Abstract
The aim of this study was to investigate the mechanisms involved in the inhibitory action of progesterone on estrogen-induced facilitatory effects of estradiol benzoate on lordosis behavior in the male rat. Intact adult male rats were given 1) 25 micrograms estradiol benzoate (EB) and 100 micrograms progesterone (P) at an interval of 42 hr. EB injected animals served as controls 2) EB followed by 3 doses of 400 micrograms dexamethasone (DEXA) and P as above. EB + DEXA injected animals served as controls. Testing for lordosis behavior was performed by 50 +/- hr after EB injection. A significant decrease in the number of the males displaying lordosis in response to the mounts of stimulus males resulted from P injection following EB treatment as compared to EB controls. DEXA treatment significantly reduced the number of EB animals showing lordosis responses but completely prevented the inhibitory effects of exogenous P to occur. Blood P values appeared to be significantly lower in EB + DEXA males than in their EB counterparts. The results provide evidence that endogenous P is involved in the display of lordosis behavior by EB-treated intact males. They mainly suggest that the effects of exogenous P on estrogen-induced lordosis behavior in the intact male rat result from sequential inhibitory mechanisms involving exposure of the animals to the successive action of endogenous and exogenous P.
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