8 Blood Brain Barrier Flashcards
Functions of the BBB
- Protects the brain from “foreign substances” in the blood that may injure the brain;
- Protects the brain from hormones and neurotransmitters in the rest of the body;
- Transport nutrients into the brain;
- Move out toxins of the brain;
- Maintains a constant environment for the brain;
- Essential for brain normal functions
Drug Delivery and the BBB
- the BBB is permeable to small and lipophilic (fat-loving) molecules (up to 400 Da);
- but larger molecules are not transported across unless there is an active transport system available;
- an additional problem is the very effective drug efflux systems (P-gly-coprotein – P-gp ), which pump the drug back out of cells
What is BBB
BBB is a membrane that controls the passage of substances from the blood into the CNS
Strategies for Drug Delivery to the Brain
GO THROUGH
The use of endogenous system
Disturbing the BBB e.g. osmotic means
Prodrugs
First, is to use endogenous transport systems, including carrier-mediated transporters such as glucose and amino acid carriers; receptor-mediated transcytosis for insulin or transferrin; and blocking of active efflux transporters such as p-glycoprotein
Second strategy is transient BBB disruption. Injection of hypertonic solution, such as mannitol, causes endothelial cell shrinkage and opening of BBB tight junctions for a period of a few hours, and this permits drug delivery to the brain. However, it has risk of entry of toxins from the blood.
Strategies for Drug Delivery to the Brain
GO BEHIND
Intraventricular /intrathecal route
Olfactory pathway
Interstitial delivery - can yield high CNS drug concentration.
One strategy for bypassing the BBB is the intralumbar injection or intraventricular infusion of drugs directly into the CSF. Because the CSF freely exchanges molecules with extracellular fluid of the brain parenchyma, delivering drugs into the CSF could result in therapeutic CNS drug conc.
An alternative CNS drugs delivery strategy is the intranasal route. Drugs delivered intranasally are transported along olfactory sensory neurons to yield significant concentration in the CSF.
How is BBB formed
Brain microvascular endothelial cells (BMEC) in brain capillaries are fused together, forming the barrier between blood and the brain
Where is BBB based and located
BBB is based on the vasculature in the brain and located in the brain capillaries.
How does blood enter the brain
Blood enters brain through large arteries, small arteries (arterioles) and capillaries.
What does the brain uptake from the blood and what does the blood uptake from the brain
Brain uptakes nutrients and oxygen from the blood whilst the blood takes away waste and carbon dioxide from the brain.
How does blood leave the brain
After the exchange blood leaves brain through small veins (venules) and large veins.
How are tight junctions formed
Endocytes of capillary forms tight junction which regulates the influx of most compounds from blood to brain
What are pericytes
Pericytes are contractile cells that have smooth muscle like properties and wrap around the endothelial cells of capillaries. Pericytes stabilise and monitor the maturation of endothelial cells by means of direct communication between the cell membrane as well as through cell signalling. Deficiency of pericytes in CNS can cause the BBB to break down.
What are astrocytes
Astrocytes are star shaped glial cells. They provide biochemical support for brain microvascular endothelial cells, influence BBB morphogenesis and organisation. Factors released by astrocytes are involved in postnatal maturation of BBB; direct contact between endothelial cella and astrocytes necessary to generate BBB.
Brain endothelial resistance in tight junction and peripheral organs
Brain endothelial tight junction resistance is between 1000-2000 ohm/cm2, while in peripheral organs it is between 5-10 ohm/cm2.
Which junction form BBB
BBB is formed by 2 junctions: tight junction and adherens junction, between capillary endothelial cells. Both attach to actin (cytoskeletal protein)