Caffeine and a selective adenosine A2A receptor antagonist induce sensitization and cross-sensitization behavior associated with increased striatal dopamine in mice.

Caffeine, a nonselective adenosine A1 and A(2A) receptor antagonist, is the most widely used psychoactive substance in the world. Evidence demonstrates that caffeine and selective adenosine A(2A) antagonists interact with the neuronal systems involved in drug reinforcement, locomotor sensitization, and therapeutic effect in Parkinson's disease (PD). Evidence also indicates that low doses of caffeine and a selective adenosine A(2A) antagonist SCH58261 elicit locomotor stimulation whereas high doses of these drugs exert locomotor inhibition. Since these behavioral and therapeutic effects are mediated by the mesolimbic and nigrostriatal dopaminergic pathways which project to the striatum, we hypothesize that low doses of caffeine and SCH58261 may modulate the functions of dopaminergic neurons in the striatum. In this study, we evaluated the neuroadaptations in the striatum by using reverse-phase high performance liquid chromatography (HPLC) to quantitate the concentrations of striatal dopamine and its metabolites, dihydroxylphenylacetic acid (DOPAC) and homovanilic acid (HVA), and using immunoblotting to measure the level of phosphorylation of tyrosine hydroxylase (TH) at Ser31, following chronic caffeine and SCH58261 sensitization in mice. Moreover, to validate further that the behavior sensitization of caffeine is through antagonism at the adenosine A(2A) receptor, we also evaluate whether chronic pretreatment with a selective adenosine A(2A) antagonist SCH58261 or a selective adenosine A1 antagonist DPCPX can sensitize the locomotor stimulating effects of caffeine. Chronic treatments with low dose caffeine (10 mg/kg) or SCH58261 (2 mg/kg) increased the concentrations of dopamine, DOPAC and HVA, concomitant with increased TH phosphorylation at Ser31 and consequently enhanced TH activity in the striatal tissues in both caffeine- and SCH58261-sensitized mice. In addition, chronic caffeine or SCH58261 administration induced locomotor sensitization, and locomotor cross-sensitization to caffeine was observed following chronic treatment of mice with SCH58261 but not with DPCPX. Our study demonstrated that low dosages of caffeine and a selective adenosine A(2A) antagonist SCH58261 elicited locomotor sensitization and cross-sensitization, which were associated with elevated dopamine concentration and TH phosphorylation at Ser31 in the striatum. Blockade of adenosine A(2A) receptor may play an important role in the striatal neuroadaptations observed in the caffeine-sensitized and SCH58261-sensitized mice.