RECEPTOR: DEFINITION, CLASSIFICATION & RECEPTOR THEORY

10 Marks Assignment | Pharmacology & Physiology

I. DEFINITION OF RECEPTOR

"Pharmacodynamics describes the actions of a drug on the body. Most drugs exert effects, both beneficial and harmful, by interacting with specialized target macromolecules called receptors, which are present on or in the cell. The drug-receptor complex initiates alterations in biochemical and/or molecular activity of a cell by a process called signal transduction."

• Lippincott Illustrated Reviews: Pharmacology, Chapter 2

"Most drugs must bind to a receptor to bring about an effect... the receptor is postulated to exist partially in the inactive, nonfunctional form (R) and partially in the activated form (R*)."

• Katzung's Basic & Clinical Pharmacology, 16th Ed., Chapter 1

• Specificity - a receptor recognizes a specific ligand
• Selectivity - the drug-receptor interaction is selective for particular agonists
• Saturability - the number of binding sites is finite
• Reversibility - drug-receptor binding is usually reversible
• Sensitivity - responses occur at very low drug concentrations

II. CLASSIFICATION OF RECEPTORS

Type I: Ligand-Gated Ion Channels (Ionotropic Receptors)

• Location: Cell membrane (transmembrane proteins)
• Structure: Oligomeric proteins forming a central pore; ligand-binding site is on the extracellular portion
• Mechanism: Agonist binding directly opens/closes an ion channel
• Response time: Milliseconds (fastest signaling)
• Second messenger: None - direct ion flux
• Examples:
  • Nicotinic acetylcholine receptor (nAChR) - allows Na+ influx, K+ efflux
  • GABA-A receptor - allows Cl- influx → hyperpolarization
  • Glycine receptor, NMDA receptor, 5-HT3 receptor
• Anesthesia relevance: Benzodiazepines and volatile anesthetics modulate GABA-A receptors; neuromuscular blocking agents act on nAChR at the neuromuscular junction

Type II: G Protein-Coupled Receptors (Metabotropic / 7-TM Receptors)

• Location: Cell membrane
• Structure: Single polypeptide with 7 transmembrane domains (7-TM); N-terminus extracellular, C-terminus intracellular; coupled to heterotrimeric G protein (Gα, Gβ, Gγ subunits)
• Mechanism:
  1. Agonist binds → receptor conformational change
  2. Gα subunit exchanges GDP for GTP → Gα-GTP dissociates from Gβγ
  3. Gα-GTP activates downstream effectors (enzymes or ion channels)
  4. Second messengers are generated (cAMP, IP3/DAG, Ca2+)
• Response time: Seconds to minutes
• Subtypes:
  • Gs - stimulates adenylyl cyclase → ↑cAMP (e.g., β-adrenoceptors)
  • Gi - inhibits adenylyl cyclase → ↓cAMP (e.g., α2-adrenoceptors, opioid receptors, M2 muscarinic)
  • Gq - activates phospholipase C → ↑IP3/DAG/Ca2+ (e.g., α1-adrenoceptors, M1/M3 muscarinic)
• Examples: α and β adrenoceptors, muscarinic receptors, opioid receptors (μ, κ, δ), dopamine receptors, histamine receptors, serotonin (5-HT1, 5-HT2)
• Anesthesia relevance: Opioid analgesics (morphine, fentanyl) act via μ-opioid Gi-coupled receptors; dexmedetomidine acts via α2-adrenoceptors (Gi-coupled)

Type III: Enzyme-Linked Receptors (Receptor Tyrosine Kinases / Kinase-Associated Receptors)

• Location: Cell membrane
• Structure: Single transmembrane domain; extracellular ligand-binding domain; intracellular catalytic (kinase) domain
• Mechanism:
  1. Ligand binding causes receptor dimerization
  2. Autophosphorylation of tyrosine residues on the intracellular domain
  3. Phosphorylated receptor recruits and activates downstream signaling molecules (e.g., MAPK, PI3K/Akt pathways)
  4. Ultimately leads to protein phosphorylation and altered gene expression
• Response time: Minutes to hours
• Examples:
  • Insulin receptor (receptor tyrosine kinase)
  • Growth hormone receptor, GH receptors, EGF receptor, PDGF receptor
  • Guanylyl cyclase receptors (e.g., ANP/BNP receptor - generates cGMP)
• Anesthesia relevance: Glucocorticoid receptors and modulation of inflammatory pathways relevant in perioperative care

Type IV: Intracellular Receptors (Nuclear / Cytosolic Receptors)

• Location: Cytoplasm or nucleus
• Mechanism:
  1. Hydrophobic ligand crosses the lipid bilayer
  2. Binds to receptor in cytoplasm or nucleus
  3. Ligand-receptor complex acts as a transcription factor
  4. Binds to specific DNA sequences (hormone response elements) → alters gene transcription → altered protein synthesis
• Response time: Hours to days (slowest)
• Examples:
  • Glucocorticoid receptor (cortisol, dexamethasone)
  • Thyroid hormone receptor
  • Estrogen, progesterone, androgen receptors
  • Vitamin D receptor, Retinoic acid receptor
• Anesthesia relevance: Corticosteroids (dexamethasone) used perioperatively for antiemesis/anti-inflammation act via intracellular glucocorticoid receptors

Summary Table: Receptor Classification

III. RECEPTOR THEORY

1. Occupancy Theory (Clark, 1926 - Modified by Ariëns, 1954)

Effect (E) = Emax × [D] / (KD + [D])

• Emax = maximum possible effect
• [D] = drug concentration
• KD = dissociation constant (concentration at which 50% receptors are occupied)

• α = 1 → Full agonist (full response when all receptors occupied)
• 0 < α < 1 → Partial agonist (submaximal response even at receptor saturation)
• α = 0 → Antagonist (no response, only blocks agonist access)

• Does not explain partial agonism fully
• Cannot explain spare receptors
• Does not account for constitutive receptor activity

2. Rate Theory (Paton, 1961)

• High rate of association-dissociation = strong agonist
• Drugs that bind and leave rapidly = agonists
• Drugs that bind and remain = antagonists (occupying but not stimulating)

3. Two-State (Conformational Selection) Theory (Del Castillo & Katz, 1957; modified further)

• R (Rq/Ri) - Inactive state - predominates at baseline
• R (Ra/Rg)* - Active state - produces biologic effect

"In the absence of drugs, the two isoforms are in equilibrium, and the Rq form is favored. Conventional full agonist drugs have a much higher affinity for the Rg conformation, and mass action thus favors the formation of the Rg-D complex with a much larger observed effect."

• Katzung's Basic & Clinical Pharmacology, 16th Ed.

4. Spare Receptor Theory (Stephenson, 1956)

• Presence of spare receptors shifts the dose-response curve to the LEFT (increases apparent potency)
• Spare receptor number varies by tissue - explains why the same drug may be a full agonist in one tissue but a partial agonist in another
• EC50 (concentration for 50% effect) is LESS than KD (concentration for 50% receptor occupancy)
• Example: In cardiac muscle, occupation of only a small fraction of β1-adrenoceptors produces maximal chronotropic effect

5. Induced-Fit Theory

IV. ADDITIONAL CONCEPTS IN RECEPTOR PHARMACOLOGY

Agonists and Antagonists

• Agonist: Drug that binds receptor and produces activation; has both affinity and intrinsic efficacy
• Full agonist: Intrinsic activity = 1 (e.g., morphine at μ-opioid receptor)
• Partial agonist: Intrinsic activity between 0 and 1 (e.g., buprenorphine, pindolol) - can act as agonist alone or antagonist in presence of full agonist
• Inverse agonist: Negative intrinsic activity - reduces constitutive receptor activity below baseline (e.g., some antihistamines as inverse agonists at H1)
• Antagonist: Has affinity but zero intrinsic efficacy; only blocks receptor access

Types of Antagonism

Receptor Regulation

• Desensitization (Tachyphylaxis): Decreased receptor response with repeated/continuous exposure; occurs within minutes. Mechanism: phosphorylation of receptor by kinases (e.g., GRKs), uncoupling from G protein, or receptor internalization
• Down-regulation: Prolonged agonist exposure → decreased total receptor number (internalization and degradation). Example: morphine tolerance via μ-opioid receptor downregulation
• Up-regulation: Prolonged antagonist exposure → increased receptor number and sensitivity. Example: beta-blocker withdrawal causing rebound tachycardia; rebound hypertension after stopping clonidine

V. ANESTHESIA-SPECIFIC RECEPTOR RELEVANCE

QUICK SUMMARY FOR 10-MARK ANSWER

1. Definition: Receptor = specialized macromolecule that binds a drug/ligand and produces a measurable response via signal transduction
2. Classification (4 families):
   • Type I: Ligand-gated ion channels (fastest; ms)
   • Type II: G protein-coupled receptors (7-TM; seconds-minutes)
   • Type III: Enzyme-linked receptors (kinase activity; minutes-hours)
   • Type IV: Intracellular nuclear receptors (slowest; hours-days)
3. Receptor Theory:
   • Occupancy Theory (Clark): Effect ∝ receptor occupancy
   • Rate Theory (Paton): Effect ∝ rate of drug-receptor association
   • Two-State Theory (del Castillo & Katz): R ⇌ R*; full/partial/inverse agonists explained
   • Spare Receptor Theory (Stephenson): Maximum effect without occupying all receptors
4. Key terms: Affinity, intrinsic efficacy, potency (EC50), efficacy (Emax), agonist, partial agonist, inverse agonist, antagonist (competitive vs non-competitive)
5. Receptor regulation: Desensitization, down-regulation, up-regulation

• Lippincott Illustrated Reviews: Pharmacology, Chapter 2 - Drug-Receptor Interactions and Pharmacodynamics
• Katzung's Basic & Clinical Pharmacology, 16th Ed., Chapters 1 & 2
• Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th Ed.
• Miller's Anesthesia, 10th Ed. - Pharmacodynamics
• Morgan & Mikhail's Clinical Anesthesiology, 7th Ed.
• Guyton & Hall Textbook of Medical Physiology - Membrane Receptors and Signaling
• Ganong's Review of Medical Physiology, 26th Ed.