Fos localization in cytosolic and nuclear compartments in neurones of the frog, Rana esculenta, brain: An analysis carried out in parallel with GnRH molecular forms

C-fos activity was determined in the brain of the frog, Rana esculenta, dur ing the annual sexual cycle. The localization of GnRH molecular forms (mamm alian- and chicken-GnRHII) was also carried out to determine whether or not the proto-oncogene and the peptides showed a functional relationship. Nort hern blot analysis of total RNA revealed the presence of a single strong si gnal of c-fos like mRNA of 1.9 Kb during February and April, This was follo wed by expression of c-Fos protein (Fos) in several brain areas during Marc h and July shown by immunocytochemistry. In particular, the olfactory regio n, the lateral and medial pallium, the nucleus lateralis septi, the ventral striatum, the caudal region of the anterior preoptic area, the suprachiasm atic nucleus, the ventral thalamus, tori semicircularis and ependymal layer s of the tectum were immunostained. There was no overlap between Fos immuno reactive perikarya and GnRH immunoreactive perikarya (e.g. gonadotrophin-re leasing hormone (GnRH) in the rostral part and Fos in the caudal region of the anterior preoptic area). Interestingly, a cytoplasmic localization of F os was also observed by immunocytochemistry and gel retardation experiments supported this observation. Cytoplasmic extracts from September-October an imals bound the API oligonucleotide. The complex was not available in the n uclear extracts from the same preparation, suggesting that, besides Fos, Ju n products were also present. Conversely, nuclear but not cytosolic binding was detected in the brain of animals collected in July. In conclusion, we show that Fos and GnRH activity does not correlate in the frog brain and, f or the first time in a vertebrate species, we give evidence of a cytoplasmi c AP1 complex in neuronal cells.