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 studie d in nine brain regions in ovariectomized rats primed with bilateral i ntracerebral implants of estradiol benzoate (1:250 cholesterol) direct ed towards the ventromedial hypothalamus (VMN). Two days after surgery , animals received P (0.5 mg SC), and were pretested for lordosis beha vior. After a 44-h recovery period, rats with LQ greater-than-or-equal -to 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 t he 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-influenc ed decreases in serotonergic activity in the VMN contribute to the fac ilitation of female receptivity, and also suggest that steroid-mediate d actions in the VMN may lead to changes in serotonergic activity in t he IMCG and PVE.