Receptor Theory

                                       Receptors 

A macromolecule or the binding site located on the surface or inside the effector cell that serves to recognize the signal drug and initiate response to it, but itself has no action is defined as a Receptor. 

The largest no. of drugs do not bind directly to the effectors, viz, enzymes, channels, transporters, structural proteins, etc, but acts through specific regulatory macromolecules which controls the above listed factors. 

These regulatory macromolecules or the site on them which binds and interacts with the drug are called 'Receptors' (enzymes and structural proteins).

These act as sensing elements on which endogenous messengers like hormones, neurotransmitters and other mediators act to bring about the desired biological response.

Terms describing drug receptor interaction:

Agonist: An agent that helps in activating a receptor for producing an effect similar to that of the physiological signal molecule.

Antagonist: An agent preventing the action of the agonist on a receptor but does not have any action of its own.

Inverse agonist: An agent that activates a receptor to produce an effect in an opposite direction to that of an agonist.

Partial agonist: An agent which activates a receptor to produce the submaximal effect.

Receptor occupation theory

This theory states that " the intensity of pharmacological action is directly proportional to the number of receptors occupied by the drug."

The pharmacological response of drug molecule is a function of dose and number of receptors available.

Receptor families:

Transmission mechanism:

1. Ligand gated ion channel/ Channel linked receptor:

*Most rapid cellular response to receptor activation.
*Especially suited for physiological processes necessitating an intermediate response, such as stimulation of nerve impulse.

2. G-Protein coupled receptor

The GPCRs are a class of large membrane bound proteins that share a well conserved structure and transduce their signal via the activation of an intracellular guanine nucleotide binding protein (G protein). 
This family of protein has 7 hydrophilic (heptahelical) domains that span the plasma membrane, therefore, it is sometimes referred to as having a serpentine structure.

3. Enzyme linked receptor/ Kinase linked receptor:

4. Intracellular receptor/  Receptor that regulate gene transcription:

For GPCRs types and detailed transduction mechanism, follow: https://receptortheorypharmacology.blogspot.com/2021/01/g-protein-coupled-receptor-types-major.html

Reference: K.D. Tripathi, "Essential of Medical Pharmacology" , Jaypee Brothers Medical Publishers Ltd, 6th edition.

HIMANGI VIG

M. Pharm (Pharmacology)