Electrical activities of neurones in the dorsomedial hypothalamic nucleus projecting to the supraoptic nucleus during milk-ejection reflex in the rat.

Using urethane-anesthetized lactating rats, extracellular action potentials were recorded from single cells in the dorsomedial hypothalamus (DMH), which were identified as projecting to the supraoptic nucleus (SON). Sixty-two DMH cells were identified as projecting to the SON. Of these 53, 4 and 5 cells had ipsilateral, contralateral and bilateral projections, respectively. Two cells (1 ipsilaterally and 1 contralaterally projecting cell) showed bursting activities preceding milk ejection that were similar to those of oxytocin (OT) cells in the SON or paraventricular nucleus. Two ipsilaterally and 2 bilaterally projecting cells reduced their firing rates preceding milk ejection. The results suggest that some of the projections from the DMH to the SON are contralateral or bilateral and that these projections may contribute to synchronized activation of OT cells bilaterally distributed in the hypothalamus during milk-ejection reflex.