Differences in the effect of 5-HTTLPR are plausible, given that the serotonergic function shows gender differences including higher synthesis rate67 and 5-HT1A and 5-HT2 receptor densities68, 69 in males, and higher cerebrospinal fluid 5-HIAA concentrations,70, 71, 72 serotonin transporter availability73 and more severe depressive symptoms following tryptophan depletion74, 75 in females. The 5-HTTLPR ss genotype was associated with higher cerebrospinal fluid 5-HIAA levels in females and lower levels in males,71 and, in response to tryptophan infusion, negative effects were more pronounced in s allele carrier females as well as l allele carrier males as opposed to other groups.70
