Redistribution of MAP2 immunoreactivity in the neurohypophysial astrocytesof adult rats during dehydration

The low-molecular-weight microtubule-associated protein-2 (LMW MAP2) is exp ressed in immature and developing brains, and decreases its content dramati cally along with maturation of the central nervous system. In our previous studies, we demonstrated through western blots and dual-labeling immunohist ochemistry that LMW MAP2 is expressed in the pituicytes, modified astrocyte s of the neurohypophysis in adult rats. The present study aimed to examine changes in the MAP2 immunoreactivity within pituicyte in adult rats under v arious hydration states using quantitative morphometrical analysis to demon strate in vivo shape conversion of the pituicyte morphology. In well-hydrat ed control rats, light microscopic observation revealed that MAP2-stained p ituicytes ramified long and well-branched processes. At electron microscopi c level, MAP2 immunoreactivity was found in the fine process and cell body of all pituicyte cytoplasm, but not in the axonal terminals containing neur osecretory vesicles. The quantitative analysis demonstrated that the cell s ize and perimeter of MAP2-stained pituicytes were significantly greater as compared with those of cells stained with glial fibrillary acidic protein ( GFAP). When the rats were dehydrated with water deprivation or drinking of 2% saline solution, the process of MAP2-stained pituicytes was less branche d due to retracting their cellular processes as compared with those of well -hydrated control and rehydrated rats. The quantitative analysis further de monstrated that water deprivation significantly reduced the cell size, peri meter and length of cellular processes of MAP2-stained pituicytes as compar ed with those of control. The present finding indicates that MAP2 staining is better method for investigating in vivo shape conversion of the pituicyt e morphology than GFAP one. Moreover, the finding that hydration states sig nificantly and reversibly alter in vivo pituicyte shape supports the hypoth esis that the plastic shape conversion of pituicyte morphology is responsib le for morphological plasticity in the neurohypophysis. (C) 1999 Elsevier S cience B.V. All rights reserved.