Antagonism of adenosine A2A receptors underlies the behavioural activating effect of caffeine and is associated with reduced expression of messenger RNA for NGFI-A and NGFI-B in caudate-putamen and nucleus accumbens

Blockade of adenosine A2A receptors inhibits Tremulous Jaw Movements as well as expression of zif-268 and GAD65 mRNAs in brain motor structures

Tremor is one of the motor symptoms of Parkinson's disease (PD), present also in neuroleptic-induced parkinsonism. Tremulous Jaw Movements (TJMs) are suggested to be a well-validated rodent model of PD resting tremor. TJMs can be induced by typical antipsychotics and are known to be reduced by different drugs, including adenosine A_2A receptor antagonists. The aim of the present study was to search for brain structures involved in the tremorolytic action of SCH58261, a selective A_2A receptor antagonist, in TJMs induced by subchronic pimozide. Besides TJMs, we evaluated in the same animals the expression of zif-268 mRNA (neuronal responsiveness marker), and mRNA levels for glutamic acid decarboxylase 65-kDa isoform (GAD65) and vesicular glutamate transporters 1 and 2 (vGluT1/2) in selected brain structures, as markers of GABAergic and glutamatergic neurons, respectively. We found that SCH58261 reduced the pimozide-induced TJMs. Pimozide increased the zif-268 mRNA level in the striatum, nucleus accumbens (NAc) core, and substantia nigra pars reticulata (SNr). Additionally, it increased GAD65 mRNA in the striatum and SNr, and vGluT2 mRNA levels in the subthalamic nucleus (STN). A positive correlation between zif-268, GAD65 and vGluT2 mRNAs and TJMs was found. SCH58261 reversed the pimozide-increased zif-268 mRNA in the striatum and NAc core and GAD65 mRNA in the striatum and SNr. In contrast, SCH58261 did not influence vGluT2 mRNA in STN. The present study suggests an importance of the striato-subthalamo-nigro-thalamic circuit in neuroleptic-induced TJMs. The tremorolytic effect of A_2A receptor blockade seems to involve this circuit bypassing, however, STN.

Orchestration of Dopamine Neuron Population Activity in the Ventral Tegmental Area by Caffeine: Comparison With Amphetamine

BACKGROUND

Among psychostimulants, the dopamine transporter ligands amphetamine and cocaine display the highest addictive potential; the adenosine receptor antagonist caffeine is most widely consumed but less addictive. Psychostimulant actions of amphetamine were correlated with its ability to orchestrate ventral tegmental dopamine neuron activity with contrasting shifts in firing after single vs repeated administration. Whether caffeine might impinge on dopamine neuron activity has remained elusive.

METHODS

Population activity of ventral tegmental area dopamine neurons was determined by single-unit extracellular recordings and set in relation to mouse behavior in locomotion and conditioned place preference experiments, respectively.

RESULTS

A single dose of caffeine reduced population activity as did amphetamine and the selective adenosine A2A antagonist KW-6002, but not the A1 antagonist DPCPX. Repeated administration of KW-6002 or amphetamine led to drug-conditioned place preference and to unaltered or even enhanced population activity. Recurrent injection of caffeine or DPCPX, in contrast, failed to cause conditioned place preference and persistently reduced population activity. Subsequent to repetitive drug administration, re-exposure to amphetamine or KW-6002, but not to caffeine or DPCPX, was able to reduce population activity.

CONCLUSIONS

Behavioral sensitization to amphetamine is attributed to persistent activation of ventral tegmental area dopamine neurons via the ventral hippocampus. Accordingly, a switch from acute A2A receptor-mediated reduction of dopamine neuron population activity to enduring A1 receptor-mediated suppression is correlated with tolerance rather than sensitization in response to repeated caffeine intake.

Why target brain adenosine receptors? A historical perspective

The quest for a non-dopaminergic approach to treating Parkinson's disease (PD) has been quietly progressing over the past several decades, and is now finding its momentum. Here, in what is more a memoir than a comprehensive review, we discuss work carried out over the past 50 years to show that adenosine acts as a critical signaling molecule via actions against a specific family of receptors. Importantly for PD, adenosine A_2A receptors have a selective localization to the basal ganglia and specifically to the indirect output pathway, offering a targeted, non-dopaminergic opportunity to modulate basal ganglia output.

Caffeine reverses the unconsciousness produced by light anesthesia in the continued presence of isoflurane in rats

Currently no drugs are employed clinically to reverse the unconsciousness induced by general anesthetics. Our previous studies showed that caffeine, when given near the end of an anesthesia session, accelerated emergence from isoflurane anesthesia, likely caused by caffeine’s ability to elevate intracellular cAMP levels and to block adenosine receptors. These earlier studies showed that caffeine did not rouse either rats or humans from deep anesthesia (≥ 1 minimum alveolar concentration, MAC). In this current crossover study, we examined whether caffeine reversed the unconsciousness produced by light anesthesia (< 1 MAC) in the continued presence of isoflurane. The primary endpoint of this study was to measure isoflurane levels at the time of recovery of righting reflex, which was a proxy for consciousness. Rats were deeply anesthetized with 2% isoflurane (~1.5 MAC) for 20 minutes. Subsequently, isoflurane was reduced to 1.2% for 10 minutes, then by 0.2% every 10 min; animals were monitored until the recovery of righting reflex occurred, in the continued presence of isoflurane. Respiration rate, heart rate and electroencephalogram (EEG) were monitored. Our results show that caffeine-treated rats recovered their righting reflex at a significantly higher inspired isoflurane concentration, corresponding to light anesthesia, than the same rats treated with saline (control). Respiration rate and heart rate increased initially after caffeine injection but were then unchanged for the rest of the anesthesia session. Deep anesthesia is correlated with burst suppression in EEG recordings. Our data showed that caffeine transiently reduced the burst suppression time produced by deep anesthesia, suggesting that caffeine altered neuronal circuit function but not to a point where it caused arousal. In contrast, under light anesthesia, caffeine shifted the EEG power to high frequency beta and gamma bands. These data suggest that caffeine may represent a clinically viable drug to reverse the unconsciousness produced by light anesthesia.

Neuronal adenosine A2A receptors signal ergogenic effects of caffeine

Caffeine is one of the most used ergogenic aid for physical exercise and sports. However, its mechanism of action is still controversial. The adenosinergic hypothesis is promising due to the pharmacology of caffeine, a nonselective antagonist of adenosine A1 and A2A receptors. We now investigated A2AR as a possible ergogenic mechanism through pharmacological and genetic inactivation. Forty-two adult females (20.0 ± 0.2 g) and 40 male mice (23.9 ± 0.4 g) from a global and forebrain A2AR knockout (KO) colony ran an incremental exercise test with indirect calorimetry (V̇O2 and RER). We administered caffeine (15 mg/kg, i.p., nonselective) and SCH 58261 (1 mg/kg, i.p., selective A2AR antagonist) 15 min before the open field and exercise tests. We also evaluated the estrous cycle and infrared temperature immediately at the end of the exercise test. Caffeine and SCH 58621 were psychostimulant. Moreover, Caffeine and SCH 58621 were ergogenic, that is, they increased V̇O2max, running power, and critical power, showing that A2AR antagonism is ergogenic. Furthermore, the ergogenic effects of caffeine were abrogated in global and forebrain A2AR KO mice, showing that the antagonism of A2AR in forebrain neurons is responsible for the ergogenic action of caffeine. Furthermore, caffeine modified the exercising metabolism in an A2AR-dependent manner, and A2AR was paramount for exercise thermoregulation.

Caffeine inhalation effects on locomotor activity in mice

It is estimated that 80% of the world's population consumes caffeine from beverages and food every day. The traditional form of caffeine intake is oral, but more recently people have been inhaling caffeine using nasal sprays. However, the effects of caffeine inhalation are not well understood. The purpose of this study was to determine whether caffeine inhalation affects mouse behavior. To test this, we compared spontaneous activity of mice following inhalation and intraperitoneal administration of caffeine. Next, we investigated whether spontaneous activity changed with the time and/or concentration of caffeine inhaled. We found that mice that inhaled caffeine increased their spontaneous activity similar to mice that were administered caffeine intraperitoneally. Furthermore, spontaneous activity increased in an inhalation time-dependent and concentration-dependent manner. These results show that caffeine-induced stimulation also occurs by inhalation in mice, which suggests that caffeine can reach the brain even by inhalation. This study is useful not only for creating new administration methods of caffeine but also for adjusting caffeine storage and management.

Profiling of behavioral effects evoked by ketamine and the role of 5HT2 and D2 receptors in ketamine-induced locomotor sensitization in mice

Ketamine has addictive potential, a troublesome fact due to its promising use as a therapeutic drug. An important phenomenon associated with drug addiction is behavioral sensitization, usually characterized as augmented locomotion. However, other behaviors may also be susceptible to sensitization, and/or interfere with locomotor activity. Thus, this study drew a comprehensive behavioral 'profiling' in an animal model of repeated administration of ketamine. Adult Swiss mice received single daily ketamine injections (30 or 50 mg/Kg, i.p.), which were followed by open field testing for 7 days (acquisition period, ACQ). A ketamine challenge (sensitization test, ST) was carried out after a 5-day withdrawal. Locomotion, rearing, grooming, rotation and falling were assessed during ACQ and ST. All behaviors were affected from the first ACQ day onwards, with no indication of competition between locomotion and the other behaviors. Only locomotion in response to 30 mg/Kg of ketamine both escalated during ACQ and expressed increased levels at ST, evidencing development and expression of locomotor sensitization. Considering the involvement of serotonin 5HT₍₂₎ and dopamine D₍₂₎ receptors on addiction mechanisms, we further tested the involvement of these receptors in ketamine-induced sensitization. Ketanserin (5HT₂ antagonist, 3 mg/Kg, s.c.) prevented ketamine-evoked development of locomotor sensitization. However, ketanserin pretreatment during ACQ failed to inhibit its expression during ST. Raclopride (D₂ antagonist, 0.5 mg/Kg, s.c.) evoked less robust reductions in locomotion but prevented the development of ketamine-evoked sensitization. Pretreatment during ACQ further inhibited the expression of sensitization during ST. These results indicate that a partial overlap in serotonergic and dopaminergic mechanisms underlies ketamine-induced locomotor sensitization.

Genetic deletion of GPR88 enhances the locomotor response to L-DOPA in experimental parkinsonism while counteracting the induction of dyskinesia

Parkinson's disease (PD) is characterized by progressive loss of midbrain dopaminergic neurons and treated with the dopamine precursor, 3,4-dihydroxy-l-phenylalanine (L-DOPA). Prolonged L-DOPA treatment is however associated with waning efficacy and the induction of L-DOPA induced dyskinesia (LID). GPR88 is an orphan G-protein Coupled Receptor (GPCR) expressed in dopaminoceptive striatal medium spiny neurons (MSNs) and their afferent corticostriatal glutamatergic neurons. Here, we studied the role of GPR88 in experimental parkinsonism and LID. Chronic L-DOPA administration to male GPR88 KO mice, subjected to unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle, resulted in more rotations than in their WT counterparts. Conversely, GPR88 KO mice had a lower abnormal involuntary movements (AIMs) score. These behavioral responses were accompanied by altered transcription of L-DOPA upregulated genes in lesioned GPR88 KO compared to WT striata. In accordance with a role for serotonin neurons in LID development, WT but not GPR88 KO striata exhibited 5-hydroxytryptamine displacement upon repeated L-DOPA treatment. Intact male GPR88 KO mice showed diminished tacrine-induced PD-like tremor and spontaneous hyperlocomotion. Dopamine and its metabolites were not increased in male GPR88 KO mice, but biosensor recordings revealed increased spontaneous/basal and evoked glutamate release in striata of male GPR88 KO mice. In conclusion, genetic deletion of GPR88 promotes l-DOPA-induced rotation and spontaneous locomotion yet suppresses the induction of LIDs and also reduces tremor. These data provide behavioral, neurochemical and molecular support that GPR88 antagonism may favour motor relief in PD patients without aggravating the induction of motor side effects.

Effects of Caffeine Supplementation on Power Performance in a Flywheel Device: A Randomised, Double-Blind Cross-Over Study

Despite the demonstrated evidence of the importance of eccentric contractions in sports performance, no research has evaluated the ergogenic effects of caffeine on this type of contraction means during flywheel exercises. Therefore, the aims of the present study were to compare the power outcomes, using different inertial loads, between caffeine and placebo conditions. Twenty-four young, healthy, and active men (age: 22.5 ± 4.8 years) took part in the study. A crossed, randomised double-blind design was used to analyse the effects of caffeine on lower limb power outcomes during a flywheel half-squat exercise. Participants completed four sets of eight all-out repetitions with a fixed three-minutes rest interval, and each set was performed using different inertial loads (i.e., 0.025, 0.050, 0.075 and 0.100 kg·m⁻²). Both the mean power (MP) and peak power (PP) in concentric (CON) and eccentric (ECC) movement phases at each inertial load were recorded after participants were administered either a caffeine supplement (6 mg·kg⁻¹) or placebo (sucrose). Participants receiving a caffeine supplementation demonstrated improvements versus the placebo in total MP (MP_total), as well as MP in CON phase (MP_con) and in ECC phase (MP_ecc) at each inertial load (22.68 to 26.53%; p < 0.01, effect size (ES) = 0.89–1.40). In addition, greater improvements with caffeine ingestion were obtained with respect to the placebo condition (18.79 to 24.98%; p < 0.01, ES = 1.03–1.40) in total PP (PP_total), as well as PP in CON phase (PP_con) and in ECC phase (PP_ecc) at each inertial load. Thus, the supplementation of 6 mg·kg⁻¹ caffeine may be considered to maximise on-field physical performance in those sports characterised by high demands of resistance.

Role of histamine H1 receptor in caffeine induced locomotor sensitization

The present study elucidated the role of histamine H₁ receptor in the caffeine induced locomotor sensitization. Intermittent administration of caffeine (15 mg/kg, i.p.) on alternate days (induction phase) i.e. 1^st, 3^rd, 5^th, 7^th, 9^th, 11^th and 13^th resulted in the development of locomotor sensitization. In addition, challenge with sub-stimulant dose of caffeine (10 mg/kg, i.p.) directly on 17^th day to induction group animals resulted in expression to locomotor sensitization to caffeine. I.c.v. injection of histaminergic agents concomitantly with caffeine during induction phase i.e. histamine H₁ receptor agonist, FMPH (6.5 μg/mouse) significantly potentiated while H₁ receptor antagonist, cetirizine (0.1 μg/mouse) attenuated the locomotor sensitization induced by caffeine (15 mg/kg, i.p.). In addition, challenge with caffeine (10 mg/kg, i.p.) on the expression day (17^th) to the induction group mice on FMPH + caffeine treatment showed enhanced, while those on cetirizine + caffeine treatment exhibited lesser expression to locomotor sensitization. Therefore, a possible contributory role of the central histaminergic system via H₁ receptor stimulation or up-regulation in the caffeine-induced locomotor sensitizing effect is proposed.

Gliotransmission and adenosinergic modulation: insights from mammalian spinal motor networks

Astrocytes are proposed to converse with neurons at tripartite synapses, detecting neurotransmitter release and responding with release of gliotransmitters, which in turn modulate synaptic strength and neuronal excitability. However, a paucity of evidence from behavioral studies calls into question the importance of gliotransmission for the operation of the nervous system in healthy animals. Central pattern generator (CPG) networks in the spinal cord and brain stem coordinate the activation of muscles during stereotyped activities such as locomotion, inspiration, and mastication and may therefore provide tractable models in which to assess the contribution of gliotransmission to behaviorally relevant neural activity. We review evidence for gliotransmission within spinal locomotor networks, including studies indicating that adenosine derived from astrocytes regulates the speed of locomotor activity via metamodulation of dopamine signaling.

Caffeine accelerates recovery from general anesthesia via multiple pathways

Various studies have explored different ways to speed emergence from anesthesia. Previously, we have shown that three drugs that elevate intracellular cAMP (forskolin, theophylline, and caffeine) accelerate emergence from anesthesia in rats. However, our earlier studies left two main questions unanswered. First, were cAMP-elevating drugs effective at all anesthetic concentrations? Second, given that caffeine was the most effective of the drugs tested, why was caffeine more effective than forskolin since both drugs elevate cAMP? In our current study, emergence time from anesthesia was measured in adult rats exposed to 3% isoflurane for 60 min. Caffeine dramatically accelerated emergence from anesthesia, even at the high level of anesthetic employed. Caffeine has multiple actions including blockade of adenosine receptors. We show that the selective A2a adenosine receptor antagonist preladenant or the intracellular cAMP ([cAMP]i)-elevating drug forskolin, accelerated recovery from anesthesia. When preladenant and forskolin were tested together, the effect on anesthesia recovery time was additive indicating that these drugs operate via different pathways. Furthermore, the combination of preladenant and forskolin was about as effective as caffeine suggesting that both A2A receptor blockade and [cAMP]i elevation play a role in caffeine's ability to accelerate emergence from anesthesia. Because anesthesia in rodents is thought to be similar to that in humans, these results suggest that caffeine might allow for rapid and uniform emergence from general anesthesia in humans at all anesthetic concentrations and that both the elevation of [cAMP]i and adenosine receptor blockade play a role in this response.NEW & NOTEWORTHY Currently, there is no method to accelerate emergence from anesthesia. Patients "wake" when they clear the anesthetic from their systems. Previously, we have shown that caffeine can accelerate emergence from anesthesia. In this study, we show that caffeine is effective even at high levels of anesthetic. We also show that caffeine operates by both elevating intracellular cAMP levels and by blocking adenosine receptors. This complicated pharmacology makes caffeine especially effective in accelerating emergence from anesthesia.

Caffeine and sleep-deprivation mediated changes in open-field behaviours, stress response and antioxidant status in mice

OBJECTIVES

Effects of daily caffeine consumption on open-field behaviours, serum corticosterone and brain antioxidant levels were investigated after six hours of total sleep-deprivation in prepubertal mice. We tested the hypothesis that daily caffeine consumption may significantly alter behaviour, stress and antioxidative response of prepubertal mice to an acute episode of total sleep-deprivation.

METHODS

Prepubertal Swiss mice of both sexes were assigned to two main groups of 120 each (subdivided into 6 groups of 10 each, based on sex), and administered vehicle or graded oral doses of caffeine (10, 20, 40, 80 and 120 mg/kg/day) for 14 days. On day 14, a main group was subjected to 6 h of total sleep-deprivation by 'gentle-handling'. Open-field behaviours were then assessed in both groups, after which animals were euthanized, and levels of corticosterone, superoxide dismutase and glutathione peroxidase assayed.

RESULTS

Horizontal locomotion, rearing and grooming increased significantly, compared to control, with sleep-deprived (SD) mice showing stronger caffeine-driven responses at higher doses; and SD female mice showing sustained response to caffeine, compared to respective males. Plasma corticosterone increased with increasing doses of caffeine in both non sleep-deprived (NSD) and SD mice; although SD mice had higher corticosterone levels. Sleep-deprivation and/or higher doses of caffeine were associated with derangements in brain antioxidant levels.

CONCLUSION

Repeated caffeine consumption and/or acute sleep-deprivation led to significant changes in pattern of open-field behaviour and stress/antioxidant response in mice. Responses seen in the study are probably due to modulatory effects of caffeine on the total body response to stressful stimuli.

Injection of Cocaine-Amphetamine Regulated Transcript (CART) peptide into the nucleus accumbens does not inhibit caffeine-induced locomotor activity: Implications for CART peptide mechanism

Much evidence suggests that intra-nucleus accumbens (NAc) CART peptide (CART 55-102) injection inhibits locomotor activity (LMA) when there is an increase in the release and activity of dopamine (DA) in the NAc. However, this hypothesis has not been fully tested. One way to examine this is to determine if there is a lack of effect of intra-NAc CART peptide on LMA that does not involve increases in DA release in the NAc. Several studies have suggested that caffeine-induced LMA does not involve extracellular DA release in the NAc core. Therefore, in this study, we have examined the effect of injections of CART peptide (2.5μg) into the NAc core on the locomotor effects of caffeine in male Sprague-Dawley rats. Several LMA relevant doses of caffeine were used (0, 10, 20mg/kg i.p.), and an inverted U response curve was found as expected. We determined, in the same animals, that intra-NAc CART peptide had no effect on caffeine-induced LMA whereas it blunted cocaine-mediated LMA, as shown by other reports. We also extended a previous observation in mice by showing that at a LMA activating dose of caffeine there is no alteration of CART peptide levels in the NAc of rats. Our study supports the hypothesis that the inhibitory effects of CART peptide in the NAc may be exerted only under conditions of increased extracellular DA release and activity in this region. Our results also suggest that intra-NAc CART 55-102 does not generally inhibit increases in LMA due to all drugs, but has a more specific inhibitory effect on dopaminergic neurotransmission.

Caffeine: cognitive and physical performance enhancer or psychoactive drug?

Caffeine use is increasing worldwide. The underlying motivations are mainly concentration and memory enhancement and physical performance improvement. Coffee and caffeine-containing products affect the cardiovascular system, with their positive inotropic and chronotropic effects, and the central nervous system, with their locomotor activity stimulation and anxiogenic-like effects. Thus, it is of interest to examine whether these effects could be detrimental for health. Furthermore, caffeine abuse and dependence are becoming more and more common and can lead to caffeine intoxication, which puts individuals at risk for premature and unnatural death. The present review summarizes the main findings concerning caffeine’s mechanisms of action (focusing on adenosine antagonism, intracellular calcium mobilization, and phosphodiesterases inhibition), use, abuse, dependence, intoxication, and lethal effects. It also suggests that the concepts of toxic and lethal doses are relative, since doses below the toxic and/or lethal range may play a causal role in intoxication or death. This could be due to caffeine’s interaction with other substances or to the individuals' preexisting metabolism alterations or diseases.

Purinergic signalling in brain ischemia

Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia a primary damage due to the early massive increase of extracellular glutamate is followed by activation of resident immune cells, i.e microglia, and production or activation of inflammation mediators. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. Extracellular concentrations of ATP and adenosine in the brain increase dramatically during ischemia in concentrations able to stimulate their respective specific P2 and P1 receptors. Both ATP P2 and adenosine P1 receptor subtypes exert important roles in ischemia. Although adenosine exerts a clear neuroprotective effect through A1 receptors during ischemia, the use of selective A1 agonists is hampered by undesirable peripheral effects. Evidence up to now in literature indicate that A2A receptor antagonists provide protection centrally by reducing excitotoxicity, while agonists at A2A (and possibly also A2B) and A3 receptors provide protection by controlling massive infiltration and neuroinflammation in the hours and days after brain ischemia. Among P2X receptors most evidence indicate that P2X7 receptor contribute to the damage induced by the ischemic insult due to intracellular Ca(2+) loading in central cells and facilitation of glutamate release. Antagonism of P2X7 receptors might represent a new treatment to attenuate brain damage and to promote proliferation and maturation of brain immature resident cells that can promote tissue repair following cerebral ischemia. Among P2Y receptors, antagonists of P2Y12 receptors are of value because of their antiplatelet activity and possibly because of additional anti-inflammatory effects. Moreover strategies that modify adenosine or ATP concentrations at injury sites might be of value to limit damage after ischemia. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Caffeine stimulates locomotor activity in the mammalian spinal cord via adenosine A1 receptor-dopamine D1 receptor interaction and PKA-dependent mechanisms

Caffeine is a potent psychostimulant that can have significant and widely variable effects on the activity of multiple neuronal pathways. The most pronounced caffeine-induced behavioral effect seen in rodents is to increase locomotor activity which has been linked to a dose-dependent inhibition of A1 and A(2A) receptors. The effects of caffeine at the level of the lumbar spinal central pattern generator (CPG) network for hindlimb locomotion are lacking. We assessed the effects of caffeine to the locomotor function of the spinal CPG network via extracellular ventral root recordings using the isolated neonatal mouse spinal cord preparation. Addition of caffeine and of an A1 receptor antagonist significantly decreased the cycle period accelerating the ongoing locomotor rhythm, while decreasing burst duration reversibly in most preparations suggesting the role of A1 receptors as the primary target of caffeine. Caffeine and an A1 receptor antagonist failed to stimulate ongoing locomotor activity in the absence of dopamine or in the presence of a D1 receptor antagonist supporting A1/D1 receptor-dependent mechanism of action. The use of caffeine or an A1 receptor blocker failed to stimulate an ongoing locomotor rhythm in the presence of a blocker of the cAMP-dependent protein kinase (PKA) supporting the need of this intracellular pathway for the modulatory effects of caffeine to occur. These results support a stimulant effect of caffeine on the lumbar spinal network controlling hindlimb locomotion through the inhibition of A1 receptors and subsequent activation of D1 receptors via a PKA-dependent intracellular mechanism.

Genetic determinants of cognitive responses to caffeine drinking identified from a double-blind, randomized, controlled trial

The widely observed between-subject variability in cognitive responses to coffee may have a genetic basis. We evaluated cognitive responses to caffeine throughout three complex cognitive tasks assessing different subdomains of attention, namely Alerting and Orienting (Categorical Search Task) and Executive Control (Stroop Task and Eriksen Flanker Task). We explored whether they are influenced by gene variants affecting adenosine metabolism or catecholamine receptors. We recruited 106 healthy male subjects who were administered, in a double-blind design, 40mL of either a decaffeinated coffee preparation plus 3mg/kg caffeine (caf) or the corresponding vehicle (decaf). The protocol was repeated 24h later with the alternative preparation. Cognitive tasks were performed between 30min and 2h after caf or decaf administration. Each subject underwent ambulatory blood pressure monitoring for 2h. Blood samples were collected for genetic evaluations and for plasma caffeine and catecholamines measures. We found a significant reduction of reaction times in two of the cognitive tasks (Categorical Search Task and Stroop Task) after caf compared with decaf, indicating that caffeine, on average, improved the attention level in the domains under investigation. We also found, however, a great inter-individual variability in the cognitive performance responses to caffeine. In exploring genetic sources for such variability, we found a relation between polymorphisms of adenosine A2A and the caffeine effects on the attentional domains of Orienting and Executive control. In conclusion, variability in the attentional response to coffee may be partly explained by genetic polymorphisms of adenosine and adrenergic receptors.

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.

Adenosine A2A receptors modulate acute injury and neuroinflammation in brain ischemia

The extracellular concentration of adenosine in the brain increases dramatically during ischemia. Adenosine A_2A receptor is expressed in neurons and glial cells and in inflammatory cells (lymphocytes and granulocytes). Recently, adenosine A_2A receptor emerged as a potential therapeutic attractive target in ischemia. Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia the early massive increase of extracellular glutamate is followed by activation of resident immune cells, that is, microglia, and production or activation of inflammation mediators. Proinflammatory cytokines, which upregulate cell adhesion molecules, exert an important role in promoting recruitment of leukocytes that in turn promote expansion of the inflammatory response in ischemic tissue. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. A_2A receptors present on central cells and on blood cells account for important effects depending on the time-related evolution of the pathological condition. Evidence suggests that A_2A receptor antagonists provide early protection via centrally mediated control of excessive excitotoxicity, while A_2A receptor agonists provide protracted protection by controlling massive blood cell infiltration in the hours and days after ischemia. Focus on inflammatory responses provides for adenosine A_2A receptor agonists a wide therapeutic time-window of hours and even days after stroke.

Caffeine accelerates recovery from general anesthesia

General anesthetics inhibit neurotransmitter release from both neurons and secretory cells. If inhibition of neurotransmitter release is part of an anesthetic mechanism of action, then drugs that facilitate neurotransmitter release may aid in reversing general anesthesia. Drugs that elevate intracellular cAMP levels are known to facilitate neurotransmitter release. Three cAMP elevating drugs (forskolin, theophylline, and caffeine) were tested; all three drugs reversed the inhibition of neurotransmitter release produced by isoflurane in PC12 cells in vitro. The drugs were tested in isoflurane-anesthetized rats. Animals were injected with either saline or saline containing drug. All three drugs dramatically accelerated recovery from isoflurane anesthesia, but caffeine was most effective. None of the drugs, at the concentrations tested, had significant effects on breathing rates, O2 saturation, heart rate, or blood pressure in anesthetized animals. Caffeine alone was tested on propofol-anesthetized rats where it dramatically accelerated recovery from anesthesia. The ability of caffeine to accelerate recovery from anesthesia for different chemical classes of anesthetics, isoflurane and propofol, opens the possibility that it will do so for all commonly used general anesthetics, although additional studies will be required to determine whether this is in fact the case. Because anesthesia in rodents is thought to be similar to that in humans, these results suggest that caffeine might allow for rapid and uniform emergence from general anesthesia in human patients.

Effects of caffeine or RX821002 in rats with a neonatal ventral hippocampal lesion

Rats with a neonatal ventral hippocampal lesion (NVHL) are used to model schizophrenia. They show enhanced locomotion and difficulties in learning after puberty. Such behavioral modifications are strengthened by dopaminergic psychostimulant drugs, which is also relevant for schizophrenia because illustrating its dopaminergic facet. But it remains questionable that only dopaminergic drugs elicit such effects. The behavioral effects could simply represent a non specific arousal, in which case NVHL rats should also be hyper-responsive to other vigilance enhancing drugs. We administered an adenosine (caffeine) or an adrenaline receptor antagonist, (RX821002) at doses documented to modify alertness of rats, respectively 5 mg/kg and 1 mg/kg. Rats were selected prior to the experiments using magnetic resonance imaging (MRI). Each group contained typical and similar NVHL lesions. They were compared to sham lesioned rats. We evaluated locomotion in a new environment and the capacity to remember a visual or acoustic cue that announced the occurrence of food. Both caffeine and RX82100 enhanced locomotion in the novel environment, particularly in NVHL rats. But, RX82100 had a biphasic effect on locomotion, consisting of an initial reduction preceding the enhancement. It was independent of the lesion. Caffeine did not modify the learning performance of NVHL rats. But, RX821002 was found to facilitate learning. Patients tend to intake much more caffeine than healthy people, which has been interpreted as a means to counter some cognitive deficits. This idea was not validated with the present results. But adrenergic drugs could be helpful for attenuating some of their cognitive deficits.

Neuroimaging over the course of Parkinson's disease: from early detection of the at-risk patient to improving pharmacotherapy of later-stage disease

Brain imaging of striatal dopamine terminal degeneration serves an important role in the clinical management of Parkinson's disease (PD). Imaging biomarkers for interrogating dopaminergic systems are used for clarifying diagnosis when only subtle motor symptoms are present. However, motor dysfunction is not the earliest symptom of PD. There is increasing interest in identifying premotor PD patients, particularly because potential disease-modifying therapies are developed and the clinical imperative becomes early and accurate diagnosis. On the other end of the spectrum of the disease course, during later stages of PD, significant clinical challenges like levo-dopa-induced dyskinesias and medication on-off phenomenon become more prevalent. In this instance, better understanding of altered PD motor pathways suggests the potential utility of novel treatments targeting neuronal systems that are impacted by degenerating dopamine neurons and chronic dopamine replacement treatment. Molecular neuroimaging serves unique roles in both very early PD and later-stage disease, in the former, potentially pushing back the time of diagnosis, and in the latter, elucidating pathology relevant to new drug development.

Influence of chronic caffeine on MDMA-induced behavioral and neuroinflammatory response in mice

RATIONALE

Previous research suggests that chronic daily caffeine administration protects against brain injury in different animal models of neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases, ischemic and traumatic brain injury, and allergic encephalitis. However, little is known about the effects of chronic caffeine administration on 3,4-methylenedioxymethamphetamine (MDMA)-induced neuroinflammation.

OBJECTIVE

The present study examines whether chronic caffeine (10, 20, or 30 mg/kg, i.p, for 21 consecutive days) protects against MDMA-induced astrocytic and microglial activation in mice striatum, impairing its neuroinflammatory effects. Additionally, locomotor activity, sensoriomotor reflexes, body temperature, and anxiety were evaluated after caffeine injection on days 0 (basal), 7, 14, and 21 of the chronic treatment in order to assess possible behavioral alterations due to caffeine administration.

METHODS

On day 22, mice pretreated with caffeine or saline received a neurotoxic regimen of MDMA (3 × 20 mg/kg, i.p., 2-h interval) or saline, and changes in body temperature were evaluated. Forty-eight hours after last MDMA or saline injection (day 24), the aforementioned behavioral parameters were investigated and microglia and astroglia activation to MDMA treatment was examined in the mouse striatum.

RESULTS

Caffeine (10 mg/kg) chronically administered completely prevented MDMA-induced glial activation without inducing physiological or behavioral alterations in any of the assays performed.

CONCLUSION

Chronic caffeine consumption at low doses exerts anti-inflammatory effects and prevents MDMA-induced neuroinflammation.

Locomotor activation by theacrine, a purine alkaloid structurally similar to caffeine: involvement of adenosine and dopamine receptors

Purine compounds, such as caffeine, have many health-promoting properties and have proven to be beneficial in treating a number of different conditions. Theacrine, a purine alkaloid structurally similar to caffeine and abundantly present in Camellia kucha, has recently become of interest as a potential therapeutic compound. In the present study, theacrine was tested using a rodent behavioral model to investigate the effects of the drug on locomotor activity. Long Evans rats were injected with theacrine (24 or 48 mg/kg, i.p.) and activity levels were measured. Results showed that the highest dose of theacrine (48 mg/kg, i.p.) significantly increased locomotor activity compared to control animals and activity remained elevated throughout the duration of the session. To test for the involvement of adenosine receptors underlying theacrine's motor-activating properties, rats were administered a cocktail of the adenosine A₁ agonist, N⁶-cyclopentyladenosine (CPA; 0.1 mg/kg, i.p.) and A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680; 0.2 mg/kg, i.p.). Pre-treatment with theacrine significantly attenuated the motor depression induced by the adenosine receptor agonists, indicating that theacrine is likely acting as an adenosine receptor antagonist. Next, we examined the role of DA D₁ and D₂ receptor antagonism on theacrine-induced hyperlocomotion. Both antagonists, D₁R SCH23390 (0.1 or 0.05 mg/kg, i.p.) and D₂R eticlopride (0.1 mg/kg, i.p.), significantly reduced theacrine-stimulated activity indicating that this behavioral response, at least in part, is mediated by DA receptors. In order to investigate the brain region where theacrine may be acting, the drug (10 or 20 μg) was infused bilaterally into nucleus accumbens (NAc). Theacrine enhanced activity levels in a dose-dependent manner, implicating a role of the NAc in modulating theacrine's effects on locomotion. In addition, theacrine did not induce locomotor sensitization or tolerance after chronic exposure. Taken together, these findings demonstrate that theacrine significantly enhances activity; an effect which is mediated by both the adenosinergic and dopaminergic systems.

Adenosine A(2A)-cannabinoid CB(1) receptor interaction: an integrative mechanism in striatal glutamatergic neurotransmission

The striatum is a subcortical area involved in sensorimotor, cognitive and emotional processes. Adenosine A(2A) receptors (A(2A)Rs) are highly expressed in the striatum, and their ability to establish functional and molecular interactions with many other receptors attributes to a pivotal role in the modulation and integration of striatal neurotransmission. This review will focus on the interaction between A(2A)Rs and cannabinoid CB(1) receptors (CB(1)Rs), taking it as a paradigmatic example of synaptic integration. Indeed, A(2A)Rs can exert an opposite (permissive vs. inhibitory) influence on CB1-dependent synaptic effect. These apparently irreconcilable functions could depend on a different role of pre- vs. postsynaptic A(2A)Rs, on their interaction with other receptors (namely adenosine A(1), metabotropic glutamate 5 and dopamine D2 receptors), and on whether A(2A)Rs form or not heteromers with CB(1)Rs. Besides providing a good example of the intricate pattern of events taking place in striatal synapses, the A(2A)/CB(1)R interaction proves very informative to understand the physiology of the basal ganglia and the mechanisms of related diseases. This article is part of a Special Issue entitled: Brain Integration.

Caffeine challenge test and panic disorder: a systematic literature review

This systematic review aimed to examine the results of studies that have investigated the induction of panic attacks and/or the anxiogenic effect of the caffeine challenge test in patients with panic disorder. The literature search was performed in PubMed, Biblioteca Virtual em Saúde and the ISI Web of Knowledge. The words used for the search were caffeine, caffeine challenge test, panic disorder, panic attacks and anxiety disorder. In total, we selected eight randomized, double-blind studies where caffeine was administered orally, and none of them controlled for confounding factors in the analysis. The percentage of loss during follow-up ranged between 14.3% and 73.1%. The eight studies all showed a positive association between caffeine and anxiogenic effects and/or panic disorder.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

Comparison of caffeine-induced locomotor activity between adolescent and adult rats

Caffeine is the psychostimulant drug most consumed in the world. This drug is present in food, beverages and medicines marketed for individuals of all ages. In spite of this, caffeine effects on adolescents are poorly understood. The aim of this study was to evaluate the differences on caffeine-induced locomotor stimulant or depressant effects in adolescent and adult rats. Adolescent (37-40 days old) or adult (70-74 days old) Wistar rats were tested for stimulant and depressant caffeine effects in two different experiments. The first was designed to evaluate the locomotor effect of caffeine in habituated rats. To this end, rats were previously habituated to test environment and had their locomotor activity registered following i.p. injections of vehicle or caffeine (3, 10, 30, 60 or 120 mg/kg). In the second experiment adolescent or adult rats were not habituated to the test environment and their locomotor activity was registered following i.p. injections of vehicle or caffeine (30, 60 or 120 mg/kg). In both experiments caffeine-induced a biphasic effect, with stimulation in small to moderate drug doses and no effect or locomotor depression in higher caffeine doses. Moreover, caffeine-induced locomotor stimulation was higher in adolescent than adult rats. Also, locomotor depression was only revealed in adult rats non-habituated to the test environment. These results suggest that adult and adolescent respond differently to caffeine indicating the need of more studies on the effects of caffeine in animals' models of adolescence.

Coca-paste seized samples characterization: chemical analysis, stimulating effect in rats and relevance of caffeine as a major adulterant

Coca-paste (CP) is a drug of abuse that so far has not been extensively characterized. CP is an intermediate product of the cocaine alkaloid extraction process from coca leaves, hence it has a high content of cocaine base mixed with other chemical substances (impurities) and it is probably adulterated when it reaches the consumers. Despite its high prevalence and distribution through South America, little is known about its effects on the central nervous system. In the present study, a chemical analysis of CP samples from different police seizures was performed to determine the cocaine base content and the presence and content of impurities and adulterants. Some CP representative samples were selected to study the effects on the locomotor activity induced after acute systemic administration in rats as a measure of its stimulant action. The behavioral response was compared to equivalent doses of cocaine. As expected, cocaine was the main component in most of the CP samples assayed. Caffeine was the only active adulterant detected. Interestingly, several CP samples elicited a higher stimulant effect compared to that observed after cocaine when administered at equivalent doses of cocaine base. Combined treatment of cocaine and caffeine, as surrogate of different CP samples mimicked their stimulant effect. We demonstrated that cocaine and caffeine are the main components responsible for the CP-induced stimulant action while the contribution of the impurities was imperceptible.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Endocannabinoid signaling mediates psychomotor activation by adenosine A2A antagonists

Adenosine A(2A) receptor antagonists are psychomotor stimulants that also hold therapeutic promise for movement disorders. However, the molecular mechanisms underlying their stimulant properties are not well understood. Here, we show that the robust increase in locomotor activity induced by an A(2A) antagonist in vivo is greatly attenuated by antagonizing cannabinoid CB(1) receptor signaling or by administration to CB(1)(-/-) mice. To determine the locus of increased endocannabinoid signaling, we measured the amount of anandamide [AEA (N-arachidonoylethanolamine)] and 2-arachidonoylglycerol (2-AG) in brain tissue from striatum and cortex. We find that 2-AG is selectively increased in striatum after acute blockade of A(2A) receptors, which are highly expressed by striatal indirect-pathway medium spiny neurons (MSNs). Using targeted whole-cell recordings from direct- and indirect-pathway MSNs, we demonstrate that A(2A) receptor antagonists potentiate 2-AG release and induction of long-term depression at indirect-pathway MSNs, but not direct-pathway MSNs. Together, these data outline a molecular mechanism by which A(2A) antagonists reduce excitatory synaptic drive on the indirect pathway through CB(1) receptor signaling, thus leading to increased psychomotor activation.

Influence of acute caffeine on 50-kHz ultrasonic vocalizations in male adult rats and relevance to caffeine-mediated psychopharmacological effects

To further characterize caffeine-mediated psychopharmacological effects, the present study investigated whether acute caffeine (3, 10, 30, 50 mg/kg i.p.) exerted any influence on the emission and features of ultrasonic vocalizations (USVs), which are thought to index changes involving emotional state, in male adult rats. The results obtained demonstrate that caffeine can trigger modifications in the maximum peak frequency and bandwidth of the 50-kHz range USVs. However, such an effect was not accompanied by a significant elevation in the number of 50-kHz USVs, relative to administration of vehicle. Under the same experimental conditions, acute amphetamine (2 mg/kg i.p.) robustly elevated the number of 50-kHz USVs emitted by rats, although it did not affect the maximum peak frequency and bandwidth of USVs. Thus, both qualitative and quantitative differences in the effects exerted by caffeine and amphetamine on 50-kHz USVs were observed. Taken together, these findings further clarify the features of caffeine-mediated psychopharmacological effects, and may help to elucidate the differences between the central effects of caffeine and those elicited by other psychostimulants.

Improvement of postural adjustment steps in hemiparkinsonian rats chronically treated with caffeine is mediated by concurrent blockade of A1 and A2A adenosine receptors

Chronic treatment with the non-selective adenosine receptor antagonist caffeine produces full recovery of the contralateral adjusting steps in hemiparkinsonian rats. In order to disclose which adenosine receptor subtype mediates this effect, a group of hemiparkinsonian rats (n=9) was treated with caffeine (5.15 mumol/kg/day), or equimolar doses of selective A1 (DPCPX) or A2A (ZM 241385) adenosine receptor antagonists, administered in a counterbalanced order over periods of 3 weeks, interspersed with equivalent washout intervals. Treatment with ZM 241385 caused full recovery (102+/-6%) of the contralateral forepaw stepping, while the maximal effect of DPCPX was only 73+/-7% of that produced by caffeine. The maximal effect of caffeine and ZM 241385 remained stable throughout the treatment period. The response to DPCPX showed more fluctuations, but tolerance did not develop. Stepping improvement was significantly faster with DPCPX than with ZM 241385, while caffeine had intermediate values. Stepping decrease after treatment interruption was faster with ZM 241385 than with caffeine, while DPCPX had intermediate values. In other experiments with the same rats, addition of the A2AR agonist CGS 21680 (5.15 mumol/kg) or the A1R agonist CCPA (2.71 mumol/kg) during the second week of caffeine treatment reversed the improvement of contralateral stepping by 59+/-4% and 30+/-3%, respectively. The combined treatment with CGS 21680 and CCPA caused complete reversal of the contralateral stepping recovery afforded by caffeine, which was more than additive (114+/-5%) compared with the sum of the maximal inhibition produced by either agonist administered alone (89+/-4%). In all cases, after interrupting the adenosine agonists, the effect of caffeine was fully restored. None of the aforementioned treatments induced significant changes in the stepping of the ipsilateral forepaw. Collectively, these results suggest that the improvement of postural adjustments induced by chronic treatment with low doses of caffeine in hemiparkinsonian rats is mediated by concurrent blockade of A1 and A2A adenosine receptors, with a larger involvement of the latter.

Does the presence of a pharmacological substance alter the placebo effect?--results of two experimental studies using the placebo-caffeine paradigm

OBJECTIVES

We employed the placebo-caffeine paradigm to test whether the presence or absence of a substance (caffeine) influences the placebo effect.

METHODS

In experiment 1 consisting of four conditions with n = 15 participants each (control, placebo, two double-blind groups, each with placebo only), we maximized the placebo effect through expectation. Effects were assessed with physiological (blood pressure, heart rate), psychomotor (response times), and well-being indicators (self-report). In experiment 2, caffeine was administered in one of the double-blind groups, and another condition was added where caffeine was given openly.

RESULTS

Effect sizes were medium to large for some outcome parameters in experiment 1 and 2, showing partial replicability of the classical placebo effect. Although not formally significant, differences between the double blind placebo conditions of the two experiments (with and without caffeine present) were medium to small. There was a significant difference (p = 0.03) between experiment 1 and experiment 2 in the physiological variables, and a near significant interaction effect between groups and experiments in the physiological variables (p = 0.06).

CONCLUSION

The question warrants further scrutiny. The presence of a pharmacological substance might change the magnitude of the placebo response.

Adenosine receptor antagonists and behavioral activation in NF-kappaB p50 subunit knockout mice

AIMS

Our previous work revealed that mice lacking the p50 subunit of transcription factor nuclear factor kappa B (NF-kappaB) (p50 KO mice) and genetically intact F2 mice have similar locomotion under basal conditions, yet p50 KO mice show greater locomotor activation after caffeine ingestion. In this report, we test whether KO mice display altered caffeine pharmacokinetics or increased caffeine-induced DA turnover relative to F2 mice, and evaluate the impact of intraperitoneal administration of specific adenosine and DA receptor antagonists on locomotor activity.

MAIN METHODS

Concentrations of DA and caffeine were measured using high performance liquid chromatography. DA turnover was measured after treatment of mice with an inhibitor of tyrosine hydroxylase. Locomotor activity was measured using telemetry.

KEY FINDINGS

The data reveal that 1) caffeine concentrations in blood and brain are similar in KO and F2 mice after oral or intraperitoneal administration; 2) KO mice show greater DA turnover under basal conditions, but turnover is similar in both strains after caffeine administration; 3) the specific A2AAR antagonist SCH 58261 induces greater locomotion in KO versus F2 mice; and 4) the activating effect of SCH 58261 in KO mice is prevented by prior treatment with the D2R antagonist raclopride.

SIGNIFICANCE

These findings support the conclusions that 1) A2AAR has a major impact on behavioral activation of p50 KO mice, and 2) D2R mediated neurotransmission is important to this effect.

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.