Ovarian steroid modulation of norepinephrine action on luteinizing hormone release: Analogous effect in male and female rats

The modulatory actions of ovarian steroids on the norepinephrine (NE) induced alterations in luteinizing hormone (LH) secretion were examined in male and female rats. Long-term (4 weeks) castrated male and female rats bearing chronic third-ventricle cannulae were implanted with intra-atrial catheters. Animals were bled sequentially at 5- or 15-min intervals for 4-6 h, and plasma LH secretory patterns were determined by radioimmunoassay. After a 2- to 3-hour control bleeding period, castrated unprimed rats received an intracerebroventricular (ICV) infusion of 0.3 μmol of arterenol bitartrate or vehicle. Blood sampling was continued for an additional 2-3 h after infusion. In female rats NE caused a rapid and potent inhibition of episodic LH secretion which was characterized by decreases in mean plasma LH, mean pulse amplitude, and pulse frequency. Similarly, ICV infusion of NE into male rats resulted in decreased mean plasma LH and pulse frequency. In a second series of experiments, castrated male and female rats were primed with estrogen and progesterone 2 days prior to the bleeding/infusion session. Animals were bled sequentially and infused with 0.3 μmol arterenol bitartrate or vehicle during a morning or afternoon session. ICV infusion of vehicle had no effect on plasma LH in either sex regardless of the time of day. NE infusion into estrogen and progesterone primed female rats resulted in significant elevations of plasma LH during both morning and afternoon periods. In estrogen and progesterone primed male rats, NE infusion resulted in a marked facilitation of LH release, similar to that observed in female rats. In both male and female groups afternoon infusion caused a slightly greater and longer lasting elevation in plasma LH than a similar morning infusion. These data demonstrate that ICV infusion of NE results in analogous inhibition of pulsatile LH secretion in both castrated male and female rats. In addition, this inhibitory effect of ICV infusion can be reversed by prior administration of estrogen and progesterone not only in female rats, but in male rats as well. This suggests that the neural mechanisms by which ovarian steroids modulate the noradrenergic regulation of LH secretion in female rats is also present in the male rat.