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See Inside June/July 2006

Crossing the Barrier

To treat neurological illnesses, researchers are learning how to smuggle drugs past the shield that guards the brain against infection

Paul Ehrlich had just injected aniline dye--used to color blue jeans--into a rat's bloodstream. For years the immunologist had been working on ways to stain cells so they would be more visible under a microscope, and aniline looked promising. Soon all the animal's muscles, blood vessels and organs were deep indigo. But for some confounding reason the central nervous system--the brain and spinal cord--remained untouched.

Ehrlich's experiment, done at Berlin's Charit hospital in 1885, provided early evidence for the blood-brain barrier--a vital wall that controls which molecules in the bloodstream can enter the brain or nerve pathways. Oxygen, sugars and amino acids are allowed in; most compounds are kept out. As a result, the brain can do its job inside a secure perimeter not available to any other organ. Which is handy, because substances in air, water and food--as well as toxins and even the body's own hormones--can severely impair the brain's functioning. Easy access would quickly lead to mental chaos.

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