Evidence for stimulatory noradrenergic and inhibitory dopaminergic regulation of oxytocin release in the lactating rat.

The present experiments tested the involvement of central catecholaminergic systems in the suckling-induced release of oxytocin (OT) during lactation in the rat. In the first experiment, female rats in midlactation were separated from their offspring for 4 h and then allowed to suckle their litters for 30 or 60 min or to remain nonsuckled. The turnover rates of norepinephrine (NE) and dopamine (DA) were calculated from the rate of decline after synthesis inhibition. Suckling decreased the turnover rate of DA in the median eminence and in the neurointermediate lobe of the pituitary gland. Suckling increased the turnover rate of NE in the rostral paraventricular and supraoptic nuclei, areas that contain most of the OT cells that project to the neural lobe of the pituitary, and in the interstitial nucleus of the stria terminalis, but not in the arcuate or caudal paraventricular nuclei, median eminence, or neurointermediate lobe. In a second experiment, midlactating females received intracerebral microinjections of the catecholamine neurotoxin 6-hydroxydopamine or of vehicle into the vicinity of the paraventricular and supraoptic nuclei 1 week before a suckling test. The release of OT was completely prevented in 6-hydroxydopamine-treated animals, and NE was significantly decreased in the paraventricular, supraoptic, and arcuate nuclei. In a third study, the increase in plasma OT in response to suckling was prevented by stimulation of DA receptors with bromocriptine, while blockade of DA receptors with domperidone significantly increased plasma OT levels in nonsuckled lactating rats. These results suggest that suckling stimulation activates the noradrenergic innervation to the rostral paraventricular nucleus and to the supraoptic nucleus, which exerts an excitatory influence on the release of OT and decreases activity of the tuberohypophyseal DA system, which provides a tonic inhibitory influence over the secretion of OT.