Hypothalamic indolamines during embryonic development and effects of steroid exposure

Abstract

The serotonin system has been implicated in the modulation of endocrine and behavioral components of reproduction. In this study, we examined endogenous hypothalamic indolamines during sexual differentiation and long-term effects of exogenous steroids during this time. In Experiment 1, Japanese quail were studied during the last half of embryonic development and early post-hatch. Samples were taken at embryonic day 10 (E10), E12, E14, E16, hatch (day 0), and days 3 and 5, post-hatch. Hypothalamic indolamines, including serotonin (5-HT) and its metabolite, 5-hydroxy indole acetic acid (5-HIAA) were measured by HPLC-EC detection. Females had relatively higher hypothalamic 5-HT at E14 than males, with both sexes showing increasing levels thereafter. By day 5, post-hatch, hypothalamic 5-HT content was higher in males than in females. When turnover was estimated by comparing relative concentrations of 5-HT to 5-HIAA, males were significantly higher at E12 and E14 than females. These data suggest that there are stage specific changes in the serotonin system, as well as sexually dimorphic patterns in the ontogeny and activity of this system. In Experiment 2, we investigated the effects of embryonic steroid hormone treatment on the serotonin system and on male sexual behavior. Birds were treated with either estradiol benzoate (EB), testosterone propionate (TP) or sesame oil (vehicle control) at selected embryonic days (E10, E12, E14, E16, 0, D3, and D5). At 4 weeks post-hatch, birds were transferred to short photoperiod (16D:8L) for 3 weeks to prevent photostimulated reproductive development. At 7 weeks of age, males were implanted with a 20 mm silastic capsule filled with testosterone and sexual behavior was tested 1 week later. Brains were collected from both males and females, and preoptic area (POA) indolamines were measured. Steroid treatment at E10 or E12 resulted in the loss of male sexual behavior. Moreover, males treated with EB or TP on E12 also had increased POA 5-HT content as adults, compared to control males. Females treated with EB on either E10 or E 12 also had higher POA 5-HT content than control or TP treated females. These data provide evidence for sexual dimorphism in the hypothalamic 5-HT system at specific stages during embryonic development. Moreover, males were sensitive to exogenous EB and TP on E12, whereas females appeared to be affected by EB only and appeared to be sensitive to steroid effects over a longer period of time in development. Moreover, exogenous steroids at E12 in males also correlated with impaired sexual behavioral. These data suggest that long-term effects of embryonic steroid exposure may be mediated in part through effects on the serotonin neurotransmitter system.