Science Questions

Is the blood-brain barrier real?

Sun, 25th Oct 2009

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Question

Chris, Cardiff asked:

Is the blood-brain barrier real?

Answer

Absolutely. People talk about this blood-brain barrier. This notional structure which in some way keeps the brain isolated, cocooned inside you biochemically and physically-away from what's happenign in you blood stream and it;s absolutely true. The history of the blood brain barrier goes back a hundred and something years to a guy called Paul Ehrlich, who was a German scientist, he was interested in dyes initially. He used to inject dyes inot animals and then see which bits of the body got stained. andhe was intrigued to see that when he put dyes into the blood stream, much of the time the dye did not get into the brain. And so he realised there must be some kind of barrier separating what goes round in the blood stream from the delicate issue inside the brain. We not understand more about what this blood-brain barrier is. It's a bit contrived, what what's going on is that you basically have special junctions between ceels that line the surfaces of the brain, that separate the brain tissue from blood vessels and these cells make very tight junctions, that's what they're called, and this effecttively means that there's a barrier which is the membranes of those cells separating what's in the blood stream from what's in the blood tissue. 

And what this means is that certain substances can move very easily into the brain, especially if they're substances that can dissolve well in fat, because of course the membranes of cells are made of fat. So lipid-solube drugs like heroin, cigarettes-nictone, cocaine, they're very oily molecules. they go into the brain beautifully and that's why they tend to be addictive. Because they move preferentially into fatty tissue like the brain. Other substances which don't dissolve in fat very well, don't get into the brain very well. But there are some exceptions. Those exceptions are things that the brain needs. So sometimes if it needs a certain chemical that wouldn't be able to diffuse in very easily, it has special transporters which can scrutinise what is going past in the blood, grab goodies that it wants and move those into the brain. This is what people found when they were giving the drug L-Dopa for Parkinson's Disease. -Dop is an amino acid, dissolves in water, doesn't dissolve in brain tissue very well but it gets into the brain much betetr than it should do and the reaon is theer are these special transporters that get hold of it and shove it into the brain.

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Chris asked the Naked Scientists: Medical shows dramatically talk of the brain blood barrier. Is this real as injected drugs seem to slip right past it. Keep up your excellent standards Chris of Cardiff What do you think? Chris, Tue, 13th Oct 2009

Not all drugs cross it.
Some of the newer "non-drowsy" antihistamies used in threating hayfever, for example, are non drowsy simply because they don't reach the brain.
This can be a blessing (as in the hay fever pills) or a nuisance- some antibiotics don't cross the barrier well so they don't work for treating infections of the brain.

As a general guide, very polar molecules won't cross it (with the exception of molecules that the brain's membrane mistakes for glucose- it has to transport glucose in specially as fuel for the brain. Bored chemist, Tue, 13th Oct 2009

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