IonotropicglutamateandATPreceptorsformtwootherionotropicreceptorsuperfamilies;thedetailsforeacharegiveninthelatercorrespondingsections.Transientreceptorpotential(TRP)channels,whichareimportantfortransductionofpainandthermalsensations,formyetanotherfamily(see 1. Neurotransmitter Neurotransmitter Ions binds 1. Neurotransmitter binds 2. Channel opens 5. Ions flow across membrane 4. Ion 3. Ions flow Ions across membrane 2. G protein is 3. G protein subunits or • Fig. 6.12 Neurotransmitter receptors. The basic structure and mechanism of action are shown for ligand-gated ion channels (ionotropic receptors) (A) and G protein–coupled (metabotropic) receptors (B). Detailed structures of cys-loop and glutamate ionotropic receptors are shown in C and D, respectively. Cys-loop receptors include ionotropic receptors for GABA, glycine, serotonin, and acetylcholine. Note the differing membrane topologies of the individual subunits of these two classes of receptors: four transmembrane domains for cys-loop receptors and three plus a pore loop for glutamate receptors. Pore loops form the internal wall of the glutamate channel, whereas transmembrane domain 2 forms the internal wall of cys-loop receptors. (A and B, From Purves D, Augustine GJ, Fitzpatrick D. Neuroscience. 2nd ed. Sunderland, MA: Sinauer Associates; 2001.)
Metabotropic receptors are not ion channels. Instead, they are protein monomers that have an extracellular binding site for a particular transmitter and an intracellular site for binding a G protein. Binding of the receptor leads to activation of a G protein, which is the first step in a signal transduction cascade that alters the function of an ion channel in the postsynaptic membrane. In contrast to ionotropic receptors, metabotropic receptors mediate post-synaptic phenomena that have a slow onset and that may persist from hundreds of milliseconds to minutes. Because of the various biochemical cascades they initiate, they have great potential to cause changes in the neuron beyond just generating a postsynaptic potential.
Acetylcholine receptors were originally classified on a pharmacological basis (being sensitive to nicotine or muscarine) into two major groups. This grouping corresponds to groupings based on structural and molecular biological studies. Nicotinic receptors are members of the ionotropic cys-loop family, and muscarinic receptors are part of the metabotropic family of receptor proteins.