Effect of progesterone on serotonin turnover in rats primed with estrogen implants into the ventromedial hypothalamus

The effect of progesterone (P) on serotonin (5-HT) turnover was studied in nine brain regions in ovariectomized rats primed with bilateral intracerebral implants of estradiol benzoate (1:250 cholesterol) directed towards the ventromedial hypothalamus (VMN). Two days after surgery, animals received P (0.5 mg SC), and were pretested for lordosis behavior. After a 44-h recovery period, rats with LQ > or = 60 were randomly divided into groups that received either a second behavior test or IP injections of saline or pargyline (4 h after P or V). P treatment decreased pargyline-induced accumulation of 5-HT in the VMN (pars lateralis), the lateral midbrain central grey (IMCG), and the periventricular region (PVE, anterior hypothalamic region). The 5-HIAA/5-HT ratios were significantly decreased in the VMN and PVE in P-treated animals. These results support the hypothesis that P-influenced decreases in serotonergic activity in the VMN contribute to the facilitation of female receptivity, and also suggest that steroid-mediated actions in the VMN may lead to changes in serotonergic activity in the IMCG and PVE.