PHYSIOLOGICALLY INDUCED FOS EXPRESSION IN THE HYPOTHALAMOHYPOPHYSEAL SYSTEM OF XENOPUS-LAEVIS

The amphibian Xenopus laevis adjusts the color of its skin to the degr ee of background illumination. The neuroendocrine mechanism responsibl e for this adaptation behavior involves various brain centers that con trol the synthesis and release of alpha-melanophore-stimulating hormon e (alpha-MSH) by the pituitary melanotrope cells. The aim of the prese nt study was to investigate the possible use of Fos as a tool to deter mine the activity of known and novel components of this mechanism. For this purpose, a quantitative Fos-immunocytochemical method (ABC) was successfully introduced for Xenopus, and the degree of specificity of background illumination as a regulator of Fos expression was tested by comparing this stimulus with two other stimuli, viz. a strong stresso r (saline immersion) and a mild stressor ('handling'). Without stimula tion basal Fos-like immunoreactivity (Fos-LI) was found in the telence phalon, the lateral pallium, the anterior, central and lateral thalami c nuclei, the suprachiasmatic nucleus, the ventral hypothalamic nucleu s and the torus semicircularis. Handling had no effect on this basal p attern of Fos-LI. Saline immersion induced Fos-LI only in the magnocel lular preoptic nucleus, the corticotrope cells and, less strongly, in the melanotrope cells. Melanotropes, and no other cells, expressed Fos -LI very strongly when Xenopus was transferred from a white to a black background. This Fos-LI expression continued to increase up to 7 days of stimulation. When such toads were returned to a white background i t took the same time before Fos-LI expression significantly dropped. I t is concluded that the ABC-Fos immunocytochemistry can be successfull y applied to assess the occurrence and degree of expression of Fos-LI in the Xenopus brain and pituitary gland. The prolonged expression of Fos-LI in the pars intermedia under black background stimulation and t he presence of an AP-1 binding site on the Xenopus proopiomelanocortin gene suggest an important role for c-Fos and/or Fos-related antigens in the control of the biosynthesis and secretion of alpha-MSH by the X enopus melanotrope cell.