FoundationalConceptsNeuroscience

Several important things emerge from this description. Ionotropic
receptors do one thing and do it very quickly: neurotransmitter binds,
ion channel opens, specific ions flow across the cell membrane, mem-
brane potential changes, and the cell experiences a very rapid change
in its state of excitability. Metabotropic receptors have a very different
and slower effect: neurotransmitter binds, G-protein attaches and
becomes activated, G-protein interacts with adenylate cyclase, intra-
cellular cAMP concentration changes, protein kinase activity changes,
channels open or close, genes are turned on or off, and so forth (Fig.
6.6). Compared with effects of ionotropic receptors, the potential
effects of metabotropic receptors are far more varied. They will also
take longer, because several molecular steps are involved. And the
effects will generally be more prolonged.
There is also very likely a substantial amplification of effect. For
ionotropic receptors, one neurotransmitter binds to one receptor pro-
tein and opens one channel—end of story. For metabotropic receptors,
one neurotransmitter binds to one receptor protein, true, but after
that several G-proteins may be activated by sequentially attaching
to that receptor. One G-protein interacting with adenylate cyclase
may increase or decrease the synthesis of hundreds or thousands of
molecules of cAMP inside the cell. This may affect numerous protein
kinases and thus numerous ion channels and transcription factors.
Thus, the binding of a single neurotransmitter molecule to a single
metabotropic receptor may result in many channels being opened or
closed and many genes being turned on or off—a big amplification
effect.