Physiologically induced Fos expression in the hypothalamo-hypophyseal system of Xenopus laevis

The amphibian Xenopus laevis adjusts the color of its skin to the degree of background illumination. The neuroendocrine mechanism responsible for this adaptation behavior involves various brain centers that control the synthesis and release of alpha-melanophore-stimulating hormone (alpha-MSH) by the pituitary melanotrope cells. The aim of the present study was to investigate the possible use of Fos as a tool to determine 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 stressor (saline immersion) and a mild stressor ('handling'). Without stimulation basal Fos-like immunoreactivity (Fos-LI) was found in the telencephalon, the lateral pallium, the anterior, central and lateral thalamic nuclei, the suprachiasmatic nucleus, the ventral hypothalamic nucleus and the torus semicircularis. Handling had no effect on this basal pattern of Fos-LI. Saline immersion induced Fos-LI only in the magnocellular 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 it took the same time before Fos-LI expression significantly dropped. It is concluded that the ABC-Fos immunocytochemistry can be successfully 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 the 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 Xenopus melanotrope cell.