Sauvagine and TRH Differentially Stimulate Proopiomelanocortin Biosynthesis in the <i>Xenopus laevis </i>Intermediate Pituitary

Abstract

In the amphibian Xenopus laevis, adaptation of the skin color to background light intensity is regulated by α-melanophore-stimulating hormone (α-MSH), a proopiomelanocortin (POMC)-derived peptide. In animals 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 α-MSH from neurointermediate lobes of white-adapted animals is stimulated in vitro by the regulatory peptides sauvagine and thyrotropin-releasing hormone (TRH), which are produced in the magnocellular nucleus of the hypothalamus. To study the role of sauvagine, cAMP, TRH and phorbol 12-myristate 13-acetate (PMA) in the regulation of POMC biosynthesis, the degree of incorporation of radioactive amino acids into the POMC protein was determined after treatment of the neurointermediate lobes with these secretagogues. When lobes of white-adapted animals are incubated in vitro, biosynthetic activity spontaneously increases because hypothalamic inhibitory control is removed by dissection. In addition to this control situation, the effects of secretagogues were tested on lobes with an inhibited level of biosynthesis, which is achieved by addition of neuropeptide Y (NPY) to 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 the stimulation of POMC biosynthesis by TRH. Sauvagine partially restored POMC biosynthesis from 27.2 to 62.5% of control level, whereas 8-Br-cAMP completely counteracted NPY inhibition from 27.8 to 97.5% of control 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 stimulated 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 still functional as was shown in superfusion experiments where TRH stimulated α-MSH secretion. The observations indicate that the neuropeptides sauvagine and TRH differently control POMC biosynthesis in the Xenopus intermediate pituitary. This differential regulation is not only apparent with regard to time aspects (sauvagine has a sustained regulatory function, whereas TRH is only effective in the initial phase of POMC biosynthesis stimulation), but also an uncoupling of biosynthetic and release processes could be shown for TRH, which did not occur with sauvagine.

Neuronal, Neurohormonal, and Autocrine Control of Xenopus Melanotrope Cell Activity

“…Thyroid‐stimulating hormone (TRH) stimulates POMC biosynthesis and αMSH release from the Xenopus melanotrope in vitro 46,49. Like CRH, it is produced in the Mg, and it is assumed to be released from neurohemal nerve terminals in the pituitary neural lobe to act on the melanotrope cells 5.…”

Melanotrope secretory cycle is regulated by physiological inputs via the hypothalamus

“…Having established that hypothalamic inhibitors can mimic in vitro the cellular changes that occur during white-background adaptation, we next asked whether TRH, the major hypothalamic stimulator for amphibian melanotrope cells (12,18,28,43), could induce the cellular changes observed during adaptation of frogs to a black background [i.e., conversion of storage cells into secretory cells (39,40)]. Indeed, in the melanotrope population of white-adapted frogs, which comprises similar proportions of both cell subtypes (39,40), longterm in vitro exposure to TRH markedly increased the number of secretory melanotropes, whereas it concurrently decreased that of hormone storage cells.…”

“…12), it is conceivable that hypothalamic neurohormones modulate the timing of this secretory cycle. Accordingly, the aim of the present study has been to ascertain whether known hypothalamic regulators of the frog intermediate lobe, such as the inhibitors dopamine (12,15,17,42) and neuropeptide Y (NPY) (3, 12-15, 17, 34, 41) and the stimulator thyrotropin-releasing hormone (TRH) (12,18,28,43), are able to induce the interconversion of cells from the two frog melanotrope subpopulations as well as to analyze the relationship between the control of the dynamics of the melanotrope secretory cycle and background color adaptation.…”

Orexis, Anorexia, and Thyrotropin-Releasing Hormone

“…AGRP mRNA levels in the hypothalamus of the ob/ob mice are increased eightfold (18), which means that leptin may normally be a suppressor of AGRP. TRH has been found to stimulate a-MSH release from the neurointermediate lobe of the amphibian Xenopus laevis (19), thus providing indirect evidence for a potential relevance of TRH to opioid-modulated orexigenic effects. However, dynorphin-A, an opioid peptide acting via the k receptor, has also been found to antagonize the MC4 receptor (20).…”