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.