ADRENAL PROGESTERONE FACILITATES THE NEGATIVE FEEDBACK OF ESTROGEN ONLH-RELEASE IN OVARIECTOMIZED RATS

There is evidence that the adrenals play a role in the regulation of t he synthesis and release of gonadotrophins in various vertebrates. The aim of this study was to determine the part played by adrenal steroid s, with special reference to progesterone, on the concentration of LH in ovariectomized (OVX) and oestrogen-primed rats. OVX rats received a single s.c. injection of vehicle or oestradiol benzoate (OB, 20 mu g/ rat). This day was designated as day 0. Three or four days later (day 3-day 4), the rats were treated with mifepristone (10 mg/kg) or with two doses of progesterone antiserum and blood samples were obtained at 13.00 and 18.00 h. OB treatment of OVX rats reduced serum LH at 13.00 h and 18.00 h on day 3 but only at 13.00 h on day 4. The administrati on of mifepristone at 08.00 h to OVX and oestrogen-treated rats induce d a significant increase in serum LH at 18.00 h on days 3 and 4, witho ut modifying the values at 13.00 h. When mifepristone was given at 13. 00 h a much larger increase in serum LH was obtained at 18.00 h. In OV X and oestrogen-treated rats, adrenalectomy on day 2 (08.00-09.00 h) i nduced an increase in serum LH at 18.00 h similar to that observed in the OVX and oestrogen-primed rats after mifepristone treatment. In ord er to determine the specificity of the effect of mifepristone, a group of OVX and oestrogen-treated rats was injected with progesterone anti serum at 08.00 and 13.00 h on day 3. Serum LH concentrations at 13.00 and 18.00 h on day 3 were similar to values obtained in OVX rats treat ed with oestrogen and mifepristone. Serum progesterone was measured at 08.00 and 13.00 h in OVX and OVX and oestrogen-primed rats. At both t imes, values were similar in OVX rats but oestrogen treatment signific antly increased serum progesterone levels. The important role of adren al progesterone on the regulation of LH secretion in OVX and oestrogen -primed rats is evident from these results. Blocking progesterone acti on at the receptor level, we showed that OB significantly increased LH values at 18.00 h. On the basis of these studies it is tempting to sp eculate on the possibility of an inhibitory or stimulatory effect of o estrogen on serum LH concentration in OVX rats, according to the prese nce or absence of adrenal progesterone action.