Distribution of pro-opiomelanocortin and its peptide end products in the brain and hypophysis of the aquatic toad, Xenopus laevis.

Using in situ hybridization with a pro-opiomelanocortin (POMC)-mRNA probe and immunocytochemistry with antisera to POMC and to various POMC-derived peptides, it is shown that melanotrope cells in the pars intermedia of the hypophysis of the South African aquatic toad Xenopus laevis contain POMC, alpha-melanophore-stimulating hormone (alpha-MSH), gamma-MSH, acetylated and non-acetylated endorphins and adrenocorticotropic hormone (ACTH). With the exception of gamma-MSH, these peptides are also found in the corticotrope cells in the rostral pars distalis. In the Xenopus brain, neuronal cell bodies in the ventral hypothalamic nucleus express POMC, alpha-MSH, gamma-MSH, non-acetylated endorphins and ACTH, neurones in the anterior preoptic area reveal POMC, alpha-MSH, gamma-MSH and non-acetylated endorphin, neurones in the suprachiasmatic nucleus contain alpha-MSH, non-acetylated endorphin and ACTH and neurones in the posterior tubercle show alpha-MSH, non-acetylated endorphin and ACTH immunoreactivities. In the locus coeruleus POMC and ACTH coexist, whereas alpha-MSH and non-acetylated endorphin occur together in the nucleus accumbens, the striatum and the nucleus of the paraventricular organ. Finally, alpha-MSH alone is present in the olfactory bulb, the medial septum, the medial and lateral parts of the amygdala, the ventromedial and posterior thalamic nuclei, the optic tectum and the anteroventral tegmental nucleus, and non-acetylated endorphin alone appears in the epiphysis. It is suggested that neurones that form POMC-derived peptides may play a direct or indirect role in the control of POMC-producing hypophyseal cells and/or in the physiological processes these endocrine cells regulate. This idea is supported by the fact that the suprachiasmatic nucleus and the locus coeruleus, both involved in melanotrope cell control, show POMC and POMC-peptide expression. A possible involvement in melanotrope and/or corticotrope control of the anterior preoptic and ventral hypothalamic nuclei, which both express POMC and various POMC-derived peptides, deserves future attention.