The surface density of glial fibrillary acidic protein immunopositive astrocytic processes in the rat supraoptic nucleus is reversibly altered by dehydration and rehydration

Ultrastructural and immunohistochemical studies of the supraoptic nucleus ( SON) have provided evidence that retraction and extension of astrocytic pro cesses from between magnocellular neuroendocrine cells (MNCs) likely plays a role in the release of oxytocin, and/or vasopressin, that accompanies par turition, lactation and dehydration. The present study estimates the surfac e density (Sv) of glial fibrillary acidic protein (GFAP)-immunoreactivity, predominantly in astrocytic processes, in the SON of normally hydrated, deh ydrated and rehydrated rats. The Sv of GFAP processes in dehydrated rats wa s significantly reduced compared with control levels. Rehydration returned Sv to control levels. The reversible reduction in Sv indicates that the pre viously observed reduction in optical density is due to a rearrangement of astrocyte processes in the SON which occurs at the same time as the selecti ve functional activation of MNCs. (C) 1999 Elsevier Science Ireland Ltd. Al l rights reserved.