NUCLEAR FOS DOMAINS IN TRANSCRIPTIONALLY ACTIVATED SUPRAOPTIC NUCLEUSNEURONS

This study has analysed by light and electron microscopy immunolocaliz ation the nuclear pattern of distribution of Fos-related proteins in s upraotic neurons. Two experimental models of transcriptional activatio n have been used: sustained, global transcriptional activation, at rel atively near physiological conditions, by six days of chronic intermit tent salt loading; and superinduction of c-fos gene by this salt loadi ng regime plus cycloheximide treatment for 4 h. In the first condition , the ultrastructural analysis showed a distribution of Fos-like immun oreactivity on the reticular network of dispersed chromatin that exten ds between the nucleolar surface and the nuclear envelope, whereas the Fos-negative adjacent interchromatin spaces appeared rich in interchr omatin granules by using a cytochemical staining for ribonucleoprotein s. The nucleolus associated heterochromatin, fibrillar centers of the nucleolus and coiled bodies were free of immunoreactivity. This immuno electron pattern seems to indicate that active genes containing activa tor protein-1 and cyclic AMP response element recognition sites are ex tensively distributed in euchromatin regions and suggests that the Fos -positive nuclear domains correspond to the actively transcribing chro matin regions, at least in supraoptic neurons. It also suggests that t hese Fos-positive transcription domains are complementary to adjacent ribonucleoprotein-rich interchromatin spaces which are involved in the processing and splicing of pre-messenger RNA. Moreover, the absence o f immunoreactivity on the fibrillar centers, the sites of pre-ribosoma l RNA synthesis, suggests that the Fos protein complexes are not invol ved in regulating the expression of ribosomal RNA genes. Following sup erinduction of c-fos gene by osmotic stimulation plus cycloheximide tr eatment, a conspicuous Fos-like immunoreactivity was detected in dispe rsed chromatin regions, whereas the heterochromatin masses, nucleoli a nd coiled bodies showed no immunoreaction. Moreover, this treatment in duced the formation of nuclear ''dense bodies'' of a fibrillar nature which were free of immunolabelling. Since Fos proteins are known to be short-lived, the expression of these nuclear constituents, under cond itions of protein synthesis inhibition induced by the cycloheximide, s uggests the stabilization of chromatin-bound Fos complexes or, alterna tively, a preferential synthesis of Fos proteins.