Noradrenergic afferents facilitate the activity of tuberoinfundibular neurons of the hypothalamic paraventricular nucleus

The role of ascending noradrenergic projections of medullary origin in regulating the activity of tuberoinfundibular neurons of the hypothalamic paraventricular nucleus (PVN) was examined in pentobarbital-anesthetized male Sprague-Dawley rats. Discrete electrical stimulation of either the A1 or the A2 noradrenaline cell group areas of the caudal medulla enhanced the probability of firing in a substantial proportion of antidromically identified tuberoinfundibular PVN cells tested. Notably, no inhibitory effects were observed. Destruction of the PVN noradrenergic terminal plexus by local application of the neurotoxin 6-hydroxydopamine 1 day prior to electrophysiological experiments abolished the effects of both A1 and A2 stimulation. These findings indicate that noradrenergic afferents can exert a facilitatory influence on the activity of a population of tuberoinfundibular PVN neurons, thus supporting earlier suggestions that central noradrenergic structures can enhance the release of certain anterior pituitary hormones.