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Dumb Question about the BBB

C

collosssus

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Mar 14, 2007
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How does the blood brain barrier, which in a textbook of mine, says that any substance that dissolves in water cannot enter the brain. It says that fat soluable substances just glide across. Now can someone explain how substances, such as LSD MDMA, amphetamines, opiods, etc get into the brain. Do they get dissolved in lipids in the blood prior to entering the brain, and do some substances(I'm talking about one that get you "high" per se), have only certain percentages of them entering through the BBB?

I'm just a high school graduate and never studied, although I'd love to, neuroscience, so I am curious.

Thanks.
 
There's lots of pharmaceutical whizzes around here, I'm sure not one.

What I know is: there's nothing special about the "blood-brain barrier", other than it's more robust and thicker than most lipid bilayer membranes. (I'm sure a little web search will turn up a nice graphic of a "lipid bilayer" membrane. It takes no special chemical knowledge to understand.)

To pass a membrane like this, a molecule must a) dissolve in the water in the bodily fluids of the creature whose membrane it is b) dissolve in the hydrocarbon "tails" of the lipid molecules in the middle of the membrane "sandwich", and c) dissolve in the water on the other side of the membrane.

Something like butane is too water insoluble to get past step (a). Something like, say, salicylic acid is too water soluble to get past step (b). Add an acetyl group to salicylic acid, now you have aspirin. That increases the lipid solubility of the molecule so that it can pass.

Morphine, and heroin, (or diacetyl morphine) have a similar relationship.

Now, alla you pharmacology experts around here explain how I got it all wrong.=D
 
to put it smply the vast majority of CNS active drugs exist are amines which can be salts where they are water soluble and as free base where they are lipid soluble, and as a small percentage of the salt exists as the freebase at the kind of pH found in the body, the drug can then cross as the freebase and then form back into the salt inside the bbb.
 
I read that mannitol which is a sugar substitute like sorbitol or xylitol can temporarily open the BBB. that seems odd seeing as how it's used as a sugar sub. Does this really work and if so, how much is needed for substantial BBB inhibition
 
fastandbulbous said:
A good guide to a drugs abilty to cross the blood brain barrier is it's 1-octanol/water partition coefficient (ratio of distribution of a drug in a two phase system of 1-octanol & water)
Click to expand...


That is so friggin' cool!

I read this paper in a book on drug QSAR one time where they actually developed an equation that got you from the octanol/water partition number to a quantitative prediction of the sedative activity of various barbiturate drugs.
 
toxide said:
I read that mannitol which is a sugar substitute like sorbitol or xylitol can temporarily open the BBB. that seems odd seeing as how it's used as a sugar sub. Does this really work and if so, how much is needed for substantial BBB inhibition
Click to expand...

I think that there are active transport mechanisms that allow the transit of carbohydrates across the BBB. Perhaps, it's possible to "fool" them.
 
fasteddie said:
I think that there are active transport mechanisms that allow the transit of carbohydrates across the BBB. Perhaps, it's possible to "fool" them.
Click to expand...

From my Physiology Book: Many nutrients, including glucose, get through the BBB by passive transport through specially shaped channels, which is called facilitated diffusion. I'd imagine that mannitol gets through this way as well. Active transport is used for some things, such as pumping potassium ions and certain amino acids out of the CNS.
 
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