Study Notes: Receptor Types and Pharmacodynamics Overview

Pharmacodynamics = What the drug does to the body.

Receptors are one of the four main types of drug targets. They are specialized proteins on or inside cells that receive signals (ligands) and trigger a cellular response.

Receptors respond to:

• Endogenous ligands: neurotransmitters, hormones, cytokines

• Exogenous ligands: drugs, toxins

Receptors mediate signal transduction — the process that converts ligand binding into a cellular effect.

Four Main Types of Receptors

1. Ligand-Gated Ion Channels (Ionotropic Receptors)

Structure & Function:

• Transmembrane proteins forming a pore/channel.

• Ligand binding → channel opens/closes → ions flow across the membrane.

• Fastest response (milliseconds).

• Found in synapses where rapid neurotransmission is required.

Mechanism:

1. Ligand binds → conformational change in channel.

2. Channel opens → ion movement → cell depolarization or hyperpolarization.

3. Direct coupling — no intermediate steps.

Examples:

• Nicotinic acetylcholine receptor (nAChR):

  • Found in skeletal muscle and autonomic ganglia.

  • Activated by acetylcholine or nicotine.

  • Pentameric structure (α, β, δ, γ subunits).

  • Causes Na⁺ influx → depolarization.

• GABA_A receptor:

  • Inhibitory; allows Cl⁻ influx → hyperpolarization.

  • Drug example: Diazepam (enhances GABA effect → sedation, anxiolysis).

Drugs Affecting These:

• Nicotine (agonist)

• Benzodiazepines (GABA agonists/modulators)

2. G-Protein Coupled Receptors (GPCRs or Metabotropic Receptors)

Structure & Function:

• Largest receptor family.

• Single polypeptide crosses the membrane 7 times (7-transmembrane receptor).

• Extracellular domain binds ligand; intracellular domain binds G-protein.

Mechanism:

1. Ligand binds → receptor conformational change.

2. GDP → GTP exchange on G-protein (α subunit).

3. G-protein splits into α (active) and βγ subunits.

4. These activate effector proteins (enzymes or ion channels).

5. Results in second messenger production (e.g., cAMP, IP₃, DAG).

6. Response time: seconds.

Example:

• β₁-adrenergic receptor (in heart)

  • Ligand: Adrenaline/Epinephrine

  • Activates adenylate cyclase → ↑ cAMP → ↑ Ca²⁺ influx → ↑ cardiac contractility.

  • Function: “Fight or flight” response.

Classes:

• Class A (small molecules like acetylcholine)

• Class B, Class C (larger ligands like hormones or peptides)

Clinical Example Drugs:

• β-blockers: antagonists (↓ HR)

• Epinephrine: agonist

• Dopamine: GPCR agonist

3. Kinase-Linked Receptors (Enzyme-Linked Receptors)

Structure:

• Single transmembrane protein.

• Three domains:

  • Extracellular ligand-binding domain

  • Transmembrane helix

  • Intracellular kinase domain

Ligands: Hormones (insulin, leptin), growth factors, cytokines.

Mechanism:

1. Ligand binds → receptor dimerization.

2. Autophosphorylation of tyrosine residues.

3. Recruitment of intracellular signaling proteins.

4. Activation of kinase cascades (MAPK, JAK-STAT).

5. Leads to changes in gene transcription, growth, differentiation, etc.

6. Response time: minutes to hours.

Subtypes:

• Receptor Tyrosine Kinases (RTKs): e.g., insulin receptor, EGFR

• Receptor Serine/Threonine Kinases: e.g., TGF-β receptor

• Cytokine Receptors: e.g., JAK/STAT pathway

Example:

• EGFR (Epidermal Growth Factor Receptor):

  • Ligand: EGF

  • Activation → dimerization → phosphorylation → cell growth/proliferation.

  • Overactivity → cancers.

  • Targeted by tyrosine kinase inhibitors (TKIs) like erlotinib.

4. Nuclear Receptors

Location: Inside the cell (cytosol or nucleus).
Ligands: Lipophilic molecules (steroid hormones, thyroid hormones, vitamin D, retinoic acid).

Mechanism:

1. Ligand diffuses into cell → binds receptor.

2. Ligand-receptor complex translocates to nucleus.

3. Binds DNA hormone response elements (HREs) → regulates gene transcription.

4. Alters protein synthesis → slow response (hours to days).

Structure:

• N-terminal domain (AF-1)

• DNA-binding domain (DBD)

• Hinge region

• Ligand-binding domain (AF-2)

• C-terminal region

Classes:

• Class I: Steroid receptors (estrogen, glucocorticoid, mineralocorticoid)

• Class II: Thyroid hormone, retinoic acid receptors

• Class III: Homodimeric orphan receptors

• Class IV: Monomeric orphan receptors

Example:

• Aldosterone receptor (Mineralocorticoid receptor):

  • Found in kidneys.

  • Aldosterone binds → increased Na⁺ reabsorption, K⁺ excretion → ↑ BP.

  • Blocked by Spironolactone (aldosterone antagonist → diuretic effect).

Summary Table

Receptor Type	Location	Ligand Examples	Mechanism	Response Time	Example Drugs
Ligand-Gated Ion Channel	Cell membrane	ACh, GABA	Direct ion flow	Milliseconds	Nicotine, Diazepam
GPCR	Cell membrane	Epinephrine, Dopamine	G-protein → 2° messengers	Seconds	β-blockers, Dopamine
Kinase-Linked	Cell membrane	Insulin, EGF	Phosphorylation cascade	Minutes–Hours	Insulin, Erlotinib
Nuclear	Intracellular	Steroids, Thyroid hormones	Gene transcription	Hours–Days	Estrogen, Spironolactone