The brain is protected from nutrient excess.
Except for L-DOPA, treatment of patients with large doses of neurotransmitter-precursors has not provided acceptable therapy for neurologic or psychiatric disorders. Indeed, neurophysiological effects generally have not followed changes in brain concentrations of the precursors or their products. A major reason for this ineffectiveness of precursor-loading may involve the very high metabolic activity of cerebrovascular endothelial cells, which can metabolize the precursor or its products before these reach the brain parenchyma. It is also noteworthy that studies purporting to examine the transport of precursors across the "blood-brain barrier" actually may not measure transport as such but rather the disappearance of precursor from the blood. The metabolic effects of the endothelial cell barrier itself would have greatly influence such studies and, heretofore, have been ignored. Thus, transport indices calculated from such experiments may need re-evaluation. Even when nutrients (precursors) are present in the blood in such excess that they do traverse the endothelial "blood-brain barrier" and enter the brain's interstitial space, other mechanisms (e.g., intraneuronal degradation) likely prevent these substances from exerting neurophysiological effects.