Effect of locally infused 2-chloroadenosine, an A1 receptor agonist, on spontaneous and evoked dopamine release in rat neostriatum

Adenosine: a partially discovered medicinal agent

Background

A plethora of chemicals exists in human body which can alter physiology in one way or other. Scientists have always been astounded by such abilities of chemicals but as the technology advances, even the chemical which was once expected to be well known changes its status to not really well known. Adenosine is one of the chemicals which is in consonance with the aforementioned statements, although previous articles have covered vast information on role of adenosine in cardiovascular physiology, bacterial pathophysiology and inflammatory diseases. In this review we have discussed adenosine and its congeners as potential promising agents in the treatment of Huntington’s disease, post-traumatic stress disorder, erectile dysfunction, viral infections (SARS-CoV) and anxiety.

Main text

Adenosine is a unique metabolite of ATP; which serves in signalling as well. It is made up of adenine (a nitrogenous base) and ribo-furanose (pentose) sugar linked by β-N9-glycosidic bond. Adenosine on two successive phosphorylation forms ATP (Adenosine Triphosphate) which is involved in several active processes of cell. It is also one of the building blocks (nucleotides) involved in DNA (Deoxy-ribonucleic Acid) and RNA (Ribonucleic Acid) synthesis. It is also a component of an enzyme called S-adenosyl-L-methionine (SAM) and cyano-cobalamin (vitamin B-12). Adenosine acts by binding to G protein-coupled receptor (GPCR: A1, A2A, A2B and A3) carries out various responses some of which are anti-platelet function, hyperaemic response, bone remodelling, involvement in penile erection and suppression of inflammation. On the other hand, certain microorganisms belonging to genus Candida, Staphylococcus and Bacillus utilize adenosine in order to escape host immune response (phagocytic clearance). These microbes evade host immune response by synthesizing and releasing adenosine (with the help of an enzyme: adenosine synthase-A), at the site of infection.

Conclusion

With the recent advancement in attribution of adenosine in physiology and pathological states, adenosine and its congeners are being looked forward to bringing a revolution in treatment of inflammation, viral infections, psychiatric and neurodegenerative disorders.

A New Look at an Old Drug: Cumulative Effects of Low Ribavirin Doses in Amphetamine-Sensitized Rats

Background

Psychotic states related to psychostimulant misuse in patients with hepatitis C virus infection may complicate acceptance and reaction to antiviral treatment. This observation equally applies to the widely used ribavirin therapy.

Objective

We examined psychomotor and body weight gain responses to low ribavirin doses after cessation of intermittent amphetamine treatment in adult rats to assess its role in neurobehavioral outcome during psychostimulant withdrawal.

Method

The model of amphetamine-induced (1.5 mg/kg/day, i.p., 7 consecutive days) motor sensitization and affected body weight gain was established in adult male Wistar rats. Then, additional cohort of amphetamine-sensitized rats was subjected to saline (0.9% NaCl; 1 mL/kg/day; i.p.) or ribavirin (10, 20 and 30 mg/kg/day, i.p.) treatment for 7 consecutive days. Animals’ motor activity in a novel environment was monitored after the 1^st and the 7^th saline/ribavirin injection. Body weight gain was calculated as appropriate. Determination and quantification of ribavirin in the brain tissue were performed also.

Results

The 1^st application of ribavirin to amphetamine-sensitized rats affected/decreased their novelty-induced motor activity only at a dose of 30 mg/kg. After the 7^th application, ribavirin 30 mg/kg/day still decreased, while 10 and 20 mg/kg/day increased novelty-induced motor activity. These behavioral effects coincided with the time required to reach maximum ribavirin concentration in the brain. Body weight gain during withdrawal was not influenced by any of the doses tested.

Conclusion

Ribavirin displays central effects that in repeated treatment, depending on the applied dose, could significantly influence psychomotor response but not body weight gain during psychostimulant/amphetamine withdrawal.

Potential antipsychotic action of the selective agonist of adenosine A1 receptors, 5'-Cl-5'-deoxy-ENBA, in amphetamine and MK-801 rat models

Background

Disturbances of dopaminergic and glutamatergic transmissions have been suggested to be involved in the pathomechanisms underlying psychotic symptoms of schizophrenia. In line with this concept, hyperlocomotion induced by the dopaminomimetic amphetamine and the uncompetitive antagonist of NMDA receptors MK-801 (dizocilpine) in rodents is a generally established model for screening of new potential antipsychotic drugs. Since recent studies have indicated that receptors for adenosine may be targets for antipsychotic therapy, the aim of the present study was to investigate an influence of 5′-Cl-5′-deoxy-ENBA, a potent and selective adenosine A₁ receptor agonist, on hyperlocomotion induced by amphetamine and MK-801.

Methods

Locomotor activity was measured by Force Plate Actimeters where four force transducers located below the corners of the floor of the cage tracked the animal position on a Cartesian plane at each time point.

Results

Hyperlocomotion induced by either amphetamine (1 mg/kg sc) or MK-801 (0.3 mg/kg ip) was inhibited by 5′-Cl-5′-deoxy-ENBA (0.1 mg/kg ip). The effect of 5′-Cl-5′-deoxy-ENBA on the amphetamine- and MK-801-induced hyperlocomotion was antagonized by the selective antagonist of adenosine A₁ receptor DPCPX at doses of 1 and 2 mg/kg ip, respectively.

Conclusion

The present study suggests that stimulation of adenosine A₁ receptors may produce antipsychotic effects.

Critical View on the Usage of Ribavirin in Already Existing Psychostimulant-Use Disorder

Substance-use disorder represents a frequently hidden non-communicable chronic disease. Patients with intravenous drug addiction are at high risk of direct exposure to a variety of viral infections and are considered to be the largest subpopulation infected with the hepatitis C virus. Ribavirin is a synthetic nucleoside analog that has been used as an integral component of hepatitis C therapy. However, ribavirin medication is quite often associated with pronounced psychiatric adverse effects. It is not well understood to what extent ribavirin per se contributes to changes in drug-related neurobehavioral disturbances, especially in the case of psychostimulant drugs, such as amphetamine. It is now well-known that repeated amphetamine usage produces psychosis in humans and behavioral sensitization in animals. On the other hand, ribavirin has an affinity for adenosine A1 receptors that antagonistically modulate the activity of dopamine D1 receptors, which play a critical role in the development of behavioral sensitization. This review will focus on the current knowledge of neurochemical/ neurobiological changes that exist in the psychostimulant drug-addicted brain itself and the antipsychotic-like efficiency of adenosine agonists. Particular attention will be paid to the potential side effects of ribavirin therapy, and the opportunities and challenges related to its application in already existing psychostimulant-use disorder.

Adenosine role in brain functions: Pathophysiological influence on Parkinson's disease and other brain disorders

Although adenosine plays a key role in multiple motor, affective, and cognitive processes, it has received less attention in the neuroscience field compared to other neurotransmitters (e.g., dopamine). In this review, we highlight the role of adenosine in behavior as well as its interaction with other neurotransmitters, such as dopamine. We also discuss brain disorders impacted by alterations to adenosine, and how targeting adenosine can ameliorate Parkinson's disease motor symptoms. We also discuss the role of caffeine (as an adenosine antagonist) on cognition as well as a neuroprotective agent against Parkinson's disease (PD).

Design and characterization of optimized adenosine A₂A/A₁ receptor antagonists for the treatment of Parkinson's disease

The design and characterization of two, dual adenosine A(2A)/A(1) receptor antagonists in several animal models of Parkinson's disease is described. Compound 1 was previously reported as a potential treatment for Parkinson's disease. Further characterization of 1 revealed that it was metabolized to reactive intermediates that caused the genotoxicity of 1 in the Ames and mouse lymphoma L51784 assays. The identification of the metabolites enabled the preparation of two optimized compounds 13 and 14 that were devoid of the metabolic liabilities associated with 1. Compounds 13 and 14 are potent dual A(2A)/A(1) receptor antagonists that have excellent activity, after oral administration, across a number of animal models of Parkinson's disease including mouse and rat models of haloperidol-induced catalepsy, mouse and rat models of reserpine-induced akinesia, and the rat 6-hydroxydopamine (6-OHDA) lesion model of drug-induced rotation.

Adenosine A(2A) Receptor Antagonists and Parkinson's Disease

This Review summarizes and updates the work on adenosine A(2A) receptor antagonists for Parkinson's disease from 2006 to the present. There have been numerous publications, patent applications, and press releases within this time frame that highlight new medicinal chemistry approaches to this attractive and promising target to treat Parkinson's disease. The Review is broken down by scaffold type and will discuss the efforts to optimize particular scaffolds for activity, pharmacokinetics, and other drug discovery parameters. The majority of approaches focus on preparing selective A(2A) antagonists, but a few approaches to dual A(2A)/A(1) antagonists will also be highlighted. The in vivo profiles of compounds will be highlighted and discussed to compare activities across different chemical series. A clinical report and update will be given on compounds that have entered clinical trials.

Dopamine release is involved in antinociceptive effect of theophylline

The methylxanthines, e.g., theophylline, are widely used for the treatment of bronchial asthma. Additionally, a pain relieving effect of theophylline has been reported in patients as well as in experimental animals. The mechanism of this antinociceptive action is not clear. In this study, involvement of dopaminergic system in theophylline-induced antinociception was evaluated using tail flick test model. Swiss albino mice, (either sex, weighing 25-30 g) with base line tail flick latencies (TFL) between 2.0 and 3.5 s, were used. TFL was recorded before and at intervals of 15, 30, 45, and 60 min. after drug treatment. The experimental protocol was duly approved by the Institutional Animal Ethics Committee, All India Institute of Medical Sciences, New Delhi, India. To determine the role of dopaminergic system, the mice were pretreated with either D1 or D2 dopaminergic receptor antagonists SCH 23390 and haloperidol, respectively, prior to treatment with theophylline. Another group of animals received apomorphine along with theophylline. The dose of theophylline used, i.e., 10 mg/kg, intraperitoneally (i.p.), had shown a significant increase in TFLs. The theophylline-induced antinociception, 10 mg/kg, i.p., was reversed by pretreatment with both D1 and D2 dopaminergic receptor antagonists SCH 23390 and haloperidol as well as with apomorphine (1 mg/kg) pretreatment. The results suggest that theophylline-induced antinociception is due to release of dopamine.

P2 receptors are involved in the mediation of motivation-related behavior

The importance of purinergic signaling in the intact mesolimbic–mesocortical circuit of the brain of freely moving rats is reviewed. In the rat, an endogenous ADP/ATPergic tone reinforces the release of dopamine from the axon terminals in the nucleus accumbens as well as from the somatodendritic region of these neurons in the ventral tegmental area, as well as the release of glutamate, probably via P2Y₁ receptor stimulation. Similar mechanisms may regulate the release of glutamate in both areas of the brain. Dopamine and glutamate determine in concert the activity of the accumbal GABAergic, medium-size spiny neurons thought to act as an interface between the limbic cortex and the extrapyramidal motor system. These neurons project to the pallidal and mesencephalic areas, thereby mediating the behavioral reaction of the animal in response to a motivation-related stimulus. There is evidence that extracellular ADP/ATP promotes goal-directed behavior, e.g., intention and feeding, via dopamine, probably via P2Y₁ receptor stimulation. Accumbal P2 receptor-mediated glutamatergic mechanisms seem to counteract the dopaminergic effects on behavior. Furthermore, adaptive changes of motivation-related behavior, e.g., by chronic succession of starvation and feeding or by repeated amphetamine administration, are accompanied by changes in the expression of the P2Y₁ receptor, thought to modulate the sensitivity of the animal to respond to certain stimuli.

A role for adenosine A(1) receptor blockade in the ability of caffeine to promote MDMA "Ecstasy"-induced striatal dopamine release

Co-administration of caffeine profoundly enhances the acute toxicity of 3,4 methylenedioxymethamphetamine (MDMA) in rats. The aim of this study was to determine the ability of caffeine to impact upon MDMA-induced dopamine release in superfused brain tissue slices as a contributing factor to this drug interaction. MDMA (100 and 300μM) induced a dose-dependent increase in dopamine release in striatal and hypothalamic tissue slices preloaded with [(3)H] dopamine (1μM). Caffeine (100μM) also induced dopamine release in the striatum and hypothalamus, albeit to a much lesser extent than MDMA. When striatal tissue slices were superfused with MDMA (30μM) in combination with caffeine (30μM), caffeine enhanced MDMA-induced dopamine release, provoking a greater response than that obtained following either caffeine or MDMA applications alone. The synergistic effects in the striatum were not observed in hypothalamic slices. As adenosine A(1) receptors are, one of the main pharmacological targets of caffeine, which are known to play an important role in the regulation of dopamine release, their role in the modulation of MDMA-induced dopamine release was investigated. 1μM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a specific A(1) antagonist, like caffeine, enhanced MDMA-induced dopamine release from striatal slices while 1μM 2,chloro-N(6)-cyclopentyladenosine (CCPA), a selective adenosine A(1) receptor agonist, attenuated this. Treatment with either SCH 58261, a selective A(2A) receptor antagonist, or rolipram, a selective PDE-4 inhibitor, failed to reproduce a caffeine-like effect on MDMA-induced dopamine release. These results suggest that caffeine regulates MDMA-induced dopamine release in striatal tissue slices, via inhibition of adenosine A(1) receptors.

A Role for Adenosine A1 Receptors in GABA and NMDA-Receptor Mediated Modulation of Dopamine Release: Studies Using Fast Cyclic Voltammetry

In the striatum many neurotransmitters including GABA, glutamate, acetylcholine, dopamine, nitric oxide and adenosine interact to regulate synaptic transmission. Dopamine release in the striatum is regulated by a number of pre- and post-synaptic receptors including adenosine. We have recently shown using isolated rat striatal slices, and the technique of fast cyclic voltammetry, that adenosine A₁ receptor-mediated inhibition of dopamine release is modulated by dopamine D₁ receptors. In the present study we have investigated the influence of NMDA and GABA receptor activation on the modulation of electrically stimulated dopamine release by adenosine. Application of the adenosine A₁ receptor agonist, N⁶-cyclopentyladenosine (CPA), concentration-dependently inhibited dopamine release to a maxiumum of 50%. Perfusion of the glutamate receptor agonist, NMDA, in low magnesium, caused a rapid and concentration-dependent inhibition of dopamine release. Prior perfusion with the adenosine A₁ receptor antagonist, DPCPX, significantly reduced the effect of 5 μM and 10 μM NMDA on dopamine release. The GABA_A receptor agonist, isoguvacine, had a significant concentration-dependent inhibitory effect on dopamine release which was reversed by prior application of the GABA_A receptor antagonist, picrotoxin, but not DPCPX. Finally inhibition of dopamine release by CPA (1μM) was significantly enhanced by prior perfusion with picrotoxin. These data demonstrate an important role for GABA, NMDA and adenosine in the modulation of dopamine release.

Differential glutamate-dependent and glutamate-independent adenosine A1receptor-mediated modulation of dopamine release in different striatal compartments

Adenosine and dopamine are two important modulators of glutamatergic neurotransmission in the striatum. However, conflicting reports exist about the role of adenosine and adenosine receptors in the modulation of striatal dopamine release. It has been previously suggested that adenosine A(1) receptors localized in glutamatergic nerve terminals indirectly modulate dopamine release, by their ability to modulate glutamate release. In the present study, using in vivo microdialysis, we provide evidence for the existence of a significant glutamate-independent tonic modulation of dopamine release in most of the analyzed striatal compartments. In the dorsal, but not in the ventral, part of the shell of the nucleus accumbens (NAc), blockade of A(1) receptors by local perfusion with the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dimethyl-xanthine or by systemic administration of the non-selective adenosine antagonist caffeine induced a glutamate-dependent release of dopamine. On the contrary, A(1) receptor blockade induced a glutamate-independent dopamine release in the core of the NAc and the nucleus caudate-putamen. Furthermore, using immunocytochemical and functional studies in rat striatal synaptosomes, we demonstrate that a fraction of striatal dopaminergic terminals contains adenosine A(1) receptors, which directly inhibit dopamine release independently of glutamatergic transmission.

Role of adenosine A1 receptors in the modulation of dopamine D1 and adenosine A2a receptor signaling in the neostriatum

Adenosine is known to modulate the function of neostriatal neurons. Adenosine acting on A(2A) receptors increases the phosphorylation of dopamine- and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32) at Thr34 (the cAMP-dependent protein kinase [PKA] site) in striatopallidal neurons, and opposes dopamine D2 receptor signaling. In contrast, the role of adenosine A(1) receptors in the regulation of dopamine/DARPP-32 signaling is not clearly understood. Here, we investigated the effect of adenosine A(1) receptors on D(1), D(2) and A(2A) receptor signaling using mouse neostriatal slices. An A(1) receptor agonist, 2-chloro-N(6)-cyclopentyladenosine (100 nM), caused a transient increase, followed by a transient decrease, in DARPP-32 Thr34 phosphorylation. Our data support the following model for the actions of the A(1) receptor agonist. The A(1) receptor-induced early increase in Thr34 phosphorylation was mediated by presynaptic inhibition of dopamine release, and the subsequent removal of tonic inhibition by D(2) receptors of A(2A) receptor/G(olf)/cAMP/PKA signaling. The A(1) receptor-induced late decrease in Thr34 phosphorylation was mediated by a postsynaptic G(i) mechanism, resulting in inhibition of D(1) and A(2A) receptor-coupled G(olf)/cAMP/PKA signaling in direct and indirect pathway neurons, respectively. In conclusion, A(1) receptors play a major modulatory role in dopamine and adenosine receptor signaling.

In vivo co-ordinated interactions between inhibitory systems to control glutamate-mediated hippocampal excitability

We present an overview of the long-term adaptation of hippocampal neurotransmission to cholinergic and GABAergic deafferentation caused by excitotoxic lesion of the medial septum. Two months after septal microinjection of 2.7 nmol alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), a 220% increase of GABA(A) receptor labelling in the hippocampal CA3 and the hilus was shown, and also changes in hippocampal neurotransmission characterised by in vivo microdialysis and HPLC. Basal amino acid and purine extracellular levels were studied in control and lesioned rats. In vivo effects of 100 mm KCl perfusion and adenosine A(1) receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on their release were also investigated. In lesioned animals GABA, glutamate and glutamine basal levels were decreased and taurine, adenosine and uric acid levels increased. A similar response to KCl infusion occurred in both groups except for GABA and glutamate, which release decreased in lesioned rats. Only in lesioned rats, DPCPX increased GABA basal level and KCl-induced glutamate release, and decreased glutamate turnover. Our results evidence that an excitotoxic septal lesion leads to increased hippocampal GABA(A) receptors and decreased glutamate neurotransmission. In this situation, a co-ordinated response of hippocampal retaliatory systems takes place to control neuron excitability.

The time-course of ribavirin-provoked changes of basal and AMPH-induced motor activities in rats

The time-course of changes of basal and amphetamine (AMPH)-induced locomotor and stereotypic activities in adult male Wistar rats after a single ribavirin injection was studied. In the first set of experiments, 10, 20 or 30 mg ribavirin/kg body weight (b.w.) were injected i.p. to rats and their basal motor activities were recorded every 10 min for 2 h and compared with those of saline-treated controls. In the second set of experiments, the animals were pretreated with ribavirin and 20 min later i.p. injected with AMPH (1.5 mg/kg b.w.). The controls received AMPH 20 min after the saline injection. Motor activity was recorded after the first injection and until 120 min after AMPH administration. Ribavirin did not significantly affect the time-course of either basal locomotor or stereotypic activities. Pretreatment with any of the applied ribavirin doses decreased the AMPH-induced hyperlocomotor response. However, the most pronounced effect was observed with ribavirin doses of 20 mg/kg and 30 mg/kg when administered during the first 10 min and 30 min after the AMPH injection respectively. In contrast, the stereotypic activities of these animals were only slightly changed. These results indicate a different susceptibility of regions in the basal ganglia to ribavirin.

Striatal adenosine A(2A) receptor blockade increases extracellular dopamine release following l-DOPA administration in intact and dopamine-denervated rats

The influence of the selective adenosine A(2A) receptor antagonist ZM 241385 on exogenous l-DOPA-derived dopamine (DA) release in intact and dopamine-denervated rats was studied using an in vivo microdialysis in freely moving animals. Local infusion of l-DOPA (2.5 microM) produced a marked increase in striatal extracellular DA level in intact and malonate-lesioned rats. Intrastriatal perfusion of ZM 241385 (50-100 microM) had no effect on basal extracellular DA level, but enhanced dose-dependently the l-DOPA-induced DA release in intact and malonate-lesioned animals. A non-selective adenosine A(2A) receptor antagonist DMPX (100 microM), similarly to ZM 241385, accelerated conversion of l-DOPA in intact and malonate-denervated rats. This effect was not produced by the adenosine A(1) receptor antagonist, CPX (10-50 microM). However, ZM 241385 did not affect the l-DOPA-induced DA release in rats pretreated with reserpine (5 mg/kg i.p.) and alpha-methyl-p-tyrosine (AMPT, 300 mg/kg i.p.). Obtained results indicate that blockade of striatal adenosine A(2A) receptors increases the l-DOPA-derived DA release possibly by indirect mechanism exerted on DA terminals, an effect dependent on striatal tyrosine hydroxylase activity. Selective antagonists of adenosine A(2A) receptors may exert a beneficial effect at early stages of Parkinson's disease by enhancing the therapeutic efficacy of l-DOPA applied exogenously.

Effects of carbamazepine on morphine-induced behavioral sensitization in mice

Effects of carbamazepine on behavioral sensitization to morphine in mice has been investigated. Mice treated daily for 7 days with morphine (10 mg/kg) induced behavioral sensitization. Carbamazepine (10, 20, 40 mg/kg, i.p.) itself dose-dependently inhibited the locomotor activity of mice, but did not affect the acute morphine induced hyperactivity. Chronic treatment with carbamazepine had no effect on the development of morphine behavioral sensitization. Co-administration of carbamazepine 30 min prior to morphine had no significant effect on the development of behavioral sensitization. After the behavioral sensitization has been established, carbamazepine (10, 20, 40mg/kg, i.p.) did not affect the expression of morphine sensitization. However, carbamazepine (10, 20, 40mg/kg, i.p.) dose-dependently potentiated the transfer of morphine sensitization. The data of the present study implies that carbamazepine may influence the maintenance process of behavioral sensitization, which results in the enhancement of the transfer of behavioral sensitization. In clinic, the present results suggest that chronic use of carbamazepine might have abuse potential in opioid abusers.

Opposite modulatory roles for adenosine A1 and A2A receptors on glutamate and dopamine release in the shell of the nucleus accumbens. Effects of chronic caffeine exposure

Previous studies have demonstrated opposing roles for adenosine A1 and A2A receptors in the modulation of extracellular levels of glutamate and dopamine in the striatum. In the present study, acute systemic administration of motor-activating doses of the A2A receptor antagonist MSX-3 significantly decreased extracellular levels of dopamine and glutamate in the shell of the rat nucleus accumbens (NAc) and counteracted both dopamine and glutamate release induced by systemic administration of motor-activating doses of either the A1 receptor antagonist CPT or caffeine. Furthermore, exposure to caffeine in the drinking water (1 mg/mL, 14 days) resulted in tolerance to the effects of systemic injection of CPT or caffeine, but not MSX-3, on extracellular levels of dopamine and glutamate in the NAc shell. The present results show: first, the existence of opposite tonic effects of adenosine on extracellular levels of dopamine and glutamate in the shell of the NAc mediated by A1 and A2A receptors; second, that complete tolerance to caffeine's dopamine- and glutamate-releasing effects which develops after chronic caffeine exposure is attributable to an A1 receptor-mediated mechanism. Development of tolerance to the dopamine-releasing effects of caffeine in the shell of the NAc may explain its weak addictive properties and atypical psychostimulant profile.

The effects of tiazofurin on basal and amphetamine-induced motor activity in rats

The effects of tiazofurin (TR; 2-beta-d-ribofuranosylthiazole-4-carboxamide), a purine nucleoside analogue on basal and amphetamine (AMPH)-induced locomotor and stereotypic activity of adult Wistar rat males were studied. The animals were injected with low (3.75, 7.5, and 15 mg/kg ip) and high (62.5, 125, and 250 mg/kg ip) TR doses. Neither low nor high TR doses influenced basal locomotor and stereotypic activity in comparison with the corresponding controls treated with saline only. However, pretreatment with TR at any dose applied, except for the lowest one, significantly decreased AMPH-induced (1.5 mg/kg ip) locomotor activity, while AMPH-induced stereotypic activity was inhibited with the two highest TR doses. In addition, TR was detected in the brain by HPLC already 15 min after the injection (125 mg/kg ip) to reach a maximum 2 h after the administration and was detectable in this tissue during the next 4 h. Our results indicate that TR modifies central regulation of the motor activity, possibly by influencing dopaminergic (DA-ergic) transmission.

Effect of the adenosine A2A receptor antagonist 8-(3-chlorostyryl)caffeine on l-DOPA biotransformation in rat striatum

In the present study, we investigated effects of the new selective adenosine A2A receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) on L-DOPA-induced dopamine (DA) release in the striatum of intact and reserpine-treated rats. CSC given in a pharmacologically effective dose of 5 mg/kg i.p. significantly increased striatal DA release after joint administration of L-DOPA (100 mg/kg, i.p.) and benserazide (50 mg/kg, i.p.) to intact and reserpine (2.5 mg/kg, s.c.)-injected rats. CSC did not change the elevated level of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in intact rats, but raised it in DA-depleted animals. The availability of exogenous L-DOPA in the extracellular space was similar and equally increased by CSC in both intact and reserpinized rats. Our results suggest that the observed effects may be mediated by striatal adenosine A2A receptors, and are probably related to the CSC action on DA metabolism and the increased transport of exogenous L-DOPA into the brain. These observations might be of relevance, considering the use of selective A2A antagonists as potential supplements to L-DOPA therapy of Parkinson's disease.

Modulation of feeding behaviour by blocking purinergic receptors in the rat nucleus accumbens: a combined microdialysis, electroencephalographic and behavioural study

The nonspecific P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), the nonspecific P1 receptor antagonist 8-(p-sulphophenyl)-theophylline (8-SPT) and the combination of both were applied by retrograde microdialysis into the nucleus accumbens (NAc) before and during feeding of 18-h food-deprived rats. In addition to the registration of behavioural parameters, such as the amount and duration of food intake, the feeding-induced changes in dopamine (DA) concentration and the concomitant changes of neuronal activity in the NAc and the ventral tegmental area (VTA) were simultaneously determined. The perfusion with PPADS (20 microm) diminished the amount of food intake and the duration of feeding. Furthermore, the P2 receptor antagonist blocked the feeding-induced DA release and prevented the feeding-elicited changes of the electroencephalography (EEG) power distribution which was characterised by an increase in the power of the 8.0-13.0-Hz frequency band in the NAc and the VTA. The effects of PPADS could be completely prevented by the concomitantly perfused adenosine receptor antagonist 8-SPT (100 microm). When given alone, 8-SPT increased the amount of food ingested, the duration of feeding and the EEG power of the higher frequency range, particularly between 19.0 and 30.0 Hz, in both the NAc and the VTA. The feeding-elicited DA release was supplemented to the enhanced DA level caused by the perfusion with 8-SPT in an additive manner. The P2 and P1 receptor antagonists interact antagonistically in the modulation of feeding behaviour and the feeding-induced changes of EEG activity suggesting that both endogenous extracellular ATP and adenosine are involved in the regulation of the feeding-associated mesolimbic neuronal activity in a functionally antagonistic manner.

Anticonvulsant dicarboxyphenylglycines differentially modulate excitatory amino acid release in the rat cerebral cortex

The 3,4-dicarboxyphenylglycines (3,4-DCPGs) have recently been shown to be effective new anticonvulsant agents in a rodent model of epilepsy, with the racemic mixture showing significantly greater potency than either isomer alone. The (R)-isomer has been identified as a competitive AMPA-type ionotropic glutamate receptor antagonist, whilst (S)-3,4-DCPG is a highly potent and selective metabotropic glutamate receptor 8 (mGlu8 receptor) agonist. We now report the inhibitory activity of (R)- and (RS)-3,4-DCPG, but not (S)-3,4-DCPG, against both 35 mM and 50 mM KCl-evoked glutamate release in the rat cerebral cortex in vitro. In contrast to the anticonvulsant actions of the 3,4-DCPGs, no evidence was obtained for a synergistic inhibitory interaction between the separate isomers. We conclude that whilst inhibition of cortical excitatory amino acid release may contribute to the anticonvulsant actions of (RS)-3,4-DCPG, it does not represent the sole mechanism of action. Synergistic interactions between ligands acting at different subtypes of ionotropic and metabotropic glutamate receptors remains a promising new strategy for the treatment of currently drug-refractory seizure states.

Enabling role of adenosine A1 receptors in adenosine A2A receptor-mediated striatal expression of c-fos

When striatal neurons are strongly activated they produce adenosine, which activates nearby adenosine A1 receptors (A1Rs) and adenosine A2A receptors (A2ARs). Although the effects of A1R or A2AR activation on neural activity in the striatum have been examined separately, the effects of coactivating both receptors has not been investigated. Using c-Fos immunohistochemistry as an indicator of neural activity, we examined the effects of coactivation of A1Rs and A2ARs on neural activity and their mechanism of interaction in the caudate-putamen, nucleus accumbens (NAc) and prefrontal cortex in rats. Administration of a motor-depressant dose of the A2AR agonist CGS 21680 (0.5 mg/kg i.p.) did not significantly induce c-fos expression in any of these brain regions. Administration of a motor-depressant dose of the A1R agonist CPA (0.3 mg/kg, i.p.) produced a small but significant induction of c-fos expression only in the shell of the NAc. Coadministration of CGS 21680 and CPA produced a synergistic induction of c-fos expression in the caudate-putamen, cingulate cortex, and especially the NAc. In the shell of the NAc administration of CPA significantly decreased extracellular dopamine levels measured by in vivo microdialysis and blocked CGS 21680-induced increases in dopamine levels. Because it has been previously shown that activation of dopamine D2 receptors (D2Rs) by endogenous dopamine blocks A2AR-mediated c-fos expression, it is hypothesized that the enabling role of A1Rs in A2AR-mediated striatal c-fos expression is related to the A1R-mediated inhibition of dopamine release.

Purinergic modulation of neuronal activity in the mesolimbic dopaminergic system in vivo

ATP and its metabolite adenosine activate membrane receptors, termed P2 and P1, respectively. In the present study, the modulation of the mesolimbic neuronal circuit by ATPergic and adenosinergic mechanisms was investigated by microdialysis in the nucleus accumbens (NAc) and by telemetrically recorded EEG from both the NAc and the ventral tegmental area (VTA) of freely moving rats. The basal extracellular dopamine concentration was enhanced after accumbal perfusion with the ATP analog 2-methylthio ATP (2-MeSATP; 100 microM); by contrast, adenosine (100 microM) caused a reduction of extracellular dopamine. When given alone, the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 20 microM) decreased the concentration of dopamine, whereas the P1 receptor antagonist 8-(p-sulfophenyl)theophylline (8-SPT; 100 microM) increased it. In the same animals, P2 receptor stimulation by 2-MeSATP caused neuronal activation, indicated by an elevation of the absolute power in the EEG of the NAc mainly by enhancement of the relative power in the alpha band (8-13 Hz) of the EEG spectrum. By contrast, adenosine led to a depression of the absolute power in the VTA accompanied by an elevation of the delta-band power (0.4-6 Hz) in the NAc corresponding to a slowing of neuronal activity. When given alone, PPADS reduced the absolute EEG power in the NAc accompanied by a decrease in the high-frequency power, but had no effects on the VTA. 8-SPT on its own enhanced the total power in both the NAc and the VTA, reflected by an enhancement in the slow and the high-frequency bands. Whereas the 8-SPT-evoked changes of EEG pattern as well as of dopamine concentration in the NAc were abolished by the co-application of PPADS, the 8-SPT-induced EEG changes in the VTA persisted under these conditions. In conclusion, the accumbal neuronal output, reflected by both dopamine release and neuronal electrical activity, is modulated in a functionally antagonistic manner by P2 and P1 receptor stimulation. It is suggested that an inhibitory GABAergic feedback projection to the VTA is stimulated by adenosine, either directly or indirectly via glutamate release.

2-chloro-N(6)-cyclopentyladenosine-elicited attenuation of evoked glutamate release is not sufficient to give complete protection against pilocarpine-induced seizures in rats

The effects of 2-chloro-N(6)-cyclopentyladenosine (CCPA) perfused intrahippocampally (1 microM) and injected intraperitoneally (0.5 mg/kg) were investigated in focally-evoked pilocarpine-induced (10 mM) seizures in freely moving rats. While the intrahippocampal perfusion of this highly selective adenosine A(1) receptor agonist gave complete protection against pilocarpine-induced seizures, systemic administration only partially protected the animals, as evaluated by concomitant behavioural and electrocorticographical (ECoG) observations and monitoring of the neurotransmitter alterations. However, pilocarpine-evoked elevation of hippocampal glutamate overflow was significantly attenuated by CCPA irrespective of the mode of administration. Acute pretreatment with systemic 8-cyclopentyl-1,3-dipropylxanthine, a selective A(1) antagonist, reversed both the partial protective effect and the attenuating effect on the extracellular glutamate elicited by systemic CCPA administration. Intrahippocampal CCPA markedly reduced basal hippocampal dopamine efflux but not GABA or glutamate and considerably attenuated the pilocarpine-evoked elevation in dopamine levels. Systemic CCPA appeared to have little influence on the overall pattern of dopamine elevation. The findings give evidence that CCPA-elicited abatement of the evoked glutamate release alone, cannot fully account for its anticonvulsant effect and may suggest that the effects mediated by adenosine on postsynaptic adenosine receptors could be more crucial for its anticonvulsant effect.

Effect of adenosine kinase, adenosine deaminase and transport inhibitors on striatal dopamine and stereotypy after methamphetamine administration

The effect of adenosine kinase (AKA), adenosine deaminase (ADA) and transport inhibitors on the release of dopamine (DA) induced by methamphetamine (MTH) in rat striatum was assessed using in vivo microdialysis in freely moving rats. MTH injected in a dose of 3 x 5 mg/kg i.p. at 2-hour intervals produced a massive release of DA. This excessive release of DA was inhibited by the ADA inhibitor 2'-deoxycoformycin (DCF), the AKA inhibitor 5'-iodotubercidin (IOT) and the adenosine uptake inhibitor dilazep (DIL), each of them given locally to the striatum via a microdialysis probe at a concentration of 100 microM. Perfusion with the same concentrations of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and 5'-amino-5'-deoxyadenosine (NH(2)dAD), ADA and AKA inhibitors, respectively, induced a considerably weaker effect on DA release. The non-selective antagonist of adenosine A(1)/A(2A) receptor caffeine (75 microM) significantly prevented the inhibitory effect of DCF, IOT and DIL on the MTH-induced DA release. Intrastriatal administration of DCF, IOT and DIL (5 nmol/microl before each injection of MTH) inhibited the stereotypy induced by MTH. The striatal content of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), decreased by MTH administration and measured 5 days after treatment with the toxin, was reversed by all the inhibitors at the order of potency as follows: IOT>DCF>DIL. Direct agonists of adenosine A(1) and A(1)/A(2A) receptors, N(6)-cyclopentyladenosine (CPA) and 5'-N-ethylcarboxamidoadenosine (NECA), respectively, given intrastriatally (5 nmol/microl) completely abolished the MTH-induced stereotypy and the fall in the striatal content of DA, DOPAC and HVA. The above results show that augmentation of endogenous adenosine in rat striatum by inhibition of its metabolism or uptake-despite the differences in the efficacy of various inhibitors-may provide neuroprotection against a toxic action of MTH.

Roles of adenosine A(1) and A(2A) receptors in the expression and development of methamphetamine-induced sensitization

We studied the effects of adenosine A(1) and A(2A) receptor agonists on the expression and development of methamphetamine-induced sensitization in rats. When animals were treated with the adenosine A(1) receptor agonist, N(6)-cyclohexyladenosine (CHA), along with methamphetamine every 3 days with a total of five administrations, the augmentation of hyperlocomotion by methamphetamine re-administration after 7-day withdrawal (methamphetamine challenge administration) was not inhibited. However, when the adenosine A(2A) receptor agonist, 2-p-(2-carboxyethyl) phenethyl-amino-5'-N-ethylcarboxy-amide adenosine (CGS21680), was administered according to the same schedule, the augmentation was significantly inhibited. On the other hand, when CHA or CGS21680 was administered 30 min before methamphetamine challenge, both drugs dose-dependently inhibited the augmentation of hyperlocomotion. These results suggested that both adenosine A(1) and A(2A) receptors play important roles in the expression of methamphetamine-induced sensitization, and that adenosine A(2A) receptors do so in the development of this sensitization.

Agonists of A1 and A2A adenosine receptors attenuate methamphetamine-induced overflow of dopamine in rat striatum

The effect of adenosine receptor agonists on the release of striatal dopamine (DA), induced by repeated doses of methamphetamine (MTH), was evaluated. Rats received three injections of MTH (5 mg/kg i.p.) at 2-h intervals. The release of DA in the striatum was measured by a microdialysis in freely moving animals. The agonist of adenosine A1 receptor, N6-cyclopentyladenosine (CPA) and the agonist of adenosine A2A receptor, 2-[p-(carboxy-ethyl)phenylethylamino]-5'-N-ethylcarboxyamidoade nosine (CGS 21680), either of them being infused locally into the striatum at concentrations of 50 and 100 microM, produced decreases in the extracellular DA level during exposure to MTH, and a weaker effect on the levels of DOPAC and HVA. The above effects were reversed by the specific antagonists of adenosine A1 and A2A receptors, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 3, 7-dimethyl-1-propargylxanthine (DMPX), respectively. Our results indicate that both the A1 and A2A adenosine receptors appear to be involved in reducing the excessive release of DA in the striatum; furthermore, they suggest a neuroprotective role of adenosine in MTH neurotoxicity.

Caffeine mimics the effect of a dopamine D2/3 receptor agonist on the expression of immediate early genes in globus pallidus

In order to evaluate the effect of caffeine on striatopallidal neurons we used in situ hybridization to examine the mRNA expression of the immediate early genes (IEGs), c-fos, fos B, c-jun, jun B, NGFI-A and NGFI-B in globus pallidus in rats given single or repeated administration of caffeine. A significant induction of c-fos mRNA, but not of any of the other IEGs, was found 2, 4 and 8 hr after a single injection of 50 mg/kg caffeine. Following repeated injections of caffeine for 2 weeks a single challenge with caffeine did not induce the expression of any of the studied genes. The ability of caffeine to increase pallidal c-fos mRNA expression was mimicked by the dopamine D2/3 receptor agonist quinpirole (1 or 3 mg/kg), whereas the dopamine D2/3 receptor antagonist raclopride (2 mg/kg) was ineffective. Caffeine and quinpirole did not have synergistic effects when given together. The caffeine-induced c-fos mRNA expression was not counteracted by concomitant treatment with raclopride. The present data provide evidence that acute treatment with caffeine reduces the activity of the striatopallidal neuron, and since this neuron is inhibitory the result is an increased activity in globus pallidus. The effect of blocking the striatal A2A receptors with caffeine is essentially identical to that observed after activation of dopamine D2 receptors, but is independent of these receptors. The fact that pallidal c-fos mRNA expression decreased upon repeated administration of caffeine may be related to the development of tolerance to locomotion stimulation that occurs following chronic caffeine ingestion.

The protective effect of 2-chloroadenosine against the development of amygdala kindling and on amygdala-kindled seizures

The influence of 2-chloroadenosine, a non-metabolizable adenosine A1 receptor agonist, was tested on the development of electrically kindled amygdala and on the seizure responses of fully kindled rats. Focal intra-amygdaloid injection of 2-chloroadenosine (1-10 nmol/0.5 microl) 20 min before applying the daily kindling stimulus prevented the development of the kindling process. The behavioural seizure score and the afterdischarge duration were reduced below their initial values. The antiepileptogenic effects of 1 and 10 nmol of 2-chloroadenosine were reversible 8-10 days after withdrawal of the drug. When 2-chloroadenosine was tested on fully developed stage 5 amygdala-kindled seizures, it increased the generalised seizure threshold in a dose-dependent manner. A maximum efficiency of 125% (P < 0.001) was achieved with 5 nmol and the median effective dose was 0.55 nmol. Higher doses resulted in the reduced anticonvulsant effect (P < 0.05). With the same daily stimulation, 2-chloroadenosine 5 nmol in 0.5 microl vehicle, significantly reduced the maximum seizure score by 90%, the afterdischarge duration by 88% and completely blocked the generalised seizure duration. The antiseizure activity of the drug lasted for 3 days. In conclusion, 2-chloroadenosine not only acts as an anticonvulsant against electrically induced kindled seizures as described here, and against audiogenic seizures, electroshock and a variety of chemical convulsants as described by others, it prevents the development of the epileptic state by kindling-stimulation, i.e., it is antiepileptogenic. We theorise here that this is due to its blockade of presynaptic glutamate release.

Systemic and intrastriatal theophylline have opposite effects on dopamine and dopamine metabolites measured by intrastriatal microdialysis in the rat

Using a model of intrastriatal microdialysis, we studied the effect of theophylline, an A1 and A2A adenosine receptor antagonist on striatal dopamine (DA) and DA metabolites. Systemic administration of theophylline (10 and 50 mg/kg) significantly reduced striatal extracellular (EC) levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxy-phenylacetic acid (HVA). Intrastriatal administration of theophylline (10(-2) M) significantly increased DA and its metabolites (DA1 + 120%; DOPAC, +28%; HVA, +30%). Contradictory effects of systemic and intrastriatal theophylline point to theophylline interactions with different receptors possibly at different locations.