membrane or within the cell cytoplasm, that terminate in the final target cell response.
Examples of second messengers are Ca^2 þ, cAMP, cGMP, 1,2-diacylglycerol and
inositol-1,4,5-trisphosphate.
However, in 1989 it was discovered that opioid receptors in NG108 cells possessed
activity in the absence of agonist. This receptor activity in the absence of agonist was
termedconstitutive activity, and synthetic ligands were identified that could bind to
the receptor and decrease its constitutive activity in the absence of physiological
agonist. Such ligands were termedinverse agonists. Subsequentin vivoandin vitro
investigations with a wide range of receptor types, many of which were G-protein-
coupled receptors, identified other examples of receptors with constitutive activity
and showed that this activity may have a physiological role thereby confirming that
constitutive activity is not solely a consequence of a mutation or overexpression of a
receptor gene. Of particular interest was the observation that certain receptor muta-
tions (known asconstitutively active mutants, CAMs) were associated with such
clinical disorders as retinitis pigmentosa, hyperthyroidism and some autoimmune
diseases.
The conformational selection model of receptor action was formulated to
rationalise the concomitant existence of active and inactive conformations. The
model envisages that in the absence of agonist, receptors exist as an equilibrium
mixture of inactive (R) and active (R*) forms and that the relative proportion of
the two forms is determined by the associated equilibrium constant. An intro-
duced ligand will preferentially bind to one conformation, thereby stabilising it,
and causing a displacement of the equilibrium between the two forms. Agonists
will preferentially bind to the R* state displacing the equilibrium to increase the
proportion of the R* form. Partial agonists are deemed to have the ability to bind
to both forms with a preference for the R* form again resulting in an increase in
the R* form but by a smaller amount than that produced by full agonists. Inverse
agonists preferentially bind to the R conformation, displacing the equilibrium and
increasing the proportion of R. Partial inverse agonists can bind to both states
with a preference for the R resulting in decrease in the proportion of R* form.
Unlike the two-state model, the conformational selection model does not require
the binding ligand to cause a conformational change in the receptor in order to
alter the activity of the receptor.
Prior to the discovery of constitutive activity in some receptors, ligands were
classified either as agonists or antagonists. This classification formed the basis of
the understanding of the pharmacological action of many therapeutic agents and the
development of new ones by the pharmaceutical industry. However, retrospective
evaluation of ligands previously classified as antagonists but using receptors pro-
duced by cloning techniques and shown to possess constitutive activity revealed
that many were actually inverse agonists and possessed negative efficacy whilst
others were neutral antagonists in that they neither increased nor decreased
the receptor activity. To date approximately 85% of all antagonists that have been
re-evaluated have been shown to be inverse agonists. These observations can be
rationalised in that:668 Cell membrane receptors and cell signalling