Inhibition of phasic but not tonic pituitary secretion by 2-hydroxyoestrone in the rat: evidence of action as an oestrogen antagonist

Abstract

Rats with 4-day oestrous cycles, implanted with intracardiac catheters, were injected with 2-hydroxyoestrone at noon on pro-oestrus and their plasma LH levels monitored at frequent intervals thereafter. A dose of 100 μg 2-hydroxyoestrone completely abolished the preovulatory LH rise in four out of ten animals tested, showing no effect in the six others. When an injection of 10 μg oestradiol 1 h before the 2-hydroxyoestrone administration was given all the rats showed an absence of the preovulatory LH surge, while it remained intact in the controls treated with oestradiol only. The principal metabolite of 2-hydroxyoestrone, 2-methoxyoestrone, exhibited no influence on the pituitary gonadotrophin release. Repeated injections of 100 pg doses of 2-hydroxyoestrone to long-term ovariectomized rats produced no change in plasma LH and prolactin levels. In animals primed with oestradiol benzoate, 2-hydroxyoestrone given 1–2 h after the priming dose blocked the phasic release of the pituitary hormones on the afternoon of the 2 subsequent days. The LH and prolactin surges in the primed animals, however, were not affected when the catechol oestrogen was injected 2 h before their appearance. These results indicate that in the cyclic rat exogenous 2-hydroxyoestrone inhibits the preovulatory LH surge when its administration is coincident with the preovulatory oestradiol rise. In the ovariectomized rat 2-hydroxyoestrone inhibits the oestrogen-dependent priming step but does not affect either the oestrogen-independent expression of the induced surges or the tonic secretion of these pituitary hormones. These results indicate a dissociation of central and peripheral activities in this oestradiol metabolite and suggest that this catechol oestrogen functions as an oestrogen antagonist in neuroendocrine events. Since catechol oestrogens can be formed in the brain these pharmacological responses may reflect physiological mechanisms.