Central functions of green tea components

Tea (Camellia sinensis) is widely consumed throughout the world and has a number of biologically active substances such as caffeine, catechins, and L-theanine (γ-glutamylethylamide). Tea consumption is generally known to induce a feeling of relaxation which may be mediated by either catechins, L-theanine, or both, since caffeine stimulates locomotor activities. The catechin (-)-epigallocatechin gallate (EGCG) occurs abundantly in tea. Moreover, frequent consumption of green tea results in high levels of EGCG in the blood and brain. Catechins, which are flavonoids, affect the central nervous system (CNS). The therapeutic effects of flavonoids may involve their binding to γ-aminobutyric acid (GABA)A receptors, which is a major inhibitory neurotransmission system. Recently, EGCG was shown to bind to (GABA)_A receptors in vitro and to induce a sedative effect through GABA_A, but not GABA_B, receptors in the brain. L-Theanine, a derivative of glutamate, is a unique amino acid occurring only in green tea and a few other plants. After administration L-theanine concentrations were increased in serum, liver and brain, suggesting that L-theanine can cross the blood-brain barrier. Intravenous administration of L-theanine was shown to affect the cortex, hippocampus and amygdala, and increase the alpha-band component of electroencephalograms in rats. More recently, it was shown that L-theanine could reduce stress via either inhibiting cortical neuron excitation in human subjects or influencing the secretion and function of neurotransmitters in the CNS. We discuss the central functions of green tea components such as EGCG and L-theanine in the CNS.