NMDA depresses glutamatergic synaptic transmission in the striatum through the activation of adenosine A1 receptors: evidence from knockout mice

Antidepressant effects of selective adenosine receptor antagonists targeting the A1 and A2A receptors administered jointly with NMDA receptor ligands: behavioral, biochemical and molecular investigations in mice

BACKGROUND

The objective of the study was to ascertain the antidepressant potential of the co-administration of NMDA receptor ligands and selective adenosine A1 and A2A receptor antagonists.

METHODS

The forced swim test (FST) and spontaneous locomotor activity test were carried out in adult male naïve mice. Before the behavioral testing, animals received DPCPX (a selective adenosine A1 receptor antagonist, 1 mg/kg) or istradefylline (a selective adenosine A2A receptor antagonist, 0.5 mg/kg) in combination with L-701,324 (a potent NMDA receptor antagonist, 1 mg/kg), D-cycloserine (a partial agonist at the glycine recognition site of NMDA receptor, 2.5 mg/kg), CGP 37849 (a competitive NMDA receptor antagonist, 0.3 mg/kg) or MK-801 (a non-competitive NMDA receptor antagonist, 0.05 mg/kg). Additionally, serum BDNF level and the mRNA level of the Adora1, Comt, and Slc6a15 genes in the murine prefrontal cortex were determined.

RESULTS

The obtained results showed that DPCPX and istradefylline administered jointly with NMDA receptor ligands (except for DPCPX + D-cycloserine combination) produced an antidepressant effect in the FST in mice without enhancement in spontaneous motility of animals. An elevation in BDNF concentration was noted in the D-cycloserine-treated group. Adora1 expression increased with L-701,324, DPCPX + D-cycloserine, and DPCPX + CGP 37849, while D-cycloserine, CGP 37849, and MK-801 led to a decrease. Comt mRNA levels dropped with DPCPX + L-701,324, istradefylline + L-701,324/CGP 37849 but increased with D-cycloserine, MK-801, CGP 37849 and DPCPX + MK-801/ CGP 37849. Slc6a15 levels were reduced by D-cycloserine, DPCPX + L-701,324 but rose with DPCPX + CGP 37849/MK-801 and istradefylline + D-cycloserine/MK-801/CGP 37849.

CONCLUSION

Our study suggests that selective antagonists of adenosine receptors may enhance the antidepressant efficacy of NMDA receptor ligands highlighting a potential synergistic interaction between the adenosinergic and glutamatergic systems. Wherein, A2A receptor antagonists are seen as more promising candidates in this context. Given the intricate nature of changes in BDNF levels and the expression of Adora1, Comt, and Slc6a15 seen after drug combinations exerting antidepressant properties, further research and integrative approaches are crucial understand better the mechanisms underlying their antidepressant action.

The Interaction of Selective A1 and A2A Adenosine Receptor Antagonists with Magnesium and Zinc Ions in Mice: Behavioural, Biochemical and Molecular Studies

The purpose of the study was to investigate whether the co-administration of Mg²⁺ and Zn²⁺ with selective A1 and A2A receptor antagonists might be an interesting antidepressant strategy. Forced swim, tail suspension, and spontaneous locomotor motility tests in mice were performed. Further, biochemical and molecular studies were conducted. The obtained results indicate the interaction of DPCPX and istradefylline with Mg²⁺ and Zn²⁺ manifested in an antidepressant-like effect. The reduction of the BDNF serum level after co-administration of DPCPX and istradefylline with Mg²⁺ and Zn²⁺ was noted. Additionally, Mg²⁺ or Zn²⁺, both alone and in combination with DPCPX or istradefylline, causes changes in Adora1 expression, DPCPX or istradefylline co-administered with Zn²⁺ increases Slc6a15 expression as compared to a single-drug treatment, co-administration of tested agents does not have a more favourable effect on Comt expression. Moreover, the changes obtained in Ogg1, MsrA, Nrf2 expression show that DPCPX-Mg²⁺, DPCPX-Zn²⁺, istradefylline-Mg²⁺ and istradefylline-Zn²⁺ co-treatment may have greater antioxidant capacity benefits than administration of DPCPX and istradefylline alone. It seems plausible that a combination of selective A1 as well as an A2A receptor antagonist and magnesium or zinc may be a new antidepressant therapeutic strategy.

Ketamine and its metabolite (2R,6R)-hydroxynorketamine induce lasting alterations in glutamatergic synaptic plasticity in the mesolimbic circuit

Low doses of ketamine trigger rapid and lasting antidepressant effects after one injection in treatment-resistant patients with major depressive disorder. Modulation of AMPA receptors (AMPARs) in the hippocampus and prefrontal cortex is suggested to mediate the antidepressant action of ketamine and of one of its metabolites (2R,6R)-hydroxynorketamine ((2R,6R)-HNK). We have examined whether ketamine and (2R,6R)-HNK affect glutamatergic transmission and plasticity in the mesolimbic system, brain regions known to have key roles in reward-motivated behaviors, mood and hedonic drive. We found that one day after the injection of a low dose of ketamine, long-term potentiation (LTP) in the nucleus accumbens (NAc) was impaired. Loss of LTP was maintained for 7 days and was not associated with an altered basal synaptic transmission mediated by AMPARs and N-methyl-D-aspartate receptors (NMDARs). Inhibition of mammalian target of rapamycin signaling with rapamycin did not prevent the ketamine-induced loss of LTP but inhibited LTP in saline-treated mice. However, ketamine blunted the increase in the phosphorylation of the GluA1 subunit of AMPARs at a calcium/calmodulin-dependent protein kinase II/protein kinase C site induced by an LTP induction protocol. Moreover, ketamine caused a persistent increased phosphorylation of GluA1 at a protein kinase A site. (2R,6R)-HNK also impaired LTP in the NAc. In dopaminergic neurons of the ventral tegmental area from ketamine- or (2R,6R)-HNK-treated mice, AMPAR-mediated responses were depressed, while those mediated by NMDARs were unaltered, which resulted in a reduced AMPA/NMDA ratio, a measure of long-term synaptic depression. These results demonstrate that a single injection of ketamine or (2R,6R)-HNK induces enduring alterations in the function of AMPARs and synaptic plasticity in brain regions involved in reward-related behaviors.

High frequency stimulation induces LTD of AMPA receptor-mediated postsynaptic responses and LTP of synaptically-evoked firing in the dorsolateral striatum

In the striatum, long term potentiation (LTP) and long-term depression (LTD) of glutamatergic transmission are believed to underlie motor learning and are impaired in animal models of Parkinson's disease. High frequency stimulation (HFS) is often used to induce synaptic plasticity in the brain. In the striatum, the polarity of HFS-induced plasticity is influenced by the recording conditions, which can differ between various studies. Here, we examined the ability of HFS to induce synaptic plasticity in the dorsolateral striatum in the presence of extracellular Mg²⁺ ions, with no GABA_A receptor blocker, and without membrane depolarization during HFS. We found that HFS induced a LTD of excitatory postsynaptic currents (EPSCs) mediated by AMPA receptors (AMPARs) in medium spiny neurons (MSNs) recorded with whole-cell voltage-clamp. However, HFS induced a LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs), which was dependent on the stimulation intensity applied. The rate of synaptically-evoked spiking in MSNs, measured with cell-attached recordings, showed LTP following HFS. LTD and LTP were impaired in the dopamine-depleted striatum of 6-hydroxydopamine (6-OHDA) lesioned mice, a model of Parkinson's disease. This study shows that HFS induces opposing forms of dopamine-dependent synaptic plasticity in the striatum, i.e. LTD of AMPAR-EPSCs and LTP of both fEPSP/PS and synaptically-evoked firing in MSNs.

CIQ, a positive allosteric modulator of GluN2C/D-containing N-methyl-d-aspartate receptors, rescues striatal synaptic plasticity deficit in a mouse model of Parkinson's disease

AIMS

To investigate if CIQ, a positive allosteric modulator of N-methyl-d-aspartate receptors (NMDARs) containing GluN2C/D subunits, rescues the loss of long-term potentiation (LTP) and forelimb-use asymmetry in a mouse model of Parkinson's disease (PD).

METHODS

We have used electrophysiology in brain slices and the cylinder test to examine the effect of CIQ on glutamatergic synaptic transmission, synaptic plasticity, and forelimb-use in the unilateral 6-hydroxydopamine-lesion mouse model of PD.

RESULTS

CIQ, applied in the perfusion solution, reversibly reduced glutamatergic synaptic transmission in the dopamine-depleted striatum and had no effect in the dopamine-intact striatum. LTP, a dopamine- and NMDAR-dependent form of synaptic plasticity, was induced in the dopamine-intact striatum but was lost in the dopamine-depleted striatum. This impaired LTP was restored in the presence of CIQ applied in the perfusion solution. This treatment, however, prevented LTP induction in control slices. In brain slices from mice which received single and chronic intraperitoneal injections of CIQ, LTP was restored in the dopamine-depleted striatum and unaffected in the dopamine-intact striatum. Forelimb-use asymmetry, a test which assesses deficits in paw usage in the unilateral lesion model of PD, was reversed by systemic chronic treatment with CIQ.

CONCLUSION

A positive allosteric modulator of GluN2C/D-containing NMDARs rescues LTP and forelimb-use asymmetry in a mouse model of PD. This study proposes GluN2D as a potential candidate for therapeutic intervention in PD.

The GABAA receptor agonist muscimol induces an age- and region-dependent form of long-term depression in the mouse striatum

Several forms of long-term depression (LTD) of glutamatergic synaptic transmission have been identified in the dorsal striatum and in the nucleus accumbens (NAc). Such experience-dependent synaptic plasticity might play important roles in reward-related learning. The GABAA receptor agonist muscimol was recently found to trigger a long-lasting depression of glutamatergic synaptic transmission in the NAc of adolescent mice, but the mechanisms that underlie this novel form of LTD were not studied. Here we examined the effect of muscimol applied in the perfusion solution on the amplitude of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in mouse brain slices. We found that muscimol depressed the fEPSP/PS in the NAc of adolescent mice but not adult mice, through both postsynaptic and presynaptic mechanisms. Indeed, muscimol altered the fEPSP/PS paired-pulse ratio, depolarized the membrane of projection neurons, and decreased the frequency, but not amplitude, of spontaneous excitatory postsynaptic currents in the NAc of adolescent mice. The LTD induced by muscimol likely involved endocannabinoids, metabotropic glutamate receptors (mGluRs), but not TRPV1 receptors. Muscimol-LTD was occluded by prior induction of LTD through low-frequency stimulation (LFS) of the slice, demonstrating a common pathway in the induction of LFS-LTD and muscimol-LTD. We also found that muscimol induced a form of LTD in the dorsolateral striatum of adult but not adolescent mice. This LTD was mediated by endocannabinoids but did not involve mGluRs or TRPV1 receptors. These results identify a novel form of synaptic plasticity, and its mechanisms of induction, which is age and region dependent. These findings may contribute to a better understanding of the increased susceptibility of the adolescent brain to long-term synaptic changes in regions associated with reward mechanisms.

Induction of cannabinoid- and N-methyl-D-aspartate receptor-mediated long-term depression in the nucleus accumbens and dorsolateral striatum is region and age dependent

BACKGROUND

The adolescent brain is sensitive to experience-dependent plasticity and might be more vulnerable than the adult brain to the effects of some drugs of abuse. The factors that contribute to these differences are not fully identified. We have examined the ability of cannabinoids to induce a form of synaptic plasticity, long-term depression, in the nucleus accumbens and dorsolateral striatum of adolescent and adult mice.

METHODS

We measured field excitatory postsynaptic potentials/population spikes in brain slices.

RESULTS

We found that the cannabinoid receptor agonist WIN 55,212-2 (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate) induced long-term depression in the nucleus accumbens of adolescent but not adult mice and failed to induce long-term depression in the dorsolateral striatum of adolescent or adult mice. Similar results were obtained with the group I metabotropic glutamate receptor agonist (S)-3,5- dihydroxyphenylglycine, which has previously been shown to promote the release of endocannabinoids. These age-related differences were associated with reduced protein levels of the cannabinoid type 1 receptor and metabotropic glutamate receptor 1 in adult nucleus accumbens and dorsolateral striatum and with an increased tone of endocannabinoids in the dorsolateral striatum of adult mice. We also found that N-methyl-D-aspartate receptor-dependent long-term depression, which was induced in the nucleus accumbens of adolescent mice, was blunted in adult mice, possibly because of decreased levels of GluN1, the obligatory subunit of N-methyl-D-aspartate receptors.

CONCLUSIONS

This study identifies region- and age-specific differences in the ability of endogenous and exogenous cannabinoids, and of N-methyl-D-aspartate receptors, to induce long-term depression in the striatal complex. These observations might contribute to a better understanding of the increased sensitivity of the adolescent brain to drug induced-plasticity.

Adenosine receptor control of cognition in normal and disease

Adenosine and adenosine receptors (ARs) are increasingly recognized as important therapeutic targets for controlling cognition under normal and disease conditions for its dual roles of neuromodulation as well as of homeostatic function in the brain. This chapter first presents the unique ability of adenosine, by acting on the inhibitory A1 and facilitating A2A receptor, to integrate dopamine, glutamate, and BNDF signaling and to modulate synaptic plasticity (e.g., long-term potentiation and long-term depression) in brain regions relevant to learning and memory, providing the molecular and cellular bases for adenosine receptor (AR) control of cognition. This led to the demonstration of AR modulation of social recognition memory, working memory, reference memory, reversal learning, goal-directed behavior/habit formation, Pavlovian fear conditioning, and effort-related behavior. Furthermore, human and animal studies support that AR activity can also, through cognitive enhancement and neuroprotection, reverse cognitive impairments in animal models of Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and schizophrenia. Lastly, epidemiological evidence indicates that regular human consumption of caffeine, the most widely used psychoactive drug and nonselective AR antagonists, is associated with the reduced cognitive decline in aging and AD patients, and with the reduced risk in developing PD. Thus, there is a convergence of the molecular studies revealing AR as molecular targets for integrating neurotransmitter signaling and controlling synaptic plasticity, with animal studies demonstrating the strong procognitive impact upon AR antagonism in normal and disease brains and with epidemiological and clinical evidences in support of caffeine and AR drugs for therapeutic modulation of cognition. Since some of adenosine A2A receptor antagonists are already in phase III clinical trials for motor benefits in PD patients with remarkable safety profiles, additional animal and human studies to better understand the mechanism underlying the AR-mediated control of cognition under normal and disease conditions will provide the required rationale to stimulate the necessary clinical investigation to rapidly translate adenosine and AR drug as a novel strategy to control memory impairment in neuropsychiatric disorders.

Ethanol inhibits excitatory neurotransmission in the nucleus accumbens of adolescent mice through GABAA and GABAB receptors

Age-related differences in various acute physiological and behavioral effects of alcohol have been demonstrated in humans and in other species. Adolescents are more sensitive to positive reinforcing properties of alcohol than adults, but the cellular mechanisms that underlie such a difference are not clearly established. We, therefore, assessed age differences in the ability of ethanol to modulate glutamatergic synaptic transmission in the mouse nucleus accumbens (NAc), a brain region importantly involved in reward mechanisms. We measured field excitatory postsynaptic potentials/population spikes (fEPSP/PS) in NAc slices from adolescent (22-30 days old) and adult (5-8 months old) male mice. We found that 50mM ethanol applied in the perfusion solution inhibits glutamatergic neurotransmission in the NAc of adolescent, but not adult, mice. This effect is blocked by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline and by the GABAB receptor antagonist CGP 55845. Furthermore, bicuculline applied alone produces a stronger increase in the fEPSP/PS amplitude in adult mice than in adolescent mice. Activation of GABAA receptors with muscimol produces a stronger and longer lasting depression of neurotransmission in adolescent mice as compared with adult mice. Activation of GABAB receptors with SKF 97541 also depresses neurotransmission more strongly in adolescent than in adult mice. These results demonstrate that an increased GABA receptor function associated with a reduced inhibitory tone underlies the depressant action of ethanol on glutamatergic neurotransmission in the NAc of adolescent mice.

Ethanol disrupts the mechanisms of induction of long-term potentiation in the mouse nucleus accumbens

BACKGROUND

Long-term changes in the efficacy of glutamatergic synaptic transmission in reward-related brain regions such as the nucleus accumbens (NAc) are proposed to contribute to neuroadaptations that lead to drug addiction. Although alcohol is a widely used addictive substance, the cellular mechanisms by which it influences synaptic plasticity in the NAc are not elucidated. We therefore examined whether acute ethanol (EtOH) alters long-term potentiation (LTP) in the core region of the NAc and investigated the possible underlying mechanisms.

METHODS

We measured field excitatory postsynaptic potential/population spike (fEPSP/PS) amplitude in mouse brain slices containing the NAc. We also used amperometry to detect, with carbon fiber electrode, evoked dopamine release in brain slices.

RESULTS

In control slices, high-frequency stimulation (HFS) induced a stable LTP. LTP was reduced in slices perfused with EtOH (50 mM). Given that induction of LTP is dependent on glutamate acting on N-methyl-d-aspartate (NMDA) receptors and group I metabotropic glutamate receptors (mGluRs), we studied the ability of EtOH to modulate these 2 classes of receptors. NMDA (20 μM) depressed the amplitude of the fEPSP/PS, but this effect was not altered by EtOH in our experimental conditions. However, EtOH reversed the ability of the group I mGluR agonist (S)-3,5-Dihydroxyphenylglycine (DHPG) (50 μM) to potentiate the depressant action of NMDA on the fEPSP/PS. We also examined whether EtOH could modulate dopamine release given that dopamine plays important roles in mediating the reinforcing actions of abused drugs and in the induction of LTP in the NAc. We found that EtOH reversibly decreased action potential-dependent dopamine release evoked by single stimulation pulses and by HFS trains in NAc slices.

CONCLUSIONS

These results show that EtOH impairs the induction of LTP possibly through several mechanisms that include inhibition of group I mGluR-mediated potentiation of NMDA receptor function and of evoked dopamine release. This study provides additional support for a key role of glutamatergic and dopaminergic neurotransmission in the NAc in mediating the reinforcing effects of acute alcohol.

Conditional BDNF release under pathological conditions improves Huntington's disease pathology by delaying neuronal dysfunction

Background

Brain-Derived Neurotrophic Factor (BDNF) is the main candidate for neuroprotective therapy for Huntington's disease (HD), but its conditional administration is one of its most challenging problems.

Results

Here we used transgenic mice that over-express BDNF under the control of the Glial Fibrillary Acidic Protein (GFAP) promoter (pGFAP-BDNF mice) to test whether up-regulation and release of BDNF, dependent on astrogliosis, could be protective in HD. Thus, we cross-mated pGFAP-BDNF mice with R6/2 mice to generate a double-mutant mouse with mutant huntingtin protein and with a conditional over-expression of BDNF, only under pathological conditions. In these R6/2:pGFAP-BDNF animals, the decrease in striatal BDNF levels induced by mutant huntingtin was prevented in comparison to R6/2 animals at 12 weeks of age. The recovery of the neurotrophin levels in R6/2:pGFAP-BDNF mice correlated with an improvement in several motor coordination tasks and with a significant delay in anxiety and clasping alterations. Therefore, we next examined a possible improvement in cortico-striatal connectivity in R62:pGFAP-BDNF mice. Interestingly, we found that the over-expression of BDNF prevented the decrease of cortico-striatal presynaptic (VGLUT1) and postsynaptic (PSD-95) markers in the R6/2:pGFAP-BDNF striatum. Electrophysiological studies also showed that basal synaptic transmission and synaptic fatigue both improved in R6/2:pGAP-BDNF mice.

Conclusions

These results indicate that the conditional administration of BDNF under the GFAP promoter could become a therapeutic strategy for HD due to its positive effects on synaptic plasticity.

Normal and abnormal functions of adenosine receptors in the central nervous system revealed by genetic knockout studies

Endogenous adenosine is a widely distributed upstream regulator of a broad spectrum of neurotransmitters, receptors, and signaling pathways that converge to contribute to the expression of an array of important brain functions. Over the past decade, the generation and characterization of genetic knockout models for all four G-protein coupled adenosine receptors, the A1 and A2A receptors in particular, has confirmed and extended the neuromodulatory and integrated role of adenosine receptors in the control of a broad spectrum of normal and abnormal brain functions. After a brief introduction of the available adenosine receptor knockout models, this review focuses on findings from the genetic knockout approach, placing particular emphasis on the most recent findings. This review is organized into two sections to separately address (i) the role of adenosine receptors in normal brain processes including neuroplasticity, sleep-wake cycle, motor function, cognition, and emotion-related behaviors; and (ii) their role in the response to various pathologic insults to brain such as ischemic stroke, neurodegeneration, or brain dysfunction/disorders. We largely limit our overview to the prominent adenosine receptor subtypes in brain-the A1 and A2A receptors-for which numerous genetic knockout studies on brain function are available. A1 and A2A receptor knockouts have provided significant new insights into adenosine's control of complex physiologic (e.g., cognition) and pathologic (e.g., neuroinflammation) phenomena. These findings extend and strengthen the support for A1 and A2A receptors in brain as therapeutic targets in several neurologic and psychiatric diseases. However, they also emphasize the importance of considering the disease context-dependent effect when developing adenosine receptor-based therapeutic strategies.

Adenosine and the auditory system

Adenosine is a signalling molecule that modulates cellular activity in the central nervous system and peripheral organs via four G protein-coupled receptors designated A₁, A_2A, A_2B, and A₃. This review surveys the literature on the role of adenosine in auditory function, particularly cochlear function and its protection from oxidative stress. The specific tissue distribution of adenosine receptors in the mammalian cochlea implicates adenosine signalling in sensory transduction and auditory neurotransmission although functional studies have demonstrated that adenosine stimulates cochlear blood flow, but does not alter the resting and sound-evoked auditory potentials. An interest in a potential otoprotective role for adenosine has recently evolved, fuelled by the capacity of A₁ adenosine receptors to prevent cochlear injury caused by acoustic trauma and ototoxic drugs. The balance between A₁ and A_2A receptors is conceived as critical for cochlear response to oxidative stress, which is an underlying mechanism of the most common inner ear pathologies (e.g. noise-induced and age-related hearing loss, drug ototoxicity). Enzymes involved in adenosine metabolism, adenosine kinase and adenosine deaminase, are also emerging as attractive targets for controlling oxidative stress in the cochlea. Other possible targets include ectonucleotidases that generate adenosine from extracellular ATP, and nucleoside transporters, which regulate adenosine concentrations on both sides of the plasma membrane. Developments of selective adenosine receptor agonists and antagonists that can cross the blood-cochlea barrier are bolstering efforts to develop therapeutic interventions aimed at ameliorating cochlear injury. Manipulations of the adenosine signalling system thus hold significant promise in the therapeutic management of oxidative stress in the cochlea.

The small-conductance calcium-activated potassium channel is a key modulator of firing and long-term depression in the dorsal striatum

The striatum is considered to be critical for the control of goal-directed action, with the lateral dorsal striatum (latDS) being implicated in modulation of habits and the nucleus accumbens thought to represent a limbic-motor interface. Although medium spiny neurons from different striatal subregions exhibit many similar properties, differential firing and synaptic plasticity could contribute to the varied behavioral roles across subregions. Here, we examined the contribution of small-conductance calcium-activated potassium channels (SKs) to action potential generation and synaptic plasticity in adult rat latDS and nucleus accumbens shell (NAS) projection neurons in vitro. The SK-selective antagonist apamin exerted a prominent effect on latDS firing, significantly decreasing the interspike interval. Furthermore, prolonged latDS depolarization increased the interspike interval and reduced firing, and this enhancement was reversed by apamin. In contrast, NAS neurons exhibited greater basal firing rates and less regulation of firing by SK inhibition and prolonged depolarization. LatDS neurons also had greater SK currents than NAS neurons under voltage-clamp. Importantly, SK inhibition with apamin facilitated long-term depression (LTD) induction in the latDS but not the NAS, without alterations in glutamate release. In addition, SK activation in the latDS prevented LTD induction. Greater SK function in the latDS than in the NAS was not secondary to differences in sodium or inwardly rectifying potassium channel function, and apamin enhancement of firing did not reflect indirect action through cholinergic interneurons. Thus, these data demonstrate that SKs are potent modulators of action potential generation and LTD in the dorsal striatum, and could represent a fundamental cellular mechanism through which habits are regulated.

Adenosine receptors and the central nervous system

The adenosine receptors (ARs) in the nervous system act as a kind of "go-between" to regulate the release of neurotransmitters (this includes all known neurotransmitters) and the action of neuromodulators (e.g., neuropeptides, neurotrophic factors). Receptor-receptor interactions and AR-transporter interplay occur as part of the adenosine's attempt to control synaptic transmission. A(2A)ARs are more abundant in the striatum and A(1)ARs in the hippocampus, but both receptors interfere with the efficiency and plasticity-regulated synaptic transmission in most brain areas. The omnipresence of adenosine and A(2A) and A(1) ARs in all nervous system cells (neurons and glia), together with the intensive release of adenosine following insults, makes adenosine a kind of "maestro" of the tripartite synapse in the homeostatic coordination of the brain function. Under physiological conditions, both A(2A) and A(1) ARs play an important role in sleep and arousal, cognition, memory and learning, whereas under pathological conditions (e.g., Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, stroke, epilepsy, drug addiction, pain, schizophrenia, depression), ARs operate a time/circumstance window where in some circumstances A(1)AR agonists may predominate as early neuroprotectors, and in other circumstances A(2A)AR antagonists may alter the outcomes of some of the pathological deficiencies. In some circumstances, and depending on the therapeutic window, the use of A(2A)AR agonists may be initially beneficial; however, at later time points, the use of A(2A)AR antagonists proved beneficial in several pathologies. Since selective ligands for A(1) and A(2A) ARs are now entering clinical trials, the time has come to determine the role of these receptors in neurological and psychiatric diseases and identify therapies that will alter the outcomes of these diseases, therefore providing a hopeful future for the patients who suffer from these diseases.

NR2A-containing NMDA receptors depress glutamatergic synaptic transmission and evoked-dopamine release in the mouse striatum

NMDA receptors play essential roles in the physiology and pathophysiology of the striatum, a brain nucleus involved in motor control and reward-motivated behaviors. NMDA receptors are composed of NR1 and NR2A-D subunits. Functional properties of NMDA receptors are determined by the type of NR2 subunit they contain. In this study, we have examined the involvement of NR2B and NR2A in the modulatory effect of NMDA on glutamatergic and dopaminergic synaptic transmission in the striatum. We found that bath application of NMDA decreased the amplitude of the field excitatory post-synaptic potential/population spike (fEPSP/PS) measured in corticostriatal mouse brain slices. This depression was not affected by the NR2B-selective antagonists Ifenprodil and Ro 25-6981, but was abolished by the NR2A antagonist NVP-AAM077. Activation of corticostriatal neurons by NMDA did not contribute to synaptic depression because similar results were obtained in decorticated striatal slices. Synaptic depression was not dependent on GABA release because the GABA(A) receptor antagonist bicuculline did not affect NMDA-induced decrease of the fEPSP/PS. NMDA also depressed evoked-dopamine release through NR2A- but not NR2B-containing NMDA receptors. Our results identify an important role for NR2A-containing NMDA receptors intrinsic to the striatum in regulating glutamatergic synaptic transmission and evoked-dopamine release.

Dopamine D1 receptors and group I metabotropic glutamate receptors contribute to the induction of long-term potentiation in the nucleus accumbens

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.

Adenosine A(1) receptor: Functional receptor-receptor interactions in the brain

Over the past decade, many lines of investigation have shown that receptor-mediated signaling exhibits greater diversity than previously appreciated. Signal diversity arises from numerous factors, which include the formation of receptor dimers and interplay between different receptors. Using adenosine A₁ receptors as a paradigm of G protein-coupled receptors, this review focuses on how receptor-receptor interactions may contribute to regulation of the synaptic transmission within the central nervous system. The interactions with metabotropic dopamine, adenosine A_2A, A₃, neuropeptide Y, and purinergic P2Y₁ receptors will be described in the first part. The second part deals with interactions between A₁Rs and ionotropic receptors, especially GABA_A, NMDA, and P2X receptors as well as ATP-sensitive K⁺ channels. Finally, the review will discuss new approaches towards treating neurological disorders.

Adenosine, an endogenous distress signal, modulates tissue damage and repair

Adenosine is formed inside cells or on their surface, mostly by breakdown of adenine nucleotides. The formation of adenosine increases in different conditions of stress and distress. Adenosine acts on four G-protein coupled receptors: two of them, A(1) and A(3), are primarily coupled to G(i) family G proteins; and two of them, A(2A) and A(2B), are mostly coupled to G(s) like G proteins. These receptors are antagonized by xanthines including caffeine. Via these receptors it affects many cells and organs, usually having a cytoprotective function. Joel Linden recently grouped these protective effects into four general modes of action: increased oxygen supply/demand ratio, preconditioning, anti-inflammatory effects and stimulation of angiogenesis. This review will briefly summarize what is known and what is not in this regard. It is argued that drugs targeting adenosine receptors might be useful adjuncts in many therapeutic approaches.