SPECIFIC BINDING OF LH-RH TO THE ANTERIOR PITUITARY GLAND DURING THE OESTROUS CYCLE IN THE RAT

The specific binding of [125I]LH-RH to partially purified homogenates of the anterior pituitary gland varied during various stages of the oestrous cycle in the rat. The quantity of [125I]LH-RH bound was maximum at pro-oestrus (150 fmole/mg protein) and minimum at oestrus (20 fmole/mg protein) and metoestrus (16 fmole/mg protein). Specific binding of LH-RH to the pituitary was undetectable in rats ovariectomized during the afternoon of dioestrus I or morning of oestrus but restored by treatment with oestradiol-17β alone or with progesterone. Peripheral plasma levels of LH-RH measured during the oestrous cycle by radioimmunoassay were 5.0 pg/ml at dioestrus I and increased to 20 pg/ml at pro-oestrus; plasma LH-RH levels at dioestrus II, oestrus, and metoestrus, were not significantly elevated. The mean plasma LH levels at measured by radioreceptor assay, were the highest (16.7 ng/ml) at 4–5 p. m. of pro-oestrus, somewhat lower (4 ng/ml) at 1–2 p. m. of pro-oestrus, and lowest (1-2 ng/ml) at other times. Pre-ovulatory LH surge was not observed in rats ovariectomized at dioestrus or killed at the expected time of pro-oestrus. However, oestradiol treatment alone elevated plasma LH levels in such rats and progesterone administration augmented this effect. These observations suggest that the ovulatory surge of LH at pro-oestrus in the rat involves the hypothalamic release of LH-RH as well as increased LH-RH binding in the pituitary gland, both of which may be stimulated by the pre-ovulatory rise in oestrogen and/or progesterone levels.