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Abstract
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 innervating the
pars intermedia. To determine the production site of neuropeptide Y involved in this
inhibitory control, the distribution of neuropeptide Y in the brain has been investigated
by immunocytochemistry and in situ hybridization. Immunoreactive cell bodies were
visualized in, among others, the ventromedial and posterior thalamic nuclei, and the
suprachiasmatic and ventral infundibular hypothalamic nuclei. A positive hybridization
signal with a Xenopus-specific probe for preproneuropeptide 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
black background showed no staining. Quantitative image analysis revealed that this
effect of background adaptation is specific for suprachiasmatic neurons because no
effect could be demonstrated of the background light condition on the ventral infundibular
nucleus (immunocytochemistry) or the ventromedial thalamic nucleus (in situ hybridization).
These results indicate that neurons in the suprachiasmatic nucleus enable the adaptation
of X. laevis to a white background, by producing and releasing neuropeptide Y that
inhibits the release of melanophore-stimulating hormone from the melanotrope cells
in the pars intermedia of the pituitary gland.