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. Lesio n and microinjection studies indicate that this action is, in part, me diated via a population of limbic neurones in the bed nuclei of the st ria terminalis and ventrolateral septum, which have been shown to poss ess oxytocin receptors and to be activated by selective oxytocin-recep tor agonists in vitro. In vivo electrophysiological 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 thi s population of limbic neurones serves to gate the activity of a pacem aker which, in turn, coordinates the bursting of hypothalamic magnocel lular neurones. The oxytocin innervation of these neurones and their e xpression of oxytocin receptors increases in the postpartum period, an d the resultant enhanced sensitivity leads to a greater facilitatory r esponse during lactation. Inhibitory opioid and noradrenergic inputs w hich converge on these oxytocin-sensitive neurones may function to swi tch off the facilitatory circuit during periods of stress. Thus, this population of limbic neurones participates in the regulation of neuroe ndocrine activity during lactation by providing an appropriate degree of feedback to alter the patterning of the milk-ejection reflex.