Facilitative role of endogenous oxytocin in noradrenaline release in the rat supraoptic nucleus.

Oxytocin is released not only from the axon terminals in the neurothypophysis but also from the dendrites in the hypothalamus. In the present study, we examined the role of dendritic oxytocin release in regulating presynaptic noradrenaline release within the hypothalamus. In vivo microdialysis experiments showed that local application of oxytocin augmented high-K+-induced noradrenaline release in the hypothalamic supraoptic nucleus. Oxytocin application to the hypothalamic synaptosomal preparation in vitro also potentiated high-K+-induced noradrenaline release. The effect of oxytocin was dose-dependent and was blocked by an oxytocin receptor antagonist. We then examined roles of oxytocin released from the dendrites using in vivo microdialysis. Local application of an oxytocin receptor antagonist impaired noradrenaline release in the supraoptic nucleus in response to high-K+ solution or noxious stimuli. An i.c.v. injection of an oxytocin receptor antagonist also impaired oxytocin release from the pituitary after noxious stimuli. These data suggest that dendritic oxytocin facilitates activation of oxytocin neurons, at least in part by augmentation of noradrenaline release via a presynaptic action.