N-Acetylserotonin is a precursor of melatonin in rats .
An experiment in 1983 found that GABA enhanced the catabolism of serotonin into N-Acetylserotonin in rats.  It is thus suspected that GABA is involved in the synthesis of melatonin and thus may exert regulatory effects on sleep and reproductive functions.
Gamma-Aminobutyric acid