Adenosine receptors are G-protein-coupled receptors (GPCRs).
Thus, adenosine’s effect as an inhibitory neurotransmitter is not
a direct effect on opening an ion channel, such as GABA acting at
ionotropic GABA receptors. Rather, adenosine’s action at adenosine
receptors results in activation of G-proteins, which then affect adeny-
late cyclase, thereby impacting intracellular cAMP concentration,
which changes the activity of protein kinases and alters the opening
of K* channels, leading to a hyperpolarization of the membrane and
decreased neuronal excitability. The action of caffeine is to prevent
all this from happening, reducing some of the inhibition that would
normally be present in the brain, thereby making the neurons more
excitable. This is presumably connected with the experience of in-
creased wakefulness and alertness.
Figure 9.1. Tobacco plant, Nicotiana tabacum.