ABSTRACT
The effect of estrogen (E) and progesterone (P) on the protein expression of the rate-limiting enzyme in serotonin synthesis, tryptophan hydroxylase (TPH), and the level of serotonin in the hypothalamic terminal field was examined in guinea pigs. In addition, we questioned whether serotonin neurons of guinea pigs contain ovarian steroid receptors (estrogen receptoralpha[ERalpha], estrogen receptor beta[ERbeta], progestin receptors [PRs]) that could directly mediate the actions of E or P. Western blot and densitometric analysis for TPH were used on raphe extracts from untreated-ovariectomized (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 and subjected to high-performance liquid chromatography analysis for serotonin, 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 ERalpha plus serotonin and PR plus serotonin with double immunohistochemistry or for expression of ERbeta mRNA with 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 increased by E+P treatment, but not by E alone. Neither ERalpha nor PR proteins were detected within serotonin neurons of the guinea pig raphe nucleus. However, ERbeta mRNA was expressed in the dorsal raphe. In summary, E alone increased TPH protein expression and the addition of P had no further effect, whereas E+P increased hypothalamic serotonin and E alone had no effect. The localization of ERbeta, but not ERalpha or PR, in the dorsal raphe nucleus suggests that E acting via ERbeta within serotonin neurons increases expression of TPH, but that P acting via other neurons and transsynaptic stimulation may effect changes in TPH enzymatic activity, which in turn, would lead to an increase in serotonin synthesis.
