Maintenance of ultrastructural plasticity of the hypothalamic supraoptic nucleus in the ovariectomized rat

In the present experiments, we examined the effect of ovariectomy on the increases in litter weight and structural plasticity of MNCs in the supraoptic nucleus (SON) during lactation. Female rats were ovariectomized 2 days after parturition, and the increases in litter weight were measured as the index of milk let-down from dams during lactation. The lactation period was elongated up to 6 weeks by providing new litter to obtain more apparent effects of the ovariectomy. There was no significant difference in the increases in litter weight between non-operated and ovariectomized females. After lactation for 6 weeks, the ultrastructures such as juxtaposition (surface membrane apposition) and multiple synapses (terminals contacting with two or more postsynaptic elements) of MNCs in the SON in nonoperated and ovariectomized females were examined to compare with those of virgins. The percentage of juxtaposition and the number of multiple synapses significantly increased in nonoperated lactating females as compared with those of virgins. Ovariectomized rats showed similar structural changes to those of nonoperated females during lactation. Therefore, we conclude that ultrastructural plasticity of MNCs in the SON is maintained even in the absence of an ovary, and direct or indirect actions of suckling stimulation may be important in maintaining the plasticity during lactation.