Functional Cross-Talk between Adenosine and Metabotropic Glutamate Receptors

Adenosinergic system and nucleoside transporters in attention deficit hyperactivity disorder: Current findings

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder with high heterogeneity that can affect individuals of any age. It is characterized by three main symptoms: inattention, hyperactivity, and impulsivity. These neurobehavioral alterations and neurochemical and pharmacological findings are mainly attributed to unbalanced catecholaminergic signaling, especially involving dopaminergic pathways within prefrontal and striatal areas. Dopamine receptors and transporters are not solely implicated in this imbalance, as evidence indicates that the dopaminergic signaling is modulated by adenosine activity. To this extent, alterations in adenosinergic signaling are probably involved in ADHD. Here, we review the current knowledge about adenosine's role in the modulation of chemical, behavioral and cognitive parameters of ADHD, especially regarding dopaminergic signaling. Current literature usually links adenosine receptors signaling to the dopaminergic imbalance found in ADHD, but there is evidence that equilibrative nucleoside transporters (ENTs) could also be implicated as players in dopaminergic signaling alterations seen in ADHD, since their involvement in other neurobehavioral impairments.

Activation of adenosine A1 receptor potentiates metabotropic glutamate receptor 1-mediated Ca2+ mobilization in the rat hippocampal marginal zone

Two subtypes of group I metabotropic glutamate receptors (mGluRs), mGluR1 and mGluR5, participate in the regulation of cell excitability and synaptic plasticity in the central nervous system. They couple to the Gq/11 protein and release Ca2+ from the intracellular stores. In the marginal zone of the neonatal hippocampus, Cajal-Retzius (CR) cells, which control radial migration of neurons, express the subtype mGluR1. The adenosine A1 receptor (A1R) is also G-protein coupled and is extensively expressed in the central nervous system. The interactions among G-protein-coupled receptors have been predicted previously, however, there is insufficient evidence of functional interactions between naturally occurring receptors. In this study, potentiation of the mGluR1-mediated response by A1R activation was demonstrated in hippocampal CR cells. Fluorescence imaging revealed that the application of A1R agonists intensified mGluR1-induced elevation of intracellular Ca2+ concentration ([Ca2+]i). Activation of A1R did not change [Ca2+]i. The potentiated responses were independent of extracellular Ca2+ and prevented by the Gi inhibitor. The potentiation of mGluR1-induced [Ca2+]i. elevation was also enhanced by mGluR2/3 activation. These results suggest that mGluR1 and A1R cooperatively influence postnatal hippocampal development by facilitating Ca2+ mobilization in CR cells.

On the participation of adenosinergic receptors in the reconsolidation of spatial long-term memory in male rats

To date, there is insufficient evidence to explain the role of adenosinergic receptors in the reconsolidation of long-term spatial memory. In this work, the role of the adenosinergic receptor family (A1, A2A, A2B, and A3) in this process has been elucidated. It was demonstrated that when infused bilaterally into the hippocampal CA1 region immediately after an early nonreinforced test session performed 24 h posttraining in the Morris water maze task, adenosine can cause anterograde amnesia for recent and late long-term spatial memory. This effect on spatial memory reconsolidation was blocked by A1 or A3 receptor antagonists and mimicked by A1 plus A3 receptor agonists, showing that this effect occurs through A1 and A3 receptors simultaneously. The A3 receptor alone participates only in the reconsolidation of late long-term spatial memory. When the memory to be reconsolidated was delayed (reactivation 5 d posttraining), the amnesic effect of adenosine became transient and did not occur in a test performed 5 d after the reactivation of the mnemonic trace. Finally, it has been shown that the amnesic effect of adenosine on spatial memory reconsolidation depends on the occurrence of protein degradation and that the amnesic effect of inhibition of protein synthesis on spatial memory reconsolidation is dependent on the activation of A3 receptors.

The role of metabotropic glutamate receptors in neurobehavioral effects associated with methamphetamine use

Metabotropic glutamate (mGlu) receptors are expressed throughout the central nervous system and act as important regulators of drug-induced neuroplasticity and behavior. Preclinical research suggests that mGlu receptors play a critical role in a spectrum of neural and behavioral consequences arising from methamphetamine (meth) exposure. However, an overview of mGlu-dependent mechanisms linked to neurochemical, synaptic, and behavioral changes produced by meth has been lacking. This chapter provides a comprehensive review of the role of mGlu receptor subtypes (mGlu1-8) in meth-induced neural effects, such as neurotoxicity, as well as meth-associated behaviors, such as psychomotor activation, reward, reinforcement, and meth-seeking. Additionally, evidence linking altered mGlu receptor function to post-meth learning and cognitive deficits is critically evaluated. The chapter also considers the role of receptor-receptor interactions involving mGlu receptors and other neurotransmitter receptors in meth-induced neural and behavioral changes. Taken together, the literature indicates that mGlu5 regulates the neurotoxic effects of meth by attenuating hyperthermia and possibly through altering meth-induced phosphorylation of the dopamine transporter. A cohesive body of work also shows that mGlu5 antagonism (and mGlu2/3 agonism) reduce meth-seeking, though some mGlu5-blocking drugs also attenuate food-seeking. Further, evidence suggests that mGlu5 plays an important role in extinction of meth-seeking behavior. In the context of a history of meth intake, mGlu5 also co-regulates aspects of episodic memory, with mGlu5 stimulation restoring impaired memory. Based on these findings, we propose several avenues for the development of novel pharmacotherapies for Methamphetamine Use Disorder based on the selective modulation mGlu receptor subtype activity.

Molecular Pathophysiological Mechanisms in Huntington's Disease

Huntington’s disease is an inherited neurodegenerative disease described 150 years ago by George Huntington. The genetic defect was identified in 1993 to be an expanded CAG repeat on exon 1 of the huntingtin gene located on chromosome 4. In the following almost 30 years, a considerable amount of research, using mainly animal models or in vitro experiments, has tried to unravel the complex molecular cascades through which the transcription of the mutant protein leads to neuronal loss, especially in the medium spiny neurons of the striatum, and identified excitotoxicity, transcriptional dysregulation, mitochondrial dysfunction, oxidative stress, impaired proteostasis, altered axonal trafficking and reduced availability of trophic factors to be crucial contributors. This review discusses the pathogenic cascades described in the literature through which mutant huntingtin leads to neuronal demise. However, due to the ubiquitous presence of huntingtin, astrocytes are also dysfunctional, and neuroinflammation may additionally contribute to Huntington’s disease pathology. The quest for therapies to delay the onset and reduce the rate of Huntington’s disease progression is ongoing, but is based on findings from basic research.

Disrupting GPCR Complexes with Smart Drug-like Peptides

G protein-coupled receptors (GPCRs) are a superfamily of proteins classically described as monomeric transmembrane (TM) receptors. However, increasing evidence indicates that many GPCRs form higher-order assemblies made up of monomers pertaining to identical (homo) or to various (hetero) receptors. The formation and structure of these oligomers, their physiological role and possible therapeutic applications raise a variety of issues that are currently being actively explored. In this context, synthetic peptides derived from TM domains stand out as powerful tools that can be predictably targeted to disrupt GPCR oligomers, especially at the interface level, eventually impairing their action. However, despite such potential, TM-derived, GPCR-disrupting peptides often suffer from inadequate pharmacokinetic properties, such as low bioavailability, a short half-life or rapid clearance, which put into question their therapeutic relevance and promise. In this review, we provide a comprehensive overview of GPCR complexes, with an emphasis on current studies using GPCR-disrupting peptides mimicking TM domains involved in multimerization, and we also highlight recent strategies used to achieve drug-like versions of such TM peptide candidates for therapeutic application.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

Tackling retinal ganglion cell apoptosis in glaucoma: role of adenosine receptors

INTRODUCTION

The role of adenosine receptors as therapeutic targets for neuroprotection is now widely recognized. Their role, however, in protection against retinal ganglion cell (RGC) apoptosis in glaucoma needs further investigation. Hence, in this review, we look into the possibility of adenosine receptors as potential therapeutic targets by exploring their role in modulating various pathophysiological mechanisms underlying glaucomatous RGC loss.

AREAS COVERED

This review presents a summary of the adenosine receptor distribution in retina and the cellular functions mediated by them. The major pathophysiological mechanisms such as excitotoxicity, vascular dysregulation, loss of neurotrophic signaling, and inflammatory responses involved in glaucomatous RGC loss are discussed. The literature showing the role of adenosine receptors in modulating these pathophysiological mechanisms is discussed. The literature search was conducted using Pubmed search engine using key words such as 'RGC apoptosis,' 'adenosine,' adenosine receptors' 'retina' 'excitotoxicity,' 'neurotrophins,' 'ischemia', and 'cytokines' individually and in various combinations.

EXPERT OPINION

Use of adenosine receptor agonists and antagonists, for preservation of the RGCs in glaucomatous eyes independent of the level of intraocular pressure seems a very useful strategy. Future application of this strategy would require appropriate designing of drug formulation for tissue and disease-specific receptor targeting. Furthermore, the modulation of physiological functions and potential adverse effects need further investigations.

Purinergic Receptors of the Central Nervous System: Biology, PET Ligands, and Their Applications

Purinergic receptors play important roles in central nervous system (CNS). These receptors are involved in cellular neuroinflammatory responses that regulate functions of neurons, microglial and astrocytes. Based on their endogenous ligands, purinergic receptors are classified into P1 or adenosine, P2X and P2Y receptors. During brain injury or under pathological conditions, rapid diffusion of extracellular adenosine triphosphate (ATP) or uridine triphosphate (UTP) from the damaged cells, promote microglial activation that result in the changes in expression of several of these receptors in the brain. Imaging of the purinergic receptors with selective Positron Emission Tomography (PET) radioligands has advanced our understanding of the functional roles of some of these receptors in healthy and diseased brains. In this review, we have accumulated a list of currently available PET radioligands of the purinergic receptors that are used to elucidate the receptor functions and participations in CNS disorders. We have also reviewed receptors lacking radiotracer, laying the foundation for future discoveries of novel PET radioligands to reveal these receptors roles in CNS disorders.

Purinergic Signaling in the Pathophysiology and Treatment of Huntington's Disease

Huntington’s disease (HD) is a devastating, progressive, and fatal neurodegenerative disorder inherited in an autosomal dominant manner. This condition is characterized by motor dysfunction (chorea in the early stage, followed by bradykinesia, dystonia, and motor incoordination in the late stage), psychiatric disturbance, and cognitive decline. The neuropathological hallmark of HD is the pronounced neuronal loss in the striatum (caudate nucleus and putamen). The striatum is related to the movement control, flexibility, motivation, and learning and the purinergic signaling has an important role in the control of these events. Purinergic signaling involves the actions of purine nucleotides and nucleosides through the activation of P2 and P1 receptors, respectively. Extracellular nucleotide and nucleoside-metabolizing enzymes control the levels of these messengers, modulating the purinergic signaling. The striatum has a high expression of adenosine A_2A receptors, which are involved in the neurodegeneration observed in HD. The P2X7 and P2Y2 receptors may also play a role in the pathophysiology of HD. Interestingly, nucleotide and nucleoside levels may be altered in HD animal models and humans with HD. This review presents several studies describing the relationship between purinergic signaling and HD, as well as the use of purinoceptors as pharmacological targets and biomarkers for this neurodegenerative disorder.

Early Effects of the Soluble Amyloid β25-35 Peptide in Rat Cortical Neurons: Modulation of Signal Transduction Mediated by Adenosine and Group I Metabotropic Glutamate Receptors

The amyloid β peptide (Aβ) is a central player in the neuropathology of Alzheimer’s disease (AD). The alteration of Aβ homeostasis may impact the fine-tuning of cell signaling from the very beginning of the disease, when amyloid plaque is not deposited yet. For this reason, primary culture of rat cortical neurons was exposed to Aβ_25-35, a non-oligomerizable form of Aβ. Cell viability, metabotropic glutamate receptors (mGluR) and adenosine receptors (AR) expression and signalling were assessed. Aβ_25-35 increased mGluR density and affinity, mainly due to a higher gene expression and protein presence of Group I mGluR (mGluR₁ and mGluR₅) in the membrane of cortical neurons. Intriguingly, the main effector of group I mGluR, the phospholipase C β₁ isoform, was less responsive. Also, the inhibitory action of group II and group III mGluR on adenylate cyclase (AC) activity was unaltered or increased, respectively. Interestingly, pre-treatment of cortical neurons with an antagonist of group I mGluR reduced the Aβ_25-35-induced cell death. Besides, Aβ_25-35 increased the density of A₁R and A_2AR, along with an increase in their gene expression. However, while A₁R-mediated AC inhibition was increased, the A_2AR-mediated stimulation of AC remained unchanged. Therefore, one of the early events that takes place after Aβ_25-35 exposure is the up-regulation of adenosine A₁R, A_2AR, and group I mGluR, and the different impacts on their corresponding signaling pathways. These results emphasize the importance of deciphering the early events and the possible involvement of metabotropic glutamate and adenosine receptors in AD physiopathology.

Glutamatergic System is Affected in Brain from an Hyperthermia-Induced Seizures Rat Model

One of the most frequent neurological disorders in children is febrile seizures (FS), a risk for epilepsy in adults. Glutamate is the main excitatory neurotransmitter in CNS acting through ionotropic and metabotropic receptors. Excess of glutamate in the extracellular space elicits excitotoxicity and has been associated with neurological disorders, such as epilepsy. The removal of extracellular glutamate by excitatory amino acid transporters (EATT) plays an important neuroprotective role. GLT-1 is the main EAAT present in the cortex brain. On the other hand, an increase in metabotropic glutamate receptors 5 (mGlu₅R) levels or their overstimulation have been related to the appearance of seizure events in different animal models and in temporal lobe epilepsy in humans. In this work, the status of several components of the glutamatergic system has been analysed in the cortex brain from an FS rat model at short (48 h) and long (20 days) term after hyperthermia-induced seizures. At the short term, we detected increased GLT-1 levels, reduced glutamate concentration, and unchanged mGlu₅R levels, without neuronal loss. However, at the long term, an increase in mGlu₅R levels together with a decrease in both GLT-1 and glutamate levels were observed. These changes were associated with the appearance of an anxious phenotype. These results suggest a neuroprotective role of the glutamatergic components mGlu₅R and GLT-1 at the short term. However, this neuroprotective effect seems to be lost at the long term, leading to an anxious phenotype and suggesting an increased vulnerability and propensity to epileptic events in adults.

Of adenosine and the blues: The adenosinergic system in the pathophysiology and treatment of major depressive disorder

Major depressive disorder (MDD) is the foremost cause of global disability, being responsible for enormous personal, societal, and economical costs. Importantly, existing pharmacological treatments for MDD are partially or totally ineffective in a large segment of patients. As such, the search for novel antidepressant drug targets, anchored on a clear understanding of the etiological and pathophysiological mechanisms underpinning MDD, becomes of the utmost importance. The adenosinergic system, a highly conserved neuromodulatory system, appears as a promising novel target, given both its regulatory actions over many MDD-affected systems and processes. With this goal in mind, we herein review the evidence concerning the role of adenosine as a potential player in pathophysiology and treatment of MDD, combining data from both human and animal studies. Altogether, evidence supports the assertions that the adenosinergic system is altered in both MDD patients and animal models, and that drugs targeting this system have considerable potential as putative antidepressants. Furthermore, evidence also suggests that modifications in adenosine signaling may have a key role in the effects of several pharmacological and non-pharmacological antidepressant treatments with demonstrated efficacy, such as electroconvulsive shock, sleep deprivation, and deep brain stimulation. Lastly, it becomes clear from the available literature that there is yet much to study regarding the role of the adenosinergic system in the pathophysiology and treatment of MDD, and we suggest several avenues of research that are likely to prove fruitful.

Adenosine Metabolism in the Cerebral Cortex from Several Mice Models during Aging

Adenosine is a neuromodulator that has been involved in aging and neurodegenerative diseases as Alzheimer’s disease (AD). In the present work, we analyzed the possible modulation of purine metabolites, 5’nucleotidase (5′NT) and adenosine deaminase (ADA) activities, and adenosine monophosphate (AMP)-activated protein kinase (AMPK) and its phosphorylated form during aging in the cerebral cortex. Three murine models were used: senescence-accelerated mouse-resistant 1 (SAMR1, normal senescence), senescence-accelerated mouse-prone 8 (SAMP8, a model of AD), and the wild-type C57BL/6J (model of aging) mice strains. Glutamate and excitatory amino acid transporter 2 (EAAT2) levels were also measured in these animals. HPLC, Western blotting, and enzymatic activity evaluation were performed to this aim. 5′-Nucleotidase (5′NT) activity was decreased at six months and recovered at 12 months in SAMP8 while opposite effects were observed in SAMR1 at the same age, and no changes in C57BL/6J mice. ADA activity significantly decreased from 3 to 12 months in the SAMR1 mice strain, while a significant decrease from 6 to 12 months was observed in the SAMP8 mice strain. Regarding purine metabolites, xanthine and guanosine levels were increased at six months in SAMR1 without significant differences in SAMP8 mice. In C57BL/6J mice, inosine and xanthine were increased, while adenosine decreased, from 4 to 24 months. The AMPK level was decreased at six months in SAMP8 without significant changes nor in SAMR1 or C57BL/6J strains. Glutamate and EAAT2 levels were also modulated during aging. Our data show a different modulation of adenosine metabolism participants in the cerebral cortex of these animal models. Interestingly, the main differences between SAMR1 and SAMP8 mice were found at six months of age, SAMP8 being the most affected strain. As SAMP8 is an AD model, results suggest that adenosinergic metabolism is involved in the neurodegeneration of AD.

Adenosine and Metabotropic Glutamate Receptors Are Present in Blood Serum and Exosomes from SAMP8 Mice: Modulation by Aging and Resveratrol

Adenosine (ARs) and metabotropic glutamate receptors (mGluRs) are G-protein coupled receptors (GPCRs) that are modulated in the brain of SAMP8 mice, an animal model of Alzheimer's disease (AD). In the present work, it is shown the presence of ARs and mGluRs in blood serum and derived exosomes from SAMP8 mice as well as its possible modulation by aging and resveratrol (RSV) consumption. In blood serum, adenosine A₁ and A_2A receptors remained unaltered from 5 to 7 months of age. However, an age-related decrease in adenosine level was observed, while 5'-Nucleotidase activity was not modulated. Regarding the glutamatergic system, it was observed a decrease in mGluR₅ density and glutamate levels in older mice. In addition, dietary RSV supplementation caused an age-dependent modulation in both adenosinergic and glutamatergic systems. These GPCRs were also found in blood serum-derived exosomes, which might suggest that these receptors could be released into circulation via exosomes. Interestingly, changes elicited by age and RSV supplementation on mGluR₅ density, and adenosine and glutamate levels were similar to that detected in whole-brain. Therefore, we might suggest that the quantification of these receptors, and their corresponding endogenous ligands, in blood serum could have predictive value for early diagnosis in combination with other distinctive hallmarks of AD.

Resveratrol Differently Modulates Group I Metabotropic Glutamate Receptors Depending on Age in SAMP8 Mice

Glutamate homeostasis is critical for neurotransmission as this excitatory neurotransmitter has a relevant role in cognition functions through ionotropic and metabotropic glutamate receptors in the central nervous system. During the last years, the role of the group I metabotropic glutamate receptors (mGluRs) in neurodegenerative diseases such as Alzheimer's disease has been intensely investigated. Resveratrol (RSV) is a natural polyphenolic compound that is thought to have neuroprotective properties for human health. However, little is known about the action of this compound on mGluR signaling. Therefore, the aim of this study was to investigate the possible modulation of group I mGluRs in SAMP8 mice five and seven months of age supplemented with RSV in the diet. Data reported herein show that RSV plays a different modulatory action on group I mGluRs: mGluR₅ is downregulated as age increases, independently of RSV presence, and mGluR₁ is upregulated or downregulated by RSV treatment depending on age (i.e., depending on mGluR₅ levels). In addition, a neuroprotective role can be inferred for RSV as lower glutamate levels, higher synapsin levels, and unchanged caspase-3 activity were detected after RSV treatment. In conclusion, our findings indicate that RSV treatment modifies the group I mGluR-mediated glutamatergic system in SAMP8 mice, which could contribute to the beneficial effects of this natural polyphenol.