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Abstract
Crosstalk between inflammatory pathways and neurocircuits in the brain can lead to
behavioural responses, such as avoidance and alarm, that are likely to have provided
early humans with an evolutionary advantage in their interactions with pathogens and
predators. However, in modern times, such interactions between inflammation and the
brain appear to drive the development of depression and may contribute to non-responsiveness
to current antidepressant therapies. Recent data have elucidated the mechanisms by
which the innate and adaptive immune systems interact with neurotransmitters and neurocircuits
to influence the risk for depression. Here, we detail our current understanding of
these pathways and discuss the therapeutic potential of targeting the immune system
to treat depression.
There is a growing body of evidence demonstrating that stress decreases the expression of brain-derived neurotrophic factor (BDNF) in limbic structures that control mood and that antidepressant treatment reverses or blocks the effects of stress. Decreased levels of BDNF, as well as other neurotrophic factors, could contribute to the atrophy of certain limbic structures, including the hippocampus and prefrontal cortex that has been observed in depressed subjects. Conversely, the neurotrophic actions of antidepressants could reverse neuronal atrophy and cell loss and thereby contribute to the therapeutic actions of these treatments. This review provides a critical examination of the neurotrophic hypothesis of depression that has evolved from this work, including analysis of preclinical cellular (adult neurogenesis) and behavioral models of depression and antidepressant actions, as well as clinical neuroimaging and postmortem studies. Although there are some limitations, the results of these studies are consistent with the hypothesis that decreased expression of BDNF and possibly other growth factors contributes to depression and that upregulation of BDNF plays a role in the actions of antidepressant treatment.
Although the fact that genetic predisposition and environmental exposures interact to shape development and function of the human brain and, ultimately, the risk of psychiatric disorders has drawn wide interest, the corresponding molecular mechanisms have not yet been elucidated. We found that a functional polymorphism altering chromatin interaction between the transcription start site and long-range enhancers in the FK506 binding protein 5 (FKBP5) gene, an important regulator of the stress hormone system, increased the risk of developing stress-related psychiatric disorders in adulthood by allele-specific, childhood trauma-dependent DNA demethylation in functional glucocorticoid response elements of FKBP5. This demethylation was linked to increased stress-dependent gene transcription followed by a long-term dysregulation of the stress hormone system and a global effect on the function of immune cells and brain areas associated with stress regulation. This identification of molecular mechanisms of genotype-directed long-term environmental reactivity will be useful for designing more effective treatment strategies for stress-related disorders.
Several studies have reported antidepressant effects of anti-inflammatory treatment; however, the results have been conflicting and detrimental adverse effects may contraindicate the use of anti-inflammatory agents. To systematically review the antidepressant and possible adverse effects of anti-inflammatory interventions. Trials published prior to December, 31, 2013, were identified searching Cochrane Central Register of Controlled Trials, PubMed, EMBASE, PsychINFO, Clinicaltrials.gov, and relevant review articles. Randomized placebo-controlled trials assessing the efficacy and adverse effects of pharmacologic anti-inflammatory treatment in adults with depressive symptoms, including those who fulfilled the criteria for depression. Data were extracted by 2 independent reviewers. Pooled standard mean difference (SMD) and odds ratios (ORs) were calculated. Depression scores after treatment and adverse effects. Ten publications reporting on 14 trials (6262 participants) were included: 10 trials evaluated the use of nonsteroidal anti-inflammatory drugs (NSAIDs) (n=4,258) and 4 investigated cytokine inhibitors (n=2,004). The pooled effect estimate suggested that anti-inflammatory treatment reduced depressive symptoms (SMD, -0.34; 95% CI, -0.57 to -0.11; I2=90%) compared with placebo. This effect was observed in studies including patients with depression (SMD, -0.54; 95% CI, -1.08 to -0.01; I2=68%) and depressive symptoms (SMD, -0.27; 95% CI, -0.53 to -0.01; I2=68%). The heterogeneity of the studies was not explained by differences in inclusion of clinical depression vs depressive symptoms or use of NSAIDs vs cytokine inhibitors. Subanalyses emphasized the antidepressant properties of the selective cyclooxygenase 2 inhibitor celecoxib (SMD, -0.29; 95% CI, -0.49 to -0.08; I2=73%) on remission (OR, 7.89; 95% CI, 2.94 to 21.17; I2=0%) and response (OR, 6.59; 95% CI, 2.24 to 19.42; I2=0%). Among the 6 studies reporting on adverse effects, we found no evidence of an increased number of gastrointestinal or cardiovascular events after 6 weeks or infections after 12 weeks of anti-inflammatory treatment compared with placebo. All trials were associated with a high risk of bias owing to potentially compromised internal validity. Our analysis suggests that anti-inflammatory treatment, in particular celecoxib, decreases depressive symptoms without increased risks of adverse effects. However, a high risk of bias and high heterogeneity made the mean estimate uncertain. This study supports a proof-of-concept concerning the use of anti-inflammatory treatment in depression. Identification of subgroups that could benefit from such treatment might be warranted.
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