Caffeine, the most widely used psychoactive compound, is an adenosine receptor antagonist. It promotes wakefulness by blocking adenosine A_2A receptors (A_2ARs) in the brain, but the specific neurons on which caffeine acts to produce arousal have not been identified. Using selective gene deletion strategies based on the Cre/loxP technology in mice and focal RNA interference to silence the expression of A_2ARs in rats by local infection with adeno-associated virus carrying short-hairpin RNA, we report that the A_2ARs in the shell region of the nucleus accumbens (NAc) are responsible for the effect of caffeine on wakefulness. Caffeine-induced arousal was not affected in rats when A_2ARs were focally removed from the NAc core or other A_2AR-positive areas of the basal ganglia. Our observations suggest that caffeine promotes arousal by activating pathways that traditionally have been associated with motivational and motor responses in the brain.

As key molecular chaperone proteins, heat shock proteins (HSPs) represent an important cellular protective mechanism against neuronal cell death in various models of neurological disorders. In this study, we investigated the effect as well as the molecular mechanism of geldanamycin (GA), an inhibitor of Hsp90, on 1-methyl-4-pheny-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic neurotoxicity, a mouse model of Parkinson disease. Neurochemical analysis showed that pretreatment with GA (via intracerebral ventricular injection 24 h prior to MPTP treatment) increased residual dopamine content and tyrosine hydroxylase immunoreactivity in the striatum 24 h after MPTP treatment. To dissect out the molecular mechanism underlying this neuroprotection, we showed that the GA-mediated protection against MPTP was associated with a reduction of cytosolic Hsp90 and an increase in Hsp70, with no significant changes in Hsp40 and Hsp25 levels. Furthermore, in parallel with the induction of Hsp70, striatal nuclear HSF1 levels and HSF1 binding to heat shock element sites in the Hsp70 promoter were significantly enhanced by the GA pretreatment. Together these results suggested that the molecular cascade leading to the induction of Hsp70 is critical to the neuroprotection afforded by GA against MPTP-induced neurotoxicity in the brain and that pharmacological inhibition of Hsp90 may represent a potential therapeutic strategy for Parkinson disease.

During brain injury, extracellular adenosine and glutamate levels increase rapidly and dramatically. We hypothesized that local glutamate levels in the brain dictates the adenosine–adenosine A_2Areceptor (A_2AR) effects on neuroinflammation and brain damage outcome. Here, we showed that, in the presence of low concentrations of glutamate, the A_2AR agonist 3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino]ethyl]phenyl]propanoic acid (CGS21680) inhibited lipopolysaccharide (LPS)-induced nitric oxide synthase (NOS) activity of cultured microglial cells, an effect that was dependent on the protein kinase A (PKA) pathway. However, in high concentrations of glutamate, CGS21680 increased LPS-induced NOS activity in a protein kinase C (PKC)-dependent manner. Thus, increasing the local level of glutamate redirects A_2AR signaling from the PKA to the PKC pathway, resulting in a switch in A_2AR effects from antiinflammatory to proinflammatory. In a cortical impact model of traumatic brain injury (TBI) in mice, brain water contents, behavioral deficits, and expression of tumor necrosis factor-α, interleukin-1 mRNAs, and inducible NOS were attenuated by administering CGS21680 at post-TBI time when brain glutamate levels were low, or by administering the A_2AR antagonist ZM241385 [4-(2-{[5-amino-2-(2-furyl)[1,2,4]triazolo[1,5-a][1,3,5]triazin-7-yl]amino}ethyl)phenol] at post-TBI time when brain glutamate levels were elevated. Furthermore, pre-TBI treatment with the glutamate release inhibitor (S)-4C3HPG [(S)-4-carboxy-3-hydroxyphenylglycine] converted the debilitating effect of CGS21680 administered at post-TBI time with high glutamate level to a neuroprotective effect. This further indicates that the switch in the effect of A_2AR activation in intact animals from antiinflammatory to proinflammatory is dependent on glutamate concentration. These findings identify a novel role for glutamate in modulation of neuroinflammation and brain injury via the adenosine–A_2AR system.

The function of striatal adenosine A_2Areceptors (A_2ARs) is well recognized because of their high expression levels and the documented antagonistic interaction between A_2ARs and dopamine D₂receptors in the striatum. However, the role of extrastriatal A_2ARs in modulating psychomotor activity is largely unexplored because of the low level of expression and lack of tools to distinguish A_2ARs in intrinsic striatal versus nonstriatal neurons. Here, we provided direct evidence for the critical role of A_2ARs in extrastriatal neurons in modulating psychomotor behavior using newly developed striatum-specific A_2AR knock-out (st-A_2AR KO) mice in comparison with forebrain-specific A_2AR KO (fb-A_2AR KO) mice. In contrast to fb-A_2AR KO (deleting A_2ARs in the neurons of striatum as well as cerebral cortex and hippocampus), st-A_2AR KO mice exhibited Cre-mediated selective deletion of the A_2AR gene, mRNA, and proteins in the neurons (but not astrocytes and microglial cells) of the striatum only. Strikingly, cocaine- and phencyclidine-induced psychomotor activities were enhanced in st-A_2AR KO but attenuated in fb-A_2AR KO mice. Furthermore, selective inactivation of the A_2ARs in extrastriatal cells by administering the A_2AR antagonist KW6002 into st-A_2AR KO mice attenuated cocaine effects, whereas KW6002 administration into wild-type mice enhanced cocaine effects. These results identify a critical role of A_2ARs in extrastriatal neurons in providing a prominent excitatory effect on psychomotor activity. These results indicate that A_2ARs in striatal and extrastriatal neurons exert an opposing modulation of psychostimulant effects and provide the first direct demonstration of a predominant facilitatory role of extrastriatal A_2ARs.