The blood-brain barrier is a protective functional separation of the circulating blood from the extracellular fluid of the CNS that limits the penetration of substances, including drugs.
It prevents solutes in the circulating blood from non-selectively crossing into the extracellular fluid of the central nervous system where neurons reside.
This separation is accomplished by the presence of tight junctions between the capillary endothelial cells as well as a surrounding layer of astrocyte end-feet.
The restriction is generally based on size and lipid solubility such that small, lipophilic molecules can cross, whereas larger, lipophobic molecules cannot.
The BBB also helps to prevent bacteria and viruses from entering the brain.
To enter the CNS, drugs must either be highly hydrophobic or engage specific transport mechanisms. For example, the second-generation antihistamines cause less drowsiness because they were developed to be significantly more polar than older antihistamines, limiting their crossing of the blood-brain barrier. Many nutrients, such as glucose and the essential amino acids, have specific transporters that allow them to cross the blood-brain barrier. l-DOPA, a precursor of the neurotransmitter dopamine, can enter the brain using an amino acid transporter, whereas dopamine cannot cross theblood-brain barrier. Thus, an orally administered drug, l-DOPA, but not dopamine, can be used to boost CNS dopamine levels in the treatment of Parkinson’s disease.
Some parts of the brain, the so-called circumventricular organs, lack a normal blood-brain barrier. These include regions that sample the blood, such as the area postrema vomiting center, and regions that secrete neurohormones into the circulation.
Specialized brain structures participating in sensory and secretory integration within brain neural circuits—the circumventricular organs and choroid plexus—have highly permeable capillaries.
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