Magnocellular neuropeptidergic neurons in hypothalamus: increases in membrane apposition and number of specialized synapses from pregnancy to lactation.

Morphological changes which have been hypothesized to accompany functional alterations in magnocellular neuropeptidergic cells (MNCs) were studied in female rats. Direct soma-somatic appositions between the MNC profiles of two nuclear groups, the supraoptic nucleus (SON) and nucleus circularis (NC) were investigated at the ultrastructural level in 4 groups of animals: virgin females, immediately pre-partum pregnant rats, post-partum and 14-day lactating animals. The percentage of SON MNC profiles in soma-somatic apposition and the amount of membrane in direct contact significantly increased over control levels by the last day of pregnancy. Further significant increases in these measures were observed in lactating rats. MNCs in NC showed steady gradual increases on these measures with significant differences from controls occurring in the post-partum group. The percentage of SON cell profiles with double synapses (i.e., presynaptic terminals making synaptic contact with two postsynaptic neurons) was significantly elevated in lactating rats (approximately 10%) over the next highest group (approximately 1% for post-partum rats). In NC, approximately 10% of the cell profiles sampled had such synapses but no differences among treatments occurred. The changes during late pregnancy suggest that close appositions may serve to enhance the metabolic activity of MNCs at a time when there is a build-up of stored oxytocin. Further increases in cell-cell contact and the addition of double synapses on possibly electrotonically coupled MNCs during lactation may serve a synchronizing function, particularly in the oxytocin cells participating in the milk ejection reflex.