Localisation and physiological regulation of corticotrophin-releasing factor receptor 1 mRNA in the Xenopus laevis brain and pituitary gland.

In Xenopus laevis, corticotrophin-releasing factor (CRF) and urocortin 1 are present in the brain and they both are potent stimulators of alpha-melanophore stimulating hormone (MSH) secretion by melanotroph cells in the pituitary gland. Because both CRF and urocortin 1 bind with high affinity to CRF receptor type 1 (CRF1) in mammals and Xenopus laevis, one of the purposes of the present study was to identify the sites of action of CRF and urocortin 1 in the Xenopus brain and pituitary gland. Moreover, we raised the hypothesis that the external light intensity is a physiological condition controlling CRF1 expression in the pituitary melanotroph cells. By in situ hybridisation, the presence of CRF1 mRNA is demonstrated in the olfactory bulb, amygdala, nucleus accumbens, preoptic area, ventral habenular nuclei, ventromedial thalamic area, suprachiasmatic nucleus, ventral hypothalamic area, posterior tuberculum, tectum mesencephali and cerebellum. In the pituitary gland, CRF1 mRNA occurs in the intermediate and distal lobe. The optical density of the CRF1 mRNA hybridisation signal in the intermediate lobe of the pituitary gland is 59.4% stronger in white-adapted animals than in black-adapted ones, supporting the hypothesis that the environmental light condition controls CRF1 mRNA expression in melanotroph cells of X. laevis, a mechanism likely to be responsible for CRF- and/or urocortin 1-stimulated secretion of alpha-MSH.