Noradrenaline Involvement in the Negative-Feedback Effects of Ovarian Steroids on Luteinising Hormone Secretion

Noradrenaline has been shown to modulate the ovarian‐steroid feedback on luteinising‐hormone (LH) release. However, despite the high amount of evidence accumulated over many years, the role of noradrenaline in LH regulation is still not clearly understood. The present study aimed to further investigate the involvement of noradrenaline in the negative‐feedback effect of oestradiol and progesterone on basal LH secretion. In experiment 1, ovariectomised (OVX) rats received a single injection of oil, oestradiol, or progesterone at 09.00–10.00 h and were decapitated 30 or 60 min later. Levels of noradrenaline and its metabolite, 3‐methoxy‐4‐hydroxyphenylglycol (MHPG), were determined in microdissections of the preoptic area (POA) and medial basal hypothalamus‐median eminence (MBH‐ME) and correlated with LH secretion. Basal LH levels were decreased 30 and 60 min after oestradiol or progesterone injection, and this hormonal response was significantly correlated with a reduction in POA MHPG levels, which reflect noradrenaline release. In addition, noradrenaline levels in the POA were increased, whereas noradrenaline turnover (MHPG/noradrenaline ratio) was decreased 60 min after the injection of both hormones. No effect was found in the MBH‐ME. In experiment 2, i.c.v. administration of noradrenaline (60 nmol), performed 15 min before oestradiol or progesterone injection in jugular vein‐cannulated OVX rats, completely prevented the ovarian steroid‐induced inhibition of LH secretion. The data obtained provide direct evidence that LH secretion in OVX rats is positively regulated by basal noradrenergic activity in the POA, and its reduction appears to play a role in the negative‐feedback effect of ovarian steroids on LH secretion in vivo.