NEGATIVE AND POSITIVE EFFECTS OF OESTRADIOL UPON LUTEINIZING HORMONE SECRETION IN THE FEMALE RAT

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Abstract
An increase in the plasma concentration of endogenous oestradiol has been shown to be responsible for luteinizing hormone (LH) release in cyclic rats (Ferin, Raziano, Tempone & Vande Wiele, 1970). Such a positive feedback effect of the steroid can also be demonstrated under various experimental conditions such as ovariectomized oestradiol-pretreated rats (Swelheim, 1965; Caligaris, Astrada & Taleisnik, 1971), or female rats shortly after ovariectomy (Ramirez & Sawyer, 1974). Similar positive feedback effects of an oestradiol injection are obtained in ovariectomized monkeys bearing a silastic oestradiol implant as described by Karsch, Dierschke, Weick, Yamaji, Hotchkiss & Knobil (1973), i.e. in pretreatment conditions where the release of LH is steadily held in check by the presence of small and constant amounts of steroid in the blood. In the present study we have investigated whether in ovariectomized rats with long-term steady levels of oestradiol in blood, as deduced from Karsch et al. (1973),

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