Oxytocin neurons in the supraoptic nucleus receive excitatory inputs from the bilateral dorsomedial hypothalamic nuclei

To examine whether inputs from the dorsomedial hypothalamic nucleus (DMH) alter the discharge of putative oxytocin (OT) neurons with hypothesis that excitation of DMH neurons would increase the activity of OT neurons, electrical stimulation was applied to the DMH in both sides of the hypothalamus while electrical activity of single OT neurons in the supraoptic nucleus (SON) was recorded in urethane-anesthetized lactating rats. About half of the OT neurons showed orthodromic excitation or inhibition followed by excitation in response to electrical stimulation of the DMH on both sides. Continuous electrical stimulation of the DMH both ipsi- and contralateral to the recording side at 10–50 Hz for 30–60 s increased firing rate in 58% of OT neurons tested. Continuous electrical stimulation of the DMH not only excited spiking activity of single OT neurons but also increased intramammary pressure. The results may suggest that some of the projections from the DMH to the SON are bilateral and possibly contribute to coordinated bilateral activation of OT neurons in the hypothalamus during the milk-ejection reflex.