Differential effects of selective adenosine A1 and A2A receptor agonists on dopamine receptor agonist-induced behavioural responses in rats

Environmental interventions reduced repetitive behavior in a mouse model

Repetitive motor behaviors are associated with several neurodevelopmental disorders including autism spectrum disorder. Non-invasive environmental interventions that can ameliorate repetitive behavior and be introduced in early development could benefit many. In Experiment 1, we characterized the development of repetitive circling in mice reared in standard and enriched environments. Environmental enrichment was associated with reduced repetitive behavior. In Experiment 2, two weekly injections of an A2A adenosine receptor agonist reduced repetitive behavior in mice fed a ketogenic diet. Together, these two approaches modified the environment and reduced repetitive behavior with potential implications for increased functioning of the indirect basal ganglia pathway.

Dopamine D1 receptor agonist alleviates acute lung injury via modulating inflammatory responses in macrophages and barrier function in airway epithelial cells

Acute lung injury (ALI) or its severe form, acute respiratory distress syndrome (ARDS) is a life-threatening illness without effective therapeutic interventions currently. Multiple lines of evidence indicated that overwhelming inflammatory responses and impaired epithelial barrier contributed to the pathogenesis of ALI/ARDS. Recently, dopamine (DA) system was identified to participate in various pulmonary diseases. Here, we discovered that dopamine D1-like receptors mainly expressed in macrophages and airway epithelial cells (AECs), which were downregulated by lipopolysaccharide (LPS) challenge in ALI mouse lung. SKF38393 (SKF) is a selective agonist for D1-like receptors and was demonstrated to inhibit excessive inflammatory responses and oxidative stress in THP-1 cell-derived macrophages and Beas-2B cells, as well as improve airway epithelial barrier dysfunction induced by LPS stimulation. Moreover, SKF administration could effectively decrease pulmonary inflammation, ameliorate tissue damage in the LPS-triggered ALI mice. The broad protective actions of SKF might be attributed to the activation of Nrf2 antioxidative system by use of the specific inhibitor, ML385. This study offers evidence of potent immunoregulatory activity of SKF in macrophages, AECs as well as ALI mouse model, which opens novel therapeutic avenues for the intervention of ALI/ARDS.

Adenosinergic Pathway in Parkinson's Disease: Recent Advances and Therapeutic Perspective

Parkinson's disease (PD) is a neurodegenerative disease characterized pathologically by α-synuclein (α-syn) aggregation. In PD, the current mainstay of symptomatic treatment is levodopa (L-DOPA)-based dopamine (DA) replacement therapy. However, the development of dyskinesia and/or motor fluctuations which is relevant to levodopa is restricting its long-term utility. Given that the ability of which is to modulate the striato-thalamo-cortical loops and function to modulate basal ganglia output, the adenosinergic pathway (AP) is qualified as a potential promising non-DA target. As an indispensable component of energy production pathways, AP modulates cellular metabolism and gene regulation in both neurons and neuroglia cells through the recognition and degradation of extracellular adenosine. In addition, AP is geared to the initiation, evolution, and resolution of inflammation as well. Besides the above-mentioned crosstalk between the adenosine and dopamine signaling pathways, the functions of adenosine receptors (A₁R, A_2AR, A_2BR, and A₃R) and metabolism enzymes in modulating PD pathological process have been extensively investigated in recent decades. Here we reviewed the emerging findings focused on the function of adenosine receptors, adenosine formation, and metabolism in the brain and discussed its potential roles in PD pathological process. We also recapitulated clinical studies and the preclinical evidence for the medical strategies targeting the Ado signaling pathway to improve motor dysfunction and alleviate pathogenic process in PD. We hope that further clinical studies should consider this pathway in their monotherapy and combination therapy, which would open new vistas to more targeted therapeutic approaches.

Impact of Chronic Risperidone Use on Behavior and Survival of 3xTg-AD Mice Model of Alzheimer's Disease and Mice With Normal Aging

Psychosis and/or aggression are common problems in dementia, and when severe or persistent, cause considerable patient distress and disability, caregiver stress, and early institutionalization. In 2005, the Food and Drug Administration (FDA) determined that atypical antipsychotics were associated with a significantly greater mortality risk compared to placebo, which prompted the addition of an FDA black-box warning. The American College of Neuropsychopharmacology (ACNP) White Paper, 2008, reviewed this issue and made clinical and research recommendations regarding the use of antipsychotics in dementia patients with psychosis and/or agitation. Increased mortality risk has also been described in cerebrovascular adverse events in elderly users of antipsychotics. In the present work, at the translational level, we used male 3xTg-AD mice (PS1M146V, APPSwe, tauP301L) at advanced stages of the disease reported to have worse survival than females, to study the behavioral effects of a low chronic dose of risperidone (0.1 mg/kg, s.c., 90 days, from 13 to 16 months of age) and its impact on long-term survival, as compared to mice with normal aging. Animals were behaviorally assessed for cognitive and BPSD (behavioral and psychological symptoms of dementia)-like symptoms in naturalistic and experimental conditions (open-field test, T-maze, social interaction, Morris water maze, and marble test) before and after treatment. Weight, basal glucose levels, and IPGTT (i.p. glucose tolerance test) were also recorded. Neophobia in the corner test was used for behavioral monitoring. Survival curves were recorded throughout the experiment until natural death. The benefits of risperidone were limited, both at cognitive and BPSD-like level, and mostly restricted to burying, agitation/vibrating tail, and other social behaviors. However, the work warns about a clear early mortality risk window during the treatment and long-lasting impact on survival. Reduced life expectancy and life span were observed in the 3xTg-AD mice, but total lifespan (36 months) recorded in C57BL/6 × 129Sv counterparts with normal aging was also truncated to 28 months in those with treatment. Sarcopenia at time of death was found in all groups, but was more severe in wild-type animals treated with risperidone. Therefore, the 3xTg-AD mice and their non-transgenic counterparts can be useful to delimitate critical time windows and for studying the physio-pathogenic factors and underlying causal events involved in this topic of considerable public health significance.

Adenosine heteroreceptor complexes in the basal ganglia are implicated in Parkinson's disease and its treatment

The adenosine homo, iso and heteroreceptor complexes in the basal ganglia play a highly significant role in modulating the indirect and direct pathways and the striosomal projections to the nigro-striatal DA system. The major adenosine receptor complexes in the striato-pallidal GABA neurons can be the A2AR-D2R and A2AR-D2R-mGluR5 receptor complexes, in which A2AR protomers and mGluR5 protomers can allosterically interact to inhibit D2R protomer signaling. Through a reorganization of these heteroreceptor complexes upon chronic dopaminergic treatment a pathological and prolonged inhibition of D2R receptor protomer signaling can develop with motor inhibition and wearing off of the therapeutic effects of levodopa and dopamine receptor agonists. The direct pathway is enriched in D1R in and around glutamate synapses enhancing the ability of these GABA neurons to be activated and increase motor initiation. The brake on these GABA neurons is in this case exerted by A1R forming A1R-D1R heteroreceptor complexes in which they allosterically inhibit D1R signaling and thereby reduce motor initiation. Upon chronic levodopa treatment a reorganization of the D1R heteroreceptor complexes develops with the formation of putative A1R-D1R-D3 in addition to D1R-D3R complexes in which D3R enhances D1R protomer signaling and may make the A1R protomer brake less effective. Alpha-synuclein monomers-dimers are postulated to form complexes with A2AR homo and heteroprotomers in the plasma membrane enhancing alpha-synuclein aggregation and toxicity. The alpha-synuclein fibrils formed in the A2AR enriched dendritic spines of the striato-pallidal GABA neurons may reach the surrounding DA terminals via extracellular-vesicle-mediated volume transmission involving internalization of the vesicles and their cargo (alpha-synuclein fibrils) into the vulnerable DA terminals, enhancing their degeneration followed by retrograde flow of these fibrils in the DA axons to the vulnerable nigral DA nerve cells.

Brain Dopamine Transmission in Health and Parkinson's Disease: Modulation of Synaptic Transmission and Plasticity Through Volume Transmission and Dopamine Heteroreceptors

This perspective article provides observations supporting the view that nigro-striatal dopamine neurons and meso-limbic dopamine neurons mainly communicate through short distance volume transmission in the um range with dopamine diffusing into extrasynaptic and synaptic regions of glutamate and GABA synapses. Based on this communication it is discussed how volume transmission modulates synaptic glutamate transmission onto the D1R modulated direct and D2R modulated indirect GABA pathways of the dorsal striatum. Each nigro-striatal dopamine neuron was first calculated to form large numbers of neostriatal DA nerve terminals and then found to give rise to dense axonal arborizations spread over the neostriatum, from which dopamine is released. These neurons can through DA volume transmission directly influence not only the striatal GABA projection neurons but all the striatal cell types in parallel. It includes the GABA nerve cells forming the island-/striosome GABA pathway to the nigral dopamine cells, the striatal cholinergic interneurons and the striatal GABA interneurons. The dopamine modulation of the different striatal nerve cell types involves the five dopamine receptor subtypes, D1R to D5R receptors, and their formation of multiple extrasynaptic and synaptic dopamine homo and heteroreceptor complexes. These features of the nigro-striatal dopamine neuron to modulate in parallel the activity of practically all the striatal nerve cell types in the dorsal striatum, through the dopamine receptor complexes allows us to understand its unique and crucial fine-tuning of movements, which is lost in Parkinson's disease. Integration of striatal dopamine signals with other transmitter systems in the striatum mainly takes place via the receptor-receptor interactions in dopamine heteroreceptor complexes. Such molecular events also participate in the integration of volume transmission and synaptic transmission. Dopamine modulation of the glutamate synapses on the dorsal striato-pallidal GABA pathway involves D2R heteroreceptor complexes such as D2R-NMDAR, A2AR-D2R, and NTSR1-D2R heteroreceptor complexes. The dopamine modulation of glutamate synapses on the striato-entopeduncular/nigral pathway takes place mainly via D1R heteroreceptor complexes such as D1R-NMDAR, A2R-D1R, and D1R-D3R heteroreceptor complexes. Dopamine modulation of the island/striosome compartment of the dorsal striatum projecting to the nigral dopamine cells involve D4R-MOR heteroreceptor complexes. All these receptor-receptor interactions have relevance for Parkinson's disease and its treatment.

Adenosine A1-Dopamine D1 Receptor Heteromers Control the Excitability of the Spinal Motoneuron

While the role of the ascending dopaminergic system in brain function and dysfunction has been a subject of extensive research, the role of the descending dopaminergic system in spinal cord function and dysfunction is just beginning to be understood. Adenosine plays a key role in the inhibitory control of the ascending dopaminergic system, largely dependent on functional complexes of specific subtypes of adenosine and dopamine receptors. Combining a selective destabilizing peptide strategy with a proximity ligation assay and patch-clamp electrophysiology in slices from male mouse lumbar spinal cord, the present study demonstrates the existence of adenosine A₁-dopamine D₁ receptor heteromers in the spinal motoneuron by which adenosine tonically inhibits D₁ receptor-mediated signaling. A₁-D₁ receptor heteromers play a significant control of the motoneuron excitability, represent main targets for the excitatory effects of caffeine in the spinal cord and can constitute new targets for the pharmacological therapy after spinal cord injury, motor aging-associated disorders and restless legs syndrome.

Emerging preclinical pharmacological targets for Parkinson's disease

Parkinson's disease (PD) is a progressive neurological condition caused by the degeneration of dopaminergic neurons in the basal ganglia. It is the most prevalent form of Parkinsonism, categorized by cardinal features such as bradykinesia, rigidity, tremors, and postural instability. Due to the multicentric pathology of PD involving inflammation, oxidative stress, excitotoxicity, apoptosis, and protein aggregation, it has become difficult to pin-point a single therapeutic target and evaluate its potential application. Currently available drugs for treating PD provide only symptomatic relief and do not decrease or avert disease progression resulting in poor patient satisfaction and compliance. Significant amount of understanding concerning the pathophysiology of PD has offered a range of potential targets for PD. Several emerging targets including AAV-hAADC gene therapy, phosphodiesterase-4, potassium channels, myeloperoxidase, acetylcholinesterase, MAO-B, dopamine, A2A, mGlu5, and 5-HT-1A/1B receptors are in different stages of clinical development. Additionally, alternative interventions such as deep brain stimulation, thalamotomy, transcranial magnetic stimulation, and gamma knife surgery, are also being developed for patients with advanced PD. As much as these therapeutic targets hold potential to delay the onset and reverse the disease, more targets and alternative interventions need to be examined in different stages of PD. In this review, we discuss various emerging preclinical pharmacological targets that may serve as a new promising neuroprotective strategy that could actually help alleviate PD and its symptoms.

The adenosine A2A receptor agonist, CGS 21680, attenuates a probabilistic reversal learning deficit and elevated grooming behavior in BTBR mice

Restricted interests and repetitive behaviors (RRBs) are a defining feature of autism spectrum disorder (ASD). To date there are limited options for treating this core symptomology. Treatments that stimulate adenosine A_2A receptors may represent a promising approach for reducing RRBs in ASD. This is because A_2A receptors are expressed on striatal neurons of the basal ganglia indirect pathway. Under activation of this pathway has been associated with RRBs while activation of A_2A receptors leads to increased activity of the indirect basal ganglia pathway. The present studies investigated whether acute, systemic treatment with CGS21680, an A_2A receptor agonist attenuates elevated self-grooming and a probabilistic reversal learning deficit in the BTBR T+ Itpr3^tf /J (BTBR) mouse model of idiopathic autism. The effects of this treatment were also investigated in C57BL/6J (B6) mice as a comparison strain. Using a spatial reversal learning test with 80/20 probabilistic feedback, comparable to one in which ASD individuals exhibit deficits, CGS 21680 (0.005 and 0.01mg/kg) attenuated a reversal learning deficit in BTBR mice. Enhancement in probabilistic reversal learning performance resulted from CGS 21680 improving the consistent maintenance of new adaptive behavioral choice patterns after reversal. CGS 21680 at 0.01 mg, but not 0.005 mg, also reduced self-grooming behavior in BTBR mice. CGS 21680 did not affect self-grooming or reversal learning in B6 mice. These findings demonstrate that A_2A receptor agonists may be a promising receptor target in the treatment of RRBs in ASD. Autism Res 2018, 11: 223-233. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.

LAY SUMMARY

The present experiments determined whether the drug, CGS 21680, that facilitates activation of adenosine A_2A receptors in the brain, would reduce repetitive and inflexible behaviors in the BTBR mouse model of idiopathic autism. CGS 21680 treatment in BTBR mice reduced repetitive and inflexible behaviors. In the control C57BL/6J (B6) mouse strain, CGS 21680 did not affect performance. These findings suggest that stimulation of brain adenosine A_2A receptors may be a promising therapeutic strategy in ASD.

Yawning-Its anatomy, chemistry, role, and pathological considerations

Yawning is a clinical sign of the activity of various supra- and infratentorial brain regions including the putative brainstem motor pattern, hypothalamic paraventricular nucleus, probably the insula and limbic structures that are interconnected via a fiber network. This interaction can be seen in analogy to other cerebral functions arising from a network or zone such as language. Within this network, yawning fulfills its function in a stereotype, reflex-like manner; a phylogenetically old function, preserved across species barriers, with the purpose of arousal, communication, and maybe other functions including respiration. Abnormal yawning with ≥3 yawns/15min without obvious cause arises from lesions of brain areas involved in the yawning zone, its trajectories causing a disconnection syndrome, or from alteration of network activity by physical or metabolic etiologies including medication.

Mechanisms of the psychostimulant effects of caffeine: implications for substance use disorders

BACKGROUND

The psychostimulant properties of caffeine are reviewed and compared with those of prototypical psychostimulants able to cause substance use disorders (SUD). Caffeine produces psychomotor-activating, reinforcing, and arousing effects, which depend on its ability to disinhibit the brake that endogenous adenosine imposes on the ascending dopamine and arousal systems.

OBJECTIVES

A model that considers the striatal adenosine A2A-dopamine D2 receptor heteromer as a key modulator of dopamine-dependent striatal functions (reward-oriented behavior and learning of stimulus-reward and reward-response associations) is introduced, which should explain most of the psychomotor and reinforcing effects of caffeine.

HIGHLIGHTS

The model can explain the caffeine-induced rotational behavior in rats with unilateral striatal dopamine denervation and the ability of caffeine to reverse the adipsic-aphagic syndrome in dopamine-deficient rodents. The model can also explain the weaker reinforcing effects and low abuse liability of caffeine, compared with prototypical psychostimulants. Finally, the model can explain the actual major societal dangers of caffeine: the ability of caffeine to potentiate the addictive and toxic effects of drugs of abuse, with the particularly alarming associations of caffeine (as adulterant) with cocaine, amphetamine derivatives, synthetic cathinones, and energy drinks with alcohol, and the higher sensitivity of children and adolescents to the psychostimulant effects of caffeine and its potential to increase vulnerability to SUD.

CONCLUSIONS

The striatal A2A-D2 receptor heteromer constitutes an unequivocal main pharmacological target of caffeine and provides the main mechanisms by which caffeine potentiates the acute and long-term effects of prototypical psychostimulants.

On the role of adenosine (A)₂A receptors in cocaine-induced reward: a pharmacological and neurochemical analysis in rats

RATIONALE

Several studies have suggested the inhibitory control of adenosine (A)2A receptor stimulation in cocaine-induced behavioral actions.

OBJECTIVES

A combination of systemic or local drug injections and in vivo neurochemical analysis investigated A2A receptors in cocaine and food reward.

METHODS

Rats, trained to self-administer cocaine or food, were tested with the selective A2A receptor antagonists KW 6002 and SCH 58261 or the selective A2A receptor agonist CGS 21680. Extracellular dopamine, glutamate, and GABA levels in the nucleus accumbens and ventral pallidum were determined following intra-accumbal CGS 21680 administration during cocaine self-administration.

RESULTS

Neither KW 6002 nor SCH 58261 (0.25-1 mg/kg) altered cocaine self-administration (0.125-0.5 mg/kg/infusion), while CGS 21680 (0.2-0.4 mg/kg) produced a downward shift in the cocaine dose-response curve under a fixed ratio schedule of reinforcement and decreased the cocaine breaking point. CGS 21680 blocked also operant responding for food, while the A2A receptor antagonists were inactive. Local steady-state infusion of CGS 21680 (10 μM) during cocaine self-administration increased the active level pressing that was accompanied with reduced dopamine and increased GABA in the nucleus accumbens in the absence of changes in GABA and glutamate levels in the ventral pallidum. Pretreatment with systemic KW 6002 counteracted the increases in number of cocaine infusions seen after intra-accumbal administration of CGS 21680.

CONCLUSION

The findings support a role of A2A receptors in modulating goal-maintained behaviors. They also indicate that increased accumbal GABA release involving an antagonistic A2A-D2 receptor interaction can participate in mediating the inhibitory effects of the A2A agonist on cocaine reward.

Dopamine heteroreceptor complexes as therapeutic targets in Parkinson's disease

INTRODUCTION

Several types of D2R and D1R heteroreceptor complexes were discovered in the indirect and direct pathways of the striatum, respectively. The hypothesis is given that changes in the function of the dopamine heteroreceptor complexes may help us understand the molecular mechanisms underlying the motor complications of long-term therapy in Parkinson's disease (PD) with l-DOPA and dopamine receptor agonists.

AREAS COVERED

In the indirect pathway, the potential role of the A2AR-D2R, A2AR-D2R-mGluR5 and D2R-NMDAR heteroreceptor complexes in PD are covered and in the direct pathway, the D1R-D3R, A1R-D1R, D1R-NMDAR and putative A1R-D1R-D3R heteroreceptor complexes.

EXPERT OPINION

One explanation for the more powerful ability of l-DOPA treatment versus treatment with the partial dopamine receptor agonist/antagonist activity to induce dyskinesias, may be that dopamine formed from l-DOPA acts as a full agonist. The field of D1R and D2R heteroreceptor complexes in the CNS opens up a new understanding of the wearing off of the antiparkinson actions of l-DOPA and dopamine receptor agonists and the production of l-DOPA-induced dyskinesias. It can involve a reorganization of the D1R and D2R heteroreceptor complexes and a disbalance of the D1R and D2R homomers versus non-dopamine receptor homomers in the direct and indirect pathways.

Dopamine D2 receptors modulate the expression of contextual conditioned fear: role of the ventral tegmental area and the basolateral amygdala

Although dopaminergic systems are more commonly associated with the reinforcing effects of various stimuli, numerous reports have demonstrated a relationship between changes in dopaminergic transmission and aversive situations. In the present study, we examined the involvement of D1-like and D2-like receptors in the expression of conditioned freezing using the context as the conditioned stimulus. Intraperitoneal injections of the D1 agonist SKF38393 or the D1 antagonist SCH23390 did not change the conditioned freezing in rats subjected to the contextual fear paradigm. In contrast, intraperitoneal injections of the D2 agonist quinpirole and the D2 antagonist sulpiride caused a significant dose-dependent reduction in the expression of contextual conditioned freezing. As these data may reflect that the systemic manipulations acted on dopaminergic receptors in different brain areas, the effects of administration of quinpirole and sulpiride into the ventral tegmental area (VTA) and the basolateral amygdala complex (BLA) on the expression of contextual conditioned freezing were also evaluated. Intra-VTA quinpirole and intra-BLA sulpiride injections reduced the conditioned freezing response; intra-VTA sulpiride and intra-BLA quinpirole injections had no significant effects. These data suggest that D2-like receptors, but not D1-like receptors, play an important role in the expression of contextual conditioned freezing. Quinpirole may act at D2 presynaptic receptors located in the VTA, decreasing dopamine levels in the terminal fields of the mesolimbic pathway. The effects of sulpiride, in contrast, appear to be triggered by an action on postsynaptic dopaminergic receptors located in the BLA. However, it cannot be totally excluded that the injected solutions did not also affect neighboring amygdalar regions. Together with previous findings, the present data suggest the need to consider dopaminergic mechanisms in the mesolimbic circuit as novel targets for the pharmacological treatment of fear-related disorders, especially post-traumatic stress disorder.

Caffeinated Alcoholic Beverages - An Emerging Trend in Alcohol Abuse

Alcohol use disorders are pervasive in society and their impact affects quality of life, morbidity and mortality, as well as individual productivity. Alcohol has detrimental effects on an individual’s physiology and nervous system, and is associated with disorders of many organ and endocrine systems impacting an individual’s health, behavior, and ability to interact with others. Youth are particularly affected. Unfortunately, adolescent usage also increases the probability for a progression to dependence. Several areas of research indicate that the deleterious effects of alcohol abuse may be exacerbated by mixing caffeine with alcohol. Some behavioral evidence suggests that caffeine increases alcohol drinking and binge drinking episodes, which in turn can foster the development of alcohol dependence. As a relatively new public health concern, the epidemiological focus has been to establish a need for investigating the effects of caffeinated alcohol. While the trend of co-consuming these substances is growing, knowledge of the central mechanisms associated with caffeinated ethanol has been lacking. Research suggests that caffeine and ethanol can have additive or synergistic pharmacological actions and neuroadaptations, with the adenosine and dopamine systems in particular implicated. However, the limited literature on the central effects of caffeinated ethanol provides an impetus to increase our knowledge of the neuroadaptive effects of this combination and their impact on cognition and behavior. Research from our laboratories indicates that an established rodent animal model of alcoholism can be extended to investigate the acute and chronic effects of caffeinated ethanol.

G protein-coupled receptor oligomerization and brain integration: focus on adenosinergic transmission

The control of glutamatergic corticostriatal transmission is essential for the induction and expression of plasticity mechanisms in the striatum, a phenomenon thickly regulated by G protein-coupled receptors (GPCRs). Interestingly, in addition to dopamine receptors, adenosine and metabotropic glutamate receptors also play a key role in striatal functioning. The existence of a supramolecular organization (i.e. oligomer) containing dopamine, adenosine and metabotropic glutamate receptors in the striatal neurons is now being widely accepted by the scientific community. Indeed, these oligomers may enhance the diversity and performance by which extracellular striatal signals are transferred to the G-proteins in the process of receptor transduction, and also may allow unpredictable receptor-receptor allosteric regulations. Overall, here we want to review how formations of adenosine, dopamine and metabotropic glutamate receptors-containing oligomers impinge into striatal functioning in both normal and pathological conditions. This article is part of a Special Issue entitled: Brain Integration.

Differential effects of the adenosine A₂A agonist CGS-21680 and haloperidol on food-reinforced fixed ratio responding in the rat

RATIONALE

Previous studies have shown that adenosine A(2A) receptors are colocalized with dopamine D(2) receptors on striatal neurons. Activation of these two receptors has antagonistic effects under a number of conditions suggesting that stimulation of adenosine A(2A) receptors may have behavioral effects resembling those produced by blockade of dopamine D(2) receptors, but this possibility has been investigated in a limited number of situations.

OBJECTIVE

We compared the effects of the adenosine A(2A) agonist CGS-21680 and the preferential D(2) dopamine antagonist haloperidol in a situation in which dopamine blockade produces a distinctive pattern of behavioral effects.

MATERIALS AND METHODS

Six rats were trained to lever press for food reward on a fixed ratio 15 schedule of reinforcement and then tested after being injected with various doses of CGS-21680 (0.064, 0.128, and 0.25 mg/kg) and haloperidol (0.25 and 0.1 mg/kg).

RESULTS

Haloperidol produced a dose-dependent suppression of lever pressing with mean response rates declining across the duration of the test session. CGS-21680 also produced a dose-dependent suppression of responding, but this effect was not temporally graded, and responding was equivalently suppressed across the duration of the session. Additionally, CGS-21680 increased post-reinforcement pause duration to a much greater extent than did haloperidol.

CONCLUSIONS

On this task, the behavioral effects of CGS-21680 do not resemble those produced by haloperidol. Several explanations of this discrepancy are possible, the most likely being that the observed behavioral effects of CGS-21680 result from an action at a site other than D(2) receptor-expressing striatal neurons.

A(2A)/D(2) receptor heteromerization in a model of Parkinson's disease. Focus on striatal aminoacidergic signaling

The present manuscript mainly summarizes the basic concepts and the molecular mechanisms underlying adenosine A(2A)-dopamine D(2) receptor-receptor interactions in the basal ganglia. Special emphasis is placed on neurochemical, behavioral and electrophysiological findings supporting the functional role that A(2A)/D(2) heteromeric receptor complexes located on striato-pallidal GABA neurons and corticostriatal glutamate terminals play in the regulation of the so called "basal ganglia indirect pathway". Furthermore, the role of A(2A)/mGluR(5) synergistic interactions in striatal neuron function and dysfunction is discussed. The functional consequences of the interactions between striatal adenosine A(2A), mGluR(5) and dopamine D(2) receptors on striatopallidal GABA release and motor behavior dysfunctions suggest the possibility of simultaneously targeting these receptors in Parkinson's disease treatment. This article is part of a Special Issue entitled Brain Integration. This article is part of a Special Issue entitled: Brain Integration.

Adenosine receptor containing oligomers: their role in the control of dopamine and glutamate neurotransmission in the brain

While the G protein-coupled receptor (GPCR) oligomerization has been questioned during the last fifteen years, the existence of a multi-receptor complex involving direct receptor-receptor interactions, called receptor oligomers, begins to be widely accepted. Eventually, it has been postulated that oligomers constitute a distinct functional form of the GPCRs with essential receptorial features. Also, it has been proven, under certain circumstances, that the GPCR oligomerization phenomenon is crucial for the receptor biosynthesis, maturation, trafficking, plasma membrane diffusion, and pharmacology and signalling. Adenosine receptors are GPCRs that mediate the physiological functions of adenosine and indeed these receptors do also oligomerize. Accordingly, adenosine receptor oligomers may improve the molecular mechanism by which extracellular adenosine signals are transferred to the G proteins in the process of receptor transduction. Importantly, these adenosine receptor-containing oligomers may allow not only the control of the adenosinergic function but also the fine-tuning modulation of other neurotransmitter systems (i.e. dopaminergic and glutamatergic transmission). Overall, we underscore here recent significant developments based on adenosine receptor oligomerization that are essential for acquiring a better understanding of neurotransmission in the central nervous system under normal and pathological conditions.

Blockade of adenosine A(1) receptors prevents methylphenidate-induced impairment of object recognition task in adult mice

Methylphenidate (MPH) is the preferred treatment used for attention-deficit/hyperactivity disorder (ADHD). Recently, misuse for MPH due to its apparent cognitive enhancer properties has been reported. Adenosine is a neuromodulator known to exert influence on the dopaminergic neurotransmission, which is the main pharmacological target of MPH. We have reported that an overdosage of MPH up-regulates adenosine A(1) receptors in the frontal cortex, but this receptor was not involved in its anxiolytic effects. In this study, the role of adenosine A(1) receptor was investigated on MPH-induced effects on aversive and recognition memory in adult mice. Adult mice received acute and chronic (15 days) administration of methylphenidate (5mg/kg, i.p.), or an acute overdosage (50mg/kg, i.p) in order to model misuse. Memory was assessed in the inhibitory avoidance and object recognition task. Acute administration 5mg/kg improved whereas 50mg/kg disrupted recognition memory and decreased performance in the inhibitory avoidance task. Chronic administration did not cause any effect on memory, but decreased adenosine A(1) receptors immunocontent in the frontal cortex. The selective adenosine A(1) receptor antagonist, (DPCPX 1mg/kg, i.p.), prevented methylphenidate-triggered recognition memory impairment. Our findings showed that recognition memory rather than aversive memory was differently affected by acute administration at both doses. Memory recognition was fully impaired by the overdosage, suggesting that misuse can be harmful for cognitive functions. The adenosinergic system via A(1) receptors may play a role in the methylphenidate actions probably by interfering with dopamine-enhancing properties of this drug.

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.

Adenosine A1 receptors are modified by acute treatment with methylphenidate in adult mice

In recent years misuse of methylphenidate (MPH) has been reported. The main pharmacological target of methylphenidate is the dopaminergic system. Adenosine is a neuromodulator that influences the dopaminergic neurotransmission, but studies on MPH and adenosine are still lacking. In this study, adult mice were acutely treated with MPH (5mg/kg, i.p.) and to model misuse, they received an acute overdosage (50mg/kg, i.p). The involvement of adenosine A(1) receptors in anxiety-related behavior and locomotor and exploratory activity was examined. The administration of methylphenidate (5 and 50mg/kg) 30 min before the exposure to open field arena did not modify locomotor activity. The anxiolytic-like behavior was observed with both doses of MPH as revealed by the increase on the number of entries and the time spent in the open arms in the elevated plus-maze. Pre treatment with selective adenosine A(1) receptor antagonist (DPCPX 1mg/kg, i.p.) did not prevent anxiolytic effect caused by MPH 50mg/kg. Immunoblotting of frontal cortex and hippocampal extracts revealed that MPH 50mg/kg increased 88% adenosine A(1) receptor density in the frontal cortex. Extracts from hippocampus did not reveal any differences in the adenosine A(1) receptor density. Our findings ruled out the participation of adenosine A(1) receptors on the MPH-triggered anxiolytic effects. However, the density of adenosine A(1) receptors increased in a brain area strictly involved in the MPH-mediated effects. Thus, the adenosinergic system may play a role in the methylphenidate actions in the central nervous system.

Adenosine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Adenosine-dopamine interactions in the central nervous system (CNS) have been studied for many years in view of their relevance for disorders of the CNS and their treatments. The discovery of adenosine and dopamine receptor containing receptor mosaics (RM, higher-order receptor heteromers) in the striatum opened up a new understanding of these interactions. Initial findings indicated the existence of A(2A)R-D(2)R heterodimers and A(1)R-D(1)R heterodimers in the striatum that were followed by indications for the existence of striatal A(2A)R-D(3)R and A(2A)R-D(4)R heterodimers. Of particular interest was the demonstration that antagonistic allosteric A(2A)-D(2) and A(1)-D(1) receptor-receptor interactions take place in striatal A(2A)R-D(2)R and A(1)R-D(1)R heteromers. As a consequence, additional characterization of these heterodimers led to new aspects on the pathophysiology of Parkinson's disease (PD), schizophrenia, drug addiction, and l-DOPA-induced dyskinesias relevant for their treatments. In fact, A(2A)R antagonists were introduced in the symptomatic treatment of PD in view of the discovery of the antagonistic A(2A)R-D(2)R interaction in the dorsal striatum that leads to reduced D(2)R recognition and G(i/o) coupling in striato-pallidal GABAergic neurons. In recent years, indications have been obtained that A(2A)R-D(2)R and A(1)R-D(1)R heteromers do not exist as heterodimers, rather as RM. In fact, A(2A)-CB(1)-D(2) RM and A(2A)-D(2)-mGlu(5) RM have been discovered using a sequential BRET-FRET technique and by using the BRET technique in combination with bimolecular fluorescence complementation. Thus, other pathogenic mechanisms beside the well-known alterations in the release and/or decoding of dopamine in the basal ganglia and limbic system are involved in PD, schizophrenia and drug addiction. In fact, alterations in the stoichiometry and/or topology of A(2A)-CB(1)-D(2) and A(2A)-D(2)-mGlu5 RM may play a role. Thus, the integrative receptor-receptor interactions in these RM give novel aspects on the pathophysiology and treatment strategies, based on combined treatments, for PD, schizophrenia, and drug addiction.

Adenosine A2A receptors and their role in drug addiction

The specific events between initial presumably manageable drug intake and the development of a drug- addicted state are not yet known. Drugs of abuse have varying mechanisms of action that create a complex pattern of behaviour related to drug consumption, drug-seeking, withdrawal and relapse. The neuromodulator adenosine has been shown to play a role in reward-related behaviour, both as an independent mediator and via interactions of adenosine receptors with other receptors. Adenosine levels are elevated upon exposure to drugs of abuse and adenosine A2A receptors are enriched in brain nuclei known for their involvement in the processing of drug-related reinforcement processing. A2A receptors are found in receptor clusters with dopamine and glutamate receptors. A2A receptors are thus ideally situated to influence the signalling of neurotransmitters relevant in the neuronal responses and plasticity that underlie the development of drug taking and drug-seeking behaviour. In this review, we present evidence for the role of adenosine and A2A receptors in drug addiction, thereby providing support for current efforts aimed at developing drug therapies to combat substance abuse that target adenosine signalling via A2A receptors.

Oral tremor induced by the muscarinic agonist pilocarpine is suppressed by the adenosine A2A antagonists MSX-3 and SCH58261, but not the adenosine A1 antagonist DPCPX

Tremulous jaw movements in rats, which can be induced by dopamine (DA) antagonists, DA depletion, and cholinomimetics, have served as a useful model for studies of tremor. Although adenosine A(2A) antagonists can reduce the tremulous jaw movements induced by DA antagonists and DA depletion, there are conflicting reports about the interaction between adenosine antagonists and cholinomimetic drugs. The present studies investigated the ability of adenosine antagonists to reverse the tremorogenic effect of the muscarinic agonist pilocarpine. While the adenosine A(2A) antagonist MSX-3 was incapable of reversing the tremulous jaw movements induced by the 4.0mg/kg dose of pilocarpine, both MSX-3 and the adenosine A(2A) antagonist SCH58261 reversed the tremulous jaw movements elicited by 0.5mg/kg pilocarpine. Systemic administration of the adenosine A(1) antagonist DPCPX failed to reverse the tremulous jaw movements induced by either an acute 0.5mg/kg dose of the cholinomimetic pilocarpine or the DA D2 antagonist pimozide, indicating that the tremorolytic effects of adenosine antagonists may be receptor subtype specific. Behaviorally active doses of MSX-3 and SCH 58261 showed substantial in vivo occupancy of A(2A) receptors, but DPCPX did not. The results of these studies support the use of adenosine A(2A) antagonists for the treatment of tremor.

Adenosine A(2A) receptors, dopamine D(2) receptors and their interactions in Parkinson's disease

Future therapies in Parkinson's disease may substantially build on the existence of intra-membrane receptor-receptor interactions in DA receptor containing heteromeric receptor complexes. The A(2A)/D(2) heteromer is of substantial interest in view of its specific location in cortico-striatal glutamate terminals and in striato-pallidal GABA neurons. Antagonistic A(2A)/D(2) receptor interactions in this heteromer demonstrated at the cellular level, and at the level of the striato-pallidal GABA neuron and at the network level made it possible to suggest A(2A) antagonists as anti-parkinsonian drugs. The major mechanism is an enhancement of D(2) signaling leading to attenuation of hypokinesia, tremor, and rigidity in models of Parkinson's disease with inspiring results in two clinical trials. Other interactions are antagonism at the level of the adenylyl cyclase; heterologous sensitization at the A(2A) activated adenylyl cyclase by persistent D(2) activation and a compensatory up-regulation of A(2A) receptors in response to intermittent Levodopa treatment. An increased dominance of A(2A) homomers over D(2) homomers and A(2A)/D(2) heteromers after intermittent Levodopa treatment may therefore contribute to development of Levodopa induced dyskinesias and to the wearing off of the therapeutic actions of Levodopa giving additional therapeutic roles of A(2A) antagonists. Their neuroprotective actions may involve an increase in the retrograde trophic signaling in the nigro-striatal DA system.

Antagonistic cannabinoid CB1/dopamine D2 receptor interactions in striatal CB1/D2 heteromers. A combined neurochemical and behavioral analysis

In vitro results show the ability of the CB(1) receptor agonist CP 55,940 to reduce the affinity of D(2) receptor agonist binding sites in both the dorsal and ventral striatum including the nucleus accumbens shell. This antagonistic modulation of D(2) receptor agonist affinity was found to remain and even be enhanced after G-protein activation by Gpp(NH)p. Using the FRET technique in living HEK-293T cells, the formation of CB(1)-D(2) receptor heteromers, independent of receptor occupancy, was demonstrated. These data thereby indicate that the antagonistic intramembrane CB(1)/D(2) receptor-receptor interactions may occur in CB(1)/D(2) formed heteromers. Antagonistic CB(1)/D(2) interactions were also discovered at the behavioral level through an analysis of quinpirole-induced locomotor hyperactivity in rats. The CB(1) receptor agonist CP 55,940 at a dose that did not change basal locomotion was able to block quinpirole-induced increases in locomotor activity. In addition, not only the CB(1) receptor antagonist rimonobant but also the specific A(2A) receptor antagonist MSX-3 blocked the inhibitory effect of CB(1) receptor agonist on D(2)-like receptor agonist-induced hyperlocomotion. Taken together, these results give evidence for the existence of antagonistic CB(1)/D(2) receptor-receptor interactions within CB(1)/D(2) heteromers in which A(2A) receptors may also participate.

Adenosine receptor–dopamine receptor interactions in the basal ganglia and their relevance for brain function

The dopamine D1 and D2 receptors are major receptors in the regulation of striatal function and striatal adenosine A1 and A2A receptors are major modulators of their signaling. The evidence suggests the existence of antagonistic A1-D1 heteromeric receptor complexes in the basal ganglia and prefrontal cortex and especially in the direct striatonigral-striatoentopeduncular GABA pathways. The neurochemical and behavioral findings showing antagonistic A1-D1 receptor interactions can be explained by the existence of such A1-D1 heteromeric receptor complexes and of antagonistic interactions at the level of the second messengers. In contrast, A2A-D2 receptor heteromers may exist in the dorsal and ventral striato-pallidal GABA pathways, where activation of A2A receptors reduces D2 receptor recognition, coupling and signaling. As a result of the A2A receptor-induced reduction of D2 receptor signaling, the activity of these GABA neurons is increased resulting in reduced motor and reward functions mediated via the indirect pathway, causing a reduced glutamate drive to the prefrontal and motor areas of the cerebral cortex. Thus, A2A receptor antagonists and A2A receptor agonists, respectively, may offer novel treatments of Parkinson's disease (reduced D2 receptor signaling) and of schizophrenia and drug addiction (increased D2 receptor signaling).

The adenosine A2A receptor agonist, CGS-21680, blocks excessive rearing, acquisition of wheel running, and increases nucleus accumbens CREB phosphorylation in chronically food-restricted rats

Adenosine A(2A) receptors are preferentially expressed in rat striatum, where they are concentrated in dendritic spines of striatopallidal medium spiny neurons and exist in a heteromeric complex with D(2) dopamine (DA) receptors. Behavioral and biochemical studies indicate an antagonistic relationship between A(2A) and D(2) receptors. Previous studies have demonstrated that food-restricted (FR) rats display behavioral and striatal cellular hypersensitivity to D(1) and D(2) DA receptor stimulation. These alterations may underlie adaptive, as well as maladaptive, behaviors characteristic of the FR rat. The present study examined whether FR rats are hypersensitive to the A(2A) receptor agonist, CGS-21680. In Experiment 1, spontaneous horizontal motor activity did not differ between FR and ad libitum fed (AL) rats, while vertical activity was greater in the former. Intracerebroventricular (i.c.v.) administration of CGS-21680 (0.25 and 1.0 nmol) decreased both types of motor activity in FR rats, and returned vertical activity levels to those observed in AL rats. In Experiment 2, FR rats given access to a running wheel for a brief period outside of the home cage rapidly acquired wheel running while AL rats did not. Pretreatment with CGS-21680 (1.0 nmol) blocked the acquisition of wheel running. When administered to FR subjects that had previously acquired wheel running, CGS-21680 suppressed the behavior. In Experiment 3, CGS-21680 (1.0 nmol) activated both ERK 1/2 and CREB in caudate-putamen with no difference between feeding groups. However, in nucleus accumbens (NAc), CGS-21680 failed to activate ERK 1/2 and selectively activated CREB in FR rats. These results indicate that FR subjects are hypersensitive to several effects of an adenosine A(2A) agonist, and suggest the involvement of an upregulated A(2A) receptor-linked signaling pathway in NAc. Medications targeting the A(2A) receptor may have utility in the treatment of maladaptive behaviors associated with FR, including substance abuse and compulsive exercise.

Intramembrane receptor–receptor interactions: a novel principle in molecular medicine

In 1980/81 Agnati and Fuxe introduced the concept of intramembrane receptor-receptor interactions and presented the first experimental observations for their existence in crude membrane preparations. The second step was their introduction of the receptor mosaic hypothesis of the engram in 1982. The third step was their proposal that the existence of intramembrane receptor-receptor interactions made possible the integration of synaptic (WT) and extrasynaptic (VT) signals. With the discovery of the intramembrane receptor-receptor interactions with the likely formation of receptor aggregates of multiple receptors, so called receptor mosaics, the entire decoding process becomes a branched process already at the receptor level in the surface membrane. Recent developments indicate the relevance of cooperativity in intramembrane receptor-receptor interactions namely the presence of regulated cooperativity via receptor-receptor interactions in receptor mosaics (RM) built up of the same type of receptor (homo-oligomers) or of subtypes of the same receptor (RM type1). The receptor-receptor interactions will to a large extent determine the various conformational states of the receptors and their operation will be dependent on the receptor composition (stoichiometry), the spatial organization (topography) and order of receptor activation in the RM. The biochemical and functional integrative implications of the receptor-receptor interactions are outlined and long-lived heteromeric receptor complexes with frozen RM in various nerve cell systems may play an essential role in learning, memory and retrieval processes. Intramembrane receptor-receptor interactions in the brain have given rise to novel strategies for treatment of Parkinson's disease (A2A and mGluR5 receptor antagonists), schizophrenia (A2A and mGluR5 agonists) and depression (galanin receptor antagonists). The A2A/D2, A2A/D3 and A2A/mGluR5 heteromers and heteromeric complexes with their possible participation in different types of RM are described in detail, especially in the cortico-striatal glutamate synapse and its extrasynaptic components, together with a postulated existence of A2A/D4 heteromers. Finally, the impact of intramembrane receptor-receptor interactions in molecular medicine is discussed outside the brain with focus on the endocrine, the cardiovascular and the immune systems.

Modulation of dopamine D1 receptor signaling by adenosine A1 receptors in Sf9 cells requires expression of Gi proteins

There are several evidences that some functions of D1 dopamine receptors can be modulated by colocalized adenosine A1 receptors. To elucidate the role of particular components of the receptor complex in the ligand binding and second messenger activation level we have used Sf9 cell expression system. The expression of D1 and A1 receptors was confirmed by proper binding of specific radioligands [3H]SCH23390 (Kd=1.1+/-0.1 nM, Bmax=2.2+/-0.1 pmol/mg protein) and [3H]DPCPX (Kd=2.1+/-0.8nM, Bmax=2.9+/-0.4 pmol/mg protein), respectively. The kinetics of [3H]SCH23390 binding corresponded to the simplest reversible bimolecular binding reaction of complex formation, with k(on)=0.20+/-0.02 min(-1)nM(-1) and k(off)=0.13+/-0.01 min(-1). Dopaminergic agonists increased the accumulation of cAMP in the transfected cells in concentration-dependent manner, indicating a correct coupling of receptor to second messenger system. The coupling of the A1 receptor to Gi proteins was confirmed by both GTPgammaS dependent agonist binding and inhibition of cAMP accumulation by N-cyclopentyladenosine (NCPA). Activation of the A1 receptor by NCPA had no significant influence on neither affinities of dopaminergic ligands nor the radioligand binding kinetics to the co-exprssed D1 receptors in Sf9 cell membranes. On the other hand, the activation of the A1 receptors inhibited the D1 receptor-specific accumulation of cAMP, but only in cells where Gi proteins were expressed with the receptors.

Involvement of adenosine in the neurobiology of schizophrenia and its therapeutic implications

Based on the neuromodulatory and homeostatic actions of adenosine, adenosine dysfunction may contribute to the neurobiological and clinical features of schizophrenia. The present model of adenosine dysfunction in schizophrenia takes into consideration the dopamine and glutamate hypotheses, since adenosine exerts neuromodulatory roles on these systems, and proposes that adenosine plays a role in the inhibitory deficit found in schizophrenia. Given the role of adenosine activation of adenosine A1 receptor (A1R) in mediating neurotoxicity in early stages of brain development, pre- and peri-natal complications leading to excessive adenosine release could induce primary brain changes (i.e., first hit). These events would lead to an adenosine inhibitory deficit through a partial loss of A1R that may emerge as reduced control of dopamine activity and increased vulnerability to excitotoxic glutamate action in the mature brain (i.e., second hit). Adenosine dysfunction is reasonably compatible with symptoms, gray and white matter abnormalities, progressive brain loss, pre- and peri-natal risk factors, age of onset, response to current treatments, impaired sensory gating and increased smoking in schizophrenia. Pharmacological treatments enhancing adenosine activity could be effective for symptom control and for alleviating deterioration in the course of the illness. Accordingly, allopurinol, which may indirectly increase adenosine, has been effective and well tolerated in the treatment of schizophrenia. Since much of the evidence for the adenosine hypothesis is preliminary and theoretical, further investigation in the field is warranted.

Involvement of adenosine A2A and dopamine receptors in the locomotor and sensitizing effects of cocaine

Recent data indicate that cocaine locomotor responses may be influenced by dopamine (DA) neurotransmission and adenosine neuromodulation involving the A2A receptor (A2AR). Male Wistar rats were injected with MSX-3 (1-25 mg/kg; an antagonist of A2AR), CGS 21680 (0.05-0.2 mg/kg; an agonist of A2AR), SCH 23390 (0.125-0.25 mg/kg; an antagonist of DA D1/5R), raclopride (0.1-0.8 mg/kg; an antagonist of DA D2/3R), nafadotride (0.2-0.4 mg/kg; an antagonist of DA D3R) or 7-OH-PIPAT (0.01-1 mg/kg; an agonist of DA D3R) to verify the hypothesis that adenosine A2AR and DA receptors and their antagonistic interactions may control locomotor and sensitizing effects of cocaine. In well-habituated animals, MSX-3 (5 mg/kg) increased, while raclopride (0.4-0.8 mg/kg) decreased basal locomotor activation; the other drugs were inactive. The locomotor hyperactivation induced by acute cocaine (10 mg/kg) was enhanced by MSX-3 (5-25 mg/kg) or nafadotride (0.4 mg/kg), while CGS 21680 (0.2 mg/kg), SCH 23390 (0.25 mg/kg), raclopride (0.2-0.8 mg/kg) or 7-OH-PIPAT (0.1 mg/kg) decreased this effect of cocaine. Given during the development of sensitization (in combination with 5-daily cocaine, 10 mg/kg, injections), MSX-3 (5-25 mg/kg) increased, but CGS 21680 (0.2 mg/kg) and raclopride (0.8 mg/kg) reduced the locomotor response to a cocaine challenge dose (10 mg/kg) on day 10. When injected acutely with a cocaine challenge dose (on day 10), CGS 21680 (0.2 mg/kg), raclopride (0.2-0.8 mg/kg) or 7-OH-PIPAT (1 mg/kg) reduced, while MSX-3 (5 mg/kg) or nafadotride (0.4 mg/kg) enhanced the expression of cocaine sensitization. The present results show that adenosine A2ARs and DA D3Rs exert inhibitory actions on acute locomotor responses to cocaine and on the expression of cocaine sensitization, while DA D2Rs had an opposing role in such effects. Pharmacological stimulation of adenosine A2ARs protected against both the development and expression of cocaine sensitization, which may offer a therapeutic potential of A2AR agonists in the treatment of cocaine dependence. The results suggest an antagonistic role of A2ARs in D2R-mediated cocaine actions based at least in part on the existence of A2A/D2 heteromeric receptor complexes.

GABA Release Modified by Adenosine Receptors in Mouse Hippocampal Slices under Normal and Ischemic Conditions

The excitatory glutamatergic neurons in the hippocampus are modulated by inhibitory GABA-releasing interneurons. The neuromodulator adenosine is known to inhibit the presynaptic release of neurotransmitters and to hyperpolarize postsynaptic neurons in the hippocampus, which would imply that it is an endogenous protective agent against cerebral ischemia and excitotoxic neuronal damage. Interactions of the GABAergic and adenosinergic systems in regulating neuronal excitability in the hippocampus is of crucial importance, particularly under cell-damaging conditions. We now characterized the effects of adenosine receptor agonists and antagonists on the release of preloaded [3H]GABA from hippocampal slices prepared from adult (3-month-old) mice, using a superfusion system. The effects were tested both under normal conditions and in ischemia induced by omitting glucose and oxygen from the superfusion medium. Basal and K+ -evoked GABA release in the hippocampus were depressed by adenosinergic compounds. Under normal conditions activation of both adenosine A1 and A2A receptors by the agonists R(-)N6-(2-phenylisopropyl)adenosine and CGS 21680 inhibited the K+ -evoked release, which effects were blocked by their specific antagonists, 8-cyclopentyl-1,3-dipropyl-xanthine and 3,7-dimethyl-1-propargylxanthine, respectively. Under ischemic conditions the release of both GABA and adenosine is markedly enhanced. The above receptor agonists then depressed both the basal and K+ -evoked GABA release, only the action of A2A receptors being however receptor-mediated. The demonstrated depression of GABA release by adenosine in the hippocampus could be deleterious to neurons and contribute to excitotoxicity.

Striatal plasticity at the network level. Focus on adenosine A2A and D2 interactions in models of Parkinson's Disease

Behavioral and microdialysis studies have been performed on antagonistic A(2A)/D(2) interactions in animal models of Parkinson's Disease. The behavioral analysis involved studies on locomotor activity in reserpinized mice, haloperidol-induced catalepsy in rats and rotational behavior in rats with unilateral 6-OHDA lesions of the ascending DA pathways (Ungerstedt model). Dual probe microdialysis studies were indirectly performed on the striatopallidal GABA neurons by studying extracellular glutamate levels in the striatum and globus pallidus of the awake freely moving rat. The striatum was perfused with A(2A) and/or D(2) agonists via reverse microdialysis. The results show that the A(2A) antagonists SCH58261 and KF17837 can increase locomotor activity in reserpinized mice and produce contralateral rotational behavior only after administration of subthreshold doses of l-DOPA or the D(2) like agonist quinpirole. Furthermore, antagonizing the A(2A) receptor (R) reduced haloperidol induced catalepsy. The behavioral results underline the view that A(2A) antagonists act by blocking A(2A) R in A(2A)/D(2) heterodimers where A(2A) R inhibits the D(2) R transduction and D(2) inhibits the adenylate cyclase (AC) activated by A(2A) R. The microdialysis studies show that the A(2A) agonist CGS21680 striatally coperfused with the D(2) agonist quinpirole more potently counteract the D(2) agonist (quinpirole) induced reduction of pallidal glutamate levels in the DA denervated vs the control striatum indicating an enhancement of the inhibitory A(2A)/D(2) interaction. In the DA denervated but not in the control striatum the A(2A) agonist CGS21680 could strongly increase striatal glutamate levels, indicating an increased receptor signaling in the A(2A) R located on the striatal glutamate terminals, where also D(2) like R exist, here probably as D(4). Thus, the signaling of this A(2A) R may be set free by the loss of D(4) tone on the AC activated by A(2A) in this postulated A(2A)/D(4) heteromer on the glutamate terminals. Taken together, the results indicate that the antiparkinsonian actions of A(2A) antagonists probably are produced by blockade of A(2A) R in the A(2A)/D(2) heterodimers mainly located in the striatopallidal GABA neurons.

Effects of adenosine A1, dopamine D1 and metabotropic glutamate 5 receptors-modulating agents on locomotion of the reserpinised rats

The pathophysiology of Parkinson's disease and l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia are characterised by an imbalance between activity of the direct and indirect pathways regulated by dopamine D1 and D2 receptors, respectively. In this study, we investigated the effects of treatments combining adenosine A(1) and metabotropic glutamate 5 (mGlu5) receptors modulators on locomotion induced by dopamine D1 receptor activation in the reserpine-treated rats. Administration of the adenosine A(1) receptor agonist and mGlu5 receptor antagonist resulted in the significant reduction of dopamine D1 receptor agonist-induced locomotion. The combination of adenosine A(1) receptor agonist with mGlu5 receptor antagonist had no greater effect than these compounds alone. However, the adenosine A(1) receptor antagonist attenuated the inhibitory effect of mGlu5 receptor antagonist. The data suggest that the effect of mGlu5 receptor blockade on locomotion elicited by dopamine D1 receptor stimulation involves activation of adenosine A(1) receptors. This interaction can improve our understanding of pathophysiology of L-DOPA-induced dyskinesia.

Effects of an adenosine A2A receptor blockade in the nucleus accumbens on locomotion, feeding, and prepulse inhibition in rats

The nucleus accumbens (NAc) subserves behaviors governed by natural rewards, i.e., feeding or exploration, and has been implicated in control of prepulse inhibition (PPI), a measure of sensorimotor gating. The present study sought to determine whether a tonic stimulation of adenosine A(2A) receptors in the rat NAc is involved in control of spontaneous locomotor activity, feeding behavior, and PPI. To this end, bilateral microinfusions of a prodrug (MSX-3) (3 microg and 5 microg in 1 microl per side) of the selective A(2A) receptor antagonist MSX-2 or vehicle (1 microl per side) were administered into the NAc. Results show that blockade of intra-NAc adenosine A(2A) receptors by a high (5 microg), but not by a low (3 microg), dose of MSX-3 increased locomotor activity in an open field, reduced food intake, and delayed intake onset in food-deprived rats examined in a test cage with standard laboratory chow. Furthermore, PPI was significantly disrupted after intra-NAc infusion of 5 microg, but not 3 microg, MSX-3. These findings suggest that locomotor activity as well as intact PPI and feeding behavior rely on tonic activation of intra-NAc A(2A) receptors. The data add further support to the view that adenosine is a tonically active modulator of striatal function through actions on A(2A) receptors.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

Involvement of adenosine A1 and A2A receptors in the motor effects of caffeine after its acute and chronic administration

The involvement of adenosine A(1) and A(2A) receptors in the motor effects of caffeine is still a matter of debate. In the present study, counteraction of the motor-depressant effects of the selective A(1) receptor agonist CPA and the A(2A) receptor agonist CGS 21680 by caffeine, the selective A(1) receptor antagonist CPT, and the A(2A) receptor antagonist MSX-3 was compared. CPT and MSX-3 produced motor activation at the same doses that selectively counteracted motor depression induced by CPA and CGS 21680, respectively. Caffeine also counteracted motor depression induced by CPA and CGS 21680 at doses that produced motor activation. However, caffeine was less effective than CPT at counteracting CPA and even less effective than MSX-3 at counteracting CGS 21680. On the other hand, when administered alone in habituated animals, caffeine produced stronger motor activation than CPT or MSX-3. An additive effect on motor activation was obtained when CPT and MSX-3 were coadministered. Altogether, these results suggest that the motor-activating effects of acutely administered caffeine in rats involve the central blockade of both A(1) and A(2A) receptors. Chronic exposure to caffeine in the drinking water (1.0 mg/ml) resulted in tolerance to the motor effects of an acute administration of caffeine, lack of tolerance to amphetamine, apparent tolerance to MSX-3 (shift to the left of its 'bell-shaped' dose-response curve), and true cross-tolerance to CPT. The present results suggest that development of tolerance to the effects of A(1) receptor blockade might be mostly responsible for the tolerance to the motor-activating effects of caffeine and that the residual motor-activating effects of caffeine in tolerant individuals might be mostly because of A(2A) receptor blockade.

Involvement of DARPP-32 phosphorylation in the stimulant action of caffeine

Caffeine has been imbibed since ancient times in tea and coffee, and more recently in colas. Caffeine owes its psychostimulant action to a blockade of adenosine A(2A) receptors, but little is known about its intracellular mechanism of action. Here we show that the stimulatory effect of caffeine on motor activity in mice was greatly reduced following genetic deletion of DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein of relative molecular mass 32,000). Results virtually identical to those seen with caffeine were obtained with the selective A(2A) antagonist SCH 58261. The depressant effect of the A(2A) receptor agonist, CGS 21680, on motor activity was also greatly attenuated in DARPP-32 knockout mice. In support of a role for DARPP-32 in the action of caffeine, we found that, in striata of intact mice, caffeine increased the state of phosphorylation of DARPP-32 at Thr 75. Caffeine increased Thr 75 phosphorylation through inhibition of PP-2A-catalysed dephosphorylation, rather than through stimulation of cyclin-dependent kinase 5 (Cdk5)-catalysed phosphorylation, of this residue. Together, these studies demonstrate the involvement of DARPP-32 and its phosphorylation/dephosphorylation in the stimulant action of caffeine.

Adenosine A2A, 5-HT1A and 5-HT7 receptor in neonatally pregnenolone-treated rats

Steroid hormones synthesized in the brain, called 'neurosteroids', modulate neuronal activity. We treated neonatal rats with a main precursor of the neurosteroidogenesis, pregnenolone, and examined adenosine A2A receptor, 5- hydroxytryptamine (5-HT)1A and 5-HT7 receptor densities in the front-parietal cortex in juvenile and adult rats. In receptor binding assay using [3H]CGS21680 and [3H]8-OH-DPAT, it was shown that neonatal pregnenolone-treatment induced a significant decrease in the adenosine A2A receptor density with no significant effects on the 5-HT1A and 5-HT7 receptor densities.

Adenosine/dopamine interaction: implications for the treatment of Parkinson's disease

Evidence for a role of dopaminergic neurotransmission in the motor effects of adenosine antagonists, such as caffeine, is reviewed, based on the existence of specific antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the striatum. Both adenosine A(1) and adenosine A(2A) receptor antagonists induce motor activation in rodents. At least a certain degree of dopaminergic activity is required to obtain adenosine antagonist-induced motor activation, with adenosine A(1) antagonists being the most sensitive and non-selective adenosine antagonists the most resistant to striatal dopamine depletion. When considering long-term treatment with adenosine antagonists concomitant administration of dopamine agonists might be required in order to obtain strong motor effects (cross-sensitization) and to avoid the development of telerance.

Electrophysiological and behavioural evidence for an antagonistic modulatory role of adenosine A2A receptors in dopamine D2 receptor regulation in the rat dopamine-denervated striatum

It has been shown that striatal adenosine A2A receptors can antagonistically interact with dopamine D2 receptors at the membrane level leading to a decrease in the affinity and efficacy of D2 receptors. Extracellular recordings and rotational behaviour were employed to obtain a correlate to these findings in an animal model of Parkinson's disease (PD). The recordings were performed in rats with unilateral 6-hydroxydopamine (6-OHDA)-induced catecholamine depletion. While recording in the dopamine-depleted striatum, local applications of the dopamine D2 agonist quinpirole reduced neuronal activity. However, when the adenosine A2A antagonist MSX-3 was applied simultaneously with quinpirole, the inhibition of neuronal firing seen after quinpirole alone was significantly potentiated (P< 0.001, n = 11). In contrast, local application of CGS 21680 attenuated the effect of quinpirole. The doses of MSX-3 and CGS 21680 used to achieve the modulation of quinpirole action had no effect per se on striatal neuronal firing. Furthermore, rotational behaviour revealed that MSX-3 dose-dependently increased the number of turns when administrated together with a threshold dose of quinpirole while no enhancement was achieved when MSX-3 was combined with SKF 38393. MSX-3 alone did not induce rotational behaviour. In conclusion, this study shows that low ineffective doses of MSX-3 enhance the effect of quinpirole on striatal firing rate, while the A2A agonist exerts the opposite action. This mechanism gives a therapeutic potential to A2A antagonists in the treatment of PD by enhancing D2 receptor function.