Steroid regulation of tryptophan hydroxylase protein in the dorsal raphe of macaques

Abstract

Background: Tryptophan hydroxylase (TPH) is the rate-limiting enzyme for the synthesis of serotonin, and serotonin is a pivotal neurotransmitter in the regulation of mood, affective behavior, pituitary hormone secretion, and numerous autonomic functions. We previously demonstrated that estradiol (E) and progesterone (P) increase TPH mRNA levels in the dorsal raphe of macaques. Methods: This study employed western blotting and densitometric quantitation to determine whether the changes observed at the level of gene expression were manifested by changes in TPH protein expression and whether modified estrogens or progestins had actions similar to the native ligands. In addition, the effect of the antiestrogen tamoxifen was examined. Ovariectomized (ovx) rhesus and cynomolgus macaques were untreated or treated with E, P, E+P, equine estrogens (EE), medroxyprogesterone (MPA), EE+MPA, or tamoxifen. The dorsal raphe region was subjected to Western analysis. Results: E treatment for 28 days increased TPH protein mass four to six fold over ovariectomized controls. Addition of P to the E regimen or treatment with P for 28 days after E priming did not alter TPH from E treatment alone. Treatment of ovx macaques with a low dose of P caused a two-fold increase in TPH protein. Treatment of ovariectomized macaques for 30 months with EE alone or MPA alone significantly increased TPH protein; however, unlike P, the addition of MPA to the EE regimen blocked the stimulatory effect of EE. Tamoxifen treatment significantly reduced TPH protein compared to EE and ovariectomized control animals. Conclusion: The stimulatory effect of E and P on TPH protein in the dorsal raphe of macaques correlates with the previously observed effect at the level of mRNA expression. P had no effect on the stimulatory action of E, whereas MPA blocked the stimulatory effect of EE. Tamoxifen acted as a potent antiestrogen on TPH protein expression. If TPH protein mass influences serotonin synthesis, then these steroids will impact many autonomic systems that are regulated by serotonin.