Which cellular structure accounts for dopamine’s inability to cross the blood-brain barrier when given intravenously?

Which cellular structure accounts for dopamine’s inability to cross the blood-brain barrier when given intravenously?

• A. Tight junctions
• B. Gap junctions
• C. Desmosomes
• D. Hemidesmosomes

Answer: (A)

The barrier that prevents dopamine from entering the brain is formed by tight junctions between brain capillary endothelial cells. These tight junctions seal the space between adjacent cells, creating a highly restrictive paracellular pathway that blocks diffusion of polar, hydrophilic molecules like dopamine. Since dopamine cannot easily cross lipid membranes, it stays in the bloodstream and does not reach brain tissue. Other junctions don’t create this barrier: gap junctions allow direct cytoplasmic connection between neighboring cells for communication, not blocking entry from blood; desmosomes and hemidesmosomes provide mechanical adhesion to neighboring cells or the basement membrane, but they don’t establish the selective barrier that prevents Dopamine’s passage into the brain.

The barrier that prevents dopamine from entering the brain is formed by tight junctions between brain capillary endothelial cells. These tight junctions seal the space between adjacent cells, creating a highly restrictive paracellular pathway that blocks diffusion of polar, hydrophilic molecules like dopamine. Since dopamine cannot easily cross lipid membranes, it stays in the bloodstream and does not reach brain tissue.

Other junctions don’t create this barrier: gap junctions allow direct cytoplasmic connection between neighboring cells for communication, not blocking entry from blood; desmosomes and hemidesmosomes provide mechanical adhesion to neighboring cells or the basement membrane, but they don’t establish the selective barrier that prevents Dopamine’s passage into the brain.