Limbic Regions Mediating Central Actions of Oxytocin on the Milk-Ejection Reflex in the Rat

Central oxytocin administration has a profound facilitatory effect on the patterning of the milk-ejection reflex in the lactating rat. Lesion and microinjection studies indicate that this action is, in part, mediated via a population of limbic neurones in the bed nuclei of the stria terminalis and ventrolateral septum, which have been shown to possess oxytocin receptors and to be activated by selective oxytocin-receptor agonists in vitro. In vivo electro-physiological recordings reveal that some of these neurones display cyclical activity which is highly correlated to each milk ejection, and are rapidly activated following i.c.v. administration of oxytocin. coincident with the facilitation of milk ejection activity. A hypothetical model is proposed in which this population of limbic neurones serves to gate the activity of a pacemaker which, in turn, coordinates the bursting of hypothalamic magnocellular neurones. The oxytocin innervation of these neurones and their expression of oxytocin receptors increases in the post-partum period, and the resultant enhanced sensitivity leads to a greater facilitatory response during lactation. Inhibitory opioid and noradrenergic inputs which converge on these oxytocin-sensitive neurones may function to switch off the facilitatory circuit during periods of stress. Thus, this population of limbic neurones participates in the regulation of neuroendocrine activity during lactation by providing an appropriate degree of feedback to alter the patterning of the milk-ejection reflex.