Able to cross the blood-brain barrier, theanine has psychoactive properties. Theanine has been shown to reduce mental and physical stress, and improves cognition and mood in a synergistic manner with caffeine.
While structurally related to the excitatory neurotransmitter glutamate, theanine only has weak affinity for the glutamate receptor on postsynaptic cells. 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. 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. It has also been found that injecting spontaneously hypertensive mice with theanine significantly lowered levels of 5-hydroxyindoles in the brain.Researchers also speculate it may inhibit glutamic acid excitotoxicity.Theanine also promotes alpha wave production in the brain.
Studies on test rats have shown even repeated, extremely high doses of theanine cause little to no harmful psychological or physical effects. Theanine showed neuroprotective effects in one rat study.
A placebo-controlled trial has shown adding theanine to ongoing antipsychotic medication is helpful in reducing some symptoms of schizophrenia.
Several beverage manufacturers are selling drinks containing theanine and are marketing them as drinks to help people focus and concentrate, while other manufacturers claim relaxing and tranquillizing properties.
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This doesn't go into the reactions on a molecular level, but its a start
This article goes into how theanine could help prevent neuronal death due to its low affinity for the glutamic acid receptors.