With this example of the GABA receptor the concept of different ligands and their responses at one single receptor with multiple binding pockets can be explained. The GABA-R is an ion channel with a binding site for GABA and additional ‘modulatory’ sites.
In case the endogenous ligand GABA binds, the ion channel opens a little allowing some chloride ions to enter the cell. This causes a minor hyperpolarisation at the post-synaptic cell (which then cannot be excitated).
Arial; font-size: medium;" />The GABA-R has additional binding pockets, either for an extra GABA molecule or another modulatory molecule than GABA. Binding of GABA plus an additional modulatory agonist (e.g. diazepam, phenobarbital) results in a higher response than GABA alone.
Addition of an antagonist at the modulatory binding pocket will bring the membrane potential back to the level similar to that when 1 GABA was bound to the receptor. The inverse agonist depolarises the membrane by completely closing the ion channel.