Effect of theanine on norepinephrine and serotonin levels in rat brain.

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Abstract
The effects of i.p. administered theanine (L-N-ethylglutamine), a constituent of Japanese green tea, on the levels of norepinephrine (NE) and serotonin (5-HT) in the brain of rats with or without coadministration of caffeine were investigated, and compared with those of glutamine. Theanine decreased the NE level, whereas no change was observed with glutamine or caffeine. The decreases of NE induced by theanine was reversed by caffeine. In rats pretreated with pargyline, a monoamine oxidase inhibitor, theanine significantly increased the NE level compared with the control. However, it did not enhance the NE levels increased by caffeine. Thus, theanine may decrease the NE levels by releasing this neurotransmitter. Theanine did not alter the levels of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in rats pretreated with without pargyline, indicating that this amide affects neither 5-HT synthesis nor its degradation. Caffeine increased the levels of 5-HT and 5-HIAA in normal rats to similar extents. This effect was depressed by theanine. In rats pretreated with pargyline, the levels of 5-HT and 5-HIAA were not altered by caffeine, and theanine did not modify the outcome. It may be concluded that the action of theanine is related to the possible inhibition of 5-HT release by caffeine. The effect of glutamine on the levels of 5-HT was somewhat different from that of theanine.