Figure 2.

The central nervous system and periphery sequestrated by the blood-brain-barrier and the microbiota-gut-brain axis. The impermeable peripheral metabolites are sequestrated from the central nervous system (CNS) by the blood-brain barrier (BBB). The gastrointestinal microbiota changes the composites of the peripheral metabolites. Tryptophan (TRP), kynurenine (KYN), 3-hydroxykynurenine (3-HK) are permeable, but kynurenic acid (KYNA) and quinolinic acid (QUIN) are impermeable to the BBB. When the integrity of the BBB is compromised such as in inflammation, the microbiota influences the brain function and thus behavior through the microbiota-gut-brain axis. TDO: tryptophan 2,3-dioxygenas; IDO: indoleamine 2,3-dioxygenase.