole blood. We found that: 1) 5-HT, 150 ng/mL, 1.5 μg/mL, and 15 μg/mL significantly decreased the IFNγ/IL-10 ratio; 2) PCPA (5 μM) significantly suppressed the production of IFNγ and IL-10; 3) flesinoxan (15 ng/mL; 1.5 μg/mL) had no significant effects on the production of the above cytokines; and 4) mCPP (2.7 μg/mL) and ritanserin (5.0 μg/mL) suppressed the IFNγ/IL-10 ratio. It is concluded that intracellular 5-HT may be necessary for an optimal synthesis of IFNγ and IL-10, and that extracellular 5-HT concentrations at or above serum values may suppress the production of the proinflammatory cytokine IFNγ. The negative immunoregulatory effects of antidepressive drugs are probably not related to their serotonergic activities.

The effect of antidepressant drugs on the HPA axis activity, glucocorticoid receptor level and FKBP51 concentration in prenatally stressed rats.

Dysregulation of hypothalamic-pituitary-adrenal (HPA) axis activity is thought to be an important factor in pathogenesis of depression. In animals, stress or glucocorticoids given in prenatal period lead to long-lasting behavioral and neuroendocrine changes similar to those observed in depressed patients. However, molecular basis for HPA disturbances in animals exposed to prenatal stress - a model of depression - have been only partially recognized. Therefore, in the present study we investigated the effect of prenatal stress on behavioral changes, blood corticosterone level, concentrations of glucocorticoid receptor (GR) and its cochaperone, FKBP51, in the hippocampus and frontal cortex in adult rats. It has been found that prenatally stressed rats display high level of immobility in the Porsolt test and anxiety-like behavior. The HPA axis hyperactivity in theses animals was evidenced by corticosterone hypersecretion at the end of the light phase and 1h following acute stress. Western blot study revealed that GR level was significantly elevated in the hippocampus but not in the frontal cortex of prenatally stressed rats, whereas concentration of FKBP51 was decreased only in the former brain structure. Chronic treatment with imipramine, fluoxetine, mirtazapine and tianeptine have diminished both behavioral and biochemical alterations observed in this animal model of depression. These data indicate that the increase in hippocampal GR level and low concentration of FKBP51 in the frontal cortex may be responsible for enhanced glucocorticoid action in depression.