30 Impact on neurotransmitter metabolism Once cytokine signals reach the brain, there is a rich literature indicating that they can interact with virtually every pathophysiologic domain relevant to depression, including marked effects on brain monoamines, which are the target of conventional antidepressant medications. Indeed, cytokines have been shown to influence central monoamine selleck compound synthesis,

release, and synaptic reuptake. Serotonin Serotonin is synthesized from tryptophan Inhibitors,research,lifescience,medical by tryptophan hydroxylase (TH) and aromatic amino acid decarboxylase (AAAD), and the amount of serotonin in brain is highly dependent on tryptophan availability.31 Specifically, depletion of tryptophan rapidly leads to reduced brain serotonin levels, which in turn can precipitate depressive symptoms in vulnerable individuals.31 Activation of the enzyme idoleamine 2,3-dioxygenase – IDO (and the related liver enzyme tryptophan 2,3dioxygenase) is an alternative pathway for tryptophan metabolism yielding kynurenine (KYN) and leading to tryptophan depletion and ultimately Inhibitors,research,lifescience,medical decreased serotonin in brain.32,33 Several Inhibitors,research,lifescience,medical cytokines and their signaling pathways have been shown to activate IDO34,35 (for a review see Shelton and Miller14). Interestingly, peripheral administration of the cytokine-inducer, lipopolysaccharide (LPS) to mice activates IDO and

is associated with depressive-like behavior.36 These LPS-induced behavioral changes can be reversed by IDO inhibition using the IDO antagonist 1-methyltryptophan. IDO activation also has other effects that may be relevant to depression. For example, KYN is metabolized to kynurenic acid (KYNA), which antagonizes α7 nicotinic acetylcholine receptors32 and can reduce striatal Inhibitors,research,lifescience,medical dopamine release (see Inhibitors,research,lifescience,medical below)37,38 KYN is also metabolized to quinolinic acid (QUIN); QUIN leads

to the generation of toxic lipid peroxides and activates N-methylD-aspartic acid (NMDA) receptors and the release of glutamate, all of which can contribute to neurotoxicity.39 The impact of QUIN on neuronal integrity has been implicated in the pathophysiology of several degenerative neurological conditions Parvulin including Alzheimer’s, Huntington’s, and Parkinson’s diseases, amyotrophic lateral sclerosis, and human immunodeficiency virusrelated dementia.40-47 Of note, IFN-α therapy has also been shown to increase KYN/tryptophan ratios in humans, and KYN has been found to access the brain in IFN-α-treated patients where it is associated with increased cerebrospinal fluid (CSF) concentrations of both QUIN and KYNA.48,49 CSF KYN and QUIN were in turn correlated with depression in during IFN-α treatment. Aside from its impact on tryptophan and serotonin synthesis, immune activation can also affect serotonin availability by acting on synaptic reuptake via the high-affinity serotonin transporter (5HTT).