Theogallin and L-theanine as active ingredients in decaffeinated green tea extract: I. electrophysiological characterization in the rat hippocampus in-vitro

The in‐vitro hippocampus slice preparation was used to mimic a physiological situation where nervous tissue is exposed directly to the water soluble extract of green tea and some of its constituents. This investigation provides evidence that L‐theanine‐ and theogallin‐enriched decaffeinated green tea extract is able to change the physiological pattern of electrical hippocampus activity in a concentration dependent manner (EC50 3 mg L−1). Of the seven fractions or single components tested (fraction containing all amino acids without L‐theanine, fractions containing all amino acids plus L‐theanine, glutamic acid, theogallin, its metabolites quinic acid and gallic acid, and L‐theanine alone), glutamic acid produced the strongest changes in terms of increased population spike amplitude after single stimuli and increased long‐term potentiation, commonly taken as representative for enhancement of spatial and time dependent memory. The presence of theogallin alone shifted the activity in the same direction. Similar results as with theogallin were obtained in the presence of quinic acid. No effect was seen with gallic acid. Opposite changes (decrease of population spike amplitude and attenuated long‐term potentiation) were observed in the presence of L‐theanine alone. No effects were detected during the addition of the amino acid mixture unless L‐theanine was added, leading to a decrease of the responses as observed for the action of L‐theanine alone. The results provide evidence for the involvement of several active principles in the action of enriched green tea extract on electrical brain activity. The overall enhancement of hippocampal pyramidal cell responses as observed for the crude extract seems to be due to the combined action of glutamic acid and theogallin (or its presumable metabolite quinic acid), whereas L‐theanine seems to have an opposite effect. However, this action was not strong enough to antagonize the effects of glutamic acid and theogallin. The results are in line with the observation that the tested green tea extract improves cognition at concomitant mental relaxation in man.