- agonismis a behaviour characteristic of a particular ligand and can be demonstrated
in the absence of any other ligand for the receptor, - antagonismcan only be demonstrated in the presence of an agonist,
- inverse agonismcan only be demonstrated when the receptor possesses constitutive
activity that can be reduced by the agent. In the absence of constitutive activity
inverse agonists can only demonstrate simple competitive antagonism of a full or
partial agonist.
Further understanding of the nature of constitutive activity and the mode of action
of different agonists on a given receptor has come from studies on the histamine H 3
receptor (H 3 R). This is a G-protein-coupled receptor, specifically coupling to Gi/Go
proteins (see Section 17.4.3). Constitutive activity has been found in both rat and
human brain in which the activity inhibits histamine release from synaptosomes.
Studies using the ligand proxyfan, previously classified as an antagonist, have
assigned to it a spectrum of activities ranging from full agonist through partial agonist
to partial inverse agonist and full inverse agonist. Such behaviour by a ligand has
been classified asprotean agonismand proxyfan as aprotean agonist.The precise
behaviour of proxyfan in a given study correlated with the level of constitutive
activity of the system and the relative efficacy of the constitutively active state R*
and that induced by the ligand, AR*. Thus in the absence of any R* or in the presence
of AR* with a lower efficacy than that of R*, the ligand will display agonist activity.
When both R* and AR* states are present with equal efficacy the ligand will display
neutral antagonism and when the AR* state has a higher efficacy than R* the ligand
will display inverse agonism. Such behaviour can only be explained by amultistate
modelin which multiple R* and AR* states of the receptor can be formed. Further-
more, studies using a range of agonists on the histamine H 3 receptor indicated that
different ligands could promote the creation of distinct active states that can display
differential signalling. A wider understanding of the mechanism by which receptors
are activated by agonists is linked to the discovery, initially made with thea1B
adrenergic receptor, that certain mutations in the sequence of G-protein-coupled
receptors caused a large increase in the constitutive activity of the receptor. This
observation had the implication that there may be domains in the receptor that are
crucial to the conservation of a receptor not displaying constitutive activity, and that
the action of agonists was to release these constraints creating the active receptor. In
mutant receptors possessing constitutive activity these constraints have been released
as a result of the mutation. Studies have shown that the activation of inactive recep-
tors by agonists proceeds by a series of conformational changes. The question as to
whether agonists and inverse agonists switch the receptor in a linear ‘on–off’ scale or
whether they operate by different mechanisms has been studied using thea2Aadren-
ergic receptor and a fluorescence resonance energy transfer (FRET) based approach.
Differences in the kinetics and character of the conformational changes induced by
these two classes of agonists provided clear evidence for distinct types of molecular
switch. Moreover, full agonists and partial agonists also showed distinct differences
indicating that receptors do not operate by a simple ‘on–off’ switch but rather that669 17.2 Quantitative aspects of receptor–ligand binding