A second tryptophan hydroxylase isoform, TPH-2 mRNA, is increased by ovarian steroids in the raphe region of macaques

Recently, a second gene that codes for the rate-limiting enzyme in serotonin synthesis was found in brain, named tryptophan hydroxylase-2 (TPH-2). We sequenced overlapping segments (251 and 510 bp) of 5' monkey TPH-2 and questioned whether TPH-2 is regulated by estrogen (E) and progesterone (P) in serotonin neurons of macaques. Monkey TPH-2 was 97% homologous to human TPH-2 and 65% homologous to monkey TPH-1 in the coding region. Spayed monkeys were administered placebo, E-only, P-only, or E + P for 1 month via Silastic implants (n = 4/treatment) and the midbrain was utilized for TPH-2 in situ hybridization (ISH). Additional monkeys (n = 3/treatment) were used to determine the relative abundance of TPH-2 mRNA with quantitative (q) RT-PCR. In the ISH assay, all of the hormone treatments caused a significant and similar increase in TPH-2 mRNA optical density (fourfold; P < 0.004) and positive pixel area (twofold; P < 0.002) over spayed controls. Treatment with E or E + P for 1 month increased the relative abundance of TPH-2 mRNA over spayed controls in the qRT-PCR assay (ANOVA P < 0.05 and P < 0.007, respectively). In conclusion, ovarian steroids stimulate TPH-2 mRNA expression, which could in turn cause an increase in serotonin synthesis. This would impact many of the neural functions that are governed by serotonin.