Blood brain barrier is crossed by?

Appeared in: RUHS, Jaipur, M.Sc Nursing Entrance Exam-2021

Explanation

The blood-brain barrier (BBB) is a lipid-rich barrier that selectively allows substances to enter the central nervous system (CNS).
The primary characteristic that allows a drug to cross the BBB is high lipid solubility (lipophilicity).
Propranolol is a classic example of a highly lipophilic beta-blocker.
Due to its lipid-soluble nature, propranolol readily passes through the BBB, leading to effects within the CNS.
This CNS penetration is also the reason for side effects like sedation, fatigue, and nightmares associated with the drug.

Option A: Dopamine is a polar catecholamine and is water-soluble, not lipid-soluble. Therefore, it cannot cross the blood-brain barrier.
Option C: Glycopyrrolate is a quaternary ammonium compound, which means it is permanently ionized (carries a positive charge). Ionized molecules are repelled by the lipid membrane of the BBB and cannot cross.
Option D: Streptomycin is an aminoglycoside antibiotic. This class of drugs consists of large, polar, water-soluble molecules that have very poor penetration into the CNS.

Clinical Relevance

Nursing practice connection: Knowing Pharmacokinetics and Blood-Brain Barrier Permeability helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Understanding a drug's ability to cross the BBB is fundamental in nursing for predicting both therapeutic outcomes and potential adverse effects.
For drugs intended to act on the brain (e.g., anesthetics, antidepressants, anticonvulsants), high lipid solubility is a desired property. Conversely, for drugs intended for peripheral action, low lipid solubility is preferred to minimize unwanted CNS side effects.
Nurses must monitor for CNS side effects (e.g., dizziness, confusion, sedation) in patients taking lipophilic drugs like propranolol.

How to Approach the Question

First, identify the core concept of the question: the permeability of the blood-brain barrier (BBB).
Recall the fundamental principle governing passage across the BBB: it is a lipid barrier.
This principle means that lipid-soluble (lipophilic, non-polar, non-ionized) substances can cross, while water-soluble (hydrophilic, polar, ionized) substances cannot.
Systematically evaluate each option based on its known chemical properties:
Dopamine: A polar catecholamine.
Propranolol: A well-known lipophilic beta-blocker.

Concept Tested & Keywords

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E6 Pharmacology Katzung 16e p. 772-774

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