84. Targeting the kynurenine blood-to-brain transport system to treat inflammation-induced fatigue and depression

Inflammation is associated with activation of the tryptophan-kynurenine metabolism pathway. Circulating kynurenine is transported into the brain at the vascular blood–brain barrier (BBB) by the large neutral amino-acid transporter LAT-1. LAT-1 transports tryptophan and kynurenine. To determine how LAT-1 responds to inflammation, we used mouse microvascular endothelial cells that form tight junctions in vitro and constitutively express the tryptophan degrading enzyme indoleamine 2,3-dioxygenase (IDO) and LAT-1. We first showed that addition of lipopolysaccharide and interferon-γ to the culture significantly increased endothelial cell IDO mRNA and the transport capacity of LAT-1 as measured with tryptophan from the luminal to the abluminal side of an in vitro model of the BBB. Before using leucine as a competitive inhibitor of LAT-1-mediated transport of kynurenine, we determined whether leucine can block the transport of tryptophan across BBB endothelial cells. Whereas leucine had no effect on tryptophan in the untreated BBB, it inhibited a portion of the enhanced transport of tryptophan conferred by pretreatment with lipopolysaccharide and interferon-γ. Consistent with this ability to block LPS-mediated brain uptake of tryptophan, leucine (administered 500 mg/kg IP before and 6 h after lipopolysaccharide) was also effective in abrogating lipopolysaccharide-induced decreases in locomotor activity and sucrose preference. Experiments are under way to assess the effect of leucine on kynurenine transport into the brain both in vivo and in vitro.