N alpha-acetylation is linked to alpha-MSH release from pars intermedia of the amphibian pituitary gland

α-TC1.9 cells--a model system for analyzing the endoproteolytic processing of POMC

The mouse α-TC1.9 endocrine cell line was used to analyze the amino acid requirements for endoproteolytic processing at the paired basic amino acid cleavage site, K(141)R(142) that is N-terminal to the ACTH sequence in the POMC proprotein of the anuran amphibian, Silurana tropicalis. Real-Time PCR analysis of non-transfected α-TC1.9 cells indicated that these cells endogenously express the pc2 (proprotein convertase 2) gene, but do not express the pc1/3 (proprotein convertase 1/3) gene or the pomc gene. In addition, immunocytochemical analysis and RIA analysis of non-transfected α-TC1.9 cells did not detect the presence of POMC-related products in these cells. For this study the open reading frame of a S. tropicalis POMC cDNA (wild-type) was placed into an expression vector and transiently transfected into α-TC1.9 cells. Two days after transfection the steady-state levels of α-MSH-related and β-endorphin-related end-products were nearly the same as the steady-state levels of these POMC-related end-products in extracts of the S. tropicalis intermediate pituitary. Transient transfection of either the R(142)/A(142)pomc construct or the K(141)/A(141)pomc construct completely blocked cleavage at this site and yielded a 6K immunoreactive product that had the ACTH(1-13)NH(2) sequence at the C-terminal end of the fusion protein. However, substitution of an alanine residue at R(137), Q(138), E(139), and N(140) had no effect on cleavage at the K(141)R(142) cleavage site. Collectively, these results indicate that secondary structure N-terminal to the K(141)R(142) does not appear to influence cleavage at this site. However, both K(141) and R(142) are required for the integrity of this cleavage site. Finally, this study indicates that α-TC1.9 cells should be useful for studying the amino acid requirements for the other endoproteolytic cleavage sites in the S. tropicalis POMC proprotein.

Analysis of the melanotrope cell neuroendocrine interface in two amphibian species, Rana ridibunda and Xenopus laevis: a celebration of 35 years of collaborative research

This review gives an overview of the functioning of the hypothalamo-hypophyseal neuroendocrine interface in the pituitary neurointermediate lobe, as it relates to melanotrope cell function in two amphibian species, Rana ridibunda and Xenopus laevis. It primarily but not exclusively concerns the work of two collaborating laboratories, the Laboratory for Molecular and Cellular Neuroendocrinology (University of Rouen, France) and the Department of Cellular Animal Physiology (Radboud University Nijmegen, The Netherlands). In the course of this review it will become apparent that Rana and Xenopus have, for the most part, developed the same or similar strategies to regulate the release of α-melanophore-stimulating hormone (α-MSH). The review concludes by highlighting the molecular and cellular mechanisms utilized by thyrotropin-releasing hormone (TRH) to activate Rana melanotrope cells and the function of autocrine brain-derived neurotrophic factor (BDNF) in the regulation of Xenopus melanotrope cell function.

About a snail, a toad, and rodents: animal models for adaptation research

Neural adaptation mechanisms have many similarities throughout the animal kingdom, enabling to study fundamentals of human adaptation in selected animal models with experimental approaches that are impossible to apply in man. This will be illustrated by reviewing research on three of such animal models, viz. (1) the egg-laying behavior of a snail, Lymnaea stagnalis: how one neuron type controls behavior, (2) adaptation to the ambient light condition by a toad, Xenopus laevis: how a neuroendocrine cell integrates complex external and neural inputs, and (3) stress, feeding, and depression in rodents: how a neuronal network co-ordinates different but related complex behaviors. Special attention is being paid to the actions of neurochemical messengers, such as neuropeptide Y, urocortin 1, and brain-derived neurotrophic factor. While awaiting new technological developments to study the living human brain at the cellular and molecular levels, continuing progress in the insight in the functioning of human adaptation mechanisms may be expected from neuroendocrine research using invertebrate and vertebrate animal models.

Roles of acetylation and other post-translational modifications in melanocortin function and interactions with endorphins

Phylogenetic, developmental, anatomic, and stimulus-specific variations in post-translational processing of POMC are well established. For melanocortins, the role of alpha-N-acetylation and the selective activities of alpha, beta, and gamma forms are of special interest. Acetylation may shift the predominant activity of POMC products between endorphinergic and melanocortinergic actions-which are often in opposition. This review addresses: (1) variations in POMC processing; (2) the influence of acetylation on the functional activity of alpha-MSH; (3) state- and stimulus-dependent effects on the proportional distribution of forms of melanocortins and endorphins; (4) divergent effects of alpha-MSH and beta-endorphin administration; (5) potential roles of beta- and gamma-MSH.

Neuronal, Neurohormonal, and Autocrine Control ofXenopusMelanotrope Cell Activity

Amphibian pituitary melanotropes are used to investigate principles of neuroendocrine translation of neural input into hormonal output. Here, the steps in this translation process are outlined for the melanotrope cell of Xenopus laevis, with attention to external stimuli, neurochemical messengers, receptor dynamics, second-messenger pathways, and control of the melanotrope secretory process. Emphasis is on the pathways that neurochemical messengers follow to reach the melanotrope. The inhibitory messengers, dopamine, gamma-aminobutyric acid, and neuropeptide Y, act on the cells by synaptic input from the suprachiasmatic nucleus, whereas the locus coeruleus and raphe nucleus synaptically stimulate the cells via noradrenaline and serotonin, respectively. Autoexcitatory actions are exerted by acetylcholine, brain-derived neurotrophic factor (BDNF), and the calcium-sensing receptor. At least six messengers released from the pituitary neural lobe stimulate melanotropes in a neurohormonal way: corticotropin-releasing hormone, thyrotropin-releasing hormone, BDNF, urocortin, mesotocin, and vasotocin. They all are produced by the magnocellular nucleus and coexist in various combinations in two types of neurohemal axon terminal. Most of the relevant receptors of the melanotropes have been elucidated. Apparently, the neural lobe has a dominant role in activating melanotrope secretory activity. The intracellular mechanisms translating the various inputs into cellular activities like biosynthesis and secretion constitute the adenylyl cyclase-cAMP pathway and Ca(2+) in the form of periodic changes of the intracellular Ca(2+) concentration, known as Ca(2+) oscillations. It is proposed that the pattern of these oscillations encodes specific regulatory information and that it is set by first messengers that control, for example, via G proteins and cAMP-related events, specific ion channel-mediated events in the membrane of the melanotrope cell.

Melanotrope cells as a model to understand the (patho)physiological regulation of hormone secretion

Regulation of hormone secretion is a complex process that comprises the sequential participation of numerous subcellular mechanisms. Hormone secretion is dictated by extracellular stimuli that are transduced intracellularly into activation/deactivation of different mechanisms, such as hormone expression, processing and exocytosis, which will ultimately determine the precise availability of hormone to be secreted. Malfunction in any of these steps may result in deficient or excessive hormone release and the subsequent appearance of endocrine disorders. Given the complexity of this system, it is difficult to find appropriate cellular models wherein to investigate the multiple components of the secretory process in a physiologically relevant, experimentally manipulable setting. In this review, we present recent evidence on the use of the intermediate lobe (IL) of the pituitary as a powerful tool to understand different aspects of the regulated secretory pathway. IL is composed of a single endocrine cell type, alpha-melanocyte stimulating hormone (alpha-MSH)-producing melanotropes, a fact that greatly facilitates its study. Furthermore, melanotropes can be separated using classic cell separation techniques into two cell subtypes showing opposite morphophysiological phenotypes of hypo- and hypersecretory cells. Comparison of their gene expression fingerprints has unveiled the existence of certain genes preferentially expressed in each melanotrope subtype. Because of their direct participation in the secretory pathway, we postulate that characterization of these gene products in an endocrine cell type may represent novel and useful markers for reliably determining the general secretory status in an endocrine gland, as well as a valuable new tool to further investigate this complex process.

A cell-specific transgenic approach in Xenopus reveals the importance of a functional p24 system for a secretory cell

The p24alpha, -beta, -gamma, and -delta proteins are major multimeric constituents of cycling endoplasmic reticulum-Golgi transport vesicles and are thought to be involved in protein transport through the early secretory pathway. In this study, we targeted transgene overexpression of p24delta2 specifically to the Xenopus intermediate pituitary melanotrope cell that is involved in background adaptation of the animal and produces high levels of its major secretory cargo proopiomelanocortin (POMC). The transgene product effectively displaced the endogenous p24 proteins, resulting in a melanotrope cell p24 system that consisted predominantly of the transgene p24delta2 protein. Despite the severely distorted p24 machinery, the subcellular structures as well as the level of POMC synthesis were normal in these cells. However, the number and pigment content of skin melanophores were reduced, impairing the ability of the transgenic animal to fully adapt to a black background. This physiological effect was likely caused by the affected profile of POMC-derived peptides observed in the transgenic melanotrope cells. Together, our results suggest that in the early secretory pathway an intact p24 system is essential for efficient secretory cargo transport or for supplying cargo carriers with the correct protein machinery to allow proper secretory protein processing.

Multiple control and dynamic response of the Xenopus melanotrope cell

Some amphibian brain-melanotrope cell systems are used to study how neuronal and (neuro)endocrine mechanisms convert environmental signals into physiological responses. Pituitary melanotropes release alpha-melanophore-stimulating hormone (alpha-MSH), which controls skin color in response to background light stimuli. Xenopus laevis suprachiasmatic neurons receive optic input and inhibit melanotrope activity by releasing neuropeptide Y (NPY), dopamine (DA) and gamma-aminobutyric acid (GABA) when animals are placed on a light background. Under this condition, they strengthen their synaptic contacts with the melanotropes and enhance their secretory machinery by upregulating exocytosis-related proteins (e.g. SNAP-25). The inhibitory transmitters converge on the adenylyl cyclase system, regulating Ca(2+) channel activity. Other messengers like thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH, from the magnocellular nucleus), noradrenalin (from the locus coeruleus), serotonin (from the raphe nucleus) and acetylcholine (from the melanotropes themselves) stimulate melanotrope activity. Ca(2+) enters the cell and the resulting Ca(2+) oscillations trigger alpha-MSH secretion. These intracellular Ca(2+) dynamics can be described by a mathematical model. The oscillations travel as a wave through the cytoplasm and enter the nucleus where they may induce the expression of genes involved in biosynthesis and processing (7B2, PC2) of pro-opiomelanocortin (POMC) and release (SNAP-25, munc18) of its end-products. We propose that various environmental factors (e.g. light and temperature) act via distinct brain centers in order to release various neuronal messengers that act on the melanotrope to control distinct subcellular events (e.g. hormone biosynthesis, processing and release) by specifically shaping the pattern of melanotrope Ca(2+) oscillations.

Dynamics and plasticity of peptidergic control centres in the retino-brain-pituitary system of Xenopus laevis

This review deals particularly with the recent literature on the structural and functional aspects of the retino-brain-pituitary system that controls the physiological process of background adaptation in the aquatic toad Xenopus laevis. Taking together the large amount of multidisciplinary data, a consistent picture emerges of a highly plastic system that efficiently responds to changes in the environmental light condition by releasing POMC-derived peptides, such as the peptide alpha-melanophore-stimulating hormone (alpha-MSH), into the circulation. This plasticity is exhibited by both the central nervous system and the pituitary pars intermedia, at the level of molecules, subcellular structures, synapses, and cells. Signal transduction in the pars intermedia of the pituitary gland of Xenopus laevis appears to be a complex event, involving various environmental factors (e.g., light and temperature) that act via distinct brain centres and neuronal messengers converging on the melanotrope cells. In the melanotropes, these messages are translated by specific receptors and second messenger systems, in particular via Ca(2+) oscillations, controlling main secretory events such as gene transcription, POMC-precursor translation and processing, posttranslational peptide modifications, and release of a bouquet of POMC-derived peptides. In conclusion, the Xenopus hypothalamo-hypophyseal system involved in background adaptation reveals how neuronal plasticity at the molecular, cellular and organismal levels, enable an organism to respond adequately to the continuously changing environmental factors demanding physiological adaptation.

Three TRH-like molecules are released from rat hypothalamus in vitro

TRH-like immunoreactivity distinct from TRH is present in various tissues and fluids. In order to determine whether TRH-like molecules are secreted by the hypothalamus, we analyzed tissues and media from hypothalamic slices incubated in Krebs Ringer bicarbonate. Media from basal or high KCl conditions contained 3 TRH-like molecules evidenced by reverse phase high performance liquid chromatography followed by TRH radioimmunoassay. Peak I corresponded to authentic TRH (73% of total immunoreactivity) and peaks II and III had a higher retention time. These additional TRH-like forms were neither detected in hypothalamic tissue nor in tissue or medium from olfactory bulb. Gel filtration analysis of hypothalamic media revealed only one TRH-like peak eluting as TRH, suggesting that the molecular weights of peaks II and III are similar to that of TRH. Peak II retention time was similar to that of pglu-phe-proNH2. We analysed if they could be produced by post secretory metabolism of TRH. Incubation of hypothalamic slices with [3H-Pro]-TRH did not produce radioactive species comigrating with peaks II or III. However, it induced rapid degradation to [3H-Pro]-his-prodiketopiperazine ([3H]-HPDKP). Inhibitor profile suggested that pyroglutamyl aminopeptidase II, but not pyroglutamyl aminopeptidase I, is responsible for [3H]-HPDKP production. These data are consistent with the hypothesis that pyroglutamyl aminopeptidase II is the main aminopeptidase degrading TRH in hypothalamic extracellular fluid. Furthermore, we suggest that the hypothalamus releases additional TRH-like molecules, one of them possibly pglu-phe-proNH2, which may participate in control of adenohypophyseal secretions.

Analysis by mass spectrometry of POMC-derived peptides in amphibian melanotrope subpopulations

We have previously shown that the melanotrope population of the pituitary intermediate lobe of Rana ridibunda is composed of two subpopulations, of low (LD) and high density (HD), that show distinct ultrastructural features and display different synthetic and secretory rates. To investigate whether LD and HD melanotrope cells also differ in proopiomelanocortin (POMC) processing, we have analyzed the POMC-end products in single cells from both subpopulations by means of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The mass spectra revealed the presence of 8 POMC-derived peptides in HD and LD melanotrope cells, indicating a similar processing of the precursor in both subpopulations. However, the relative abundance of three POMC-end products (i.e. lys-gamma1-MSH, acetyl-alpha-MSH, and CLIP fragment) was higher in the HD subset. Moreover, two peptides with molecular weights of 1030 and 1818 Da, respectively, were detected that could not be assigned to any product deduced from the frog POMC sequence. The relative amount of the 1030 Da peptide was higher in LD melanotrope cells. Taken together, our results suggest that POMC processing is differentially regulated in the two melanotrope cell subsets.

Differential effects of dopamine on two frog melanotrope cell subpopulations

The frog intermediate lobe consists of a single endocrine cell type, the melanotrope cells, which are under the tonic inhibitory control of dopamine. Separation of dispersed pars intermedia cells in a Percoll density gradient has revealed the existence of two melanotrope cell subpopulations, referred to as high-density (HD) and low-density (LD) cells. The aim of the present study was to investigate the effects of dopamine on each of these melanotrope cell subsets. Increasing doses of dopamine, ranging from 10(-9)-10(-6) M, inhibited the release of alpha-melanocyte-stimulating hormone (alpha-MSH) in LD (but not in HD) melanotrope cells. In addition, dopamine provoked a significant reduction of the rate of acetylation of alpha-MSH in LD cells but not in HD cells. Similarly, dopamine significantly decreased the accumulation of POMC messenger RNA in LD cells, whereas it did not affect POMC gene expression in the HD melanotrope subset. On the other hand, microfluorimetric studies revealed that dopamine induced a significant reduction of KCl-stimulated cytosolic free calcium concentration in both LD and HD cells. The present study provides additional evidence for functional heterogeneity of melanotrope cells in the frog pars intermedia. Because dopamine plays a pivotal role in the regulation of alpha-MSH secretion, these data suggest the involvement of cell heterogeneity in the physiological process of background color adaptation in amphibians.

Background adaptation by Xenopus laevis: a model for studying neuronal information processing in the pituitary pars intermedia

This review is concerned with recent literature on the neural control of the pituitary pars intermedia of the amphibian Xenopus laevis. This aquatic toad adapts skin colour to the light intensity of its environment, by releasing the proopiomelanocortin (POMC)-derived peptide alpha-MSH (alpha-melanophore-stimulating hormone) from melanotrope cells. The activity of these cells is controlled by brain centers of which the hypothalamic suprachiasmatic and magnocellular nuclei, respectively, inhibit and stimulate both biosynthesis and release of alpha-MSH. The suprachiasmatic nucleus secretes dopamine, GABA, and NPY from synaptic terminals on the melanotropes. The structure of the synapses depends on the adaptation state of the animal. The inhibitory transmitters act via cAMP. Under inhibition conditions, melanotropes actively export cAMP, which might have a first messenger action. The magnocellular nucleus produces CRH and TRH. CRH, acting via cAMP, and TRH stimulate POMC-biosynthesis and POMC-peptide release. ACh is produced by the melanotrope cell and acts in an autoexcitatory feedback on melanotrope M1 muscarinic receptors to activate secretory activity. POMC-peptide secretion is driven by oscillations of the [Ca2+]i, which are initiated by receptor-mediated stimulation of Ca2+ influx via N-type calcium channels. The hypothalamic neurotransmitters and ACh control Ca2+ oscillatory activity. The structural and functional aspects of the various neural and endocrine steps in the regulation of skin colour adaptation by Xenopus reveal a high degree of plasticity, enabling the animal to respond optimally to the external demands for physiological adaptation.

Two frog melanotrope cell subpopulations exhibiting distinct biochemical and physiological patterns in basal conditions and under thyrotropin-releasing hormone stimulation

Cell heterogeneity designates the phenomenon by which a particular cell type is composed of morphologically and physiologically distinct cell subpopulations. We have previously isolated two subsets of melanotrope cells in the intermediate lobe of the frog pituitary by means of a separation procedure based on a Percoll density gradient High density (HD) melanotrope cells were found to exhibit a more granulated cytoplasm and a lower secretory rate than low density (LD) cells. In the present study, we have investigated the biochemical and functional characteristics of each melanotrope cell subpopulation by using various approaches, including chromatographic analysis for the measurement of the proportion of acetylated alpha MSH, microfluorimetric measurement of the cytosolic free calcium concentration ([Ca2+]i) and in situ hybridization for quantification of POMC messenger RNA (mRNA). Under basal conditions, LD melanotrope cells showed higher secretory activity, acetylation rate, [Ca2+]i, and POMC mRNA content compared to HD cells. Incubation of the cells with 100 nM TRH for 2 h induced a more pronounced activation of alpha MSH secretion, [Ca2+]i mobilization, and POMC mRNA accumulation in LD than in HD melanotrope cells. Conversely, TRH increased the rate of acetylation of alpha MSH in HD cells, but did not affect acetylation in LD cells. Taken together, these results demonstrate that the frog intermediate lobe is composed of two subsets of endocrine cells with distinct biochemical and functional characteristics. The coexistence of two cell subpopulations in the frog pars intermedia is consistent with the idea of a cell secretory cycle, in which each melanotrope subset represents a specific state of cellular activity.

Identification of POMC processing products in single melanotrope cells by matrix-assisted laser desorption/ionization mass spectrometry

The use of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) in identifying proopiomelanocortin (POMC) processing products in melanotrope cells of the pituitary intermediate lobe of Xenopus laevis was explored. Mass spectra were obtained with such a high sensitivity of detection that the peptides could be identified in a single melanotrope cell. In addition to known POMC processing products of the Xenopus melanotrope cell, the presence of previously unidentified POMC-derived peptides was demonstrated. Together these POMC processing products accounted for the entire length of the POMC precursor. Furthermore, Xenopus possesses two genes for POMC and the sensitivity and accuracy of the MALDI-MS technique allowed identification of processing products of both the POMCA and POMCB gene. In addition, differences were obtained between the mass spectra of melanotrope cells from Xenopus laevis adapted to different conditions of background illumination. These results show that MALDI-MS is a valuable tool in the study of the expression of peptides in single (neuroendocrine) cells.

POMC-related products in the intermediate pituitary of the amphibian, Bufo marinus: differential subcellular processing in the Golgi and secretory granules

In the intermediate pituitary of the anuran amphibian, Bufo marinus, the N-acetylation of ACTH(1-13)-NH2 to yield alpha-MSH occurs as a cosecretory processing event, whereas the N-acetylation of beta-endorphin occurs as a posttranslational processing event. To understand how these two N-acetylation reactions are segregated, B. marinus intermediate pituitary cells were analyzed by immunogold labeling electron microscopy, and by using an ultracentrifugation procedure. The immunogold labeling studies indicated that ACTH(1-13)-NH2-related immunoreactivity was colocalized with N-acetylated beta-endorphin-related immunoreactivity in secretory granules. Furthermore, ACTH(1-13)-NH2-related immunoreactivity was not detected in either the ER or the Golgi. N-Acetylated beta-endorphin-related immunoreactivity, however, was detected in the Golgi. Ultracentrifugation analysis revealed that in an ER/microsomal fraction, beta-LPH-sized and nonacetylated beta-endorphin-sized immunoreactive material were present in a molar ratio of 1:2. No N-acetylated forms of beta-endorphin were detected in the ER/microsomal fraction. In a Golgi/secretory granule fraction, the molar ratio of beta-LPH to beta-endorphin was 1:9 with 58% of the beta-endorphin being N-acetylated. Collectively, these data support the following hypotheses. The proteolytic cleavage of ACTH (1-39) to yield ACTH (1-13)-NH2 is a late processing event occurring in secretory granules. The cleavage of beta-LPH to yield nonacetylated beta-endorphin is an early processing event that may occur in the ER or the Golgi. Because N-acetylated beta-endorphin and nonacetylated ACTH(1-13)-NH2 are colocalized in secretory granules, it appears, therefore, that the N-acetylation of beta-endorphin is completed prior to loading into secretory granules. Thus, there is a spatial and temporal separation of the posttranslational processing events associated with the beta-LPH portion and ACTH portion of the POMC biosynthetic pathway in amphibian intermediate pituitary cells.

Effects of background adaptation on alpha-MSH and beta-endorphin in secretory granule types of melanotrope cells of Xenopus laevis

Placing the clawed toad Xenopus laevis on a black background stimulates the melanotrope cells in the pars intermedia of the pituitary gland to release proopiomelanocortin (POMC)-derived peptides, including alpha-MSH and N-acetyl-beta-endorphin. In this study three types of secretory granules, electron-dense (approximately 130 nm phi), moderately electron-dense (approximately 160 nm phi) and electron-lucent (approximately 180 nm phi), have been identified in these cells. Apparently, only dark granules are formed by the Golgi apparatus and lucent granules release their contents via exocytosis. Immuno-electron microscopy (immunogold double labelling) of glutaraldehyde-fixed and freeze-substituted material shows that desacetyl-alpha-MSH and N-acetyl-beta-endorphin coexist in all three granule types. Quantification of immunostaining revealed that immunoreactivities to these peptides are lowest in the dark granules and highest in the light ones. It is proposed that intragranular processing of POMC to immunoreactive desacetyl-alpha-MSH and N-acetyl-beta-endorphin involves an increase in granule size and a decrease in granule electron density. Black background-induced activation of the melanotrope cell is reflected by an increase in immunoreactivity of the secretory granules to each of the antisera. This suggests that cell activation stimulates the formation of peptides by intragranular processing of POMC and/or of intermediate POMC-processing products. In addition, cell activation evoked an increase in the percentage of the granule population that reacts with anti-N-acetyl-beta-endorphin, probably by stimulating intragranular acetylation of beta-endorphin. Apparently, this acetylation is a regulated event that occurs in the cytoplasm, independently from the acetylation of desacetyl-alpha-MSH which takes place near the plasmalemma at the time of granule exocytosis.

Alpha-melanocyte-stimulating hormone (alpha-MSH) in the brain of the African lungfish, Protopterus annectens: immunohistochemical localization and biochemical characterization

The distribution of alpha-melanocyte-stimulating hormone (alpha-MSH) containing neurons and the molecular forms of alpha-MSH-related peptides exhibit substantial differences in the brains of fish and amphibians. Lungfishes, which share similarities with both fishes and tetrapods, represent a valuable group in which to investigate the neuroanatomical and neurochemical facets of evolution. In the present study, we have localized and characterized alpha-MSH-immunoreactive peptides in the central nervous system of the African lungfish Protopterus annectens. Perikarya exhibiting alpha-MSH-like immunoreactivity were observed in two distinct regions of the hypothalamus: the rostral part of the preoptic nucleus and the caudal part of the hypothalamus. In the caudal hypothalamus most alpha-MSH-immunopositive perikarya were located in both the subependymal and deepest layers of the ventral periventricular region. Scattered alpha-MSH-immunopositive cells were occasionally detected in the dorsal side of the caudal hypothalamus. The alpha-MSH-immunoreactive material localized in the brain was characterized by combining high-performance liquid chromatography (HPLC) analysis and radioimmunological detection. The displacement curves obtained with synthetic alpha-MSH and serial dilutions of brain and pituitary extracts were parallel. HPLC analysis of lungfish hypothalamic extracts showed that the major immunoreactive peak coeluted with synthetic desacetyl alpha-MSH and its sulfoxide derivative. An additional peak coeluted with synthetic sulfoxide alpha-MSH. In contrast, in the pituitary, the predominant form of alpha-MSH-like material coeluted with the N,O-diacetyl alpha-MSH standard. These results provide the first evidence for the presence of alpha-MSH-related peptides in the brain of a lungfish. The distribution of alpha-MSH neuronal systems in the lungfish is very similar to that reported in amphibians, supporting the existence of phylogenetic convergences between these two vertebrate groups.

Detection and partial characterization of proopiomelanocortin-related end-products from the pars intermedia of the toad, Bombina orientalis

Steady-state analyses were performed on the proopiomelanocortin (POMC)-related end-products present in acid extracts of the pars intermedia of the anuran amphibian, Bombina orientalis. Sephadex G-75 gel filtration chromatography indicated that immunoreactive alpha-MSH-sized material and N-acetylated beta-endorphin-related material are the major POMC-related products present in this tissue. The alpha-MSH-sized immunoreactivity was further fractionated by reversed phase HPLC. The major peak of immunoreactivity isolated by this procedure eluted with the same retention time as synthetic ACTH(1-13)amide. Cation exchange chromatography supported the conclusion that the major storage form of alpha-MSH in the pars intermedia of Bombina is ACTH(1-13)amide. Analysis of Bombina pars intermedia in culture indicated that mono-acetylated and di-acetylated alpha-MSH were the major forms of alpha-MSH secreted into the medium. The major peak of N-acetylated beta-endorphin-related material was further analyzed by cation exchange chromatography and Sephadex G-25 gel filtration column chromatography. The major storage form of beta-endorphin in this tissue is N-acetylated, has a net positive charge at pH 2.75 of +1, and has an apparent molecular weight of 1.2K. The beta-endorphin present in the pars intermedia of this tissue does not undergo further N-acetylation at the time of secretion. These results indicate that in the pars intermedia of the archaeobatrachian, Bombina orientalis, the N-acetylation of alpha-MSH is a cosecretory processing event, whereas N-acetylation of beta-endorphin is a post-translational processing event. These results are compared to other archaeobatrachian and neobatrachian pituitary POMC systems that have been analyzed.

Detection of a novel sequence change in the major form of alpha-MSH isolated from the intermediate pituitary of the reptile, Anolis carolinensis

Intermediate pituitaries of the reptile, Anolis carolinensis, were separately pulse labeled with [3H]Trp and [3H]Tyr. The major form of alpha-MSH was purified by immunoprecipitation and isolated by reverse phase HPLC. Tryptic peptide analysis indicated that the [3H]Trp-labeled C-terminal fragment of Anolis alpha-MSH had the same retention time as mammalian ACTH(9-13) amide; however, the [3H]Tyr-labeled N-terminal fragment did not coelute with either mammalian ACTH(1-8) or N-acetyl-ACTH(1-8). Purification of alpha-MSH from 76 Anolis intermediate pituitaries confirmed that a sequence change had occurred in the N-terminal region of Anolis alpha-MSH. The tissues were acid extracted and purified by Sephadex G-25 chromatography and reverse phase HPLC to yield 4.5 micrograms of purified Anolis alpha-MSH for amino acid composition analysis and automated Edman degradation sequence analysis. The major form of Anolis alpha-MSH is nonacetylated and has the following novel primary sequence: Ser-Tyr-Ala-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro(Val-amide). The presence of Val-amide was verified by immunological analysis, tryptic peptide analysis and amino acid composition analysis.

[The intermediate lobe of the pituitary, model of neuroendocrine communication]

The intermediate lobe of the pituitary is composed of a homogeneous population of endocrine cells, the melanotrophs, which secrete several bioactive peptides including alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. In contrast to most endocrine glands which are richly vascularized, the intermediate lobe of the pituitary contains very few blood vessels; in some species, the pars intermedia is virtually totally avascular. In contrast, pituitary melanotrophs are richly supplied by nerve fibers originating from the hypothalamus. The pars intermedia thus appears as a pure model of neuroendocrine communication, i.e. it is an archetype of the mode of transducing interface between the central nervous system and endocrine effectors. In mammalian species, different types of nerve terminals containing dopamine, norepinephrine, gamma-aminobutyric acid (GABA) and serotonin have been identified. In lower vertebrates, particularly in fish and amphibians, the pars intermedia is also innervated by peptidergic fibers which are though to take part in regulation of the secretory activity of the melanotroph. In these animals, the pars intermedia is regarded as a major center of neuroendocrine integration and an exceptional model to investigate the process of communication between the brain and the endocrine glands. The purpose of the present review is to summarize our current knowledge on the synthesis, processing and release of peptide hormones from pars intermedia cells and to survey the multiple regulatory mechanisms which are involved in the control of the activity of pituitary melanotrophs. Proopiomelanocortin, a multifunctional precursor. Pituitary melanotrophs synthetise a major precursor protein called proopiomelanocortin (POMC) which generates through proteolytic cleavage several biologically active peptides including adrenocorticotropic hormone (ACTH), endorphins and MSHs. In lower vertebrates, alpha-MSH is generally considered as the major hormone secreted by melanotrophs, in that it is involved in the process of skin colour adaptation. The post-translational processing of POMC, which yields to the mature hormones released by melanotrophs, includes a number of steps: glycosylation, phosphorylation, tissue-specific proteolytic cleavage, amidation and acetylation. Some of these posttranslational modifications can be regulated by neuroendocrine factors. For instance, in frogs, it has been shown that dopamine inhibits acetylation of alpha-MSH and thus reduces the secretion of the biologically active form of the peptide. The intermediate lobe of the pituitary: a model of neuroendocrine integration. In most vertebrate species, the intermediate lobe of the pituitary is innervated by catecholamine-containing fibers. In particular, the presence of dopaminergic nerve fibers has been observed in the pars intermedia of mammals and poikilotherms.(ABSTRACT TRUNCATED AT 400 WORDS)

Isoproterenol-stimulated release of beta-endorphin and related peptides from the rat pituitary neurointermediate lobe in vitro: evidence for preferential release of certain molecular forms of beta-endorphin

The intermediate lobe of the pituitary gland synthesizes the multifactorial precursor molecule pro-opiomelanocortin (POMC), from which, through a process of post-translational enzymatic processing, beta-endorphin-(1-31) (beta E) and a variety of N alpha-acetylated and C-terminally shortened forms of this peptide are generated. Using an in vitro superfusion system, the release of these endorphins from intact rat neurointermediate lobes (NILs) was investigated under basal and isoproterenol (ISO) stimulated conditions. Superfusion of NILs with the beta-adrenergic agonist ISO (30 min pulse) resulted in a rapid, sustained and concentration-dependent stimulation of the release of beta E-like immunoreactivity (beta E-IR) over basal as determined with an antiserum directed against the C-terminus of the beta E- (1-31) sequence (10(-6) M: + 145%; 10(-7) M: + 73%; 10(-8) m: + 41%). The release of N(alpha)-acetylated-endorphin-like immunoreactivity (AcE-IR) was stimulated to a similar extent. These effects of ISO were antagonized by the competitive alpha-adrenoceptor antagonist propranolol in a concentration-dependent manner, indicating the involvement of alpha-adrenoceptors. The beta-related peptides released from the NILs under basal and ISO-stimulated conditions were further characterized, based on their retention times in a reversed-phase HPLC system and their reactivity with specific antisera recognizing respectively the midportion of beta E, the N-terminus of acetylated endorphins, the C-terminus of tau-endorphin (beta E-(1-17); tau E), or the C-terminus of alpha-endorphin (beta E-(1-16); alpha E). In HPLC fractionated superfusates 10 peaks were resolved that reacted with the midportion beta E antiserum. In superfusates collected under basal conditions, three major peaks possessed chromatographical and immunological characteristics of Ac beta E-(1-26), Ac beta E- (1-27) Ac beta E-(1-31). In addition, a prominent peak was found eluting around the retention time of beta E-(1-31), that contained both acetylated and non-acetylated material. Six smaller peaks were observed, with the characteristics of beta E-(1-26) and beta E-(1-27) (these peptides were not resolved with the HPLC system used), Ac tau E, tau E, Aa alpha E, and des-tyrosine-alpha E (DT alpha E), respectively. In superfusates collected during superfusion of NILs with ISO (10(-6) M) all peaks were increased. However, those eluting as beta E-(1-31), beta E-(1-26)/beta E-(1-27), Ac beta E-(1-26) and Ac tau E appeared to be preferentially stimulated.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of chronic treatment of rats with dopamine receptor drugs on the post-translational processing of Beta-endorphin in the neurointermediate lobe of the pituitary gland

Abstract To investigate whether chronic changes in the activity of proopiomelanocortin cells in the neurointermediate lobe (NIL) of the pituitary gland are associated with changes in the enzymatic processing of beta-endorphin (betaE), the effects of treatment of rats with the dopamine receptor antagonist haloperidol or the dopamine receptor agonist bromocriptine (2.5 mg.kg(-1) sc, once daily for 21 days) were studied on the content of betaE-related peptides in the NIL and on the release of these peptides from NILs in an in vitro superfusion system. Treatment with haloperidol increased, and with bromocriptine decreased the tissue content and the release of N(alpha)-acetyl-, beta-, gamma- and alpha-endorphin-immunoreactivity (AcE-, betaE-, gammaE, and alphaE-IR). The endorphin-IR was further characterized using reversed-phase high-performance liquid chromatography and specific radioimmunoassay systems, and the following peptides were identified: des-tyrosine alpha-endorphin (DTalphaE), alphaE, AcalphaE, gammaE, AcgammaE, betaE-(1-31), AcbetaE-(1-31), AcbetaE-(1-27), AcbetaE-(1-26) and betaE-(1-26)/betaE-(1-27) (the latter peptides were not separated with the high-performance liquid chromatography system used). Analysis of NIL superfusates indicated that all peptides found in the tissue were released in vitro. In addition, an as yet unidentified acetylated IR-endorphin component was found which was not observed in extracts of NIL tissue, and therefore was probably formed during release. Following haloperidol treatment, the levels of all betaE-related peptides detected were increased in the tissues as well as superfusates, the increase in AcbetaE-(1-27) being most and that in betaE-(1-26)/betaE-(1-27) least pronounced. Following bromocriptine treatment, the concentrations of all peptides in tissues and superfusates were decreased as compared to vehicle controls. The acetylated endorphins, in particular AcbetaE-(1-27), were most affected and betaE-(1-26)/betaE-(1-27) least affected. The results indicate that chronic modulation of the synthesizing and secretory activity of proopiomelanocortin cells in the NIL is parallelled by changes in the enzymatic processing of betaE.

Detection of N-acetylated forms of beta-endorphin and nonacetylated alpha-MSH in the intermediate pituitary of the toad, Bufo marinus

Steady-state analysis of the acid extracts of the intermediate pituitary of the toad, Bufo marinus, revealed the presence of multiple forms of beta-endorphin and alpha-MSH. Approximately 98% of the immunoreactive beta-endorphin was N-acetylated. The major form of N-acetylated beta-endorphin, which represented 81.5% of the total beta-endorphin recovered from this tissue, had an apparent molecular weight of 1.2 kDa and a net charge of +1 at pH 2.75. Approximately 98% of the immunoreactive alpha-MSH present in the Bufo intermediate pituitary had reverse phase HPLC properties similar to the nonacetylated form of alpha-MSH, ACTH(1-13)amide. These observations are in agreement with studies on the intermediate pituitary of the frog, Xenopus laevis, which have shown that the N-acetylation of alpha-MSH in this species is a cosecretory processing event, whereas the N-acetylation of beta-endorphin is a posttranslational processing event (2, 5, 15). These observations indicate that the N-acetylation of beta-endorphin and alpha-MSH occurs at distinct subcellular sites in intermediate pituitary cells of anuran amphibians. The Bufo intermediate pituitary will serve as a good model system for studying these novel N-acetyltransferase reactions.

Characterization of alpha-melanocyte-stimulating hormone (alpha-MSH)-like peptides in discrete regions of the rat brain. In vitro release of alpha-MSH from perifused hypothalamus and amygdala

The neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) is synthesized by discrete populations of hypothalamic neurons which project in different brain regions including the cerebral cortex, hippocampus and amygdala nuclei. The purpose of the present study was to identify the alpha-MSH-immunoreactive species contained in these different structures and to compare the ionic mechanisms underlaying alpha-MSH release at the proximal and distal levels, i.e. within the hypothalamus and amygdala nuclei, respectively. The molecular forms of alpha-MSH-related peptides stored in discrete areas of the brain were characterized by combining high-performance liquid chromatography (HPLC) separation and radioimmunoassay detection. In mediobasal and dorsolateral hypothalamic extracts, HPLC analysis confirmed the existence of a major immunoreactive peak which co-eluted with the synthetic des-N alpha-acetyl alpha-MSH standard. In contrast, 3 distinct forms of immunoreactive alpha-MSH, which exhibited the same retention times as synthetic des-, mono- and di-acetyl alpha-MSH, were resolved in amygdala nuclei, hippocampus, cortex and medulla oblongata extracts. The proportions of acetylated alpha-MSH (authentic alpha-MSH plus diacetyl alpha-MSH) contained in these extrahypothalamic structures were, respectively, 78, 80, 60 and 92% of the total alpha-MSH immunoreactivity. In order to compare the ionic mechanisms underlaying alpha-MSH release from hypothalamic and extrahypothalamic tissues, we have investigated in vitro the secretion of alpha-MSH by perifused slices of hypothalamus and amygdala nuclei. High potassium concentrations induced a marked increase of alpha-MSH release from both tissue preparations. However, a higher concentration of KCl was required to obtain maximal stimulation of amygdala nuclei (90 mM) than hypothalamic tissue (50 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamics of background adaptation in Xenopus laevis: role of catecholamines and melanophore-stimulating hormone

The pars intermedia of the pituitary gland in Xenopus laevis secretes alpha-melanophore-stimulating hormone (alpha-MSH), which causes dispersion of pigment in dermal melanophores in animals on a black background. In the present study we have determined plasma levels of alpha-MSH in animals undergoing adaptation to white and black backgrounds. Plasma values of black-adapted animals were high and decreased rapidly after transfer to a white background, as did the degree of pigment dispersion in dermal melanophores. Plasma MSH values of white-adapted animals were below the detection limit of our radioimmunoassay. Transfer of white animals to a black background resulted in complete dispersion of melanophore pigment within a few hours, but plasma MSH levels remained low for at least 24 hr. This discrepancy between plasma MSH and degree of pigment dispersion suggested the involvement of an additional factor for stimulating dispersion. Results of in vitro and in vivo experiments with receptor agonists and antagonists indicated that a beta-adrenergic mechanism, functioning at the level of the melanophore, is involved in the stimulation of pigment dispersion during the early stages of background adaptation.

Characterization of alpha-MSH-related peptides released from rat hypothalamic neurons in vitro

Reverse-phase high-performance liquid chromatography analysis, coupled with a sensitive radioimmunoassay for alpha-melanocyte-stimulating hormone (alpha-MSH), was used to characterize the alpha-MSH-related peptides stored in the rat hypothalamus or released from perifused hypothalamic slices. Four peaks of alpha-MSH-like immunoreactivity (alpha-MSH-LI) co-eluting with synthetic des-N alpha-acetyl alpha-MSH, alpha-MSH and their respective sulfoxide derivatives were resolved and quantified. In hypothalamic extract, deacetyl alpha-MSH which was the predominant peptide represented 94.4% of total alpha-MSH-LI content, while the relative amount of alpha-MSH was only 5.6%. Analysis of alpha-MSH-related peptides contained in effluent perifusates showed that deacetyl alpha-MSH and its oxidized form were the major peptides released from neurons in basal conditions or under KCl-induced depolarization (50 mM KCl for 75 min). However, the proportion of acetylated peptide was 3-4 times higher in the perifusion medium than in hypothalamic extracts. Our data indicate that acetylation of des-N alpha-acetyl alpha-MSH may occur during the process of exocytosis. Since acetylation of alpha-MSH markedly increases the behavioural potency of the peptide, these results suggest that regulation of the acetyltransferase activity could be a key mechanism to modulate the bioactivity of alpha-MSH-related peptides in the brain.

Immunocytochemical Localization and Biochemical Characterization of alpha-Melanocyte-Stimulating Hormon in the Brain of the Rainbow Trout, Salmo gairdneri

Abstract The distribution of alpha-melanocyte-stimulating hormone (alpha-MSH)-like immunoreactivity in the central nervous system of the rainbow trout Salmo gairdneri was investigated by indirect immunofluorescence and peroxidase-antiperoxidase techniques, using a highly specific antiserum generated in rabbits against synthetic alpha-MSH. Immunoreactive perikarya were exclusively observed in the basal hypothalamus within the pars anterioris of the nucleus lateralis tuberis. In this region, a moderate number of small stained cell bodies were observed surrounding the dorsal wall of the anterior infundibular recess. These immunoreactive cells were organized in rostro-caudal rows extending over the whole portion of the nucleus. Positive fibres originating from these perikarya were visualized in the dorsal posterior lobe and the ventral hypothalamus. A dense tract of immunoreactive fibres projected ventrally through the pituitary stalk and terminated in the neurohypophysis. The concentrations of alpha-MSH in different regions of the brain were measured by means of a sensitive and specific radioimmunoassay. The dilution curves obtained with synthetic alpha-MSH and serial dilutions of diencephalon, mesencephalon, medulla oblongata, telencephalon or pituitary extracts were strictly parallel. The highest concentration of alpha-MSH in brain was found in the diencephalon (1.31 +/- 0.07 ng/mg protein). In contrast alpha-MSH was not detectable in cerebellar extracts. Reverse-phase high-performance liquid chromatography and radioimmunoassay were used to characterize alpha-MSH-like peptides in the trout brain and pituitary. Two major forms of immunoreactive alpha-MSH were resolved by high performance liquid chromatography in hypothalamic extracts; these peptides exhibited the same retention times as des-Na-acetyl alpha-MSH and its sulfoxide derivative, respectively. Additional peaks of alpha-MSH immunoreactive material were detected in pituitary extract. These latter peptides coeluted with authentic alpha-MSH, diacetyl alpha-MSH and their sulfoxide forms. These results provide the first evidence for the presence of alpha-MSH in the brain of a teleostean fish. Our data indicate that, in the brain, the immunoreactivity corresponds to the non-acetylated form of alpha-MSH, while three different types of alpha-MSH-like molecules (namely deacetylated, monoacetylated, and diacetylated forms) coexist in the pituitary. It thus appears that, in salmonoid fish, mono- or diacetylation of the N-terminal serine residue of aL-MSH only occurs at the pituitary level.

Immunological evidence for multiple forms of alpha-melanotropin (alpha-MSH) in the pars intermedia of the Australian lungfish, Neoceratodus forsteri

Acid extracts of individual pars intermedia from the Australian lungfish, Neoceratodus forsteri, were fractionated by gel filtration chromatography and analyzed for alpha-melanotropin (alpha-MSH) immunoreactivity. In these studies a C-terminal-specific alpha-MSH radioimmunoassay (RIA) was used. Following gel filtration chromatography on a Sephadex G-75 column, a major peak of immunoreactive alpha-MSH-sized material was detected. On the average there was 338 +/- 72 pmol (SD) of immunoreactive alpha-MSH per lungfish pars intermedia (n = 3). Following gel filtration the immunoreactive alpha-MSH was further analyzed by reverse-phase high-performance liquid chromatography (HPLC). Three peaks of immunoreactivity were detected. These peaks were designated Peaks 1, 2, and 3. The retention times of these peaks corresponded to, respectively, mammalian ACTH(1-13)amide, N-acetyl-ACTH(1-13)-amide, and N,O-diacetyl-ACTH(1-13)amide. Peaks 2 and 3 represented approximately 95% of the immunoreactive alpha-MSH recovered. Analysis of immunoreactive Peaks 2 and 3 by cation-ion-exchange indicated that both peaks had a net charge of +3 at pH 2.5. Since O-acetyl groups are sensitive to high pH, Peak 3 was incubated for 1 hr at 37 degrees in 0.01 N NaOH, pH 12. Under these conditions, Peak 3 eluted with the same retention time as untreated Peak 2. Collectively, these results indicate that Peaks 2 and 3 correspond to mono- and diacetylated lungfish alpha-MSH, respectively.

Alpha-melanocyte-stimulating hormone (alpha-MSH) in the brain of the cartilagenous fish. Immunohistochemical localization and biochemical characterization

The distribution of immunoreactive alpha-melanocyte-stimulating hormone (alpha-MSH) in the central nervous system and pituitary of the elasmobranch fish Scyliorhinus canicula was determined by the indirect immunofluorescence and the peroxidase-antiperoxidase methods using a highly specific antiserum. Perikarya containing alpha-MSH-like immunoreactivity were localized in the dorsal portion of the posterior hypothalamus, mainly in the tuberculus posterioris and sacci vasculosus nuclei. Immunoreactive alpha-MSH cell bodies were found in the dorsal wall and ventral region of the caudal part of the tuberculum posterioris. These structures were densely innervated by fine beaded immunoreactive fibers. Some alpha-MSH immunoreactive cells were occasionally detected in the ventral part of the nucleus periventricularis. Scattered cell bodies and fibers were also observed in the dorsal wall of the posterior recess. Outside the hypothalamus very few fibers were detected in the dorsal thalamus and mesencephalon. No immunoreactivity was found in any other parts of the brain. The alpha-MSH immunoreactive material localized in the brain was characterized by combining high-performance liquid chromatography (HPLC) analysis and radioimmunological detection. Brain and pituitary extracts exhibited displacement curves which were parallel to that obtained with synthetic alpha-MSH. The concentrations of alpha-MSH immunoreactive material were determined in 5 different regions of the brain. The highest concentration was found in the hypothalamus. HPLC analysis resolved two major forms of immunoreactive alpha-MSH in the hypothalamus, which had been same retention times as des-N alpha-acetyl-alpha-MSH and its sulfoxide derivative. These results provide the first evidence for the presence of alpha-MSH-like peptides in the fish brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of alpha-melanotropin from the pars intermedia of the larval amphibian, Ambystoma tigrinum

The effect of background adaptation on the steady-state levels of alpha-melanotropin in the pars intermedia of the larval amphibian. Ambystoma tigrinum, was investigated. Acid extracts of pars intermedia obtained from light-adapted and dark-adapted animals were analyzed by radioimmunoassay following Sephadex gel filtration chromatography, reverse-phase HPLC, and Sulfopropyl Sephadex cation-ion-exchange chromatography. For both background adaptation conditions similar results were obtained. The major form of alpha-melanotropin present in the pars intermedia has the following properties: (1) an apparent molecular mass of 1.5 kDa; (2) a net charge at pH 3.5 of +4; and (3) a retention time following reverse-phase HPLC similar to that of synthetic ACTH(1-13)amide. In dark-adapted animals a minor form of alpha-melanotropin which has a net charge of +3 at pH 3.5 was also detected. The latter form represented approximately 10% of the total alpha-melanotropin immunoreactivity in the pars intermedia of dark-adapted animals. These results strongly suggest that the predominant form of alpha-melanotropin in the pars intermedia of larval A. tigrinum is a nonacetylated ACTH(1-13)amide-like polypeptide.

Isolation of immunoreactive beta-endorphin-related and Met-enkephalin-related peptides from the posterior pituitary of the amphibian, Xenopus laevis

Acid extracts of the posterior pituitary of the amphibian, Xenopus laevis, were analyzed with two heterologous region specific beta-endorphin RIAs. Following gel filtration chromatography and cation exchange chromatography four peaks of immunoreactivity were detected. All four peaks were detected with a N-acetyl specific beta-endorphin RIA. Peak I represented 92% of the total immunoreactivity isolated following cation exchange chromatography. This peak had a net positive charge at pH 2.5 of +1 and an apparent molecular weight of 1.4 Kd. Following reverse phase HPLC, Peak I fractionated into two peaks: Peak Ia and Peak Ib. Both peaks were detected with the N-acetyl specific beta-endorphin RIA and a Met-enkephalin RIA, however, neither peak co-migrated with either Met-enkephalin or N-acetyl-beta-endorphin(1-16). At present it is not clear whether Peak I is derived from pro-opiomelanocortin or one of the other opioid polyproteins. Peaks II, III, and IV represented 8% of the total immunoreactivity recovered following cation exchange chromatography. These peaks had net positive charges of +3, +4, and +5, respectively, and apparent molecular weights of 2.8, 3.2, and 3.5 Kd, respectively. These apparently N-acetylated beta-endorphin-sized forms are minor end products of the pro-opiomelanocortin biosynthetic pathway.

Steady-state analysis of alpha-melanotropin in the pars intermedia of Anolis carolinensis: effect of background adaptation

The steady-state levels of alpha-melanotropin-stimulating hormone (alpha-MSH)-related peptides were examined in the pars intermedia of the reptile Anolis carolinensis as a function of background adaptation. After a 7-day period, the content of immunoreactive alpha-MSH-related material in the pars intermedia of light-adapted animals was approximately fourfold higher than that of animals maintained on a dark background for the same period. The immunoreactive alpha-MSH-related material present in the pars intermedia of light-adapted and dark-adapted animals was separately analyzed by gel filtration chromatography, reverse-phase HPLC, and cation-exchange chromatography. For light-adapted animals the major form of alpha-MSH had an apparent molecular weight of 1.5 kDa and a net charge of +4 at pH 3.5. Following reverse-phase HPLC this material eluted as a single peak of immunoreactivity with a retention time distinct from that of both mammalian ACTH(1-13)amide and N-acetyl-ACTH(1-13)amide. For dark-adapted animals a peak of alpha-MSH-sized material with an apparent molecular weight of 1.5 kDa was also detected. Following reverse-phase HPLC analysis this material eluted as an apparent single peak of immunoreactivity with a retention time distinct from that of the mammalian standards. Subsequent analysis of this major HPLC peak by cation-exchange chromatography revealed the presence of at least two forms of immunoreactive alpha-MSH. These forms differed in relative proportions. The major peak of immunoreactivity had a net charge of +4, whereas the minor peak had a net charge of +3. The +3 immunoreactive form was not detected to any appreciable degree in light-adapted animals.

GABA and dopamine act directly on melanotropes of Xenopus to inhibit MSH secretion

The release of melanophore stimulating hormone (MSH) from the pars intermedia of the amphibian Xenopus laevis is regulated by multiple factors of hypothalamic origin. The aim of this study was to determine if potential secretagogues function through a direct action on the melanotrope cell. For this purpose an in vitro superfusion system containing isolated melanotropes (cell suspension) was utilized. The viability of the cells in suspension was tested by examining their ability to synthesize, process and release pro-opiomelanocortin (POMC) related peptides. All biosynthetic functions appeared normal, with the exception that the isolated melanotropes are unable to N-terminally acetylate MSH. Release of immunoreactive-MSH from these cells was shown to be Ca2+-dependent, and high K+ stimulated release. Both the neurotransmitters dopamine and gamma-aminobutyric acid (GABA), which are thought to be physiologically important MSH-release inhibiting factors, were shown to inhibit MSH release from isolated melanotropes. Dopamine appeared to function through a dopamine D2 type receptor mechanism while for GABA, both a GABAa and GABAb receptor mechanism are involved.

Circulating forms of alpha MSH in the carp and trout blood: an HPLC and RIA study

The three forms of MSH(desacetyl-, monoacetyl-, and diacetyl) circulate in the carp and trout blood. Their proportions there are as follows: in the carp, the desacetylated form is 2.5% of total MSH and the monoacetylated form 72% of the acetylated forms of alpha MSH. In the trout, similar proportions were found: desacetyl alpha MSH was 5% of total alpha MSH and monoacetyl-, 70% of the acetylated MSHs.

Localization and identification of alpha-melanocyte-stimulating hormone (alpha-MSH) in the frog brain

The distribution of alpha-melanocyte-stimulating hormone (alpha-MSH) in the central nervous system of the frog Rana ridibunda was determined by immunofluorescence using a highly specific antiserum. alpha-MSH-like containing perikarya were localized in the infundibular region, mainly in the ventral hypothalamic nucleus. A rich plexus of immunoreactive fibers directed towards the ventral telencephalic region was detected. Reverse-phase high-performance liquid chromatography and radioimmunoassay were used to characterize alpha-MSH-like peptides in the frog brain. Chromatographic separation revealed that immunoreactive alpha-MSH coeluted with synthetic des-N alpha-acetyl alpha-MSH, authentic alpha-MSH and their sulfoxide derivatives. The heterogeneity of alpha-MSH-like material in the frog brain was in marked contrast with the figure observed in the intermediate lobe of the pituitary gland where only des-N alpha-acetyl alpha-MSH is present. These findings support the existence of discrete alpha-MSH immunoreactive neurons in the frog brain containing both desacetyl and authentic alpha-MSH.

Proportions of mono- and diacetylated forms of alpha MSH in individual neurointermediate lobe extracts of Cyprinus carpio L

The proportions of the mono- and diacetylated forms of alpha MSH in individual carp neurointermediate lobe (NIL) extracts, as assessed by HPLC and RIA, fluctuate within a narrow range (mono/mono- + diacetyl alpha MSH = 3-14%). These results obtained on individuals of different sex and age show that the diacetylated form predominates in the NIL of all individuals where it represents in the mean 90% of the acetylated forms. The relatively low fluctuation of the proportions of the acetylated forms suggests that the possibility for a physiological modulation of the diacetylation step may be limited in this species of fish.

Characterization of endorphins from the pituitary of the spiny dogfish Squalus acanthias

Opioid-like immunoreactive material was extracted from the pituitary and brain of the Spiny Dogfish Shark Squalus acanthias. The immunoreactive material in the pituitary extracts was purified to apparent homogeneity by reverse phase high performance liquid chromatography and subsequently characterized by amino acid analysis, Edman degradation and fast atom bombardment mass spectrometry. The largest opioid-like peptide isolated contained 30 amino acids and showed 80 percent homology with salmon endorphin-II but less than 50 percent homology with human beta-endorphin. Three structural variants of this molecule were also characterized. These variants were shown to be shorter N-terminal fragments, two of which corresponded to cleavage products at the single basic residues arginine and lysine. Cleavage at a single lysine residue has not been reported for posttranslational processing of beta-endorphin in mammals and could represent a modification seen only in lower vertebrates. The remaining fragment corresponded to a loss of 3 residues from the C-terminus of the parent molecule. No alpha-N-acetylated peptides were detected. These results provide the first unequivocal confirmation of beta-endorphin in an elasmobranch and provide evidence of novel N-terminal variants of beta-endorphin.

Adrenocorticotropin and alpha-melanotropin in the myenteric plexus of the rat duodenum: an electron microscopic study

Adrenocorticotropin (ACTH) immunoreactivity was localized at the ultrastructural level as positive 'cores' within large dense-cored vesicles (LDVs) of axons and dendrites of the rat duodenum. The immunostained vesicle 'cores' were 35-50 nm in mean diameter, corresponding to 'cores' of LDVs with a mean diameter of 80-90 nm. alpha-melanotropin (alpha-MSH) was detected also within LDVs, expressing the same mean diameter as ACTH-stained vesicles. alpha-MSH and ACTH were localized only within structures belonging to the enteric nervous system of the rat duodenum. alpha-MSH and ACTH, as detected by immunostaining, were absent in endocrine cells of the rat duodenum. These findings suggest the possibility that these peptides may have important physiological roles in the rat duodenum.

Separation and partial characterization by high-performance liquid chromatography and radioimmunoassay of different forms of melanocyte-stimulating hormone from fish (Cyprinidae) neurointermediate lobes

From neurointermediate lobe (NIL) extracts of two species of Cyprinidae, Carassius auratus and Cyprinus carpio, several peptides were separated by high-performance liquid chromatography (HPLC) on a C18 muBondapak column eluted with a methanol/acetic acid/triethylamine mixture. Monitoring all fractions by radioimmunoassay (RIA) with an antibody against melanocyte-stimulating hormone (MSH) C terminal gave positive reactions for fractions 7, 11-12, 15-16, 23-24, and 25-27. For further characterization, the elution positions of these peaks were compared to those of known synthetic reference substances. Peak 7 elutes in the same position as oxidized alpha MSH, whereas peak 15-16 matches the elution position of des-acetyl alpha MSH and 23-24 that of alpha MSH. The product from peak 26-27 has several characteristics of the diacetylated form of alpha MSH: its immunoreactivity in RIA, its sensitivity to weak bases and to HCl and its mass spectrum which is identical with that of mammalian diacetyl alpha MSH. In both species, the diacetylated form is predominant in the intracellular pool. This study establishes the coexistence of three different forms of alpha MSH, a des-acetylated, monoacetylated, and diacetylated in the cyprinid NIL extracts.

The mechanism of N-terminal acetylation of proteins

N alpha-acetylation is almost exclusively restricted to eukaryotic structural proteins. As a rule it is a post-initiational process, requiring the presence of the enzyme N alpha-acetyltransferase and the acetyl donor acetylcoenzyme A. N alpha-acetyltransferases appear to have a narrow substrate specificity, which is very similar for enzymes from different tissues and species. Amino acids predominantly present at the N terminus of N alpha-acetylated proteins are alanine, serine, and methionine. The occurrence of these residues is apparently a prerequisite for acetylation. The region following these amino acids is also important. If methionine is at the N terminus, the second position is always occupied by a strongly hydrophilic amino acid. Two- and three-dimensional structural characteristics of the protein do not seem to play a major role in N alpha-acetylation. Up to now the exact function for N alpha-acetylation is not known.

The involvement of melanotrophins in physiological colour change in the dogfish Scyliorhinus canicula

Desacetylated alpha-, beta-, and gamma-MSH purified from the neurointermediate lobe of the pituitary gland of the dogfish Squalus acanthias were all equipotent in turning partially hypophysectomized dogfish, Scyliorhinus canicula, dark. However, each possessed less than 2% of the melanotrophic activity of either acetylated mammalian alpha-MSH or synthetic acetylated Squ. acanthias alpha-MSH. These results suggest that acetylation is much more important than amidation in determining melanotrophic potency in vivo in the dogfish. Intravenous administration into grey-adapted dogfish of an antibody capable of binding the alpha-MSH-like peptides present in the blood was very effective at inducing pallor. In contrast, a gamma-MSH antibody had no effect on the melanophores of Scy. canicula. Overall the results suggest that it is alpha-melanotrophins that regulate physiological colour change in the dogfish Scy. canicula.