Divergent glial fibrillary acidic protein and its mRNA in the activated supraoptic nucleus

Previous studies have shown decreased immunoreactive glial fibrillary acidic protein (GFAP) in the supraoptic nucleus (SON) when magnocellular neuroendocrine cells (MNCs) are activated by lactation or dehydration. This is thought to underlie structural plasticity of glial processes that occurs during these times. Here, we investigated how this apparent reduction in protein relates to GFAP mRNA expression in the dehydrated rat as visualized by in situ hybridization. Densitometry of silver grains in the SON revealed low levels of mRNA expression in control, 2-day dehydrated and 21-day rehydrated (R21) animals. Conversely, the SON from 7-day dehydrated (D7) subjects displayed significantly more silver grains. Thus, the pattern of GFAP mRNA expression is the inverse of what we previously observed for GFAP immunoreactivity in tissue sections of the SON. No differences in mRNA levels due to hydration state were seen in the lateral hypothalamic area, suggesting that increases in GFAP mRNA at D7 were specifically related to MNC activation. These data indicate a divergence in GFAP mRNA and protein expression in the SON.