Dynamics of proopiomelanocortin and prohormone convertase 2 gene expression in Xenopus melanotrope cells during long-term background adaptation.

The toad Xenopus laevis is able to adapt its skin color to background light intensity. In this neuroendocrine reflex, the proopiomelanocortin (POMC)-derived peptide alpha-melanophore-stimulating hormone (alphaMSH) is a key regulatory factor. In animals adapting to a black background, release of alphaMSH from the pituitary pars intermedia causes dispersal of melanin in skin melanophores. To investigate the long-term in vivo dynamics of alphaMSH production during black background adaptation, the biosynthetic rate of POMC and the contents of POMC, alphaMSH and the POMC processing enzyme precursor convertase 2 (PC2) have been studied in the pars intermedia using pulse-labeling, Western blot and radioimmunoassay. In control animals, adapted to a white background, the rate of POMC biosynthesis and the POMC content were low, while high alphaMSH and PC2 contents were found. After 1 week of adaptation to a black background, the rate of POMC biosynthesis and the POMC protein content had increased 19- and 3.7-fold respectively. These parameters attained a maximum level (28- and 5. 8-fold higher than control) after 3 weeks and remained at these elevated levels for at least 12 weeks. After 1 week, the pars intermedia content of alphaMSH was only 30% of the control level, but after 6 and 12 weeks, the alphaMSH level had increased to the control level. The PC2 content decreased to 52% of control after 1 week and stabilized after 3 weeks at a level slightly lower than the control value. The results show that during long-term background adaptation a steady-state situation is reached, with a balance between the biosynthesis, enzymatic processing and release of alphaMSH. The in vivo dynamics of the processing enzyme PC2 suggest a parallel storage and release of alphaMSH and mature PC2 in the Xenopus pituitary pars intermedia.

Specific expression of secretogranin II in magnocellular vasopressin neurons of the rat supraoptic and paraventricular nucleus in response to osmotic stimulation.

As secretogranin II is considered to be a marker for the regulated secretory pathway, its distribution in the hypothalamo-neurohypophyseal system of salt-loaded Wistar rats was studied in detail by immunocytochemistry. Although after an osmotic challenge both vasopressin and oxytocin neurons are stimulated, secretogranin II was exclusively expressed in a subpopulation of vasopressinergic magnocellular neurons in the supraoptic and paraventricular nucleus of Wistar rats. Secretogranin II was only surely visualized after a combination of osmotic challenge and blockade of axonal transport by colchicine treatment. When these pre-treatments were not performed, only punctate fibers situated around the magnocellular neurons within the paraventricular and supraoptic nucleus were observed. Oxytocinergic magnocellular neurons never displayed any secretogranin II immunoreactivity, not even during lactation and after colchicine treatment. These findings suggest that secretogranin II is of functional importance during enhanced secretory activity within vasopressinergic neurons.

Sauvagine and TRH differentially stimulate proopiomelanocortin biosynthesis in the Xenopus laevis intermediate pituitary.

In the amphibian Xenopus laevis, adaptation of the skin color to background light intensity is regulated by alpha-melanophore-stimulating hormone (alpha-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 alpha-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 alpha-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.

Differential action of secreto-inhibitors on proopiomelanocortin biosynthesis in the intermediate pituitary of Xenopus laevis.

In the South African clawed toad Xenopus laevis, background adaptation is regulated by alpha MSH, a POMC-derived peptide. After transfer of the animal from a black to a white background, secretion of alpha MSH from the intermediate pituitary lobe is inhibited by the hypothalamic neurotransmitter neuropeptide Y (NPY). The neurointermediate lobe in vitro is also subject to inhibitory regulation by dopamine and gamma-aminobutyric acid (GABA). In the nerve terminals contacting the intermediate lobe of the pituitary, GABA is contained in electron-lucent vesicles, whereas dopamine and NPY coexist in electron-dense vesicles. To study the role of these secreto-inhibitors in the regulation of POMC biosynthesis, the rate of incorporation of radioactive amino acids into POMC protein was determined after in vitro treatment of the neurointermediate pituitary with NPY, apomorphine (dopamine D2 receptor agonist), isoguvacine (GABAA receptor agonist) and baclofen (GABAB receptor agonist). After 24 h of treatment, inhibition of POMC biosynthesis by NPY and apomorphine was 77% and 74%, respectively. Isoguvacine treatment resulted in an inhibition of 59%, whereas no significant effect of baclofen was observed. When neurointermediate lobes were treated for 3 days, inhibition of POMC biosynthesis by NPY was maintained, and inhibition by apomorphine was even stronger, whereas isoguvacine gave an inhibition of 52%, and baclofen produced 34% inhibition. Superfusion experiments on alpha MSH secretion showed that prolonged treatment with the GABA receptor agonists results in a desensitization of GABA receptor-mediated signal transduction mechanisms, whereas the NPY receptor does not show desensitization. The observations indicate differential actions of the secreto-inhibitors NPY, apomorphine, and GABA agonists on POMC biosynthesis in the Xenopus intermediate pituitary, suggesting a major role for dopamine and NPY, whereas GABA, acting via two receptor types, does not seem to have a major function in long term control of POMC biosynthesis.

Differential effects of chronic agonist administration on mu-opioid receptor- and muscarinic receptor-regulated adenylate cyclase in rat striatal neurons.

In cultured rat striatal neurons exposed to 10 microM morphine or oxotremorine for 24 hours, we observed an increased (about 30%) dopamine D1 receptor-stimulated cyclic AMP production, whereas no desensitization of mu-opioid receptor or muscarinic cholinergic receptor was found. However, whereas upregulation of dopamine D1 receptor-stimulated adenylate cyclase activity upon 7 days morphine exposure was at least as pronounced as observed after 24 hours of exposure to the opioid, this adaptive phenomenon was virtually absent following one week of oxotremorine treatment. This reduced adenylate cyclase sensitization upon 7 days oxotremorine exposure appeared to coincide with desensitization of muscarinic cholinergic receptors. A possible role of the resistance of mu receptors to desensitization and the (resulting) upregulation of the neuronal adenylate cyclase system upon chronic receptor activation in the development of opiate tolerance and dependence is suggested.