Regulation of biosynthesis and release of pars intermedia peptides in Rana ridibunda: dopamine affects both acetylation and release of alpha-MSH

The Melanocortin and Endorphin Neuropeptides in Patients with Restless Legs Syndrome

OBJECTIVE

Based upon similarities between the urge to move and sensory discomfort of restless legs syndrome (RLS) and properties of melanocortin hormones, including their incitement of movement and hyperalgesia, we assessed plasma and cerebrospinal fluid (CSF) α-melanocyte-stimulating hormone (α-MSH) and β-endorphin in RLS patients and controls.

METHODS

Forty-two untreated moderate-to-severe RLS patients and 44 matched controls underwent venipuncture at 19:00, 20:30, and 22:00; 37 RLS and 36 controls had lumbar puncture at 21:30. CSF and plasma were analyzed for pro-opiomelanocortin (POMC), adrenocorticotropin hormone (ACTH), α-MSH, β-MSH, and β-endorphin by immunoassay. RLS severity was assessed by International RLS Study Group Severity Scale.

RESULTS

RLS participants were 52.7 ± 12.0 years old, 61.9% were women, 21.4% had painful RLS, and RLS severity was 24.8 ± 9.0. Controls had similar age and sex. Plasma ACTH, α-MSH, and β-endorphin were similar between groups. Plasma POMC was significantly greater in RLS than controls (17.0 ± 11.5 vs 12.7 ± 6.1fmol/ml, p = 0.048). CSF ACTH was similar between groups. CSF β-MSH was significantly higher in painful than nonpainful RLS or controls (48.2 ± 24.8 vs 32.1 ± 14.8 vs 32.6 ± 15.2pg/ml, analysis of variance [ANOVA] p = 0.03). CSF α-MSH was higher in RLS than controls (34.2 ± 40.9 vs 20.3 ± 11.0pg/ml, p = 0.062). CSF β-EDP was lowest in painful RLS, intermediate in nonpainful RLS, and highest in controls (8.0 ± 3.4 vs 10.8 ± 3.1 vs 12.3 ± 5.0pg/ml, ANOVA p = 0.049). The ratio of the sum of CSF α- and β-MSH to CSF β-endorphin was highest, intermediate, and lowest in painful RLS, nonpainful RLS, and controls (p = 0.007).

INTERPRETATION

CSF β-MSH is increased and CSF β-endorphin decreased in RLS patients with painful symptoms. ANN NEUROL 2024;95:688-699.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Tyrosine hydroxylase-immunoreactive neurons in the brain of tadpole of the narrow mouthed frog Microhyla ornata

Catecholamines serve as a neuromodulators of many behavioral and endocrine responses in different vertebrates including amphibians. However, the neuroanatomical studies on catecholamines, especially in the tadpole brain are limited. In this study, we report the distribution of catecholaminergic neurons in different areas of the brain in the tadpole of Microhyla ornata at metamorphic climax stage. Application of antisera against tyrosine hydroxylase (TH) revealed the presence of catecholaminergic cells and fibres in the olfactory bulb, the telencephalon, the diencephalon, the mesencephalon, the spinal cord and the pituitary gland. Whereas densest aggregations of TH-immunoreactive (TH-ir) fibres were noticed in the nucleus accumbens and the amygdala pars medialis regions of the telencephalon, highest population of TH-ir cells with dorsolaterally and rostrocaudally oriented fibres was observed in the preoptic area. Larger and distinct TH-ir cell bodies along with few dorsolaterally oriented TH-ir fibres were scattered throughout the suprachiasmatic nucleus. While moderate to intensely stained clusters of TH-ir cells were observed in dorsal and ventral hypothalamic regions, conspicuous TH-ir cells and fibres were seen in the pars distalis of the pituitary gland. In the nucleus tuberculi posterioris, numerous moderate sized TH-ir cells were found along the margin of the third ventricle and the fibres from this region were oriented dorsolaterally towards the torus semicircularis and tectal regions, whereas well organized largest TH-ir cells and fibres were seen in the tegmentum. In the spinal cord, medium sized TH-ir cells along with numerous laterally running fibres were encountered. Overall, widespread distribution of the TH-ir cells and fibres in the brain and the pituitary gland of the tadpole suggest diverse roles for the catecholamines in regulation of locomotion, olfaction, skin pigmentation and endocrine responses during final stages of metamorphosis in M. ornata.

Seasonal plasticity of the pituitary pars intermedia of the one-humped camel (Camelus dromedarius)

The pituitary pars intermedia of Camelus dromedarius is well developed and completely surrounds the pars nervosa. Two major groups of cells are present: endocrine (ec) and glial-like cells (glc). The ec group is composed of three morphologically distinct cell types. Type I, or polyhedral light cells (LC-I) and type II, or polyhedral dark cells (DC-II), have secretory granules of heterogeneous electron density whose size ranges from 170 to 300nm. Type III cells are elongated with homogeneous electron-dense secretory granules of 80-200nm. The glc make up an organized network, form follicles in the centrolobular zones and are positive for vimentin and S-100β immunolabelling. The nerve fibres penetrating the lobe are numerous, and can be classified into two types according to the membrane bound vesicles found in their endings (ne). Ultrastructural quantitative analysis revealed significant variations in PI elements between winter and summer seasons (F=8.014, p=0.006). DC-II cells characterized by developed biosynthetic machinery and a large pool of secretory granules storage are increased with the ne in winter. However, LC-I cells showing frequent cytoplasmic degranulation are predominant with glc in summer. Thus, important cellular remodelling occurs in the dromedary PI that may depend upon, or perhaps anticipate, external living conditions.

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.

Maternal influence of prolyl endopeptidase on fat mass of adult progeny

Background

Maternal genotype has lifetime effects on progeny, but few specific genes, and no proteases, are known to underlie maternal effects. Prolyl endopeptidase (PREP) is a serine protease with putative substrates that regulate appetite or milk production.

Objective

To test effects of PREP on obesity phenotypes in mice.

Design

Mice with a gene-trap of PREP (PREP^gt/gt) on the C57BL/6J (B6) background were generated. Minimal PREP protein was detected by Western blot. In experiment 1, direct effects of PREP were measured in littermate mice derived from intercrosses of heterozygotes (PREP^WT/gt). In experiment 2, maternal effects of PREP were measured in reciprocal crosses of heterozygous (PREP^WT/gt) and wild-type (PREP^WT/WT) males and females.

Diets

Mice were fed either low-fat (LF, Experiments 1 and 2) or high-fat (HF, Experiment 1) defined diets.

Measurements

Adiposity index (AI) was calculated from body weight and weights of four fat depots measured in 120 day old mice. Fasting plasma glucose, insulin and leptin were measured. In vivo plasma α-MSH levels were measured by targeted quantitative peptidomics.

Results

Experiment 1. In intercross mice there were significant diet effects, but few genotype effects. Body weight and AI in females on the LF diet were marginally affected by pup genotype. There were no genotype effects in males on either diet or in females on the HF diet.

Experiment 2. In contrast, reciprocal crosses of heterozygous males or females with wild-type B6 revealed highly significant parent of origin effects on all traits except body length. Progeny (wild-type and heterozygous genotypes and both sexes) born to female PREP^WT/gt heterozygotes had fat pads that weighed as much as 2-fold more at 120 days old than progeny born to male heterozygotes.

In vivo multiplex quantitative analysis of 3 forms of alpha melanocyte stimulating hormone in pituitary of prolyl endopeptidase deficient mice

Background

In vitro reactions are useful to identify putative enzyme substrates, but in vivo validation is required to identify actual enzyme substrates that have biological meaning. To investigate in vivo effects of prolyl endopeptidase (PREP), a serine protease, on alpha melanocyte stimulating hormone (α-MSH), we developed a new mass spectrometry based technique to quantitate, in multiplex, the various forms of α-MSH.

Methods

Using Multiple Reaction Monitoring (MRM), we analyzed peptide transitions to quantify three different forms of α-MSH. Transitions were first confirmed using standard peptides. Samples were then analyzed by mass spectrometry using a triple quadrupole mass spectrometer, after elution from a reverse phase C18 column by a gradient of acetonitrile.

Results

We first demonstrate in vitro that PREP digests biological active alpha melanocyte stimulating hormone (α-MSH_1–13), by cleaving the terminal amidated valine and releasing a truncated alpha melanocyte stimulating hormone (α-MSH_1–12) product – the 12 residues α-MSH form. We then use the technique in vivo to analyze the MRM transitions of the three different forms of α-MSH: the deacetylated α-MSH_1–13, the acetylated α-MSH_1–13 and the truncated form α-MSH_1–12. For this experiment, we used a mouse model (PREP-GT) in which the serine protease, prolyl endopeptidase, is deficient due to a genetrap insertion. Here we report that the ratio between acetylated α-MSH_1–13 and α-MSH_1–12 is significantly increased (P-value = 0.015, N = 6) in the pituitaries of PREP-GT mice when compared to wild type littermates. In addition no significant changes were revealed in the relative level of α-MSH_1–13 versus the deacetylated α-MSH_1–13. These results combined with the demonstration that PREP digests α-MSH_1–13 in vitro, strongly suggest that α-MSH_1–13 is an in vivo substrate of PREP.

Conclusion

The multiplex targeted quantitative peptidomics technique we present in this study will be decidedly useful to monitor several neuropeptide enzymatic reactions in vivo under varying conditions.

Plasticity in the melanotrope neuroendocrine interface of Xenopus laevis

Melanotrope cells of the amphibian pituitary pars intermedia produce alpha-melanophore-stimulating hormone (alpha-MSH), a peptide which causes skin darkening during adaptation to a dark background. The secretory activity of the melanotrope of the South African clawed toad Xenopus laevis is regulated by multiple factors, both classical neurotransmitters and neuropeptides from the brain. This review concerns the plasticity displayed in this intermediate lobe neuroendocrine interface during physiological adaptation to the environment. The plasticity includes dramatic morphological plasticity in both pre- and post-synaptic elements of the interface. Inhibitory neurons in the suprachiasmatic nucleus, designated suprachiasmatic melanotrope-inhibiting neurons (SMINs), possess more and larger synapses on the melanotrope cells in white than in black-background adapted animals; in the latter animals the melanotropes are larger and produce more proopiomelanocortin (POMC), the precursor of alpha-MSH. On a white background, pre-synaptic SMIN plasticity is reflected by a higher expression of inhibitory neuropeptide Y (NPY) and is closely associated with postsynaptic melanotrope plasticity, namely a higher expression of the NPY Y1 receptor. Interestingly, melanotrope cells in such animals also display higher expression of the receptors for thyrotropin-releasing hormone (TRH) and urocortin 1, two neuropeptides that stimulate alpha-MSH secretion. Possibly, in white-adapted animals melanotropes are sensitized to neuropeptide stimulation so that, when the toad moves to a black background, they can immediately initiate alpha-MSH secretion to achieve rapid adaptation to the new background condition. The melanotrope cell also produces brain-derived neurotrophic factor (BDNF), which is co-sequestered with alpha-MSH in secretory granules within the cells. The neurotrophin seems to control melanotrope cell plasticity in an autocrine way and we speculate that it may also control presynaptic SMIN plasticity.

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.

N-acetylation of hypothalamic alpha-melanocyte-stimulating hormone and regulation by leptin

The central melanocortin system is critical in the regulation of appetite and body weight, and leptin exerts its anorexigenic actions partly by increasing hypothalamic proopiomelanocortin (POMC) expression. The POMC-derived peptide alpha-melanocyte-stimulating hormone (alphaMSH) is a melanocortin 4 receptor agonist, and its potency in reducing energy intake is strongly increased by N-acetylation. The reason for the higher biological activity of N-acetylated alphaMSH (Act-alphaMSH) compared with that of N-desacetylated alphaMSH (Des-alphaMSH) is unclear, and regulation of acetylation by leptin has not been investigated. We show here that total hypothalamic alphaMSH levels are decreased in leptin-deficient ob/ob mice and increased in leptin-treated ob/ob and C57BL/6J mice. The increase in total alphaMSH occurred as soon as 3 h after leptin injection and was entirely due to an increase in Act-alphaMSH. Consistent with this observation, leptin rapidly induced the enzymatic activity of a N-acetyltransferase in the hypothalamus of mice. In 293T cells expressing the melanocortin 4 receptor, Act-alphaMSH is far more potent than Des-alphaMSH in stimulating cAMP accumulation, an effect caused by a dramatically increased stability of Act-alphaMSH. Moreover, Des-alphaMSH is rapidly degraded in the hypothalamus after intracerebroventricular injection in rats and was less potent in inhibiting energy intake. The results suggest that leptin activates a N-acetyltransferase in POMC neurons, leading to increased hypothalamic levels of Act-alphaMSH. Due to its increased stability, this posttranslational modification of alphaMSH may play a critical role in leptin action via the central melanocortin pathway.

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.

Isolation and structural characterization of peptides related to alpha- and gamma-melanocyte-stimulating hormone (MSH) from the frog brain

Peptides that are derived from the processing of proopiomelanocortin were isolated in pure form from the brain of the frog Rana ridibunda. The primary structure of the most abundant of those peptides was established as: Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val. This amino acid sequence is identical to that of mammalian and frog pituitary alpha-melanocyte-stimulating hormone (MSH) and the peptide co-eluted with synthetic desacetyl alpha-MSH, indicating that it is COOH-terminally alpha-amidated. A second component, which exhibited a shorter retention time, co-eluted with the glycine-extended form of desacetyl alpha-MSH [ACTH(1-14)]. The primary structure of the third peptide isolated in pure form from the brain extract was established as: Lys-Tyr-Val-Met-Ser-His-Phe-Arg-Trp-Asn-Lys-Phe-NH2. This sequence corresponds to Lys-gamma 1-MSH as predicted from the nucleotide sequence of frog proopiomelanocortin. The presence of substantial amounts of desacetyl alpha-MSH and Lys-gamma 1-MSH in the frog brain supports the concept that, in amphibia, melanotropins may act as neurotransmitters and/or neuromodulators as well as hormonal peptides.

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.

[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)

Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism

Recently we have demonstrated that dopamine inhibits action potentials in cultured frog melanotrophs through D2 receptor-mediated activation of hyperpolarizing potassium current and reduction of calcium and sodium currents. Herein, the respective roles of G proteins, guanosine-5'-triphosphate and adenosine-3':5'-cyclic-monophosphate in dopamine-induced electrical responses were investigated using the whole-cell patch-clamp technique. Pretreatment of melanotrophs with pertussis toxin (1 microgram/ml) abolished the hyperpolarization and arrest of action potentials evoked by dopamine (1 microM) in 77% of the cells studied. Addition of guanosine-5'-O-(2-thiodiphosphate) (500 microM) to the intracellular solution did not alter the effects of a first exposure to dopamine, but completely blocked the response of cultured melanotrophs to subsequent pulses of dopamine. In cells which were dialysed with guanosine-5'-O-(3-thiotriphosphate) (100 microM) dopamine caused a sustained hyperpolarization and an irreversible inhibition of spikes. Voltage-clamp recordings with electrodes containing guanosine-5'-O-(3-thiotriphosphate), showed that the increase of potassium current and decrease of calcium and sodium currents caused by dopamine were irreversible. These effects were not modified when the pipette contained, in addition to guanosine-5'-O-(3-thiotriphosphate), a high concentration of adenosine-3':5'-cyclic-monophosphate (100 microM) together with the inhibitor of phosphodiesterases 3-isobutyl-1-methylxanthine (100 microM). It is concluded that, in cultured frog melanotrophs, a pertussis toxin-sensitive G protein is implicated in the coupling of dopamine D2 receptors to activation of potassium channels and inhibition of calcium and sodium channels. Our results also indicate that the G protein-mediated signal transduction does not involve the adenylate cyclase system.

Dopamine-induced inhibition of action potentials in cultured frog pituitary melanotrophs is mediated through activation of potassium channels and inhibition of calcium and sodium channels

A patch-clamp study was conducted in order to investigate the effects of dopamine on the ionic currents in cultured frog melanotrophs. Brief applications of dopamine (1 microM) hyperpolarized the cell and inhibited the spontaneous action potentials. The hyperpolarization was accompanied by an increase in membrane conductance. Under voltage clamp, dopamine evoked a net outward current. The dopamine-induced outward current was negligible at the equilibrium potential for potassium ions. It was also observed that dopamine increased the intensity of a voltage-dependent outward potassium current monitored by constant depolarizing pulses. In addition, voltage-dependent L- and N-like calcium currents and sodium current were reduced. In the cell-attached configuration, two distinct channel types were activated and one channel type was blocked by dopamine exposure to the extrapatch membrane, which indicates the involvement of an intracellular factor in the signal transduction pathway. A higher conductance channel (100 pS) was characterized by a very low basal activity which rapidly increased upon dopamine application. A lower conductance channel (30 pS) displayed a basal activity with frequent opening events, and a delayed (30-40 s) increase of activity in response to dopamine. Both currents reversed at a deduced potential corresponding to the equilibrium potential for potassium ions. The channel type inhibited by dopamine had a low conductance of 15 pS. The inhibition of the electrical activity induced by dopamine was totally blocked by the D2 receptor antagonist S(-)-sulpiride (1 microM) but was not affected by the D1 receptor antagonist SKF-83566 (1 microM). It is concluded that dopamine activates potassium channels and inhibits calcium and sodium channels in frog melanotrophs. The results also indicate that stimulus-response coupling is mediated by intracellular messenger system(s).

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)

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.

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.

Hypothalamic alpha-melanocyte-stimulating hormone (alpha-MSH) is not under dopaminergic control

A possible dopaminergic regulation of hypothalamic proopiomelanocortin (POMC)-containing neurons has been investigated in rats by means of in vivo and in vitro approaches. Acute or 3-weeks chronic in vivo treatments with the dopaminergic agonists apomorphine (1 mg/kg: s.c.) and 2-Br-alpha-ergocriptine (2.5 mg/kg; s.c.) or the dopaminergic antagonist haloperidol (0.15-3 mg/kg; i.p.) had no significant effect on the concentration of alpha-melanocyte-stimulating hormone (alpha-MSH) in two hypothalamic regions: arcuate nucleus (AN) and dorsolateral area (DLH). In the same way, chronic administration of the dopaminergic agonists or antagonist did not induce any change in hypothalamic contents of beta-endorphin, another peptide derived from POMC. Reverse-phase high-performance liquid chromatographic analysis revealed that acetic acid extracts of AN and DLH both contained two major forms of alpha-MSH-like peptides: deacetylated alpha-MSH and authentic alpha-MSH. The ratio between these two forms was not altered after acute haloperidol treatment (3 mg/kg, i.p.). The possible effect of dopamine on the release of hypothalamic alpha-MSH was studied in vitro using perifused rat hypothalamic slices. Infusion of dopamine (10(-7)-10(-5)M) or its antagonist haloperidol (10(-5)M) had no effect on spontaneous alpha-MSH release from hypothalamic tissue. In addition, none of these drugs had any effect on potassium (50 mM)-induced alpha-MSH release. It is concluded that dopaminergic neurons are not involved in the regulation of synthesis, post-translational processing (acetylation) or release of hypothalamic alpha-MSH.

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.

Biphasic effect of thyrotropin-releasing factor (TRH) on alpha-melanotropin secretion from frog intermediate lobe in vitro

The kinetics of alpha-MSH secretion induced by prolonged TRH infusion were studied using perfused frog neurointermediate lobe (NIL). During a 2 h administration of TRH (10(-8) M), the secretion rate of alpha-MSH displayed two phases. During the first phase, secretion of alpha-MSH increased rapidly reaching a maximum within 20 min and then, despite continued TRH infusion, this secretion slowly declined. The second phase was characterized as plateau of elevated release (relative to basal secretion); within this second phase there was often a small peak of released alpha-MSH occurring at about 100 min. Exposure of NIL to another TRH (10(-8) M) pulse 90 min later induced a normal stimulation of alpha-MSH secretion, thus demonstrating the viability of tissue in perifusion. Continuous infusion of cycloheximide (10(-5) M) during a 5 h period totally inhibited the biosynthetic activity of NIL but did not influence TRH-induced alpha-MSH secretion. In particular, cycloheximide had no effect on the second phase of the response to prolonged infusion of TRH. Similarly, during continuous infusion of the monovalent carboxylic ionophore monensin (10(-6) M), the biphasic response to prolonged infusion of TRH (10(-8) M) was still observed. Administration of a short pulse of TRH (10(-7) M) during the declining part of the first phase or during the second phase of prolonged TRH (10(-8) M) infusion induced a significant enhancement of alpha-MSH stimulation. From these results we conclude that prolonged TRH infusion causes alpha-MSH release in a biphasic manner; attenuation of the secretory response to continuous TRH administration does not result from exhaustion of the releasable pool of alpha-MSH.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA-ergic control of alpha-melanocyte-stimulating hormone (alpha-MSH) release by frog neurointermediate lobe in vitro

Measurement of glutamate decarboxylase (GAD) activity in the intermediate lobe of the frog pituitary and brain showed that neurointermediate lobe extracts represented 12% of the GAD activity detected in the whole brain. No significant activity was measured in distal lobe extracts. Immunocytochemical studies revealed GAD-containing fibers among the parenchymal cells of the pars intermedia. The localization of GAD-like material in the intermediate lobe of the frog pituitary suggested a possible role of gamma-aminobutyric acid (GABA) in the regulation of melanotropic cell secretion. Administration of GABA (10(-6) to 10(-4) M), to perifused neurointermediate lobes caused a brief stimulation of alpha-melanocyte stimulating hormone (alpha-MSH) release followed by an inhibition. Picrotoxin (10(-4) M), a Cl- channel blocker, abolished only the stimulatory effect of GABA (10(-4) M), whereas bicuculline (10(-4) M), a specific antagonist of GABAA receptors, totally inhibited the effects of GABA (both stimulatory and inhibitory phases). Bicuculline induced by itself a slight stimulation of alpha-MSH release, suggesting that GABA-ergic nerve fibers present in the intermediate lobe are functionally active in vitro. The GABAA agonist muscimol (10(-7) to 10(-4) M) mimicked the biphasic effect of GABA on alpha-MSH release. Administration of baclofen, a specific GABAB agonist (10(-7) to 10(-4) M) induced a dose-dependent inhibition of alpha-MSH secretion. In contrast to GABA or muscimol, baclofen did not cause any stimulatory effect whatever the dose. Taken together these result suggested that GABAA and GABAB receptors were present on frog melanotrophs.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of two proopiomelanocortin genes in the pituitary gland of Xenopus laevis: complete structures of the two preprohormones

A number of cDNA clones corresponding to Xenopus POMC mRNA was isolated from a cDNA library constructed from Xenopus pituitary polyadenylated RNA. Characterization of the cDNA inserts revealed two groups of structurally different proopiomelanocortin mRNAs, indicating that two proopiomelanocortin genes are expressed to virtually the same level in Xenopus pituitary glands. From the mRNA structures the complete amino acid sequences of the two Xenopus preproopiomelanocortins could be deduced. Comparison with proopiomelanocortin mRNA and protein sequences from other species shows regions of high homology (including the portion of the prohormone located N-terminally of gamma-melanophore-stimulating hormone) and regions of extremely low homology (including the signal sequence).

Effect of tunicamycin on biosynthesis, processing and release of proopiomelanocortin-derived peptides in the intermediate lobe of the frog Rana ridibunda

The intermediate lobe of the pituitary gland synthesizes a glycoprotein, proopiomelanocortin (POMC), which is cleaved by specific proteolytic enzymes to generate several hormonal peptides. The purpose of the present study was to examine the possible role of the carbohydrate moiety in the synthesis, intracellular processing and release of POMC-derived peptides in frog (Rana ridibunda) intermediate lobe cells. In vitro incorporation of [3H]-labelled glucosamine gave rise to three major radioactive products. Trypsin digestion of each of these glycopeptides gave a single glucosamine-labelled tryptic fragment with identical chromatographic characteristics. We conclude that Rana POMC is glycosylated in only one site (its gamma-MSH region) and that intracellular processing of this prohormone gives rise to smaller glycopeptides including glycosylated gamma-MSH. Treatment with the antibiotic tunicamycin (10 micrograms/ml, 6 hr) inhibited the glycosylation of POMC but did not significantly alter the neosynthesis of the peptide moiety of the precursor. Pulse-chase experiments combined with high-performance liquid chromatography analysis of the peptides derived from POMC revealed that inhibition of glycosylation by tunicamycin had no effect on the enzymatic cleavage of the precursor nor on the release of mature peptides. Thus, it is concluded that, in the frog, glycosylation of POMC has no influence on the biosynthesis, processing and release of intermediate lobe hormones.

In vitro study of frog (Rana ridibunda Pallas) interrenal function by use of a simplified perifusion system. VIII. Structure-activity relationship of synthetic ACTH fragments and gamma-MSH

The present study was undertaken to determine the structure-activity relationships of ACTH analogs on corticosteroid production by frog adrenal gland. Rana ridibunda interrenal dice were perifused with amphibian culture medium for 10 hr. Corticosterone and aldosterone concentrations were measured in the effluent perifusate using sensitive and specific radioimmunoassay methods. Perifusion of interrenal fragments with increasing concentrations of synthetic human ACTH 1-39 (ranging from 6.25 X 10(-11) to 10(-9) M) led to a linear log-dose increase in both corticosterone and aldosterone secretion. Thus, this model made it possible to compare the steroidogenic potency of several ACTH analogs. Synthetic alpha-MSH and its des-N alpha-acetyl derivative were found to be approximately equipotent, and 5 X 10(3) times less active than authentic ACTH. The short-chain analog ACTH 1-10 was 2 X 10(4) times less potent than ACTH whereas ACTH 4-10 was totally inactive. A fragment of the N-terminal region of the proopiomelanocortin molecule, gamma 3-MSH, caused a dose-related stimulation of steroid secretion. However, in contrast to what has been observed in the rat, gamma 3-MSH did not potentiate the corticotropic action of ACTH on frog interrenal gland. Since processing of proopiomelanocortin in frog intermediate lobe generates high amounts of alpha-MSH and des-N alpha-acetyl alpha-MSH, these results suggest that in amphibians, several peptides other than ACTH may be involved in the control of corticosteroidogenesis.