Alterations in supraoptic nucleus ultrastructure of maternally behaving virgin rats.

Adult, nulliparous female rats were induced to behave maternally via constant cohousing with rat pups. After exhibiting maternal behaviors for 3 days, the animals were transcardially perfused, the supraoptic nuclei (SON) excised and examined quantitatively by transmission electron microscopy. Relative to virgin controls, the animals behaving maternally were found to have significant increases in: a) mean number of dendrites in large (9-12) dendritic bundles, b) mean area per single dendritic profile, c) area of the dendritic zone occupied by dendritic profiles and d) size of the dendritic zone. No significant changes were observed in the cell body zone or in the number of double synapses in the dendritic region. The observed changes are likely to be at least in part associated with the oxytocin-containing cells of the SON. These observations suggest a role for the SON in promoting maternal behaviors and constitute a novel demonstration of a neural modification in the mammalian central nervous system that appears conjointly with a complex set of behaviors.