Prolactin releasing peptides modulate background firing rate and milk-ejection related burst of oxytocin cells in the supraoptic nucleus

Abstract

The hypothalamic dorsomedial nucleus is suggested to be a final relay site for the afferent pathway of milk-ejection reflex. Existence of prolactin releasing peptide-immunoreactive cells in the dorsomedial nucleus and synaptic contact of prolactin releasing peptide-immunoreactive terminals with oxytocin cells was reported. Experiments were done to test the effect of prolactin releasing peptide on the electrical activity of oxytocin cells in the supraoptic nucleus. In rat brain slice preparations, oxytocin cells were unresponsive to the peptide. In lactating rats, although lateral ventricular injection of prolactin releasing peptide (20 nmol) was ineffective, a hundred nanomoles of the peptide increased basal activity and amplitude of milk-ejection related burst firing of oxytocin cells. Cells responded to lateral ventricular injection of peptides were unresponsive to direct application of peptides by pressure ejection from the recording electrode. These results suggest that prolactin releasing peptide may modulate electrical activity of oxytocin cells not through its direct action on oxytocin cells but through its action on area other than supraoptic nucleus.