AN OXYTOCIN RECEPTOR ANTAGONIST INFUSED INTO THE SUPRAOPTIC NUCLEUS ATTENUATES INTRANUCLEAR AND PERIPHERAL RELEASE OF OXYTOCIN DURING SUCKLING IN CONSCIOUS RATS

We tested the hypothesis that oxytocin (OXT) released within the supra optic nucleus (SON) during suckling in lactating rats facilitates its own intranuclear release and thereby stimulates its secretion into blo od during the milk ejection reflex. In the first study, to examine the positive feedback action of OXT released within the SON, microdialysa tes of the SON area were sampled before and during suckling. Either ve hicle (1.0 mu l) or an OXT receptor antagonist (50 ng/mu l) was infuse d adjacent to the microdialysis probe into the SON area before sucklin g began. Compared to the corresponding presuckling values (100%), the OXT content in microdialysates sampled during the suckling period incr eased after vehicle treatment (to 501 +/- 174%; P < 0.01), whereas inf usion of the OXT antagonist significantly (P < 0.05) attenuated the ri se in the release of OXT during suckling (130 +/- 22.0%). In the secon d study, the influence of intranuclear OXT on release of OXT into the blood during suckling was examined. Compared to those in rats infused with vehicle into both SON before suckling started (100%), injection o f the OXT antagonist decreased the concentration of OXT in plasma (to 32.1 +/- 5.50%; P < 0.05) and the milk transfer to the pups during the suckling period (to 30.2 +/- 13.3%; P < 0.01). Although the neuroendo crine response to suckling was attenuated in rats receiving the OXT an tagonist, maternal behavior, assessed as the time at which six to eigh t pups began suckling and the time from the onset until the occurrence of the third milk ejection reflex, was unaffected. These findings pro vide evidence for a receptor-mediated positive feedback action of OXT on its own release within the SON during suckling. The amplification o f intranuclear release of OXT is needed for the release of appropriate amounts of OXT into the bloodstream to stimulate milk transfer to the suckling pups.