Biomedical subjects

C Ledent

Publications and source records attributed to C Ledent.

At least 19 recordsLinked to original sources

PubMed2000

Characterisation of central adenosine A(1) receptors and adenosine transporters in mice lacking the adenosine A(2a) receptor.

The present study was designed to assess whether adenosine A(2a) receptor knockout mice exhibit altered purine utilisation in brain nuclei. Specifically, the properties of adenosine transporters and adenosine A(1) receptors were characterised in brain membranes and on slide-mounted sections. The B(MAX) for [(3)H]nitrobenzylthioinosine ([(3)H]NBTI) binding (adenosine transporter density) was significantly reduced in brainstem membranes of homozygotes (560+/-52 fmol/mg protein, n=5, P<0.05, Kruskal-Wallis ANOVA) compared to wildtype (1239+/-213 fmol/mg protein) and heterozygous mice (1300+/-558 fmol/mg protein). Quantitative autoradiography data indicated that [(3)H]NBTI binding in the medulla oblongata of heterozygous mice was seen to decrease significantly (P<0.05) in the subpostremal nucleus tractus solitarius (NTS), medial NTS, inferior olive and area postrema (AP). On the other hand, in the homozygous mice a decrease was seen in the medial NTS and AP. In the pons, [(3)H]1, 3-dipropyl-8-cyclopentylxanthine ([(3)H]DPCPX) (adenosine A(1) receptor density) binding increased significantly (P<0.05, Kruskal-Wallis ANOVA) in the lateral parabrachial nucleus, caudal pontine reticular nucleus and locus coeruleus of homozygotes compared to wildtype. In higher brain centres, [(3)H]NBTI binding was reduced in the paraventricular thalamic nucleus of both heterozygous and homozygous mice, whereas [(3)H]DPCPX binding was reduced in the hippocampus and lateral hypothalamus of heterozygotes. In homozygotes, [(3)H]DPCPX binding in the hippocampus increased compared to wildtype mice. The present study indicates that deletion of the A(2a) receptor may have contributed to region-specific compensatory changes in purine utilisation in brain nuclei associated with autonomic, neuroendocrine and behavioural regulation.

Affinity Labels↗

PubMed2000

Visualisation of AMPA binding sites in the brain of mice lacking the adenosine A(2a) receptor.

Adenosine A(2a) receptor knockout mice (A(2a)R k/o) have been characterised as hypertensive, aggressive, anxious and hypoalgesic [12]. Thus, the present study was designed to investigate markers of glutamatergic transmission in specific brain nuclei associated with autonomic regulation. Visualisation of alpha-amino-3-hydroxy-5-methylisoxasole-4-propionic acid binding sites in the brains of A(2a)R k/o mice was achieved by utilising quantitative receptor autoradiography with (S)-[(3)H]-5-fluorowillardiine ([(3)H]FW:10 nM) on slide-mounted sections of mouse-brain. [(3)H]FW binding significantly increased in the nucleus tractus solitarius and area postrema of A(2a)R k/o mice compared with wildtype CD-1 mice. In other autonomic nuclei examined, binding was unchanged between wildtype and knockout mice. The present study suggests that glutamatergic neurotransmission within certain neural pathways involved in autonomic and motor control is altered in the brains of A(2a)R k/o mice.

Animals↗

PubMed2000

SCH 58261 and ZM 241385 differentially prevent the motor effects of CGS 21680 in mice: evidence for a functional 'atypical' adenosine A(2A) receptor.

The acute motor effects elicited by drugs acting upon adenosine A(2A) receptors, namely the highly selective agonist CGS 21680 or the antagonists SCH 58261 and ZM 241385, were investigated in mice. CGS 21680 dose-dependently (0.1-2.5 mg/kg i.p.) decreased horizontal and vertical motor activities. The depressant effect of CGS 21680 (0. 5 mg/kg i.p.) was maintained in mice pretreated by the adenosine receptor antagonist 8-(p-sulfophenyl)-theophylline (10-30 mg/kg i.p. ), which poorly penetrates the blood-brain barrier, but was completely lost in adenosine A(2A) receptor knockout mice. Thus, the adenosine A(2A) receptor is critically involved in motor activity. SCH 58261 (1-10 mg/kg i.p.) increased locomotion and rearing with a quick onset, but for a shorter period in mice habituated to the environment than in mice unfamiliar to it. ZM 241385 (7.5-60 mg/kg i. p.) stimulated horizontal and vertical activities with a slow onset at the two highest tested doses, similarly in naive and in habituated mice. The increase in locomotion elicited by ZM 241385 (15-30 mg/kg i.p. and 10-20 nM i.c.v.) was retained in mice treated by CGS 21680 (0.5 mg/kg i.p.) but that elicited by SCH 58261 (1-3-10 mg/kg i.p. and 10-20 nM i.c.v.) partially subsided. In conclusion, both 'striatal-like'/'SCH 58261-sensitive' adenosine A(2A) receptors and 'ZM 241385-sensitive'/'atypical' CGS 21680 binding sites may mediate CGS 21680-induced motor effects. Moreover, our results suggest that 'atypical' CGS 21680 binding sites could be adenosine A(2A) receptors with a peculiar pharmacological profile.

Adenosine↗

PubMed2000

Regulation of the phosphorylation of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa in vivo by dopamine D1, dopamine D2, and adenosine A2A receptors.

Dopamine D(1), dopamine D(2), and adenosine A(2A) receptors are highly expressed in striatal medium-sized spiny neurons. We have examined, in vivo, the influence of these receptors on the state of phosphorylation of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). DARPP-32 is a potent endogenous inhibitor of protein phosphatase-1, which plays an obligatory role in dopaminergic transmission. A dose-dependent increase in the state of phosphorylation of DARPP-32 occurred in mouse striatum after systemic administration of the D(2) receptor antagonist eticlopride (0.1-2.0 mg/kg). This effect was abolished in mice in which the gene coding for the adenosine A(2A) receptor was disrupted by homologous recombination. A reduction was also observed in mice that had been pretreated with the selective A(2A) receptor antagonist SCH 58261 (10 mg/kg). The eticlopride-induced increase in DARPP-32 phosphorylation was also decreased by pretreatment with the D(1) receptor antagonist SCH 23390 (0.125 and 0.25 mg/kg) and completely reversed by combined pretreatment with SCH 23390 (0.25 mg/kg) plus SCH 58261 (10 mg/kg). SCH 23390, but not SCH 58261, abolished the increase in DARPP-32 caused by cocaine (15 mg/kg). The results indicate that, in vivo, the state of phosphorylation of DARPP-32 and, by implication, the activity of protein phosphatase-1 are regulated by tonic activation of D(1), D(2), and A(2A) receptors. The results also underscore the fact that the adenosine system plays a role in the generation of responses to dopamine D(2) antagonists in vivo.

Animals↗

PubMed2000

The anxiogenic-like effect of caffeine in two experimental procedures measuring anxiety in the mouse is not shared by selective A(2A) adenosine receptor antagonists.

RATIONALE: The elevated plus-maze and the light/dark box are two established anxiety tests in rodents, which are useful to screen putative anxiogenic effects of drugs. OBJECTIVE: Caffeine is well known to promote anxious behaviour in humans and animal models, but the precise site of action of the drug is still a matter of debate. The present study investigated whether the anxiogenic effects of caffeine observed in mice depend on the blockade of A(2A) receptor. First, the effects induced by the non-selective drug caffeine were compared with those elicited by two selective A(2A) receptor antagonists over a wide range of doses in the same experimental conditions. The effects of A(2A) or A(1 )adenosine receptor agonists and of a selective A(1) adenosine receptor antagonist were also investigated. Second, wild-type and A(2A) receptor knockout mice offered another approach to delineate the role played by A(2A) receptor in caffeine's anxiogenic effects. METHODS: Mice were exposed to the elevated plus-maze or to the light/dark box for 5 min after acute or chronic administration of tested drugs. RESULTS: Caffeine acutely administered (50 or 100 mg/kg IP) induced anxiety-like effects in both procedures. Its chronic administration (50 mg/kg IP twice daily) for 1 week or consumption in the drinking water (0.3 g/l) for 8 days or 2 months were also anxiogenic in the plus-maze test. The A(2A) receptor antagonists ZM241385 (up to 60 mg/kg IP) and SCH58261 (up to 10 mg/kg IP) were devoid of acute effects in both tests. One week administration of ZM241385 (30 mg/kg IP) or SCH58261 (3 mg/kg IP) had no effects in the plus-maze test. An antagonist (DPCPX) and an agonist (CPA) at A(1) receptors had no acute effects on anxiety-related indices, whereas an A(2A) receptor agonist (CGS 21680) displayed non-specific motor effects in the plus-maze test. Acute administration of caffeine (50 mg/kg IP) induced no clear-cut anxiety-like effects in the plus-maze test in A(2A) receptor knockout mice that exhibited higher basal anxiety levels than wild-type mice. Chronic administration (50 mg/kg IP twice daily) for 1 week elicited less anxiety-like behaviour in A(2A) receptor knockout than in wild-type mice. CONCLUSIONS: Adaptative mechanisms following mutation in A(2A) receptors or their long-term blockade after chronic ingestion of caffeine may be responsible for increase proneness to anxiety. However, the short-term anxiety-like effect of caffeine in mice might not be related solely to the blockade of adenosine A(2A) receptors, since it is not shared by A(2A) selective antagonists.

Adenosine↗

PubMed2000

Enhanced long-term potentiation in mice lacking cannabinoid CB1 receptors.

Marijuana is known to affect learning and memory in humans, and cannabinoids block long-term potentiation in the hippocampus, a model for the synaptic changes that are believed to underlie memory at the cellular level. We have now examined the physiological properties of the Schaffer collateral-CA1 synapses in mutant mice in which the CB1 receptor gene has been invalidated and found that these animals exhibit a half-larger long-term potentiation than wild-type controls. Other properties of these synapses, such as paired-pulse facilitation, remained unchanged. This indicates that disrupting CB1 receptor-mediated neurotransmission at the genome level produces mutant mice with an enhanced capacity to strengthen synaptic connections in a brain region crucial for memory formation.

Animals↗

PubMed2000

Early effect of ultrarush venom immunotherapy on the IgG antibody response.

BACKGROUND: We have previously shown in several allergy models that allergic and tolerance status with respect to allergens is associated with a somewhat different dominant specificity of IgG antibodies. The objective was to test this hypothesis in the compelling model of ultrarush venom immunotherapy (VIT), which induces clinical tolerance after only a few hours of treatment. METHODS: Antibody titers and specificity were evaluated through solid-phase ELISA using streptavidin-biotin technology in 12 patients allergic to wasp venom before and during the ultrarush procedure (at 12 h, 24 h, and 15 days). The results were compared with those from another group of 20 patients treated with venom injections for at least 2 years. RESULTS: No significant change was observed in IgG titers during the early phase of VIT. The capacity of individual sera to prevent the antigen binding of pooled IgG from allergic patients changed rapidly, with mean percentage inhibitions falling from 80+/-15%, before starting VIT, to 26+/-14%, 35+/-15%, and 34+/-5% after 12 h, 24 h, and 15 days of treatment, respectively (P<0.001 by one-way ANOVA). The capacity of individual sera to prevent the antigen binding of pooled IgG from patients receiving prolonged VIT changed, with mean percent inhibitions increasing from 47+/-8%, before starting VIT, to 76+/-7%, 83+/-6%, and 87+/-6% after 12 h, 24 h, and 15 days of treatment, respectively (P<0.001 by one-way ANOVA). CONCLUSIONS: During the initial phase of ultrarush VIT, a change in IgG specificity, i.e., a change in the set of epitopes dominantly recognized by IgG on wasp-venom antigens, occurred concomitantly with early clinical tolerance and was already detectable a few hours after the onset of treatment. Although it may be an epiphenomenon, this change represents the earliest humoral modification described so far during this procedure. The mechanism is unknown, but it appears to be a selective depletion of the highest avidity antibody fraction by the venom injected in large doses at this stage of therapy. Finally, our data now show the previously documented association between a particular IgG specificity and the clinical status (allergy vs tolerance) to be true also with ultrarush VIT, a model in which the clinical ability to display allergic symptoms is rapidly reversed.

Adult↗

PubMed2000

The stimulant effects of caffeine on locomotor behaviour in mice are mediated through its blockade of adenosine A(2A) receptors.

1. The locomotor stimulatory effects induced by caffeine (1,3, 7-trimethylxanthine) in rodents have been attributed to antagonism of adenosine A(1) and A(2A) receptors. Little is known about its locomotor depressant effects seen when acutely administered at high doses. The roles of adenosine A(1) and A(2A) receptors in these activities were investigated using a Digiscan actimeter in experiments carried out in mice. Besides caffeine, the A(2A) antagonist SCH 58261 (5-amino-7-(beta-phenylethyl)-2-(8-furyl)pyrazolo[4,3-e]-1,2, 4-triazolo[1,5-c]pyrimidine), the A(1) antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine), the A(1) agonist CPA (N(6)-cyclopentyladenosine) and A(2A) receptor knockout mice were used. 2. Caffeine had a biphasic effect on locomotion of wild-type mice not habituated to the open field, stimulating locomotion at 6.25 - 25 mg kg(-1) i.p. doses, while depressing it at 100 mg kg(-1). In sharp contrast, caffeine dose-dependently decreased locomotion in A(2A) receptor knockout mice over the whole range of tested doses. 3. The depressant effects induced by high doses of caffeine were lost in control CD1 mice habituated to the open field. 4. The A(1) agonist CPA depressed locomotion at 0.3 - 1 mg kg(-1) i.p. doses. 5. The A(1) antagonist DPCPX decreased locomotion of A(2A) receptor knockouts and CD1 mice at 5 mg kg(-1) i.p. and 25 mg kg(-1) i.p. respectively. 6. DPCPX (0.2 - 1 mg kg(-1) i.p.) left unaltered or even reduced the stimulant effect of SCH 58261 (1 - 3 mg kg(-1) i.p.) on CD1 mice. 7. These results suggest therefore that the stimulant effect of low doses of caffeine is mediated by A(2A) receptor blockade while the depressant effect seen at higher doses under some conditions is explained by A(1) receptor blockade.

Analysis of Variance↗

PubMed2000

Cannabinoids inhibit hippocampal GABAergic transmission and network oscillations.

Using a new antibody developed against the C-terminus of the cannabinoid receptor (CB1), the immunostaining in the hippocampus revealed additional axon terminals relative to the pattern reported previously with an N-terminus antibody. Due to a greater sensitivity of this antibody, a large proportion of boutons in the dendritic layers displaying symmetrical (GABAergic) synapses were also strongly immunoreactive for CB1 receptors, as were axon terminals of perisomatic inhibitory cells containing cholecystokinin. Asymmetrical (glutamatergic) synapses, however, were always negative for CB1. To investigate the effect of presynaptic CB1 receptor activation on hippocampal inhibition, we recorded inhibitory postsynaptic currents (IPSCs) from principal cells. Bath application of CB1 receptor agonists (WIN55,212-2 and CP55,940) suppressed IPSCs evoked by local electrical stimulation, which could be prevented or reversed by the CB1 receptor antagonist SR141716A. Action potential-driven IPSCs, evoked by pharmacological stimulation of a subset of interneurons, were also decreased by CB1 receptor activation. We also examined the effects of CB1 receptor agonists on Ca2+-independent miniature IPSCs (mIPSC). Both agonists were without significant effect on the frequency or amplitude of mIPSCs. Synchronous gamma oscillations induced by kainic acid in the CA3 region of hippocampal slices were reversibly reduced in amplitude by the CB1 receptor agonist CP 55,940, which is consistent with an action on IPSCs. We used CB1-/- knock-out mice to confirm the specificity of the antibody and of the agonist (WIN55,212-2) action. We conclude that activation of presynaptic CB1 receptors decreases Ca2+-dependent GABA release, and thereby reduces the power of hippocampal network oscillations.

Action Potentials↗

PubMed2000

Reduction of stress-induced analgesia but not of exogenous opioid effects in mice lacking CB1 receptors.

CB1 cannabinoid receptors are widely distributed in the central nervous system where they mediate most of the cannabinoid-induced responses. Here we have evaluated the interactions between the CB1 cannabinoid receptors and the endogenous opioid system by assaying a number of well-characterized opioid responses, e.g. antinociception and stress-mediated effects, on mutant mice in which the CB1 receptor gene was invalidated. The spontaneous responses to various nociceptive stimuli (thermal, mechanical and visceral pain) were not changed in mutant CB1 mice. Furthermore, the absence of the CB1 cannabinoid receptor did not modify the antinociceptive effects induced by different opioid agonists: morphine (preferential mu opioid agonist), D-Pen2-D-Pen5-enkephalin (DPDPE) and deltorphin II (selective delta opioid agonists), and U-50,488H (selective kappa opioid agonist) in the hot-plate and tail-immersion tests. In contrast, the stress-induced opioid mediated responses were modified in CB1 mutants. Indeed, these mutants did not exhibit antinociception following a forced swim in water at 34 degrees C and presented a decrease in the immobility induced by the previous exposure to electric footshock. However, the antinociception induced by a forced swim in water at 10 degrees C was preserved in CB1 mutants. These results indicate that CB1 receptors are not involved in the antinociceptive responses to exogenous opioids, but that a physiological interaction between the opioid and cannabinoid systems is necessary to allow the development of opioid-mediated responses to stress.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗

PubMed1999

Lack of morphine-induced dopamine release in the nucleus accumbens of cannabinoid CB(1) receptor knockout mice.

Morphine (10 and 20 mg/kg, s.c.) does not modify dopamine release in the nucleus accumbens of cannabinoid CB(1) knock-out mice under conditions where it dose-dependently stimulates the release of dopamine in the corresponding wild-type mice. These results demonstrate that cannabinoid CB(1) receptors, regulate mesolimbic dopaminergic transmission in brain areas known to be involved in the reinforcing effects of morphine.

Analgesics, Opioid↗

PubMed1999

Enhancement of memory in cannabinoid CB1 receptor knock-out mice.

We have used cannabinoid CB knock-out mice in a two-trial object recognition test to assess the role of cannabinoid CB receptors in memory. Cannabinoid CB1 knock-out mice are able to retain memory for at least 48 h after the first trial whereas the wild-type controls lose their capacity to retain memory after 24 h. These results suggest that endogenous cannabinoid CB receptors play a crucial role in the process of memory storage and retrieval.

Animals↗

PubMed1999

Unresponsiveness to cannabinoids and reduced addictive effects of opiates in CB1 receptor knockout mice.

The function of the central cannabinoid receptor (CB1) was investigated by invalidating its gene. Mutant mice did not respond to cannabinoid drugs, demonstrating the exclusive role of the CB1 receptor in mediating analgesia, reinforcement, hypothermia, hypolocomotion, and hypotension. The acute effects of opiates were unaffected, but the reinforcing properties of morphine and the severity of the withdrawal syndrome were strongly reduced. These observations suggest that the CB1 receptor is involved in the motivational properties of opiates and in the development of physical dependence and extend the concept of an interconnected role of CB1 and opiate receptors in the brain areas mediating addictive behavior.

Analgesics, Opioid↗

PubMed1998

Differential patterns of cell cycle regulatory proteins expression in transgenic models of thyroid tumours.

Cell cycle proteins regulate the transitions from G1 to S and G2 to M phases. In higher eukaryotes, their function is controlled by intracellular cascades regulated by extracellular growth factors. We have studied in previously described transgenic mouse models for thyroid proliferative diseases the expression of the key proteins regulating the cell cycle by Western blotting and immunohistochemistry, and have correlated the observations with the known actions of the transgenes on the signal transduction cascades. In the adenosine A2a receptor model, the cyclic AMP pathway, upstream of the Rb family cell division block, is constitutively activated. In the model expressing HPV 16 E7 protein, the Rb-like proteins are inhibited. Cyclin-dependent kinases cdk4, cdk2 and cdc2, and the associated cyclins D, E and A have been studied. Cyclin D3 appears as the major cyclin D subtype expressed in mouse thyroid epithelial cells in normal and transgenic mice. In the adenosine A2aR model, all cell cycle proteins tested were accumulated. In the E7 model, all cell cycle proteins except for D-type cyclins and cdk4 were also accumulated. A similar pattern was observed in thyroids coexpressing both transgenes, suggesting a dominant effect of E7 over the consequences of the cAMP cascade activation. The cyclin-dependent kinase inhibitors p21cip1/waf1 and p27kip1 were not downregulated in these proliferating thyroids which suggest other roles than the inhibition of the cell cycle progression.

Animals↗

PubMed1998

Modified antigenic reactivity of anti-phospholipase A2 IgG antibodies in patients allergic to bee venom: conversion with immunotherapy and relation to subclass expression.

BACKGROUND: We have previously reported that, in addition to modifying IgG levels and subclass distributions, wasp venom immunotherapy (VIT) rapidly changes IgG antibody specificity. OBJECTIVES: We investigated whether such a change can be documented in the IgG response to the major bee venom allergen, phospholipase A2 (PLA2), from patients allergic to bees treated with VIT; whether it is coupled to the shift in IgG subclass distribution (IgG4 predominance) usually observed during VIT; and whether it restores the specificity displayed by IgG antibodies from nonallergic individuals. METHODS: Antibody specificity was evaluated in 17 patients allergic to bee venom in competitive ELISAs by using streptavidin biotin technology. Patients were tested before and during specific immunotherapy (at 15 days and 6 months) and compared with another group of 17 patients treated with venom injections for at least 2 years (VIT patients) and 30 healthy individuals. RESULTS: The capacity of individual sera to prevent PLA2 binding of pooled IgG from allergic patients changed rapidly with mean percentage inhibitions falling from 84% +/- 14% before starting VIT to 27% +/- 13% and 28% +/- 7% after 15 days and 6 months of treatment, respectively (p < 0.001 by one-way analysis of variance [ANOVA]). IgG titers were only slightly increased. The capacity of individual sera to prevent the binding of pooled IgG from patients receiving VIT changed rapidly with mean percentage inhibition increasing from 60% +/- 12% before starting VIT to 85% +/- 6% and 82% +/- 6% after 15 days and 6 months of treatment, respectively (p < 0.001 by one-way ANOVA). Similar results were found regardless of whether pooled IgG1 or pooled IgG4 were used. CONCLUSION: VIT results in a rapid change in the antigenic reactivity of anti-PLA2 IgG antibody of human allergic sera, restoring, although not completely, the specificity peculiar to lgG from healthy individuals. This suggests that allergic status and immunoprotection correlate with the preferential expression of distinct IgG specificities, which appear equally distributed over the IgG1 and IgG4 antibody subclasses. It is, however, not known whether the shift in IgG specificity is one of the operative mechanisms of VIT.

Adult↗

PubMed1998

[Transgenic mouse models. Their interest in thyroid tumors].

Mouse transgenic models that develop thyroid diseases were generated. All transgenes were driven by the thyroid specific promoter of the thyroglobulin gene. The tissue specificity of the promoter was investigated by using the bacterial chloramphenicol acetyltransferase gene as reporter. The expression of the adenosine A2a receptor resulted in the permanent activation of the cAMP cascade. As a consequence, transgenic mice developed severe hyperthyroidism and a large goiter, demonstrating in vivo the role of the cAMP cascade in the promotion of both function and proliferation of the thyroid cell. These mice constitute a model for autonomous hyperfunctional adenoma and non autoimmune familial hyperthyroidism, where mutant of thyrotropin receptors stimulate the cAMP cascade constitutively. The expression of a mutant of the alpha 1B adrenergic receptor resulted in the constitutive activation of both the cAMP and IP3-CA++ cascades, growth stimulation, hyperfunction, cell degeneracy attributed to the overproduction of free radicals, and development of malignancies. The expression of the SV40 large T antigen promoted the development of aggressive undifferentiated tumors mimicking the phenotype of human anaplastic carcinomas and embryonal tumors. In another transgenic model, the function of the retinoblastoma susceptibility gene product RB1 (and of related proteins) was inhibited by expressing the E7 oncoprotein of human papillomavirus type 16. The result was the development of a differentiated and normofunctional colloid goiter, with progressive development of differentiated malignant lesions. This model suggests the essential role of RB1 and related proteins in the negative control of proliferation that characterizes thyroid cells in the adult. Other transgenic models of thyroid diseases are discussed.

Adenoma↗

PubMed1998

[Thyroid insular carcinoma].

This review article illustrates the several histological and immunohistochemical patterns of thyroid insular carcinoma and their associated disease. Differential diagnosis are also discussed. Poorly differentiated thyroid carcinomas are overviewed. The interest of transgenic mice models is presented.

Animals↗

PubMed1997

Aggressiveness, hypoalgesia and high blood pressure in mice lacking the adenosine A2a receptor.

Adenosine is released from metabolically active cells by facilitated diffusion, and is generated extracellularly by degradation of released ATP. It is a potent biological mediator that modulates the activity of numerous cell types, including various neuronal populations, platelets, neutrophils and mast cells, and smooth muscle cells in bronchi and vasculature. Most of these effects help to protect cells and tissues during stress conditions such as ischaemia. Adenosine mediates its effects through four receptor subtypes: the A1, A2a, A2b and A3 receptors. The A2a receptor (A2aR) is abundant in basal ganglia, vasculature and platelets, and stimulates adenylyl cyclase. It is a major target of caffeine, the most widely used psychoactive drug. Here we investigate the role of the A2a receptor by disrupting the gene in mice. We found that A2aR-knockout (A2aR-/-) mice were viable and bred normally. Their exploratory activity was reduced, whereas caffeine, which normally stimulates exploratory behaviour, became a depressant of exploratory activity. Knockout animals scored higher in anxiety tests, and male mice were much more aggressive towards intruders. The response of A2aR-/- mice to acute pain stimuli was slower. Blood pressure and heart rate were increased, as well as platelet aggregation. The specific A2a agonist CGS 21680 lost its biological activity in all systems tested.

Adenosine↗