The role of norepinephrine in the neuroendocrine regulation of luteinizing hormone release in the rat

An excitatory role for norepinephrine (NE) in the regulation of luteinizing hormone (LH) release was first suggested when it was demonstrated that noradrenergic receptor antagonists were able to block ovulation. More recently it has been proposed that NE has both an excitatory role and an inhibitory role in the neuroendocrine regulation of LH release. The excitatory effects may be mediated by alpha-adrenergic receptors and the inhibitory effects may be mediated via beta-adrenergic receptors. These experiments were performed to better understand the role of NE, the receptor type through which NE exerts its effects, and the role of the two major NE pathways in the brain, on LH secretion in the rat. To further understand the role of NE in pulsatile LH release, NE or one of its agonists was infused into the third ventricle of ovariectomlzed rats pretreated with an adrenergic antagonist. In the second set of experiments ascending noradrenergic pathways were electrically stimulated to determine their effect on pulsatile LH release. These experiments demonstrated that the inhibitory effect of NE on pulsatile LH release is blocked when alpha-1- or alpha-2- receptors are blocked but not when beta-receptors are blocked. Electrical stimulation experiments in unprimed ovariectomlzed rats demonstrated that activation of the dorsal noradrenergic tract (DNT) but not the ventral noradrenergic tract (VNT) inhibited pulsatile LH release. Another series of experiments were performed to determine the role NE in the regulation of LH release in the steroid-primed ovariectomlzed rat. These experiments demonstrated that activation of alpha- or beta-adrenergic receptors inhibited the LH surge when adrenergic agonists are infused during the rising phase of the surge. In a similar manner electrical stimulation of either the DNT or VNT inhibited LH release if stimulation occured during the rising phase of the surge. The inhibitory effects of the DNT appear to be via activation of alpha-adrenergic receptors since inhibition was prevented by an alpha-adrenergic antagonist. Under a variety of steroidal conditions and stimulation parameters, activation of the DNT or VNT did not enhance LH release. The lone exception to this was stimulation of the VNT in anaesthetized, steroid-primed ovariectomized rats pretreated with an alpha-adrenergic antagonist. In this case stimulation of the VNT did enhance LH release over non-stimulated and electrically stimulated, saline-treated controls. These results suggest that LH release is enhanced by stimulation of the VNT only when alpha-adrenergic receptors are blocked. In conclusion, it is evident from these studies that activation of alpha-adrenergic receptors either by intraventricular infusion of NE or alpha-agonists, as well as electrical stimulation of noradrenergic tracts inhibits LH secretion. This suggests that the inhibitory effects of NE may be more of a factor in the regulation of LH release than has been previously proposed. In conclusion, NE, in addition to its well established excitatory role, may also have an important inhibitory role in the regulation of LH release. It appears that both inhibitory and excitatory effects of NE on LH release may be mediated by both alpha- and beta-receptors.Medicine, Faculty ofCellular and Physiological Sciences, Department ofGraduat