Effects of intraventricular norepinephrine on preoptic-anterior hypothalamic electrical activity in the freely-moving rat: modulation by ovarian steroid hormones.

In freely-moving female rats the effects of intraventricular infusion of norepinephrine (NE) on multiunit activity (MUA) were examined in the Diagonal Band of Broca (DBB), medial preoptic area (MPOA) and anterior hypothalamic area (AHA), regions containing neurons producing luteinizing hormone-releasing hormone (LHRH). NE was infused at a dosage known to depress plasma LH levels in ovariectomized (OVX) rats and elevate plasma LH concentrations in OVX-estrogen-progesterone-primed (EBP) animals. It was found that in adult OVX rats MUA in the brain areas listed above was almost invariably inhibited by NE (20 of 23 cases or 87%; the other three cases showed no change in electrical activity). However, after priming with estrogen and progesterone only about one-third of the OVX-EBP rats gave an inhibitory response, with another one-third showing no change in MUA and the final one-third of the cases actually giving an excitatory MUA response to NE--the DBB neurons being the most positive in the regard. Thus it appears that responsiveness of LHRH and/or adjacent neurons to the modulatory action of NE may itself be modulated by the influence of gonadal steroids.