A receptor is a protein molecule usually found embedded within the plasma membrane surface of a cell that receives chemical signals from outside the cell and when such chemical signals bind to a receptor, they cause some form of cellular/tissue response

2. Receptor
These are macromolecule or binding site located on the surface
or inside the cell, that recognize the signal molecule or drug
and initiate the response.
 Located mostly in the cell membrane
 Receive messages from chemical messengers coming from
other cells
 Transmit a message into the cell leading to a cellular effect
 Different receptors specific for different chemical messengers

3.  Each cell has a range of receptors in the cell membrane making it
responsive to different chemical messengers
 Receptor are protein in nature.
 Receptor have a definite life span after which the recptors are
degraded by the cells
Binding site
 A hydrophobic hollow or cleft on the receptor surface -
equivalent to the active site of an enzyme
 Accepts and binds a chemical messenger
 Contains amino acids which bind the messenger
 No reaction or catalysis takes place

4. Messenger binding
 Binding site is nearly the correct shape for the messenger
 Binding alters the shape of the receptor (induced fit)
 Altered receptor shape leads to further effects - signal
transduction
Receptor
Nerve
Nucleus
Cell Cell
Nerve

5. RECEPTOR THEORIES
Several theories has been used to explain the pharmacology of
receptor which includes:-
 Lock-key theory
 Occupational theory
 Rate theory
 Two state Model Theory
Lock-Key- Here receptor act as a lock and drug/signal as key
Rate-theory- Magnitude of response=agonist-receptor
association and dissociation

6. Occupation theory - given by Clark .
Magnitude of drug response depends on proportion of
receptor occupied by the drug.
Two state Model - Receptor exist in two state-
•Resting
•Activated
Affinity
Ability of a drug to bind with a receptor
Efficacy
Ability of a drug to elicit a response after binding with a
receptor

7. Agonist
 Agonist binds reversibly to the binding site
 Agonist has both affinity and efficacy.
 Similar intermolecular bonds formed as to natural messenger
 Induced fit alters the shape of the receptor in the same way as the
normal messenger
 Receptor is activated
 Agonists are often similar in structure to the natural messenger
Antagonist
 Antagonist binds to the binding site
 Antagonist has only affinity.
 Antagonist prevent the action of an agonist on a receptor

8. Non competitive (reversible) allosteric antagonists
 Antagonist binds reversibly to an allosteric site
 Intermolecular bonds formed between antagonist and
binding site
 Induced fit alters the shape of the receptor
 Binding site is distorted and is not recognised by the
messenger
 Increasing messenger concentration does not reverse
antagonism

9. Classification of Receptor
There are 2 types of receptors.
1. Internal receptor.
2. Cell surface receptor.
1. Internal /Intracellular/Cytoplasmic receptors :
 found in the cytoplasm of the cell
 respond to hydrophobic ligand molecules
 Hydrophobic signaling molecules typically diffuse
across the plasma membrane
 interact with intracellular receptors in the cytoplasm.

11. 2. Cell-surface /transmembrane receptors/cellspecific
proteins
performs signal transduction, converting an extracellular
signal into an intracellular signal.
There are three general categories of cell-surface receptors:
1. Ion channel-linked receptors,
2. G-protein-linked receptors,
3. Enzyme-linked receptors.

12. Ion Channel-Linked Receptors
 Ion channels are also known as ligand gated ion channel.
 Receptors bind with ligand.
Ex:Nicotinic Receptor
 Ion channel act as the target for the drug.
 Open a channel through the membrane that allows
specific ions to pass through.
 Conformational change in the protein's structure that allows
ions such as Na,Ca, Mg, and H2 to pass through

13. Ion channel
receptors
Structure:
•Protein pores in
the plasma
membrane

14. G-Protein Linked Receptors
 Binds with a ligand and activate a membrane protein called
a G-protein.
 The activated G-protein then interacts with either an ion
channel or an enzyme in the membrane.
 Each receptor has its own specific extracellular domain and G-
protein-binding site.
Example : Beta-adrenergic receptor

16. Enzyme-Linked Receptors
 Cell surface receptors with intracellular domains that are
associated with an enzyme.
 Normally have large extracellular and intracellular
domains.
 When a ligand binds to the extracellular domain, a signal is
transferred through the membrane and activates the
enzyme, which eventually leads to a response.
Example : Tyrosine Kinase receptor