A naturally occurring xanthine derivative with central nervous system (CNS) stimulating activity. Due to the structural similarity to adenosine, caffeine binds to and blocks adenosine receptors, thereby preventing the inhibitory effects of adenosine on nerve cells. This leads to stimulation of medullary, vagal, vasomotor, and respiratory centers in the brain; and the release of epinephrine. Physiologic responses can include bradycardia, tachycardia, vasoconstriction, CNS excitablility, increased respiratory rate, increased blood pressure, increased blood flow to muscles, decreased blood flow to skin and inner organs, and release of glucose by the liver. Due to the interaction between adenosine A2A and dopamine D2 receptors, caffeine can also indirectly increase the levels of dopamine in the brain.