Adenosine-dopamine interactions in the brain

The role of dopamine in the behavioral effects of caffeine in animals and humans

Dopamine has been proposed to mediate some of the behavioral effects of caffeine. This review discusses cellular mechanisms of action that could explain the role of dopamine in the behavioral effects of caffeine and summarizes the results of behavioral studies in both animals and humans that provide evidence for a role of dopamine in these effects. Caffeine is a competitive antagonist at adenosine receptors and produces a range of central and physiological effects that are opposite those of adenosine. Recently, caffeine has been shown to enhance dopaminergic activity, presumably by competitive antagonism at adenosine receptors that are colocalized and interact functionally with dopamine receptors. Thus, caffeine, as a competitive antagonist at adenosine receptors, may produce its behavioral effects by removing the negative modulatory effects of adenosine from dopamine receptors, thus stimulating dopaminergic activity. Consistent with this interpretation, preclinical behavioral studies show that caffeine produces behavioral effects similar to classic dopaminergically mediated stimulants such as cocaine and amphetamine, including increased locomotor activity, increased turning behavior in 6-hydroxydopamine-lesioned animals, stimulant-like discriminative stimulus effects, and self-administration. Furthermore, caffeine potentiates the effects of dopamine-mediated drugs on these same behaviors, and some of caffeine's effects on these behaviors can be blocked by dopamine receptor antagonists. Although more limited in scope, human studies also show that caffeine produces subjective, discriminative stimulus and reinforcing effects that have some similarities to those produced by cocaine and amphetamine.

Adenosine A2A receptor agonists increase Fos-like immunoreactivity in mesolimbic areas

Expression of the early-gene c-fos is an useful method for studying potential sites of action of drugs active in the CNS. Stimulation of adenosine A2A receptors by CGS 21680 (5 mg/kg) induced an increase in Fos-like immunoreactivity in the rat nucleus accumbens shell, while in the rostral pole and core CGS 21680 induced Fos-like immunoreactivity only after a high dose. CGS 21680 (5 mg/kg) stimulated c-fos expression also in the lateral septal nucleus and dorso-medial striatum, but not in the dorso-lateral striatum. A similar pattern of Fos-like immunoreactivity was obtained after administration of the A2A agonist HENECA (5 mg/kg) which displays higher selectivity for A2A receptors than CGS 21680. Administration of the selective A2A antagonist SCH 58261 counteracted CGS 21680-induced Fos-like immunoreactivity. Lesions of the dopaminergic mesostriatal projection by 6-hydroxydopamine and stimulation of dopamine D2/D3 receptors by quinpirole, prevented CGS 21680-induced Fos-like immunoreactivity in the nucleus accumbens shell. The present results show that stimulation of A2A receptors induces a profile of c-fos expression similar to that of atypical neuroleptics. A2A receptor stimulation has been reported to have dopamine antagonistic actions, it is therefore suggested that A2A agonists might have antipsychotic activity without producing extrapyramidal side effects.

Adenosine A2A receptor antagonism potentiates L-DOPA-induced turning behaviour and c-fos expression in 6-hydroxydopamine-lesioned rats

In order to investigate the role of adenosine A2A receptor blockade on dopamine-mediated motor responses, contralateral turning behaviour and expression of the early-gene c-fos was evaluated in rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway. SCH 58261, (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1 , 5-c]pyrimidine) a potent and selective antagonist of adenosine A2A receptors (5 mg/kg i.p.), induced a 70-fold increase in the contralateral turning behaviour induced by a low dose (2 mg/kg i.p.) of the dopamine precursor L-DOPA (L-3, 4-dihydroxyphenylalanine). Expression of c-fos as measured by Fos-like immunoreactivity after SCH 58261 plus L-DOPA was also potentiated as compared with L-DOPA alone, both in striatum and globus pallidus of the 6-hydroxydopamine-lesioned side of the brain. SCH 58261 induced a less marked potentiation (7-fold) of turning behaviour induced by dopamine D2 receptor stimulation with quinpirole, while Fos-like immunoreactivity in the striatum and globus pallidus was not affected. Previous studies have shown that SCH 58261 strongly potentiated dopamine D1 receptor-mediated responses. The results of the present study therefore indicate that the positive interaction between SCH 58261 and L-DOPA, in 6-hydroxydopamine-lesioned rats, is mainly due to an interaction with dopamine D1 receptors. The data also suggest that adenosine A2A receptor antagonists might be useful for potentiating the effects of L-DOPA in Parkinson's disease.

In vivo biodistribution of [N-11C-methyl]KF 17837 using 3-D-PET: evaluation as a ligand for the study of adenosine A2A receptors

(KF 17837, (E)-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine, was 11C-labelled by methylation at N-7 of the nor-compound, KF 17440, using [11C]methyl iodide. Radiochemical conversions of 50% or 70-80% were obtained using sodium hydride or potassium carbonate, respectively, as base. Total synthesis time was 40-45 min, including isolation by semipreparative liquid chromatography. Cerebral uptake of [N-11C-methyl]KF 17837 in Cynomolgus monkeys, evaluated using positron emission tomography (PET), was so low that regional differences in distribution kinetics were revealed first after increasing injected dose 3-fold and using 3-D mode of data acquisition. At all times, the relative regional retention (maximum striatum:cerebellum: cortex approximately 1.1:1:0.8 at 20 min) was considerably different from the known relative density of A2A receptors in these regions. Radioactivity decreased more rapidly in the cortex than in the striatum and cerebellum (by 20% vs. 3-7%, respectively, between 5 and 50 min). Addition of carrier to [N-11C-methyl]KF 17837 only marginally affected the cerebral radiotracer uptake. By contrast, in the heart the initial tracer uptake was high and the elimination kinetics was enhanced by adding unlabelled carrier. We have thus shown that KF 17837 passes the blood-brain barrier, though to a very low extent. This fact and the apparently high nonspecific binding in vivo of [N-11C-methyl]KF 17837 in regions with low receptor densities limits its usefulness as a ligand for quantification of the adenosine A2A receptors in the primate brain.

Motor depressant effects of systemically administered polyamines in mice: involvement of central NMDA receptors

The systemic administration of polyamines (s.c.) produced a dose-dependent motor depression. With high doses the depressant effect was long-lasting and the animals showed signs of toxicity. ED50 values for spermine, spermidine and putrescine were 38, 90 and 251 mg/kg respectively. The motor depression induced by the systemic administration of N-methyl-D-aspartate (NMDA; 25 mg/kg i.p.) was used as a model for studying the interactions between polyamines and the NMDA receptor. Results indicate that (1) the motor effects elicited by NMDA are very similar to those induced by polyamines at ED50 doses; (2) polyamines, even at non-active doses, potentiate the motor depressant effect induced by NMDA; (3) the NMDA receptor antagonist, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine (MK-801; 0.5 mg/kg i.p.), abolishes the depressant effect elicited by NMDA and by polyamines, even at toxic doses; (4) amphetamine (1.5 mg/kg i.p.) does not counteract the motor depressant effects of NMDA or polyamines. On the other hand, the adenosine receptor antagonist, theophylline (30 mg/kg i.p.), counteracts NMDA- but not polyamine-induced motor depression. The concentration of polyamines in the brain is modified after their systemic administration at high doses and at the ED50 dose of putrescine. In conclusion, the data suggest that the NMDA receptor could be a target mediating the motor effect elicited by polyamines. They also show that the quantitative analysis of the motor effects elicited by non-convulsant doses of NMDA might be a powerful tool for studying in vivo the interaction between neurotransmission systems involved in the regulation of motor activity.

Synergistic interaction between an adenosine antagonist and a D1 dopamine agonist on rotational behavior and striatal c-Fos induction in 6-hydroxydopamine-lesioned rats

The interaction between adenosine and D1 dopamine systems in regulating motor behavior and striatal c-Fos expression was examined in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. These results were compared to the synergistic interaction between D1 and D2 dopamine systems in 6-OHDA rats. Coadministration of the adenosine antagonist 3,7-dimethyl-1-propargylxanthine (DMPX: 10 mg/kg) and the D1 dopamine agonist SKF38393 (0.5 mg/kg) to 6-OHDA-lesioned rats produced significant contralateral rotation and c-Fos expression in the ipsilateral striatum compared to 6-OHDA rats treated with either drug alone. However, the regional pattern of striatal c-Fos activation following treatment of 6-OHDA rats with SKF38393 and DMPX was different from the dorsolateral pattern of striatal c-Fos induction observed after coadministration of D1 and D2 dopamine agonists (SKF38393: 0.5 mg/kg + quinpirole: 0.05 mg/kg). These data are consistent with a functional interaction between D1 dopamine and adenosine systems in the striatum, but suggest that activation of different subsets of striatal neurons underlie the behavioral synergy observed following combined adenosine antagonist-D1 dopamine agonist and combined D1 dopamine agonist-D2 dopamine agonist treatment.

Adenosine A2A receptors modulate the binding characteristics of dopamine D2 receptors in stably cotransfected fibroblast cells

In membrane preparations from rat striatum, where adenosine A2A and dopamine D2 receptors are coexpressed, stimulation of adenosine A2A receptors was found to decrease the affinity of dopamine D2 receptors for dopamine agonists. We now demonstrate the existence of this antagonistic interaction in a fibroblast cell line (Ltk-) stably transfected with the human dopamine D2 (long-form) receptor and the dog adenosine A2A receptor cDNAs (A2A-D2 cells). In A2A-D2 cells, but not in control cells only containing dopamine D2 receptors (D2 cells), the selective adenosine A2A agonist 2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethyl-carboxamido adenosine (CGS 21680) induced a 2-3-fold decrease in the affinity of dopamine D2 receptors for dopamine, as shown in competition experiments with dopamine versus the selective dopamine D2 antagonist [3H]raclopride. By contrast, activation of the constitutively expressed adenosine A2B receptors with 5'-N-ethyl-carboxamidoadenosine (NECA) did not modify dopamine D2 receptor binding. In A2A-D2 cells CGS 21680 failed to induce or induced only a small increase in adenosine-3',5'-cyclic-monophosphate (cAMP) accumulation. In D2 cells NECA- or forskolin-induced adenylyl cyclase activation was not associated with any change in dopamine D2 receptor binding. These results indicate that adenylyl cyclase activation is not involved in the adenosine A2A receptor-mediated modulation of the binding characteristics of the dopamine D2 (long-form) receptor.

Adenosine A1 receptor blockade selectively potentiates the motor effects induced by dopamine D1 receptor stimulation in rodents

An antagonistic interaction between adenosine A1 and dopamine D1 receptors has previously been found in the basal ganglia. However, direct evidence for a selective adenosine A1 antagonist-induced potentiation of dopamine D1-mediated motor activation is lacking. The systemic administration of the adenosine A1 antagonist 8-cyclopentyl-1,3-dimethylxanthine significantly potentiated the motor activating properties of the systemically administered dopamine D1 agonist SKF 38393 in both reserpinized mice and unilaterally 6-hydroxy-dopamine-lesioned rats. However, 8-cyclopentyl-1, 3-dimethylxanthine did not modify the motor effects of the dopamine D2 agonist quinpirole. The present work shows that an antagonistic interaction between adenosine A1 and dopamine D1 receptors may be involved in the motor activating effects of adenosine antagonists, like caffeine.

Kindled seizures do not affect adenosinergic inhibition of DA or ACh release in rat accumbens or PFC

Epileptic seizures are thought to terminate largely as a result of the extracellular accumulation of the purinergic neuromodulator, adenosine, released by discharging neurons. However, the postictal surge in extracellular adenosine and its widespread inhibitory effects are limited in time to only a few minutes and cannot directly account for increased resistance to seizures and the complex behavioural and motivational effects that may persist for hours or days after a seizure. The present study examined whether kindled seizures might alter the sensitivity or efficacy of inhibitory presynaptic adenosine receptors, and thereby induce more enduring changes in downstream transmitter systems. Rats were kindled in the amygdala of the dominant cerebral hemisphere, contralateral to the preferred direction of rotation, and their brains were removed either 2 h or 28 days after completion of kindling. Inhibition of electrically stimulated release of dopamine (DA) and acetylcholine (ACh) by the A1 adenosine-receptor agonist, R-phenylisopropyladenosine (R-PIA) was then measured in the prefrontal cortex (PFC) and nucleus accumbens. R-PIA (1.0 microM) inhibited [1H]DA release from PFC and nucleus accumbens tissue, and [14C]ACh release from nucleus accumbens tissue, but release was unaffected by prior kindling, regardless of the intervening interval. These results do not support suggestions that DA or ACh might mediate the effects of seizure-induced changes in purinergic inhibitory tone so as to cause long-term shifts in seizure threshold and postictal behavior.

A2 adenosine receptors: their presence and neuromodulatory role in the central nervous system

1. Adenosine is an endogenous neuromodulator that exerts its depressant effect on neurons by acting on the A1 adenosine receptor subtype. Excitatory actions of adenosine, mediated by the activation of the A2 adenosine receptor subtype, have also been shown in the central nervous system. 2. Adenosine A2a receptors are highly localized in the striatum, as demonstrated by the binding assay of the A2a selective agonist, CGS2680, and by analysis of the A2 receptor mRNA localization with in situ hybridization histochemistry. However, adenosine A2a, receptors, albeit at lower levels, are also localized in other brain regions, such as the cortex and the hippocampus. 3. In the striatum, adenosine A2a, receptors are implicated in the control of motor activity. Evidences exists of an antagonistic interaction between adenosine A2a and dopamine D2 receptors. 4. Utilizing selective agonists and antagonists for adenosine A2a receptors, their role in the modulation of the release of several neurotransmitters (acetylcholine, dopamine, glutamate, GABA) has been extensively studied in the brain (striatum, cortex, hippocampus). Controversial results have been obtained and, because the overall effect of endogenous adenosine in the brain is that of an inhibitory tonus, the physiological meaning of the excitatory A2 receptor remains to be clarified.

Dopamine D1 receptor-mediated facilitation of GABAergic neurotransmission in the rat strioentopenduncular pathway and its modulation by adenosine A1 receptor-mediated mechanisms

By using in vivo microdialysis it was found that one of the main functions of striatal dopamine D1 receptors is to selectively facilitate GABAergic neurotransmission in the 'direct' strioentopeduncular pathway. D1 receptors localized in the entopeduncular nucleus were also found to facilitate GABA release. However, results obtained from in vivo microdialysis, in vivo electrochemistry, immunohistochemistry and confocal laser microscopy suggested that entopeduncular D1 receptors could only be activated under pharmacological conditions. Adenosine A1 receptors were found to antagonistically modulate the D1-mediated regulation of the strioentopeduncular pathway. Furthermore, using in situ hybridization D1 and A1 receptors were shown to be colocalized in medium-sized striatal neurons. These results show that the strioentopeduncular neuron is a main locus for adenosine-dopamine interactions in the brain.

Stimulation of adenosine A1 receptors prevents the EEG arousal due to dopamine D1 receptor activation in rabbits

The influence of adenosine A1 (N6-cyclopentyladenosine, CPA) and A2 (2-[4-(2-carboxylethyl)phenethylamino]-5'-N-ethylcarboxamido -adenosine hydrochloride, CGS 21680) receptor agonists on SKF 38393-induced electroencephalographic (EEG) arousal was studied in rabbits. While CPA (0.1 mg/kg i.v.) significantly prevented the EEG effects of SKF 38393, CGS 21680 (0.2 mg/kg i.v.) did not affect them. These results demonstrate that adenosine A1 receptors can modulate dopamine D1 receptor-induced EEG arousal and show, for the first time, that adenosine-dopamine interactions are involved in brain functions other than motor activity.

Comparison of the effects of prototypical behavioral stimulants on locomotor activity and rotational behavior in rats

The present study was performed to characterize on rotational behavior the dose- and time-effect relationship of four prototypical behavioral stimulants that interact with dopamine systems via different mechanisms of action. Drug effects on rotational behavior was compared with effects on locomotor activity. The drugs examined were apomorphine (0.03-1.0 mg/kg), d-amphetamine (0.1-3.0 mg/kg), cocaine (3.0-56 mg/kg), and caffeine (10-100 mg/kg). SKF-38393 (0.3-10 mg/kg), a dopamine receptor agonist that has only modest effects on locomotor activity, was tested as a comparison. In rats with unilateral 6-hydroxydopamine (6-OHDA)-induced lesions of the nigrostriatal tract, d-amphetamine and cocaine dose dependently increased both the duration and the maximum number of turns/10 min, whereas apomorphine and caffeine increased only the duration of turning. There was a significant correlation of the effects of the four drugs on rotational behavior with effects on locomotor activity, but effects across drugs were not identical. Dose-response curves revealed potency differences among drugs in their effects on the two behaviors (e.g., apomorphine stimulated rotational behavior at a lower dose than it stimulated locomotor activity, whereas the converse was true with caffeine). Different mechanisms of action of these drugs might account for the differences in their effects on these behaviors.

Blockade of A2a adenosine receptors positively modulates turning behaviour and c-Fos expression induced by D1 agonists in dopamine-denervated rats

In rats with unilateral 6-hydroxydopamine lesions of the dopaminergic nigrostriatal pathway, administration of the A2a adenosine antagonist SCH 58261 alone did not induce any motor asymmetry but strongly potentiated the contralateral turning behaviour induced by the dopamine D1 agonist SKF 38393. SCH 58261 also increased the number of Fos-like positive nuclei induced by SKF 38393 in the 6-hydroxydopamine-lesioned striatum. Intense potentiation of D1-dependent turning behaviour and c-Fos expression was also observed after administration of the A2a/A1 antagonist CGS 15943. Administration of the A1 adenosine receptor antagonist DPCPX induced a small potentiation of D1-mediated contralateral turning while c-Fos expression induced by SKF 38393 was not modified. The results suggest that endogenous adenosine acting on A2a receptors can exert an inhibitory influence on the functional expression of D1-mediated responses in dopamine-denervated rats, and propose new possible therapeutic approaches in the treatment of Parkinson's disease.

Brain maturation of high-affinity adenosine A2 receptors and their coupling to G-proteins

The neuromodulator adenosine is acting through specific receptors, A1 and A2, coupled to their effector systems via G-proteins. The regulatory effects of adenosine on locomotor activity have been attributed to an interaction with A2 striatal receptors. The postnatal development of adenosine A2a receptors was analysed in rat striatal membranes and by quantitative autoradiography in brain sections using [3H]CGS 21680 as specific probe. At the concentration of radioligand used (5 nM), A2a sites were concentrated in the striatum at all ages, with minor developmental alterations in the expression pattern within the striatal regions. In membrane preparations, Scatchard analysis showed that the density of CGS 21680 binding sites was low at birth, around 3% of the adult value, and then increased, mostly between birth and 5 days and then from 15 days to adulthood. Concomitantly, the receptor affinity decreased sharply during brain development, Kd values varying from 2 to 15.5 nM. The addition of a GTP analogue, guanylyl-5'-imidodiphosphate (Gpp(NH)p, 10 microM), to the assay medium reduced significantly the receptor affinity throughout the postnatal development, reflecting a coupling to G-proteins at all ages, but it also suggested a weaker association at birth. These data show that the developmental properties of A2a receptors contrast with those of A1 receptors, and emphasize the role played by adenosine through its A2 receptors in the maturation of striatum-related cerebral pathways.

Adenosine A1 receptor-dopamine D1 receptor interaction in the rat limbic system: modulation of dopamine D1 receptor antagonist binding sites

Antagonistic interactions between adenosine A2a and dopamine D2 receptors and between adenosine A1 and dopamine D1 receptors have been previously found in the basal ganglia. Those interactions have been proposed to be key mechanisms of action responsible for the motor depressant effects of adenosine agonists and the motor activating effects of adenosine antagonists, like caffeine. By using quantitative receptor autoradiography, the selective adenosine A1 receptor agonist N6-cyclopentyladenosine was found to decrease the affinity of dopamine D1 receptors for the specific D1 antagonist [(125)I]SCH 23982 in both the nucleus accumbens and the medial prefrontal cortex of the rat brain. The present results suggest that dopamine neurotransmission, through an A1-D1 interaction, might also he involved in the behavioural effects of adenosine agonists and antagonists not related to motor activity, like the sedative-hypnogenic properties of adenosine analogues and the psychostimulant effects of caffeine.

Adenosine receptor ligands: differences with acute versus chronic treatment

Adenosine receptors have been the target of intense research with respect to potential use of selective ligands in a variety of therapeutic areas. Caffeine and theophylline are adenosine receptor antagonists, and over the past three decades a wide range of selective agonists and antagonists for adenosine receptor subtypes have been developed. A complication to the therapeutic use of adenosine receptor ligands is the observation that the effects of acute administration of a particular ligand can be diametrically opposite to the chronic effects of the same ligand. This 'effect inversion' is discussed here by Ken Jacobson and colleagues, and has been observed for effects on cognitive processes, seizures and ischaemic damage.

Adenine nucleotide metabolism in primary rat neuronal cultures

The metabolism of adenine nucleotides (AdRN) has been studied previously in whole brains, brain slices and brain extracts, containing mixed populations of neurons and glia. The availability of primary neuronal cultures enables us to study these pathways in almost pure neuronal preparations. The aim of the present study was to characterize the relative importance of the pathways of AdRN metabolism in the neurons. The metabolic fate of (8-14C) adenine and of AdRN prelabeled with (8-14C)adenine were studied in immature and mature primary rat neuronal cultures. Specific inhibitors were used to clarify the various metabolic fluxes, which were evaluated based on the time-related changes in the distribution of label (the cellular nucleotide content did not change during incubation). The turnover rate of AdRN was found to reflect mainly conversion of label to acid insoluble derivatives (AID) and partly degradation to hypoxanthine. The turnover was faster in the immature neurons. The combined addition of 2'-deoxycoformycin (2'-dCF) and of 5'-amino-5'-deoxyadenosine, inhibiting adenosine metabolism, resulted in both cultures in enhanced loss of label from AdRN, mainly to adenosine and adenine. This finding indicates the activity of the futile cycle AMP-->adenosine-->AMP. In both cultures, in the presence of these inhibitors, the ratio (hypoxanthine + inosine)/(adenine + adenosine) was 1.1, indicating that the fluxes through AMP deamination and AMP dephosphorylation are about equal. Addition of L-alanosine, inhibiting the conversion of IMP to AMP, resulted in both cultures, but especially in the mature neurons, in enhanced loss of label from AdRN to hypoxanthine and inosine. This finding indicates the functioning of the adenine nucleotide cycle (AMP-->IMP-->adenylosuccinic acid-->AMP). Under conditions of enhanced degradation of ATP (induced by iodoacetate and antimycin A), addition of 2'-dCF resulted in the immature cultures in lowering the ratio (hypoxanthine + inosine + IMP)/(adenine + adenosine) to 0.62, indicating a shift in favor of AMP dephosphorylation.

Adenosine physiology and pharmacology: how about A2 receptors?

Adenosine participates in the physiology of central and peripheral tissues through several subtypes of G-protein-coupled receptors. Positively linked to adenylate cyclase, A2 receptors have been subdivided into A2a and A2b sites on the basis of their molecular, biochemical and pharmacological properties. They exhibit selective distribution, and are implicated in the modulation of psychomotor activity, circulation, respiration, and metabolism. Recent data support the evidence that adenosine A2 receptor properties may prove useful in future drug development, and selective manipulation of receptor-associated biologic effects might be relevant in the treatment of various disorders, including psychiatric diseases, hypoxia/ischemia, inflammation or erythrocytosis.

Functional characterization of adenosine receptors in the nucleus tractus solitarius mediating hypotensive responses in the rat

1. The aim of this study was to characterize adenosine receptors located in the nucleus tractus solitarius (NTS) that mediate decreases in blood pressure in the anaesthetized rat. To determine the adenosine receptor subtype involved, a range of selective agonists and antagonists were studied and their relative potencies evaluated. 2. The rank order of agonist potency in inducing decreases in diastolic blood pressure was N6-cyclopentyladenosine (CPA) > N6-cyclohexyladenosine (CHA) > N-ethyl-carboxamidoadenosine (NECA) > or = 2-phenylaminoadenosine (CV1808) > 2-p-(carboxyethyl)phenethylamino-5' N-ethylcarboxamidoadenosine (CGS 21680) > N6-(2-(4-aminophenyl)ethyl)-adenosine (APNEA). 3. The hypotensive action of CPA following microinjection into the NTS was antagonized by i.v. infusions (50 micrograms kg-1 min-1) of adenosine receptor antagonists, 8-cyclopentyl-1,3 dipropylxanthine (DPCPX), 8-phenyltheophylline (8-PT), 8-(p-sulphophenyl)theophylline (8-SPT), and 1,3-dipropyl-8-N-(2-diethylamino)ethyl)-N methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo) benzenesulphonamidexanthine (PD 115199). The antagonist potency order was DPCPX > PD115199 > or = 8-PT. Intravenous infusion of 8-SPT had no effect on blood pressure responses to microinjection of CPA into the NTS. 4. The results suggest that adenosine A1 receptors in the NTS mediate hypotensive responses in the anaesthetized rat preparation.

Involvement of adenosine and glutamate receptors in the induction of c-fos in the striatum by haloperidol

The psychostimulant drugs amphetamine and cocaine induce the expression of immediate early genes, such as c-fos, in the striatum via D1 dopamine receptor activation. This occurs primarily in the striato-nigral neurons. Conversely, neuroleptic drugs, such as haloperidol, which block D2-type dopamine receptors, induce c-fos expression in striatal neurons projecting to the globus pallidus. In order to gain insight into the neurochemical substrates of neuroleptic-induced c-fos expression, we examined the effects of adenosine A2 and N-methyl-D-aspartate (NMDA) receptor antagonists as well as inhibition of nitric oxide synthase, on haloperidol-induced Fos immunoreactivity in the striatum. While blockade of D1 receptors had no effect on haloperidol-induced Fos expression, adenosine A2 receptor antagonists decreased the number of neurons in the striatum expressing haloperidol-induced Fos by half. NMDA receptor antagonists also potently blocked the induction of Fos immunoreactivity by haloperidol, while inhibition of nitric oxide synthase activity had no effect. These results indicate that in the presence of a dopamine D2 antagonist, Fos expression in striato-pallidal neurons is mediated in part through activation of A2 receptors by adenosine, and via NMDA receptor activation by glutamate.

Cerebral ischemia in gerbils: effects of acute and chronic treatment with adenosine A2A receptor agonist and antagonist

Despite significant progress in understanding of the potential of adenosine A1 receptor-based therapies in treatment of cerebral ischemia and stroke, very little is known about the effect of selective stimulation of adenosine A2A receptors on the outcome of a cerebrovascular arrest. In view of a major role played by adenosine A2 receptors in the regulation of cerebral blood flow, we have investigated the effect of both acute and chronic administration of the selective adenosine receptor agonist 2-[(2-aminoethylamino)-carbonylethylphenylethylamino]-5'-N- ethylcarboxoamidoadenosine (APEC) and antagonist 8-(3-chlorostyryl)caffeine (CSC) on the outcome of 10 min ischemia in gerbils. Acute treatment with APEC improved recovery of postischemic blood flow and survival without affecting neuronal preservation in the hippocampus. Acute treatment with CSC had no effect on the cerebral blood flow but resulted in a very significant protection of hippocampal neurons. Significant improvement of survival was present during the initial 10 days postischemia. Due to subsequent deaths of animals treated acutely with CSC, the end-point mortality (14 days postischemia) in this group did not differ statistically from that seen in the controls. It is, however, possible that the late mortality in the acute CSC group was caused by the systemic effects of brain ischemia that are not subject to the treatment with this drug. Chronic treatment with APEC resulted in a statistically significant improvement in all studied measures. Although chronic treatment with CSC improved postischemic blood flow, its effect on neuronal preservation was minimal and statistically insignificant. Mortality remained unaffected. The results indicate that the acute treatment with adenosine A2A receptor antagonists may have a limited value in treatment of global ischemia. However, since administered CSC has no effect on the reestablishment of postischemic blood flow, treatment of stroke with adenosine A2A receptor antagonists may not be advisable. Additional studies are necessary to elucidate whether chronically administered drugs acting at adenosine A2 receptors may be useful in treatment of stroke and other neurodegenerative disorders.

Adenosine A2 receptor-mediated modulation of contralateral rotation induced by metabotropic glutamate receptor activation

Systemic pretreatment with the adenosine receptor antagonist theophylline significantly decreases contralateral rotation induced by unilateral intrastriatal 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD). Intrastriatal or intrasubthalamic nucleus coadministration of theophylline and 1S,3R-ACPD significantly decreases contralateral rotation suggesting that metabotropic glutamate (mGlu) receptors and adenosine receptors interact locally. These appear to be adenosine A2 receptor effects as the adenosine A2 receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) also decreases contralateral rotation induced by unilateral intrastriatal and intrasubthalamic nucleus administration of 1S,3R-ACPD, while the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) has no effect. Pretreatment with the adenosine A2 receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS 21680) potentiates contralateral rotation induced by unilateral striatal 1S,3R-ACPD, whereas pretreatment with the adenosine A1 receptor agonist N6-cyclopentyl-adenosine (CPA) has no effect. These results suggest that mGlu receptor effects may be due, in part, to modulation of adenosine action.

Reduction of dopamine D2 receptor transduction by activation of adenosine A2a receptors in stably A2a/D2 (long-form) receptor co-transfected mouse fibroblast cell lines: studies on intracellular calcium levels

A stably co-transfected mouse fibroblast cell line, which expresses the long form of the human dopamine D2 receptor and the dog adenosine A2a receptor, was used to analyse the mechanism underlying changes in the cytosolic free calcium concentration ([Ca2+]i) induced by activation of D2 (long-form) receptors and its modulation by the A2a receptor agonist CGS 21680 by means of fura-2 imaging. Quinpirole (1-1000 nM), a D2 receptor agonist, caused a concentration-dependent increase in [Ca2+]i. Haloperidol, a preferential D2 receptor antagonist, completely blocked this [Ca2+]i response to quinpirole. Preincubation with Ca(2+)-free medium containing 2 mM EGTA or a medium containing 320 mM ethanol, an inositol 1,4,5-triphosphate receptor antagonist, substantially diminished the increase in [Ca2+]i evoked by quinpirole. Furthermore, quinpirole totally failed to elevate [Ca2+]i in a medium containing both 2 mM EGTA and 320 mM ethanol. CGS 21680 (1-500 nM) did not, by itself, exert any significant effect on [Ca2+]i. However, 100 nM of CGS 21680 substantially counteracted the [Ca2+]i responses to quinpirole (10-1000 nM). Moreover, this counteraction still occurred after blocking Ca2+ mobilization from intracellular stores with ethanol, but disappeared when the cells were pretreated with the Ca(2+)-free medium containing 2 mM EGTA. Our findings imply that the D2 (long-form) receptors in the present fibroblast cell line can raise [Ca2+]i both via Ca2+ influx from the extracellular medium and Ca2+ release from intracellular stores via activation of inositol 1,4,5-triphosphate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine antagonists potentiate D2 dopamine-dependent activation of Fos in the striatopallidal pathway

Adenosine antagonists potentiate dopamine-mediated behaviours. A2a adenosine and D2 dopamine receptors are abundantly co-expressed within the striatopallidal subset of striatal neurons, suggesting that this is the site of interaction between A2a and D2 receptors. We show that the D2-dependent induction of the immediate early gene c-Fos occurs in striatopallidal neurons 3 h following injection of reserpine (10 mg/kg). We used this paradigm to test whether adenosine antagonists modulate D2-dependent activation of striatopallidal neurons. The non-selective A1-A2 adenosine antagonists theophylline (25 mg/kg) or 3,7-dimethyl-1-propargylxanthine (25 mg/kg) potentiated the effect of a submaximal dose of the D2 dopamine agonist quinpirole (0.05 mg/kg) to prevent the induction of striatal c-Fos following reserpine. Co-administration of the A2a receptor antagonist 8-(3-chlorostyryl) caffeine (5 mg/kg) with quinpirole (0.05 mg/kg) also attenuated striatal c-Fos induction following reserpine to a greater extent than 0.05 mg/kg quinpirole alone. When administered prior to reserpine, theophylline (25 mg/kg) or 3,7-dimethyl-1-propargylxanthine (25 mg/kg) partially attenuate the induction of striatal c-fos. These results demonstrate that systemic administration of adenosine antagonists enhance D2 dopamine receptor-dependent regulation of c-Fos in the striatopallidal pathway. These results support a functional interaction between A2a adenosine and D2 dopamine receptors in striatopallidal neurons.

Chronic NMDA receptor stimulation: therapeutic implications of its effect on adenosine A1 receptors

It is known that stimulation of adenosine A1 receptors has a modulatory effect on the excitability of postsynaptic NMDA receptors. Conversely, acute stimulation of NMDA receptors results in release of adenosine via calcium-independent mechanisms. These findings indicate a close functional relationship between these receptors. It is, therefore, possible that chronic, low level stimulation of the NMDA receptor may have a negative impact on these modulatory processes. To investigate this possibility, we have subjected C57BL mice either to an acute injection of a N6-cyclopentyladenosine (CPA, 0.01 mg/kg) or deoxycoformycin (1 mg/kg) followed by a convulsant dose of N-methyl-D-aspartate (NMDA) (60 mg/kg) or to chronic, low level (20 mg/kg i.p. daily) exposure to NMDA for 8 weeks. One day after the last injection of NMDA, animals were injected either with a convulsant dose of NMDA alone, or with either CPA at 0.001 or 0.01 mg/kg, or with 1 mg/kg deoxycoformycin followed 15 min later by 60 mg/kg NMDA. Neither CPA nor deoxycoformycin were protective when NMDA was given acutely at 60 mg/kg. Chronic treatment with NMDA alone or chronic administration of NMDA followed by 0.001 mg/kg CPA had no significant effect on mortality following a convulsant dose of NMDA. However, when the chronic regimen of NMDA was followed by either 0.01 mg/kg CPA or 1 mg/kg deoxycoformycin, mortality was reduced to 10% (CPA), or eliminated completely (deoxycoformycin). Moreover, combination of chronic NMDA treatment with either CPA (both doses) or deoxycoformycin produced a significant improvement in other measures, i.e., seizure onset, intensity of neurological impairment, and extension of time to death.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of CGS 21680 in vivo on dopamine D2 agonist binding in the rat brain

To investigate whether adenosine A2a agonists modulate dopamine D2 receptor binding in vivo, we have analyzed the effects of intraperitoneally administered 2-[p-(carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosin e (CGS 21680) on the ability of dopamine to compete at [125I]iodosulpride (0.25 nM) binding sites in filter-wiped cryostat sections of the rat forebrain and on [3H]L-(-)-N-propylnorapomorphine ([3H]NPA) binding (1 nM) using quantitative receptor autoradiography. CGS 21680 (1-3 mg/kg) decreased the IC50 value of dopamine on [125I]iodosulpride binding, and the decrease at 1 mg/kg was blocked by the A2 antagonist 3,7-dimethyl-1-propargylxanthine (DMPX; 5 mg/kg). The decrease in the IC50 value of dopamine was due to a decrease in the KL value whereas the KH value and the proportion of high-affinity binding sites were unaffected. The binding of [3H]NPA was significantly increased in the rostral and caudal parts of the caudate-putamen and in the rostral part of the olfactory tubercle, whereas no change could be demonstrated in the nucleus accumbens and in the caudal part of the olfactory tubercle. These results indicate that stimulation of A2a receptors in vivo causes alterations in the binding characteristics of D2 receptors in certain regions of the basal ganglia.

A brief appraisal on some aspects of the receptor-receptor interaction

Receptor-receptor interaction is being recognized as a key cellular mechanism responsible for the integration of signals between different transmission lines at the membrane level. Receptor-receptor interaction extends the classical concept of integration of depolarization and hyperpolarization effects to the integration of chemical events at membrane level. In the present paper only some hypotheses on the possible mechanistic aspects of the interactions between the membrane receptors will be discussed. Furthermore, possible functional consequences of receptor-receptor interaction will be explored. The limitations of the present experimental approaches to the study of receptor-receptor interaction will also be analysed.

Possible role of striatal adenosine in the modulation of acute ethanol-induced motor incoordination in rats

Several reports from our laboratory have suggested the involvement of the brain adenosinergic system in ethanol-induced motor incoordination (EIMI). This study is an extension of the previous work and pertains to the evaluation of the role of the striatal adenosine in EIMI in male Sprague-Dawley rats. Using the motor incoordination induced by 1.5 g/kg of ethanol (ip) as a test response, the possible behavioral interactions between ethanol and adenosine agonists and antagonists in the striatum were investigated. Intrastriatal (IST) administration of adenosine A1-, A1 = A2-, and As-selective agonists, R(-)N6-(2-phenylisopropyl)adenosine (R-PIA), 5'-N-ethylcarboxamido-adenosine (NECA), and 5'-(N-cyclopropyl)-carboxamidoadenosine, respectively, significantly and dose-dependently accentuated EIMI when evaluated by rotorod test, suggesting the striatal adenosinergic modulation of EIMI. No significant change in normal motor coordination was noted, even when the highest IST doses of adenosine agonists were followed by saline instead of ethanol, suggesting that the observed behavioral interactions of these drugs were selective to ethanol. Hippocampus, which is known not to be involved in the normal motor functions, was selected as a control brain area because of the presence of high density of adenosine receptors, as well as the high levels of adenosine. Intrahippocampal NECA failed to alter EIMI, indicating the specific role of striatal and not hippocampal adenosinergic system in the modulation of EIMI. The potentiating effects of adenosine agonists N6-cyclohexyladenosine (CHA) and CGS-21680 on EIMI were blocked by adenosine A1- and A2-selective antagonists, 8-cyclopentyl-1,3-dipropylxanthine and 3,7-dimethyl-1-propargylxanthine, respectively, suggesting the participation of specific adenosine receptors in this functional interaction. A role for the adenosine A1 receptor in the striatal adenosinergic modulation of EIMI was favored based on the rank-order potency of adenosine agonists. IST pretreatment with pertussis toxin (PT), but not with PT beta-oligomer, nearly completely eliminated the accentuation of EIMI by CHA, further supporting the favored role of adenosine A1 receptors in EIMI. Histological and IST [3H]R-PIA distribution data confirmed that the observed behavioral effects were caused by exclusive striatal distribution of intrastriatally microinjected drugs. Data obtained suggested modulation of acute EIMI by striatal adenosine receptor-mediated mechanism(s) and the coupling of these adenosine receptor to the PT-sensitive Gi protein.

Effects of the systemic administration of kainic acid and NMDA on exploratory activity in rats

In spite of growing evidence for the involvement of the glutamatergic system in mammal's locomotion, studies on behavioral effects induced by the systemic administration of excitatory amino acids not associated to convulsions are lacking. In the present work, the effect of one single systemic administration of kainic acid (KA) (9 mg/kg, IP) or NMDA (100 mg/kg, IP) on exploratory activity in the rat during 6 consecutive days was studied. Separation of exploratory activity in fast (FM) and slow movements (SM) and rearings (R), together with the analysis of those variables during both the light and dark periods of the light-dark cycle, allowed finding specific drug-induced effects. KA produced an acute short-lasting increase in exploratory activity, only significant for FM. On the other hand, NMDA produced an acute short-lasting depressant effect on FM, SM, and R, followed during the next 2 days by a long-lasting increase in exploratory activity, only significant for FM during the dark period. These results underline the importance of using repeated testing during both light and dark periods of the light-dark cycle when analyzing drug-induced changes on exploratory activity.

Caffeine antinociception in the rat hot-plate and formalin tests and locomotor stimulation: involvement of noradrenergic mechanisms

The present study examined antinociception produced by systemic administration of caffeine in the rat hot-plate (HP) and formalin tests and addressed several aspects of the mechanism of action of caffeine. Locomotor activity was monitored throughout. Caffeine produced a dose-related antinociception the HP (50-100 mg/kg) and formalin tests (12.5-75 mg/kg). When observed during the formalin test, caffeine stimulated locomotor activity between 12.5 and 50 mg/kg; this was followed by a depression in activity at 75 mg/kg. Caffeine did not produce an anti-inflammatory effect as determined by hindpaw plethysmometry, suggesting that antinociception was not secondary to an anti-inflammatory action. Peripheral co-administration of caffeine with the formalin did not produce antinociception, suggesting a predominant central rather than peripheral site of action for caffeine. Naloxone (10 mg/kg) did not reduce the antinociceptive or locomotor stimulant effects of caffeine, suggesting a lack of involvement of endogenous opioids in these actions. Phentolamine (5 mg/kg) enhanced antinociception by caffeine in both the HP and formalin tests, but inhibited locomotor stimulation. Prazosin (0.15 mg/kg) mimicked the action of phentolamine on locomotor stimulation, but idazoxan (0.5 mg/kg) mimicked the action of phentolamine on antinociception in the formalin test. These observations suggest an involvement of different alpha-adrenergic receptors in the two actions of phentolamine. Microinjection of 6-hydroxydopamine (6-OHDA) into the locus coeruleus, which depleted noradrenaline (NA) in the spinal cord and forebrain, inhibited the action of caffeine in the HP test. This was mimicked by intrathecal 6-OHDA which depleted NA in the spinal cord, but not by microinjection of 6-OHDA into the dorsal bundle which depleted NA in the forebrain. These results suggest an integral involvement of noradrenergic mechanisms in the antinociceptive action of caffeine in the HP and formalin tests and in locomotor stimulation, but the nature of this involvement differs for the 3 end points.

Effect of different xanthines and phosphodiesterase inhibitors on c-fos expression in rat striatum

It has previously been shown that caffeine, in a dose-dependent manner, increases the expression of the protooncogene c-fos in the rat brain, predominantly in the caudate-putamen and tuberculum olfactorium. In this study we examined the effect of related xanthines and of selective phosphodiesterase inhibitors on c-fos expression. The effect of caffeine (75 mg kg-1) was mimicked by 3-isobutyl-1-methyl xanthine (IBMX) (25 mg kg-1) and theophylline (100 mg kg-1) but not by 8-p-sulfophenyltheophylline (10 mg kg-1), enprofylline, theobromine or paraxanthine (each at 100 mg kg-1). Moreover, the cyclic AMP-selective phosphodiesterase inhibitors rolipram (10 or 20 mg kg-1) and SQ 20,006 (25 mg kg-1) and the cyclic GMP-selective phosphodiesterase inhibitor zaprinast (10 mg kg-1) failed to induce c-fos in striatum, but caused a clear-cut induction in the overlying cerebral cortex. Thus, c-fos is induced in rat striatum following administration of caffeine and other xanthines that (provided they enter the brain) block adenosine receptors, suggesting an involvement of central adenosine receptors. Inhibition of cyclic nucleotide phosphodiesterase does not appear to play any important role in c-fos induction by the xanthines.

Purinoceptors in the nervous system

The purine nucleoside adenosine and the purine nucleotide ATP play different roles in the nervous system. Adenosine acts on a family of G protein coupled receptors, collectively called adenosine receptors or P1 purinoceptors. Four members of this family have been cloned and pharmacologically characterized: A1, A2A, A2B and A3. Their distribution, pharmacology and biological roles are briefly discussed. In particular, the evidence that adenosine acting at A1 receptors regulates the release of several neurotransmitters and that adenosine acting at A2A receptors modulates dopaminergic transmission is summarized. ATP acts on receptors called P2 purinoceptors, which appear to fall into at least two main families--G protein coupled receptors and intrinsic ion channels. Their subclassification is becoming clearer as receptors are cloned and new selective agonists and/or antagonists are becoming available. There is an interesting potential for development of drugs targeted at purines or their receptors.

Glutamate/dopamine D1/D2 balance in the basal ganglia and its relevance to Parkinson's disease

The recent availability of selective ligands for NMDA and AMPA receptors has enabled neuroscientists to test the hypothesis that Parkinson's disease is a glutamate hyperactivity disorder and hence treatable with glutamate antagonists. This review takes a critical look at the motor characteristics of this new class of drugs in rodent and primate models of parkinsonism and assesses the clinical potential and pitfalls of this radical new approach. Monotherapy of Parkinson's disease with glutamate antagonists appears impractical at the present time, due to their low efficacy and unacceptable side effects, but polypharmacy with L-DOPA and a glutamate antagonist as adjuvant is a more realistic prospect. This review will focus on the ways in which glutamate receptor blockade facilitates motor recovery with L-DOPA and will examine whether the basis for this beneficial effect can be traced to a specific interaction with dopamine at D1 or D2 receptors, and therefore to discrete motor pathways within the basal ganglia.

Dopamine-independent and adenosine-dependent mechanisms involved in the effects of N-methyl-D-aspartate on motor activity in mice

The involvement of dopamine and adenosine mechanisms in the motor effects of systemically administered N-methyl-D-aspartate (NMDA) was studied in non-reserpinized and in reserpinized mice. In non-reserpinized mice NMDA induced motor depression (with 8, 25 and 75 mg/kg i.p.) during the first hour and motor activation (with 25 and 75 mg/kg i.p.) during the second hour after its administration. The non-selective adenosine antagonist, theophylline (3, 10 and 30 mg/kg i.p) induced motor activation during both 1-h periods of observation. NMDA-induced motor depression in non-reserpinized mice was antagonized by theophylline. Higher doses of theophylline were needed to counteract the motor depressant effect induced by higher doses of NMDA. The motor activation induced by NMDA and theophylline in non-reserpinized mice was not additive and theophylline did not enhance the motor activation induced by high doses of NMDA. Both NMDA (25 and 75 mg/kg i.p.) and theophylline (10 and 30 mg/kg) induced motor activation in reserpinized mice and, when coadministered, NMDA counteracted the effect of theophylline. NMDA (8 and 25 mg/kg i.p.) antagonized and theophylline (3, 10 and 30 mg/kg i.p.) potentiated the motor activation induced by the non-selective dopamine agonist, apomorphine (0.1 mg/kg s.c.), in reserpinized mice. In reserpinized mice, the non-selective dopamine antagonist, haloperidol (0.5 mg/kg s.c.) antagonized the motor activation induced by apomorphine (0.1 mg/kg s.c.) and the induced by theophylline (10 mg/kg i.p.) and did not modify NMDA-induced motor activation.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Adenosine has been shown to inhibit dopamine release from striatal slices and synaptosomes. Recently, a direct interaction between the adenosine A2 receptor and dopamine D2 receptor has been provided. Activation of striatal adenosine A1 receptors is known to partially inhibit the release of dopamine (DA), but some aspects of this mechanism remain unclear. We have studied the participation of adenosine A1 receptors in the control of DA release 'in vivo' in awake, freely moving rats using microdialysis. To this end, the effects of 2-chloroadenosine (2-CADO), a non-metabolizable adenosine A1 receptor agonist, were studied on basal and stimulated striatal DA release. Basal levels were found to be slightly decreased by a maximal concentration of 2-CADO without any changes in DA metabolites. Haloperidol stimulated DA release was fully counteracted by 2-CADO. However, high K+ (100 mM) or (+)-amphetamine stimulated DA release was not altered by 2-CADO. Altogether, these data suggest that adenosine acting through A1 receptors possibly localized on striatal dopaminergic nerve terminals can block an induced D2 receptor blockade, but not the releasing effects caused by (+)-amphetamine and high K+ concentration. It is postulated that the increase in DA release by haloperidol is mainly due to an increased firing rate of the DA neurons and that A1 receptor activation can block the DA release observed in response to the action potential activation of DA nerve terminals.

Astra Award Lecture. Adenosine, adenosine receptors and the actions of caffeine

Of the known biochemical actions of caffeine, only inhibition of adenosine receptors occurs at concentrations achieved during normal human consumption of the drug. Under normal physiological conditions, adenosine is present in sufficient concentrations to activate A1 and A2a receptors. Via actions on A1 receptors, adenosine decreases neuronal firing and the release of neurotransmitters. The exact mechanisms are not known, but several possibilities are discussed. Via actions on A2a receptors, adenosine--and hence caffeine--can influence dopaminergic neurotransmission. Caffeine can induce rapid changes in gene expression and, somewhat later, marked adaptive changes. These include antiepileptic and neuroprotective changes. Thus, caffeine has a number of central effects directly or indirectly related to adenosine receptors. Some of these are potentially useful, and drug development based on the actions of caffeine should be interesting.

Simultaneous high-performance liquid chromatographic determination of adenosine and dopamine in rat striatal tissue with combined ultraviolet absorbance and electrochemical detection

A method is described for the simultaneous determination of biogenic amines, adenosine and their metabolites in rat striatal tissue using high-performance liquid chromatography with ultraviolet spectrophotometric and electrochemical detection. Peaks in the chromatograms of striatal tissue extracts were identified by retention times and by on-line analysis of peak spectra for adenosine and its metabolites, and by comparing current ratios of the dual-electrode coulometric detector for monoamines and metabolites. The assay gives a linear response over the concentration range of 0.15-0.60 micrograms/ml for biogenic amines, 0.5-2.0 micrograms/ml for serotonin, 5-20 micrograms/ml for hypoxanthine, adenosine and N-methyladenosine, and 10-40 micrograms/ml for inosine. The limit of detection for striatal homogenates was 3.5 ng/g for monoamines, 9 ng/g for serotonine, 140 ng/g for hypoxanthine, 290 ng/g for inosine and 80 ng/g for adenosine. The recovery ranged from 88.5% for vanillylmandelic acid to 110.3% for dopamine. The method was used to measure biogenic amines, adenosine and related metabolites in rat striatal tissues.

Antagonistic interaction between adenosine A2A receptors and dopamine D2 receptors in the ventral striopallidal system. Implications for the treatment of schizophrenia

Recent studies have shown the existence of a specific antagonistic interaction between adenosine A2a receptors and dopamine D2 receptors in the brain. This A2a-D2 interaction seems to be essential for the behavioural effects of adenosine agonists and antagonists, like caffeine. In the present study quantitative receptor autoradiography and brain microdialysis were combined to demonstrate a powerful antagonistic A2a-D2 interaction in the ventral striopallidal system. In the presence of the A2a agonist (2-p-carboxyethyl)phenylamino-5'-N carboxamidoadenosine, dopamine exhibited a lower efficacy in displacing the radiolabelled D2 receptor antagonist [125I]iodosulpiride from the rat ventral striatum, specially in the nucleus accumbens. A tonic dopaminergic modulation of the striopallidal neurons from the ventral striopallidal system was demonstrated by a dual-probe approach, by infusing selective dopamine agonists and antagonists in the nucleus and by measuring dopamine extracellular levels in the nucleus accumbens and GABA extracellular levels in the nucleus accumbens and in the ipsilateral ventral pallidum. The infusion of (2-p-carboxyethyl)phenylamino-5'-N-carboxamidoadenosine in the nucleus accumbens induced the same postsynaptic changes as the D2 antagonist raclopride, i.e. an increase in pallidal GABA extracellular levels, without changing those levels in the nucleus accumbens. Furthermore, the coinfusion in the nucleus accumbens of low concentrations of (2-p-carboxyethyl) phenylamino-5'-N-carboxamido-adenosine and raclopride, which were ineffective when administered alone, induced a significant increase in pallidal gamma-aminobutyric acids extracellular levels. These results suggest that A2a agonists, alone or in combination with D2 antagonists, could be advantageous antischizophrenic drugs, as blockage of D2 receptors in the ventral striopallidal system appears to be associated with the antipsychotic activity of neuroleptics but not with their extrapyramidal motor-side effects.

A2a/D2 receptor interactions are not observed in COS-7 cells transiently transfected with dopamine D2 and adenosine A2a receptor cDNA

The rat D2 receptor and the dog A2a receptor subcloned into the pXM vector were transiently transfected into COS-7 cells using the DEAE-dextran method. The transfected cells expressed approx. 200 fmol D2 receptors/mg protein and approx. 5 pmol/mg protein of the A2a receptor as judged by binding experiments with [3H]raclopride [or[3H]-N-propyl-apomorphine (NPA)] and [3H]-CGS 21680, respectively. The high affinity KD values were 0.43 and 19 nM for D2 and A2a receptors, respectively, in agreement with results obtained from other cells and tissues. The non-selective adenosine receptor agonist NECA stimulated cAMP accumulation both in non-transfected and transfected COS-7 cells with only a slight difference in potency, suggesting that most of the stimulation is due to activation of A2b receptors known to be present on virtually every cell. The two A2a selective agonists CGS 21680 and CV-1808 were essentially inactive in transfected COS-7 cells, but were very active in PC-12 cells known to possess functional A2a receptors. Dopamine did not decrease cAMP accumulation in the transfected COS-7 cells. CGS 21680 (30 nM) did not affect the binding characteristics of D2 receptors in the co-transfected COS-7 cells in contrast to the increased KH, KL and RH values found previously in rat striatal membranes after CGS 21680 treatment. The present findings indicate that transiently transfected A2a and D2 receptors in COS-7 cells have normal binding properties, but couple poorly to adenylyl cyclase, despite the presence of Gs protein and adenylyl cyclase in these cells. Our results also demonstrate that the previously reported interactions between A2a receptors and D2 receptors do not occur when only the receptor proteins are expressed in COS-7 cells, suggesting that the two receptor molecules do not interact directly to influence binding characteristics.

Chronic haloperidol treatment leads to an increase in the intramembrane interaction between adenosine A2 and dopamine D2 receptors in the neostriatum

Stimulation of adenosine A2 receptors (with the selective adenosine A2 agonist CGS 21680) in rat striatal membrane preparations, produces a decrease in both the affinity of D2 receptors and the transduction of the signal from the D2 receptor to the G protein. This intramembrane A2-D2 interaction might be responsible for the behavioural depressant effects of adenosine agonists and for the behavioural stimulant effects of adenosine antagonists such as caffeine and theophylline. Dopamine denervation induces an increase in the intramembrane A2-D2 interaction, which may underlie the observed higher sensitivity to the behavioural effects induced by adenosine antagonists found in these animals. The present study was designed to examine if chronic treatment with haloperidol, which also produces dopamine receptor supersensitivity, is also associated with an increase in the intramembrane A2-D2 interaction in the neostriatum and with a higher sensitivity to the behavioural effects induced by adenosine antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Toward the complete genomic map and molecular pathology of human chromosome 4

The identification of disease genes via molecular DNA cloning has revolutionized human genetics and medicine. Both the candidate gene approach and positional cloning have been used successfully. The defects causing Huntington's disease, facioscapulohumeral muscular dystrophy, piebaldism, Hurler/Scheie syndrome, one form of autosomal recessive retinitis pigmentosa, and a second locus for autosomal dominant polycystic kidney disease have recently been localized to chromosome 4. In addition to the rapid progress in the cloning of the 203-megabase chromosome, the presence of more than 60 closely spaced microsatellites on this chromosome will undoubtedly lead to the localization of additional disease genes. In order to consider cloned genes as potential candidates for disorders assigned to chromosome 4, it is important to collect and order all genes with respect to their chromosomal localization. Analysis of cytogenetically visible interstitial and terminal deletions should also be helpful in defining new disease gene loci and in mapping novel genes. These data represent the status quo of the integrated molecular map for chromosome 4.

Changes in adenosine receptors in the neonatal rat brain following hypoxic ischemia

We used quantitative in situ hybridization and receptor autoradiography to study changes in adenosine receptors following hypoxia-ischemia (H-I) in the neonatal rat brain. Seven-day-old rat pups were subjected to a unilateral ligation of the common carotid artery followed by a 2 h 15 min hypoxic period (7.7% O2 in N2). Adenosine A1 receptor mRNA in cortex and several parts of hippocampus, and A2a mRNA was decreased in the ligated hemisphere 0 h, 1 h and 2 h following hypoxia. The binding of the A1 receptor selective antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine (DPCPX) in the presence or in the absence of GTP decreased immediately after the hypoxic period in both hemispheres and returned thereafter gradually towards control. These results show that there are rapid changes in A1 receptor number on both sides of the brain, and of adenosine A1 and A2a receptor mRNA in the hemisphere that would later develop infarction. Decreases in adenosine receptors may worsen H-I brain damage and have consequences for the use of adenosine directed therapy.

Modulation of striatal adenosine A1 and A2 receptors induces rotational behaviour in response to dopaminergic stimulation in intact rats

The intraperitoneal injection of d-amphetamine (5 mg/kg i.p.), preceded (10 min before) by intrastriatal injection of an adenosine A2 receptor agonist (CGS 21680, 5-10 micrograms) or followed (5 min later) by an intrastriatal adenosine A1 receptor agonist (N6-cyclopentyladenosine, CPA, 30 micrograms), induced ipsilateral rotations in rats. The opposite effect (contralateral rotations) was observed with adenosine receptor antagonists (A2 antagonist, 3,7-dimethyl-1-propargylxanthine, DMPX, 10 micrograms; A1 antagonist, 8-cyclopentyl-1,3-dimethylxanthine, CPT, 2.5 micrograms). These results confirm that both adenosine A2 and A1 receptors modulate striatal dopaminergic neurotransmission.

CGS 21680 antagonizes motor hyperactivity in a rat model of Huntington's disease

The influence of CGS 21680 (2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido-adenos ine), an adenosine A2 receptor agonist, was tested in an animal model of Huntington's disease. Male Wistar rats received bilateral intrastriatal injections of quinolinic acid and then, 1 and 2 weeks later, they were treated with intrastriatal CGS 21680 (3 micrograms/2 microliters) or saline. While quinolinic acid-lesioned rats not treated with CGS 21680 showed the typical motor hyperresponsiveness to d-amphetamine (1 mg/kg i.p.), the intrastriatal injection of CGS 21680 completely prevented this effect.