Regulation of serotonin synthesis.

The regulation of serotonin synthesis in the central nervous system seems to involve mechanisms which control the intracellular concentration of tryptophan and the hydroxylation step . Numerous situations are reviewed which demonstrate that changes in the concentration of free tryptophan in plasma and/or in the uptake of the amino acid in brain may alter tryptophan concentration and 5-HT synthesis . In other cases, modifications in 5-HT synthesis were still detected after tryptophan loading, clearly indicating that the tryptophan goncentration is not the only critical parameter . In particular, changes in 5-HT synthesis dependent on nerve impulse flow seem to be regulated at the hydroxylation step . In addition, long term regulations of 5-HT synthesis involving changes in tryptophan hydroxylase concentration have been recently demonstrated . How serotoninergic neurons integrate all these regulating factors to modulate 5-HT synthesis is still an open question .