Oxytocin neurons in the supraoptic nucleus receive synaptic inputs from the contralateral supraoptic nucleus in cyclic female and male rats

Abstract

The present experiments were undertaken to examine whether oxytocin cells in the supraoptic nucleus receive synaptic inputs from the contralateral supraoptic nucleus in cyclic female and male rats. Using urethane-anesthetized cyclic female or male 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, 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 did not cause antidromic excitation in neither oxytocin nor vasopressin cells but caused orthodromic responses. Although analysis by peri-stimulus time histogram showed that electrical stimulation of the contralateral supraoptic nucleus caused orthodromic excitation in both oxytocin and vasopressin cells, the proportion of excited oxytocin cells was greater than that of vasopressin cells in both cyclic female and male rats. In male rats, train electrical stimulation applied to the contralateral supraoptic nucleus at 10Hz increased firing rates of oxytocin cells. In cyclic female rats, train electrical stimulation applied to the contralateral supraoptic nucleus at 10 and 5Hz increased firing rates of oxytocin cells. The results of the present experiments indicate that oxytocin cells in the supraoptic nucleus of cyclic female and male rats also receive mainly excitatory synaptic inputs from the contralateral supraoptic nucleus similar to those of lactating rats. The present experiments suggest that excitatory neural connections between oxytocin cells in separate magnocellular nuclei in the hypothalamus may not be specific in a period of lactation.