FoundationalConceptsNeuroscience

commercially manufactured sweetened and flavored beverages gener-
ally contain more caffeine than traditional soft drinks.
Caffeine is a powerful stimulant drug, acting on the central nervous
system (CNS) to increase wakefulness and alertness. In addition,
caffeine has stimulant effects on the cardiovascular system, increas-
ing heart rate and blood pressure. Caffeine consumption is pervasive
in the modern world and meshes well with the modern lifestyle of
extreme production and consumption. Whether or not you yourself
are a regular consumer of caffeine, we all live in a highly caffeinated
world. So pervasive is caffeine consumption that we have, to large ex-
tent, lost touch with what a powerful stimulant it is.
Caffeine’s molecular structure is quite similar to the structures of
two of the nucleotides of DNA, adenine and guanine. Indeed, plants
synthesize caffeine (or theophylline or theobromine) from adenine
and guanine precursor molecules. Adenosine is also made from ade-
nine by the attachment of a ribose sugar. Thus, there is a molecular
similarity between caffeine and adenosine. It turns out that one of the
several roles adenosine plays in the body is as a neurotransmitter. And
caffeine acts at adenosine receptors as an antagonist.
Adenosine receptors in the heart mediate a slowing of heart rate,
adenosine receptors on blood vessels mediate vasodilation (opening
up of the vessels), and adenosine receptors in the brain act to decrease
neuronal excitability. Antagonizing these receptors gives rise to
caffeine’s stimulant effects: increasing heart rate, vasoconstriction
contributing to increased blood pressure, and increasing excitability
in the central nervous system. This illustrates how the blocking of
neural effects that are normally in one direction produces an outcome
in the opposite direction. Blocking the normal neuronal inhibitory
effect of adenosine produces neuronal excitation, because the usual
balance between excitation and inhibition is altered.