A Possible Neuroprotective
and Cognitive Enhancing Agent
Pradeep J. Nathan, PhD
Kristy Lu, BSc (Hons)
M. Gray, BSc (Hons)
C. Oliver, BAgSc
ABSTRACT. L-theanine (N-ethyl-L-glutamine) or theanine is a major
amino acid uniquely found in green tea. L-theanine has been historically
reported as a relaxing agent, prompting scientific research on its phar-
macology. Animal neurochemistry studies suggest that L-theanine in-
creases brain serotonin, dopamine, GABA levels and has micromolar
affinities for AMPA, Kainate and NMDA receptors. In addition has
been shown to exert neuroprotective effects in animal models possibly
through its antagonistic effects on group 1 metabotrophic glutamate re-
ceptors. Behavioural studies in animals suggest improvement in learn-
ing and memory. Overall, L-theanine displays a neuropharmacology
suggestive of a possible neuroprotective and cognitive enhancing agent
Pradeep J. Nathan is affiliated with Behavioural Neuroscience Laboratory, Depart-
ment of Physiology, Monash Centre for Brain and Behaviour, Monash University,
Kristy Lu and M. Gray are affiliated with Brain Sciences Institute, Swinburne Uni-
versity of Technology, Melbourne Victoria, Australia.
ern Cross University, Lismore NSW, Australia.
Address correspondence to: Pradeep J. Nathan, Associate Professor, Behavioural
Neuroscience Laboratory, Department of Physiology, Monash Centre for Brain and
Behaviour, PO Box 13F, Monash University, VIC. 3800, Australia (E-mail: Pradeep.
Journal of Herbal Pharmacotherapy, Vol. 6(2) 2006
Available online at http://jhp.haworthpress.com
© 2006 by The Haworth Press, Inc. All rights reserved.
and warrants further investigation in animals and humans. doi:10.1300/
J157v06n02_02 [Article copies available for a fee from The Haworth Docu-
ment Delivery Service: 1-800-HAWORTH. E-mail address: <docdelivery@
haworthpress.com> Website: <http://www.HaworthPress.com> © 2006 by
The Haworth Press, Inc. All rights reserved.]
KEYWORDS. Theanine, monoamines, pharmacology, neuropharma-
cology, cognition and anxiety, neuroprotection, cerebral infarct
ed green tea. Green tea is primarily consumed as beverage since it was
brought to Japan from China in the eighth century during China’s Tang
Dynasty. More recently, the popularity of green tea has spread through a
ing scientific evaluation of its proposed benefits (Graham, 1992).
L-theanine or theanine is a major free amino acid uniquely found in
green tea (Konishi & Takahashi, 1969; Neumann & Montag, 1983;
Selvendran, 1970). It was first isolated and identified in green tea leaves,
Camellias sinensis, in 1949 by Sakato (1949) and in the mushroom,
Xerocomus badius, in the early 1950s (Casimir et al., 1960). Theanine
about 1-2% of the total dry weight of the green tea leaves (Goto et al.,
1996). L-theanine is synthesised in the roots of tea plants from glutamic
acid (glutamate) and ethylamine before being transported to the leaves
(Buckowski et al., 1999).
While, the historic use is well documented, the neuropharmacology of
L-theanine has not been described. There have only been a few experi-
mental studies of L-theanine and evidence from these studies suggests
that L-theanine may modulate key brain neurotransmitters and their re-
mals and humans describing effects on neurotransmitters levels, their re-
ceptors and behavior.
L-theanine (N-ethyl-L-glutamine) is structurally similar to glutamic
acid, which is one of the primary excitatory neurotransmitters in the cen-
tral nervous system. The structure of L-theanine and glutamic acid is
shown in Figure 1.
22 JOURNAL OF HERBAL PHARMACOTHERAPY
6.6andshows goodstabilityinbothneutral(pH =6.5)andacidic(pH =
3.0) conditions in 12 months storage with temperature below 25?C
(Juneja et al., 1999).
Kitaoka et al. (1996) reported that oral administration of L-theanine
to guinea pigs results in absorption of L-theanine through the intestinal
tract via a common Na?-coupled co-transporter in the brush-border
membrane in the same manner as glutamine. Similarly, both oral and
intraperitoneal administration of L-theanine administered to rats, leads
to absorption through the intestinal tract and then is subsequently hy-
drolysed to glutamic and ethylamine in the rat kidney (Unno et al.,
1999; Desai et al., 2005). The latter finding suggests that the kidney is
the most effective site for the enzymatic hydrolysis of L-theanine to
glutamic acid and ethylamine (Unno et al., 1999).
Studies suggest that L-theanine also crosses the blood-brain barrier
and is transported in a dose-dependent manner into the brain. Transport
into the brain occurs via the leucine-preferring transport system of the
blood-brain barrier (Yokogoshi et al., 1998a). The plasma concentra-
tions of L-theanine and ethylamine reach their highest levels about 0.5
and 2 hours, respectively (Unno et al., 1999). Kimura and Murata
(1971) reported that L-theanine (administered intraperitoneally) was
taken up by brain tissue within 30 minutes after its administrationwith-
out any metabolic change in mice. In animals, L-theanine concentra-
tions reaches maximal levels at 1hour post-administration in blood
Nathan et al.23
FIGURE 1. Chemical Structure of Theanine and Glutamic Acid.
serum and liver, and 5 hours post-administration in the central nervous
system (Terashima et al., 1999). Concentrations were shown to decline
gradually over the course of 24 hours (Terashima et al., 1999). No
pharmacokinetic studies have been conducted in humans.
EFFECTS ON BRAIN NEUROTRANSMITTERS
Effects on Monoamines
There are limited studies on the effects of L-theanine on brain neuro-
transmitters. Yokogoshi et al. (1998a) reported a dose-related increase
in striatal dopamine release after both intra gastric and direct injection
ated by the utilizationof Ca2?by simulation of NMDA receptors as the
effect of L-theanine on dopamine was inhibited by the NMDA receptor
antagonist AP-5 (Yokogoshi et al., 1998a). The concentrations of nor-
adrenaline was found to be unaffected by L-theanine administration
(Yokogoshi et al., 1998a), suggesting selective effects on dopamine
neurons in the brain. However an earlier study in rats reported that L-
theanine may affect noradrenaline-dependent signaling pathways as it
phosphate (cAMP) formation (Kimura & Murata, 1980). This could,
however, be a general effect on second messenger pathways as in the
same study, the histamine stimulated cAMP formation was also found
to be inhibited by L-theanine.
An initial study showed that theanine was found to increase trypto-
phan, the precursor of serotonin, but it decreased both serotonin and the
serotonin metabolite 5-hydroxy indole acetic acid (5-HIAA) in the
brain (Yokogoshi et al., 1998a). However, a subsequent study by the
same authors investigating specific brain regions reported that intra-
gastric L-theanine significantly increased serotonin levels in the stria-
tum, hippocampus and hypothalamus (Yokogoshi et al., 1998b). The
discrepancy in the serotonin findings could be due to the fact that the
former study reported whole brain concentrations, whereas the latter
study reported changes within specific brain areas.
Effects on GABA
One study reported an increase in brain ?-aminobutyric acid (GABA)
concentrations following L-theanine administration in mice (Kimura &
24 JOURNAL OF HERBAL PHARMACOTHERAPY
Murata, 1971). In the same study, the convulsive effect of caffeine was
inhibited by L-theanine suggesting a possible GABA related anti-con-
EFFECTS ON NEUROTRANSMITTER RECEPTORS
A recent study suggests that L-theaninehas low affinity for a number
of glutamate receptors including AMPA (?-Amono-3-hydroxy-5-
Methylisoxazole-4-Propionic Acid), kainate, and the glycine site of the
NMDA (N-Methyl-D-Aspartate) receptor (Kakuda et al., 2002). The
reported affinities were in the micromolar range (AMPA = 19.2 μM;
Kainate = 0.373 μM; NMDA glycine site = 329 μM), with the binding
activity (antagonistic) for AMPA and kainite receptors being 10-fold
higher than that for NMDA receptor glycine sites. While the affinity of
theanine was 80-fold lower than glutamate, the reported affinities are
thought to play some role in neuroprotection (Kakuda et al., 2002).
receptors including receptors for monoamines.
Kakuda et al. (2000) examined the neuroprotective effect of L-
campus. Ischemic neural death in the field CA1 of hippocampus was
significantly suppressed when L-theanine was pre-treated in a dose-de-
of cerebral infarction in mice, theanine (administered before and 3
hours after the occlusion) reduced the size of the cerebral infarcts
remains relatively unknown. Since L-theanine is a natural glutamate
analogue, one mechanism responsible for the neuroprotective effects
may be related to L-theanine’s affinity to glutamate receptor subtypes
such as AMPA, kainite and NMDA (Kakuda, 2002). It has been sug-
gested that L-theanine may act on glutamate receptors as an antagonist
contribute towards its neuroprotective effects (Kakuda et al., 2000).
effects was blocked by group 1 metabotrophic glutamate receptor
Nathan et al. 25
in the neuroprotective effects of theanine.
BEHAVIOURAL PHARMACOLOGY OF L-THEANINE
L-Theanine and Cognition
Studies have reported potential cognitive enhancing effects of L-
theanine, particularly concerning learning and memory. Juneja et al.
(1999) examined chronic effects (4 months) of L-theanine administra-
tion(180 mg/day)on memoryandlearningabilityinrats.Thestudy ap-
plied an operant conditioning paradigm to examine learning ability.
During operant conditioning paradigm, food was delivered when rats
pushed a lever and a light turned on. Learning ability was significantly
responses when compared to the control rat group). Learning ability
ance test. The avoidance conditions assessed general tendency of the
rats to move from a light to a dark compartment. In the passive avoid-
ance test, the electric shock was applied immediately after a rat moved
from a light to a dark compartment. Rats administered L-theanine
(180 mg/day) showed greater cognitive ability, reporting hesitation to
move to the dark compartment, and hence remained longer in the light
compartment compared with control group. The active avoidance test
examined the escape behaviour of rats from an electric shock. Rats
treated with L-theanine showed an increase in avoidance behaviour,
which signified an improvement of their memory ability.
Similarly, Yokogoshi and Terashima (2000) reported significant im-
provement of avoidance learning ability during both passive and active
avoidance tasks, following long-term L-theanine administration (3 mon-
ths). In addition, memory ability as estimated by the Morris Water Maze
(MWM) transfer test was also improved by the chronic administration of
L-theanine (Yokogoshi & Terashima, 2000). The MWM has been exten-
sively used as an optimal preparation for assessing cognitive functions in
animals. The MWM is a widely accepted test of spatial learning in ro-
eral centimetres below the water level in a circular pool filled with water
(Morris, 1981; Graziano et al., 2002; Higgins et al., 2002). Rats adminis-
tered L-theanine were faster and more consistent than the control rats in
finding the hidden platform in the MWM experiment.
26 JOURNAL OF HERBAL PHARMACOTHERAPY
The findings of the above studies suggest that L-theanine may have
positive behavioural effects on learning and memory. These findings
support the neurochemical findings showing increases in monoamines
(Yokogoshi et al., 1998a,b). Given that serotonin and dopamine have
been shown to improve cognitive processes such as attention, learning
and memory (Andre, 2002; Menesses, 1999), it is possible that the
learning and memory effects of L-theaninemay be mediatedvia manip-
ulation of serotonin and dopamine.
L-Theanine and Anxiety
Whilethereis historicalevidencefor possibleanxiolyticeffectsof L-
theanine, very few studies have examined this experimentally in ani-
mals or humans. Ito et al. (1998) examined the effects of L-theanine
(200 mg) on brain ?-activity in eight human volunteers. Participants
were divided into two equal groups: high anxiety and low anxiety,
based on the Manifest Anxiety Scale. The study reported relaxation
effects resulting from the generation of ?-activity in the occipital and
generated around 30 minutes after L-theanine administration, which is
consistent with the pharmacokinetic finding in rats showing that L-
ministration (Kimura & Murata, 1971). While, L-theanine may modu-
as similar effects on ?-activity are observed with non-anxiolytics (i.e.,
amphetamines) (Montagu, 1968). Thus modulating ?-activity may not
be a direct measure of possible anxiolytic effects. In addition the latter
study failed to examine subjective anxiety levels and hence it is unclear
increases both serotonin and GABA (Kimura & Murata, 1971; Yoko-
goshi et al., 1998a,b), and both these neurotransmitters play an impor-
tant role in the pathophysiology of anxiety disorders and anxiolytic
effects (Gonzalezet al., 1998; Graeff, 2002; Kent et al., 2002), it is pos-
sible L-theanine may have anxiolytic effects.
We recently examined the acute effects of L-theanine (200 mg) in
comparison with the benzodiazepine, alprazolam on anticipatory anxi-
ety in humans (Lu et al., 2004). The effects were assessed under a re-
laxed and experimentally induced anxiety (anticipation of an electric
shock) condition. The findings suggested that while L-theanine may
have some relaxing effects under resting conditions as evidence by re-
duction in anxiety level on the tranquil-troubled subscale of the visual
Nathan et al.27
analogue mood scale (VAMS), both L-theanine and alprazolam did not
demonstrate any acute anxiolytic effects under conditions of increased
the chronic effects of L-theanine of anxiety are warranted.
TOXICOLOGY OF L-THEANINE
There are no reported side effects in studies investigating L-theanine
within animals (Kakuda et al., 2000) or humans (Ito et al., 1998). L-
theanine has also shown to produce no drowsiness in humans (Ito et al.,
1998). Sudzuka et al. (1996) showed studies on cancer chemotherapy
demonstrating that L-theanine enhanced the therapeutic efficacy of
doxorubicin without inducing side effects.
Pre-clinical studies suggest that L-theanine increases a number of
neurotransmittersincluding serotonin, dopamine and GABA levels and
havioural studies in animals demonstrate improvementsin learning and
memory following L-theanine administration. In addition L-theanine
bly through its antagonisticeffectson group 1 metabotrophicglutamate
receptors. While studies in humans are lacking, there is some evidence
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