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Journal of Alzheimer’s Disease 20 (2010) S3–S15 S3
DOI 10.3233/JAD-2010-1379
IOS Press
Review Article
Caffeine and Adenosine
Joaquim A. Ribeiro∗and Ana M. Sebasti˜
ao
Institute of Pharmacology and Neurosciences, Facultyof Medicine and Unit of Neurosciences, Institute of
Molecular Medicine, University of Lisbon, Lisbon, Portugal
Abstract. Caffeine causes most of its biological effects via antagonizing all types of adenosine receptors (ARs): A1, A2A, A3,
and A2B and, as does adenosine, exerts effects on neurons and glial cells of all brain areas. In consequence, caffeine, when
acting as an AR antagonist, is doing the opposite of activation of adenosine receptors due to removal of endogenous adenosinergic
tonus. Besides AR antagonism, xanthines, including caffeine, have other biological actions: they inhibit phosphodiesterases
(PDEs) (e.g., PDE1, PDE4, PDE5), promote calcium release from intracellular stores, and interfere with GABA-A receptors.
Caffeine, through antagonism of ARs, affectsbrain functions such as sleep, cognition, learning, and memory, and modifies brain
dysfunctions and diseases: Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, Epilepsy, Pain/Migraine, Depression,
Schizophrenia. In conclusion, targeting approaches that involve ARs willenhance the possibilities to correct brain dysfunctions,
via the universally consumed substance that is caffeine.
Keywords: Adenosine, Alzheimer’s disease, anxiety, caffeine, cognition, Huntington’s disease, migraine, Parkinson’s disease,
schizophrenia, sleep
INTRODUCTION
Caffeine causes most of its biological effects via
antagonizing all types of adenosine receptors (ARs).
When acting as an AR antagonist, caffeine, used acute-
ly, is doing the opposite of activation of adenosine re-
ceptors,dueto removalofthe adenosinergictonus. The
adenosine A1 and A2A receptors have high affinity for
adenosine and are those responsible for tonic actions
of endogenous adenosine. So, in the present review
we will focus on A1 and A2A adenosine receptors and
on the mechanisms they operate in order to infer how
caffeine exerts most of its actions in the brain. There
are many studies reporting actions of caffeine in hu-
mans where it is not completely clear if those actions
are mediated by adenosine receptors. These studies,
in spite of being relevant for caffeine research per se,
∗Correspondence to: J.A. Ribeiro, Institute ofPharmacology and
Neurosciences, Faculty of Medicine and Unit of Neurosciences, In-
stitute of Molecular Medicine, University of Lisbon, Av Prof Egas
Moniz, 1649–028 Lisbon, Portugal. Tel.: +351 217985183; E-mail:
jaribeiro@fm.ul.pt.
were considered out of the scope of the present work.
For more detailed analysis of the actions of caffeine in
humans, namely cognition, dementia, and Alzheimer’s
disease, the reader may refer to other papers published
in the present issue.
The broad caffeine intake in common beverages, to-
gether with the impact of xanthines on biomedical re-
search, prompted many studies that focus on specif-
ic caffeine effects rather than using it as a tool to an-
tagonize adenosine receptors (ARs) [1–3]. Caffeine
is mainly present in coffee, which also contains trace
amounts of theophylline, but no theobromine. Tea is
another common source of caffeine. As a pharmaco-
logical tool, caffeine is not very useful since its affinity
for ARs is low and its selectivity towards the different
ARs is also very poor. Caffeine is an antagonist of all
subtypes of ARs, andchronicor acute intake of caffeine
may affect ARs in different and even opposite ways.
Having similar affinity for A1 and A2A Rs [1], acute
caffeineactionsata givenbrainareawill reflect the pre-
ponderant AR activationin that area, since most of the
adenosinergic tonus are exerted through that receptor.
ISSN 1387-2877/10/$27.50 2010 – IOS Press and the authors. All rights reserved
S4 J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine
ADO
Sleep and level of arousal
Neuronal maturation/
development
Control of ventilation
ADO, neuronal functions, dysfunctions & diseases
Anxiety
Stroke
Alzheimer s disease
Huntington s disease
Schizophrenia
Drug addiction
Pain
Parkinson s disease
Myasthenia gravis
Epilepsy
Depression
ALS
Cognition and memory
Fig. 1. Actions proposed for adenosine on the central nervous system, including on brain functions, dysfunctions and diseases. As caffeine is
a non-selective adenosine antagonist and crosses easily the blood brain barrier, it is likely that the caffeine effects on these adenosine receptors
mirrorthosecausedbyadenosineactions.ForfurtherdetailsseeSebasti˜aoandRibeiro,2009–HandbookofExperimentalPharmacology, 193,
471–534.
Besides the high affinity A1 and A2A receptors, the
cloned adenosine receptors also include the high affini-
tyA3receptor,andthelowaffinityA2Breceptor. Other
entities have been proposed based on functional and/or
binding studies (e.g., an atypical A2A receptor in the
rat hippocampus [4]; the A3 receptor in the frog motor
nerve endings [5]). The first proposal for the existence
of an A3 AR was based upon pharmacologicalcharac-
teristics, namely high affinity for agonists and xanthine
sensitivity [5]. Cloning and cellular expression of the
rat A3 AR [6] challenged these criteria since the rat
A3 receptor is xanthine-insensitive and has low ago-
nist affinity. Cloning and expression of the human A3
AR [7] reversed the situation again since the human A3
AR is xanthine sensitiveand is a high affinity receptor
for A3 AR ligands. Although research on the relevance
of the A3 AR under pathological conditions is gain-
ing progressive interest, these receptors are poorlyex-
pressed in the brain and studies involving them on the
actions of caffeine are scarce. Therefore, we decided
not to discuss this aspect in the present review.
Adenosine is ubiquitously present in all cells, with
receptors distributed inall brain cells; anyimbalance of
such a widespread system is expected to lead to neuro-
logicaldysfunctions/diseases(see Fig. 1). When acting
as an AR antagonist, caffeine is doing the opposite of
adenosine receptors activation, whenever the levels of
endogenous adenosine are tonically activating recep-
tors. So caffeine, like adenosine, can potentiallyexert
effects on all brain areas, providing that endogenous
adenosine is tonically activating its receptors. As a re-
sult of its psychoactive effects, caffeine is considered
by some religions (e.g., Mormons, Adventists, Hin-
dus), along with alcohol, nicotine, and other drugs, to
cloud the mind and over-stimulate the senses.
In 1819 the German chemist Friedrich Ferdinand
Runge isolated caffeine at the behest of Johann Wolf-
gang von Goethe. As in the work ‘Faust’ by Goethe,
the soul of Faust has been sold to the devil in exchange
for ‘jeunesse’, it appears that Goethe was anticipating,
in almost 200 years, the use of caffeine to treat diseases
that predominate during aging, such as neurodegener-
ative diseases.
The structure of caffeine was elucidated near the end
of the 19th century by Hermann Fischer, and it is sim-
ilar to that of adenosine. Caffeine is metabolized in
J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine S5
CAFFEINE
ARs
Intracellular
Ca2+
release
Phosphodiesterases
(PDE1, PDE4, PDE5) inhibition
GABARs
Fig. 2. Sites/mechanisms of action of caffeine. ARs: adenosine
receptors. GABARs: GABA receptors.
the liver by the cytochrome P450 oxidase enzyme sys-
tem into three dimethylxanthines: paraxanthine, which
increases lipolysis, leading to elevated glycerol and
free fatty acid levels in the blood plasma; theobromine,
whichdilatesbloodvesselsandincreasesurinevolume;
and theophylline, which relaxes smooth muscles of the
bronchi, and is used to treat asthma. The therapeutic
dose of theophylline, however, is many times greater
than the amount resulting from caffeine metabolism
taken in non-toxic amounts. Each of those xanthines
is further metabolized and then excreted into the urine.
For an extensive review including consumption and
metabolism of caffeine, see [2].
Adenosine is able to regulate synapses through tun-
ing and fine-tuning. Tuning synapses occurs when
adenosine, by activating its receptors, is controlling,
e.g.therelease of neurotransmitters, by interfering with
Ca2+ or other mechanisms directly related to neu-
rotransmitter release [8]. In the case of fine-tuning,
adenosine is interfering with receptors for other neu-
romodulators [9]. Besides AR antagonism, xanthines,
including caffeine, have other biological actions (see
Fig. 2), such as 1) inhibition of phosphodiesterases
(PDEs) (e.g., PDE1, PDE4, PDE5). These effects (up
to 40 % inhibition of phosphodiesterases), according
to Daly (2007) [1], are observed in concentrations well
below those that cause toxic effects. In relation to PDE
inhibition, it is interesting to note that caffeine, being
a PDE5 inhibitor, operates through a mechanism also
used by sildenafil, which is a vasodilator, via selective
PDE5 inhibition. So, the potential effects related to
these actions need to be investigated to see whether
consequent vasodilation might contribute to net caf-
feine effects. 2) Promotion of calcium release from
intracellular stores. Application of caffeine-halothane
contracture test in the diagnosis of malignant hyper-
thermia is an example of application of this effect. 3)
Interfering with GABA-A receptors [1]. According to
Daly [1], caffeine analogues can be developed to tar-
get any of these mechanisms rather than ARs, and this
may be explored therapeutically [1]. However, in the
case of caffeine, the effects seen at very low doses,
achieved duringnormalhuman consumption,are most-
ly due to AR antagonism [2]. Because of its safety,
its ability to antagonize ARs and to readily cross the
blood brain barrier, caffeine has therapeutic potential
in central nervous system dysfunctions (see below and
Fig. 1). Adverse effects of caffeine may include anx-
iety, hypertension, drug interactions, and withdrawal
symptoms [1]. Caffeine improves cognition [1]; how-
ever,it also affects sleep [3]. Moreover, a relationship
between adenosine A2A ARs and genetic variability in
caffeine metabolism associated with habitual caffeine
consumption, has been proposed [10], which provides
a biological basis for caffeine consumption. In that
study, personswith the ADORA2A TT genotype were
significantly more likely to consume less caffeine than
carriers of the C allele.
Thetherapeuticoradverseeffectsofcaffeinearecon-
siderably different, dependingon whether it is adminis-
tered chronically or acutely. For example, chronic caf-
feine intake, which increases plasma concentrations of
adenosine [11], may be neuroprotective. This is in con-
trast with the consequences of acutely antagonizing A1
ARs [12]. Chronic AR antagonism with caffeine may
alsoinfluencecognitionandmotoractivityinawaythat
resembles the acute effects of AR agonists [13]. Such
opposed actions of chronic versus acute treatment not
only have important implications in the development
of xanthine- based compounds as therapeutic agents,
but also constitute a frequently confounding parameter
for research. Up-regulation of A1 ARs after chronic
AR antagonism with xanthines occurs, but A2A AR
levels apparently do not change. In addition there are
changes in the levels of receptors for neurotransmit-
ters with chronic administration of xanthines, namely
a marked decrease in β-adrenergic receptors and an in-
crease in 5-HT and GABA-A receptors [13]. The in-
creased expression of A1 ARs in response to chronic
antagonism of ARs by caffeine, as compared with A2A
ARs, may lead to a shift in the A1/A2A AR balance
S6 J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine
after prolonged caffeine intake [3]. Moreover, chron-
ic caffeine treatment may lead to modifications in the
function of the A1R–A2AR heteromer and this may, in
part, be the scientific basis for the strong tolerance to
the psychomotor effects of chronic caffeine [14]. Al-
teration of astrocytogenesis via A2A AR blockade dur-
ing brain development has been reported [15], raising
the possibility that postnatal caffeine treatment could
have long-term consequences on brain function, and
therefore care should be taken during breast feeding.
Tolerance/Withdrawal
Tolerance develops very quickly, after heavy doses,
e.g. tolerance to sleep disruption (400 mg of caffeine 3
times a day for 7 days), tolerance to subjective effects
of caffeine (300 mg 3 times per day for 18 days), and
withdrawal symptoms, including inability to concen-
trate, headache, irritability, drowsiness, insomnia, and
paininthe stomach,upperbody,andjoints(within 12 to
24 hours after discontinuation of caffeine intake, peak
being at roughly 48 hours, and usually lasting from one
to five days, see Fredholm et al. [2]). This is the time
required for the number of adenosine receptors in the
brain to revert to “normal” levels. Analgesics, such
as aspirin, can relieve the pain withdrawal symptoms,
as can a small dose of caffeine [1]. Overuse and de-
pendency occurs after consumptionof caffeine in large
amounts, and in particular over extended periods of
time, inducing caffeinism. Caffeinism combines caf-
feinedependencywith a wide range of unpleasant phys-
ical and mental conditions including nervousness, irri-
tability, anxiety, tremulousness, muscle twitching, hy-
perreflexia, insomnia, headaches, respiratory alkalosis,
and heart palpitations. Caffeine increases production
of stomach acid; highusage over timecan lead to peptic
ulcers,erosiveesophagitis, and gastroesophagealreflux
disease.
The influence of caffeine-adenosine receptor inter-
actions upon brain functions and dysfunctions will be
discussed below.
ANXIETY
Caffeineis well known to promoteanxiousbehaviour
in humans and animal models, and can precipitate pan-
ic attacks [16]. It is of interest that patients suffering
from panic disorder, a serious form of anxiety disorder,
appear to be particularly sensitive to small amounts of
caffeine [17]. It is, however, worthwhile to note that
chronic and acute caffeine consumption may lead to
quite different consequences with respect to the func-
tion of ARs [18,19]. Short-term anxiety-like effect
of caffeine in mice might not be related solely to the
blockade of A1 and A2A ARs, since it is not shared by
selective antagonists of each receptor [20]. In contrast,
anxiolytic effects of xanthine derivatives containingan
arylpiperazine moiety have been reported, but this is
most probably related to agonist activity at serotonin
receptors rather than antagonism of adenosine recep-
tors [1].
The possibility that drugs which facilitate A1 AR-
mediatedactionscouldbeeffectiveforanxietywassup-
ported by the observations that A1 AR agonists have
anxiolytic actions in rodents [20,21]. The inhibitory
action of A1 ARs on the nervous system, together with
the identification of cross-talk mechanisms between
benzodiazepines and ARs [22] and transporters [23],
soon suggested that adenosine could mediate the anxi-
olytic action of several centrally active drugs [24]. Ac-
cordingly,A1 AR KO mice showed increased anxiety-
related behaviour [25], but this also holds true for
A2A AR KO mice [26]. A1 and A2A ARs are in-
volved in benzodiazepine withdrawal signs. In mice,
these signs of withdrawal are manifested by increased
seizure susceptibility, and agonists of A1 ARs [27]
or A2A ARs [28] attenuate them. The potential of
A1 AR agonists to reduce the anxiogenic effects dur-
ing ethanol withdrawal have also been suggested [29].
The caffeine-induced anxiety disorder, which can re-
sultfromlong-termexcessivecaffeineintake, canmim-
ic organic mental disorders, such as panic disorder,
generalized anxiety disorder, bipolar disorder, or even
schizophrenia. Caffeine-intoxicated people might be
misdiagnosed and unnecessarily medicated when the
treatment for caffeine-induced psychosis would simply
be to stop further caffeine intake. Other adverse effects
of caffeine besides anxiety, sleep disorders, withdraw-
al symptoms and hypertension, include drug interac-
tions [1].
A significant association between self-reported anx-
iety after caffeine administration and two linked poly-
morphismsof theA2AAR gene has been reported [30].
Furthermore, evidence for a susceptibility locus for
panic disorder, either within the A2A AR gene or in a
nearbyregion of chromosome22, was reported[31,32].
This positive association between A2A AR gene poly-
morphism and panic disorder may, however, not occur
in the Asian population [33] suggesting an ethnicity-
dependent association.
J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine S7
SLEEP
Most studies on ARs and sleep regulationin humans
relyuponconsequences of caffeine ingestion by human
volunteers, and it is now widely accepted that caffeine
prolongs wakefulness by interfering with the key role
of adenosine upon sleep homeostasis [34]. In a review
on the role of adenosine upon sleep regulation, Porkka-
Heiskanen et al. [35] proposed adenosine as a sleep-
ing factor and hypothesized that adenosine works as a
neuroprotector against energy depletion. In the criti-
cal arousal area (basal forebrain), extracellular adeno-
sine levels start to rise in response to prolonged neu-
ronal activity during wakeful periods. This increase
leads to a decrease in neuronal activity, and sleep is
induced before the energybalance, in the whole brain,
is affected. Microdialysis measurements performed in
freely moving cats showed an increase in the concen-
trations of adenosine during spontaneous wakefulness,
and adenosine transport inhibitors mimicked the sleep-
wakefulness profile occurringafter prolonged wakeful-
ness [36]. In contrast, AR antagonists, like caffeine,
increase wakefulness. Prolonged wakefulness induces
signs of energy depletion in the brain, which causes
facilitation of sleep [37]. Molecular imaging showed
that there is A1 receptor upregulation in cortical and
subcortical brain regions after prolonged wakefulness
in humans [38]. Adenosinergic mechanisms contribute
to individual differencesassociated with sleep depriva-
tion sensitivity in humans [39]. Furthermore, a genet-
ic variation in the adenosine A2A AR gene may con-
tribute to individual sensitivity to the effects of caffeine
on sleep [40].
It is well documented that A1ARs are involved in
sleep regulation by inhibiting ascending cholinergic
neurons of the basal forebrain [41]. However, more
recent studies, which include experiments with A2A
and A1 AR KO mice, indicate that A2A ARs (most
probably localized in the ventrolateral preoptic area
of the hypothalamus) also play a crucial role in the
sleep-promoting effects of adenosine and the arousal-
enhancing effects of caffeine [42]. These studies sug-
gestthatA2A AR antagonists may represent a novelap-
proach as potential treatments for narcolepsy and other
sleep-related disorders [43]. Adenosine A2A ARs in
the pontine reticular formation promote acetylcholine
release, rapid eye movement (REM) and non-REM
sleep in mice. This effect on non-REM sleep is proba-
bly due to A2A AR-induced enhancement of GABAer-
gic inhibition of arousal promoting neurons [44]. In
addition to its action in the basal forebrain, adeno-
sine exerts its sleep-promoting effect in the lateral hy-
pothalamus by A1 AR-mediated inhibition of hypocre-
tin/orexin neurons [45,46]. According to the American
Psychiatric Association (APA), the caffeine-induced
sleep disorder is sufficiently severe to warrant clinical
attention.
In summary, the two high affinity ARs, the A1 and
the A2A ARs affect multiple mechanisms in several
brain areas involved in regulation of sleep and arousal.
Therefore, the influences of caffeineupon sleep felt by
manyhumans, and as mentioned above,also document-
ed in controlled studies in healthy volunteers, can be
attributed to both A1 and A2A AR blockade. Chronic
caffeine consumption may alter AR function and the
A1/A2A AR balance, and as a consequence influences
the involvementof both ARs upon sleep.
COGNITION, LEARNING, AND MEMORY
Endogenous adenosine, through A1 ARs, inhibits
long-term synaptic plasticity phenomena, such as long
term potentiation (LTP) [47], long term depression
(LTD), and depotentiation [48]. In accordance, A1 AR
antagonists have for a long time been proposedto treat
memory disorders [49]. Cognitive effects of caffeine
are mostly due to its ability to antagonise adenosine
A1 ARs in the hippocampus and cortex, the brain ar-
eas mostly involved in cognition, but as discussed in
detail [2], positive actions of caffeine on information
processing and performance might also be attributed to
improvementof behavioural routines, arousal enhance-
ment and sensorimotor gating, and these actions may
be not solely related to A1 receptor function (see be-
low). Theophylline enhances spatial memory perfor-
mance only during the light period,which is the time of
sleepiness in rats [50]. Independently of the processes
used by caffeine or theophyllineto improve cognition,
there is a consensus that the beneficial effects most of
us feel after a fewcups of coffee ortea are due to the ac-
tions of these psychoactive substances upon ARs. Re-
cent evidence that blockade of A1 receptors improves
cognition came from a study using a mixed A1/A2A
receptor antagonist, ASP5854 [51]. This orally ac-
tive drug could reverse scopolamine-induced memory
deficits in rats, whereas a specific adenosine A2A AR
antagonist, KW-6002, did not. Reduced A2A AR acti-
vationmay alsobe relevantforcognitiveimprovements
since A2A AR KO mice have improvedspatial recog-
nition memory [52]. Accordingly, over-expression of
A2A ARs leads to memory deficits [53].
S8 J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine
Thereis the possibility that chronic intake of caffeine
during one’s lifetime might protect from cognitive de-
cline associated with aging. Elderly women who drank
relatively large amounts of coffee over their lifetimes
have better performances on memory and other cog-
nitive tests than non-drinkers [54]. A case – control
study was specifically designed to evaluate if chron-
ic intake of caffeine might be related to a lower risk
of Alzheimer’s disease [55], the most common form
of dementia. Levels of caffeine consumption in the
20 years that preceded the diagnosis in patients were
compared with those taken by age- and sex-matched
controls with no signs of cognitive impairment. Data
analysis showed that caffeine intake was inversely as-
sociated with the risk of Alzheimer’s disease and that
this association was not explained by several possible
confounding variables related to habits and medical
disorders [55]. This was confirmed in a larger scale
study (4,197 women and 2,820 men) with similar ob-
jectives, showing that the psychostimulant properties
of caffeine appear to reduce cognitive decline in aged
women without dementia [56].
Long-term protective effects of dietary caffeine in-
take were also shown in a controlled longitudinalstudy
involving a transgenic murine model of Alzheimer’s
disease. Caffeine was added to the drinking water of
mice between 4 and 9 months of age, with behavioural
testing done during the final 6 weeks of treatment; the
results revealed that moderate daily intake of caffeine
may delay or reduce the risk of cognitive impairment in
these mice [57]. Amnesia can be induced experimen-
tally in mice by central administration of beta-amyloid
peptides, a process that involves cholinergic dysfunc-
tion [58]. Acute intravenous administration of caf-
feine or A2A AR antagonists affords protection against
beta amyloid-induced amnesia [59]. These acute ef-
fects of A2A AR blockade are somehow unexpect-
ed because A2A ARs are known to facilitate cholin-
ergic function mainly in the hippocampus [60], and
therefore, either adenosine A2A AR agonists or A1
AR antagonists, which prevent A1 AR-mediated inhi-
bition of acetylcholine release, were more likely ex-
pected to be cognitive enhancers. Indeed, the most
widely used drugs in Alzheimer’s disease are directed
towards an increase in cholinergic function by inhibit-
ing acetylcholinesterase [61]. These apparent discrep-
ancies point toward the need of more basic research
to understand the biological basis and the potential
benefit for the emerging adenosine-based therapies for
Alzheimer’s disease. It is interesting to note the very
recent reports by Arendash’s groupon caffeine protec-
tion in Alzheimer’sdisease transgenic mice [62,63]. In
a very recent study [64] it has been shown that human
coffeedrinkingat midlife is associated with a decreased
risk of dementia/AD later in life. This finding further
supports possibilities for preventionof dementia/AD.
PARKINSON’S DISEASE
A significant association between higher caffeine in-
take and lower incidence of Parkinson’s disease was
reported some years ago [65]. Moreover, the beneficial
effects of caffeine in Parkinson’s disease patients was
also reported [66]. Furthermore, caffeine administered
before levodopa may improve its pharmacokinetics in
some patients with Parkinson’sdisease [67].
Caffeine has well-known stimulatory actions upon
locomotion due to the antagonism of A2A and A1 ARs
inthestriatum[3],and in most animal models ofParkin-
son’s disease, antagonizingA2A ARs attenuates some
disease symptoms, which has been matter of several
reviews published as proceedings of a meeting on the
topic [68–71]. So, we will highlight a point that is less
focused, which concerns interactions between adeno-
sine and neurotrophic factors. The putative role of the
neurotrophic factor, GDNF, in slowing or halting dis-
ease progression through facilitation of neuronal sur-
vival [72], and the facilitatory action of A2A ARs up-
on GDNF actions in striatal dopaminergic nerve end-
ings [73], raise the need of great cautionwhen blocking
A2A ARs in the early phases of Parkinson’s disease.
If trophic GDNF actions on dopaminergic neurons will
also prove to be dependent upon co-activation of A2A
ARs, as it has been observed in relationto fast synaptic
actions of this neurotrophic factor [73], it is possible
that blockade of A2A ARs will be deleterious during a
window of time when it is possible to rescue neurons
with trophic support.
Another relevant consideration is related to the re-
cent finding [74] that deep brain stimulation, a proce-
dure now used to reduce tremor in Parkinson’s disease
patients, involves the release of considerable amounts
of ATP with its subsequent extracellular metabolism to
adenosine. Activation of A1 ARs by adenosine dur-
ing this procedure is an essential step to reduce tremor
and control spread of excitability, thereby reducingthe
side effects of deep brain stimulation. However, since
A2A ARs are expressed in thalamic areas, it may be
expected that A2A ARs are also activated during deep
brain stimulation. A2A receptors attenuate A1 recep-
tor functioning [75]. Furthermore, they attenuate D2
J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine S9
dopaminergic responses [3]. Thus, in late stages of
the disease, where it is desirable to prevent A2A AR-
mediated inhibition of dopamine D2 receptor function,
the use of an A2A AR antagonist in combination with
deep brain stimulation may be beneficial.
HUNTINGTON’S DISEASE
The role played by ARs in Huntington’s disease was
recently reviewed and discussed [76]. The complexity
inherent to a genetically – based, slowly progressing
neurodegenerative disease, the different experimental
models which are very frequently non-chronic or sub-
chronic models, as well as changes in receptor lev-
els due to cell loss or to prolonged drug administra-
tion, give an apparent contradictorypicture on the AR
involvement in this disease. The pre – versus post-
synaptic localization ofARs, in particular ofA2A ARs,
which have highly distinct roles in striatal functionac-
cording to their synaptic localization, may also con-
tribute to conflicting neuroprotective/neurotoxic con-
sequences of AR manipulation [77]. Indeed, A1 AR
agonists [78], A2A AR agonists [79], as well as A2A
AR antagonists [79], are all able to influence diverse
symptoms in experimental models of Huntington’s dis-
ease. For a detailed discussion of the causes for this
conflicting evidence see [76].
Another aspect that applies to all neurodegenerative
diseases, and that may be particularly relevant in the
case of Huntington’sdisease, is related to loss of neu-
rotrophic support. Huntington’s disease is caused by
a mutation in a protein named huntingtin that in its
mutated form is neurotoxic. It happens that wild-type
huntingtin up-regulates transcription of BDNF [80],
and decreased BDNF levels may be an initial cause of
neuronal death in this disease. A2A AR activation can
facilitate or even trigger BDNF actions in the brain [76,
81–83], pointing toward the possibility that A2A AR
activation,at least in the early stages ofthe disease, may
rescue striatal neurons from death due to diminished
trophic support by BDNF. It is worth noting that A2A
ARs have a dual action in Huntington’s disease [76].
The ability of A2A ARs to facilitate actions of BDNF,
which is clearly deficient in this neurodegenerative dis-
ease [84], is most likely part of the positive influences
of A2A ARs against the disease.
EPILEPSY
There are several clinical reports on caffeine or theo-
phylline intake and seizure susceptibility [86,87], but
surprisingly, no mention is made of the main cause of
seizure induction by these drugs, i.e., AR antagonism.
Indeed, after the initial observation that adenosine
has anticonvulsant actions [87], the therapeutic poten-
tial of adenosine related compounds in epilepsy was
immediately pointed out [88], and it is now widely ac-
cepted that adenosine is an endogenousanticonvulsant,
an action mediated by inhibitory A1 ARs that restrain
excessive neuronal activity. Other ARs are, however,
involvedinseizurecontrol,though their role is most fre-
quently related to exacerbation of seizures. The influ-
ence of A3 and A2 ARs on GABAA receptor stability
hasbeen recently suggested [89],based on the observa-
tion that A3 or A2B AR antagonists, acutely applied to
oocytes transfected with human GABAA receptors, re-
duce rundown of GABAA currents. A2A ARs, by pro-
moting neuronal excitability, may also increase seizure
susceptibility. Indeed, A2A ARs KO mice are less
sensitive to pentylenetetrazol-inducedseizures [90].
It has been shown that A1 AR activation by local-
ly released adenosine is an efficient way to keep an
epileptic focus localized [91]. Therefore, attention is
now focused on the development of biocompatible ma-
terials for adenosine-releasing intrahippocampal im-
plants [92]. In line with the evidence for the anti-
epilepticroleof A1 ARs,A1 AR KO mice are more sus-
ceptibletoseizuresanddeveloplethal status epilepticus
after experimental traumatic brain injury [93]. There
are, however, limitations in the use of A1 AR agonists
asanticonvulsantdrugs due to theirpronouncedperiph-
eral side effects like cardiacasystole, as well as central
side effects like sedation [94]. A possibility would be
the use of partial agonists, which are more likely to dis-
play tissue selectivity. A N6,C8-disubstituted adeno-
sine derivative with low efficacy towards A1 AR acti-
vationin wholebrainmembranesbutwith high efficacy
as an inhibitor of hippocampal synaptic transmission
was identified [95]. Another approach that has been
more intensely explored is the use of compounds that
increase the extracellular concentrations of adenosine.
This has been attempted with adenosine kinase (AK)
inhibitors, which showed beneficial effects in animal
models of epilepsy, and an improved preclinical ther-
apeutic index over direct acting AR agonists [96]. An
evenmore refined approachwasthe local reconstitution
of the inhibitory adenosinergic tone by intracerebral
implantation of cells engineered to release adenosine,
S10 J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine
and this has been done using AK deficient cells [97].
The reverse also holds true, since transgenic mice over-
expressing AK in the brain have increased seizure sus-
ceptibility [91]. Furthermore, intrahippocampal im-
plants of AK-deficient stem cell-derived neural precur-
sors suppress kindling epileptogenesis [98]. The above
evidence suggests that adenosine-augmentingcell and
gene therapies may lead to improved treatment options
for patients suffering from intractable epilepsy [99].
AKismostly expressed in astrocytes [100],and over-
expression of AK after seizures, with consequent re-
duced adenosine inhibitory tone, contributes to seizure
aggravation [91]. However, release of interleukin-
6 (IL-6) from astrocytes induces an upregulation of
A1 ARs both in astrocytes [101] and neurons [102].
This leads to an amplification of A1 AR function, en-
hances the response to readily released adenosine, en-
ables neuronal rescue from glutamate-induced death,
andprotects animals from chemicallyinducedconvuls-
ingseizures [102]. Indeed, IL-6KOmice are moresus-
ceptible to seizures and lack the well known seizure-
induced up-regulation of A1 ARs [102].
Seizure-induced release of neurotrophic factors,
such as BDNF, may have beneficial and aggravating
actions upon epilepsy, the beneficial ones being mostly
related to promotion of cell survival, the deleterious
ones being related to excessive cell proliferation and
neuronal sprouting [103]. Adenosine, through A2A
AR activation, triggers and facilitates BDNF actions in
neurons [82,83], but the relevance of this interplay for
epilepsy remains to be explored.
PAIN/MIGRAINE
Caffeine makes pain relievers 40% more effective
in alleviating headaches and helps the body to absorb
headache medications more quickly,bringing faster re-
lief. Many headache drugs include caffeine in their
formula. It is also used with the vasoconstrictor er-
gotamine in the treatment of migraine and cluster
headachesaswellastoovercomethe drowsinesscaused
by antihistaminics. It is well established that A2A re-
ceptors are potent vasodilators, and therefore the influ-
ence of caffeine in migraine probably occurs through
A2A receptor antagonism [2]. However, other mech-
anisms might also coexist. Calcitonin Gene Related
Peptide – Receptors (CGRP-R) antagonists are useful
in the treatment of migraine, and as CGRP effects on
synaptic transmission are greatly enhanced by A2A re-
ceptors activation [104], one is tempted to speculate
thatA2Areceptorblockadeandconsequentattenuation
of CGRP-R activation might also contributeto the abil-
ity of caffeine to alleviate migraine. It is likely that its
association with CGRP receptor antagonists, useful to
treat migraine, will substantially increase the efficacy
of these drugs in the treatment of headache/migraine.
DEPRESSION
A2A AR KO mice and wild-type mice injected with
A2A AR antagonists were found to be less sensitive to
‘depressant’ challenges than controls [105], suggesting
that blockade of adenosine A2A ARs might be an in-
teresting target for the development of antidepressant
agents. Thisantidepressant-likeeffectofselectiveA2A
AR antagonists is probably linked to an interaction
with dopaminergic transmission, possibly in the frontal
cortex, since administration of the dopamine D2 re-
ceptorantagonist,haloperidol, prevents antidepressant-
like effects elicited by selective A2A AR antagonists
in the forced swim test (putatively involving cortex),
whereas it had no effect on stimulant motor effects
of selective A2A AR antagonists (e.g. caffeine, puta-
tively linked to ventral striatum) [106]. Depression is
frequently associated to loss of motivation and psy-
chomotor slowing. In this context, it is interesting to
note that A2A AR in the nucleus accumbens appear to
regulate effort-related processes, an action that could
be related to modulation of the ventral striatopallidal
pathway [107].
Besides A2A ARs, A1 ARs are also probably
involved in the antidepressant-like effect of adeno-
sine [108], which may be consequence of interactions
with the opioid system [109].
It is worthwhile to note that deep brain stimulation,
now widely used by neurosurgeons to treat tremor and
other movementdisorders, as well as a number of psy-
chiatric diseases including obsessive-compulsive dis-
orders and depression, produces its effects by induc-
ing the release of ATP which is subsequently converted
extracellularly to adenosine [74,110].
Results from clinical and basic studies have demon-
strated that stress and depression decrease BDNF ex-
pression and neurogenesis, leading to the neurotrophic
hypothesis of depression [111,112]. How adenosine
A2A AR-dependent facilitation of BDNF actions on
hippocampal synapses, namely enhancement of synap-
tictransmission[81]andenhancementofsynapticplas-
ticity [83], may contribute to some antidepressive ac-
tions of adenosine remains to be established.
J.A. Ribeiro and A.M. Sebasti˜
ao / Caffeine and Adenosine S11
SCHIZOPHRENIA
No study so far has directly evaluated the influence
of caffeine in schizophrenia, but there is growing ev-
idence that adenosine dysfunction may contribute to
the neurobiological and clinical features of schizophre-
nia [113]. Indeed, adenosine, via activation of A1
and A2A ARs, is uniquely positioned to influence glu-
tamatergic and dopaminergic neurotransmission, two
neurotransmitter systems that are mostly affected by
the disease. It is possible that an adenosine inhibito-
ry deficit may emerge, resulting in reduced control of
dopamine activity and increased vulnerability to exci-
totoxic glutamate action in the mature brain. Interac-
tions between A2A ARs and D2 receptors allow fur-
ther opportunity for mutual modulation between the
adenosine and dopamine systems [114]. These mech-
anisms could provide a rationale for an antipsychotic-
like profile of AR agonists, in particular A2A AR ag-
onists, to promote a reduction in D2 receptor signal-
ing [114], and A1 AR agonists to promote a reduc-
tion in dopamine release [113]. Indeed, dipyridamole,
a well known inhibitor of adenosine transporters, and
therefore an enhancer of extracellular adenosine levels,
may be of some therapeutic interest in schizophrenic
patients [115].
Reduced NMDA receptor function may contribute
to the cognitive and negative symptoms of schizophre-
nia [116]. The relationships between adenosine and
NMDA receptor functionare complex and may operate
in opposite ways. Thus, NMDA receptor activation in-
duces adenosine release [117,118], and therefore NM-
DA receptor hypofunction may induce a decrease in
adenosine-mediated actions. On the other hand, NM-
DA receptor activation suppresses neuronal sensitivity
toadenosine[119]. In addition,both A1[120] andA2A
ARs [121] can influence NMDA receptor functioning,
both receptors being able to inhibit NMDA currents in
different brain areas
CONCLUSION
Adenosine builds its influence on neuronal commu-
nicationviafine-tuning,‘synchronizing’ or ‘desynchro-
nizing’ receptor activation [9]. On the other hand, ab-
normal neural synchronization is considered to be cen-
tral to and the underlying basis for several neurolog-
ical diseases such as epilepsy, schizophrenia, autism,
Alzheimer’s disease, and Parkinson’s disease [122]. It
is well established that adenosine is involved in brain
homeostasis, and recently proposed to be crucial to
the effects of deep brain stimulation [74], which aims
to affect neuronal ‘synchronization’and, therefore, in-
fluence several psychiatric and neurodegenerativedis-
eases. One is, therefore, temptedto propose thatadeno-
sineworksas a sort of “universalmodulator”ora “mae-
stro”, being the main molecule involved in coordinat-
ing and controlling the synchronization of the release
and actions of many synaptic mediators. These ac-
tions of adenosine are operated by high affinity A1 and
A2A receptors, and caffeine affects both. Chronic caf-
feine may up-regulate adenosine receptors and exac-
erbate adenosine levels and some adenosine actions in
the brain.
In conclusion, targeting approaches that involve ab-
normal synchronization, namelyARs, will enhance our
possibilities to interfere in and/or correct brain dys-
functions. An efficient way may be through the use of
the universally consumed substance caffeine.
ACKNOWLEDGMENTS
The work in the authors’ laboratory is supported by
research grants from Fundac¸˜
ao para a Ciˆ
encia e Tec-
nologia (FCT), Gulbenkian Foundation and European
Union (COST B30).
Authors’ disclosures available online (http://www.j-
alz.com/disclosures/view.php?id=212).
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