Oestrogen priming for the positive oestrogen feedback: site of action

The significance of oestrogen priming for efficacy of the positive, ovulation-inducing oestrogen feedback has been known for more than 15 years, but the site and mechanism of oestrogen action in the priming effect have not yet been elucidated. Long-term ovariectomized adult female rats were injected once or twice with 20 micrograms oestradiol benzoate (OB), and the serum LH concentration was estimated. Whereas a single injection of OB induced significant inhibition of LH secretion, high circulating LH levels were recorded in rats injected twice with the hormone at an interval of 48 h. This increase was prevented in ovariectomized females fitted with guide cannulae, if the antioestrogen clomiphene citrate was implanted into the medial preoptic area (MPOA) before the first injection of oestrogen and removed prior to the second. On the other hand, replacement of the first oestrogen administration by the implantation of a very low dose of OB into the MPOA resulted in stimulation of LH secretion. OB implants placed into the hypothalamic ventromedial-arcuate region were ineffective in this regard. Taken together, the findings suggest that the priming effect of oestrogen is mediated in rats, at least in part, by the MPOA.