Distribution and postnatal ontogeny of adenosine A2A receptors in rat brain: comparison with dopamine receptors

Role of purines in brain development, from neuronal proliferation to synaptic refinement

The purinergic system includes P1 and P2 receptors, which are activated by ATP and its metabolites. They are expressed in adult neuronal and glial cells and are crucial in brain function, including neuromodulation and neuronal signaling. As P1 and P2 receptors are expressed throughout embryogenesis and development, purinergic signaling also has an important role in the development of the peripheral and central nervous system. In this review, we present the expression pattern and activity of purinergic receptors and of their signaling pathways during embryonic and postnatal development of the nervous system. In particular, we review the involvement of the purinergic signaling in all the crucial steps of brain development i.e. in neural stem cell proliferation, neuronal differentiation and migration as well as in astrogliogenesis and oligodendrogenesis. Then, we review data showing a crucial role of the ATP and adenosine signaling pathways in the formation of the peripheral neuromuscular junction and of central GABAergic and glutamatergic synapses. Finally, we examine the consequences of deregulation of the purinergic system during development and discuss the therapeutic potential of targeting it at adult stage in diseases with reactivation of the ATP and adenosine pathway.

A Pattern to Link Adenosine Signaling, Circadian System, and Potential Final Common Pathway in the Pathogenesis of Major Depressive Disorder

Several studies have reported separate roles of adenosine receptors and circadian clockwork in major depressive disorder. While less evidence exists for regulation of the circadian clock by adenosine signaling, a small number of studies have linked the adenosinergic system, the molecular circadian clock, and mood regulation. In this article, we review relevant advances and propose that adenosine receptor signaling, including canonical and other alternative downstream cellular pathways, regulates circadian gene expression, which in turn may underlie the pathogenesis of mood disorders. Moreover, we summarize the convergent point of these signaling pathways and put forward a pattern by which Homer1a expression, regulated by both cAMP-response element binding protein (CREB) and circadian clock genes, may be the final common pathogenetic mechanism in depression.

The adenosine A2A receptor alleviates postoperative delirium-like behaviors by restoring blood cerebrospinal barrier permeability in rats

Postoperative delirium (POD) is a common post-operative complication in elderly patients that is associated with increased morbidity and mortality. However, the neuropathogenesis of this complication remains unknown. The blood-cerebrospinal fluid barrier (BCB) and brain-blood barrier (BBB) are composed of tight junctions between cells that form physical barriers, and BBB damage plays an important role in the neuropathogenesis of POD. Nevertheless, the role of BCB in POD remains to be elucidated. Herein, we investigated the effect of adenosine A_2A receptor (A_2A R), a key regulator of the permeability of barriers, on surgery-induced increased permeability of BCB and POD-like behaviors. Open field, buried food, and Y maze tests were used to evaluate behavioral changes in rats after surgery. Levels of tight junction proteins, adherens junction proteins, A_2A R, GTP-RhoA, and ROCK2 in the choroid plexus were assessed by western blotting. The concentrations of NaFI and FITC-dextran in the cerebrospinal fluid (CSF) were detected by fluorescence spectrophotometry. Transmission electron microscopy was applied to observe the ultrastructure of the choroid plexus. Surgery/anesthesia decreased the levels of tight junction (e.g., ZO-1, occludin, and claudin1) proteins, increased concentrations of NaFI and FITC-dextran in CSF, damaged the ultrastructure of choroid plexus, and induced POD-like behaviors in rats. An A_2A R antagonist alleviated POD-like behaviors in rats. Furthermore, the A_2A R antagonist increased the levels of tight junction proteins and restored the permeability of BCB in rats with POD. Fasudil, a selective Rho-associated protein kinase 2 (ROCK2) inhibitor, ameliorated POD-like behaviors induced by A_2A R activation. Moreover, fasudil also abolished the increased levels of GTP-RhoA/ROCK2, decreased levels of tight junction proteins, and increased permeability of BCB caused by A_2A R activation. Our findings demonstrate that A_2A R might participate in regulating BCB permeability in rats with POD via the RhoA/ROCK2 signaling pathway, which suggests the potential of A_2A R as a therapeutic target for POD.

Caffeine and Clinical Outcomes in Premature Neonates

Caffeine is the most widely used drug by both adults and children worldwide due to its ability to promote alertness and elevate moods. It is effective in the management of apnea of prematurity in premature infants. Caffeine for apnea of prematurity reduces the incidence of bronchopulmonary dysplasia in very-low-birth-weight infants and improves survival without neurodevelopmental disability at 18-21 months. Follow-up studies of the infants in the Caffeine for Apnea of Prematurity trial highlight the long-term safety of caffeine in these infants, especially relating to motor, behavioral, and intelligence skills. However, in animal models, exposure to caffeine during pregnancy and lactation adversely affects neuronal development and adult behavior of their offspring. Prenatal caffeine predisposes to intrauterine growth restriction and small growth for gestational age at birth. However, in-utero exposure to caffeine is also associated with excess growth, obesity, and cardio-metabolic changes in children. Caffeine therapy is a significant advance in newborn care, conferring immediate benefits in preterm neonates. Studies should help define the appropriate therapeutic window for caffeine treatment along with with the mechanisms relating to its beneficial effects on the brain and the lung. The long-term consequences of caffeine in adults born preterm are being studied and may depend on the ability of caffeine to modulate both the expression and the maturation of adenosine receptors in infants treated with caffeine.

Adenosine A1 Receptor Agonist 2-chloro-N6-cyclopentyladenosine and Hippocampal Excitability During Brain Development in Rats

Objective: The adenosinergic system may influence excitability in the brain. Endogenous and exogenous adenosine has anticonvulsant activity presumably by activating A1 receptors. Adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) may thus bolster anticonvulsant effects, but its action and the number of A1 receptors at different developmental stages are not known.

Methods: Hippocampal epileptic afterdischarges (ADs) were elicited in 12-, 15-, 18-, 25-, 45-, and 60-day-old rats. Stimulation and recording electrodes were implanted into the dorsal hippocampus. The A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA, 0.5 or 1 mg/kg) was administered intraperitoneally 10 min before the first stimulation. Control animals were injected with saline. All rats were stimulated with a 2-s series of 1-ms biphasic pulses delivered at 60 Hz with increasing stepwise intensity (0.05–0.6 mA). Each age and dose group contained 9–14 animals. The AD thresholds and durations were evaluated, and the A1 receptors were detected in the hippocampus in 7-, 10-, 12-, 15-, 18-, 21-, 25-, 32-, and 52-day-old rats.

Results: Both CCPA doses significantly increased hippocampal AD thresholds in 12-, 15-, 18-, and 60-day-old rats compared to controls. In contrast, the higher dose significantly decreased AD threshold in the 25-day-old rats. The AD durations were significantly shortened in all age groups except for 25-day-old rats where they were significantly prolonged. A1 receptor expression in the hippocampus was highest in 10-day-old rats and subsequently decreased.

Significance: The adenosine A1 receptor agonist CCPA exhibited anticonvulsant activity at all developmental stages studied here except for 25-day-old rats. Age-related differences might be due to the development of presynaptic A1 receptors in the hippocampus.

Purinergic Signaling in the Vertebrate Olfactory System

Adenosine 5'-triphosphate (ATP) is an ubiquitous co-transmitter in the vertebrate brain. ATP itself, as well as its breakdown products ADP and adenosine are involved in synaptic transmission and plasticity, neuron-glia communication and neural development. Although purinoceptors have been demonstrated in the vertebrate olfactory system by means of histological techniques for many years, detailed insights into physiological properties and functional significance of purinergic signaling in olfaction have been published only recently. We review the current literature on purinergic neuromodulation, neuron-glia interactions and neurogenesis in the vertebrate olfactory system.

Role of Ectonucleotidases in Synapse Formation During Brain Development: Physiological and Pathological Implications

BACKGROUND

Extracellular adenine nucleotides and nucleosides, such as ATP and adenosine, are among the most recently identified and least investigated diffusible signaling factors that contribute to the structural and functional remodeling of the brain, both during embryonic and postnatal development. Their levels in the extracellular milieu are tightly controlled by various ectonucleotidases: ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPP), alkaline phosphatases (AP), ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) and ecto-5'- nucleotidase (eN).

METHODS

Studies related to the expression patterns of ectonucleotidases and their known features during brain development are reviewed, highlighting involvement of these enzymes in synapse formation and maturation in physiological as well as in pathological states.

RESULTS

During brain development and in adulthood all ectonucleotidases have diverse expression pattern, cell specific localization and function. NPPs are expressed at early embryonic days, but the expression of NPP3 is reduced and restricted to ependymal area in adult brain. NTPDase2 is dominant ectonucleotidase existing in the progenitor cells as well as main astrocytic NTPDase in the adult brain, while NTPDase3 is fully expressed after third postnatal week, almost exclusively on varicose fibers. Specific brain AP is functionally associated with synapse formation and this enzyme is sufficient for adenosine production during neurite growth and peak of synaptogenesis. eN is transiently associated with synapses during synaptogenesis, however in adult brain it is more glial than neuronal enzyme.

CONCLUSION

Control of extracellular adenine nucleotide levels by ectonucleotidases are important for understanding the role of purinergic signaling in developing tissues and potential targets in developmental disorders such as autism.

Caffeine and Selective Adenosine Receptor Antagonists as New Therapeutic Tools for the Motivational Symptoms of Depression

Major depressive disorder is one of the most common and debilitating psychiatric disorders. Some of the motivational symptoms of depression, such anergia (lack of self-reported energy) and fatigue are relatively resistant to traditional treatments such as serotonin uptake inhibitors. Thus, new pharmacological targets are being investigated. Epidemiological data suggest that caffeine consumption can have an impact on aspects of depressive symptomatology. Caffeine is a non-selective adenosine antagonist for A1/A2A receptors, and has been demonstrated to modulate behavior in classical animal models of depression. Moreover, selective adenosine receptor antagonists are being assessed for their antidepressant effects in animal studies. This review focuses on how caffeine and selective adenosine antagonists can improve different aspects of depression in humans, as well as in animal models. The effects on motivational symptoms of depression such as anergia, fatigue, and psychomotor slowing receive particular attention. Thus, the ability of adenosine receptor antagonists to reverse the anergia induced by dopamine antagonism or depletion is of special interest. In conclusion, although further studies are needed, it appears that caffeine and selective adenosine receptor antagonists could be therapeutic agents for the treatment of motivational dysfunction in depression.

Striatal dopamine D1-type receptor availability: no difference from control but association with cortical thickness in methamphetamine users

Chronic methamphetamine use poses potentially devastating consequences for directly affected individuals and for society. Lower dopamine D2-type receptor availability has been observed in striata of methamphetamine users as compared with controls, but an analogous comparison of D1-type receptors has been conducted only on post-mortem material, with no differences in methamphetamine users from controls in the caudate nucleus and putamen and higher D1-receptor density in the nucleus accumbens. Released from neurons when methamphetamine is self-administered, dopamine binds to both D1- and D2-type receptors in the striatum, with downstream effects on cortical activity. Thus, both receptor subtypes may contribute to methamphetamine-induced alterations in cortical morphology and behavior. In this study, 21 methamphetamine-dependent subjects and 23 healthy controls participated in positron emission tomography and structural magnetic resonance imaging for assessment of striatal D1- and D2-type receptor availability and cortical gray-matter thickness, respectively. Although D2-type receptor availability (BPnd) was lower in the methamphetamine group, as shown previously, the groups did not differ in D1-type BPnd. In the methamphetamine group, mean cortical gray-matter thickness was negatively associated with cumulative methamphetamine use and craving for the drug. Striatal D1-type but not D2-type BPnd was negatively associated with global mean cortical gray-matter thickness in the methamphetamine group, but no association was found between gray-matter thickness and BPnd for either dopamine receptor subtype in the control group. These results suggest a role of striatal D1-type receptors in cortical adaptation to chronic methamphetamine use.

Adenosine A1 receptor activates background potassium channels and modulates information processing in olfactory bulb mitral cells

KEY POINTS

Adenosine is a widespread neuromodulator in the mammalian brain, but whether it affects information processing in sensory system(s) remains largely unknown. Here we show that adenosine A₁ receptors hyperpolarize mitral cells, one class of principal neurons that propagate odour information from the olfactory bulb to higher brain areas, by activation of background K⁺ channels. The adenosine-modulated background K⁺ channels belong to the family of two-pore domain K⁺ channels. Adenosine reduces spontaneous activity of mitral cells, whereas action potential firing evoked by synaptic input upon stimulation of sensory neurons is not affected, resulting in a higher ratio of evoked firing (signal) over spontaneous firing (noise) and hence an improved signal-to-noise ratio. The study shows for the first time that adenosine influences fine-tuning of the input-output relationship in sensory systems.

ABSTRACT

Neuromodulation by adenosine is of critical importance in many brain regions, but the role of adenosine in olfactory information processing has not been studied so far. We investigated the effects of adenosine on mitral cells, which are projection neurons of the olfactory bulb. Significant expression of A₁ and A_2A receptors was found in mitral cells, as demonstrated by in situ hybridization. Application of adenosine in acute olfactory bulb slices hyperpolarized mitral cells in wild-type but not in adenosine A₁ receptor knockout mice. Adenosine-induced hyperpolarization was mediated by background K⁺ currents that were reduced by halothane and bupivacaine, which are known to inhibit two-pore domain K⁺ (K2P) channels. In mitral cells, electrical stimulation of axons of olfactory sensory neurons evoked synaptic currents, which can be considered as input signals, while spontaneous firing independent of sensory input can be considered as noise. Synaptic currents were not affected by adenosine, while adenosine reduced spontaneous firing, leading to an increase in the signal-to-noise ratio of mitral cell firing. Our findings demonstrate that A₁ adenosine receptors activate two-pore domain K⁺ channels, which increases the signal-to-noise ratio of the input-output relationship in mitral cells and thereby modulates information processing in the olfactory bulb.

ATP released from astrocytes modulates action potential threshold and spontaneous excitatory postsynaptic currents in the neonatal rat prefrontal cortex

Maternal immune activation during pregnancy is a risk factor for neurodevelopmental disorders, such as schizophrenia; however, a full mechanistic understanding has yet to be established. The activity of a transient cell population, the subplate neurons, is critical for the development of cortical inhibition and functional thalamocortical connections. Sensitivity of these cells to factors released during inflammation, therefore, may offer a link between maternal immune activation and the aberrant cortical development underlying some neuropsychiatric disorders. An elevated extracellular ATP concentration is associated with inflammation and has been shown to have an effect on neuronal activity. Here, we investigated the effect of ATP on the electrophysiological properties of subplate neurons. Exogenous ATP increased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) at micromolar concentrations. Further, ATP released by astrocytes activated by the PAR-1 agonist, TFLLR-NH₂, also increased the amplitude and frequency of sEPSCs in subplate neurons. The electrophysiological properties of subplate neurons recorded from prefrontal cortical (PFC) slices from neonatal rats were also disrupted in a maternal immune activation rat model of schizophrenia, with a suramin-sensitive increase in frequency and amplitude of sEPSCs. An alternative neurodevelopmental rat model of schizophrenia, MAM-E17, which did not rely on maternal immune activation, however, showed no change in subplate neuron activity. Both models were validated with behavioral assays, showing schizophrenia-like endophenotypes in young adulthood. The purinergic modulation of subplate neuron activity offers a potential explanatory link between maternal immune activation and disruptions in cortical development that lead to the emergence of neuropsychiatric disorders.

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Alpha2-adrenoceptor and adenosine A1 receptor within the nucleus tractus solitarii in hypertension development

Alpha2-adrenoceptor and A1 adenosine receptor systems within the nucleus tractus solitarii (NTS) play an important role in cardiovascular control. Deregulation of these systems may result in an elevated sympathetic tone, one of the root causes of neurogenic hypertension. The dorsomedial/dorsolateral and subpostremal NTS subnuclei of spontaneously hypertensive rats (SHR) show density changes in both receptors, even at 15 days of age, prior to the onset of hypertension. In addition, adenosine A1 receptors have been specifically reported to modulate alpha2-adrenoceptors in several brain regions, including the NTS, via a PLC-dependent pathway involving cross regulation between sympathetic neurons and astrocytes. The physiological cross talk between these receptor systems is also deregulated in SHR suggesting that alpha2-adrenoceptor and A1 adenosine receptor might be germane to the development of hypertension. In this review, we will focus on these systems within the NTS during development, pointing out some interesting modulations in processes, and chemical changes within specific subnuclei of NTS circuitry, that might have implications for neurogenic hypertension.

A1 not A2A adenosine receptors play a role in cortical epileptic afterdischarges in immature rats

Endo- as well as exogenous adenosine exhibits anticonvulsant action. Participation of individual types of adenosine receptors was studied in present experiments in immature rats. Cortical epileptic afterdischarges were used as a model in rat pups 12, 18 and 25 days old. CCPA, an agonist of A1 adenosine receptors, decreased markedly duration of afterdischarges whereas DPCPX, an antagonist of A1 receptors, exhibited strong proconvulsant action. Action of either drug was best expressed in 12-day-old rats and it decreased with age. Drugs influencing A2A adenosine receptors (agonist CGS21680 and antagonist ZM241385) did not exhibit systematic effects in our model. Motor phenomena accompanying cortical stimulation or epileptic afterdischarge were never influenced by any of the four drugs studied. A1 adenosine receptors are important in the model of cortical seizures, especially in the youngest group studied.

Early caffeine exposure: transient and long-term consequences on brain excitability

The influence of pre- and postnatal caffeine treatment on brain excitability during development and adulthood is reviewed. Pre- and postnatal exposure to caffeine induces sex- and age-specific long-term neurochemical alterations in the brain and the behavior of rodents. Because adenosine neuromodulation is closely related to the regulation of brain excitability the increased expression in adenosine receptor system due to neonatal caffeine treatment should cause transient and permanent changes in seizure susceptibility. So far, findings have been focused on primarily developmental changes of the brain adenosine modulatory system and have demonstrated that the alterations are not restricted to a single brain region. Neurobehavioral changes and the anticonvulsant effect of early caffeine exposure are dependent on the caffeine dose, developmental stage of exposure and age of testing. Although outcomes of caffeine treatment are still a matter of debate, our review raise questions concerning the impact of early caffeine treatment on regulation of seizure susceptibility during development and adulthood.

Developmental increase in ecto-5'-nucleotidase activity overlaps with appearance of two immunologically distinct enzyme isoforms in rat hippocampal synaptic plasma membranes

Ecto-5'-nucleotidase (e-5NT), a glycosylphosphatidylinositol-linked membrane protein, catalyzes a conversion of AMP to adenosine, which influences nearly every aspect of brain physiology, including embryonic and postnatal brain development. The present study aimed to investigate a pattern of expression, activity and kinetic properties of e-5NT in the hippocampal formation and synaptic plasma membrane (SPM) preparations in rats at postnatal days (PDs) 7, 15, 20, 30 and 90. By combining gene expression analysis and enzyme histochemistry, we observed that e-5NT mRNA reached the adult level at PD20, while the enzyme activity continued to increase beyond this age. Further analysis revealed that hippocampal layers rich in synapses expressed the highest levels of e-5NT activity, while in layers populated with neuronal cell bodies, the enzyme activity was weak or absent. Therefore, activity and expression of e-5NT were analyzed in SPM preparations isolated from rats at different ages. The presence of two protein bands of about 65 and 68 kDa was determined by immunoblot analysis. The 65-kDa band was present at all ages, and its abundance increased from PD7 to PD20. The 68-kDa band appeared at PD15 and increased until PD30, coinciding with the increase of e-5NT activity, substrate affinity and enzymatic efficiency. Since distinct e-5NT isoforms may derive from different patterns of the enzyme protein N-glycosylation, we speculate that long-term regulation of e-5NT activity in adulthood may be effectuated at posttranslational level and without overall change in the gene and protein expression.

CB1 Cannabinoid Receptor Expression in the Striatum: Association with Corticostriatal Circuits and Developmental Regulation

Corticostriatal circuits mediate various aspects of goal-directed behavior and are critically important for basal ganglia-related disorders. Activity in these circuits is regulated by the endocannabinoid system via stimulation of CB1 cannabinoid receptors. CB1 receptors are highly expressed in projection neurons and select interneurons of the striatum, but expression levels vary considerably between different striatal regions (functional domains). We investigated CB1 receptor expression within specific corticostriatal circuits by mapping CB1 mRNA levels in striatal sectors defined by their cortical inputs in rats. We also assessed changes in CB1 expression in the striatum during development. Our results show that CB1 expression is highest in juveniles (P25) and then progressively decreases toward adolescent (P40) and adult (P70) levels. At every age, CB1 receptors are predominantly expressed in sensorimotor striatal sectors, with considerably lower expression in associative and limbic sectors. Moreover, for most corticostriatal circuits there is an inverse relationship between cortical and striatal expression levels. Thus, striatal sectors with high CB1 expression (sensorimotor sectors) tend to receive inputs from cortical areas with low expression, while striatal sectors with low expression (associative/limbic sectors) receive inputs from cortical regions with higher expression (medial prefrontal cortex). In so far as CB1 mRNA levels reflect receptor function, our findings suggest differential CB1 signaling between different developmental stages and between sensorimotor and associative/limbic circuits. The regional distribution of CB1 receptor expression in the striatum further suggests that, in sensorimotor sectors, CB1 receptors mostly regulate GABA inputs from local axon collaterals of projection neurons, whereas in associative/limbic sectors, CB1 regulation of GABA inputs from interneurons and glutamate inputs may be more important.

In vivo dopaminergic and behavioral responses to acute cocaine are altered in adenosine A(2A) receptor knockout mice

Adenosine, acting on adenosine A(2A) receptors (A2ARs), regulates addictive processes induced by drugs of abuse. This study investigates the role of A(2A) adenosine receptors in neurochemical and behavioral responses to an acute cocaine challenge. Changes in the extracellular levels of dopamine (DA) in the nucleus accumbens (NAc) of mice lacking A(2A) adenosine receptors and wild type (WT) littermates after an acute cocaine (20 mg/kg) administration were evaluated by in vivo microdialysis studies. Locomotor effects induced by cocaine were measured during the microdialysis procedure. Cocaine-evoked increases in extracellular DA were not sustained in mice lacking A(2A) Rs in comparison with wild-type mice (P < 0.05). Cocaine administration significantly increased ambulatory activity in both genotypes. However, overall locomotor activity was further increased, whereas rest and small local movement measures were significantly attenuated in the A(2A) R knockout mice compared with WT littermates (P < 0.05). Our findings support an important role for adenosine A(2A) R in modulating the acute effects of cocaine, as demonstrated by the decrease in cocaine-evoked dopaminergic transmission in the NAc. Furthermore, the results support an important antagonistic role of A(2A) R in vivo in regulating psychostimulant-induced hyperlocomotion.

GPR88 - a putative signaling molecule predominantly expressed in the striatum: Cellular localization and developmental regulation

GPR88 is a putative G-protein-coupled receptor that is highly and almost exclusively expressed in the striatum. Its function remains unknown. We investigated GPR88 cellular localization and expression levels across development in different functional domains of the striatum in juvenile (P25), adolescent (P40), and adult (P70) rats, by in situ hybridization histochemistry. At all ages, GPR88 mRNA expression was most robust in the sensorimotor (lateral) striatum and was detected in virtually every neuron. Expression was highest in juveniles and decreased thereafter with regionally distinct trajectories. Thus, in the dorsal striatum, there was a progressive decrease from juveniles to adolescents to adults. In contrast, in the nucleus accumbens, the only (modest) decrease occurred between juveniles and adolescents. These findings indicate that GPR88 is expressed in all striatal neurons, but is differentially regulated across development in different striatal regions.

An essential role for adenosine signaling in alcohol abuse

In the central nervous system (CNS), adenosine plays an important role in regulating neuronal activity and modulates signaling by other neurotransmitters, including GABA, glutamate, and dopamine. Adenosine suppresses neurotransmitter release, reduces neuronal excitability, and regulates ion channel function through activation of four classes of G protein-coupled receptors, A(1), A(2A), A(2B), and A(3). Central adenosine are largely controlled by nucleoside transporters, which transport adenosine levels across the plasma membrane. Adenosine has been shown to modulate cortical glutamate signaling and ventral-tegmental dopaminergic signaling, which are involved in several aspects of alcohol use disorders. Acute ethanol elevates extracellular adenosine levels by selectively inhibiting the type 1 equilibrative nucleoside transporter, ENT1. Raised adenosine levels mediate the ataxic and sedative/hypnotic effects of ethanol through activation of A(1) receptors in the cerebellum, striatum, and cerebral cortex. Recently, we have shown that pharmacological inhibition or genetic deletion of ENT1 reduces the expression of excitatory amino acid transporter 2 (EAAT2), the primary regulator of extracellular glutamate, in astrocytes. These lines of evidence support a central role for adenosine-mediated glutamate signaling and the involvement of astrocytes in regulating ethanol intoxication and preference. In this paper, we discuss recent findings on the implication of adenosine signaling in alcohol use disorders.

Pharmacologic neuroimaging of the ontogeny of dopamine receptor function

Characterization of the ontogeny of the cerebral dopaminergic system is crucial for gaining a greater understanding of normal brain development and its alterations in response to drugs of abuse or conditions such as attention-deficit hyperactivity disorder. Pharmacological MRI (phMRI) was used to determine the response to dopamine transporter (DAT) blockers cocaine and methylphenidate (MPH), the dopamine releaser D-amphetamine (AMPH), the selective D1 agonist dihydrexidine, and the D2/D3 agonist quinpirole in young (<30 days old) and adult (>60 days old) rats. In adult rats, cocaine (0.5 mg/kg i.v.) or MPH (2 mg/kg) induced primarily positive cerebral blood volume (rCBV) changes in the dopaminergic circuitry, but negative rCBV changes in the young animals. Microdialysis measurements in the striatum showed that young rats have a smaller increase in extracellular dopamine in response to cocaine than adults. The young rats showed little rCBV response to the selective D1 agonist dihydrexidine in contrast to robust rCBV increases observed in the adults, whereas there was a similar negative rCBV response in the young and adult rats to the D2 agonist quinpirole. We also performed a meta-analysis of literature data on the development of D1 and D2 receptors and the DAT. These data suggest a predominance of D2-like over D1-like function between 20 and 30 days of age. These combined results suggested that the dopamine D1 receptor is functionally inhibited at young age.

Pathophysiological roles for purines: adenosine, caffeine and urate

The motor symptoms of Parkinson's disease (PD) are primarily due to the degeneration of the dopaminergic neurons in the nigrostriatal pathway. However, several other brain areas and neurotransmitters other than dopamine such as noradrenaline, 5-hydroxytryptamine and acetylcholine are affected in the disease. Moreover, adenosine because of the extensive interaction of its receptors with the dopaminergic system has been implicated in the pathophysiology of the disease. Based on the involvement of these non-dopaminergic neurotransmitters in PD and the sometimes severe adverse effects that limit the mainstay use of dopamine-based anti-parkinsonian treatments, recent assessments have called for a broadening of therapeutic options beyond the traditional dopaminergic drug arsenal. In this review we describe the interactions between dopamine and adenosine receptors that underpin the pre-clinical and clinical rationale for pursuing adenosine A(2A) receptor antagonists as symptomatic and potentially neuroprotective treatment of PD. The review will pay particular attention to recent results regarding specific A(2A) receptor-receptor interactions and recent findings identifying urate, the end product of purine metabolism, as a novel prognostic biomarker and candidate neuroprotectant in PD.

Impaired active avoidance learning in infant rats appears to be related to insufficient metabolic recruitment of the lateral septum

The temporal dissociation between early information acquisition and output of complex behaviors is a common principle during development. Thus, although infant rats are not able to generate sufficient avoidance behavior during two-way active avoidance (TWA) training they obviously deposit a certain "memory trace" (Schäble, Poeggel, Braun, & Gruss, 2007). The ontogeny of learning is probably mirrored by the maturing functionality of different basal forebrain regions. Two of the basal forebrain regions involved in TWA learning are the medial septum/diagonal band of Broca (MS/DB), which is essential for the encoding and retrieval of memory and the lateral septum (LS) that plays a role in the generation of behavior. Mapping 2-fluoro-deoxy-glucose utilization in freely behaving animals, the aim of this study was to assess the functional recruitment of the MS/DB and LS in infant (P17-P21) and adolescent (P38-P42) rats during the first (acquisition) and fifth (retrieval) TWA training. Metabolic activity in the MS/DB was similar in both age groups during acquisition and retrieval indicating that this region is already mature in the infant rat. In contrast, metabolic activity in the LS was generally lower in the infant rats suggesting that this region is not yet fully functional during P17 and P21. This insufficient recruitment may be one reason for the poor TWA performance of infant rats. Finally, the LS displayed significantly higher activity during acquisition than during retrieval indicating that the highest amount of energy is consumed during the initial learning phase.

Perinatal caffeine, acting on maternal adenosine A(1) receptors, causes long-lasting behavioral changes in mouse offspring

BACKGROUND

There are lingering concerns about caffeine consumption during pregnancy or the early postnatal period, partly because there may be long-lasting behavioral changes after caffeine exposure early in life.

METHODOLOGY/PRINCIPAL FINDINGS

We show that pregnant wild type (WT) mice given modest doses of caffeine (0.3 g/l in drinking water) gave birth to offspring that as adults exhibited increased locomotor activity in an open field. The offspring also responded to cocaine challenge with greater locomotor activity than mice not perinatally exposed to caffeine. We performed the same behavioral experiments on mice heterozygous for adenosine A(1) receptor gene (A(1)RHz). In these mice signaling via adenosine A(1) receptors is reduced to about the same degree as after modest consumption of caffeine. A(1)RHz mice had a behavioral profile similar to WT mice perinatally exposed to caffeine. Furthermore, it appeared that the mother's genotype, not offspring's, was critical for behavioral changes in adult offspring. Thus, if the mother partially lacked A(1) receptors the offspring displayed more hyperactivity and responded more strongly to cocaine stimulation as adults than did mice of a WT mother, regardless of their genotype. This indicates that long-term behavioral alterations in the offspring result from the maternal effect of caffeine, and not a direct effect on fetus. WT offspring from WT mother but having a A(1)R Hz grandmother preserved higher locomotor response to cocaine.

CONCLUSIONS/SIGNIFICANCE

We suggest that perinatal caffeine, by acting on adenosine A(1) receptors in the mother, causes long-lasting behavioral changes in the offspring that even manifest themselves in the second generation.

The effects of adenosine A2A receptor antagonists on haloperidol-induced movement disorders in primates

RATIONALE

Adenosine and dopamine interact within the striatum to control striatopallidal output and globus pallidus GABA release. Manipulating striatal adenosine transmission via blockade of the A2A receptor subtype can compensate for the reduced dopamine activity within the striatum that underlies movement disorders such as antipsychotic-induced extrapyramidal syndrome (EPS) and Parkinson's disease (PD). Preclinical studies in the rat have demonstrated that adenosine A2A receptor antagonists can attenuate behaviors reflecting reduced dopamine activity, such as haloperidol-induced catalepsy and hypoactivity.

OBJECTIVES

In the present studies using nonhuman primates, adenosine antagonists were tested against haloperidol-induced EPS in Cebus apella and haloperidol-induced catalepsy in Saimiri sciureus (squirrel monkey). Specifically, the A2A receptor antagonists, SCH 412348 (0.3-30 mg/kg PO) and KW-6002 (3-100 mg/kg PO); the A1/A2A receptor antagonist, caffeine (1-30 mg/kg PO and IM); and the A1 receptor antagonist, DPCPX (3-30 mg/kg PO) were tested in at least one of these models.

RESULTS

SCH 412348 (10-30 mg/kg), KW-6002 (57-100 mg/kg), and caffeine (30 mg/kg) significantly increased the time to EPS onset. Additionally, SCH 412348, KW-6002, and caffeine afforded protection from the onset of EPS for at least 6 h in some of the primates. SCH 412348 (10 mg/kg) and caffeine (10 mg/kg) significantly reduced haloperidol-induced catalepsy. DPCPX produced a very slight attenuation of EPS at 30 mg/kg, but had no effect on catalepsy.

CONCLUSIONS

These findings suggest that adenosine A2A receptor antagonists may represent an effective treatment for the motor impairments associated with both antipsychotic-induced EPS and PD.

Safety pharmacology assessment of central nervous system function in juvenile and adult rats: effects of pharmacological reference compounds

INTRODUCTION

Recent EU/US pediatric legislation and FDA/EMEA guidelines recognize the potential differences in safety profiles of drugs in adults versus young patients. Hence safety studies are recommended to investigate key functional domains of e.g. the developing CNS.

METHODS

Selected psychoactive stimulants (caffeine, d-amphetamine, scopolamine) and depressants (baclofen, diazepam, haloperidol, chlorpromazine, imipramine, morphine) were characterized upon single administration with regard to behavioural parameters, locomotor activity, body temperature, pro-/anti-convulsive activity (pentylenetetrazole, PTZ), and nocifensive responses (hotplate) in neonatal (2 weeks), juvenile (4 weeks) and adult rats (8-9 weeks).

RESULTS

In vehicle-treated rats, behavioural patterns matured with age, locomotor activity and handling-induced rise in body temperature were enhanced, whereas PTZ convulsion threshold dose and nocifensive response latency decreased. Single test compound treatment elicited behavioural effects characteristic for psychoactive drugs with stimulating and depressing properties regardless of age. However, incidence of certain behaviours, and magnitude of effects on locomotor activity and body temperature varied with age and became generally more pronounced in adult rats. Pro-/anti-convulsive effects and delayed nocifensive responses did not differ between juvenile and adult rats.

CONCLUSION

CNS effects of selected psychoactive reference compounds were qualitatively similar, but quantitatively different in neonatal, juvenile and adult rats.

Age-dependent changes in adenosine A1 receptor distribution and density within the nucleus tractus solitarii of normotensive and hypertensive rats

This study shows the distribution and density of adenosine A1 receptor (A1R) within the nucleus tractus solitarii (NTS) of Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) from birth to adulthood (1, 15, 30 and 90 days old). The NTS shows heterogeneous distribution of A1R in dorsomedial/dorsolateral, subpostremal and medial/intermediate subnuclei. A1R decrease from rostral to caudal within dorsomedial/dorsolateral subnucleus in 15-, 30- and 90-day-old WKY and SHR. A1R increase from rostral to caudal subpostremal subnucleus in 30- and 90-day-old WKY, and in 15-, 30- and 90-day-old SHR. Furthermore, A1Rs are increased in SHR as compared with WKY within dorsomedial/dorsolateral in 30- and 90-day-old and within subpostremal of 15-, 30- and 90-day-old rats. Finally, A1Rs increase from 1- to 30-day-old rats. Medial/intermediate did not show any changes in A1R from rostral to caudal levels, age or strain. In summary, our result highlights the importance of A1 adenosine system regarding the neural control of blood pressure and the development of hypertension.

New evidence for purinergic signaling in the olfactory bulb: A2A and P2Y1 receptors mediate intracellular calcium release in astrocytes

Purinergic receptors play a key role in neuron-glia and glia-neuron interactions. In the present study, we have recorded cytosolic Ca(2+) responses using confocal imaging in astrocytes of acute olfactory bulb slices from mice (postnatal days 3-8). By application of agonists and antagonists, we identified two types of receptors, P2Y(1) and A(2A), that mediated Ca(2+) responses attributable to Ca(2+) release from intracellular stores in the astrocytes. Both receptor types were activated by application of ATP and ADP; however, when enzymatic ATP degradation was suppressed by the alkaline phosphatase inhibitor levamisole, ATP only activated MRS2179-sensitive P2Y(1) but not ZM241385-sensitive A(2A) receptors. The dose-response curve for A(2A) receptors activated by adenosine revealed an EC(50) of 0.3 microM, one order of magnitude smaller than the EC(50) of 5 microM determined for P2Y(1) receptors activated by ADP. Electrical stimulation of the olfactory nerve in the presence of glutamate receptor blockers to suppress excitation of postsynaptic neurons evoked Ca(2+) responses in most of the astrocytes, which were inhibited by blocking both P2Y(1) and A(2A) receptors. Our results indicate that olfactory nerve terminals release not only glutamate, but also ATP, which activates P2Y(1) receptors and, after degradation of ATP to adenosine, A(2A) receptors in astrocytes.

Adenosine A2A receptors in ventral striatum, hypothalamus and nociceptive circuitry implications for drug addiction, sleep and pain

Adenosine A2A receptors localized in the dorsal striatum are considered as a new target for the development of antiparkinsonian drugs. Co-administration of A2A receptor antagonists has shown a significant improvement of the effects of l-DOPA. The present review emphasizes the possible application of A2A receptor antagonists in pathological conditions other than parkinsonism, including drug addiction, sleep disorders and pain. In addition to the dorsal striatum, the ventral striatum (nucleus accumbens) contains a high density of A2A receptors, which presynaptically and postsynaptically regulate glutamatergic transmission in the cortical glutamatergic projections to the nucleus accumbens. It is currently believed that molecular adaptations of the cortico-accumbens glutamatergic synapses are involved in compulsive drug seeking and relapse. Here we review recent experimental evidence suggesting that A2A antagonists could become new therapeutic agents for drug addiction. Morphological and functional studies have identified lower levels of A2A receptors in brain areas other than the striatum, such as the ventrolateral preoptic area of the hypothalamus, where adenosine plays an important role in sleep regulation. Although initially believed to be mostly dependent on A1 receptors, here we review recent studies that demonstrate that the somnogenic effects of adenosine are largely mediated by hypothalamic A2A receptors. A2A)receptor antagonists could therefore be considered as a possible treatment for narcolepsy and other sleep-related disorders. Finally, nociception is another adenosine-regulated neural function previously thought to mostly involve A1 receptors. Although there is some conflicting literature on the effects of agonists and antagonists, which may partly be due to the lack of selectivity of available drugs, the studies in A2A receptor knockout mice suggest that A2A receptor antagonists might have some therapeutic potential in pain states, in particular where high intensity stimuli are prevalent.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Extracellular cAMP inhibits D1 dopamine receptor expression in CAD catecholaminergic cells via A2a adenosine receptors

The expression of D1 dopamine (DA) receptor gene is regulated during development, aging, and pathophysiology. The extracellular factors and signaling mechanisms that modulate the expression of D1 DA receptor have not been well characterized. Here, we present novel evidence that endogenous D1 DA receptor expression is inhibited by extracellular cAMP in the Cath.A Derived (CAD) catecholaminergic neuronal cell line. CAD cells express the multi-drug resistance protein 5 transporters and secrete cAMP. Addition of exogenous cAMP decreases D1 receptor mRNA and protein greater than fourfold in 24 h. The cAMP-induced decrease of D1 receptor mRNA levels is blocked by cGMP and by 1,3-dipropyl-8-(p-sulfo-phenyl)xanthine, an inhibitor of ecto-phosphodiestrase. Extracellular AMP, a metabolite of cAMP, also independently decreased D1 receptor mRNA levels. Inhibitors of ecto-nucleotidases, alpha,beta-methyleneadenosine 5'-di-phosphate and GMP, completely blocked the decrease of D1 receptor mRNA by extracellular cAMP, but only partially blocked the decrease induced by extracellular AMP. Levamisole, an inhibitor of tissue non-specific alkaline phosphatase, completely blocked the AMP-induced decrease of D1 receptor mRNA. The extracellular cAMP, AMP, and adenosine (ADO)-induced decrease in D1 receptor mRNA expression are mediated by A2a ADO receptor subtype. The results suggest a novel molecular mechanism linking activation of A2a ADO receptors with inhibition of D1 DA receptor expression.

Improved spatial recognition memory in mice lacking adenosine A2A receptors

Adenosine receptors play an important role in learning and memory as their antagonists have been found to facilitate learning and memory in various tasks in rodents. However, few studies have examined the effect of adenosine A2A receptor deficiency on cognition. In the present study, we therefore used the Y-maze, a simple two-trial recognition test to measure spatial recognition memory in mice lacking adenosine A2A receptors. The results showed that adenosine A2A receptor knockout mice had a higher percentage of novel arm visits as first choice than wild-type CD1 mice. Moreover, these mice showed longer duration of visits in the novel arm when compared with controls, suggesting that the lack of adenosine A2A receptors improved spatial recognition memory. On the other hand, mice lacking the adenosine A2A receptors had low scores in the number of arm visits, suggesting that they were hypoactive. In conclusion, these data suggest the involvement of adenosine receptors in modulating spatial recognition memory in mice, consistent with earlier findings using adenosine receptor antagonists.

Absence of quasi-morphine withdrawal syndrome in adenosine A2A receptor knockout mice

RATIONALE

Caffeine and other methylxanthines induce behavioral activation and anxiety responses in mice via antagonist action at A2A adenosine receptors. When combined with the opioid antagonist naloxone, methylxanthines produce a characteristic quasi-morphine withdrawal syndrome (QMWS) in opiate-naive animals.

OBJECTIVES

The aim of this study was to establish the role of A2A receptors in the quasi-morphine withdrawal syndrome induced by co-administration of caffeine and naloxone and in the behavioral effects of caffeine.

METHODS

We have used A2A receptor knockout (A(2A)R(-/-)) mice in comparison with their wild-type and heterozygous littermates to measure locomotor activity in the open field and withdrawal symptoms induced by caffeine and naloxone. Naïve wild-type and knockout mice were also examined for enkephalin and dynorphin mRNA expression by in situ hybridization and for mu-opiate receptor by ligand binding autoradiography to check for possible opiate receptor changes induced by A2A receptor inactivation.

RESULTS

Caffeine increases locomotion and anxiety in wild-type animals, but it has no psychomotor effects in A(2A)R(-/-) mice. Co-administration of caffeine (20 mg/kg) and naloxone (2 mg/kg) resulted in a severe quasi-morphine withdrawal syndrome in wild-type mice that was almost completely abolished in A(2A)R(-/-) mice. Heterozygous animals exhibited a 40% reduction in withdrawal symptoms, suggesting that there is no genetic/developmental compensation for the inactivation of one of the A(2A)R alleles. A(2A)R(-/-) and wild-type mice have similar levels of striatal mu-opioid receptors, thus the effect is not due to altered opioid receptor expression.

CONCLUSIONS

Our results demonstrate that A2A receptors are required for the induction of quasi-morphine withdrawal syndrome by co-administration of caffeine and naloxone and implicate striatal A2A receptors and mu-opiate receptors in tonic inhibition of motor activity in the striatum.

Adenosine A2A receptors are expressed by GABAergic neurons of medulla oblongata in developing rat

During early development, adenosine contributes to the occurrence of respiratory depression and recurrent apneas. Recent physiological studies indicate that GABAergic mechanisms may be involved in this inhibitory action of adenosine, via their A(2A) receptors. In the present study, in situ hybridization with ribonucleotide probes for A(2A) receptor (A(2A)R) mRNA was combined with the immunolabeling technique for parvalbumin and transneuronal retrograde tracing method using green fluorescent protein expressing pseudorabies virus (GFP-PRV) to (1) characterize age-dependent changes in the expression of adenosine A(2A)Rs mRNA in brain stem regions where GABAergic neurons are located; (2) determine whether GABA-containing neurons express A(2A)R mRNA traits, and (3) identify whether bulbospinal GABAergic neurons projecting to phrenic nuclei contain A(2A)R mRNA. Results revealed expression of A(2A) receptors in regions of medulla oblongata containing GABAergic neurons, namely in the ventral aspect of the medulla, within the Bötzinger region and caudal to it, the gigantocellular reticular nucleus, midline neurons and the caudal ventrolateral medulla oblongata. Furthermore, a subpopulation of identified GABAergic neurons, projecting to the phrenic motor nuclei, possess A(2A)R mRNA. It is concluded that adenosine A(2A)Rs expressed by GABAergic neurons are likely to play a role in mediating adenosine-induced respiratory depression.

Neuroprotection induced by the adenosine A2A antagonist CSC in the 6-OHDA rat model of parkinsonism: effect on the activity of striatal output pathways

In Parkinson's disease (PD), the striatal dopamine depletion and the following overactivation of the indirect pathway of the basal ganglia leads to very early disinhibition of the subthalamic nucleus (STN) that may contribute to the progression of PD by glutamatergic overstimulation of the dopaminergic neurons in the substantia nigra. Adenosine A2A antagonism has been demonstrated to attenuate the overactivity of the striatopallidal pathway. To investigate whether neuroprotection exerted by the A2A antagonist 8-(3-chlorostyryl)caffeine (CSC) correlates with a diminution of the striatopallidal pathway activity, we have examined the changes in the mRNA encoding for enkephalin, dynorphin, and adenosine A2A receptors by in situ hybridization induced by subacute systemic pretreatment with CSC in rats with striatal 6-hydroxydopamine(6-OHDA) administration. Animals received CSC for 7 days until 30 min before 6-OHDA intrastriatal administration. Vehicle-treated group received a solution of dimethyl sulfoxide. CSC pretreatment partially attenuated the decrease in nigral tyrosine hydroxylase immunoreactivity induced by 6-OHDA, whereas no modification of the increase in preproenkephalin mRNA expression in the dorsolateral striatum was observed. The neuroprotective effect of the adenosine A2A antagonist CSC in striatal 6-OHDA-lesioned rats does not result from a normalization of the increase in striatal PPE mRNA expression in the DL striatum, suggesting that other different mechanisms may be involved.

Neuroprotection by adenosine in the brain: From A(1) receptor activation to A (2A) receptor blockade

Adenosine is a neuromodulator that operates via the most abundant inhibitory adenosine A(1) receptors (A(1)Rs) and the less abundant, but widespread, facilitatory A(2A)Rs. It is commonly assumed that A(1)Rs play a key role in neuroprotection since they decrease glutamate release and hyperpolarize neurons. In fact, A(1)R activation at the onset of neuronal injury attenuates brain damage, whereas its blockade exacerbates damage in adult animals. However, there is a down-regulation of central A(1)Rs in chronic noxious situations. In contrast, A(2A)Rs are up-regulated in noxious brain conditions and their blockade confers robust brain neuroprotection in adult animals. The brain neuroprotective effect of A(2A)R antagonists is maintained in chronic noxious brain conditions without observable peripheral effects, thus justifying the interest of A(2A)R antagonists as novel protective agents in neurodegenerative diseases such as Parkinson's and Alzheimer's disease, ischemic brain damage and epilepsy. The greater interest of A(2A)R blockade compared to A(1)R activation does not mean that A(1)R activation is irrelevant for a neuroprotective strategy. In fact, it is proposed that coupling A(2A)R antagonists with strategies aimed at bursting the levels of extracellular adenosine (by inhibiting adenosine kinase) to activate A(1)Rs might constitute the more robust brain neuroprotective strategy based on the adenosine neuromodulatory system. This strategy should be useful in adult animals and especially in the elderly (where brain pathologies are prevalent) but is not valid for fetus or newborns where the impact of adenosine receptors on brain damage is different.