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INI-CET EXAM -2026
Pharmacology
Easy

All the following are ligand-gated ion channels EXCEPT:

Appeared in: INI-CET EXAM -2026

Explanation

  • The H2 receptor is a type of G-protein coupled receptor (GPCR), also known as a metabotropic receptor.
  • GPCRs do not have an intrinsic ion channel. Instead, when a ligand (like histamine) binds, the receptor activates an intracellular G-protein.
  • This activation initiates a slower, more prolonged signaling cascade involving second messengers (e.g., cAMP for H2 receptors) to produce a cellular effect.
  • This mechanism is fundamentally different from the direct and rapid channel opening seen in ligand-gated ion channels.

Why Other Options Were Wrong

  • Option B: This is a ligand-gated ion channel. It is the only serotonin receptor subtype that functions as an ionotropic receptor (a non-selective cation channel).
  • Option C: This is a classic example of a ligand-gated ion channel. It is an anion-selective channel that allows chloride ions (Cl-) to enter the neuron, causing hyperpolarization and inhibition.
  • Option D: This is a well-known ligand-gated cation channel. Acetylcholine binding opens the channel, allowing sodium (Na+) influx and causing cell excitation.

Related Visual

Visual explanation — Related Visual
Clinical Relevance
  • Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of cellular receptors: Ligand-gated ion channels vs. G-protein coupled receptors as background academic context rather than a clinical decision trigger.
  • Understanding receptor types is crucial for pharmacology. H2 receptor antagonists (e.g., ranitidine, famotidine) are used to decrease stomach acid, while 5-HT3 antagonists (e.g., ondansetron) are powerful anti-emetics used in chemotherapy.
  • The speed of response is clinically significant. Fast synaptic transmission needed for muscle control relies on ionotropic receptors like the nicotinic receptor. Slower, modulatory functions like hormone responses often use GPCRs.
  • What if the question asked for a receptor that uses a second messenger system? The answer would be the H2 receptor, as this is the hallmark of GPCRs.
How to Approach the Question
  • First, identify the keyword 'EXCEPT'. This signals that three options will share a common characteristic, and one will be the outlier.
  • The common characteristic is 'ligand-gated ion channels'. Your task is to find the receptor that is NOT in this category.
  • Recall the two major classes of membrane receptors: ionotropic (ligand-gated ion channels) and metabotropic (G-protein coupled receptors).
  • Systematically evaluate each option by classifying it: 5-HT3 (ionotropic), GABA-A (ionotropic), Nicotinic (ionotropic), and H2 (metabotropic).
  • Conclude that the H2 receptor is the only one that is not a ligand-gated ion channel, making it the correct answer.
Concept Tested & Keywords
  • Concept Tested: Classification of cellular receptors: Ligand-gated ion channels vs. G-protein coupled receptors.
  • Stem keywords: ligand-gated ion channels, receptor
  • Lead-in keywords: EXCEPT
  • Negative lead-in flag: The question uses a negative frame ('EXCEPT'), asking to identify the option that does not belong to the specified category.

Question ID

Qdlxy5yg5ACIV50KP1R1HZ

Reference Book

E6 Robert Boland, Marcia L. Verduin - Kaplan and Sadock's Comprehensive Text of Psychiatry-Wolters Kluwer Health (2024) (pp 1-16525 of 16525) pp. 9790-9792, 392-394, 9793-9795

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