TIME-COURSE OF PHOSPHORYLATED CREB AND FOS-LIKE IMMUNOREACTIVITY IN THE HYPOTHALAMIC SUPRAOPTIC NUCLEUS AFTER SALT LOADING

Phosphorylation of the cAMP response element binding protein (CREB) pr ecedes the induction of immediate early gene expression. Using antibod ies that distinguish CREB from phosphorylated CREB (PCREB), we studied the appearance of PCREB-like immunoreactivity (PCREB-LI) and Fos-LI i n the hypothalamic supraoptic nucleus (SON) of rats treated with hyper tonic or normal saline and uninjected controls. Fifteen minutes after injection, increased numbers of PCREB-LI cells were seen in both norma l and hypertonic saline-treated rats as compared with uninjected contr ols. Forty-five minutes after injection, levels of c-fos mRNA in the S ON were elevated in hypertonic saline-treated rats as compared with no rmal saline-treated rats, and were minimally detectable in uninjected rats. At this time period, the hypertonic saline-treated rats showed i ncreased number of Fos-LI cells in the SON, whereas normal saline-trea ted rats showed little or no Fos-LI cells. The discrepancy between lev els of PCREB-LI and c-fos mRNA suggests that injection of hypertonic s aline may activate additional transcriptional factors besides CREB. Th e 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 exce ed a threshold before increases in Fos-LI cells are detectable by immu nostaining of the SON. Such a threshold might permit neuronal cells to activate diverse genes, through phosphorylation of CREB, without indu cing the constellation of Fos-responsive genes.