Localization of kappa opioid receptors in oxytocin magnocellular neurons in the paraventricular and supraoptic nuclei.

It has been demonstrated previously that kappa opioid receptor agonists, such as dynorphin, inhibit oxytocin secretion in the rat. To determine whether kappa agonists act directly on oxytocin-containing magnocellular neurons to inhibit hormone secretion, we utilized immunofluorescence to examine the cellular localization of kappa opioid receptors in the rat paraventricular and supraoptic nuclei. kappa Opioid receptor immunoreactivity co-localized with oxytocin-containing cell bodies, their axons and axon terminals. Thus, our results suggest that kappa opioid receptor agonists can exert direct inhibitory actions on oxytocin magnocellular neurons.