[Dual noradrenergic control of oxytocin release during the milk ejection reflex in the rat]

During suckling in the Rat, inhibition of oxytocin release by alpha-noradrenergic antagonist and beta-noradrenergic agonist is well established, but no result pinpointed the site where the drugs acted. Simultaneous recordings of: (1) the electrical activity of the oxytocinergic (OT) neurons in the paraventricular nucleus; (2) the intramammary pressure (p.i.m.) with observations of the behaviour of the litter, have shown two levels of control. Intraventricular injections of phentolamine (an alpha antagonist) significantly depressed the spontaneous and reflex firing of OT-cells, and during this inhibition, neither p.i.m. increases nor behavioural responses of young Rats could be detected. On the contrary, after isoproterenol (a beta agonist), the reflex OT releases did not occur but the basal firing rate of OT cells and the magnitude and frequency of bursts were not affected. The action level of the two drugs is discussed.