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Functional Assessment of Urinary Neuro-biogenic Amines—A COMPREHENSIVE GUIDE 

13

Blood-Brain Barrier

The blood-brain barrier (BBB) is a

semi-permeable membrane that sepa-

rates the central nervous system (CNS)

from peripheral blood circulation. (See

Figure 2.) Normal BBB function is nec-

essary for normal brain function. The

BBB functions to:

Protect the brain from foreign

substances and infectious agents

Buffer fluctuations of neuro-active

compounds, nutrients and elements

in the systemic circulation to

maintain a constant environment for

the brain

Keep out hormones and

neurotransmitters released into

systemic circulation; excess that

might over-stimulate brain receptors

and disrupt central nervous system

signaling

Regulate the migration of circulating

immune cells into the brain

The BBB exists as tight junctions be-

tween specialized capillary endotheli-

al cells that line the blood vessels and

capillaries of the brain. Astroglia (astro-

cyte) cells surround the blood vessels.

Astroglia also act as a partial barrier

while providing nutrient support to the

capillary endothelial cells and the nerve

cells of the brain. The endothelial capil-

lary wall further employs efflux pumps,

which actively transport unwanted mol-

ecules back into blood circulation. In the

CNS astroglia may release neuro-active

molecules (cross-talk), supply neurons

with neurotransmitter precursors, se-

quester or metabolize extracellular neu-

rotransmitter molecules or respond to

neurotransmitter signaling.

Water and lipid (fat)-soluble sub-

stances pass through the BBB easily, and

a variety of transport mechanisms exist

to ensure that the brain receives the nu-

trients it needs. Other transport mech-

anisms ensure that CNS wastes are re-

leased back into the circulation. Large

molecules, polar (charged) molecules

and charged ions cross the BBB with

difficulty unless they are specially trans-

ported. In addition to the barrier and ef-

flux pumps, enzymes found on the cap-

illary endothelial cell walls further filter

the substances passing into the brain.

Mutations or single nucleotide poly-

morphisms may affect the structure or

function of these enzymes or transport

mechanisms.

There are several areas of the brain

where the BBB is more permeable or ab-

sent. These areas, called cirumventricu-

lar organs, allow the brain to monitor the