INICET Pharmacology Previous Year Questions (2017–2026)

The correct answer is H2 receptor.

Ligand-gated ion channels (ionotropic receptors) open an ion channel directly when a ligand binds, giving very fast signalling. The classic examples are the nicotinic acetylcholine, GABA-A, glycine, 5-HT3 and ionotropic glutamate (NMDA/AMPA) receptors. The nicotinic, GABA-A and 5-HT3 receptors in this question are all ligand-gated channels (nicotinic and 5-HT3 are cation channels, GABA-A a chloride channel). The histamine H2 receptor, in contrast, is a G-protein-coupled receptor (GPCR) - it couples to Gs and acts through cAMP (stimulating gastric acid secretion), not by gating an ion channel. So H2 is the odd one out.

This is the key distinction: ionotropic receptors are fast ion channels, whereas metabotropic GPCRs like H2 signal more slowly through G-proteins and second messengers.

Incorrect Options:

• Nicotinic receptor: a ligand-gated cation channel, the classic ionotropic receptor at the neuromuscular junction and ganglia.
• GABA A receptor: a ligand-gated chloride channel, the main inhibitory ionotropic receptor.
• 5HT3 receptor: a ligand-gated cation channel, the only ionotropic serotonin receptor.

High-Yield Facts:

• Ligand-gated ion channels: nicotinic, GABA-A, glycine, 5-HT3, ionotropic glutamate.
• H2 (and all other histamine receptors) are GPCRs.
• H2 couples to Gs and raises cAMP, stimulating gastric acid.
• 5-HT3 is the only ionotropic serotonin receptor, and the rest are GPCRs.

Figure 1: A ligand-gated ion channel - ligand binding opens the channel and ions flow (nicotinic, GABA-A, 5-HT3).

Figure 2: A seven-transmembrane GPCR - the histamine H2 receptor is this type, signalling via Gs and cAMP rather than an ion channel.

The correct answer is Beta1 adrenergic receptor.

Gs (stimulatory G-protein) activates adenylyl cyclase, raising cAMP and protein kinase A. Among the autonomic receptors, the beta-adrenergic receptors (beta-1, beta-2, beta-3) couple to Gs, so beta-1 is the Gs-linked receptor here (in the heart this raises rate and contractility). The other options use different G-proteins: M1 and alpha-1 couple to Gq (phospholipase C, IP3/DAG), while M2 and alpha-2 couple to Gi, which inhibits adenylyl cyclase and lowers cAMP.

A quick rule: beta receptors with stimulatory Gs raise cAMP, M2 and alpha-2 (Gi) lower cAMP, and M1/M3/alpha-1 (Gq) work through IP3 and DAG with calcium.

Incorrect Options:

• Muscarinic M1 receptor: couples to Gq (phospholipase C, IP3/DAG), not Gs.
• Muscarinic M2 receptor: couples to Gi, inhibiting adenylyl cyclase and lowering cAMP.
• Alpha1 adrenergic receptor: couples to Gq (phospholipase C pathway), not Gs.

High-Yield Facts:

• Gs raises cAMP via adenylyl cyclase, and beta-1/2/3 adrenergic receptors couple to Gs.
• M1, M3 and alpha-1 couple to Gq (IP3, DAG, calcium).
• M2 and alpha-2 couple to Gi (lower cAMP).
• Memory aid: beta = Gs, M2 and alpha-2 = Gi, M1/M3 and alpha-1 = Gq.

Figure 1: The Gs pathway used by beta-adrenergic receptors - Gs stimulates adenylyl cyclase, raising cAMP and activating PKA.

Figure 2: For contrast, the Gi pathway (M2, alpha-2) inhibits adenylyl cyclase and lowers cAMP.

The correct answer is Decreased activity of mitochondrial aldehyde dehydrogenase.

Organic nitrates are prodrugs that must be bioactivated to nitric oxide (NO), which raises cGMP in vascular smooth muscle and causes vasodilation. The key bioactivating enzyme is mitochondrial aldehyde dehydrogenase (ALDH2). With continuous nitrate use, the drug generates reactive oxygen species that inactivate ALDH2, so less NO is formed and the response falls - this impaired bioactivation from decreased ALDH2 activity is the most important mechanism of nitrate tolerance.

ROS acting on mitochondria is the upstream trigger, but the proximate, best-defined mechanism is the resulting loss of ALDH2 activity. Tolerance is avoided clinically with a daily nitrate-free interval of 10 to 12 hours. Older theories (sulfhydryl depletion, neurohormonal activation, plasma volume expansion) are now considered less important.

Incorrect Options:

• Reactive oxygen species act on mitochondria: ROS is the upstream cause, but it produces tolerance specifically by inactivating ALDH2 (the proximate mechanism).
• Peripheral vascular resistance: a haemodynamic effect, not the molecular cause of nitrate tolerance.
• Down regulation of Beta adrenergic receptor: relevant to beta-agonist tolerance, not to nitrates.

High-Yield Facts:

• Nitrates are bioactivated to NO mainly by mitochondrial ALDH2.
• Tolerance is mainly due to ROS-mediated inactivation of ALDH2 (impaired bioactivation).
• NO raises cGMP via soluble guanylyl cyclase to cause vasodilation.
• Prevent tolerance with a daily nitrate-free interval of 10 to 12 hours.

Figure 1: Nitroglycerin (glyceryl trinitrate) - a prodrug bioactivated to NO by mitochondrial ALDH2.

Figure 2: NO activates guanylyl cyclase to raise cGMP and relax vessels; reactive oxygen species impair this NO pathway, underlying tolerance.