Positive and negative feedback effects of ovarian steroids on luteinizing hormone release in ovariectomized rats following chronic depletion of hypothalamic norepinephrine

Abstract

The involvement of central noradrenergic mechanisms in the feedback control of LH secretion was studied in ovariectomized rats. Permanent depletion of hypothalamic norepinephrine (NE) was accomplished by transecting the ascending noradrenergic pathway (ANP) in the midbrain. A postcastration elevation in serum LH was observed in rats with complete transections of the ANP, and although the rise was somewhat slower than that seen in sham-cut animals, there was no significant difference between the two groups 5 weeks or more after ovariectomy. Both the positive and negative feedback actions of ovarian steroids remained intact after chronic depletion of NE, and animals with transections of the ANP actually had greater responses than sham-cut controls. In addition, phenoxybenzamine, an alpha-adrenergic receptor blocker which effectively blocked the steroid-induced LH surge in six of eight sham-cut ahimals, was ineffective in NE-depleted rats. These results support the concept that NE acts as a modulator of LH secretion but is not indispensable for feedback control mechanisms.