Neuronal/glial plasticity in the supraoptic dendritic zone: dendritic bundling and double synapse formation at parturition.

The magnocellular neurosecretory cells of the supraoptic nucleus increase production and release of oxytocin and/or vasopressin under such conditions as parturition, lactation and dehydration. These stimuli have been shown to result in increased direct apposition of neuronal membranes and the formation of double synapses (one presynaptic terminal contacting two postsynaptic elements) within the supraoptic nucleus at the level of the cell bodies. These morphological changes are due to the retraction of the thin glial processes which are normally interposed between adjacent neurons. The present study was undertaken to ascertain whether, and to what extent, neuronal/glial plasticity occurs in the dendritic zone (i.e. the ventral glial laminar area) of the supraoptic nucleus. The instances of two or more dendrites with membrane in direct apposition (dendritic bundles), the number of dendrites per bundle, the amount of dendritic membrane in direct apposition and the percentage of dendrites contacted by double synapses were quantified at the ultrastructural level in virgin female, prepartum (21 days of gestation), postpartum (day of parturition) and lactating rats. All parameters measured varied significantly with the hormone demand states created by pregnancy and lactation, apparently due to glial retraction. Moreover, in the 2-24 h period between pre- and postpartum there was a significant increase in the number of dendrites per bundle, dendritic membrane in direct apposition and the percentage of dendrites contacted by double synapses. This time course corresponds to the known increased release of oxytocin and vasopressin at parturition. These findings constitute the first demonstration that dendritic bundles and double synapses occur in the ventral glial lamina/dendritic zone of the supraoptic nucleus and vary under the physiological conditions of pregnancy, parturition and lactation.