M. El Yacoubi

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.

Abstract 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 A2A receptor. First, the effects induced by the non-selective drug caffeine were compared with those elicited by two selective A2A receptor antagonists over a wide range of doses in the same experimental conditions. The effects of A2A or A1 adenosine receptor agonists and of a selective A1 adenosine receptor antagonist were also investigated. Second, wild-type and A2A receptor knockout mice offered another approach to delineate the role played by A2A 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 A2A 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 A1 receptors had no acute effects on anxiety-related indices, whereas an A2A 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 A2A 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 A2A receptor knockout than in wild-type mice. Conclusions: Adaptative mechanisms following mutation in A2A 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 A2A receptors, since it is not shared by A2A selective antagonists.

Caffeine reduces hypnotic effects of alcohol through adenosine A2A receptor blockade

The role of the adenosine A(2A) receptor in the hypnotic effects of ethanol was assessed in mice. The duration of the loss of righting reflex following acute ethanol administration was shorter for A(2A) receptor-deficient mice (A(2A)R KO) than for wild-type mice (A(2A)R WT), whereas the fall in body temperature was not different between the two phenotypes. In contrast, the duration of the loss of righting reflex was increased in A(2A)R KO mice versus controls after administration of pentobarbital. Dipyridamole, an inhibitor of adenosine uptake, increased the sleep time observed following administration of ethanol in CD1 mice and in A(2A)R WT but not in A(2A)R KO mice. SCH 58261, a selective A(2A) receptor antagonist, unlike DPCPX, a selective A(1) receptor antagonist, shortened the duration of the loss of righting reflex induced by ethanol, thus mimicking the lack of receptor in deficient mice. Finally, the non-selective adenosine receptor antagonist caffeine (25 mg/kg) reduced ethanol-induced hypnotic effects. These results indicate that the activation of A(2A) receptors that follows an increase in extracellular adenosine levels caused by the administration of high doses of ethanol plays a role in its hypnotic effects. Thus, A(2A) receptor antagonists may be useful therapeutic agents for alleviating ethylic coma.

Adenosine A2A receptor gene disruption provokes marked changes in melanocortin content and pro-opiomelanocortin gene expression.

A2A receptor knockout (A2AR−/−) mice are more anxious and aggressive, and exhibit reduced exploratory activity than their wild‐type littermates (A2AR+/+). Because α‐melanocyte‐stimulating hormone (α‐MSH) influences anxiety, aggressiveness and motor activity, we investigated the effect of A2AR gene disruption on α‐MSH content in discrete brain regions and pro‐opiomelanocortin (POMC) expression in the hypothalamus and pituitary. No modification in α‐MSH content was observed in the hypothalamus and medulla oblongata where POMC‐expressing perikarya are located. In the arcuate nucleus of the hypothalamus, POMC mRNA levels were not affected by A2AR disruption. Conversely, in A2AR−/− mice, a significant increase in α‐MSH content was observed in the amygdala and cerebral cortex, two regions that are innervated by POMC terminals. In the pars intermedia of the pituitary, A2AR disruption provoked a significant reduction of POMC mRNA expression associated with a decrease in α‐MSH content. By contrast, in the anterior lobe of the pituitary, a substantial increase in POMC mRNA and adrenocorticotropin hormone concentrations was observed, and plasma corticosterone concentration was significantly higher in A2AR−/− mice, revealing hyperactivity of their pituitary‐adrenocortical axis. Together, these results suggest that adenosine, acting through A2A receptors, may modulate the release of α‐MSH in the cerebral cortex and amygdala. The data also indicate that A2A receptors are involved in the control of POMC gene expression and biosynthesis of POMC‐derived peptides in pituitary melanotrophs and corticotrophs.

P.2.028 Enhanced vulnerability to cocaine reinforcing effects in female H/Rouen mice selectively bred for depressive-like behaviour

leads to good memory performance of rats (1 h interval ORT, repeated measures ANOVA; MLA (F3,51 = 7.33; P< 0.001). This indicates that MLA could act both as a cognition enhancer and as a deficit model in this paradigm. Among other possibilities, one explanation for these findings could be that a7 nAChR antagonists promote a7 nAChR resensitisation. Since a7 nAChRs desensitise fast, by occupying a subset of a7 nAChRs with selective antagonists without intrinsic value, these receptors could have an opportunity to recover or resensitise. This hypothesis is currently investigated. In summary, while the main focus of the a7nAChR as a target for cognition enhancement lies on agonists and positive modulators, the antagonists of these receptors might prove to be a valuable tool for cognition enhancement in AD or schizophrenia.