Bloodbrain barrier

Hyoscyamine is a tertiary amine. It is the levo-isomer to atropine. Tetertiary amines have the potential to cross the bloodbrain barrier and their oral absorption is also considerably better. Other synthetic tertiary amines used for their antispasmodic properties are dicyclomine and phencyclimine. 

Luer MS, Hamani C, Dujovny M, et al. Saturable transport of gabapentin at the bloodbrain barrier. Neurol Res 1999 21 559-562. 

The importance of dopamine in neural transmission is emphasized by the number of major neurological diseases that are associated with improper dopamine regulation. The earliest indication of this type of defect was the finding that dopamine levels are abnormally low in a particular region of the brain of patients with Parkinsonism, a severe neurological disorder. Attempts to treat such patients with dopamine were futile, because after injection, dopamine does not cross the bloodbrain barrier. However, the dopamine precursor, dopa, does cross the bloodbrain barrier. For many individuals with Parkinsonism, daily doses of dopa have provided dramatic clinical improvement. 

Betz. A.L., Firth, JA. and Goldstein, G.W. (1980) Polarity of the bloodbrain barrier Distribution of enzymes between the luminal and antiluminal membranes of brain capillary endothelial cells. Brain Res. 192 17-28. 

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