Three predictive scores compared in a retrospective multicenter study of nonunion tibial shaft fracture.

BACKGROUND: Delayed union, malunion, and nonunion are serious complications in the healing of fractures. Predicting the risk of nonunion before or after surgery is challenging. AIM: To compare the most prevalent predictive scores of nonunion used in clinical practice to determine the most accurate score for predicting nonunion. METHODS: We collected data from patients with tibial shaft fractures undergoing surgery from January 2016 to December 2020 in three different trauma hospitals. In this retrospective multicenter study, we considered only fractures treated with intramedullary nailing. We calculated the tibia FRACTure prediction healING days (FRACTING) score, Nonunion Risk Determination score, and Leeds-Genoa Nonunion Index (LEG-NUI) score at the time of definitive fixation. RESULTS: Of the 130 patients enrolled, 89 (68.4%) healed within 9 months and were classified as union. The remaining patients (n = 41, 31.5%) healed after more than 9 months or underwent other surgical procedures and were classified as nonunion. After calculation of the three scores, LEG-NUI and FRACTING were the most accurate at predicting healing. CONCLUSION: LEG-NUI and FRACTING showed the best performances by accurately predicting union and nonunion.

Rewarding, reinforcing and incentive salient events involve orexigenic hypothalamic neuropeptides regulating mesolimbic dopaminergic neurotransmission.

The hypothalamus is an integrated energy sensing system interfacing with higher motivational structures of the mesocorticolimbic dopamine (DA) system. This interconnectivity is strictly regulated by a number of orexigenic hypothalamic neuropeptides, including especially ghrelin, orexins and neuropeptide Y (NPY), enabling the latter to modulate salient events of natural and chemical reinforcers. In this review we aim to analyse the current knowledge on these three orexigenic neuropeptide systems that are involved in the DAergic regulation of psychostimulant behaviours. We will first review the co-existing interactions between ghrelin, orexins and NPY in hypothalamic nuclei. We will next outline whether these neuropeptides can affect DAergic neurotransmission by either regulating the firing rate of DA neurons in the ventral tegmental area (VTA) or by presynaptically interacting on the DAergic nerve terminals. Finally, we will underscore the main studies that outlined the involvement of ghrelin, orexins and NPY with rewarding, reinforcing and incentive properties of natural reinforcers and drugs of abuse. The reciprocal hypothalamic interaction of ghrelin, orexins and NPY might represent a new central view on neuronal mechanisms regulating the behavioural phenomenology of addiction maintained by the DA system.

Serotonin antagonists in the five-choice serial reaction time task and their interactions with nicotine.

Much research has implicated the serotonin (5-HT) system in cognitive functioning and psychomotor stimulant abuse, but its role depends on the subtypes of 5-HT receptors involved and the nature of the behavioural task. Here we aimed to extend previous studies by examining the role of 5-HT1A and 5-HT2C receptors in attentional performance. The effects of the selective 5-HT antagonists WAY-100635 and SB-242084 were assessed alone and for interactions with nicotine in the five-choice serial reaction time task in rats. The effects of several doses of WAY-100635 were tested in combination with a fixed dose of nicotine, and then various doses of nicotine were tested in combination with SB-242084. Systemic administration of WAY-100635 and SB-242084 induced opposing effects on speed-related measures in the five-choice serial reaction time task, with antagonism at 5-HT1A receptors increasing omission errors and response latency, and antagonism at 5-HT2C receptors reducing both omissions and latency, and also increasing anticipatory responses; neither drug affected accuracy. Nicotine itself improved all main indices of attention, and there was preliminary evidence that the detrimental effects of WAY-100635 on response latency were weakened by nicotine. Conversely, treatment with SB-242084 enhanced all speed-related indices of performance to above the levels seen under the influence of nicotine alone, thus suggesting that 5-HT2C antagonists might be useful to decrease reaction times if used as an add-on therapy to treat attentional decline.

The efficacy of sodium channel blockers to prevent phencyclidine-induced cognitive dysfunction in the rat: potential for novel treatments for schizophrenia.

Sodium channel inhibition is a well precedented mechanism used to treat epilepsy and other hyperexcitability disorders. The established sodium channel blocker and broad-spectrum anticonvulsant lamotrigine is also effective in the treatment of bipolar disorder and has been evaluated in patients with schizophrenia. Double-blind placebo-controlled clinical trials found that the drug has potential to reduce cognitive symptoms of the disorder. However, because of compound-related side-effects and the need for dose titration, a conclusive evaluation of the drug's efficacy in patients with schizophrenia has not been possible. (5R)-5-(4-{[(2-Fluorophenyl)methyl]oxy}phenyl)-l-prolinamide (GSK2) and (2R,5R)-2-(4-{[(2-fluorophenyl)methyl]oxy}phenyl)-7-methyl-1,7-diazaspiro[4.4]nonan-6-one (GSK3) are two new structurally diverse sodium channel blockers with potent anticonvulsant activity. In this series of studies in the rat, we compared the efficacy of the two new molecules to prevent a cognitive deficit induced by the N-methyl-d-aspartic acid receptor antagonist phencyclidine (PCP) in the reversal-learning paradigm in the rat. We also explored the effects of the drugs to prevent brain activation and neurochemical effects of PCP. We found that, like lamotrigine, both GSK2 and GSK3 were able to prevent the deficit in reversal learning produced by PCP, thus confirming their potential in the treatment of cognitive symptoms of schizophrenia. However, higher doses than those required for anticonvulsant efficacy of the drugs were needed for activity in the reversal-learning model, suggesting a lower therapeutic window relative to mechanism-dependent central side effects for this indication.

Orexin-1 receptor antagonist SB-334867 reduces the acquisition and expression of cocaine-conditioned reinforcement and the expression of amphetamine-conditioned reward.

Preclinical evidence suggests an important role of the brain orexin system in behaviours related to drug addiction. This study aimed at assessing the effect of the orexin-1 receptor antagonist SB-334867 on aspects of psychostimulant-conditioned behaviours that are thought to contribute to the maintenance of and relapse to psychostimulant drug use. Rats were first allowed to nose poke for cocaine infusions associated with a cue light presentation (conditioned stimulus; CS) over five daily sessions. Subsequently, drug-free rats were tested for the acquisition of a new response in which presses on a novel active lever led to the presentation of the previously paired CS. We tested SB-334867 in two conditions, SB-334867 was given either before each cocaine self-administration or before the initial four sessions of acquisition for a novel instrumental responding paired with the CS (conditioned reinforcement). The effect of SB-334867 was also tested on the expression of conditioned place preference to d-amphetamine. The rats treated with SB-334867 before each cocaine self-administration session subsequently showed reduced active lever pressing compared with controls in the initial days of the conditioned reinforcement. In the second study, untreated rats showed normal acquisition of discriminated responding preferential for the lever providing the cocaine cue. In contrast, SB-334867 decreased the number of active lever pressing (compared with the control) with significant effects in all sessions. Finally, SB-334867 blocked the expression of d-amphetamine-induced conditioned place preference. These results suggest that orexin-1 receptor antagonism could offer therapeutic potential in reducing the impact of psychostimulant-predictive stimuli that contribute to compulsive drug seeking in human drug users.

The GABA-B positive modulator GS39783 decreases psychostimulant conditioned-reinforcement and conditioned-reward.

Baclofen, a γ-amino-butyric-acid (GABA)(B) receptor agonist, can reduce cue-enhanced cocaine-seeking in rats and attenuate cue-evoked craving in cocaine addicts. However, baclofen also has sedative effects that might interfere with its efficacy in reducing cocaine's rewarding effects. The present study aimed at comparing the effects of baclofen with the GABA(B) -receptor positive allosteric modulator GS39783 on psychostimulant conditioned cues. Two identically trained groups of male Lister-Hooded rats were baselined on a new responding for a light stimulus previously paired with cocaine self-administration. One group was treated with the GABA(B) -receptor positive allosteric modulator GS39783 (0, 10, 30, 100 mg/kg, i.p.), the other with baclofen (0, 0.6, 1.25, 1.9, 2.5 mg/kg, i.p.). In another series of experiments, male Wistar rats received GS39783 (0, 10, 30, 100 mg/kg, i.p.) or baclofen (1.25 mg/kg) prior to the expression of a conditioned place preference (CPP) to amphetamine (2 mg/kg i.p.). Both GS39783 (30 and 100 mg/kg) and baclofen (2.5 mg/kg) significantly decreased responding for the cocaine cue; however, only GS39783 (30 mg/kg) reduced lever pressing responding without interfering with locomotor activity. Both GS39783 (30 and 100 mg/kg) and baclofen (1.25 mg/kg), significantly blocked the expression of amphetamine CPP without affecting locomotor activity. These findings suggest that GABA(B) positive allosteric modulators can modulate discrete and contextual psychostimulant conditioned stimuli in a manner dissociable from unwanted sedative effects and may offer a novel therapeutic approach to treat cravings and relapse to drug-taking triggered by stimuli associated with psychostimulant use.

Reinforcing and neurochemical effects of cannabinoid CB1 receptor agonists, but not cocaine, are altered by an adenosine A2A receptor antagonist.

Several recent studies suggest functional and molecular interactions between striatal adenosine A(2A) and cannabinoid CB(1) receptors. Here, we demonstrate that A(2A) receptors selectively modulate reinforcing effects of cannabinoids. We studied effects of A(2A) receptor blockade on the reinforcing effects of delta-9-tetrahydrocannabinol (THC) and the endogenous CB(1) receptor ligand anandamide under a fixed-ratio schedule of intravenous drug injection in squirrel monkeys. A low dose of the selective adenosine A(2A) receptor antagonist MSX-3 (1 mg/kg) caused downward shifts of THC and anandamide dose-response curves. In contrast, a higher dose of MSX-3 (3 mg/kg) shifted THC and anandamide dose-response curves to the left. MSX-3 did not modify cocaine or food pellet self-administration. Also, MSX-3 neither promoted reinstatement of extinguished drug-seeking behavior nor altered reinstatement of drug-seeking behavior by non-contingent priming injections of THC. Finally, using in vivo microdialysis in freely-moving rats, a behaviorally active dose of MSX-3 significantly counteracted THC-induced, but not cocaine-induced, increases in extracellular dopamine levels in the nucleus accumbens shell. The significant and selective results obtained with the lower dose of MSX-3 suggest that adenosine A(2A) antagonists acting preferentially at presynaptic A(2A) receptors might selectively reduce reinforcing effects of cannabinoids that lead to their abuse. However, the appearance of potentiating rather than suppressing effects on cannabinoid reinforcement at the higher dose of MSX-3 would likely preclude the use of such a compound as a medication for cannabis abuse. Adenosine A(2A) antagonists with more selectivity for presynaptic versus postsynaptic receptors could be potential medications for treatment of cannabis abuse.

Effects of lamotrigine on PCP-evoked elevations in monoamine levels in the medial prefrontal cortex of freely moving rats.

Lamotrigine is suggested to have potential as an add-on treatment for patients with schizophrenia. Supporting evidence comes from the efficacy of the drug in models of psychotic-like behaviour induced by N-methyl-D-aspartate (NMDA) receptor antagonists, such as phencyclidine (PCP). These drugs enhance levels of the monoamines in the cortex, which may contribute to their psychotomimetic effects. The ability of lamotrigine to prevent these neurochemical changes has not been examined. We studied PCP-evoked overflow of noradrenaline, dopamine and serotonin in the medial prefrontal cortex of awake rats using microdialysis. Rats were administered lamotrigine or vehicle, followed by PCP. Locomotor activity was also recorded before and after drug treatment. Lamotrigine did not have an influence on basal levels of the monoamines, but significantly reduced PCP-evoked overflow of dopamine and serotonin; PCP-evoked overflow of noradrenaline was also reduced by lamotrigine, but not to a significant degree. In contrast, PCP-induced hyperactivity was unaffected by lamotrigine. It is concluded that lamotrigine can modify PCP-evoked monoamine overflow in the cortex, consistent with an ability to prevent the psychotomimetic effects of NMDA receptor antagonists in rodents and humans. The dissociation between monoamine overflow and locomotor activity suggests the involvement of different brain circuits; relevance to the treatment of schizophrenia is also discussed.

Increased phasic activity of VTA dopamine neurons in mice 3 weeks after repeated social defeat.

Social defeat is an ethologically relevant stress inducing neuroadaptive changes in the mesocorticolimbic dopaminergic system. Three weeks after 10 days of daily defeat salient behaviors and in vivo dopamine (DA) neuron firing were evaluated in mice. Prior defeat induced social avoidance and hyperphagia and increased ventral tegmental area (VTA) DA neuron bursting activity. These data extend previous studies and suggest that increased phasic DA neuron firing in the VTA could be considered amongst the features defining the lasting imprint of social defeat stress.

The orexin-1 receptor antagonist SB-334867 reduces amphetamine-evoked dopamine outflow in the shell of the nucleus accumbens and decreases the expression of amphetamine sensitization.

Orexin-expressing neurons are present in hypothalamic nuclei and send projections toward mesolimbic regions such as the nucleus accumbens (NAc), a key brain region implicated in the processing of the motivational significance of reinforcers. Recent evidence found that activation of the orexin system can lead to a state of hyperarousal that may facilitate drug craving or contribute to vulnerability to drug relapse. This study aimed at assessing the effects of the orexin-1 receptor antagonist SB-334867 [1-(2-methylbenzoxazol-6-yl)-3-[1,5]naphthyridin-4-yl-urea hydrochloride] on amphetamine-induced dopamine (DA) release in the shell subregion of the NAc by means of in vivo microdialysis in freely moving rats. Since behavioral sensitization is thought to play a role in the maintenance of compulsive drug use, we also tested the effect of SB-334867 on the expression of sensitization to the locomotor activating effects of amphetamine. Acute administration of SB-334867 (30 mg/kg SC) significantly reduced the acute effects of amphetamine (1 mg/kg IP) on extracellular DA levels in the NAc shell. The expression of amphetamine sensitization was also significantly reduced by acute SB-334867 treatment. Altogether our findings show that selective orexin-1 antagonism both reduces the acute effects of amphetamine on DA outflow in the NAc shell and decreases the expression of locomotor sensitization to the repeated, intermittent administration of amphetamine.

Systemic administration of ghrelin increases extracellular dopamine in the shell but not the core subdivision of the nucleus accumbens.

The gut-hormone ghrelin endogenously binds to the ghrelin receptor (GHS-R) to promote foraging and feeding behaviours mainly via the hypothalamic arcuate nucleus (ARC). GHS-Rs are also expressed in midbrain dopaminergic neurons of the ventral tegmental area (VTA) suggesting that ghrelin may modulate the mesolimbic dopamine (DA) system. In support of this hypothesis, previous results have shown that intraventricular administration of ghrelin in rats increases DA levels in the nucleus accumbens (NAc). In the present study, the systemic doses of ghrelin capable of triggering central activation were first determined, and growth hormone (GH) levels were used as a marker of ghrelin-induced activation. Similar dose regimen was then used to measure ghrelin-induced effects on extracellular levels of monoamines in the shell and core subdivisions of the NAc using microdialysis in freely moving rats. We show that subcutaneous (s.c.) administration of ghrelin produced an increase in basal plasmatic ghrelin concentrations that was paralleled by enhanced GH secretion. In addition, a significant increase in extracellular levels of DA was observed specifically in the NAc shell, with no effect in the core subdivision. Extracellular serotonin (5-HT) levels were also affected in the shell subregion, but without reaching statistical significance. Increased extracellular DA levels in the NAc shell have been typically associated with the acute reinforcing effects of addictive drugs. The present findings therefore suggest that systemic ghrelin may modulate the valence of reinforcers such as food and drugs of abuse by interfering with mesolimbic DA activity.

Drug discrimination and neurochemical studies in alpha7 null mutant mice: tests for the role of nicotinic alpha7 receptors in dopamine release.

RATIONALE: The nicotine discriminative stimulus has been linked to beta2-containing (beta2*) nicotinic receptors, with little evidence of a role for alpha7 nicotinic receptors, because nicotine discrimination was very weak in beta2 null mutant mice but normal in alpha7 mutants. OBJECTIVES: As both alpha7 and beta2* nicotinic receptors have been implicated in nicotine-stimulated dopamine overflow, this study focused on the dopamine-mediated element in the nicotine stimulus by examining cross-generalisation between amphetamine and nicotine. MATERIALS AND METHODS: Male alpha7 nicotinic receptor null mutant mice and wild-type controls were bred in-house and trained to discriminate nicotine (0.8 mg/kg) or (+)-amphetamine (0.6 mg/kg) from saline in a two-lever procedure with a tandem VI-30 FR-10 schedule of food reinforcement. Dopamine release from striatal slices was determined in parallel experiments. RESULTS: An alpha7 nicotinic receptor-mediated component of dopamine release was demonstrated in tissue from wild-type mice using choline as a selective agonist. This response was absent in tissue from null mutant animals. The mutation did not influence acquisition of drug discriminations but subtly affected the results of cross-generalisation tests. In mice trained to discriminate nicotine or amphetamine, there was partial cross-generalisation in wild-type mice and, at certain doses, these effects were attenuated in mutants. Further support for an alpha7 nicotinic receptor-mediated component was provided by the ability of the alpha7 nicotinic receptor antagonist methyllycaconitine to attenuate responses to nicotine and amphetamine in wild-type mice. CONCLUSIONS: These findings support the concept of an alpha7 nicotinic receptor-mediated dopaminergic element in nicotine discrimination, warranting further tests with selective dopamine agonists.

Adenosine A1-A2A receptor heteromers: new targets for caffeine in the brain.

The contribution of blockade of adenosine A1 and A2A receptor to the psychostimulant effects of caffeine is still a matter of debate. When analyzing motor activity in rats, acutely administered caffeine shows a profile of a non-selective adenosine receptor antagonist, although with preferential A1 receptor antagonism. On the other hand, tolerance to the effects of A1 receptor blockade seems to be mostly responsible for the tolerance to the motor-activating effects of caffeine, while the residual motor-activating effects of caffeine in tolerant individuals seem to involve A2A receptor blockade. These behavioral studies correlate with in vivo microdialysis experiments that suggest that A1 receptor-mediated modulation of striatal glutamate release is involved in the psychostimulant effects of caffeine. Experiments in transfected cells demonstrate the ability of A1 receptors to heteromerize with A2A receptors and the A1-A2A receptor heteromer can be biochemically identified in the striatum, in striatal glutamatergic terminals. The striatal A1-A2A receptor heteromer provides a "concentration-dependent switch" mechanism by which low and high concentrations of synaptic adenosine produce the opposite effects on glutamate release. The analysis of the function of A1-A2A receptor heteromers during chronic treatment with caffeine gives new clues about the well-known phenomenon of tolerance to the psychostimulant effects of caffeine.

Different effects of ionotropic and metabotropic glutamate receptor antagonists on attention and the attentional properties of nicotine.

Distinct lines of evidence indicate that glutamate plays a primary role in modulating cognitive functions. Notably, competitive glutamate receptor antagonists acting at ionotropic N-methyl-d-aspartate (NMDA) or metabotropic glutamate 5 (mGlu5) receptors impair cognitive performance. Conversely, nicotine and other psychostimulants stimulate glutamatergic mechanisms and can act as cognitive enhancers. Hence we analysed the role of glutamate in performance of an attentional task and in nicotine-induced enhancement of attention by using the rodent five-choice serial reaction time task (5-CSRTT). Rats were trained to criterion performance and were then pre-dosed with either vehicle, the NMDA receptor antagonist (+)3-(2-carboxypiperazin-4-propyl)-1-propenyl-1-phosphonic acid (CPP, 0.3-2.0 mg/kg) or the mGlu5 antagonist 2-methyl-6-phenylethynyl-pyridine (MPEP, 1.0-9.0 mg/kg) and challenged with nicotine (0.2 mg/kg). Nicotine improved attentional performance, an effect that was weakened by doses of CPP that themselves had little impact on performance; importantly, CPP dose-dependently blunted the ability of nicotine to improve response accuracy, the major measure of signal detection in the paradigm. MPEP dose-dependently impaired signal detection under conditions with a high attentional load, an effect that was reversed by nicotine; thus, MPEP did not block nicotine-induced attentional enhancement. Co-administration of either CPP or MPEP with nicotine also produced a general slowing of performance characterised by increases in omission errors and response latencies and reduced anticipatory responding. It is concluded that activation of NMDA receptors may be an important determinant of the effects of nicotine in the 5-CSRTT. Stimulation of nicotinic receptors may also reverse attentional deficits associated with the impaired function of the glutamate network.

The serotonin 2C receptor agonist Ro-60-0175 attenuates effects of nicotine in the five-choice serial reaction time task and in drug discrimination.

RATIONALE: There is evidence that serotonin(2C) (5-HT(2C)) receptors can modulate some behavioural effects of nicotine, but the generality of this action is not known. OBJECTIVE: To analyse the influence of the 5-HT(2C) agonist Ro-60-0175 on responses to nicotine in the five-choice serial reaction time task (5-CSRTT) and on its discriminative stimulus effect; these procedures constitute models for attention-enhancing and subjective effects of nicotine, respectively. MATERIALS AND METHODS: In the 5-CSRTT, rats were trained to obtain food reinforcers by detecting light stimuli and then challenged with Ro-60-0175 (0.3-0.8 mg/kg) and nicotine (0.2 mg/kg). For drug discrimination studies, rats were trained to discriminate nicotine (0.2 mg/kg) from saline in a two-lever procedure using a tandem schedule of food reinforcement. RESULTS: In the 5-CSRTT, nicotine positively influenced most response indices, confirming previous results. Ro-60-0175 increased response latencies and omission errors and reduced anticipatory responding but had little effect on response accuracy; importantly, it counteracted the effects of nicotine on response speed and omission errors. Pentobarbitone (10-14 mg/kg) also slowed performance of the 5-CSRTT but did not weaken the nicotine-induced enhancement of performance. In the drug discrimination procedure, Ro-60-0175 was not generalised with nicotine but shifted the nicotine dose-response curve to the right in a dose-related manner. CONCLUSIONS: The data suggest that selective occupancy of 5-HT(2C) receptors can attenuate some effects of nicotine in the 5-CSRTT and weaken the nicotine discriminative stimulus; these effects cannot be explained by a sedative action of Ro-60-0175.

Heteromeric nicotinic acetylcholine-dopamine autoreceptor complexes modulate striatal dopamine release.

In the striatum, dopamine and acetylcholine (ACh) modulate dopamine release by acting, respectively, on dopamine D(2) autoreceptors and nicotinic ACh (nACh) heteroreceptors localized on dopaminergic nerve terminals. The possibility that functional interactions exist between striatal D(2) autoreceptors and nACh receptors was studied with in vivo microdialysis in freely moving rats. Local perfusion of nicotine in the ventral striatum (shell of the nucleus accumbens) produced a marked increase in the extracellular levels of dopamine, which was completely counteracted by co-perfusion with either the non-alpha(7) nACh receptor antagonist dihydro-beta-erythroidine or the D(2-3) receptor agonist quinpirole. Local perfusion of the D(2-3) receptor antagonist raclopride produced an increase in the extracellular levels of dopamine, which was partially, but significantly, counteracted by coperfusion with dihydro-beta-erythroidine. These findings demonstrate a potent crosstalk between G protein-coupled receptors and ligand-gated ion channels in dopaminergic nerve terminals, with the D(2) autoreceptor modulating the efficacy of non-alpha(7) nACh receptor-mediated modulation of dopamine release. We further demonstrate physical interactions between beta(2) subunits of non-alpha(7) nicotinic acetylcholine receptors and D(2) autoreceptors in co-immunoprecipitation experiments with membrane preparations from co-transfected mammalian cells and rat striatum. These results reveal that striatal non-alpha(7) nicotinic acetylcholine receptors form part of heteromeric dopamine autoreceptor complexes that modulate dopamine release.

Involvement of adenosine A1 receptors in the discriminative-stimulus effects of caffeine in rats.

RATIONALE: Caffeine is a non-selective adenosine receptor antagonist in vitro, but involvement of different adenosine receptor subtypes, particularly adenosine A1 and A 2A receptors, in the central effects of caffeine remains a matter of debate. OBJECTIVE: Investigate the role of adenosine A1 and A 2A receptors in the discriminative-stimulus effects of caffeine. METHODS: Rats were trained to discriminate an injection of 30 mg/kg (i.p.) caffeine from saline. The selective A1 receptor antagonist CPT, the selective A 2A receptor antagonist MSX-3 and the non-selective adenosine receptor antagonist DMPX were assessed for their ability to produce caffeine-like discriminative effects. The ability of CPT, MSX-3, the A1 receptor agonist CPA and the A 2A receptor agonist CGS21680 to reduce the discriminative effects of caffeine was also tested. Radioligand binding experiments with membrane preparations from rat striatum and transfected mammalian cell lines were performed to characterize binding affinity profiles of the different adenosine antagonists used in the present study (caffeine, DMPX, CPT and MSX-3) in relation to all known adenosine receptors (A1, A 2A, A 2B, A3). RESULTS: DMPX and CPT, but not MSX-3, produced significant caffeine-like discriminative effects. MSX-3, but not CPT, markedly reduced the discriminative effects of caffeine and the caffeine-like discriminative effects of CPT. Furthermore, the A1 receptor agonist CPA, but not the A 2A agonist CGS21680, reduced caffeine's discriminative effects. CONCLUSIONS: Adenosine A1 receptor blockade is involved in the discriminative-stimulus effects of behaviorally relevant doses of caffeine; A 2A receptor blockade does not play a central role in caffeine's discriminative effects and counteracts the A1 receptor-mediated discriminative-stimulus effects of caffeine.

Adenosine receptor-mediated modulation of dopamine release in the nucleus accumbens depends on glutamate neurotransmission and N-methyl-D-aspartate receptor stimulation.

Adenosine, by acting on adenosine A(1) and A(2A) receptors, exerts opposite modulatory roles on striatal extracellular levels of glutamate and dopamine, with activation of A(1) inhibiting and activation of A(2A) receptors stimulating glutamate and dopamine release. Adenosine-mediated modulation of striatal dopaminergic neurotransmission could be secondary to changes in glutamate neurotransmission, in view of evidence for a preferential colocalization of A(1) and A(2A) receptors in glutamatergic nerve terminals. By using in vivo microdialysis techniques, local perfusion of NMDA (3, 10 microm), the selective A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 3, 10 microm), the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 300, 1000 microm), or the non-selective A(1)-A(2A) receptor antagonist in vitro caffeine (300, 1000 microm) elicited significant increases in extracellular levels of dopamine in the shell of the nucleus accumbens (NAc). Significant glutamate release was also observed with local perfusion of CGS 21680, CPT and caffeine, but not NMDA. Co-perfusion with the competitive NMDA receptor antagonist dl-2-amino-5-phosphonovaleric acid (APV; 100 microm) counteracted dopamine release induced by NMDA, CGS 21680, CPT and caffeine. Co-perfusion with the selective A(2A) receptor antagonist MSX-3 (1 microm) counteracted dopamine and glutamate release induced by CGS 21680, CPT and caffeine and did not modify dopamine release induced by NMDA. These results indicate that modulation of dopamine release in the shell of the NAc by A(1) and A(2A) receptors is mostly secondary to their opposite modulatory role on glutamatergic neurotransmission and depends on stimulation of NMDA receptors. Furthermore, these results underscore the role of A(1) vs. A(2A) receptor antagonism in the central effects of caffeine.

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

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

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

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