Caffeine and adenosine.

@article{Ribeiro2010CaffeineAA,
  title={Caffeine and adenosine.},
  author={Joaquim Alexandre Ribeiro and Ana M. Sebasti{\~a}o},
  journal={Journal of Alzheimer's disease : JAD},
  year={2010},
  volume={20 Suppl 1},
  pages={
          S3-15
        }
}
Caffeine causes most of its biological effects via antagonizing all types of adenosine receptors (ARs): A1, A2A, A3, and A2B and, as does adenosine, exerts effects on neurons and glial cells of all brain areas. In consequence, caffeine, when acting as an AR antagonist, is doing the opposite of activation of adenosine receptors due to removal of endogenous adenosinergic tonus. Besides AR antagonism, xanthines, including caffeine, have other biological actions: they inhibit phosphodiesterases… 

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