SAUVAGINE AND TRH DIFFERENTIALLY STIMULATE PROOPIOMELANOCORTIN BIOSYNTHESIS IN THE XENOPUS-LAEVIS INTERMEDIATE PITUITARY

In the amphibian Xenopus laevis, adaptation of the skin color to backg round light intensity is regulated by alpha-melanophore-stimulating ho rmone (alpha-MSH), a proopiomelanocortin (POMC)-derived peptide. In an imals adapted to a white background, the level of POMC biosynthesis in the intermediate pituitary is much lower than in animals adapted to a black background. Release of alpha-MSK from neurointermediate lobes o f white-adapted animals is stimulated in vitro by the regulatory pepti des sauvagine and thyrotropin-releasing hormone (TRH), which are produ ced in the magnocellular nucleus of the hypothalamus. To study the rol e of sauvagine, cAMP, TRH and phorbol 12-myristate 13-acetate (PMA) in the regulation of POMC biosynthesis, the degree of incorporation of r adioactive amino acids into the POMC protein was determined after trea tment of the neurointermediate lobes with these secretagogues. When lo bes of white-adapted animals are incubated in vitro, biosynthetic acti vity spontaneously increases because hypothalamic inhibitory control i s removed by dissection. In addition to this control situation, the ef fects of secretagogues were tested on lobes with an inhibited level of biosynthesis, which is achieved by addition of neuropeptide Y (NPY) t o the incubation medium. After 24 h of treatment, TRH stimulated POMC biosynthesis in NPY-inhibited lobes of white-adapted animals from 40.2 to 95.3 % of control level. This stimulation could not be reduced by adding PMA, which indicates that protein kinase C is not involved in t he stimulation of POMC biosynthesis by TRH. Sauvagine partially restor ed POMC biosynthesis from 27.2 to 62.5 % of control level, whereas 8-B r-cAMP completely counteracted NPY inhibition from 27.8 to 97.5% of co ntrol level. After 3 days of treatment, stimulation by sauvagine and 8 -Br-cAMP was maintained (sauvagine increased POMC biosynthesis in NPY- inhibited lobes from 7.4 to 36.2% of control level and 8-Br-cAMP stimu lated from 6.5 to 82.5 % of control level). TRH had no effect on POMC biosynthesis after 3 days of treatment, although its receptor was stil l functional as was shown in superfusion experiments where TRH stimula ted alpha-MSH secretion. The observations indicate that the neuropepti des sauvagine and TRH differently control POMC biosynthesis in the Xen opus intermediate pituitary. This differential regulation is not only apparent with regard to time aspects (sauvagine has a sustained regula tory function, whereas TRH is only effective in the initial phase of P OMC biosynthesis stimulation), but also an uncoupling of biosynthetic and release processes could be shown for TRK, which did not occur with sauvagine.