Pharmacology for Dentistry

Pharmacodynamics (Mode of Action of Drugs) 43

nist prevents the action of an agonist on a
receptor, but does not have any effect of its
own (Fig. 1.5.1).

Receptors are mostly protein macro-
molecules. The cholinergic, adrenergic, hista-
minergic and other receptors are ‘physiological
receptors’ on which certain drugs act which
mediate the responses to transmitters, autacoids
etc. Another type of receptor is ‘drug receptors’,
which do not have any known physiological
ligands, for example thiazide receptor, benzo-
diazepine receptor etc.

+

Agonist Receptor Agonist-

receptor

interaction

Response

+

Antagonist Receptor Antagonist-

receptor

interaction

No

response

Fig. 1.5.1: Drug-receptor interaction: The agonist
completely fits (interact) with the receptor site to
produce a pharmacological response and antagonist
only partially fit with the receptor site and is unable to
produce any pharmacological response, and also
prevents the agonist from combining with the
receptors.
The ability of a drug to get bound to a
receptor is termed as the ‘affinity of the
drug’ for the receptor. The ability of the drug
to produce a pharmacological response after
its interaction with the receptor is known as
‘intrinsic activity of the drug’, it also
determines the degree of receptor response.
The agonists produce a maximal receptor
response (high intrinsic activity), partial
agonists have intermediate intrinsic activity
and antagonists have low intrinsic activity
and high affinity for the receptors.

On the basis of affinity and efficacy, the

drugs can be classified as agonists, which

have both affinity as well as high intrinsic

activity and can mimic the effects of the

endogenous substance after combining with

the receptor (e.g. methacholine produces the

effect of acetylcholine at cholinergic

receptors). Antagonists have only the affinity

but no intrinsic activity. These drugs bind to

the receptor, but do not mimic (rather block)

or interfere with the binding of an

endogenous agonist (e.g. atropine block the

effect of acetylcholine on cholinergic-

muscarinic receptors). Partial agonists have

full affinity to the receptor but with lower

intrinsic activity [e.g. pentazocine is a partial

agonist at μ receptor (subtype of opioid

receptor)]. Inverse agonist or negative

antagonists have full affinity towards the

receptor but their intrinsic activity is

absolutely negligible and may be zero or in

minus.

RECEPTOR TYPE

On the basis of molecular structure and

nature of transduction mechanism, receptor

can be classified into following categories:

i. G-protein and second messengers.

ii. Ligand-gated channels.

iii. Cytokine receptors.

iv. Tyrosine kinases.

v. Intracellular receptors.

COMBINED EFFECT OF DRUGS

SYNERGISM

When two drugs are given simulta-

neously, and the action of one drug is in-

creased by the other, they are treated as