Low Temperature Stimulates α-Melanophore-Stimulating Hormone Secretion and Inhibits Background Adaptation in Xenopus laevis

• Published in: Journal of neuroendocrinology 2004-11, Vol.16 (11), p.894-905
• Main Authors: Tonosaki, Y., Cruijsen, P. M. J. M., Nishiyama, K., Yaginuma, H., Roubos, E. W.
• Format: Article
• Language: English
• Subjects: Adaptation, Physiological; alpha-MSH - blood; alpha-MSH - secretion; Animals; Biological and medical sciences; c-Fos; Cold Temperature; Fundamental and applied biological sciences. Psychology; Hypothalamus - metabolism; In Vitro Techniques; magnocellular nucleus; neuropeptide Y; Neuropeptide Y - physiology; pars intermedia; Pituitary Gland, Anterior - secretion; Proto-Oncogene Proteins c-fos - metabolism; Skin Pigmentation - physiology; suprachiasmatic nucleus; thyrotrophin-releasing hormone; Thyrotropin-Releasing Hormone - physiology; Vertebrates: endocrinology; Xenopus laevis - physiology
• ISSN: 0953-8194; 1365-2826
• DOI: 10.1111/j.1365-2826.2004.01247.x

It is well‐known that α‐melanophore‐stimulating hormone (α‐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 α‐MSH secretion, respectively. We show that temperatures below 8 °C stimulate α‐MSH secretion and skin darkening, with a maximum at 5 °C, independent of the illumination state of the background. No significant stimulatory effect of low temperature on the MI and α‐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 α‐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 α‐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 °C to 5 °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 α‐MSH release. We conclude that temperatures around 5 °C inactivate the SMIN in the SC and activate TRH‐neurones in the magnocellular nucleus, resulting in enhanced α‐MSH secretion from the PI, darkening the skin of white‐adapted X. laevis.