In the mediobasal hypothalamus (MBH) of prooestrous rats or acutely ovariectomized oestrogentreated adults a marked but short-lived increase in adrenergic activity occurs at 16.00 h, 2 h before the oestrogen-dependent surge of gonadotrophins at 18.00 h. In this study oestrogen-stimulated (noon on day 1) 22-day-old female rats were used which are known to produce surge levels of prolactin at 18.00 h on day 2 and surges of both prolactin and LH at 18.00 h on day 3; although similar treatment of 18-day-old animals or oil-treated 22-day-old rats failed to produce these effects. Radioenzymatic assays of adrenaline concentrations and of the activity of its synthesizing enzyme (phenylethanolamine-N-methyl transferase; PNMT, EC 126.96.36.199) in the MBH of oestrogen-treated 22-day-old rats showed significant (P< 0·05–0·01) increases in both parameters at 16.00 h (i.e. 2 h before surge levels of gonadotrophins) on days 2 and 3 when compared with other times of day. Such effects were not seen in oil-treated 22-day-old animals or in oestrogen-treated 16-day-old rats. Noradrenaline and dopamine concentrations in the MBH of oestrogen-treated 22-day-old rats remained at baseline levels on days 2 and 3 with the exception of noradrenaline at 17.00 h on day 3 when levels appeared higher (P<0·05) than at either 15.00 or 16.00 h. Subsequent measurements of PNMT activity in oestrogen-treated 22-day-old rats at 4-hourly intervals throughout days 2 and 3 showed the presence of a clear circadian rhythm with peak levels occurring at 16.00 h. In conclusion, a temporal relationship (not necessarily specific) exists between increased adrenergic activity in the MBH of oestrogen-treated 22-day-old rats and a surge of gonadotrophins (LH and/or prolactin) 2 h later. This relationship apparently depends on an oestrogen-stimulated circadian rhythm of PNMT activity.
J. Endocr. (1986) 109, 45–51