Motor stimulant effects of caffeine in 6-hydroxydopamine-lesioned rats are dependent on previous stimulation of dopamine receptors: a different role of D1 and D2 receptors

Craving espresso: the dialetics in classifying caffeine as an abuse drug

Caffeine is the most consumed psychoactive substance in the world; in general, it is not associated to potentially harmful effects. Nevertheless, few studies were performed attempting to investigate the caffeine addiction. The present review was mainly aimed to answer the following question: is caffeine an abuse drug? To adress this point, the effects of caffeine in preclinical and clinical studies were summarized and critically analyzed taking account the abuse disorders described in the Diagnostic and Statistical Manual of Mental Disorders (DSM-V). We concluded that the diagnostic criteria evidenced on DSM-V to intoxication-continued use and abstinence are not well supported by clinical studies. The fact that diagnostic criteria is not widely supported by preclinical or clinical studies may be due specially to a controversy in its exactly mechanism of action: recent literature point to an indirect, rather than direct modulation of dopamine receptors, and auto-limitant consumption due to adverse sensations in high doses. On the other hand, it reports clear withdrawal-related symptoms. Thus, based on a classical action on reward system, caffeine only partially fits its mechanism of action as an abuse drug, especially because previous research does not report a clear effect of dopaminergic activity enhance on nucleus accumbens; despite this, there are reports concerning dopaminergic modulation by caffeine on the striatum. However, based on human and animal research, caffeine withdrawal evokes signals and symptoms, which are relevant enough to include this substance among the drugs of abuse.

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

BACKGROUND

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

OBJECTIVES

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

HIGHLIGHTS

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

CONCLUSIONS

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

Caffeine and a selective adenosine A2A receptor antagonist induce sensitization and cross-sensitization behavior associated with increased striatal dopamine in mice

Background

Caffeine, a nonselective adenosine A₁ and A_2A receptor antagonist, is the most widely used psychoactive substance in the world. Evidence demonstrates that caffeine and selective adenosine A_2A antagonists interact with the neuronal systems involved in drug reinforcement, locomotor sensitization, and therapeutic effect in Parkinson's disease (PD). Evidence also indicates that low doses of caffeine and a selective adenosine A_2A antagonist SCH58261 elicit locomotor stimulation whereas high doses of these drugs exert locomotor inhibition. Since these behavioral and therapeutic effects are mediated by the mesolimbic and nigrostriatal dopaminergic pathways which project to the striatum, we hypothesize that low doses of caffeine and SCH58261 may modulate the functions of dopaminergic neurons in the striatum.

Methods

In this study, we evaluated the neuroadaptations in the striatum by using reverse-phase high performance liquid chromatography (HPLC) to quantitate the concentrations of striatal dopamine and its metabolites, dihydroxylphenylacetic acid (DOPAC) and homovanilic acid (HVA), and using immunoblotting to measure the level of phosphorylation of tyrosine hydroxylase (TH) at Ser31, following chronic caffeine and SCH58261 sensitization in mice. Moreover, to validate further that the behavior sensitization of caffeine is through antagonism at the adenosine A_2A receptor, we also evaluate whether chronic pretreatment with a selective adenosine A_2A antagonist SCH58261 or a selective adenosine A₁ antagonist DPCPX can sensitize the locomotor stimulating effects of caffeine.

Results

Chronic treatments with low dose caffeine (10 mg/kg) or SCH58261 (2 mg/kg) increased the concentrations of dopamine, DOPAC and HVA, concomitant with increased TH phosphorylation at Ser31 and consequently enhanced TH activity in the striatal tissues in both caffeine- and SCH58261-sensitized mice. In addition, chronic caffeine or SCH58261 administration induced locomotor sensitization, and locomotor cross-sensitization to caffeine was observed following chronic treatment of mice with SCH58261 but not with DPCPX.

Conclusions

Our study demonstrated that low dosages of caffeine and a selective adenosine A_2A antagonist SCH58261 elicited locomotor sensitization and cross-sensitization, which were associated with elevated dopamine concentration and TH phosphorylation at Ser31 in the striatum. Blockade of adenosine A_2A receptor may play an important role in the striatal neuroadaptations observed in the caffeine-sensitized and SCH58261-sensitized mice.

Caffeine and a selective adenosine A2A receptor antagonist induce reward and sensitization behavior associated with increased phospho-Thr75-DARPP-32 in mice

RATIONALE

Caffeine, an antagonist of adenosine A(1) and A(2A) receptor, is the most widely used psychoactive substance in the world. Evidence indicates that caffeine interacts with the neuronal systems involved in drug reinforcing. Although adenosine A(1) and/or A(2A) receptor have been found to play important roles in the locomotor stimulation and probably reinforcing effect of caffeine, the relative contribution of the A(1) and/or A(2A) receptors to the acute and chronic motor activation and reinforcing effects of caffeine has not been completely investigated.

OBJECTIVE

The roles of adenosine A(1) and/or A(2A) receptor and the association of phospho-Thr75-dopamine- and cAMP-regulated phosphoprotein of molecular weight 32 kDa (DARPP-32) in the motor activation and reinforcing effects of caffeine, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) antagonist, and 5-amino-7-(beta-phenylethyl)-2-(8-furyl) pyrazolol [4,3-e]-1,2,4-triazolol [1,5-c] pyrimidine (SCH58261), a selective A(2A) receptor antagonist were examined.

METHODS

Locomotor stimulation and behavioral sensitization of caffeine, DPCPX, and SCH58261 were studied in C57BL/6 male mice following acute and chronic administration. Conditioned place preference (CPP) paradigm was used to evaluate the drug-seeking potential of these compounds. Furthermore, the expression of phospho-Thr75-DARPP-32 in striatal membrane from behaviorally sensitized mice was analyzed by Western blot.

RESULTS

Caffeine and SCH58261 but not DPCPX induced CPP and locomotor sensitization in C57BL/6 mice. The locomotor sensitization after chronic treatment was associated with increased DARPP-32 phosphorylation at Thr75 in the striatum.

CONCLUSION

Caffeine-induced reinforcing effect and behavioral sensitization are mediated by antagonism at adenosine A(2A) receptor. These effects are associated with phosphorylation of DARPP-32 at Thr75 in the striatum.

The effects of systemic, intrastriatal, and intrapallidal injections of caffeine and systemic injections of A2A and A1 antagonists on forepaw stepping in the unilateral 6-OHDA-lesioned rat

RATIONALE AND OBJECTIVES

Given that adenosine A2A antagonists appear to be therapeutic in several animal models of Parkinson's disease (PD), we examined the extent to which caffeine and selective A2A and A1 antagonists could enhance contralateral forepaw stepping in the unilateral 6-OHDA-lesioned rat.

MATERIALS AND METHODS

Following unilateral injections of 12 microg 6-OHDA into the medial forebrain bundle (MFB), frequency of stepping with both front paws was counted separately as the paws were dragged anteriorally and laterally by a treadmill.

RESULTS

The MFB lesions decreased contralateral stepping by 74-83%, and 8 mg/kg 3,4-dihydroxy-L-phenylalanine (L-DOPA) increased contralateral stepping by 25-26%. Caffeine given systemically (15 mg/kg) or into the dorsal striatum or external globus pallidus (GPE; 20-40 microg) increased contralateral forepaw stepping by 14%, 27%, and 26%, respectively, and enhanced the effect of 8 mg/kg L-DOPA on stepping. The selective A(2A) antagonist SCH-58261 (2 mg/kg) also increased stepping by 13% and enhanced the therapeutic effect of L-DOPA, whereas the selective A(1) [corrected] antagonist 8-cyclopentyltheophylline (3-7 mg/kg) and A(1) agonist N(6)-cyclopentyladenosine (0.03-0.2 mg/kg) had no effect. None of these drugs appeared to produce dyskinesic effects.

CONCLUSIONS

In this well-validated animal model of the akinesic effects of PD, caffeine and a selective A2A, but not an A1, antagonist were able to provide both monotherapeutic and adjunctive therapeutic effects. These data are consistent with the hypothesis that A2A antagonists may be therapeutic in human PD patients and indicate that the dorsal striatum and GPE are critical sites of therapeutic action.

Role of adenosine A2A receptors in parkinsonian motor impairment and l-DOPA-induced motor complications

Adenosine A2A receptors have a unique cellular and regional distribution in the basal ganglia, being particularly concentrated in areas richly innervated by dopamine such as the caudate-putamen and the globus pallidus. Adenosine A2A receptors are selectively located on striatopallidal neurons and are capable of forming functional heteromeric complexes with dopamine D2 and metabotropic glutamate mGlu5 receptors. Based on the unique cellular and regional distribution of this receptor and in line with data showing that A2A receptor antagonists improve motor symptoms in animal models of Parkinson's disease (PD) and in initial clinical trials, A2A receptor antagonists have emerged as an attractive non-dopaminergic target to improve the motor deficits that characterize PD. Experimental data have also shown that A2A receptor antagonists do not induce neuroplasticity phenomena that complicate long-term dopaminergic treatments. The present review provides an updated summary of results reported in the literature concerning the biochemical characteristics and basal ganglia distribution of A2A receptors. We subsequently aim to examine the effects of adenosine A2A antagonists in rodent and primate models of PD and of l-DOPA-induced dyskinesia. Finally, concluding remarks are made on post-mortem human brains and on the translation of adenosine A2A receptor antagonists in the treatment of PD.

Sustained improvement of motor function in hemiparkinsonian rats chronically treated with low doses of caffeine or trihexyphenidyl

The effects of chronic oral treatment with low doses of caffeine (1-3 mg/kg) and trihexyphenidyl (0.1-0.2 mg/kg) were tested on hemiparkinsonian rats, which received the following treatments in a counterbalanced order: vehicle, caffeine, trihexyphenidyl, and caffeine plus trihexyphenidyl. Three preclinical models were used: the stepping test, the cylinder test, and the staircase test. Compared to pre-lesion values, the forepaw contralateral to the dopamine-denervated side showed impaired stepping, fewer wall contacts in the cylinder test, and fewer pellets retrieved in the staircase test. In the stepping test both doses of caffeine produced a complete recovery of motor function (100%), whereas the effect of trihexyphenidyl was less intense (77-80%). In this same test the maximal effect of drugs did not develop tolerance during 2-3 weeks, and was completely reversible after drug cessation. In the cylinder test only the wall contacts performed simultaneously with both forepaws were significantly increased by caffeine (3 mg/kg) and trihexyphenidyl (0.2 mg/kg), and this effect was also reversible. In the staircase test none of the treatments improved food pellet retrieval with the contralateral forepaw. Altogether, these results show that chronic treatment with caffeine, at doses similar to daily human consumption, produces a sustained improvement in the use of the contralateral forelimb in unilaterally 6-hydroxydopamine denervated rats, without the development of tolerance. Although the combined administration of caffeine plus trihexyphenidyl showed no synergism in these models, the results suggest that low doses of caffeine (1-3 mg/kg/day) could be of therapeutic value for the reversal of motor symptoms in parkinsonian patients.

Sensitization to caffeine and cross-sensitization to amphetamine: Influence of individual response to caffeine

The present study evaluated the ability of a subchronic intermittent administration of caffeine to induce a sensitized motor response and correlated the individual susceptibility of rats to acute caffeine to the development of sensitization. Moreover, individual susceptibility to caffeine and development of motor behaviour sensitization were correlated to the behavioural response obtained after a challenge with amphetamine. To this end, rats were subdivided in "low" and "high" responders according to their individual susceptibility to acute caffeine established on the basis of the motor activity observed after the first caffeine administration. "Low" and "high" responder rats were then repeatedly and intermittently treated with caffeine (15 mg/kg, i.p.), or vehicle, every other day for fourteen days. Three days after treatment discontinuation, behavioural activation induced by acute amphetamine (0.5 mg/kg, s.c.) was measured in vehicle- and caffeine-pretreated rats. Subchronic caffeine resulted in motor sensitization of a variable degree among rats and no difference were observed between "low" and "high" responders. Moreover, caffeine pretreatment potentiated the behavioural effects of amphetamine according to the degree of caffeine sensitization but not to individual susceptibility to acute caffeine. These results demonstrate that individual susceptibility to acute caffeine does not influence the modifications in caffeine motor effects produced by its subchronic administration and does not affect the enhancement of acute behavioural effects of amphetamine in caffeine-pretreated rats, rather sensitization to subchronic caffeine administration critically influences the behavioural effects of amphetamine.

Subchronic-intermittent caffeine amplifies the motor effects of amphetamine in rats

Caffeine, the most widely consumed psychostimulant drug, acutely stimulates motor behaviour and enhances dopamine agonists actions whilst chronically it induces tolerance to either caffeine- or dopamine agonist-induced motor activating effects. The present study examined whether subchronic caffeine administration (15 mg/kg, on alternate days for 14 days) induces enduring modifications in caffeine- and amphetamine-mediated motor activity. To this end, motor activation and rotational behaviour stimulated by either caffeine or D-amphetamine (0.5, 2 mg/kg), given 3 days after the last caffeine administration, were evaluated in neurologically intact and unilaterally 6-hydroxydopamine-lesioned rats respectively. Subchronic caffeine resulted in an increase in caffeine-induced motor and turning behaviour. Furthermore, caffeine pretreatment potentiated the motor effects of amphetamine in both intact and 6-hydroxydopamine-lesioned rats. These results suggest that subchronic caffeine treatment results in an enhancement of its motor stimulant effects, rather than in tolerance, and induces neuroadaptive facilitatory changes in dopamine transmission.

Potentiation of amphetamine-mediated responses in caffeine-sensitized rats involves modifications in A2A receptors and zif-268 mRNAs in striatal neurons

Subchronic intermittent administration of caffeine induces sensitization of motor behaviour and promotes cross-sensitization to amphetamine motor activity. In order to evaluate the possible mechanisms at the basis of these effects, modifications in A(2A) receptor and zif-268 mRNAs were evaluated in rats subchronically treated with caffeine (15 mg/kg i.p.) and challenged with caffeine (15 mg/kg i.p.) or amphetamine (0.5, 1 mg/kg s.c.) 3 days after discontinuation of treatment. Results showed that the sensitized motor response to caffeine was associated with a decrease of adenosine A(2A) receptor and zif-268 mRNA levels in the striatum and nucleus accumbens, whereas cross-sensitization to amphetamine was linked to a more pronounced increase of zif-268 mRNA levels in the striatum, but not in the nucleus accumbens. Single-cell analysis showed that zif-268 mRNA modifications occurred in Enk(+) striatopallidal neurons after acute or subchronic treatment with caffeine and in Enk(-) striatonigral neurons after acute amphetamine administration. Potentiation of amphetamine effects was not associated with modifications of amphetamine-induced dopamine release in nucleus accumbens in caffeine-pretreated rats compared with vehicle-pretreated rats. Results demonstrate that sensitization to caffeine and cross-sensitization to amphetamine are associated with post-synaptic neuroadaptive changes in selective neuronal populations of the striatum.

Cross-sensitization between caffeine- and l-dopa-induced behaviors in hemiparkinsonian mice

Adenosine A(2A) receptor (A(2A)R) antagonists, including the non-specific adenosine antagonist caffeine, have been proposed as a novel, non-dopaminergic treatment strategy for Parkinson's disease (PD). However, the long-term interaction between caffeine and L-dopa treatment in PD models has not been characterized. We examined the interaction between caffeine and L-dopa following a repeated treatment paradigm in hemiparkinsonian mice. In contrast to the progressively sensitized rotational behavior induced by daily L-dopa (2.0 mg/kg) treatment, tolerance for the rotational response to daily caffeine (2.5 or 10 mg/kg) treatment tended to develop over several weeks. However, after a subsequent two-week washout, challenge with same drug demonstrated an extinction of the sensitized L-dopa-induced rotation, but a sensitization of the caffeine-induced rotation. In a cross-challenge paradigm, daily treatment of mice with L-dopa (compared to daily saline) produced a three-fold enhancement in the rotational response to a subsequent re-challenge with caffeine. Similarly, daily treatment of mice with caffeine produced a six-fold enhancement in the rotational response to a subsequent re-challenge with L-dopa. Furthermore, daily co-administration of caffeine plus L-dopa produced enhanced rotational behavior, compared to caffeine or L-dopa alone, indicating an additive or synergistic interaction between caffeine and L-dopa during repeated treatment. Cross-sensitization between caffeine and L-dopa following repeated treatment and their positive interaction during chronic co-adminstration in hemiparkinsonian mice suggest that repeated exposure to caffeine may alter L-dopa responses in PD.

Subchronic intermittent caffeine administration to unilaterally 6-hydroxydopamine-lesioned rats sensitizes turning behaviour in response to dopamine D1 but not D2 receptor agonists

The effects of caffeine, an antagonist of adenosine A(1) and A(2A) receptors, are significantly influenced by modifications in dopamine transmission. Administration of caffeine to unilaterally 6-hydroxydopamine-lesioned rats induces ipsilateral turning behaviour in rats never exposed to a dopamine receptor agonist, whereas contralateral turning is elicited if rats are repeatedly primed with a dopamine receptor agonist. In this study, rats unilaterally lesioned with 6-hydroxydopamine and subchronically treated with an intermittent administration of caffeine (15 mg/kg) or vehicle, were administered, 3 days after discontinuations of the treatment, with the dopamine D(1) receptor agonist 1-phenyl 1,2,3,4,5-tetrahydro(1H)-3-benzazepine-7,8-diolhydrochloride (SKF 38393), the D(2)/D(3) receptor agonist quinpirole, the D(2) receptor agonist R(-)-propylnorapomorphine or the dopamine precursor L-3,4-dihydroxyphenyl-alanine. Administration of SKF 38393 (1.5 mg/kg) or L-3,4-dihydroxyphenyl-alanine (6 mg/kg), but not quinpirole (0.15 mg/kg) or R(-)-propylnorapomorphine (0.01 mg/kg), induced a significantly higher contralateral turning behaviour in rats subchronically treated with caffeine than in vehicle-pretreated rats. The results show that repeated intermittent caffeine exposure enhances the motor stimulant effects elicited by dopamine agonists by a preferential sensitization of dopamine D(1) receptors.

Caffeine and the dopaminergic system

Caffeine is the most widely consumed psychostimulant substance, being self-administered throughout a wide range of conditions and present in numerous dietary products. Due to its widespread use and low abuse potential, caffeine is considered an atypical drug of abuse. The main mechanism of action of caffeine occurs via the blockade of adenosine A1 and A2A receptors. Adenosine is a modulator of CNS neurotransmission and its modulation of dopamine transmission through A2A receptors has been implicated in the effects of caffeine. This review provides an updated summary of the results reported in the literature concerning the behavioural pharmacology of caffeine and the neurochemical mechanisms underlying the psychostimulant effects elicited by caffeine. The review focuses on the effects of caffeine mediated by adenosine A2A receptors and on the influence that pre-exposure to caffeine may exert on the effects of classical drugs of abuse.

New adenosine A2A receptor antagonists: Actions on Parkinson's disease models

The 8-substituted 9-ethyladenine derivatives: 8-bromo-9-ethyladenine (ANR 82), 8-ethoxy- 9-ethyladenine (ANR 94), and 8-furyl-9-ethyladenine (ANR 152) have been characterized in vitro as adenosine receptor antagonists. Adenosine is deeply involved in the control of motor behaviour and substantial evidences indicate that adenosine A(2A) receptor antagonists improve motor deficits in animal models of Parkinson's disease. On this basis, the efficacy of ANR 82, ANR 94, and ANR 152 in rat models of Parkinson's disease was evaluated. All compounds tested reversed the catalepsy induced by haloperidol. However, in unilaterally 6-hydroxydopamine-lesioned rats, only ANR 94 and ANR 152 potentiated l-dihydroxy-phenylalanine (l-DOPA) effect on turning behaviour and induced contralateral turning behaviour in rats sensitised to l-DOPA. Taken together the results of this study indicate that some 8-substituted 9-ethyladenine derivatives ameliorate motor deficits in rat models of Parkinson's disease, suggesting a potential therapeutic role of these compounds.

Subthalamic high frequency stimulation induced rotations are differentially mediated by D1 and D2 receptors

High frequency stimulation (HFS) of the subthalamic nucleus (STN) has clinically emerged as a promising approach in the treatment of Parkinson's disease, epilepsy, dystonia as well as compulsive and possibly other mood disorders. The underlying mechanisms are incompletely understood, but are definitely related to high frequency and likely to involve the dopamine (DA)-system. To further test this hypothesis the present study investigated the modulation of STN-HFS-induced circling by systemic and intracerebral injection of drugs acting on DA receptors in naive freely moving rats. Within this experimental setup, unilateral STN-HFS alone induced intensity-dependent circling. Systemic injections of selective D1- (SCH-23390) and D2-((-)-sulpiride) antagonists as well as the mixed D1 and D2 agonist apomorphine dose-dependently reduced STN-HFS-induced rotational behavior. Intracerebral microinjections of (-)-sulpiride but not SCH-23390 decreased circling when injected intrastriatally and increased the number of rotations when injected intranigrally (pars reticulata (SNr)). These data reveal that STN-HFS-induced contralateral circling is differentially modulated by D1 and D2 receptors. While D2 receptor-mediated effects involve the dorso-/ventrolateral striatum and the SNr, D1 receptors probably exert their actions via brain areas outside the striatum and SNr. These findings suggest the nigrostriatal DA-system to be specifically involved in the mediation of STN-HFS-induced motor effects.

Reverse microdialysis of a dopamine D2 receptor antagonist alters extracellular adenosine levels in the rat nucleus accumbens

Recent evidence suggests that modulation of dopaminergic transmission alters striatal levels of extracellular adenosine. The present study used reverse microdialysis of the selective dopamine D(2) receptor antagonist raclopride to investigate whether a blockade of dopamine D(2) receptors modifies extracellular adenosine concentrations in the nucleus accumbens. Results reveal that perfusion of raclopride produced an increase of dialysate adenosine which was significant with a high (10 mM) and intermediate (1 mM) drug concentration, but not with lower drug concentrations (10 and 100 microM). Thus, the present study demonstrates that a selective blockade of dopamine D(2) receptors in the nucleus accumbens produced a pronounced increase of extracellular adenosine. The cellular mechanisms underlying this effect are yet unknown. It is suggested that the increase of extracellular adenosine might be related to a homeostatic modulatory mechanism proposed to be a key function of adenosine in response to neuronal metabolic challenges.

Subchronic caffeine exposure induces sensitization to caffeine and cross-sensitization to amphetamine ipsilateral turning behavior independent from dopamine release

We have recently shown that repeated exposure to caffeine sensitizes rats to the motor activating effects of dopamine D(1) and D(2) receptor agonists. In order to study the role of dopamine in this effect, sensitization to caffeine and cross-sensitization between caffeine and amphetamine was evaluated by studying turning behavior and in vivo striatal dopamine release in unilaterally 6-hydroxydopamine-lesioned rats. Administration of caffeine (15 mg/kg) for 2 weeks, on alternate days, induced a significant increase in ipsilateral turning behavior during the course of treatment, indicating that sensitization to caffeine took place in the intact striatum. Caffeine modestly increased dopamine release in the intact dorsa-lateral striatum and no significant difference between the first (+38%) and the last (+51%) injection was observed. Amphetamine (2 mg/kg) induced a significantly higher ipsilateral turning behavior in caffeine-sensitized rats than in vehicle-pretreated rats, however, a similar increase in dopamine release (+900 and +800%) was observed in the two groups. The results are the first demonstration that caffeine pre-exposure sensitizes the motor-stimulant effects of caffeine itself and of amphetamine. Sensitized ipsilateral turning after caffeine and amphetamine are not correlated to modification in striatal dopamine release, rather, postsynaptic modifications in dopamine and adenosine receptor interaction might be involved in the sensitization phenomena observed.

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

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

Inactivation of adenosine A2A receptors selectively attenuates amphetamine-induced behavioral sensitization

Repeated treatment with the psychostimulant amphetamine produces behavioral sensitization that may represent the neural adaptations underlying some features of psychosis and addiction in humans. In the present study we investigated the role of adenosine A(2A) receptors in psychostimulant-induced locomotor sensitization using an A(2A) receptor knockout (A(2A) KO) model. Daily treatment with amphetamine for 1 week resulted in an enhanced motor response on day 8 (by two-fold compared to that on day 1), and remained enhanced at day 24 upon rechallenge with amphetamine. By contrast, locomotor sensitization to daily amphetamine did not develop in A(2A) KO mice on day 8 or 24, and this absence was not the result of a nonspecific threshold effect. The absence of behavioral sensitization was selective for amphetamine since daily treatment with the D(1) agonist SKF81297 (2.5 mg/kg) or the D(2) agonist quinpirole (1.0 mg/kg) produced similar behavioral sensitization in both WT and A(2A) KO mice. Furthermore, coinjection of SKF81297 and quinpirole also resulted in indistinguishable locomotor sensitization in A(2A) KO and WT mice, suggesting normal D(1) and D(2) receptor responsiveness. Finally, at the cellular level A(2A) receptor inactivation abolished the increase in striatal dynorphin mRNA induced by repeated amphetamine administration. The selective absence of amphetamine-induced behavioral sensitization in A(2A) KO mice suggests a critical role of the A(2A) receptor in the development of psychostimulant-induced behavioral sensitization, and supports the pharmacological potential of A(2A) adenosinergic agents to modulate adaptive responses to repeated psychostimulant exposure.

Modification of adenosine extracellular levels and adenosine A(2A) receptor mRNA by dopamine denervation

Adenosine A(2A) receptor antagonists have been proposed as an effective therapy in the treatment of Parkinson's disease. To explore the possibility that dopamine denervation may produce modifications in adenosine A(2A) transmission, we measured the extracellular concentration of adenosine and adenosine A(2A) receptor mRNA in the striatum of rats infused unilaterally with 6-hydroxydopamine in the medial forebrain bundle. Fifteen days after 6-hydroxydopamine infusion, extracellular adenosine levels, measured by in vivo microdialysis, were significantly lower (-35%) in the dopamine-denervated striatum. At the time of the decrease in adenosine levels, an increase in striatal adenosine A(2A) receptor mRNA levels (+20%), measured by in situ hybridization, was observed. Modifications in adenosine A(2A) transmission, following nigrostriatal dopamine neuron degeneration, establish a potential neural basis for the effectiveness of adenosine A(2A) receptor antagonists in the treatment of Parkinson's disease.

Adenosine A(2a) receptor antagonists: potential therapeutic and neuroprotective effects in Parkinson's disease

The most effective treatment of Parkinson's disease (PD) is, at present, the dopamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA), however a number of disadvantages such as a loss of drug efficacy and severe side-effects (psychoses, dyskinesias and on-off phenomena) limit long-term effective utilisation of this drug. Recent experimental studies in which selective antagonists of adenosine A(2A) receptors were used, have shown an improvement in motor disabilities in animal models of PD. The A(2A) antagonist [7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-(4,3-e)-1,2,4-triazolo(1,5-c) pyrimidine] (SCH 58261) potentiated the contralateral turning behavior induced by a threshold dose of L-DOPA or direct dopamine receptor agonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, an effect accompanied by an increase in Fos-like-immunoreactivity in neurons of the lesioned striatum. Likewise, other A(2A) receptor antagonists such as (3,7-dimethyl-1-propargylxanthine) (DMPX), [E-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine] (KF 17837) and [E-1,3-diethyl-8(3,4-dimethoxystyryl-7-methyl-3,7-dihydro-1H-purine-2,6-dione] (KW 6002) antagonized catalepsy induced by haloperidol or reserpine in the rat, whereas in non-human primate models of PD, KW 6002 reduced the rigidity and improved the disability score of MPTP-treated marmosets and cynomolgus monkeys. Moreover, in contrast to L-DOPA, selective A(2A) receptor antagonists administered chronically did not produce dyskinesias and did not evoke tolerance in 6-OHDA and MPTP models of PD. An additional therapeutic potential of adenosine A(2A) antagonists emerged from studies showing neuroprotective properties of these compounds in animal models of cerebral ischemia and excitotoxicity, as well as in the MPTP model of PD. Adenosine A(2A) receptor antagonists by reversing motor impairments in animal models of PD and by contrasting cell degeneration are some of the most promising compounds for the treatment of PD.

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

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

Lack of tolerance to motor stimulant effects of a selective adenosine A(2A) receptor antagonist

It is well known that tolerance develops to the actions of caffeine, which acts as an antagonist on adenosine A(1) and A(2A) receptors. Since selective adenosine A(2A) antagonists have been proposed as adjuncts to 3,4-dihydroxyphenylalanine (L-DOPA) therapy in Parkinson's disease we wanted to examine if tolerance also develops to the selective A(2A) receptor antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2, 4-triazolo [1,5-c]pyrimidine (SCH 58261). SCH 58261 (0.1 and 7.5 mg/kg) increased basal locomotion and the motor stimulation afforded by apomorphine. Neither effect was subject to tolerance following long-term treatment with the same doses given intraperitoneally twice daily. There were no adaptive changes in A(1) and A(2A) adenosine receptors or their corresponding messenger RNA or in dopamine D(1) or D(2) receptors. These results demonstrate that the tolerance that develops to caffeine is not secondary to its inhibition of adenosine A(2A) receptors. The results also offer hope that long-term treatment with an adenosine A(2A) receptor antagonist may be possible in man.

Cross-sensitization between the motor activating effects of bromocriptine and caffeine: role of adenosine A(2A) receptors

The acute motor response to caffeine was studied in rats repeatedly treated with vehicle or the dopamine D(2) agonist bromocriptine either in a novel cage or in the home cage. Rats receiving bromocriptine (5 mg/kg i.p.) in a novel cage were sensitized to the motor stimulating effects of bromocriptine itself and showed cross-sensitization to the acute administration of low (10 mg/kg s.c. ) but not high (25 mg/kg s.c.) doses of caffeine, no matter if the novel cage was identical or different from the test cage. In contrast, caffeine (10 mg/kg i.p.) administered to rats which had received bromocriptine (5 mg/kg i.p.) in the home cage and which showed no sign of a sensitized response to bromocriptine, failed to show an increased locomotor and stereotyped response as compared to vehicle pretreated rats. Similarly to caffeine, the selective adenosine A(2A) antagonist SCH 58261 (3 mg/kg i.p.) showed an increased motor response in bromocriptine sensitized rats. The sensitized response to caffeine or SCH 58261 did not appear to be due to an higher basal motor activity of bromocriptine sensitized rats since acute administration of vehicle induced a similar motor response in bromocriptine and vehicle pretreated rats. Dopamine D(2) and adenosine A(2A) receptors are colocalized in striatal efferent neurons where they control in an opposite direction motor behavior. The results of the present study showed that changes in the sensitivity of D(2) receptors influenced the sensitivity of the adenosine antagonist caffeine through an action on A(2A) receptors. D(2) and A(2A) receptors, therefore, not only acutely interact in the mediation of motor behavior but long-term modification of the D(2) receptors, such as sensitization, affected the response of adenosine A(2A) receptors.

Functional uncoupling of adenosine A(2A) receptors and reduced responseto caffeine in mice lacking dopamine D2 receptors

Dopamine D(2) receptors (Rs) and adenosine A(2A)Rs are coexpressed on striatopallidal neurons, where they mediate opposing actions. In agreement with the idea that D(2)Rs tonically inhibit GABA release from these neurons, stimulation-evoked GABA release was significantly greater from striatal/pallidal slices from D(2)R null mutant (D(2)R(-/-)) than from wild-type (D(2)R(+/+)) mice. Release from heterozygous (D(2)R(+/-)) slices was intermediate. However, contrary to predictions that A(2A)R effects would be enhanced in D(2)R-deficient mice, the A(2A)R agonist CGS 21680 significantly increased GABA release only from D(2)R(+/+) slices. CGS 21680 modulation was observed when D(2)Rs were antagonized by raclopride, suggesting that an acute absence of D(2)Rs cannot explain the results. The lack of CGS 21680 modulation in the D(2)R-deficient mice was also not caused by a compensatory downregulation of A(2A)Rs in the striatum or globus pallidus. However, CGS 21680 significantly stimulated cAMP production only in D(2)R(+/+) striatal/pallidal slices. This functional uncoupling of A(2A)Rs in the D(2)R-deficient mice was not explained by reduced expression of G(s), G(olf), or type VI adenylyl cyclase. Locomotor activity induced by the adenosine receptor antagonist caffeine was significantly less pronounced in D(2)R(-/-) mice than in D(2)R(+/+) and D(2)R(+/-) mice, further supporting the idea that D(2)Rs are required for caffeine activation. Caffeine increased c-fos only in D(2)R(-/-) globus pallidus. The present results show that a targeted disruption of the D(2)R reduces coupling of A(2A)Rs on striatopallidal neurons and thereby responses to drugs that act on adenosine receptors. They also reinforce the ideas that D(2)Rs and A(2A)Rs are functionally opposed and that D(2)R-mediated effects normally predominate.