The effect of ovarian steroids on hypothalamic 5-hydroxytryptamine neuronal activity.

The effect of ovarian steroids on the concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) in the anterior hypothalamus, as well as on the total decline of 5-HIAA levels after the administration of the monoamine oxidase (MAO) inhibitor pargyline, was studied in ovariectomized (OVX) rats at different times of the day. The concentration of 5-HT and 5-HIAA and the total decline of 5-HIAA levels were higher in the morning than in the afternoon in OVX rats. Three days after the injection of 20 microgram estradiol benzoate (EB), the concentration of 5-HT and its metabolite were reduced in the morning but not in the afternoon, resulting in a disappearance of the daily changes. The total decline of 5-HIAA levels after pargyline was significantly reduced by EB administration only in the morning. The injection of 2 mg progesterone (P) into OVX EB-primed rats restored all parameters to the levels present in OVX rats. These findings suggest that ovarian steroids have a modulatory action on the activity of hypothalamic serotonin-containing neurons.