Oxytocin cells in the supraoptic nucleus receive excitatory synaptic inputs from the contralateral supraoptic and paraventricular nuclei in the lactating rat.

The present experiments were undertaken to examine whether oxytocin cells in the supraoptic nucleus receive synaptic inputs from the contralateral supraoptic nucleus or paraventricular nucleus. Using urethane-anesthetized lactating rats, extracellular action potentials were recorded from single oxytocin or vasopressin cells in the supraoptic nucleus. Electrical stimulation was applied to the contralateral supraoptic nucleus or paraventricular nucleus, and responses of oxytocin or vasopressin cells were analyzed by peri-stimulus time histogram or by change in firing rate of oxytocin or vasopressin cells. Electrical stimulation of the contralateral supraoptic nucleus or paraventricular nucleus did not cause antidromic excitation in oxytocin or vasopressin cells but caused orthodromic responses. Although analysis by peri-stimulus time histogram showed that electrical stimulation of the contralateral supraoptic nucleus or paraventricular nucleus caused orthodromic excitation in both oxytocin and vasopressin cells, the proportion of excited oxytocin cells was greater than that of vasopressin cells. Train stimulation applied to the contralateral supraoptic nucleus or paraventricular nucleus at 10 Hz increased firing rates of oxytocin cells and decreased those of vasopressin cells. The results of the present experiments suggest that oxytocin cells in the supraoptic nucleus receive mainly excitatory synaptic inputs from the contralateral supraoptic nucleus and paraventricular nucleus. Receipt these synaptic inputs to oxytocin cells may contribute to the synchronized activation of oxytocin cells during the milk ejection reflex.