Neurohumoral transmission in ANS: adrenaline and acetylcholine MCQs With Answer

Introduction: This quiz collection on neurohumoral transmission in the autonomic nervous system focuses on adrenaline (epinephrine) and acetylcholine—two central neurotransmitters that determine sympathetic and parasympathetic responses. Designed for M.Pharm students, the set emphasizes molecular synthesis, vesicular handling, receptor subtypes, intracellular signaling (Gs, Gi, Gq pathways), enzymatic metabolism (MAO, COMT, cholinesterases), transporters, pharmacokinetics, receptor desensitization, and clinically relevant agonists/antagonists. Questions probe mechanism-based pharmacology, receptor distribution, presynaptic modulation, and therapeutic implications such as adrenergic drugs, cholinesterase inhibitors, and adverse effect profiles. Use these MCQs to deepen understanding, prepare for exams, and integrate pharmacological principles with clinical applications.

Q1. Which enzyme catalyzes the conversion of dopamine to norepinephrine in sympathetic nerve terminals?

• Tyrosine hydroxylase
• DOPA decarboxylase
• Phenylethanolamine N-methyltransferase
• Dopamine β-hydroxylase

Correct Answer: Dopamine β-hydroxylase

Q2. Which transporter is primarily responsible for reuptake of norepinephrine into presynaptic sympathetic neurons?

• DAT (Dopamine transporter)
• SERT (Serotonin transporter)
• NET (Norepinephrine transporter)
• VMAT2 (Vesicular monoamine transporter 2)

Correct Answer: NET (Norepinephrine transporter)

Q3. Which of the following best describes the primary intracellular signaling pathway activated by β1-adrenergic receptors?

• Activation of phospholipase C and IP3/DAG formation
• Stimulation of guanylate cyclase and increased cGMP
• Gs-mediated activation of adenylyl cyclase and increased cAMP
• Gi-mediated inhibition of adenylyl cyclase and decreased cAMP

Correct Answer: Gs-mediated activation of adenylyl cyclase and increased cAMP

Q4. Which enzyme chiefly inactivates acetylcholine in the synaptic cleft of autonomic ganglia and neuromuscular junctions?

• Monoamine oxidase (MAO)
• Acetylcholinesterase (AChE)
• Butyrylcholinesterase (BuChE) only
• Catechol-O-methyltransferase (COMT)

Correct Answer: Acetylcholinesterase (AChE)

Q5. Which adrenergic receptor subtype predominantly mediates vasoconstriction in peripheral arterioles?

• β2-adrenergic receptor
• α1-adrenergic receptor
• β1-adrenergic receptor
• α2-adrenergic receptor

Correct Answer: α1-adrenergic receptor

Q6. Which drug is a direct agonist at β2-adrenergic receptors and is commonly used as a bronchodilator?

• Dobutamine
• Isoproterenol
• Albuterol (salbutamol)
• Phenylephrine

Correct Answer: Albuterol (salbutamol)

Q7. Which mechanism explains the action of organophosphate poisoning on autonomic transmission?

• Irreversible inhibition of acetylcholinesterase leading to increased ACh at synapses
• Blockade of nicotinic receptors preventing ACh binding
• Inhibition of tyrosine hydroxylase reducing catecholamine synthesis
• Stimulation of MAO increasing catecholamine breakdown

Correct Answer: Irreversible inhibition of acetylcholinesterase leading to increased ACh at synapses

Q8. Phenylethanolamine N-methyltransferase (PNMT) in the adrenal medulla converts norepinephrine into which hormone?

• Dopamine
• Epinephrine (adrenaline)
• Serotonin
• Acetylcholine

Correct Answer: Epinephrine (adrenaline)

Q9. Which receptor subtype is a ligand-gated ion channel found at autonomic ganglia and at the neuromuscular junction?

• Muscarinic M2 receptor
• Nicotinic receptor
• β3-adrenergic receptor
• α2-adrenergic receptor

Correct Answer: Nicotinic receptor

Q10. Which compound blocks vesicular uptake of monoamines by inhibiting VMAT, depleting norepinephrine stores and producing orthostatic hypotension?

• Reserpine
• Methyldopa
• Tyramine
• Ephedrine

Correct Answer: Reserpine

Q11. Which muscarinic receptor subtype primarily mediates decreased heart rate when activated by acetylcholine?

• M1
• M2
• M3
• M4

Correct Answer: M2

Q12. A patient receives an MAO inhibitor and eats aged cheese, leading to hypertensive crisis. This reaction is primarily due to accumulation of which substance?

• Tyramine causing excessive norepinephrine release
• Excess acetylcholine at parasympathetic synapses
• Accumulation of dopamine in peripheral tissues
• Excess serotonin causing vasoconstriction

Correct Answer: Tyramine causing excessive norepinephrine release

Q13. Which property best differentiates epinephrine from norepinephrine in receptor selectivity at physiological concentrations?

• Epinenphrine has higher affinity for α receptors only
• Epinephrine activates β2 receptors more potently than norepinephrine
• Norepinephrine is a stronger β2 agonist than epinephrine
• Both have identical receptor selectivity

Correct Answer: Epinephrine activates β2 receptors more potently than norepinephrine

Q14. Which physiological mechanism causes rapid termination of acetylcholine action at the neuromuscular junction?

• Endocytosis of acetylcholine receptors
• Diffusion of acetylcholine into blood
• Hydrolysis by acetylcholinesterase localized in the synaptic cleft
• Reuptake of acetylcholine into presynaptic vesicles via a high-affinity transporter

Correct Answer: Hydrolysis by acetylcholinesterase localized in the synaptic cleft

Q15. Clonidine lowers blood pressure primarily by which mechanism?

• α1-adrenergic receptor antagonism in vasculature
• Central α2-adrenergic receptor agonism reducing sympathetic outflow
• β1-adrenergic receptor blockade at the heart
• Peripheral blockade of acetylcholine release

Correct Answer: Central α2-adrenergic receptor agonism reducing sympathetic outflow

Q16. Which drug increases synaptic acetylcholine levels by reversibly inhibiting acetylcholinesterase and is used in myasthenia gravis?

• Neostigmine
• Botulinum toxin
• Atropine
• Tubocurarine

Correct Answer: Neostigmine

Q17. Which statement regarding receptor desensitization of β-adrenergic receptors is correct?

• Desensitization involves phosphorylation by GRKs and β-arrestin-mediated internalization
• Desensitization only occurs at muscarinic receptors, not adrenergic receptors
• It is irreversible and permanently abolishes receptor function
• It increases receptor affinity for Gs protein

Correct Answer: Desensitization involves phosphorylation by GRKs and β-arrestin-mediated internalization

Q18. Which presynaptic receptor type provides negative feedback to inhibit further norepinephrine release?

• Presynaptic α2-adrenergic receptors
• Presynaptic β2-adrenergic receptors
• Presynaptic M1 muscarinic receptors
• Presynaptic nicotinic receptors

Correct Answer: Presynaptic α2-adrenergic receptors

Q19. Which pharmacologic agent acts as a direct muscarinic antagonist and is used to treat bradycardia by blocking vagal effects on the heart?

• Physostigmine
• Atropine
• Pilocarpine
• Edrophonium

Correct Answer: Atropine

Q20. Which metabolic pathway predominantly degrades circulating epinephrine released from the adrenal medulla?

• Acetylcholinesterase hydrolysis
• Monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT)
• Hydrolysis by plasma cholinesterase
• Direct renal excretion without metabolism

Correct Answer: Monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT)

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