Immunocytochemical studies showed the presence of a fiber system containing a CRF-like peptide in the median eminence and in the neural lobe of the pituitary gland of Xenopus laevis. During in vitro superfusion of neurointermediate lobe tissue, CRF, sauvagine and urotensin I induced a rapid and dose-dependent stimulation of secretion of MSH and endorphin. Tissue of white-background adapted animals displayed a remarkably higher sensitivity to CRF and sauvagine than tissue from animals that were adapted to a black background. During superfusion of isolated melanotrope cells in suspension, it was shown that CRF and sauvagine exerted their effect directly on the melanotrope cell. We therefore conclude that there is morphological and biochemical evidence to consider a CRF-like peptide as a physiological MSH-releasing factor.

The pars intermedia of the pituitary gland in Xenopus laevis secretes alpha-melanophore-stimulating hormone (alpha-MSH), which causes dispersion of pigment in dermal melanophores in animals on a black background. In the present study we have determined plasma levels of alpha-MSH in animals undergoing adaptation to white and black backgrounds. Plasma values of black-adapted animals were high and decreased rapidly after transfer to a white background, as did the degree of pigment dispersion in dermal melanophores. Plasma MSH values of white-adapted animals were below the detection limit of our radioimmunoassay. Transfer of white animals to a black background resulted in complete dispersion of melanophore pigment within a few hours, but plasma MSH levels remained low for at least 24 hr. This discrepancy between plasma MSH and degree of pigment dispersion suggested the involvement of an additional factor for stimulating dispersion. Results of in vitro and in vivo experiments with receptor agonists and antagonists indicated that a beta-adrenergic mechanism, functioning at the level of the melanophore, is involved in the stimulation of pigment dispersion during the early stages of background adaptation.

It is well-known that alpha-melanophore-stimulating hormone (alpha-MSH) release from the amphibian pars intermedia (PI) depends on the light condition of the animal's background, permitting the animal to adapt the colour of its skin to background light intensity. In the present study, we carried out nine experiments on the effect of low temperature on this skin adaptation process in the toad Xenopus laevis, using the skin melanophore index (MI) bioassay and a radioimmunoassay to measure skin colour adaptation and alpha-MSH secretion, respectively. We show that temperatures below 8 degrees C stimulate alpha-MSH secretion and skin darkening, with a maximum at 5 degrees C, independent of the illumination state of the background. No significant stimulatory effect of low temperature on the MI and alpha-MSH plasma contents was noted when the experiment was repeated with toads from which the neurointermediate lobe (NIL) had been surgically extirpated. This indicates that low temperature stimulates alpha-MSH release from melanotrope cells located in the PI. An in vitro superfusion study with the NIL demonstrated that low temperature does not act directly on the PI. A possible role of the central nervous system in cold-induced alpha-MSH release from the PI was tested by studying the hypothalamic expression of c-Fos (as an indicator for neuronal activity) and the coexistence of c-Fos with the regulators of melanotrope cell activity, neuropeptide Y (NPY) and thyrotrophin-releasing hormone (TRH), using double fluorescence immunocytochemistry. Upon lowering temperature from 22 degrees C to 5 degrees C, in white-adapted animals c-Fos expression decreased in NPY-producing suprachiasmatic-melanotrope-inhibiting neurones (SMIN) in the ventrolateral area of the suprachiasmatic nucleus (SC) but increased in TRH-containing neurones of the magnocellular nucleus. TRH is known to stimulate melanotrope alpha-MSH release. We conclude that temperatures around 5 degrees C inactivate the SMIN in the SC and activate TRH-neurones in the magnocellular nucleus, resulting in enhanced alpha-MSH secretion from the PI, darkening the skin of white-adapted X. laevis.

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