Furthermore, stress is associated with increased levels

of cytokines.79-82 Activation of proinflammatory cytokines in MS may be a route through which stress contributes to depression. Furthermore, cytokines and stressors appear to act synergistically in some studies.83,84 Immune activation may have enhanced effects when there is concomitant stress. Indeed, this possibility may underlie the observation that stress is associated with immune exacerbations and lesion burden in MS.85 Cytokines interact with the HPA axis As previously Inhibitors,research,lifescience,medical discussed, hyper-reactivity of the HPA axis is a hallmark of depression, and cytokines are potent activators of the HPA axis. Indeed, three cytokines – TNF-α, IL-1, and IL-6 – selleck chemicals account for most of the activity

in plasma that stimulates the HPA axis.86 Furthermore, clinical research findings suggest that the action of cytokines on the HPA axis contributes to the development of depression. Inhibitors,research,lifescience,medical For example, HPA axis reactivity in patients with depression correlates with cytokine levels.87 Furthermore, both IL-6 and IL-1β production correlate with Cortisol elevations after the dexamethasone Inhibitors,research,lifescience,medical suppression test.88 In addition, patients who are treated with INF-α are more likely to develop major depressive symptoms if the initial dose results in a large increase in ACTH and Cortisol.89 It has been suggested that cytokines may mediate the impairment of negative feedback, which normally acts to prevent excess levels of Cortisol, which can occur in depressed subjects.63,90 Observations from animal models are also consistent with cytokine-mediated alterations of HPA function: administration of endotoxin, which evokes “sickness behavior” and is considered to be an animal model of depression, Inhibitors,research,lifescience,medical no longer stimulates the HPA axis when coadministered with antibodies against IL-6.91 Thus, elevations of proinflammatory cytokines in MS may facilitate depression via actions on the HPA axis and associated stress hormones. Cytokines interact with serotonergic systems Cytokines can influence Inhibitors,research,lifescience,medical serotonin (5-HT) neurotransmission by altering the metabolism of tryptophan (TRP), the

metabolic precursor of 5-HT IFN-γ, in particular, is known to activate the TRP-metabolizing enzyme indoleamine-2,3-dioxygenase (IDO),92 recruiting TRP away from the 5-HT-synthesizing indolamine pathway to the alternate kynurenine (KYN) pathway. Activation of IDO thus results in increased production of 3-hydroxy-kynurenine (KYN) and quinolinic acid old (QUIN).93 Increased levels of KYN and QUIN have been proposed to contribute to the development of depressive symptoms.94 Enhanced production of the neurotoxic metabolite QUIN may result in excess stimulation of N-methyl-D-aspartate (NMDA) receptors, causing hippocampal damage and the loss of corticosteroid receptors which mediate negative feedback of the HPA axis, thereby accounting for changes in hippocampal volume and HPA axis regulation seen in depression.