Long-term Effect of Convulsive Behavior on the Density of Adenosine A1 and A2A Receptors in the Rat Cerebral Cortex

In silico exploration of cholinergic activity and neuroprotection of novel caffeine analogues

Alzheimer's disease (AD) is characterized by a cholinergic deficit, prompting conventional therapies to elevate acetylcholine levels as a compensatory measure. Two main strategies involve the inhibition of acetylcholinesterase (AChE) and/or the stimulation of acetylcholine receptors (AChR). Caffeine (CFF), known as a partial agonist of nAChR and an AChE inhibitor, acts as a cholinergic enhancer. Additionally, it is suggested that CFF may exhibit neuroprotective capabilities through the inhibition of the human adenosine receptor type 2A (hA2AR) in the brain's striatum, potentially preventing cellular apoptosis. This study explores on the design and prediction of the bioactivity of CFF analogues with the aim of enhancing cholinergic signaling and providing neuroprotection to improve their therapeutic potential. We employed tools to predict pharmacokinetic and bioactivity properties, molecular docking, molecular dynamics, and target prediction to identify potential candidates among the designed CFF analogues capable of enhancing neurotransmission and providing cellular protection. In a novel approach, a normalized index is proposed for the combined analysis of the pharmacokinetic parameters and molecular docking binding affinities, which facilitates the systematic evaluation and comparison of the synthesized analogues and minimizes subjectivity in the selection of promising candidates. Results indicated that some analogues show promise in improving cholinergic activity and providing neuroprotection. These findings instill optimism, encouraging further research to corroborate their effects, while also representing a significant step towards the development of new therapeutic agents for AD.

Lateral septum adenosine A2A receptors control stress-induced depressive-like behaviors via signaling to the hypothalamus and habenula

Major depressive disorder ranks as a major burden of disease worldwide, yet the current antidepressant medications are limited by frequent non-responsiveness and significant side effects. The lateral septum (LS) is thought to control of depression, however, the cellular and circuit substrates are largely unknown. Here, we identified a subpopulation of LS GABAergic adenosine A2A receptors (A2AR)-positive neurons mediating depressive symptoms via direct projects to the lateral habenula (LHb) and the dorsomedial hypothalamus (DMH). Activation of A2AR in the LS augmented the spiking frequency of A2AR-positive neurons leading to a decreased activation of surrounding neurons and the bi-directional manipulation of LS-A2AR activity demonstrated that LS-A2ARs are necessary and sufficient to trigger depressive phenotypes. Thus, the optogenetic modulation (stimulation or inhibition) of LS-A2AR-positive neuronal activity or LS-A2AR-positive neurons projection terminals to the LHb or DMH, phenocopied depressive behaviors. Moreover, A2AR are upregulated in the LS in two male mouse models of repeated stress-induced depression. This identification that aberrantly increased A2AR signaling in the LS is a critical upstream regulator of repeated stress-induced depressive-like behaviors provides a neurophysiological and circuit-based justification of the antidepressant potential of A2AR antagonists, prompting their clinical translation.

The impact of inosine on hippocampal synaptic transmission and plasticity involves the release of adenosine through equilibrative nucleoside transporters rather than the direct activation of adenosine receptors

Inosine has robust neuroprotective effects, but it is unclear if inosine acts as direct ligand of adenosine receptors or if it triggers metabolic effects indirectly modifying the activity of adenosine receptors. We now combined radioligand binding studies with electrophysiological recordings in hippocampal slices to test how inosine controls synaptic transmission and plasticity. Inosine was without effect at 30 μM and decreased field excitatory post-synaptic potentials by 14% and 33% at 100 and 300 μM, respectively. These effects were prevented by the adenosine A₁ receptor antagonist DPCPX. Inosine at 300 (but not 100) μM also decreased the magnitude of long-term potentiation (LTP), an effect prevented by DPCPX and by the adenosine A_2A receptor antagonist SCH58261. Inosine showed low affinity towards human and rat adenosine receptor subtypes with K_i values of > 300 µM; only at the human and rat A₁ receptor slightly higher affinities with K_i values of around 100 µM were observed. Affinity of inosine at the rat A₃ receptor was higher (K_i of 1.37 µM), while it showed no interaction with the human orthologue. Notably, the effects of inosine on synaptic transmission and plasticity were abrogated by adenosine deaminase and by inhibiting equilibrative nucleoside transporters (ENT) with dipyridamole and NBTI. This shows that the impact of inosine on hippocampal synaptic transmission and plasticity is not due to a direct activation of adenosine receptors but is instead due to an indirect modification of the tonic activation of these adenosine receptors through an ENT-mediated modification of the extracellular levels of adenosine.

Adenosine A2A receptors control synaptic remodeling in the adult brain

The molecular mechanisms underlying circuit re-wiring in the mature brain remains ill-defined. An eloquent example of adult circuit remodelling is the hippocampal mossy fiber (MF) sprouting found in diseases such as temporal lobe epilepsy. The molecular determinants underlying this retrograde re-wiring remain unclear. This may involve signaling system(s) controlling axon specification/growth during neurodevelopment reactivated during epileptogenesis. Since adenosine A_2A receptors (A_2AR) control axon formation/outgrowth and synapse stabilization during development, we now examined the contribution of A_2AR to MF sprouting. A_2AR blockade significantly attenuated status epilepticus(SE)-induced MF sprouting in a rat pilocarpine model. This involves A_2AR located in dentate granule cells since their knockdown selectively in dentate granule cells reduced MF sprouting, most likely through the ability of A_2AR to induce the formation/outgrowth of abnormal secondary axons found in rat hippocampal neurons. These A_2AR should be activated by extracellular ATP-derived adenosine since a similar prevention/attenuation of SE-induced hippocampal MF sprouting was observed in CD73 knockout mice. These findings demonstrate that A_2AR contribute to epilepsy-related MF sprouting, most likely through the reactivation of the ability of A_2AR to control axon formation/outgrowth observed during neurodevelopment. These results frame the CD73-A_2AR axis as a regulator of circuit remodeling in the mature brain.

Adenosine-A2A Receptor Signaling Plays a Crucial Role in Sudden Unexpected Death in Epilepsy

Adenosinergic activities are suggested to participate in SUDEP pathophysiology; this study aimed to evaluate the adenosine hypothesis of SUDEP and specifically the role of adenosine A_2A receptor (A_2AR) in the development of a SUDEP mouse model with relevant clinical features. Using a combined paradigm of intrahippocampal and intraperitoneal administration of kainic acid (KA), we developed a boosted-KA model of SUDEP in genetically modified adenosine kinase (ADK) knockdown (Adk^+/-) mice, which has reduced ADK in the brain. Seizure activity was monitored using video-EEG methods, and in vivo recording of local field potential (LFP) was used to evaluate neuronal activity within the nucleus tractus solitarius (NTS). Our boosted-KA model of SUDEP was characterized by a delayed, postictal sudden death in epileptic mice. We demonstrated a higher incidence of SUDEP in Adk^+/- mice (34.8%) vs. WTs (8.0%), and the ADK inhibitor, 5-Iodotubercidin, further increased SUDEP in Adk^+/- mice (46.7%). We revealed that the NTS level of ADK was significantly increased in epileptic WTs, but not in epileptic Adk^+/- mutants, while the A_2AR level in NTS was increased in epileptic (WT and Adk^+/-) mice vs. non-epileptic controls. The A_2AR antagonist, SCH58261, significantly reduced SUDEP events in Adk^+/- mice. LFP data showed that SCH58261 partially restored KA injection-induced suppression of gamma oscillation in the NTS of epileptic WT mice, whereas SCH58261 increased theta and beta oscillations in Adk^+/- mutants after KA injection, albeit with no change in gamma oscillations. These LFP findings suggest that SCH58261 and KA induced changes in local neuronal activities in the NTS of epileptic mice. We revealed a crucial role for NTS A_2AR in SUDEP pathophysiology suggesting A_2AR as a potential therapeutic target for SUDEP risk prevention.

Adenosine A2A Receptors as Biomarkers of Brain Diseases

Extracellular adenosine is produced with increased metabolic activity or stress, acting as a paracrine signal of cellular effort. Adenosine receptors are most abundant in the brain, where adenosine acts through inhibitory A₁ receptors to decrease activity/noise and through facilitatory A_2A receptors (A_2AR) to promote plastic changes in physiological conditions. By bolstering glutamate excitotoxicity and neuroinflammation, A_2AR also contribute to synaptic and neuronal damage, as heralded by the neuroprotection afforded by the genetic or pharmacological blockade of A_2AR in animal models of ischemia, traumatic brain injury, convulsions/epilepsy, repeated stress or Alzheimer's or Parkinson's diseases. A_2AR overfunction is not only necessary for the expression of brain damage but is actually sufficient to trigger brain dysfunction in the absence of brain insults or other disease triggers. Furthermore, A_2AR overfunction seems to be an early event in the demise of brain diseases, which involves an increased formation of ATP-derived adenosine and an up-regulation of A_2AR. This prompts the novel hypothesis that the evaluation of A_2AR density in afflicted brain circuits may become an important biomarker of susceptibility and evolution of brain diseases once faithful PET ligands are optimized. Additional relevant biomarkers would be measuring the extracellular ATP and/or adenosine levels with selective dyes, to identify stressed regions in the brain. A_2AR display several polymorphisms in humans and preliminary studies have associated different A_2AR polymorphisms with altered morphofunctional brain endpoints associated with neuropsychiatric diseases. This further prompts the interest in exploiting A_2AR polymorphic analysis as an ancillary biomarker of susceptibility/evolution of brain diseases.

Increased ATP release and CD73-mediated adenosine A2A receptor activation mediate convulsion-associated neuronal damage and hippocampal dysfunction

Extracellular ATP is a danger signal to the brain and contributes to neurodegeneration in animal models of Alzheimer's disease through its extracellular catabolism by CD73 to generate adenosine, bolstering the activation of adenosine A_2A receptors (A_2AR). Convulsive activity leads to increased ATP release, with the resulting morphological alterations being eliminated by A_2AR blockade. However, it is not known if upon convulsions there is a CD73-mediated coupling between ATP release and A_2AR overactivation, causing neurodegeneration. We now show that kainate-induced convulsions trigger a parallel increase of ATP release and of CD73 and A_2AR densities in synapses and astrocytes of the mouse hippocampus. Notably, the genetic deletion of CD73 attenuates neuronal degeneration but has no impact on astrocytic modifications in the hippocampus upon kainate-induced convulsions. Furthermore, kainate-induced convulsions cause a parallel deterioration of hippocampal long-term potentiation (LTP) and hippocampal-dependent memory performance, which is eliminated by knocking out CD73. This demonstrates the key role of the ATP release/CD73/A_2AR pathway to selectively control synaptic dysfunction and neurodegeneration following an acute brain insult, paving the way to consider CD73 as a new therapeutic target to prevent neuronal damage upon acute brain damage.

ATP and adenosine-Two players in the control of seizures and epilepsy development

Despite continuous advances in understanding the underlying pathogenesis of hyperexcitable networks and lowered seizure thresholds, the treatment of epilepsy remains a clinical challenge. Over one third of patients remain resistant to current pharmacological interventions. Moreover, even when effective in suppressing seizures, current medications are merely symptomatic without significantly altering the course of the disease. Much effort is therefore invested in identifying new treatments with novel mechanisms of action, effective in drug-refractory epilepsy patients, and with the potential to modify disease progression. Compelling evidence has demonstrated that the purines, ATP and adenosine, are key mediators of the epileptogenic process. Extracellular ATP concentrations increase dramatically under pathological conditions, where it functions as a ligand at a host of purinergic receptors. ATP, however, also forms a substrate pool for the production of adenosine, via the action of an array of extracellular ATP degrading enzymes. ATP and adenosine have assumed largely opposite roles in coupling neuronal excitability to energy homeostasis in the brain. This review integrates and critically discusses novel findings regarding how ATP and adenosine control seizures and the development of epilepsy. This includes purine receptor P1 and P2-dependent mechanisms, release and reuptake mechanisms, extracellular and intracellular purine metabolism, and emerging receptor-independent effects of purines. Finally, possible purine-based therapeutic strategies for seizure suppression and disease modification are discussed.

Purines and Pyrimidines: Metabolism, Function and Potential as Therapeutic Options in Neurodegenerative Diseases

Various neurodegenerative disorders have various molecular origins but some common molecular mechanisms. In the current scenario, there are very few treatment regimens present for advanced neurodegenerative diseases. In this context, there is an urgent need for alternate options in the form of natural compounds with an ameliorating effect on patients. There have been individual scattered experiments trying to identify potential values of various intracellular metabolites. Purines and Pyrimidines, which are vital molecules governing various aspects of cellular biochemical reactions, have been long sought as crucial candidates for the same, but there are still many questions that go unanswered. Some critical functions of these molecules associated with neuromodulation activities have been identified. They are also known to play a role in foetal neurodevelopment, but there is a lacuna in understanding their mechanisms. In this review, we have tried to assemble and identify the importance of purines and pyrimidines, connecting them with the prevalence of neurodegenerative diseases. The leading cause of this class of diseases is protein misfolding and the formation of amyloids. A direct correlation between loss of balance in cellular homeostasis and amyloidosis is yet an unexplored area. This review aims at bringing the current literature available under one umbrella serving as a foundation for further extensive research in this field of drug development in neurodegenerative diseases.

Integrating metabolomics and network pharmacology to explore the protective effect of gross saponins of Tribulus terrestris L. fruit against ischemic stroke in rat

ETHNOPHARMACOLOGICAL RELEVANCE

Tribulus terrestris L. belongs to the family Zygophyllaceae and has been widely used as a folk medicine for a long history in Asian countries. Gross saponins of Tribulus terrestris L. fruit (GSTTF) has an obvious neuroprotective effect on the treatment of ischemic stroke, but its potential therapeutic mechanisms have not been thoroughly studied.

AIM OF THE STUDY

To investigate the protective effect of GSTTF against ischemic stroke in rat.

MATERIALS AND METHODS

The combination of metabolomics and network pharmacology analysis was applied to investigate the protective effects of GSTTF on ischemic stroke and its putative mechanism. The related pathway of the biomarkers highlighted from metabolomics analysis was explored, then the possible targets of GSTTF were further revealed by network pharmacology analysis. Molecular docking was conducted to investigate the interaction between the active compound and target protein.

RESULTS

Metabolomics analysis showed that metabolic disturbances were observed in serum for the rats in middle cerebral artery occlusion (MCAO). These MCAO-induced deviations in serum metabolism can be reversely changed by GSTTF via metabolic pathways regulation. Twenty-four proteins with the connectivity degree larger than 15 were selected by the network pharmacology analysis, which are considered as the possible therapeutic targets of the GSTTF against ischemic stroke. The results of molecular docking showed that the active compounds were capable of binding to the representative potential targets HSD11B1 and AR, respectively. And the docking mode of two compounds with the lowest binding energy to their target protein was illustrated by the ribbon binding map.

CONCLUSION

The present study combines metabolomics and network pharmacology analysis to investigate the mechanism of MCAO-induced ischemic stroke and reveal the efficiency and possible mechanisms of GSTTF for ischemic stroke. Further studies on the bioactive saponin as well as their synergistic action on ischemic stroke will be conducted to better reveal the underlying mechanisms.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

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.

Activation of adenosine receptors modulates the efflux transporters in brain capillaries and restores the anticonvulsant effect of carbamazepine in carbamazepine resistant rats developed by window-pentylenetetrazole kindling

Up-regulation of efflux transporters in brain capillaries may lead to the decreased therapeutic efficacy of antiepileptic drugs in patients with Drug Resistant Epilepsy. Adenosine receptor activation in brain capillaries can modulate blood-brain barrier permeability by decreasing the protein levels and function of efflux transporters. Therefore, we aimed to investigate whether the activation of adenosine receptors improves convulsions outcome in carbamazepine (CBZ) resistant animals and modulates the protein levels of efflux transporters (P-GP, MRP1, MRP2) in brain capillaries. We employed the window-pentylenetetrazol (PTZ) kindling model to develop CBZ resistant rats by CBZ administration during the post-kindling phase, and tested if these animals displayed subsequent resistance to other antiepileptic drugs. Crucially, we investigated if the administration of a broad-spectrum adenosine agonist (NECA) improves convulsions control in CBZ resistant rats. Of potential therapeutic relevance, in CBZ resistant rats NECA restored the anticonvulsant effect of CBZ. We also evaluated how the resistance to CBZ and the activation of adenosine receptors with NECA affect protein levels of efflux transporters in brain capillaries, as quantified by western blot. While CBZ resistance was associated with the up-regulation of both P-GP/MRP2 in brain capillaries, with the administration of NECA in CBZ resistant rats, we observed a decrease of P-GP and an increase of MRP2 levels, in brain capillaries. Since the activation of adenosine receptors improves the outcome of convulsions probably through the modulation of the efflux transporters protein levels in brain capillaries, adenosine agonists could be useful as an adjunct therapy for the control of Drug Resistant Epilepsy.

Enhanced ATP release and CD73-mediated adenosine formation sustain adenosine A2A receptor over-activation in a rat model of Parkinson's disease

BACKGROUND AND PURPOSE

Parkinson's disease (PD) involves an initial loss of striatal dopamine terminals evolving into degeneration of dopamine neurons in substantia nigra (SN), which can be modelled by 6-hydroxydopamine (6-OHDA) administration. Adenosine A_2A receptor blockade attenuates PD features in animal models, but the source of the adenosine causing A_2A receptor over-activation is unknown. As ATP is a stress signal, we have tested if extracellular catabolism of adenine nucleotides into adenosine (through ecto-5'-nucleotidase or CD73) leads to A_2A receptor over-activation in PD.

EXPERIMENTAL APPROACH

Effects of blocking CD73 with α,β-methylene ADP (AOPCP) were assayed in 6-OHDA-treated rats and dopamine-differentiated neuroblastoma SH-SY5Y cells.

KEY RESULTS

6-OHDA increased ATP release and extracellular conversion into adenosine through CD73 up-regulation in SH-SY5Y cells. Removing extracellular adenosine with adenosine deaminase, blocking CD73 with AOPCP, or blocking A_2A receptors with SCH58261 were equi-effective in preventing 6-OHDA-induced damage in SH-SY5Y cells. In vivo striatal exposure to 6-OHDA increased ATP release and extracellular formation of adenosine from adenosine nucleotides and up-regulated CD73 and A_2A receptors in striatal synaptosomes. Intracerebroventricular administration of AOPCP phenocopied effects of SCH58261, attenuating 6-OHDA-induced (a) increase of contralateral rotations after apomorphine, (b) reduction of dopamine content in striatum and SN, (c) loss of TH staining in striatum and SN, (d) motor dysfunction in the cylinder test, and (e) short-term memory impairment in the object recognition test.

CONCLUSION AND IMPLICATIONS

Our data indicate that increased ATP-derived adenosine formation is responsible for A_2A receptor over-activation in PD, suggesting CD73 as a new target to manage PD.

Complex regulation of ecto-5'-nucleotidase/CD73 and A2AR-mediated adenosine signaling at neurovascular unit: A link between acute and chronic neuroinflammation

The review summarizes available data regarding the complex regulation of CD73 at the neurovascular unit (NVU) during neuroinflammation. Based on available data we propose the biphasic pattern of CD73 regulation at NVU, with an early attenuation and a postponed up-regulation of CD73 activity. Transient attenuation of CD73 activity on leukocyte/vascular endothelium and leukocyte/astrocyte surface, required for the initiation of a neuroinflammatory response, may be effectuated either by catalytic inhibition of CD73 and/or by shedding of the CD73 molecule from the cell surface, while postponed induction of CD73 is effectuated by transcriptional up-regulation of Nt5e and posttranslational modifications. Neuroinflammatory conditions are also associated with significant enhancement and gain-of-function of A_2AR-mediated adenosine signaling. However, in contrast to the temporary prevalence of A_2AR over A₁R signaling during an acute inflammatory response, prolonged induction of A_2AR and resulting perpetual CD73/A_2AR coupling may be a contributing factors in the transition between acute and chronic neuroinflammation. Thus, pharmacological targeting of the CD73/A_2AR axis may attenuate inflammatory response and ameliorate neurological deficits in chronic neuroinflammatory conditions.

Neuronal Adenosine A2A Receptors Are Critical Mediators of Neurodegeneration Triggered by Convulsions

Neurodegeneration is a process transversal to neuropsychiatric diseases and the understanding of its mechanisms should allow devising strategies to prevent this irreversible step in brain diseases. Neurodegeneration caused by seizures is a critical step in the aggravation of temporal lobe epilepsy, but its mechanisms remain undetermined. Convulsions trigger an elevation of extracellular adenosine and upregulate adenosine A_2A receptors (A_2AR), which have been associated with the control of neurodegenerative diseases. Using the rat and mouse kainate model of temporal lobe epilepsy, we now tested whether A_2AR control convulsions-induced hippocampal neurodegeneration. The pharmacological or genetic blockade of A_2AR did not affect kainate-induced convulsions but dampened the subsequent neurotoxicity. This neurotoxicity began with a rapid A_2AR upregulation within glutamatergic synapses (within 2 h), through local translation of synaptic A_2AR mRNA. This bolstered A_2AR-mediated facilitation of glutamate release and of long-term potentiation (LTP) in CA1 synapses (4 h), triggered a subsequent synaptotoxicity, heralded by decreased synaptic plasticity and loss of synaptic markers coupled to calpain activation (12 h), that predated overt neuronal loss (24 h). All modifications were prevented by the deletion of A_2AR selectively in forebrain neurons. This shows that synaptic A_2AR critically control synaptic excitotoxicity, which underlies the development of convulsions-induced neurodegeneration.

Early-life hyperthermic seizures upregulate adenosine A2A receptors in the cortex and promote depressive-like behavior in adult rats

Febrile seizures (FS) represent one of the most frequent convulsive disorders in children which can be classified into simple and prolonged depending on the duration. Although simple FS are generally considered as benign, there is controversy about the outcome of prolonged FS. Here, we have used an animal model of prolonged FS to investigate persistent neurochemical and behavioral alterations in adult rats. Hyperthermic seizures were induced in 12-day-old rats using a warmed air stream from a hair dryer. Neonates exhibited arrest of heat-induced hyperkinesis followed by body flexion and rearing and falling over associated with hindlimb clonus seizures (stage 5 on Racine scale criteria) after hyperthermic induction. After 48 days, the animals were assayed on dark-light box and forced swim tests in order to detect if rats will show signs of anxiety or depression. Finally, animals were sacrificed 56 days after hyperthermia-induced seizures (HIS), and their effects on adenosine A_2A receptor signaling and 5'-nucleotidase activity were studied in plasma membranes from the cerebral cortex by using radioligand-binding assay and by measuring the activities of adenylate cyclase and 5'-nucleotidase. Results obtained have shown that adult rats submitted to HIS during the neonatal period showed depressive-like behavior. Furthermore, animals exposed to hyperthermic insult showed an increase in A_2A receptor level which was also accompanied by an increase in A_2A receptor functionality.

Impact of Coffee and Cacao Purine Metabolites on Neuroplasticity and Neurodegenerative Disease

Increasing evidence suggests that regular consumption of coffee, tea and dark chocolate (cacao) can promote brain health and may reduce the risk of age-related neurodegenerative disorders. However, the complex array of phytochemicals in coffee and cacao beans and tea leaves has hindered a clear understanding of the component(s) that affect neuronal plasticity and resilience. One class of phytochemicals present in relatively high amounts in coffee, tea and cacao are methylxanthines. Among such methylxanthines, caffeine has been the most widely studied and has clear effects on neuronal network activity, promotes sustained cognitive performance and can protect neurons against dysfunction and death in animal models of stroke, Alzheimer's disease and Parkinson's disease. Caffeine's mechanism of action relies on antagonism of various subclasses of adenosine receptors. Downstream xanthine metabolites, such as theobromine and theophylline, may also contribute to the beneficial effects of coffee, tea and cacao on brain health.

Caffeine Controls Glutamatergic Synaptic Transmission and Pyramidal Neuron Excitability in Human Neocortex

Caffeine is the most widely used psychoactive drug, bolstering attention and normalizing mood and cognition, all functions involving cerebral cortical circuits. Whereas studies in rodents showed that caffeine acts through the antagonism of inhibitory A₁ adenosine receptors (A₁R), neither the role of A₁R nor the impact of caffeine on human cortical neurons is known. We here provide the first characterization of the impact of realistic concentrations of caffeine experienced by moderate coffee drinkers (50 μM) on excitability of pyramidal neurons and excitatory synaptic transmission in the human temporal cortex. Moderate concentrations of caffeine disinhibited several of the inhibitory A₁R-mediated effects of adenosine, similar to previous observations in the rodent brain. Thus, caffeine restored the adenosine-induced decrease of both intrinsic membrane excitability and excitatory synaptic transmission in the human pyramidal neurons through antagonism of post-synaptic A₁R. Indeed, the A₁R-mediated effects of endogenous adenosine were more efficient to inhibit synaptic transmission than neuronal excitability. This was associated with a distinct affinity of caffeine for synaptic versus extra-synaptic human cortical A₁R, probably resulting from a different molecular organization of A₁R in human cortical synapses. These findings constitute the first neurophysiological description of the impact of caffeine on pyramidal neuron excitability and excitatory synaptic transmission in the human temporal cortex, providing adequate ground for the effects of caffeine on cognition in humans.

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

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".

Aberrant adenosine A2A receptor signaling contributes to neurodegeneration and cognitive impairments in a mouse model of synucleinopathy

Synucleinopathy is characterized by abnormal accumulation of misfolded α-synuclein (α-Syn)-positive cytoplasmic inclusions and by neurodegeneration and cognitive impairments, but the pathogenesis mechanism of synucleinopathy remains to be defined. Using a transmission model of synucleinopathy by intracerebral injection of preformed A53T α-Syn fibrils, we investigated whether aberrant adenosine A2A receptor (A2AR) signaling contributed to pathogenesis of synucleinopathy. We demonstrated that intra-hippocampal injection of preformed mutant α-Syn fibrils triggered a striking and selective induction of A2AR expression which was closely co-localized with pSer129 α-Syn-rich inclusions in neurons and glial cells of hippocampus. Importantly, by abolishing aberrant A2AR signaling triggered by mutant α-Syn, genetic deletion of A2ARs blunted a cascade of pathological events leading to synucleinopathy, including pSer129 α-Syn-rich and p62-positive aggregates, NF-κB activation and astrogliosis, apoptotic neuronal cell death and working memory deficits without affecting motor activity. These findings define α-Syn-triggered aberrant A2AR signaling as a critical pathogenesis mechanism of synucleinopathy with dual controls of cognition and neurodegeneration by modulating α-Syn aggregates. Thus, aberrant A2AR signaling represents a useful biomarker as well as a therapeutic target of synucleinopathy.

Adenosine A1 receptor-mediated suppression of carbamazepine-resistant seizure-like events in human neocortical slices

OBJECTIVE

The need for alternative pharmacologic strategies in treatment of epilepsies is pressing for about 30% of patients with epilepsy who do not experience satisfactory seizure control with present treatments. In temporal lobe epilepsy (TLE) even up to 80% of patients are pharmacoresistant, and surgical resection of the ictogenic tissue is only possible for a minority of TLE patients. In this study we investigate purinergic modulation of drug-resistant seizure-like events (SLEs) in human temporal cortex slices.

METHODS

Layer V/VI field potentials from a total of 77 neocortical slices from 17 pharmacoresistant patients were recorded to monitor SLEs induced by application of 8 mM [K(+) ] and 50 μm bicuculline.

RESULTS

Activating A1 receptors with a specific agonist completely suppressed SLEs in 73% of human temporal cortex slices. In the remaining slices, incidence of SLEs was markedly reduced. Because a subportion of slices can be pharmacosensitive, we tested effects of an A1 agonist, in slices insensitive to a high dose of carbamazepine (50 μm). Also in these cases the A1 agonist was equally efficient. Moreover, ATP and adenosine blocked or modulated SLEs, an effect mediated not by P2 receptors but rather by adenosine A1 receptors.

SIGNIFICANCE

Selective activation of A1 receptors mediates a strong anticonvulsant action in human neocortical slices from pharmacoresistant patients. We propose that our human slice model of seizure-like activity is a feasible option for future studies investigating new antiepileptic drug (AED) candidates.

Adenosine receptor neurobiology: overview

Adenosine is a naturally occurring nucleoside that is distributed ubiquitously throughout the body as a metabolic intermediary. In the brain, adenosine functions as an important upstream neuromodulator of a broad spectrum of neurotransmitters, receptors, and signaling pathways. By acting through four G-protein-coupled receptors, adenosine contributes critically to homeostasis and neuromodulatory control of a variety of normal and abnormal brain functions, ranging from synaptic plasticity, to cognition, to sleep, to motor activity to neuroinflammation, and cell death. This review begun with an overview of the gene and genome structure and the expression pattern of adenosine receptors (ARs). We feature several new developments over the past decade in our understanding of AR functions in the brain, with special focus on the identification and characterization of canonical and noncanonical signaling pathways of ARs. We provide an update on functional insights from complementary genetic-knockout and pharmacological studies on the AR control of various brain functions. We also highlight several novel and recent developments of AR neurobiology, including (i) recent breakthrough in high resolution of three-dimension structure of adenosine A2A receptors (A2ARs) in several functional status, (ii) receptor-receptor heterodimerization, (iii) AR function in glial cells, and (iv) the druggability of AR. We concluded the review with the contention that these new developments extend and strengthen the support for A1 and A2ARs in brain as therapeutic targets for neurologic and psychiatric diseases.

ATP as a multi-target danger signal in the brain

ATP is released in an activity-dependent manner from different cell types in the brain, fulfilling different roles as a neurotransmitter, neuromodulator, in astrocyte-to-neuron communication, propagating astrocytic responses and formatting microglia responses. This involves the activation of different ATP P2 receptors (P2R) as well as adenosine receptors upon extracellular ATP catabolism by ecto-nucleotidases. Notably, brain noxious stimuli trigger a sustained increase of extracellular ATP, which plays a key role as danger signal in the brain. This involves a combined action of extracellular ATP in different cell types, namely increasing the susceptibility of neurons to damage, promoting astrogliosis and recruiting and formatting microglia to mount neuroinflammatory responses. Such actions involve the activation of different receptors, as heralded by neuroprotective effects resulting from blockade mainly of P2X7R, P2Y1R and adenosine A_2A receptors (A_2AR), which hierarchy, cooperation and/or redundancy is still not resolved. These pleiotropic functions of ATP as a danger signal in brain damage prompt a therapeutic interest to multi-target different purinergic receptors to provide maximal opportunities for neuroprotection.

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.

Glial adenosine kinase--a neuropathological marker of the epileptic brain

Experimental research over the past decade has supported the critical role of astrocytes activated by different types of injury and the pathophysiological processes that underlie the development of epilepsy. In both experimental and human epileptic tissues astrocytes undergo complex changes in their physiological properties, which can alter glio-neuronal communication, contributing to seizure precipitation and recurrence. In this context, understanding which of the molecular mechanisms are crucially involved in the regulation of glio-neuronal interactions under pathological conditions associated with seizure development is important to get more insight into the role of astrocytes in epilepsy. This article reviews current knowledge regarding the role of glial adenosine kinase as a neuropathological marker of the epileptic brain. Both experimental findings in clinically relevant models, as well as observations in drug-resistant human epilepsies will be discussed, highlighting the link between astrogliosis, dysfunction of adenosine homeostasis and seizure generation and therefore suggesting new strategies for targeting astrocyte-mediated epileptogenesis.

The imbalanced expression of adenosine receptors in an epilepsy model corrected using targeted mesenchymal stem cell transplantation

Adenosine inhibits epileptic episodes by interacting with G-protein-coupled receptors. This study examined the mechanism by which the inhibitory effect of adenosine becomes impaired during epileptogenesis. Dynamic changes in adenosine A1 receptors (A1Rs) and A2a receptors (A2aRs) were investigated in a kindling model of epilepsy. RT-PCR, Western blotting, and immunofluorescence results indicated that expression of A1Rs was increased in the hippocampus 24 h after kindling, but progressively decreased 1 and 6 months after kindling. Opposite changes were seen in the expression of A2aRs. This bidirectional change resulted in an imbalance between A1Rs and A2aRs and dysregulation of the adenosine system. Autologous mesenchymal stem cell (MSC) transplantation was used to correct this disorder and avoid side effects of systematic adenosine therapy. Paramagnetic iron oxide particles were used to mark and track the MSCs in vivo using MRI. The results indicated that the transplanted cells migrated along the callosum and settled at the ependymal layer. The MSCs displayed a relatively long survival time, at least 3 months. The improved AR expression and EEG findings suggested that MSC transplantation was a potentially effective means of treating refractory epilepsy.

Studies on the effect of adenosine on calcium oscillation in hippocampal neurons

Adenosine (Ade) is an antiepileptic agent. In order to investigate the possible mechanism of action of Ade, its effect on calcium (Ca²⁺) oscillations in hippocampal neurons of Sprague Dawley (SD) rats was explored. Primary hippocampal neurons were cultured from suckling neonatal SD rats. Cells were cultured for 7–9 days and the Ca²⁺ oscillations in response to perfusion with Ade were detected using confocal laser scanning microscopy in combination with Fluo-3/AM labeling. This study found that Ade inhibits the spontaneous synchronized Ca²⁺ oscillation frequency and amplitude in mature hippocampal neurons and such inhibition depends on the Ade dosage level to a certain extent. Ade also had a significant inhibitory effect on high potassium-induced Ca²⁺ oscillation frequency and amplitude. Ade had a significant inhibitory effect on high-voltage-activated Ca²⁺ channel-mediated Ca²⁺ influx and Ca²⁺ oscillations in neurons. This may be one of the mechanisms for Ade to exert antiepileptic effects as an endogenous substance.

Effect of adenosine A2A receptor antagonist ZM241385 on amygdala-kindled seizures and progression of amygdala kindling

The purpose of this study was to evaluate the effect of adenosine A2A receptor antagonist ZM241385 on amygdala-kindled seizures and its roles in epileptogenesis. Electrodes were implanted into the right amygdala of male adult Wistar rats. Kindling was accomplished by using stimulus strength of 500 μA applied daily to the amygdala until 10 consecutive stage 5 seizues were induced. Then effect of ZM241385 was studied in fully kindled rats after intracerebroventricular administration of the drug. In addition, the effect on kindling progression was evaluated through ZM241385 injection before daily stimulation. In all experiments, behavioral changes in the rats in response to ZM241385 were monitored closely. The results showed that, in fully amygdala-kindled rats, ZM241385 (0.001-0.1 nmol/L) decreased afterdischage duration (ADD), motor seizure duration (MSD), stage 5 duration (S5D) and seizure duration (SD), but only the effect on ADD was dose-dependent. The doses of 0.001-0.1 nmol/L had no influence on stage 4 latency (S4L) and seizure stage (SS). The dosages of 0.0001 and 1 nmol/L of ZM241385 did not exert any effect on all seizure parameters. In contrast to the results in fully amygdala-kindled rats, ZM241385 (0.001-0.1 nmol/L) had minimal or no effects on the progression of amygdala-kindled seizures. We are led to the conclusion that although ZM241385 had no influence on the progression of amygdala-kindled seizures, it had potent anti-convulsant profile and little adverse effects at the dosage of 0.001-0.1 nmol/L, suggesting that the agent is effective against the amygdala-kindled seizures.

Local disruption of glial adenosine homeostasis in mice associates with focal electrographic seizures: a first step in epileptogenesis?

Astrogliosis and associated dysfunction of adenosine homeostasis are pathological hallmarks of the epileptic brain and thought to contribute to seizure generation in epilepsy. The authors hypothesized that astrogliosis-an early component of the epileptogenic cascade-might be linked to focal seizure onset. To isolate the contribution of astrogliosis to ictogenesis from other pathological events involved in epilepsy, the authors used a minimalistic model of epileptogenesis in mice, based on a focal onset status epilepticus triggered by intra-amygdaloid injection of kainic acid. The authors demonstrate acute neuronal cell loss restricted to the injected amygdala and ipsilateral CA3, followed 3 weeks later by focal astrogliosis and overexpression of the adenosine-metabolizing enzyme adenosine kinase (ADK). Using synchronous electroencephalographic recordings from multiple depth electrodes, the authors identify the KA-injected amygdala and ipsilateral CA3 as two independent foci for the initiation of non-synchronized electrographic subclinical seizures. Importantly, seizures remained focal and restricted to areas of ADK overexpression. However, after systemic application of a non-convulsive dose of an adenosine A(1) -receptor antagonist, seizures in amygdala and CA3 immediately synchronized and spread throughout the cortex, leading to convulsive seizures. This focal seizure phenotype remained stable over at least several weeks. We conclude that astrogliosis via disruption of adenosine homeostasis per se and in the absence of any other overt pathology, is associated with the emergence of spontaneous recurrent subclinical seizures, which remain stable over space and time. A secondary event, here mimicked by brain-wide disruption of adenosine signaling, is likely required to turn pre-existing subclinical seizures into a clinical seizure phenotype.

Adenosine 2A receptor: a crucial neuromodulator with bidirectional effect in neuroinflammation and brain injury

This review summarizes recent developments that have contributed to our understanding of how adenosine 2A receptors (A2ARs) modulate brain damage in various animal models of acute neurological injuries, including brain ischemia, traumatic brain injury, spinal cord injury and hemorrhage stroke. The main conclusions are: (1) pharmacological, neurochemical and molecular/genetic approaches to the complex actions of A2AR in different cellular elements suggest that A2AR activation exerts bidirectional effect (detrimental or protective) after brain insults; (2) modulation of glutamate excitotoxicity and neuroinflammation are involved in the protection of A2AR agonists or antagonists, but the bidirectional effect of A2AR is largely due to the bidirectional regulation of neuroinflammation (anti-inflammation or proinflammation) by A2AR on immune cells such as microglia cells and peripheral bone marrow cells; and (3) the bidirectional effect of A2AR on neuroinflammation and brain injury depends on the distinct and sometimes opposite actions of A2AR in various cellular elements and on different injury models and associated pathological conditions. The local glutamate level in the brain injury is one of the crucial factors that contribute to the direction of A2AR effect on neuroinflammation and brain injury outcome. These developments presented here clearly highlight the complexity of using A2AR agents therapeutically in acute neuronal injuries and confirm that A2AR ligands have many promising characteristics that encourage the pursuit of their full therapeutic potential.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

Differential trafficking of adenosine receptors in hippocampal neurons monitored using GFP- and super-ecliptic pHluorin-tagged receptors

Adenosine receptors (ARs) modulate many cellular and systems-level processes in the mammalian CNS. However, little is known about the trafficking of ARs in neurons, despite their importance in controlling seizure activity and in neuroprotection in cerebral ischaemia. To address this we examined the agonist-dependent internalisation of C-terminal GFP-tagged A(1)Rs, A(2A)Rs and A(3)Rs in primary hippocampal neurons. Furthermore, we developed a novel super-ecliptic pHluorin (SEP)-tagged A(1)R which, via the N-terminal SEP tag, reports the cell-surface expression and trafficking of A(1)Rs in real-time. We demonstrate the differential trafficking of ARs in neurons: A(3)Rs internalise more rapidly than A1Rs, with little evidence of appreciable A(2A)R trafficking over the time-course of the experiments. Furthermore, the novel SEP-A(1)R construct revealed the time-course of internalisation and recovery of cell-surface expression to occur within minutes of agonist exposure and removal, respectively. These observations highlight the labile nature of A(1)R and A(3)Rs when expressed at the neuronal plasma membrane. Given the high levels of adenosine in the brain during ischaemia and seizures, internalisation of the inhibitory A(1)R may result in hyperexcitability, increased brain damage and the development of chronic epileptic states.

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.

E. coli lipopolysaccharide attenuates adenosine A(1) receptor-mediated increase in plasma exudation from the hamster cheek pouch

OBJECTIVE AND DESIGN

To determine whether exposure to E. coli lipopolysaccharide (LPS) modulates adenosine A(1) receptor-induced increase in plasma exudation from the intact hamster cheek pouch microcirculation.

METHODS AND RESULTS

Using intravital microscopy, we found that suffusion of R(-)-N(6)-(2-phenylisopropyl)-adenosine (R(-)-PIA) (1.0 and 10.0 nM), a selective adenosine A(1) receptor agonist, onto the intact cheek pouch elicited significant, concentration-dependent leaky site formation and increase in clearance of fluorescein thioisocyanate-dextran (mol mass, 70 kDa) from post-capillary venules (p < 0.05). These responses were significantly attenuated by pre-treatment of hamsters with LPS (p < 0.05). By contrast, LPS had no significant effects on CGS-21680-, a selective adenosine A(2A) receptor agonist, bradykinin- and substance P-induced increases in plasma exudation from the cheek pouch.

CONCLUSION

These data indicate that LPS attenuates adenosine A(1) receptor-induced increase in plasma exudation in vivo in a specific fashion. We suggest that this phenomenon represents an endogenous anti-inflammatory cue to avoid excessive inflammation during Gram-negative bacterial infections.