Functional organization of the suprachiasmatic nucleus of Xenopus laevis in relation to background adaptation

The process of background adaptation in the toad Xenopus Laevis is controll ed by neurons in the suprachiasmatic nucleus (SC) that inhibit the release of ol-melanophore-stimulating hormone from the neuroendocrine melanotrope c ells in the pituitary gland. We have identified the structural and function al organization of different neuropeptide Y (NPY)-containing cell groups in the Xenopus SC in relation to background adaptation. A ventrolateral, a do rsomedial, and a caudal group were distinguished, differing in location as well as in number, size, and shape of their cells. They also show different degrees of NPY immunoreactivity in response to different background adapta tion conditions. In situ hybridization using a Xenopus mRNA probe for the e xocytosis protein DOC2 revealed that melanotrope cells of black-adapted ani mals have a much higher expression of DOC2-mRNA than white-adapted ones. Th is establishes that the degree of DOC2-mRNA expression is a good parameter to measure cellular secretory activity in Xenopus. We show that in the vent rolateral SC group, more NPY-positive neurons express DOC2-mRNA in white-th an in black-adapted animals. In contrast, NPY-positive neurons in the dorso medial group have a high secretory activity under the black-adaptation cond ition. We propose that in black-adapted animals, NPY-positive neurons in th e ventrolateral group, known to inhibit the melanotrope cells in white-adap ted animals synaptically, are inhibited by NPY-containing interneurons in t he dorsmedial group. NPY-positive neurons in the caudal group have similar secretory dynamics as the dorsomedial NPY neurons, indicating that they als o play a role in background adaptation, distinct from that exerted by the v entrolateral and dorsomedial group. (C) 2001 Wiley-Liss, Inc.