Bioassay of serotonin (5-HT) MCQs With Answer

Introduction

Bioassay of serotonin (5-HT) is a core pharmacology topic for B. Pharm students, covering experimental techniques to measure 5-HT activity, potency and efficacy using isolated tissues and receptor-specific responses. This introduction links concepts such as receptor subtypes (5-HT1–5-HT7), agonists and antagonists, dose–response curves, EC50, pA2, Schild analysis, organ bath assays (guinea pig ileum, rabbit jejunum), platelet aggregation, and factors affecting assay sensitivity. Understanding sample preparation, buffers, temperature, and ethical tissue handling is essential for reliable results. Mastery of these principles is critical for drug development and therapeutic evaluation. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which isolated tissue is classically used to measure contractile responses to serotonin (5-HT) in bioassays?

• Guinea pig ileum
• Rat diaphragm
• Mouse heart
• Dog trachea

Correct Answer: Guinea pig ileum

Q2. In a dose–response curve for a 5-HT agonist, the EC50 represents:

• The maximum effect produced by the drug
• The dose producing 50% of the maximal response
• The slope of the dose–response curve
• The antagonist concentration producing 50% inhibition

Correct Answer: The dose producing 50% of the maximal response

Q3. Which receptor subtype is primarily associated with serotonin-induced vasoconstriction and smooth muscle contraction mediated by IP3/DAG?

• 5-HT1A
• 5-HT2A
• 5-HT3
• 5-HT4

Correct Answer: 5-HT2A

Q4. The pA2 value in antagonist studies indicates:

• The potency of an agonist
• The affinity of a competitive antagonist
• The intrinsic activity of a partial agonist
• The rate of receptor internalization

Correct Answer: The affinity of a competitive antagonist

Q5. Which procedure is essential to maintain viable isolated tissue during a 5-HT bioassay?

• Continuous oxygenation with carbogen and stable temperature
• Cooling to 4°C throughout the assay
• Adding high concentrations of protease
• Using distilled water as the organ bath medium

Correct Answer: Continuous oxygenation with carbogen and stable temperature

Q6. Which antagonist is commonly used to identify 5-HT2 receptor-mediated responses?

• Atropine
• Ketanserin
• Propranolol
• Phentolamine

Correct Answer: Ketanserin

Q7. In Schild analysis for a competitive antagonist of 5-HT, a linear Schild plot with slope ≈1 indicates:

• Non-competitive antagonism
• Irreversible antagonism
• Competitive reversible antagonism
• Allosteric modulation

Correct Answer: Competitive reversible antagonism

Q8. Platelet aggregation assays for 5-HT measure which primary endpoint?

• Change in membrane potential
• Extent of platelet aggregation (light transmission)
• ATP production rate
• Serotonin synthesis rate

Correct Answer: Extent of platelet aggregation (light transmission)

Q9. A partial agonist at a 5-HT receptor will typically show:

• Higher maximal response than full agonist
• Lower intrinsic efficacy than a full agonist
• No receptor binding
• Irreversible receptor activation

Correct Answer: Lower intrinsic efficacy than a full agonist

Q10. Which buffer is commonly used in organ bath experiments for 5-HT bioassays?

• Distilled water
• Phosphate-buffered saline without glucose
• Krebs-Henseleit solution
• Acetate buffer pH 4.0

Correct Answer: Krebs-Henseleit solution

Q11. Sumatriptan is an agonist primarily at which 5-HT receptors relevant to bioassay interpretation?

• 5-HT3
• 5-HT1B/1D
• 5-HT2C
• 5-HT4

Correct Answer: 5-HT1B/1D

Q12. In cumulative dosing protocols, what is the key advantage for constructing dose–response curves?

• Reduces tissue fatigue by long intervals between doses
• Allows building a full curve on a single tissue with increasing concentrations
• Ensures antagonist pre-treatment only
• Prevents receptor desensitization entirely

Correct Answer: Allows building a full curve on a single tissue with increasing concentrations

Q13. Which measurement indicates drug potency in a bioassay comparing two 5-HT agonists?

• Higher Emax
• Lower EC50
• Longer duration of action
• Slower onset time

Correct Answer: Lower EC50

Q14. Which enzyme primarily metabolizes serotonin and can affect bioassay outcomes if inhibited?

• Monoamine oxidase (MAO)
• Acetylcholinesterase (AChE)
• Cyclooxygenase (COX)
• Phosphodiesterase (PDE)

Correct Answer: Monoamine oxidase (MAO)

Q15. Non-competitive antagonism in a 5-HT assay typically causes:

• Rightward parallel shift of curve without reduced Emax
• Reduced Emax regardless of agonist concentration
• No change in the dose–response curve
• Leftward shift of the dose–response curve

Correct Answer: Reduced Emax regardless of agonist concentration

Q16. Which of the following is a practical step to minimize variability in 5-HT bioassays?

• Use tissues from different species in the same experiment
• Standardize tissue loading tension and equilibration time
• Vary bath temperature between runs
• Change organ bath solution composition mid-experiment

Correct Answer: Standardize tissue loading tension and equilibration time

Q17. 5-HT3 receptors differ from other 5-HT receptors because they are:

• G-protein coupled receptors linked to cAMP
• Ligand-gated ion channels mediating fast responses
• Nuclear receptors affecting gene transcription
• Tyrosine kinase receptors

Correct Answer: Ligand-gated ion channels mediating fast responses

Q18. When performing antagonist shift experiments, the term “dose ratio” refers to:

• The ratio of antagonist to agonist concentrations
• The fold shift in agonist EC50 in presence vs absence of antagonist
• The ratio of two different tissues’ responses
• The time ratio between doses

Correct Answer: The fold shift in agonist EC50 in presence vs absence of antagonist

Q19. Which control is critical to confirm serotonin-specific responses in an organ bath?

• Use of a non-specific protease
• Application of a selective 5-HT receptor antagonist
• Adding glucose-free buffer
• Exposing tissue to ultraviolet light

Correct Answer: Application of a selective 5-HT receptor antagonist

Q20. Desensitization of 5-HT receptors during repeated exposure is mainly due to:

• Receptor phosphorylation and internalization
• Increased receptor synthesis
• Decreased ligand affinity by temperature alone
• Immediate genetic mutation

Correct Answer: Receptor phosphorylation and internalization

Q21. Which pharmacological parameter measures intrinsic activity of an agonist?

• Potency (EC50)
• Efficacy (Emax)
• Affinity constant (KD)
• pA2

Correct Answer: Efficacy (Emax)

Q22. Which experimental factor can falsely increase apparent potency of serotonin in a bioassay?

• Maintaining correct pH and ionic composition
• Presence of monoamine oxidase inhibitors in the bath
• Using freshly prepared agonist solutions
• Consistent tissue equilibration

Correct Answer: Presence of monoamine oxidase inhibitors in the bath

Q23. A Schild slope significantly different from 1 suggests:

• Simple competitive antagonism
• Possible non-competitive or complex antagonism
• High agonist efficacy
• Perfect experimental conditions

Correct Answer: Possible non-competitive or complex antagonism

Q24. Which readout is most appropriate for assessing 5-HT-induced smooth muscle relaxation?

• Increase in contractile force
• Decrease in pre-contracted tension
• Rise in intracellular calcium only
• ATP depletion

Correct Answer: Decrease in pre-contracted tension

Q25. Which drug class increases extracellular 5-HT by blocking reuptake and can affect bioassay baselines?

• Monoamine oxidase activators
• Selective serotonin reuptake inhibitors (SSRIs)
• Calcium channel blockers
• Beta blockers

Correct Answer: Selective serotonin reuptake inhibitors (SSRIs)

Q26. The use of cumulative versus non-cumulative dosing is decided based on:

• Tissue recovery time and receptor desensitization properties
• Only the solubility of the drug
• Availability of antagonist compounds
• Whether the receptor is ionotropic

Correct Answer: Tissue recovery time and receptor desensitization properties

Q27. Which observation suggests irreversible antagonism in a 5-HT bioassay?

• Rightward shift reversible by increasing agonist concentration
• Persistent depression of response after antagonist washout
• Increased Emax with antagonist
• No change in any curve parameter

Correct Answer: Persistent depression of response after antagonist washout

Q28. In receptor characterization, radioligand binding provides complementary data to bioassay by measuring:

• Functional contractile response directly
• Receptor number (Bmax) and affinity (KD)
• Serotonin synthesis rate in tissue
• Intracellular calcium oscillations

Correct Answer: Receptor number (Bmax) and affinity (KD)

Q29. Which 5-HT receptor is linked to increase in intracellular cAMP via Gs coupling?

• 5-HT1A
• 5-HT2A
• 5-HT4
• 5-HT3

Correct Answer: 5-HT4

Q30. Ethical considerations in 5-HT bioassays using animal tissues include:

• Avoiding informed consent from animals
• Minimizing animal use, following humane euthanasia and approvals
• Using any available tissue regardless of welfare regulations
• Performing assays without institutional oversight to save time

Correct Answer: Minimizing animal use, following humane euthanasia and approvals

G S Sachin

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.

Mail- Sachin@pharmacyfreak.com