Able to cross the blood-brain barrier, theanine has psychoactive properties.[6] Theanine has been shown to reduce mental and physical stress,[7] and improves cognition[8] and mood in a synergistic manner with caffeine.[9][10]
While structurally related to the excitatory neurotransmitter glutamate, theanine only has weak affinity for the glutamate receptor on postsynaptic cells.[11] Rather, its primary effect seems to increase the overall level of the brain inhibitory transmitter GABA. Theanine also increases brain dopamine levels and has a low affinity for AMPA, kainate and NMDA receptors.[12] Its effect on serotonin is still a matter of debate in the scientific community, with studies showing increases and decreases in brain serotonin levels using similar experimental protocols.[13][14] It has also been found that injecting spontaneously hypertensive mice with theanine significantly lowered levels of 5-hydroxyindoles in the brain.[15] Researchers also speculate it may inhibit glutamic acid excitotoxicity.[12] Theanine also promotes alpha wave production in the brain.[6]
