Somatodendritic autoreceptors on oxytocin neurones.

Oxytocin (OT) is known to exert a facilitatory effect on its own pulsatile release during the milk ejection reflex. Several experimental approaches strongly suggested that this effect occurred in the supraoptic and paraventricular magnocellular hypothalamic nuclei where the OT neurones are concentrated. In these nuclei autoradiographic techniques failed however to detect OT receptors. In the presently reported data, OT binding sites were visualized in the supraoptic, paraventricular and accessory magnocellular nuclei in lactating rats, suckled or not, after acute intracerebroventricular injection of an OT antagonist. No such labelling occurred either in untreated lactating rats or in OT antagonist treated male rats. The combined application of histoautoradiographic and immunocytochemical techniques revealed that the OT binding sites were concentrated on magnocellular OT cell bodies and dendrites but not on their axons and endings in the pituitary neural lobe. The presently detected somatodendritic autoreceptors on OT neurones probably mediate the facilitatory effect of OT on its own release during the milk ejection reflex.