Shifting the balance of brain tryptophan metabolism elicited by isolation housing and systemic administration of lipopolysaccharide in mice

The kynurenine (KYN) pathway, which is initiated by indoleamine 2,3-dioxygenase, is a key tryptophan (TRP) metabolic pathway. It shares TRP mainly with the serotonin (5-HT) pathway. Activation of the KYN pathway by stimulation of the inflammatory response system (IRS) is known to induce depressive symptoms. Thus, we considered that shifting the balance between the KYN and 5-HT systems in the brain to the KYN pathway closely relate to the etiology of depression. In the present study, we investigated the influence of environmental risk factors for depression, such as social isolation and activation of the IRS, on brain TRP metabolism. Male ICR mice (postnatal day 21) were divided into two housing conditions, isolation and group housing, reared for 4 weeks, and then given an intraperitoneal injection of lipopolysaccharide (LPS). We measured the TRP, KYN, and 5-HT levels in the prefrontal cortex, hippocampus, amygdala, and dorsal raphe nuclei. Isolation housing decreased the KYN/5-HT ratio in the amygdala and dorsal raphe nuclei. LPS increased the KYN/5-HT ratio in all regions except the dorsal raphe nuclei. Thus, isolation housing shifted the balance between the KYN and 5-HT pathways to the 5-HT pathway, whereas systemic administration of LPS shifted it to the KYN pathway.