Persistent reduction of cocaine seeking by pharmacological manipulation of adenosine A1 and A 2A receptors during extinction training in rats

Effects of adenosine A2A receptors on cognitive function in health and disease

Adenosine A2A receptors have been studied extensively in the context of motor function and movement disorders such as Parkinson's disease. In addition to these roles, A2A receptors have also been increasingly implicated in cognitive function and cognitive impairments in diverse conditions, including Alzheimer's disease, schizophrenia, acute brain injury, and stress. We review the roles of A2A receptors in cognitive processes in health and disease, focusing primarily on the effects of reducing or enhancing A2A expression levels or activities in animal models. Studies reveal that A2A receptors in neurons and astrocytes modulate multiple aspects of cognitive function, including memory and motivation. Converging evidence also indicates that A2A receptor levels and activities are aberrantly increased in aging, acute brain injury, and chronic disorders, and these increases contribute to neurocognitive impairments. Therapeutically targeting A2A receptors with selective modulators may alleviate cognitive deficits in diverse neurological and neuropsychiatric conditions. Further research on the exact neural mechanisms of these effects as well as the efficacy of selective A2A modulators on cognitive alterations in humans are important areas for future investigation.

Interaction between cocaine use and sleep behavior: A comprehensive review of cocaine's disrupting influence on sleep behavior and sleep disruptions influence on reward seeking

Dopamine, orexin (hypocretin), and adenosine systems have dual roles in reward and sleep/arousal suggesting possible mechanisms whereby drugs of abuse may influence both reward and sleep/arousal. While considerable variability exists across studies, drugs of abuse such as cocaine induce an acute sleep loss followed by an immediate recovery pattern that is consistent with a normal response to loss of sleep. Under more chronic cocaine exposure conditions, an abnormal recovery pattern is expressed that includes a retention of sleep disturbance under withdrawal and into abstinence conditions. Conversely, experimentally induced sleep disturbance can increase cocaine seeking. Thus, complementary, sleep-related therapeutic approaches may deserve further consideration along with development of non-human models to better characterize sleep disturbance-reward seeking interactions across drug experience.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Adenosine A2AReceptors in Substance Use Disorders: A Focus on Cocaine

Several psychoactive drugs can evoke substance use disorders (SUD) in humans and animals, and these include psychostimulants, opioids, cannabinoids (CB), nicotine, and alcohol. The etiology, mechanistic processes, and the therapeutic options to deal with SUD are not well understood. The common feature of all abused drugs is that they increase dopamine (DA) neurotransmission within the mesocorticolimbic circuitry of the brain followed by the activation of DA receptors. D₂ receptors were proposed as important molecular targets for SUD. The findings showed that D₂ receptors formed heteromeric complexes with other GPCRs, which forced the addiction research area in new directions. In this review, we updated the view on the brain D₂ receptor complexes with adenosine (A)2A receptors (A_2AR) and discussed the role of A_2AR in different aspects of addiction phenotypes in laboratory animal procedures that permit the highly complex syndrome of human drug addiction. We presented the current knowledge on the neurochemical in vivo and ex vivo mechanisms related to cocaine use disorder (CUD) and discussed future research directions for A_2AR heteromeric complexes in SUD.

Functional and Neuroprotective Role of Striatal Adenosine A2A Receptor Heterotetramers

In the striatum, adenosine A_2A receptors (A_2AR) are mainly expressed within the soma and dendrites of the striatopallidal neuron. A predominant proportion of these striatal postsynaptic A_2AR form part of the macromolecular complexes that include A_2AR-dopamine D₂ receptor (D₂R) heteromers, G_olf and G_i/o proteins, and the effector adenylyl cyclase (AC), subtype AC5. The A_2AR-D₂R heteromers have a tetrameric structure, constituted by A_2AR and D₂R homomers. By means of reciprocal antagonistic allosteric interactions and antagonistic interactions at the effector level between adenosine and dopamine, the A_2AR-D₂R heterotetramer-AC5 complex acts an integrative molecular device, which determines a switch between the adenosine-facilitated activation and the dopamine-facilitated inhibition of the striatopallidal neuron. Striatal adenosine also plays an important presynaptic modulatory role, driving the function of corticostriatal terminals. This control is mediated by adenosine A₁ receptors (A₁R) and A_2AR, which establish intermolecular interactions forming A₁R-A_2AR heterotetramers. Here, we review the functional role of both presynaptic and postsynaptic striatal A_2AR heterotetramers as well as their possible neuroprotective role. We hypothesize that alterations in the homomer/heteromer stoichiometry (i.e., increase or decrease in the proportion of A_2AR forming homomers or heteromers) are pathogenetically involved in neurological disorders, specifically in Parkinson's disease and restless legs syndrome.

Effects of adenosine A2A receptor antagonists on cocaine-induced locomotion and cocaine seeking

RATIONALE AND OBJECTIVES

Adenosine signaling through adenosine A_2A receptors (A_2ARs) is known to influence cocaine-induced behaviors. These studies sought to elucidate how two A_2AR antagonists distinguished by their antagonist effects at presynaptic and postsynaptic A_2AR influence cocaine-induced locomotion and cocaine seeking.

METHODS

Sprague-Dawley rats were used to assess the differential effects of SCH 442416 and istradefylline that antagonize presynaptic and postsynaptic A_2AR, respectively. We evaluated the effects of these antagonists on both basal and cocaine-induced locomotion in cocaine-naïve rats and rats that received seven daily cocaine treatments. The effects of SCH 442416 or istradefylline on cocaine seeking were measured in animals extinguished from cocaine self-administration. We assessed the effects of the A_2AR antagonists to induce cocaine seeking when administered alone and their effects on cocaine seeking induced by a cocaine-priming injection. Lastly, we evaluated the effects of the antagonists on sucrose seeking in animals extinguished from sucrose self-administration.

RESULTS

Neither istradefylline nor SCH 442416 significantly altered basal locomotion. Istradefylline enhanced acute cocaine-induced locomotion but had no effect on the expression of locomotor sensitization. SCH 44216 had no effect on acute cocaine-induced locomotion but inhibited the expression of locomotor sensitization. Istradefylline was sufficient to induce cocaine seeking and augmented both cocaine-induced seeking and sucrose seeking. SCH 442416 inhibited cocaine-induced seeking, but had no effect on sucrose seeking and did not induce cocaine seeking when administered alone.

CONCLUSIONS

These findings demonstrate differential effects of two A_2AR antagonists distinguished by their effects at pre- and postsynaptic A_2AR on cocaine-induced behaviors.

A2AR-D2R Heteroreceptor Complexes in Cocaine Reward and Addiction

The concept of allosteric receptor-receptor interactions in G protein-coupled receptor homo- and heteroreceptor complexes in which they physically interact provides a new dimension to molecular integration in the brain. The receptor-receptor interactions dynamically change recognition, pharmacology, signaling, and trafficking of the participating receptors. Among the receptor complexes, disruption of the A2A receptor-dopamine D2 receptor (A2AR-D2R) complex by an A2AR agonist has been shown to fully block the inhibition of cocaine self-administration. Cocaine induced pathological A2AR-D2R-Sigma1R complexes may form a long-term memory with a strong and permanent D2R brake, leading to cocaine addiction. These heteroreceptor complexes can potentially be targeted for future pharmacotherapy of cocaine addiction by using heterobivalent compounds or A2AR-D2R receptor interface-interfering peptides that disrupt the A2AR-D2R-Sigma1R complexes.

Disruption of A2AR-D2R Heteroreceptor Complexes After A2AR Transmembrane 5 Peptide Administration Enhances Cocaine Self-Administration in Rats

Antagonistic allosteric A2AR-D2R receptor-receptor interactions in heteroreceptor complexes counteract cocaine self-administration and cocaine seeking in rats as seen in biochemical and behavioral experiments. It was shown that the human A2AR transmembrane five (TM5) was part of the interface of the human A2AR-D2R receptor heteromer. In the current paper, the rat A2AR synthetic TM5 (synthTM5) peptide disrupts the A2AR-D2R heteroreceptor complex in HEK293 cells as shown by the bioluminescence resonance energy transfer method. Rat A2AR synthTM5 peptide, microinjected into the nucleus accumbens, produced a complete counteraction of the inhibitory effects of the A2AR agonist CGS21680 on cocaine self-administration. It was linked to a disappearance of the accumbal A2AR-D2R heteroreceptor complexes and the A2AR agonist induced inhibition of D2R recognition using proximity ligation assay and biochemical binding techniques. However, possible effects of the A2AR synthTM5 peptide on accumbal A2AR-D3R and A2AR-D4R heteroreceptor complexes remain to be excluded. Evidence is provided that accumbal A2AR-D2R-like heteroreceptor complexes with their antagonistic receptor-receptor interactions can be major targets for treatment of cocaine use disorder.

Essential Control of the Function of the Striatopallidal Neuron by Pre-coupled Complexes of Adenosine A2A-Dopamine D2 Receptor Heterotetramers and Adenylyl Cyclase

The central adenosine system and adenosine receptors play a fundamental role in the modulation of dopaminergic neurotransmission. This is mostly achieved by the strategic co-localization of different adenosine and dopamine receptor subtypes in the two populations of striatal efferent neurons, striatonigral and striatopallidal, that give rise to the direct and indirect striatal efferent pathways, respectively. With optogenetic techniques it has been possible to dissect a differential role of the direct and indirect pathways in mediating "Go" responses upon exposure to reward-related stimuli and "NoGo" responses upon exposure to non-rewarded or aversive-related stimuli, respectively, which depends on their different connecting output structures and their differential expression of dopamine and adenosine receptor subtypes. The striatopallidal neuron selectively expresses dopamine D₂ receptors (D2R) and adenosine A_2A receptors (A2AR), and numerous experiments using multiple genetic and pharmacological in vitro, in situ and in vivo approaches, demonstrate they can form A2AR-D2R heteromers. It was initially assumed that different pharmacological interactions between dopamine and adenosine receptor ligands indicated the existence of different subpopulations of A2AR and D2R in the striatopallidal neuron. However, as elaborated in the present essay, most evidence now indicates that all interactions can be explained with a predominant population of striatal A2AR-D2R heteromers forming complexes with adenylyl cyclase subtype 5 (AC5). The A2AR-D2R heteromer has a tetrameric structure, with two homodimers, which allows not only multiple allosteric interactions between different orthosteric ligands, agonists, and antagonists, but also the canonical Gs-Gi antagonistic interaction at the level of AC5. We present a model of the function of the A2AR-D2R heterotetramer-AC5 complex, which acts as an integrative device of adenosine and dopamine signals that determine the excitability and gene expression of the striatopallidal neurons. The model can explain most behavioral effects of A2AR and D2R ligands, including the psychostimulant effects of caffeine. The model is also discussed in the context of different functional striatal compartments, mainly the dorsal and the ventral striatum. The current accumulated knowledge of the biochemical properties of the A2AR-D2R heterotetramer-AC5 complex offers new therapeutic possibilities for Parkinson's disease, schizophrenia, SUD and other neuropsychiatric disorders with dysfunction of dorsal or ventral striatopallidal neurons.

The Role of Adenosine Receptors in Psychostimulant Addiction

Adenosine receptors (AR) are a family of G-protein coupled receptors, comprised of four members, named A1, A2A, A2B, and A3 receptors, found widely distributed in almost all human body tissues and organs. To date, they are known to participate in a large variety of physiopathological responses, which include vasodilation, pain, and inflammation. In particular, in the central nervous system (CNS), adenosine acts as a neuromodulator, exerting different functions depending on the type of AR and consequent cellular signaling involved. In terms of molecular pathways and second messengers involved, A1 and A3 receptors inhibit adenylyl cyclase (AC), through Gi/o proteins, while A2A and A2B receptors stimulate it through Gs proteins. In the CNS, A1 receptors are widely distributed in the cortex, hippocampus, and cerebellum, A2A receptors are localized mainly in the striatum and olfactory bulb, while A2B and A3 receptors are found at low levels of expression. In addition, AR are able to form heteromers, both among themselves (e.g., A1/A2A), as well as with other subtypes (e.g., A2A/D2), opening a whole range of possibilities in the field of the pharmacology of AR. Nowadays, we know that adenosine, by acting on adenosine A1 and A2A receptors, is known to antagonistically modulate dopaminergic neurotransmission and therefore reward systems, being A1 receptors colocalized in heteromeric complexes with D1 receptors, and A2A receptors with D2 receptors. This review documents the present state of knowledge of the contribution of AR, particularly A1 and A2A, to psychostimulants-mediated effects, including locomotor activity, discrimination, seeking and reward, and discuss their therapeutic relevance to psychostimulant addiction. Studies presented in this review reinforce the potential of A1 agonists as an effective strategy to counteract psychostimulant-induced effects. Furthermore, different experimental data support the hypothesis that A2A/D2 heterodimers are partly responsible for the psychomotor and reinforcing effects of psychostimulant drugs, such as cocaine and amphetamine, and the stimulation of A2A receptor is proposed as a potential therapeutic target for the treatment of drug addiction. The overall analysis of presented data provide evidence that excitatory modulation of A1 and A2A receptors constitute promising tools to counteract psychostimulants addiction.

Cocaine self-administration specifically increases A2AR-D2R and D2R-sigma1R heteroreceptor complexes in the rat nucleus accumbens shell. Relevance for cocaine use disorder

Adenosine 2A receptor (A2AR) agonists were indicated to reduce cocaine reward and cocaine seeking mainly through activation of antagonistic allosteric A2AR-dopamine D2R (D2R) interactions in A2AR-D2R heteroreceptor complexes. Furthermore, it was shown that modulation of cocaine reward involves antagonistic A2AR-D2R interactions in the ventral but not the dorsal striatum in rats. In the current work the proximity ligation assay (PLA) was used to further study the A2AR-D2R heteroreceptor complexes in the nucleus accumbens shell and core as well as the dorsal striatum under the influence of cocaine self-administration in rats. A significant increase in the A2AR-D2R PLA positive clusters was observed in the nucleus accumbens shell but not in the other regions vs yoked saline controls using the duolink software. Additionally, cocaine self-administration evoked a selective and significant increase in the density of D2R-sigma1R positive clusters in the nucleus accumbens shell vs yoked saline controls, while a significant reduction of the density of the D2R-sigma1R positive clusters was found in the dorsal part of the dorsal striatum. The results suggest that cocaine self-administration can reorganize A2AR and D2R into increased A2AR-D2R heteroreceptor complexes in the nucleus accumbens shell associated with increases in the D2R-sigma1R heteroreceptor complexes in this region. This reorganization can contribute to the demonstrated anti-cocaine actions of A2A receptor agonists and the putative formation of A2AR-D2R-sigma1R heterocomplexes.

Hypothermia in mouse is caused by adenosine A1 and A3 receptor agonists and AMP via three distinct mechanisms

Small mammals have the ability to enter torpor, a hypothermic, hypometabolic state, allowing impressive energy conservation. Administration of adenosine or adenosine 5'-monophosphate (AMP) can trigger a hypothermic, torpor-like state. We investigated the mechanisms for hypothermia using telemetric monitoring of body temperature in wild type and receptor knock out (Adora1-/-, Adora3-/-) mice. Confirming prior data, stimulation of the A3 adenosine receptor (AR) induced hypothermia via peripheral mast cell degranulation, histamine release, and activation of central histamine H1 receptors. In contrast, A1AR agonists and AMP both acted centrally to cause hypothermia. Commonly used, selective A1AR agonists, including N6-cyclopentyladenosine (CPA), N6-cyclohexyladenosine (CHA), and MRS5474, caused hypothermia via both A1AR and A3AR when given intraperitoneally. Intracerebroventricular dosing, low peripheral doses of Cl-ENBA [(±)-5'-chloro-5'-deoxy-N6-endo-norbornyladenosine], or using Adora3-/- mice allowed selective stimulation of A1AR. AMP-stimulated hypothermia can occur independently of A1AR, A3AR, and mast cells. A1AR and A3AR agonists and AMP cause regulated hypothermia that was characterized by a drop in total energy expenditure, physical inactivity, and preference for cooler environmental temperatures, indicating a reduced body temperature set point. Neither A1AR nor A3AR was required for fasting-induced torpor. A1AR and A3AR agonists and AMP trigger regulated hypothermia via three distinct mechanisms.

Caffeine, a common active adulterant of cocaine, enhances the reinforcing effect of cocaine and its motivational value

RATIONALE

Caffeine is one of the psychoactive substances most widely used as an adulterant in illicit drugs, such as cocaine. Animal studies have demonstrated that caffeine is able to potentiate several cocaine actions, although the enhancement of the cocaine reinforcing property by caffeine is less reported, and the results depend on the paradigms and experimental protocols used.

OBJECTIVES

We examined the ability of caffeine to enhance the motivational and rewarding properties of cocaine using an intravenous self-administration paradigm in rats. Additionally, the role of caffeine as a primer cue during extinction was evaluated.

METHODS

In naïve rats, we assessed (1) the ability of the cocaine (0.250-0.125 mg/kg/infusion) and caffeine (0.125-0.0625 mg/kg/infusion) combination to maintain self-administration in fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement compared with cocaine or caffeine alone and (2) the effect of caffeine (0.0625 mg/kg/infusion) in the maintenance of responding in the animals exposed to the combination of the drugs during cocaine extinction.

RESULTS

Cocaine combined with caffeine and cocaine alone was self-administered on FR and PR schedules of reinforcement. Interestingly, the breaking point determined for the cocaine + caffeine group was significantly higher than the cocaine group. Moreover, caffeine, that by itself did not maintain self-administration behavior in naïve rats, maintained drug-seeking behavior of rats previously exposed to combinations of cocaine + caffeine.

CONCLUSIONS

Caffeine enhances the reinforcing effects of cocaine and its motivational value. Our results highlight the role of active adulterants commonly used in cocaine-based illicit street drugs.

Connectome and molecular pharmacological differences in the dopaminergic system in restless legs syndrome (RLS): plastic changes and neuroadaptations that may contribute to augmentation

Restless legs syndrome (RLS) is primarily treated with levodopa and dopaminergics that target the inhibitory dopamine receptor subtypes D3 and D2. The initial success of this therapy led to the idea of a hypodopaminergic state as the mechanism underlying RLS. However, multiple lines of evidence suggest that this simplified concept of a reduced dopamine function as the basis of RLS is incomplete. Moreover, long-term medication with the D2/D3 agonists leads to a reversal of the initial benefits of dopamine agonists and augmentation, which is a worsening of symptoms under therapy. The recent findings on the state of the dopamine system in RLS that support the notion that a dysfunction in the dopamine system may in fact induce a hyperdopaminergic state are summarized. On the basis of these data, the concept of a dynamic nature of the dopamine effects in a circadian context is presented. The possible interactions of cell adhesion molecules expressed by the dopaminergic systems and their possible effects on RLS and augmentation are discussed. Genome-wide association studies (GWAS) indicate a significantly increased risk for RLS in populations with genomic variants of the cell adhesion molecule receptor type protein tyrosine phosphatase D (PTPRD), and PTPRD is abundantly expressed by dopamine neurons. PTPRD may play a role in the reconfiguration of neural circuits, including shaping the interplay of G protein-coupled receptor (GPCR) homomers and heteromers that mediate dopaminergic modulation. Recent animal model data support the concept that interactions between functionally distinct dopamine receptor subtypes can reshape behavioral outcomes and change with normal aging. Additionally, long-term activation of one dopamine receptor subtype can increase the receptor expression of a different receptor subtype with opposite modulatory actions. Such dopamine receptor interactions at both spinal and supraspinal levels appear to play important roles in RLS. In addition, these interactions can extend to the adenosine A₁ and A_2A receptors, which are also prominently expressed in the striatum. Interactions between adenosine and dopamine receptors and dopaminergic cell adhesion molecules, including PTPRD, may provide new pharmacological targets for treating RLS. In summary, new treatment options for RLS that include recovery from augmentation will have to consider dynamic changes in the dopamine system that occur during the circadian cycle, plastic changes that can develop as a function of treatment or with aging, changes in the connectome based on alterations in cell adhesion molecules, and receptor interactions that may extend beyond the dopamine system itself.

Cocaine self-administration differentially affects allosteric A2A-D2 receptor-receptor interactions in the striatum. Relevance for cocaine use disorder

In the current study behavioral and biochemical experiments were performed to study changes in the allosteric A2AR-D2R interactions in the ventral and dorsal striatum after cocaine self-administration versus corresponding yoked saline control. By using ex vivo [(3)H]-raclopride/quinpirole competition experiments, the effects of the A2AR agonist CGS 21680 (100 nM) on the KiH and KiL values of the D2-like receptor (D2-likeR) were determined. One major result was a significant reduction in the D2-likeR agonist high affinity state observed with CGS 21680 after cocaine self-administration in the ventral striatum compared with the yoked saline group. The results therefore support the hypothesis that A2AR agonists can at least in part counteract the motivational actions of cocaine. This action is mediated via the D2-likeR by targeting the A2AR protomer of A2AR-D2-like R heteroreceptor complexes in the ventral striatum, which leads to the reduction of D2-likeR protomer recognition through the allosteric receptor-receptor interaction. In contrast, in the dorsal striatum the CGS 21680-induced antagonistic modulation in the D2-likeR agonist high affinity state was abolished after cocaine self-administration versus the yoked saline group probably due to a local dysfunction/disruption of the A2AR-D2-like R heteroreceptor complexes. Such a change in the dorsal striatum in cocaine self-administration can contribute to the development of either locomotor sensitization, habit-forming learning and/or the compulsive drug seeking by enhanced D2-likeR protomer signaling. Potential differences in the composition and stoichiometry of the A2AR-D2R heteroreceptor complexes, including differential recruitment of sigma 1 receptor, in the ventral and dorsal striatum may explain the differential regional changes observed in the A2A-D2-likeR interactions after cocaine self-administration.

Caffeine enhances and accelerates the expression of sensitization induced by coca paste indicating its relevance as a main adulterant

BACKGROUND AND OBJECTIVES

Caffeine is an active adulterant found in several drugs of abuse including coca paste (CP). We had previously demonstrated that caffeine potentiated the acute stimulant effect induced by CP seized samples. The role of caffeine in the expression of sensitization elicited by a CP seized sample (CP1) was here evaluated.

METHODS

CP1 (equivalent dose of 10 mg/kg of cocaine), cocaine (pure, 10 mg/kg), a combination of cocaine 10 mg/kg plus caffeine 2.5 mg/kg (CP1-surrogate) and saline (control) were intraperitoneally injected in male rats under two different sensitization schedules. Ambulatory locomotion was recorded in 58 animals.

RESULTS

After five daily CP1 injections and 5 days of withdrawal, CP1-challenged animals displayed a more robust sensitization than cocaine-treated animals. When a 3 injections-regime of CP1-surrogate or cocaine was assayed, only CP1-surrogate was able to elicit sensitization.

DISCUSSION AND CONCLUSIONS

Caffeine enhances and accelerates the CP1-induced sensitization.

SCIENTIFIC SIGNIFICANCE

Results may shed light on the fast and high dependence observed in CP users.

Effects of adolescent caffeine consumption on cocaine sensitivity

Caffeine is the most commonly used psychoactive substance, and consumption by adolescents has risen markedly in recent years. We identified the effects of adolescent caffeine consumption on cocaine sensitivity and determined neurobiological changes within the nucleus accumbens (NAc) that may underlie caffeine-induced hypersensitivity to cocaine. Male Sprague-Dawley rats consumed caffeine (0.3 g/l) or water for 28 days during adolescence (postnatal day 28-55; P28-P55) or adulthood (P67-P94). Testing occurred in the absence of caffeine during adulthood (P62-82 or P101-121). Cocaine-induced and quinpirole (D2 receptor agonist)-induced locomotion was enhanced in rats that consumed caffeine during adolescence. Adolescent consumption of caffeine also enhanced the development of a conditioned place preference at a sub-threshold dose of cocaine (7.5 mg/kg, i.p.). These behavioral changes were not observed in adults consuming caffeine for an equivalent period of time. Sucrose preferences were not altered in rats that consumed caffeine during adolescence, suggesting there are no differences in natural reward. Caffeine consumption during adolescence reduced basal dopamine levels and augmented dopamine release in the NAc in response to cocaine (5 mg/kg, i.p.). Caffeine consumption during adolescence also increased the expression of the dopamine D2 receptor, dopamine transporter, and adenosine A1 receptor and decreased adenosine A2A receptor expression in the NAc. Consumption of caffeine during adulthood increased adenosine A1 receptor expression in the NAc, but no other protein expression changes were observed. Together these findings suggest that caffeine consumption during adolescence produced changes in the NAc that are evident in adulthood and may contribute to increases in cocaine-mediated behaviors.

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSION

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