Phosphorylation of the cAMP response element binding protein (CREB) precedes the induction of immediate early gene expression. Using antibodies that distinguish CREB from phosphorylated CREB (PCREB), we studied the appearance of PCREB-like immunoreactivity (PCREB-LI) and Fos-LI in the hypothalamic supraoptic nucleus (SON) of rats treated with hypertonic or normal saline and uninjected controls. Fifteen minutes after injection, increased numbers of PCREB-LI cells were seen in both normal and hypertonic saline-treated rats as compared with uninjected controls. Forty-five minutes after injection, levels of c-fos mRNA in the SON were elevated in hypertonic saline-treated rats as compared with normal saline-treated rats, and were minimally detectable in uninjected rats. At this time period, the hypertonic saline-treated rats showed increased number of Fos-LI cells in the SON, whereas normal saline-treated rats showed little or no Fos-LI cells. The discrepancy between levels of PCREB-LI and c-fos mRNA suggests that injection of hypertonic saline may activate additional transcriptional factors besides CREB. The lack of Fos-LI in the presence of modest increases in c-fos mRNA in normal saline-treated rats implies that levels of c-fos mRNA must exceed a threshold before increases in Fos-LI cells are detectable by immunostaining of the SON. Such a threshold might permit neuronal cells to activate diverse genes, through phosphorylation of CREB, without inducing the constellation of Fos-responsive genes.
