Methotrexate reduces hippocampal blood vessel density and activates microglia in rats but does not elevate central cytokine release.

Methotrexate is a cytostatic drug applied in adjuvant chemotherapy and associated with cognitive impairment in part of the cancer patients. In this paper we studied in rats whether a reduction in blood supply to the brain or neuroinflammation are possible mediators of this cognitive dysfunctionality. Methotrexate reduced hippocampal blood vessel density 1 week and 3 weeks after treatment as measured immunohistochemically with an endothelial barrier antigen. Since reduced brain vascularization may relate to lowered central glucose metabolism [(18)F]FDG PET was performed. Methotrexate reduced tracer uptake in the hippocampal region 1 week after treatment, which was not seen 3 weeks after treatment. Neuroinflammatory processes were explored via a number of methods: a microglia immunohistochemical marker was applied to hippocampal sections, [(11)C]PK11195 PET was performed, and cytokine levels in plasma and homogenized hippocampal tissue were measured. Methotrexate activated microglia in the hippocampus 1 week and 3 weeks after treatment. PET analysis, however, did not show an increase in hippocampal tracer uptake and the multiplex analysis of various cytokines showed that hippocampal cytokine levels were not increased after methotrexate administration. Methotrexate did reduce plasma cytokine levels indicating a suppression of peripheral immune functioning. Methotrexate reduces hippocampal blood vessel density, indicative of a reduced brain glucose metabolism, which may contribute to the cognitive impairment following methotrexate administration. Although methotrexate activates microglia activation in the hippocampus, no effects were seen in [(11)C]PK11195 tracer uptake or hippocampal cytokine levels. This suggests that the microglial activation in this study is not a marker for neuroinflammation.