N-Acetylserotonin is a precursor of melatonin in rats .
An experiment in 1983 found that GABA enhanced the catabolism of serotonin into N-Acetylserotonin ( the precursor of melatonin 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