Receptor Theories, Types of Receptors, Agonists, Antagonists and Spare Receptors – Short Notes

These are concise notes and MCQs on receptor theories, receptor classification, receptor regulation, spare receptors, agonists, partial agonists, inverse agonists, and antagonists for pharmacy students preparing for GPAT, NIPER, Drug Inspector, AIIMS Pharmacist, ESIC, Railway Pharmacist, and State Pharmacist examinations.

Receptors are specialised cellular macromolecules, mainly proteins, that recognise endogenous substances or drugs and convert their binding into a biological response. Understanding receptor theories, receptor structure, classification, regulation, spare receptors, agonists, partial agonists, inverse agonists, and antagonists is important for students preparing for B.Pharm examinations and competitive pharmacy exams such as GPAT, NIPER, Drug Inspector, AIIMS Pharmacist, ESIC, Railway Pharmacist, and State Pharmacist examinations.

Receptor Theories

The occupancy theory proposes that the magnitude of drug response is related to the number of receptors occupied by the drug. Modified receptor theories also consider the efficacy of the drug. The two-state receptor theory proposes that receptors exist in inactive (R) and active (R*) states. Agonists preferentially stabilise the active state, whereas inverse agonists stabilise the inactive state.

Structure and Classification of Receptors

Receptors are broadly classified into four major types: ligand-gated ion channels, G-protein-coupled receptors (GPCRs), enzyme-linked receptors, and intracellular receptors. Ligand-gated ion channels produce very rapid responses. GPCRs possess seven transmembrane domains and activate intracellular signalling pathways through G proteins. Enzyme-linked receptors, such as the insulin receptor, have enzymatic activity or are associated with enzymes. Intracellular or nuclear receptors regulate gene transcription and generally produce slower but longer-lasting effects.

Regulation and Spare Receptors

Continuous stimulation by an agonist may cause desensitisation and downregulation, whereas prolonged receptor blockade may cause upregulation and increased sensitivity. Spare receptors, or receptor reserve, are receptors that do not need to be occupied to produce the maximum biological response. Thus, maximal response may occur even when only a fraction of receptors is occupied.

Agonists and Antagonists

A full agonist has affinity and high efficacy and can produce the maximum response. A partial agonist has affinity but lower efficacy and produces a submaximal response even at full receptor occupancy. In the presence of a full agonist, a partial agonist may behave as a functional antagonist. A neutral antagonist has affinity but essentially no intrinsic efficacy and blocks agonist action. An inverse agonist decreases constitutive receptor activity by preferentially stabilising the inactive receptor state.

Dr. Alok Singh

Click the link to access short notes and MCQs based on previous examination questions.

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