From Nose to Brain?
A direct pathway may NOT exist

June 8, 2003

Last year, a study suggested that drugs could enter the brain directly from the nose. Such intranasal delivery of drugs would bypass the blood-brain barrier (BBB) and might prevent the side effects that may occur when drugs enter the bloodstream. New research, however, has found no evidence of a direct nose-to-brain connection and suggests that chemicals placed in the nose do NOT bypass the bloodstream.

In the recent experiments, patients who received melatonin (a hormone) or vitamin B12 two ways:

  1. Intravenous: injected into a vein
  2. Intranasal: puffed into the nose

The researchers then measured the levels of melatonin and vitamin B12 in the patients' blood and cerebrospinal fluid (CSF) after the chemicals were given. Melatonin and vitamin B12 were found in both blood and CSF samples. Melatonin appeared in CSF samples at the same rate after intravenous and intranasal administration. Although the level of vitamin B12 appeared in the CSF slightly later after intranasal administration compared with intravenous administration, the rate at which it increased in the blood and CSF was the same. This suggests that a little extra time is needed for vitamin B12 to cross through the BBB.

These data suggest that chemicals placed in the nose enter the brain through the blood supply, not through a direct nose-to-brain pathway. Olfactory receptors are located in an area of the nose that contains many blood vessels. Chemicals may be absorbed into the bloodstream through these blood vessels and then enter the brain across the BBB.


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