Modification of adenosine modulation of acetylcholine release in the hippocampus of aged rats

Blackcurrant (Ribes nigrum L.) and Its Association with Donepezil Restore Cognitive Impairment, Suppress Oxidative Stress and Pro-inflammatory Responses, and Improve Purinergic Signaling in a Scopolamine-Induced Amnesia Model in Mice

Purinergic signaling plays a major role in aging and neurodegenerative diseases, which are associated with memory decline. Blackcurrant (BC), an anthocyanin-rich berry, is renowned for its antioxidant and neuroprotective activities. However, evidence on the effects of BC on purinergic signaling is lacking. This study investigated the effects of BC and its association with Donepezil (DNPZ) on learning and memory, on the modulation of purinergic signaling, pro-inflammatory responses, and oxidative markers in a mouse model of cognitive impairment chronically induced by scopolamine (SCO). Animals were divided into twelve groups and treated with BC (50 or 100 mg/kg), and/or DNPZ (5 mg/kg), and/or SCO (1 mg/kg). Results showed that SCO decreased spatial learning and memory as assessed by the Morris Water Maze test, and treatment with BC and/or DNPZ restored these effects. Furthermore, BC and/or DNPZ treatments also prevented changes in ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) and adenosine deaminase (ADA) activities and restored the increased density of P2X7 and A2A receptors in synaptosomes of the cerebral cortex of SCO-induced mice. Moreover, the increased Nod-like receptor protein 3 (NLRP3) and interleukin-1β expression, and the oxidative stress markers levels were reduced by BC and/or DNPZ treatments, compared with the SCO group. Overall, BC and/or DNPZ treatments ameliorated SCO-induced cognitive decline, alleviated oxidative stress and pro-inflammatory responses, and improved purinergic signaling. These findings underscore the potential of BC, especially when in combination with DNPZ, as a therapeutic agent for the prevention of memory deficits associated with aging or neurological diseases.

Feedback facilitation by adenosine A2A receptors of ATP release from mouse hippocampal nerve terminals

The adenosine modulation system is mostly composed by inhibitory A1 receptors (A1R) and the less abundant facilitatory A2A receptors (A2AR), the latter selectively engaged at high frequency stimulation associated with synaptic plasticity processes in the hippocampus. A2AR are activated by adenosine originated from extracellular ATP through ecto-5'-nucleotidase or CD73-mediated catabolism. Using hippocampal synaptosomes, we now investigated how adenosine receptors modulate the synaptic release of ATP. The A2AR agonist CGS21680 (10-100 nM) enhanced the K+-evoked release of ATP, whereas both SCH58261 and the CD73 inhibitor α,β-methylene ADP (100 μM) decreased ATP release; all these effects were abolished in forebrain A2AR knockout mice. The A1R agonist CPA (10-100 nM) inhibited ATP release, whereas the A1R antagonist DPCPX (100 nM) was devoid of effects. The presence of SCH58261 potentiated CPA-mediated ATP release and uncovered a facilitatory effect of DPCPX. Overall, these findings indicate that ATP release is predominantly controlled by A2AR, which are involved in an apparent feedback loop of A2AR-mediated increased ATP release together with dampening of A1R-mediated inhibition. This study is a tribute to María Teresa Miras-Portugal.

Adenosine A2A Receptor Blockade Provides More Effective Benefits at the Onset Rather than after Overt Neurodegeneration in a Rat Model of Parkinson's Disease

Adenosine A2A receptor (A2AR) antagonists are the leading nondopaminergic therapy to manage Parkinson's disease (PD) since they afford both motor benefits and neuroprotection. PD begins with a synaptic dysfunction and damage in the striatum evolving to an overt neuronal damage of dopaminergic neurons in the substantia nigra. We tested if A2AR antagonists are equally effective in controlling these two degenerative processes. We used a slow intracerebroventricular infusion of the toxin MPP+ in male rats for 15 days, which caused an initial loss of synaptic markers in the striatum within 10 days, followed by a neuronal loss in the substantia nigra within 30 days. Interestingly, the initial loss of striatal nerve terminals involved a loss of both dopaminergic and glutamatergic synaptic markers, while GABAergic markers were preserved. The daily administration of the A2AR antagonist SCH58261 (0.1 mg/kg, i.p.) in the first 10 days after MPP+ infusion markedly attenuated both the initial loss of striatal synaptic markers and the subsequent loss of nigra dopaminergic neurons. Strikingly, the administration of SCH58261 (0.1 mg/kg, i.p. for 10 days) starting 20 days after MPP+ infusion was less efficacious to attenuate the loss of nigra dopaminergic neurons. This prominent A2AR-mediated control of synaptotoxicity was directly confirmed by showing that the MPTP-induced dysfunction (MTT assay) and damage (lactate dehydrogenase release assay) of striatal synaptosomes were prevented by 50 nM SCH58261. This suggests that A2AR antagonists may be more effective to counteract the onset rather than the evolution of PD pathology.

A2AR and traumatic brain injury

Accumulating evidence has revealed the adenosine 2A receptor is a key tuner for neuropathological and neurobehavioral changes following traumatic brain injury by experimental animal models and a few clinical trials. Here, we highlight recent data involving acute/sub-acute and chronic alterations of adenosine and adenosine 2A receptor-associated signaling in pathological conditions after trauma, with an emphasis of traumatic brain injury, including neuroinflammation, cognitive and psychiatric disorders, and other severe consequences. We expect this would lead to the development of therapeutic strategies for trauma-related disorders with novel mechanisms of action.

The role of adenosine A2A receptors in Alzheimer's disease and tauopathies

Adenosine signals through four distinct G protein-coupled receptors that are located at various synapses, cell types and brain areas. Through them, adenosine regulates neuromodulation, neuronal signaling, learning and cognition as well as the sleep-wake cycle, all strongly impacted in neurogenerative disorders, among which Alzheimer's Disease (AD). AD is a complex form of cognitive deficits characterized by two pathological hallmarks: extracellular deposits of aggregated β-amyloid peptides and intraneuronal fibrillar aggregates of hyper- and abnormally phosphorylated Tau proteins. Both lesions contribute to the early dysfunction and loss of synapses which are strongly associated to the development of cognitive decline in AD patients. The present review focuses on the pathophysiological impact of the A_2ARs dysregulation observed in cognitive area from AD patients. We are reviewing not only evidence of the cellular changes in A_2AR levels in pathological conditions but also describe what is currently known about their consequences in term of synaptic plasticity, neuro-glial miscommunication and memory abilities. We finally summarize the proof-of-concept studies that support A_2AR as credible targets and the clinical interest to repurpose adenosine drugs for the treatment of AD and related disorders. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Increased Synaptic ATP Release and CD73-Mediated Formation of Extracellular Adenosine in the Control of Behavioral and Electrophysiological Modifications Caused by Chronic Stress

Increased ATP release and its extracellular catabolism through CD73 (ecto-5'-nucleotidase) lead to the overactivation of adenosine A_2A receptors (A_2AR), which occurs in different brain disorders. A_2AR blockade blunts mood and memory dysfunction caused by repeated stress, but it is unknown if increased ATP release coupled to CD73-mediated formation of extracellular adenosine is responsible for A_2AR overactivation upon repeated stress. This was now investigated in adult rats subject to repeated stress for 14 consecutive days. Frontocortical and hippocampal synaptosomes from stressed rats displayed an increased release of ATP upon depolarization, coupled to an increased density of vesicular nucleotide transporters and of CD73. The continuous intracerebroventricular delivery of the CD73 inhibitor α,β-methylene ADP (AOPCP, 100 μM) during restraint stress attenuated mood and memory dysfunction. Slice electrophysiological recordings showed that restraint stress decreased long-term potentiation both in prefrontocortical layer II/III-layer V synapses and in hippocampal Schaffer fibers-CA1 pyramid synapses, which was prevented by AOPCP, an effect occluded by adenosine deaminase and by the A_2AR antagonist SCH58261. These results indicate that increased synaptic ATP release coupled to CD73-mediated formation of extracellular adenosine contributes to mood and memory dysfunction triggered by repeated restraint stress. This prompts considering interventions decreasing ATP release and CD73 activity as novel strategies to mitigate the burden of repeated stress.

A Review on Pathophysiological Aspects of Sleep Deprivation

Sleep deprivation (SD) (also referred as insomnia) is a condition in which individuals fail to get enough sleep due to excessive yawning, facing difficulty to learn new concepts, experiencing forgetfulness as well as depressed mood. This could occur due to several possible reasons, including medications and stress (caused by shift work). Despite the fact that sleep is important for normal physiology, it currently affects millions of people around the world, especially the US (70 million) and Europe (45 million). Due to increased work demand nowadays, lots of people are experiencing sleep deprivation hence, this could be the reason for several car accidents followed by death and morbidity. This review highlighted the impact of SD on neurotransmitter release and functions, theories (Flip-flop theory, oxidative stress theory, neuroinflammation theory, neurotransmitter theory, and hormonal theory) associated with SD pathogenesis; apart from this, it also demonstrates the molecular pathways underlying SD (PI3K and Akt, NF-κB, Nrf2, and adenosine pathway. However, this study also elaborates on the SD-induced changes in the level of neurotransmitters, hormonal, and mitochondrial functions. Along with this, it also covers several molecular aspects associated with SD as well. Through this study, a link is made between SD and associated causes, which will further help to develop a potential therapeutic strategy against SD.

Adenosine A2A receptor modulates microglia-mediated synaptic pruning of the retinogeniculate pathway during postnatal development

Synapse pruning is essential not only for the developmental establishment of synaptic connections in the brain but also for the pathogenesis of neurodevelopmental and neurodegenerative disorders. However, there are no effective pharmacological means to regulate synaptic pruning during early development. Using the eye-specific segregation of the dorsal lateral geniculate nucleus (dLGN) as a model of synaptic pruning coupled with adenosine A2A receptor (A2AR) antagonism and knockout, we demonstrated while genetic deletion of the A2AR throughout the development attenuated eye-specific segregation with the attenuated microglial phagocytosis at postnatal day 5 (P5), selective treatment with the A2AR antagonist KW6002 at P2-P4 facilitated synaptic pruning of visual pathway with microglial activation, increased lysosomal activity in microglia and increased microglial engulfment of retinal ganglion cell (RGC) inputs in the dLGN at P5 (but not P10). Furthermore, KW6002-mediated facilitation of synaptic pruning was activity-dependent since tetrodotoxin (TTX) treatment abolished the KW6002 facilitation. Moreover, the A2AR antagonist also modulated postsynaptic proteins and synaptic density at early postnatal stages as revealed by the reduced immunoreactivity of postsynaptic proteins (Homer1 and metabotropic glutamate receptor 5) and colocalization of presynaptic VGlut2 and postsynaptic Homer1 puncta in the dLGN. These findings suggest that A2AR can control pruning by multiple actions involving the retinal wave, microglia engulfment, and postsynaptic stability. Thus, A2AR antagonists may represent a novel pharmacological strategy to modulate microglia-mediated synaptic pruning and treatment of neurodevelopmental disorders associated with dysfunctional pruning.

Emerging Roles of Microglia in Neuro-vascular Unit: Implications of Microglia-Neurons Interactions

Microglia, which serve as the defensive interface of the nervous system, are activated in many neurological diseases. Their role as immune responding cells has been extensively studied in the past few years. Recent studies have demonstrated that neuronal feedback can be shaped by the molecular signals received and sent by microglia. Altered neuronal activity or synaptic plasticity leads to the release of various communication messages from neurons, which in turn exert effects on microglia. Research on microglia-neuron communication has thus expanded from focusing only on neurons to the neurovascular unit (NVU). This approach can be used to explore the potential mechanism of neurovascular coupling across sophisticated receptor systems and signaling cascades in health and disease. However, it remains unclear how microglia-neuron communication happens in the brain. Here, we discuss the functional contribution of microglia to synapses, neuroimmune communication, and neuronal activity. Moreover, the current state of knowledge of bidirectional control mechanisms regarding interactions between neurons and microglia are reviewed, with a focus on purinergic regulatory systems including ATP-P₂RY₁₂R signaling, ATP-adenosine-A₁Rs/A_2ARs, and the ATP-pannexin 1 hemichannel. This review aims to organize recent studies to highlight the multifunctional roles of microglia within the neural communication network in health and disease.

Motor Deficits Coupled to Cerebellar and Striatal Alterations in Ube3am-/p+ Mice Modelling Angelman Syndrome Are Attenuated by Adenosine A2A Receptor Blockade

Angelman syndrome (AS) is a neurogenetic disorder involving ataxia and motor dysfunction, resulting from the absence of the maternally inherited functional Ube3a protein in neurons. Since adenosine A2A receptor (A2AR) blockade relieves synaptic and motor impairments in Parkinson's or Machado-Joseph's diseases, we now tested if A2AR blockade was also effective in attenuating motor deficits in an AS (Ube3am-/p+) mouse model and if this involved correction of synaptic alterations in striatum and cerebellum. Chronic administration of the A2AR antagonist SCH58261 (0.1 mg/kg/day, ip) promoted motor learning of AS mice in the accelerating-rotarod task and rescued the grip strength impairment of AS animals. These motor impairments were accompanied by synaptic alterations in cerebellum and striatum typified by upregulation of synaptophysin and vesicular GABA transporters (vGAT) in the cerebellum of AS mice along with a downregulation of vGAT, vesicular glutamate transporter 1 (vGLUT1) and the dopamine active transporter in AS striatum. Notably, A2AR blockade prevented the synaptic alterations found in AS mice cerebellum as well as the downregulation of striatal vGAT and vGLUT1. This provides the first indications that A2AR blockade may counteract the characteristic motor impairments and synaptic changes of AS, although more studies are needed to unravel the underlying mechanisms.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

Adenosine A2A receptors format long-term depression and memory strategies in a mouse model of Angelman syndrome

Angelman syndrome (AS) is a neurodevelopmental disorder caused by loss of function of the maternally inherited Ube3a neuronal protein, whose main features comprise severe intellectual disabilities and motor impairments. Previous studies with the Ube3a^m-/p+ mouse model of AS revealed deficits in synaptic plasticity and memory. Since adenosine A_2A receptors (A_2AR) are powerful modulators of aberrant synaptic plasticity and A_2AR blockade prevents memory dysfunction in various brain diseases, we tested if A_2AR could control deficits of memory and hippocampal synaptic plasticity in AS. We observed that Ube3a^m-/p+ mice were unable to resort to hippocampal-dependent search strategies when tested for learning and memory in the Morris water maze; this was associated with a decreased magnitude of long-term depression (LTD) in CA1 hippocampal circuits. There was an increased density of A_2AR in the hippocampus of Ube3a^m-/p+ mice and their chronic treatment with the selective A_2AR antagonist SCH58261 (0.1 mg/kg/day, ip) restored both hippocampal-dependent learning strategies, as well as LTD deficits. Altogether, this study provides the first evidence of a role of A_2AR as a new prospective therapeutic target to manage learning deficits in AS.

Role of Adenosine in Epilepsy and Seizures

Adenosine is an endogenous anticonvulsant and neuroprotectant of the brain. Seizure activity produces large quantities of adenosine, and it is this seizure-induced adenosine surge that normally stops a seizure. However, within the context of epilepsy, adenosine plays a wide spectrum of different roles. It not only controls seizures (ictogenesis), but also plays a major role in processes that turn a normal brain into an epileptic brain (epileptogenesis). It is involved in the control of abnormal synaptic plasticity and neurodegeneration and plays a major role in the expression of comorbid symptoms and complications of epilepsy, such as sudden unexpected death in epilepsy (SUDEP). Given the important role of adenosine in epilepsy, therapeutic strategies are in development with the goal to utilize adenosine augmentation not only for the suppression of seizures but also for disease modification and epilepsy prevention, as well as strategies to block adenosine A2A receptor overfunction associated with neurodegeneration. This review provides a comprehensive overview of the role of adenosine in epilepsy.

Novel Players in the Aging Synapse: Impact on Cognition

While neuronal loss has long been considered as the main contributor to age-related cognitive decline, these alterations are currently attributed to gradual synaptic dysfunction driven by calcium dyshomeostasis and alterations in ionotropic/metabotropic receptors. Given the key role of the hippocampus in encoding, storage, and retrieval of memory, the morpho- and electrophysiological alterations that occur in the major synapse of this network-the glutamatergic-deserve special attention. We guide you through the hippocampal anatomy, circuitry, and function in physiological context and focus on alterations in neuronal morphology, calcium dynamics, and plasticity induced by aging and Alzheimer's disease (AD). We provide state-of-the art knowledge on glutamatergic transmission and discuss implications of these novel players for intervention. A link between regular consumption of caffeine-an adenosine receptor blocker-to decreased risk of AD in humans is well established, while the mechanisms responsible have only now been uncovered. We review compelling evidence from humans and animal models that implicate adenosine A2A receptors (A2AR) upsurge as a crucial mediator of age-related synaptic dysfunction. The relevance of this mechanism in patients was very recently demonstrated in the form of a significant association of the A2AR-encoding gene with hippocampal volume (synaptic loss) in mild cognitive impairment and AD. Novel pathways implicate A2AR in the control of mGluR5-dependent NMDAR activation and subsequent Ca2+ dysfunction upon aging. The nature of this receptor makes it particularly suited for long-term therapies, as an alternative for regulating aberrant mGluR5/NMDAR signaling in aging and disease, without disrupting their crucial constitutive activity.

Adenosine A2A receptors modulate the dopamine D2 receptor-mediated inhibition of synaptic transmission in the mouse prefrontal cortex

Prefrontal cortex (PFC) circuits are modulated by dopamine acting on D₁ - and D₂ -like receptors, which are pharmacologically exploited to manage neuropsychiatric conditions. Adenosine A_2A receptors (A_2A R) also control PFC-related responses and A_2A R antagonists are potential anti-psychotic drugs. As tight antagonistic A_2A R-D₂ R and synergistic A_2A R-D₁ R interactions occur in other brain regions, we now investigated the crosstalk between A_2A R and D₁ /D₂ R controlling synaptic transmission between layers II/III and V in mouse PFC coronal slices. Dopamine decreased synaptic transmission, a presynaptic effect based on the parallel increase in paired-pulse responses. Dopamine inhibition was prevented by the D₂ R-like antagonist sulpiride but not by the D₁ R antagonist SCH23390 and was mimicked by the D₂ R agonist sumanirole, but not by the agonists of either D₄ R (A-412997) or D₃ R (PD128907). Dopamine inhibition was prevented by the A_2A R antagonist, SCH58261, and attenuated in A_2A R knockout mice. Accordingly, triple-labelling immunocytochemistry experiments revealed the co-localization of A_2A R and D₂ R immunoreactivity in glutamatergic (vGluT1-positive) nerve terminals of the PFC. This reported positive A_2A R-D₂ R interaction controlling PFC synaptic transmission provides a mechanistic justification for the anti-psychotic potential of A_2A R antagonists.

Distinct sensitivity to caffeine-induced insomnia related to age

Caffeine acts by antagonizing the effect of the endogenous homeostatic sleep factor adenosine. In the current study we aimed to evaluate the pattern of caffeine-induced insomnia and its relation to age and sex in a general population sample derived from a web survey. The sample included 75,534 participants (28.1% men) from 18 to 75 years who answered self-report questionnaires by accessing a website in Brazilian Portuguese (BRAINSTEP project). In our sample, 3620 (17.0%) men and 9920 (18.3%) women reported insomnia due to caffeine intake. Caffeine-induced insomnia increased with aging in both men and women. This difference remained after adjusting for sociodemographic, psychiatric and sleep related variables as well as caffeine intake. Women showed higher proportion of caffeine-induced insomnia than men, but this difference did not remain after controlling for covariates. Also, individuals with caffeine-induced insomnia reported poorer sleep quality, higher latency to fall asleep and a higher proportion of psychiatric diagnoses and daily use of hypnotic drugs. In conclusion, our results show an age-associated increase in caffeine-induced insomnia and poorer mental health indicators among people with caffeine-induced insomnia complaints.

Effects of cordycepin on spontaneous alternation behavior and adenosine receptors expression in hippocampus

Cordycepin, an adenosine analogue, has been reported to improve cognitive function. Important roles on learning and memory of adenosine and its receptors, such as adenosine A1 and A2A receptors (A1R and A2AR), also have been shown. Therefore, we assume that the improvement of learning and memory induced by cordycepin is likely related to hippocampal adenosine content and adenosine receptor density. Here we investigated the effects of cordycepin on the short-term spatial memory by using a spontaneous alternation behavior (SAB) test in Y-maze, and then examined hippocampal adenosine content and A1R and A2AR densities. We found that orally administrated cordycepin (at dosages of 5 and 10mg/kg twice daily for three weeks) significantly increased the percent of relative alternation of mice in SAB but not altered body weight, hippocampus weight and hippocampal adenosine content. Furthermore, cordycepin decreased A2AR density in hippocampal subareas; however, cordycepin only reduced the A1R density in DG but not CA1 or CA3 region. Our results suggest that cordycepin exerts a nootropic role possibly through modulating A2AR density of hippocampus, which further support the concept that it is mostly A2AR rather than A1R to control the adaptive processes of memory performance. These findings would be helpful to provide a new window into the pharmacological properties of cordycepin for cognitive promotion.

How does adenosine control neuronal dysfunction and neurodegeneration?

The adenosine modulation system mostly operates through inhibitory A₁ (A₁ R) and facilitatory A_2A receptors (A_2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A₁ R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: high-frequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A_2A R; A_2A R switch off A₁ R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A₁ R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A₁ R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A_2A R, probably to bolster adaptive changes, but this heightens brain damage since A_2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A_2A R, whereas astrocytic and microglia A_2A R might control the spread of damage. The A_2A R signaling mechanisms are largely unknown since A_2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A₁ R preconditioning and preventing excessive A_2A R function might afford maximal neuroprotection. The main physiological role of the adenosine modulation system is to sharp the salience of information encoding through a combined action of adenosine A_2A receptors (A_2A R) in the synapse undergoing an alteration of synaptic efficiency with an increased inhibitory action of A₁ R in all surrounding synapses. Brain insults trigger an up-regulation of A_2A R in an attempt to bolster adaptive plasticity together with adenosine release and A₁ R desensitization; this favors synaptotocity (increased A_2A R) and decreases the hurdle to undergo degeneration (decreased A₁ R). Maximal neuroprotection is expected to result from a combined A_2A R blockade and increased A₁ R activation. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Adenosine A1 Receptor-Mediated Endocytosis of AMPA Receptors Contributes to Impairments in Long-Term Potentiation (LTP) in the Middle-Aged Rat Hippocampus

Aging causes multiple changes in the mammalian brain, including changes in synaptic signaling. Previous reports have shown increased extracellular adenosine in the aging brain, and we recently reported that activation of adenosine A1 receptors (A1Rs) induces AMPA receptor (AMPAR) internalization in rat hippocampus. This study investigated whether aging-related changes in the rat hippocampus include altered surface expression of adenosine A1 and A2A receptors, and whether these changes correspond to changes in AMPAR surface expression and altered synaptic plasticity. We found reduced A1R surface expression in middle-aged rat hippocampus, and also reduced GluA1 and GluA2 AMPAR subunit surface expression. Using a chemically-induced LTP (cLTP) experimental protocol, we recorded fEPSPs in young (1 month old) and middle-aged (7-12 month old) rat hippocampal slices. There were significant impairments in cLTP in middle-aged slices, suggesting impaired synaptic plasticity. Since we previously showed that the A1R agonist N(6)-cyclopentyladenosine (CPA) reduced both A1Rs and GluA2/GluA1 AMPARs, we hypothesized that the observed impaired synaptic plasticity in middle-aged brains is regulated by A1R-mediated AMPAR internalization by clathrin-mediated endocytosis. Following cLTP, we found a significant increase in GluA1 and GluA2 surface expression in young rats, which was blunted in middle-aged brains or in young brains pretreated with CPA. Blocking A1Rs with 8-cyclopentyl-1,3-dipropylxanthine or AMPAR endocytosis with either Tat-GluA2-3Y peptide or dynasore (dynamin inhibitor) similarly enhanced AMPAR surface expression following cLTP. These data suggest that age-dependent alteration in adenosine receptor expression contributes to increased AMPAR endocytosis and impaired synaptic plasticity in aged brains.

Optogenetic activation of intracellular adenosine A2A receptor signaling in the hippocampus is sufficient to trigger CREB phosphorylation and impair memory

Human and animal studies have converged to suggest that caffeine consumption prevents memory deficits in aging and Alzheimer's disease through the antagonism of adenosine A2A receptors (A2ARs). To test if A2AR activation in the hippocampus is actually sufficient to impair memory function and to begin elucidating the intracellular pathways operated by A2AR, we have developed a chimeric rhodopsin-A2AR protein (optoA2AR), which retains the extracellular and transmembrane domains of rhodopsin (conferring light responsiveness and eliminating adenosine-binding pockets) fused to the intracellular loop of A2AR to confer specific A2AR signaling. The specificity of the optoA2AR signaling was confirmed by light-induced selective enhancement of cAMP and phospho-mitogen-activated protein kinase (p-MAPK) (but not cGMP) levels in human embryonic kidney 293 (HEK293) cells, which was abolished by a point mutation at the C terminal of A2AR. Supporting its physiological relevance, optoA2AR activation and the A2AR agonist CGS21680 produced similar activation of cAMP and p-MAPK signaling in HEK293 cells, of p-MAPK in the nucleus accumbens and of c-Fos/phosphorylated-CREB (p-CREB) in the hippocampus, and similarly enhanced long-term potentiation in the hippocampus. Remarkably, optoA2AR activation triggered a preferential p-CREB signaling in the hippocampus and impaired spatial memory performance, while optoA2AR activation in the nucleus accumbens triggered MAPK signaling and modulated locomotor activity. This shows that the recruitment of intracellular A2AR signaling in the hippocampus is sufficient to trigger memory dysfunction. Furthermore, the demonstration that the biased A2AR signaling and functions depend on intracellular A2AR loops prompts the possibility of targeting the intracellular A2AR-interacting partners to selectively control different neuropsychiatric behaviors.

Adenosine A(2A) receptors are necessary and sufficient to trigger memory impairment in adult mice

BACKGROUND AND PURPOSE

Caffeine (a non-selective adenosine receptor antagonist) prevents memory deficits in aging and Alzheimer's disease, an effect mimicked by adenosine A2 A receptor, but not A1 receptor, antagonists. Hence, we investigated the effects of adenosine receptor agonists and antagonists on memory performance and scopolamine-induced memory impairment in mice.

EXPERIMENTAL APPROACH

We determined whether A2 A receptors are necessary for the emergence of memory impairments induced by scopolamine and whether A2 A receptor activation triggers memory deficits in naïve mice, using three tests to assess short-term memory, namely the object recognition task, inhibitory avoidance and modified Y-maze.

KEY RESULTS

Scopolamine (1.0 mg·kg(-1) , i.p.) impaired short-term memory performance in all three tests and this scopolamine-induced amnesia was prevented by the A2 A receptor antagonist (SCH 58261, 0.1-1.0 mg·kg(-1) , i.p.) and by the A1 receptor antagonist (DPCPX, 0.2-5.0 mg·kg(-1) , i.p.), except in the modified Y-maze where only SCH58261 was effective. Both antagonists were devoid of effects on memory or locomotion in naïve rats. Notably, the activation of A2 A receptors with CGS 21680 (0.1-0.5 mg·kg(-1) , i.p.) before the training session was sufficient to trigger memory impairment in the three tests in naïve mice, and this effect was prevented by SCH 58261 (1.0 mg·kg(-1) , i.p.). Furthermore, i.c.v. administration of CGS 21680 (50 nmol) also impaired recognition memory in the object recognition task.

CONCLUSIONS AND IMPLICATIONS

These results show that A2 A receptors are necessary and sufficient to trigger memory impairment and further suggest that A1 receptors might also be selectively engaged to control the cholinergic-driven memory impairment.

Inactivation of adenosine A2A receptors reverses working memory deficits at early stages of Huntington's disease models

Cognitive impairments in Huntington's disease (HD) are attributed to a dysfunction of the cortico-striatal pathway and significantly affect the quality of life of the patients, but this has not been a therapeutic focus in HD to date. We postulated that adenosine A(2A) receptors (A(2A)R), located at pre- and post-synaptic elements of the cortico-striatal pathways, modulate striatal neurotransmission and synaptic plasticity and cognitive behaviors. To critically evaluate the ability of A(2A)R inactivation to prevent cognitive deficits in early HD, we cross-bred A(2A)R knockout (KO) mice with two R6/2 transgenic lines of HD (CAG120 and CAG240) to generate two double transgenic R6/2-CAG120-A(2A)R KO and R6/2-CAG240-A(2A)R KO mice and their corresponding wild-type (WT) littermates. Genetic inactivation of A(2A)R prevented working memory deficits induced by R6/2-CAG120 at post-natal week 6 and by R6/2-CAG240 at post-natal month 2 and post-natal month 3, without modifying motor deficits. Similarly the A2(A)R antagonist KW6002 selectively reverted working memory deficits in R6/2-CAG240 mice at post-natal month 3. The search for possible mechanisms indicated that the genetic inactivation of A(2A)R did not affect ubiquitin-positive neuronal inclusions, astrogliosis or Thr-75 phosphorylation of DARPP-32 in the striatum. Importantly, A(2A)R blockade preferentially controlled long-term depression at cortico-striatal synapses in R6/2-CAG240 at post-natal week 6. The reported reversal of working memory deficits in R6/2 mice by the genetic and pharmacological inactivation of A(2A)R provides a proof-of-principle for A(2A)R as novel targets to reverse cognitive deficits in HD, likely by controlling LTD deregulation.

Adenosine A2b receptors control A1 receptor-mediated inhibition of synaptic transmission in the mouse hippocampus

Adenosine is a neuromodulator mostly acting through A1 (inhibitory) and A2A (excitatory) receptors in the brain. A2B receptors (A(2B)R) are G(s/q)--protein-coupled receptors with low expression in the brain. As A(2B)R function is largely unknown, we have now explored their role in the mouse hippocampus. We performed electrophysiological extracellular recordings in mouse hippocampal slices, and immunological analysis of nerve terminals and glutamate release in hippocampal slices and synaptosomes. Additionally, A(2B)R-knockout (A(2B)R-KO) and C57/BL6 mice were submitted to a behavioural test battery (open field, elevated plus-maze, Y-maze). The A(2B)R agonist BAY60-6583 (300 nM) decreased the paired-pulse stimulation ratio, an effect prevented by the A(2B)R antagonist MRS 1754 (200 nM) and abrogated in A(2B)R-KO mice. Accordingly, A(2B)R immunoreactivity was present in 73 ± 5% of glutamatergic nerve terminals, i.e. those immunopositive for vesicular glutamate transporters. Furthermore, BAY 60-6583 attenuated the A(1)R control of synaptic transmission, both the A(1)R inhibition caused by 2-chloroadenosine (0.1-1 μM) and the disinhibition caused by the A(1)R antagonist DPCPX (100 nM), both effects prevented by MRS 1754 and abrogated in A(2B)R-KO mice. BAY 60-6583 decreased glutamate release in slices and also attenuated the A(1)R inhibition (CPA 100 nM). A(2B)R-KO mice displayed a modified exploratory behaviour with an increased time in the central areas of the open field, elevated plus-maze and the Y-maze and no alteration of locomotion, anxiety or working memory. We conclude that A(2B)R are present in hippocampal glutamatergic terminals where they counteract the predominant A(1)R-mediated inhibition of synaptic transmission, impacting on exploratory behaviour.

Peripheral blood mononuclear cells as a laboratory to study dementia in the elderly

The steady and dramatic increase in the incidence of Alzheimer's disease (AD) and the lack of effective treatments have stimulated the search for strategies to prevent or delay its onset and/or progression. Since the diagnosis of dementia requires a number of established features that are present when the disease is fully developed, but not always in the early stages, the need for a biological marker has proven to be urgent, in terms of both diagnosis and monitoring of AD. AD has been shown to affect peripheral blood mononuclear cells (PBMCs) that are a critical component of the immune system which provide defence against infection. Although studies are continuously supplying additional data that emphasize the central role of inflammation in AD, PBMCs have not been sufficiently investigated in this context. Delineating biochemical alterations in AD blood constituents may prove valuable in identifying accessible footprints that reflect degenerative processes within the Central Nervous System (CNS). In this review, we address the role of biomarkers in AD with a focus on the notion that PBMCs may serve as a peripheral laboratory to find molecular signatures that could aid in differential diagnosis with other forms of dementia and in monitoring of disease progression.

Cellular prion protein is present in dopaminergic neurons and modulates the dopaminergic system

Cellular prion protein (PrP(C) ) is widely expressed in the brain. Although the precise role of PrP(C) remains uncertain, it has been proposed to be a pivotal modulator of neuroplasticity events by regulating the glutamatergic and serotonergic systems. Here we report the existence of neurochemical and functional interactions between PrP(C) and the dopaminergic system. PrP(C) was found to co-localize with dopaminergic neurons and in dopaminergic synapses in the striatum. Furthermore, the genetic deletion of PrP(C) down-regulated dopamine D1 receptors and DARPP-32 density in the striatum and decreased dopamine levels in the prefrontal cortex of mice. This indicates that PrP(C) affects the homeostasis of the dopaminergic system by interfering differently in different brain areas with dopamine synthesis, content, receptor density and signaling pathways. This interaction between PrP(C) and the dopaminergic system prompts the hypotheses that the dopaminergic system may be implicated in some pathological features of prion-related diseases and, conversely, that PrP(C) may play a role in dopamine-associated brain disorders.

Adenosine A(2A) Receptors as novel upstream regulators of BDNF-mediated attenuation of hippocampal Long-Term Depression (LTD)

Hippocampal Long-Term Potentiation (LTP) is facilitated by BDNF, through the activation of tropomyosin-related kinase B (TrkB) receptors. However, an influence of BDNF upon Long-Term Depression (LTD) was also shown. The present work aimed to further evaluate the effect of BDNF and TrkB receptors upon CA1 hippocampal LTD and to elucidate whether this effect is under the upstream control of other signalling processes, such as the adenosine A(2A)Receptors (A(2A)Rs). LTD, induced by a Low-Frequency Stimulation (LFS, 900 pulses, 1 Hz) in the CA1 area of rat hippocampal slices, was significantly attenuated when these slices were exposed to BDNF (60-100 ng/mL). A lower BDNF concentration (20 ng/ml) was only effective to inhibit LTD if A(2A)Rs were activated by a selective agonist, CGS 21680 (10 nM), or if the extracellular adenosine level was increased by 5-iodotubercidin (100 nM). BDNF (100 ng/ml) effect upon LTD was prevented by K252a (200 nM), which is known to prevent TrkB transphosphorylation, hence suggesting that this action requires TrkB receptor activation. BDNF (100 ng/ml) lacked effect on an adenosine-depleted background (adenosine deaminase, 2 U/ml) or under selective A(2A)R blockade (SCH 58261, 100 nM), indicating that it relies on tonic A(2A)R activation. Forskolin (10 μM), a cell-permeable activator of adenylate cyclase, rescued BDNF (100 ng/ml) effect in slices where A(2A)Rs were blocked with SCH 58261 (100 nM), whereas a PKA inhibitor, H-89 (1 μM), prevented LTD attenuation by BDNF (100 ng/ml). We conclude that the influence of BDNF TrkB receptors upon LTD is under the strict control of A(2A)Rs activation, through a mechanism that requires the cAMP/PKA transducing system.

Adenosine A₂A receptors in striatal glutamatergic terminals and GABAergic neurons oppositely modulate psychostimulant action and DARPP-32 phosphorylation

Adenosine A_2A receptors (A_2AR) are located postsynaptically in striatopallidal GABAergic neurons, antagonizing dopamine D₂ receptor functions, and are also located presynaptically at corticostriatal terminals, facilitating glutamate release. To address the hypothesis that these two A_2AR populations differently control the action of psychostimulants, we characterized A_2AR modulation of cocaine-induced effects at the level of DARPP-32 phosphorylation at Thr-34 and Thr-75, c-Fos expression, and psychomotor activity using two lines of cell-type selective A_2AR knockout (KO) mice with selective A_2AR deletion in GABAergic neurons (striatum-A_2AR-KO mice), or with A_2AR deletion in both striatal GABAergic neurons and projecting cortical glutamatergic neurons (forebrain-A_2AR-KO mice). We demonstrated that striatum-A_2AR KO mice lacked A_2ARs exclusively in striatal GABAergic terminals whereas forebrain-A_2AR KO mice lacked A_2ARs in both striatal GABAergic and glutamatergic terminals leading to a blunted A_2AR-mediated facilitation of synaptosomal glutamate release. The inactivation of A_2ARs in GABAergic neurons reduced striatal DARPP-32 phosphorylation at Thr-34 and increased its phosphorylation at Thr-75. Conversely, the additional deletion of corticostriatal glutamatergic A_2ARs produced opposite effects on DARPP-32 phosphorylation at Thr-34 and Thr-75. This distinct modulation of DARPP-32 phosphorylation was associated with opposite responses to cocaine-induced striatal c-Fos expression and psychomotor activity in striatum-A_2AR KO (enhanced) and forebrain-A_2AR KO mice (reduced). Thus, A_2ARs in glutamatergic corticostriatal terminals and in GABAergic striatal neurons modulate the action of psychostimulants and DARPP-32 phosphorylation in opposite ways. We conclude that A_2ARs in glutamatergic terminals prominently control the action of psychostimulants and define a novel mechanism by which A_2ARs fine-tune striatal activity by integrating GABAergic, dopaminergic and glutamatergic signaling.

Genetic deletion of the adenosine A(2A) receptor prevents nicotine-induced upregulation of α7, but not α4β2* nicotinic acetylcholine receptor binding in the brain

Considerable evidence indicates that adenosine A(2A) receptors (A(2A)Rs) modulate cholinergic neurotransmission, nicotinic acetylcholine receptor (nAChR) function, and nicotine-induced behavioural effects. To explore the interaction between A(2A) and nAChRs, we examined if the complete genetic deletion of adenosine A(2A)Rs in mice induces compensatory alterations in the binding of different nAChR subtypes, and whether the long-term effects of nicotine on nAChR regulation are altered in the absence of the A(2A)R gene. Quantitative autoradiography was used to measure cytisine-sensitive [¹²⁵I]epibatidine and [¹²⁵I]α-bungarotoxin binding to α4β2* and α7 nAChRs, respectively, in brain sections of drug-naïve (n = 6) or nicotine treated (n = 5-7), wild-type and adenosine A(2A)R knockout mice. Saline or nicotine (7.8 mg/kg/day; free-base weight) were administered to male CD1 mice via subcutaneous osmotic minipumps for a period of 14 days. Blood plasma levels of nicotine and cotinine were measured at the end of treatment. There were no compensatory developmental alterations in nAChR subtype distribution or density in drug-naïve A(2A)R knockout mice. In nicotine treated wild-type mice, both α4β2* and α7 nAChR binding sites were increased compared with saline treated controls. The genetic ablation of adenosine A(2A)Rs prevented nicotine-induced upregulation of α7 nAChRs, without affecting α4β2* receptor upregulation. This selective effect was observed at plasma levels of nicotine that were within the range reported for smokers (10-50 ng ml⁻¹). Our data highlight the involvement of adenosine A(2A)Rs in the mechanisms of nicotine-induced α7 nAChR upregulation, and identify A(2A)Rs as novel pharmacological targets for modulating the long-term effects of nicotine on α7 receptors.

Anti-cholinergics for axial symptoms in Parkinson's disease after subthalamic stimulation

OBJECTIVE

We studied the effect of anti-cholinergic therapy on axial symptoms that show a tendency to worsen over time after deep brain stimulation of the subthalamic nucleus (STN-DBS) in patients with Parkinson's disease (PD).

PATIENTS AND METHODS

We conducted a prospective study of 20 consecutive patients treated with the anti-cholinergic agent trihexyphenidyl after bilateral STN-DBS and assessed the effect of anti-cholinergic therapy on parkinsonism 1 month after its initiation using the Unified Parkinson's Disease Rating Scale (UPDRS).

RESULTS

After a mean post-operative follow-up period of 22.3 months, the scores of axial symptoms on UPDRS part II (ADL score) and part III (motor score) deteriorated by 87% and 54% (baseline), respectively, compared with the pre-operative scores (P < 0.001 for both comparisons). After adding trihexyphenidyl to dopaminergic medication with stimulation, the scores of axial symptoms on UPDRS part II and part III improved from baseline by 33% and 39%, respectively (P < 0.001 for both comparisons).

CONCLUSIONS

Our findings demonstrated that the anti-cholinergic agent trihexyphenidyl shows positive effect for a patient population developing deterioration of axial symptoms after STN-DBS. The results in the present study may provide insights into the mechanism of emergence or progression of axial symptoms in patients with PD after STN-DBS.

Caffeine consumption prevents diabetes-induced memory impairment and synaptotoxicity in the hippocampus of NONcZNO10/LTJ mice

Diabetic conditions are associated with modified brain function, namely with cognitive deficits, through largely undetermined processes. More than understanding the underlying mechanism, it is important to devise novel strategies to alleviate diabetes-induced cognitive deficits. Caffeine (a mixed antagonist of adenosine A(1) and A(2A) receptors) emerges as a promising candidate since caffeine consumption reduces the risk of diabetes and effectively prevents memory deficits caused by different noxious stimuli. Thus, we took advantage of a novel animal model of type 2 diabetes to investigate the behavioural, neurochemical and morphological modifications present in the hippocampus and tested if caffeine consumption might prevent these changes. We used a model closely mimicking the human type 2 diabetes condition, NONcNZO10/LtJ mice, which become diabetic at 7-11 months when kept under an 11% fat diet. Caffeine (1 g/l) was applied in the drinking water from 7 months onwards. Diabetic mice displayed a decreased spontaneous alternation in the Y-maze accompanied by a decreased density of nerve terminal markers (synaptophysin, SNAP25), mainly glutamatergic (vesicular glutamate transporters), and increased astrogliosis (GFAP immunoreactivity) compared to their wild type littermates kept under the same diet. Furthermore, diabetic mice displayed up-regulated A(2A) receptors and down-regulated A(1) receptors in the hippocampus. Caffeine consumption restored memory performance and abrogated the diabetes-induced loss of nerve terminals and astrogliosis. These results provide the first evidence that type 2 diabetic mice display a loss of nerve terminal markers and astrogliosis, which is associated with memory impairment; furthermore, caffeine consumption prevents synaptic dysfunction and astrogliosis as well as memory impairment in type 2 diabetes.

Adenosine A2A receptors modulate glutamate uptake in cultured astrocytes and gliosomes

Glutamate is the primary excitatory neurotransmitter in the central nervous system, where its toxic build-up leads to synaptic dysfunction and excitotoxic cell death that underlies many neurodegenerative diseases. Therefore, efforts have been made to understand the regulation of glutamate transporters, which are responsible for the clearance of extracellular glutamate. We now report that adenosine A(2A) receptors (A(2A) R) control the uptake of D-aspartate in primary cultured astrocytes as well as in an ex vivo preparation enriched in glial plasmalemmal vesicles (gliosomes) from adult rats, whereas A(1) R and A(3) R were devoid of effects. Thus, the acute exposure to the A(2A) R agonist, CGS 21680, inhibited glutamate uptake, an effect prevented by the A(2A) R antagonist, SCH 58261, and abbrogated in cultured astrocytes from A(2A) R knockout mice. Furthermore, the prolonged activation of A(2A) R lead to a cAMP/protein kinase A-dependent reduction of GLT-I and GLAST mRNA and protein levels, which leads to a sustained decrease of glutamate uptake. This dual mechanism of inhibition of glutamate transporters by astrocytic A(2A) R provides a novel candidate mechanism to understand the ability of A(2) (A) R to control synaptic plasticity and neurodegeneration, two conditions tightly associated with the control of extracellular glutamate levels by glutamate transporters.

The role of extracellular adenosine in chemical neurotransmission in the hippocampus and Basal Ganglia: pharmacological and clinical aspects

Now there is general agreement that the purine nucleoside adenosine is an important neuromodulator in the central nervous system, playing a crucial role in neuronal excitability and synaptic/non-synaptic transmission in the hippocampus and basal ganglia. Adenosine is derived from the breakdown of extra- or intracellular ATP and is released upon a variety of physiological and pathological stimuli from neuronal and non-neuronal sources, i.e. from glial cells and exerts effects diffusing far away from release sites. The resultant elevation of adenosine levels in the extracellular space reaches micromolar level, and leads to the activation A₁, A_2A, A_2B and A₃ receptors, localized to pre- and postsynaptic as well as extrasynaptic sites. Activation of presynaptic A₁ receptors inhibits the release of the majority of transmitters including glutamate, acetylcholine, noradrenaline, 5-HT and dopamine, whilst the stimulation of A_2A receptors facilitates the release of glutamate and acetylcholine and inhibits the release of GABA. These actions underlie modulation of neuronal excitability, synaptic plasticity and coordination of neural networks and provide intriguing target sites for pharmacological intervention in ischemia and Parkinson’s disease. However, despite that adenosine is also released during ischemia, A₁ adenosine receptors do not participate in the modulation of excitotoxic glutamate release, which is nonsynaptic and is due to the reverse operation of transporters. Instead, extrasynaptic A1 receptors might be responsible for the neuroprotection afforded by A₁ receptor activation.

Enhancement of LTP in aged rats is dependent on endogenous BDNF

Long-term potentiation (LTP), considered the neurophysiological basis for learning and memory, is facilitated by brain-derived neurotrophic factor (BDNF), an action more evident when LTP is evoked by weak θ-burst stimuli and dependent on co-activation of adenosine A(2A) receptors (A(2A)R), which are more expressed in aged rats. As θ-burst stimuli also favor LTP in aged animals, we hypothesized that enhanced LTP in aging could be related to changes in neuromodulation by BDNF. The magnitude of CA1 LTP induced by a weak θ-burst stimuli delivered to the Schaffer collaterals was significantly higher in hippocampal slices taken from 36 to 38 and from 70 to 80-week-old rats, when compared with LTP magnitude in slices from 4 or 10 to 15-week-old rats; this enhancement does not impact in cognitive improvement as aged rats revealed an impairment on hippocampal-dependent learning and memory performance, as assessed by the Morris water maze tests. The scavenger for BDNF, TrkB-Fc, and the inhibitor of Trk phosphorylation, K252a, attenuated LTP in slices from 70 to 80-week-old rats, but not from 10 to 15-week-old rats. When exogenously added, BDNF significantly increased LTP in slices from 4 and 10 to 15-week-old rats, but did not further increased LTP in 36 to 38 or 70 to 80-week-old rats. The effects of exogenous BDNF on LTP were prevented by the A(2A)R antagonist, SCH58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine). These results indicate that the higher LTP magnitude observed upon aging, which does not translate into improved spatial memory performance, is a consequence of an increase in the tonic action of endogenous BDNF.

Adenosine receptors and brain diseases: neuroprotection and neurodegeneration

Adenosine acts in parallel as a neuromodulator and as a homeostatic modulator in the central nervous system. Its neuromodulatory role relies on a balanced activation of inhibitory A(1) receptors (A1R) and facilitatory A(2A) receptors (A2AR), mostly controlling excitatory glutamatergic synapses: A1R impose a tonic brake on excitatory transmission, whereas A2AR are selectively engaged to promote synaptic plasticity phenomena. This neuromodulatory role of adenosine is strikingly similar to the role of adenosine in the control of brain disorders; thus, A1R mostly act as a hurdle that needs to be overcame to begin neurodegeneration and, accordingly, A1R only effectively control neurodegeneration if activated in the temporal vicinity of brain insults; in contrast, the blockade of A2AR alleviates the long-term burden of brain disorders in different neurodegenerative conditions such as ischemia, epilepsy, Parkinson's or Alzheimer's disease and also seem to afford benefits in some psychiatric conditions. In spite of this qualitative agreement between neuromodulation and neuroprotection by A1R and A2AR, it is still unclear if the role of A1R and A2AR in the control of neuroprotection is mostly due to the control of glutamatergic transmission, or if it is instead due to the different homeostatic roles of these receptors related with the control of metabolism, of neuron-glia communication, of neuroinflammation, of neurogenesis or of the control of action of growth factors. In spite of this current mechanistic uncertainty, it seems evident that targeting adenosine receptors might indeed constitute a novel strategy to control the demise of different neurological and psychiatric disorders.

Protective effect of caffeine and a selective A2A receptor antagonist on impairment of memory and oxidative stress of aged rats

In this study, the effects of caffeine (CAF) and SCH58261, a selective A(2A) receptor antagonist, on memory impairment and oxidative stress generated by aging in rats were investigated. Young and aged rats were treated daily per 10 days with CAF (30 mg/kg p.o.) or SCH58261 (0.5mg/kg, p.o.) or vehicle (1 ml/kg p.o.). Rats were trained and tested in a novel object recognition task. After the behavioral test, ascorbic acid and oxygen and nitrogen reactive species levels as well as Na(+)K(+) ATPase activity were determined in rat brain. The results demonstrated that the age-related memory deficit was reversed by treatment with CAF or SCH58261. Treatment with CAF or SCH58261 significantly normalized oxygen and nitrogen reactive species levels increased in brains of aged rats. Na(+)K(+) ATPase activity inhibited in brains of aged rats was also normalized by CAF or SCH58261 treatment. A decrease in basal ascorbic acid levels in brains of aged rats was not changed by CAF or SCH58261. These results demonstrated that CAF and SCH58261, modulators of adenosinergic receptors, were able to reverse age-associated memory impairment and to partially reduce oxidative stress.

Caffeine and an adenosine A(2A) receptor antagonist prevent memory impairment and synaptotoxicity in adult rats triggered by a convulsive episode in early life

Seizures early in life cause long-term behavioral modifications, namely long-term memory deficits in experimental animals. Since caffeine and adenosine A(2A) receptor (A(2A)R) antagonists prevent memory deficits in adult animals, we now investigated if they also prevented the long-term memory deficits caused by a convulsive period early in life. Administration of kainate (KA, 2 mg/kg) to 7-days-old (P7) rats caused a single period of self-extinguishable convulsions which lead to a poorer memory performance in the Y-maze only when rats were older than 90 days, without modification of locomotion or anxiety-like behavior in the elevated-plus maze. In accordance with the relationship between synaptotoxicity and memory dysfunction, the hippocampus of these adult rats treated with kainate at P7 displayed a lower density of synaptic proteins such as SNAP-25 and syntaxin (but not synaptophysin), as well as vesicular glutamate transporters type 1 (but not vesicular GABA transporters), with no changes in PSD-95, NMDA receptor subunits (NR1, NR2A, NR2B) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor subunits (GluR1, GluR2) compared with controls. Caffeine (1 g/L) or the A(2A)R antagonist, KW6002 (3 mg/kg) applied in the drinking water from P21 onwards, prevented these memory deficits in P90 rats treated with KA at P7, as well as the accompanying synaptotoxicity. These results show that a single convulsive episode in early life causes a delayed memory deficit in adulthood accompanied by a glutamatergic synaptotoxicity that was prevented by caffeine or adenosine A(2A)R antagonists.

Modification upon aging of the density of presynaptic modulation systems in the hippocampus

Different presynaptic neuromodulation systems have been explored as possible targets to manage neurodegenerative diseases. However, most studies used young adult animals whereas neurodegenerative diseases are prevalent in the elderly. Thus, we now explored by Western blot analysis how the density of different presynaptic markers and receptors changes with aging in rat hippocampal synaptosomes (purified nerve terminals). Compared to synaptosomal membranes from 2-month-old rats, the density of presynaptic proteins (synaptophysin or SNAP-25) decreased at 18-24 months. In parallel, markers of glutamatergic terminals (vGluT1 or vGluT2) and cholinergic terminal markers (vAChT) constantly decreased with aging from 12 to 18 months onwards, whereas the densities of GABAergic (vGAT) only decreased after 24 months. Inhibitory A(1) and CB(1) receptor density tended to decrease with aging, whereas facilitatory mGluR5 and P2Y1 receptor density was roughly constant and facilitatory A(2A) receptor density increased at 18-24 months. Thus aging causes an imbalance of excitatory versus inhibitory nerve terminal markers and causes a predominant decrease of inhibitory rather than facilitatory presynaptic modulation systems.