A selective role of brainstem noradrenergic neurons in oxytocin release from the neurohypophysis following noxious stimuli in the rat.

Noxious stimuli facilitate oxytocin release from the neurohypophysis. Oxytocin-secreting hypothalamic magnocellular neurosecretory neurons receive excitatory synaptic inputs from noradrenergic neurons in the medulla oblongata. The medulla oblongata includes the A2 noradrenergic and the A1 noradrenergic cells. Here we investigated whether medullary noradrenergic neurons mediate oxytocin release after noxious stimuli in male rats. 5-Amino-2,4-dihydroxy-alpha-methylphenylethylamine, a neurotoxin selective for noradrenergic fibers, was injected into the lateral cerebral ventricle or the medulla. Seven days after the injection, the hypothalamic content of noradrenaline was decreased. In the rats injected with the neurotoxin, the release of oxytocin but not vasopressin after footshocks was impaired. Surgical ablation by suction of the caudal dorsomedial medulla including the A2 cell region did not significantly affect oxytocin release after footshocks, though the surgery abolished oxytocin release after i.v. injection of cholecystokinin octapeptide. In the rats whose A2 cell region had been ablated, an i.c.v. injected alpha 1 adrenoreceptor antagonist, benoxathian, blocked oxytocin release after footshocks. These results demonstrate that brainstem noradrenergic neurons mediate oxytocin release following noxious stimuli in the rat and suggest that responsible noradrenergic neurons are the A1 cells in the caudal ventrolateral medulla.