Ovarian steroid action on tryptophan hydroxylase protein and serotonin compared to localization of ovarian steroid receptors in midbrain of guinea pigs

The effect of estrogen (E) and progesterone (P) on the protein expression o f the rate-limiting enzyme in serotonin synthesis, tryptophan hydroxylase ( TPH), and the level of serotonin in the hypothalamic terminal field was exa mined in guinea pigs. In addition, we questioned whether serotonin neurons of guinea pigs contain ovarian steroid receptors (estrogen receptor-alpha[E R alpha], estrogen receptor beta[ER beta], progestin receptors [PRs]) that could directly mediate the actions of E or P. Western blot and densitometri c analysis for TPH were used on raphe extracts from untreated-ovariectomize d (OVX), OVX-E-treated (28 d), and OVX-E+P-treated (14 d E+14 d E+P) guinea pigs. The medial basal hypothalami from the same animals were extracted an d subjected to high-performance liquid chromatography analysis for serotoni n, dopamine, 5-hydroxyindole acetic acid, and homovanillic acid. The brains from other animals treated in an identical manner were perfusion fixed and examined for the colocalization of ER alpha plus serotonin and PR plus ser otonin with double immunohistochemistry or for expression of ER beta mRNA w ith in situ hybridization. E and E+P treatment significantly increased TPH protein levels compared to the untreated control group (p < 0.05), but TPH levels were similar in the E and E+P-treated groups. By contrast, serotonin (nanogram/milligram of protein) in the hypothalamus was significantly incr eased by E+P treatment, but not by E alone. Neither ER alpha nor PR protein s were detected within serotonin neurons of the guinea pig raphe nucleus. H owever, ER beta mRNA was expressed in the dorsal raphe. In summary, E alone increased TPH protein expression and the addition of P had no further effe ct, whereas E+P increased hypothalamic serotonin and E alone had no effect. The localization of ER beta, but not ER alpha or PR, in the dorsal raphe n ucleus suggests that E acting via ER beta within serotonin neurons increase s expression of TPH, but that P acting via other neurons and transsynaptic stimulation may effect changes in TPH enzymatic activity, which in turn, wo uld lead to an increase in serotonin synthesis.