Opioid-noradrenergic interactions in the neurohypophysis. II. Does noradrenaline mediate the actions of endogenous opioids on oxytocin and vasopressin release?

The function of noradrenaline in the rat neurohypophysis was investigated by examining the effects of selective adrenergic receptor agents on electrically evoked release of oxytocin, arginine vasopressin, and noradrenaline using the [3H]-noradrenaline technique. Since endogenous opioids in the neurohypophysis suppress release of both neurohormones and of noradrenaline, we assessed the role of noradrenaline in mediating opioid actions on neurohormone secretion by examining modification of the action of the opioid antagonist naloxone by adrenergic receptor agents. The data suggest (1) that in addition to opioid receptors, 'presynaptic' alpha 2-receptors regulate release from neurohypophysial noradrenaline terminals; (2) noradrenaline released from neurohypophysial terminals acts on beta- and alpha 1-receptors to facilitate both oxytocin and arginine vasopressin release: this action only becoming evident at elevated levels of endogenous noradrenaline release attained following removal of presynaptic opioid or alpha 2-regulation, and (3) opioid peptides within the neurohypophysis act to inhibit oxytocin and, to a lesser extent, arginine vasopressin secretion, partly through inhibiting release of facilitatory noradrenaline. We propose a model in which opioids act in the neurohypophysis both independently of noradrenaline via kappa-receptors on neurosecretory terminals or pituicytes and also via interaction with the noradrenaline system.