Chapter 16
Web Text Box 1
Ionotropic and metabotropic receptors for the same transmitter
In the book we meet a number of transmitter molecules that bind to receptors on target
cells. For example we have described how acetylcholine binds to and opens the
nicotinic acetylcholine receptor, an ion channel permeable to sodium potassium and
sometimes calcium, while ADP binds to and activates a metabotropic receptor that acts
through the trimeric G protein Gq to activate phospholipase Cβ and hence generate a
cytosolic calcium signal.
In fact many transmitters act at more than one receptor and more than one receptor
type. Acetylcholine has another evolutionarily unrelated receptor called the muscarinic
acetylcholine receptor. Like the nicotinic acetylcholine receptor this is named for a toxin
that binds to it, in this case muscarine from the fly agaric mushroom Amanita muscaria.
Many muscarinic acetylcholine receptors activate Gq and hence trigger a calcium signal.
There is no ionotropic receptor for ADP, but there is a whole family of ionotropic
receptors for ATP, called the P2X receptors (P stands for purine). These have a wide
range of functions and even mediate synaptic transmission in some locations in the
body (for a review see Burnstock. 2006. Trends in Pharmacological Sciences, 27:166).
The table below shows some other cases where multiple receptor types exist for the
same transmitter. Black text indicates receptors mentioned in the book while red text
indicates another receptor for the same transmitter.
It is because there are often more than one receptor for a transmitter that the names of
receptors are often cumbersome. For example we need to specify “nicotinic
acetylcholine receptor” to distinguish this protein from the muscarinic acetylcholine
receptor.
