Oxytocin receptors on oxytocin neurones: histoautoradiographic detection in the lactating rat.

1. The purpose of the present study was the detection at the cellular scale of the oxytocin (OT) receptors involved in the facilitatory effect of this neuropeptide on its own release during the milk ejection reflex. 2. OT binding sites were demonstrated in brain sections by using a highly selective 125I-labelled OT antagonist detected by film- and histoautoradiography. 3. Film autoradiographs revealed the presence of OT binding sites in the hypothalamic magnocellular (supraoptic, paraventricular and anterior commissural) nuclei in lactating rats, suckled or not. This detection was only possible after acute i.c.v. injection of OT antagonist which probably induced an upregulation of the OT binding sites to autoradiographically detectable levels. 4. Combined application of histoautoradiographic and immunohistochemical techniques showed that the OT binding sites were concentrated on OT magnocellular neurones. Labelling concerned cell bodies and dendrites but not the axons and endings in the pituitary neural lobe. 5. The presently detected somatodendritic autoreceptors on OT neurones probably mediate the facilitatory effect of OT on its own release during the milk ejection reflex.