IMMUNOCYTOCHEMISTRY AND IN-SITU HYBRIDIZATION OF NEUROPEPTIDE-Y IN THE HYPOTHALAMUS OF XENOPUS-LAEVIS IN RELATION TO BACKGROUND ADAPTATION

The amphibian Xenopus laevis is able to adapt to a dark background by releasing melanophore-stimulating hormone from the pars intermedia of the pituitary gland. The inhibition of melanophore-stimulating hormone release is accomplished by neuropeptide Y-containing axons innervatin g the pars intermedia. To determine the production site of neuropeptid e Y involved in this inhibitory control, the distribution of neuropept ide Y in the brain has been investigated by immunocytochemistry and in situ hybridization. Immunoreactive cell bodies were visualized in, am ong others, the ventromedial and posterior thalamic nuclei, and the su prachiasmatic and ventral infundibular hypothalamic nuclei. A positive hybridization signal with a Xenopus-specific probe for preproneuropep tide Y-RNA was found in the diencephalic ventromedial thalamic nucleus and in the suprachiasmatic nucleus. With both immunocytochemistry and in situ hybridization, suprachiasmatic neurons appeared to be stained only in animals adapted to a white background; animals adapted to a b lack background showed no staining. Quantitative image analysis reveal ed that this effect of background adaptation is specific for suprachia smatic neurons because no effect could be demonstrated of the backgrou nd light condition on the ventral infundibular nucleus (immunocytochem istry) or the ventromedial thalamic nucleus (in situ hybridization). T hese results indicate that neurons in the suprachiasmatic nucleus enab le the adaptation of X. laevis to a white background, by producing and releasing neuropeptide Y that inhibits the release of melanophore-sti mulating hormone from the melanotrope cells in the pars intermedia of the pituitary gland.