NITRIC-OXIDE SYNTHASE AND BACKGROUND ADAPTATION IN XENOPUS-LAEVIS

Adaptation of the skin colour to the background light condition in the amphibian Xenopus laevis is achieved by migration of pigment granules in the skin melanophores, a process regulated by alpha-MSH secretion from melanotrope cells in the pituitary pars intermedia (PI). alpha-MS H secretion in turn, is regulated by various stimulatory and inhibitor y messengers synthesized in brain nuclei, especially the hypothalamic suprachiasmatic and magnocellular nuclei and the locus coeruleus in th e hindbrain. In the present study, the roles in background adaptation of nitric oxide (NO) and NO synthase (NOS) enzyme activity were evalua ted. In situ, using both immunohistochemistry with anti-human brain NO S (bNOS) serum in paraffin-embedded material and using nicotinamide ad enine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry in cr yo-sections, we showed NOS in neurons in the optic tectum and in the l ocus coeruleus. NADPH-d reactivity was also found in neurons in the la teral amygdala, the ventral hypothalamic nucleus and in fibers in the median eminence. Using a Western blot stained with an anti-human bNOS serum, we demonstrated a 150 kDa band in Xenopus hindbrain lysates, wh ich is similar to the NOS protein present in the rat anterior pituitar y, but which was not detectable in the lysates from both the neurointe rmediate and distal lobes in Xenopus. No differences in histochemical staining pattern or on Western blotting were observed between animals adapted to a black or a white background. Paraffin sections of the end ocrine PI and pars distalis did not reveal bNOS-like immunoreactivity. NADPH-d reactivity was observed in the endothelia of this gland. Howe ver, using a new procedure of thin cryo-sections of pituitary neuroint ermediate lobes, we observed bNOS-immunoreactive fibers as well as cyc lic 3',5' guanosine monophosphate (cGMP)-accumulating fibers in the PI . The PI may be regulated by NOergic neurons from higher brain centers . The possibility that NOergic neurons in the locus coeruleus are invo lved in the innervation of the PI needs further investigation. The lat ter neurons are probably not noradrenergic because double labeling stu dies show no co-localization of NADPH-d reactivity and tyrosine hydrox ylase immunoreactivity in locus coeruleus neurons. (C) 1997 Elsevier S cience B.V.