Introduction: Screening models for ANS activity — parasympathomimetics are essential in pharmacology practicals for B. Pharm students. These models include in vitro organ bath assays (guinea pig ileum, rabbit jejunum, rat detrusor), in vivo tests (salivation, miosis, bradycardia) and biochemical assays (Ellman’s cholinesterase assay). Understanding cholinergic mechanisms, muscarinic and nicotinic receptor subtypes (M1–M5, Nm, Nn), agonists, indirect agonists (acetylcholinesterase inhibitors), dose–response analysis (EC50, Emax, pA2) and antagonist profiling is critical for drug screening and interpretation. Practical skills include tissue preparation, controls, agonist/antagonist protocols, Schild analysis and data interpretation. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which in vitro tissue preparation is classically used to study muscarinic smooth muscle contractions and screening of parasympathomimetics?
- Isolated guinea pig ileum
- Rat vas deferens
- Isolated frog skeletal muscle
- Rabbit thoracic aorta
Correct Answer: Isolated guinea pig ileum
Q2. Which muscarinic receptor subtype primarily mediates bronchoconstriction and glandular secretion in airways?
- Muscarinic M1 receptor
- Muscarinic M2 receptor
- Muscarinic M3 receptor
- Muscarinic M4 receptor
Correct Answer: Muscarinic M3 receptor
Q3. In a dose–response curve, EC50 is defined as:
- The concentration producing 100% of maximal effect
- The concentration producing 50% of maximal effect
- The maximal possible concentration tested
- The minimum effective concentration
Correct Answer: The concentration producing 50% of maximal effect
Q4. A competitive muscarinic antagonist like atropine produces which characteristic change in an agonist dose–response curve?
- Leftward shift with increased Emax
- Rightward parallel shift without change in Emax
- Decrease in Emax with no shift
- Complete blockade that cannot be overcome
Correct Answer: Rightward parallel shift without change in Emax
Q5. Which of the following is a reversible indirect parasympathomimetic (acetylcholinesterase inhibitor) commonly used in screening assays?
- Bethanechol
- Physostigmine
- Atropine
- Pilocarpine
Correct Answer: Physostigmine
Q6. In organ bath experiments, potassium chloride (KCl) is used to:
- Specifically activate muscarinic receptors
- Depolarize smooth muscle causing receptor-independent contraction
- Inhibit calcium influx to relax tissue
- Measure nicotinic receptor function only
Correct Answer: Depolarize smooth muscle causing receptor-independent contraction
Q7. Which G-protein is primarily coupled to muscarinic M3 receptors and mediates smooth muscle contraction via IP3 and DAG?
- Gs (stimulatory)
- Gi (inhibitory)
- Gq (phospholipase C activation)
- G12/13
Correct Answer: Gq (phospholipase C activation)
Q8. Which preparation is widely used to assess nicotinic neuromuscular blockers in screening models?
- Isolated guinea pig ileum
- Isolated phrenic nerve–hemidiaphragm preparation
- Rabbit jejunum strip
- Rat urinary bladder
Correct Answer: Isolated phrenic nerve–hemidiaphragm preparation
Q9. Which biochemical assay is standard for measuring acetylcholinesterase inhibition in drug screening?
- Bradford protein assay
- Ellman’s colorimetric assay using acetylthiocholine and DTNB
- ELISA for acetylcholine
- Western blot for cholinesterase protein
Correct Answer: Ellman’s colorimetric assay using acetylthiocholine and DTNB
Q10. The pA2 value obtained from Schild analysis represents:
- The potency of an agonist
- The negative log of antagonist concentration that requires a twofold shift in agonist concentration to maintain the same response
- The maximum effect achievable by an antagonist
- The time for half-maximal inhibition
Correct Answer: The negative log of antagonist concentration that requires a twofold shift in agonist concentration to maintain the same response
Q11. Activation of cardiac muscarinic M2 receptors produces which primary effect on the heart?
- Increased heart rate (tachycardia)
- Decreased heart rate (bradycardia)
- Increased contractility in ventricles
- Vasoconstriction of coronary arteries
Correct Answer: Decreased heart rate (bradycardia)
Q12. In screening assays, a partial agonist is characterized by:
- Producing a greater maximal response than a full agonist
- Having no intrinsic activity but blocking receptors
- Producing a submaximal Emax compared with a full agonist even at full receptor occupancy
- Always acting as an inverse agonist
Correct Answer: Producing a submaximal Emax compared with a full agonist even at full receptor occupancy
Q13. To distinguish muscarinic from nicotinic responses in an isolated tissue, which antagonist would you use to block muscarinic effects selectively?
- Hexamethonium
- Atropine
- D-tubocurarine
- Strychnine
Correct Answer: Atropine
Q14. Which synthetic muscarinic agonist is resistant to acetylcholinesterase and commonly used to test direct muscarinic responses?
- Acetylcholine
- Bethanechol
- Physostigmine
- Neostigmine
Correct Answer: Bethanechol
Q15. Which muscarinic receptor subtype is most associated with cognitive function in the central nervous system?
- Muscarinic M1 receptor
- Muscarinic M2 receptor
- Muscarinic M4 receptor
- Muscarinic M5 receptor
Correct Answer: Muscarinic M1 receptor
Q16. Hexamethonium is used in autonomic pharmacology screening to:
- Stimulate muscarinic receptors selectively
- Block nicotinic receptors at autonomic ganglia
- Inhibit acetylcholinesterase irreversibly
- Activate nicotinic receptors at the NMJ
Correct Answer: Block nicotinic receptors at autonomic ganglia
Q17. Which isolated tissue is commonly used to study bladder (detrusor) contractions mediated by muscarinic M3 receptors?
- Isolated rat urinary bladder (detrusor)
- Guinea pig ileum
- Rabbit aorta
- Isolated frog skeletal muscle
Correct Answer: Isolated rat urinary bladder (detrusor)
Q18. Organophosphate insecticides produce prolonged cholinergic overstimulation by:
- Reversibly inhibiting acetylcholinesterase
- Irreversibly phosphorylating and inactivating acetylcholinesterase
- Blocking muscarinic receptors directly
- Blocking nicotinic receptors at the NMJ
Correct Answer: Irreversibly phosphorylating and inactivating acetylcholinesterase
Q19. In Schild plot analysis, a slope near unity (≈1) typically indicates:
- Noncompetitive antagonism
- Allosteric modulation
- Simple reversible competitive antagonism
- Irreversible antagonism
Correct Answer: Simple reversible competitive antagonism
Q20. Which receptor subtype often acts as a presynaptic autoreceptor to inhibit acetylcholine release?
- Muscarinic M1 receptor
- Muscarinic M2 receptor
- Muscarinic M3 receptor
- Muscarinic M5 receptor
Correct Answer: Muscarinic M2 receptor
Q21. In an isolated tissue experiment, pretreatment with physostigmine increases responses to endogenous acetylcholine by:
- Blocking muscarinic receptors
- Inhibiting acetylcholinesterase and prolonging ACh action
- Blocking calcium channels directly
- Activating beta-adrenergic receptors
Correct Answer: Inhibiting acetylcholinesterase and prolonging ACh action
Q22. Which parameter is most appropriate to compare the potency of two parasympathomimetic agonists in screening assays?
- Emax
- EC50
- pA2
- Therapeutic index
Correct Answer: EC50
Q23. Which drug is commonly used as a reference antagonist to confirm muscarinic-mediated responses in bioassays?
- Pilocarpine
- Atropine
- Neostigmine
- Bethanechol
Correct Answer: Atropine
Q24. Which compound is a classical nicotinic (NMJ) antagonist used historically in neuromuscular blockade studies?
- Physostigmine
- D-tubocurarine
- Bethanechol
- Pilocarpine
Correct Answer: D-tubocurarine
Q25. An in vivo model to assess ocular muscarinic agonist activity commonly measures:
- Pupil constriction (miosis) in rabbit or guinea pig
- Salivary flow in mice only
- Skin temperature changes
- Tail flick latency
Correct Answer: Pupil constriction (miosis) in rabbit or guinea pig
Q26. After exposure to an irreversible acetylcholinesterase inhibitor in an organ bath, which observation confirms irreversible inhibition?
- Complete recovery of responses after washing the tissue
- Persistent inhibition of ACh breakdown despite extensive washout
- Immediate increase in baseline contraction only
- Sensitivity to atropine is lost
Correct Answer: Persistent inhibition of ACh breakdown despite extensive washout
Q27. To identify muscarinic receptor subtypes in a tissue during screening, which approach is most informative?
- Using subtype-selective muscarinic antagonists such as pirenzepine, methoctramine and 4-DAMP
- Measuring only total protein content of the tissue
- Applying only potassium to stimulate contraction
- Using broad-spectrum antibiotics
Correct Answer: Using subtype-selective muscarinic antagonists such as pirenzepine, methoctramine and 4-DAMP
Q28. Application of an irreversible muscarinic antagonist before agonist in a concentration–response study typically results in:
- A parallel rightward shift without change in Emax
- An increase in Emax
- A decrease in Emax (reduced maximal response)
- No change in the concentration–response curve
Correct Answer: A decrease in Emax (reduced maximal response)
Q29. Which parameter primarily reflects the intrinsic efficacy of an agonist in organ bath experiments?
- EC50
- Emax (maximum response)
- pA2
- Binding affinity only
Correct Answer: Emax (maximum response)
Q30. For reliable organ bath screening of parasympathomimetics, which experimental conditions are critical to maintain tissue viability and reproducible responses?
- Any room temperature and no aeration
- Physiological temperature, proper oxygenation (95% O2/5% CO2 or equivalent) and appropriate ionic medium
- Complete darkness and no solution changes
- High concentrations of ethanol as solvent for all drugs
Correct Answer: Physiological temperature, proper oxygenation (95% O2/5% CO2 or equivalent) and appropriate ionic medium
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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