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The presence of adrenaline within the central nervous system appears to be essential for the occurrence of an oestrogen-stimulated surge of LH in both pro-oestrous and ovariectomized rats. Previous measurements of adrenergic activity based on the rate of decline of adrenaline (RDA) 2 h after injection of a synthesis inhibitor (SKF 64139) suggested that adrenergic activity increases in both the medial preoptic area (MPOA) and the mediobasal hypothalamus (MBH) at the start of the LH surge (15.00–17.00 h). The purpose of the present studies was to see whether oestrogen and progesterone affected this increase in adrenergic activity at the sites of the cell bodies (MPOA) and axon terminals (MBH) of the gonadotrophin-releasing hormone-synthesizing neurone. Rats ovariectomized at dioestrus showed a reduction in both plasma LH concentrations and RDA (P < 0·01) in the MBH between 15.00 and 17.00 h on the day of expected pro-oestrus. Oestrogen replacement at operation restored LH levels and the RDA to values found in intact rats. Injection of an antioestrogen (Cl-628), with or without oestrogen, also reduced plasma LH levels and the RDA (P < 0·01) in the MBH. Removal or replacement of oestrogen apparently had little effect on the RDA in the MPOA. In ovariectomized oestrogen-primed rats an injection of progesterone on the morning of expected pro-oestrus advanced and enhanced not only the LH surge but also the RDA in the MBH (P< 0·001). Moreover, concentrations of adrenaline in the MBH (but not the MPOA) decreased (P < 0·001) consistently over a 2-h period following injection of progesterone. These results support the suggestion that steroid-stimulated adrenergic activity in the MBH may be concerned with the preovulatory release of gonadotrophins.

J. Endocr. (1985) 104, 129–135