Reevaluation of the plasticity in the rat supraoptic nucleus after chronic dehydration using immunogold for oxytocin and vasopressin at the ultrastructural level.

It has been shown that during physiological stimuli, such as dehydration, supraoptic nucleus (SON) neurons undergo profound morphological changes. However, little is known about how much each type of cell, oxytocin (OT) or vasopressin (VP), contributes to this plasticity during dehydration. Using postembedding immunogold cytochemistry for both OT and VP hormones at the electron microscopic level, we address this question. Rats were chronically dehydrated (given 2% saline to drink for 10 days) and their SON neurons were studied morphologically. The results were compared to control animals with free access to water. Both VP and OT somata showed an enlargement in size in dehydrated animals. Percentage of somasomatic/dendritic membrane contact increased significantly in both VP and OT neurons, with no significant changes in percentage of coverage of the cells by astrocytic membrane. Only the VP cells had a lesser amount of axosomatic membrane contact after dehydration, possibly due to an increase in cell size rather than a decrease in synaptic contact. Multiple synapses (MSs) (i.e., terminals that form more than one synapse with adjacent somata and or dendrites) occurred only between positively labeled cells and between negatively labeled cells, but not between positively and negatively labeled cells. The number of MSs per 100 microns OT somatic membrane or per 100 OT cells was significantly higher in dehydrated rats but was unchanged with regard to VP neurons. These findings indicate that both VP and OT neurons undergo morphological changes during chronic dehydration and, thus, that plasticity is not limited to OT cells as some earlier reports have suggested.