The teleost melanin-concentrating hormone -- a pituitary hormone of hypothalamic origin

Characterization of melanin concentrating hormone in teleost hypothalamus

Melanin concentrating hormone (MCH) is a heptadecapeptide isolated from chum salmon (Oncorhynchus keta) pituitaries. The peptide has been isolated from whole brain extract at a low yield of 1.2 micrograms/1300 brains. MCH activity in the hypothalamus was characterised by in vitro scale bioassay and radioimmunoassay. Specificity of these assay systems was examined with neurotransmitters such as epinephrine, norepinephrine, and dopamine, hypothalamic hormones such as somatostatin, isotocin, Arg-vasotocin, oxytocin, and Arg-vasopressin, and salmon prolactin and its chymotryptic peptide or salmon PRL176-187. Among them only salmon PRL176-187 exhibited weak activities in both assays. The neurotransmitters were 10(4) to 10(5) times less potent than MCH in the bioassay. MCH concentrations in a pituitary and a hypothalamus were estimated as 5300 +/- 750 ng (ca. 106 micrograms/g) and 48 +/- 9.5 ng (ca. 1.6 micrograms/g), respectively, by radioimmunoassay. Lysyl endopeptidase digestion of the hypothalamic extract resulted in a significant increase of biological activity as well as of immunoreactivity. Gel filtration of the hypothalamic extract and subsequent enzymatic digestion revealed that the fractions at higher molecular weight were contributory to the increase in the activities.

Melanin concentrating hormone inhibits the release of alpha MSH from teleost pituitary glands

A radioimmunoassay was developed for salmonid melanin concentrating hormone (MCH) and used to measure immunoreactive (ir)MCH in the hypothalamus and pituitary of trout (Salmo gairdneri) and eels, (Anguilla anguilla) maintained under different regimes of background color. In trout, 95% of the total irMCH was located in the pituitary gland. The amount of MCH in both pituitary and hypothalamus was increased when white-adapted trout were transferred to a black background. In eels, a similar change of background led to an accumulation of MCH in the pituitary but not in the hypothalamus. The results suggest that MCH is released from the neurohypophysis in association with physiological color change. Neurointermediate lobes of trout and eels released both ir alpha MSH and irMCH when they were cultured in vitro. The release of alpha MSH was significantly enhanced when endogenous MCH was immunoabsorbed by MCH antiserum added to the culture medium. The results indicate that MCH can induce pallor in fish not only by its peripheral effect on the melanophores but also by an inhibitory action on the release of alpha MSH from the pituitary.

Bidirectional pigment granule movements of melanophores are regulated by protein phosphorylation and dephosphorylation

Studies were conducted to investigate the molecular basis for bidirectional pigment granule transport in digitonin-lysed melanophores. Pigment granule dispersion, but not aggregation, required cAMP and resulted in the phosphorylation of a 57 kd polypeptide. cAMP-dependent protein kinase inhibitor prevented this phosphorylation as well as pigment dispersal. In contrast, both pigment aggregation and the concomitant dephosphorylation of the 57 kd polypeptide were blocked by phosphatase inhibitors. These data support a model in which pigment dispersion and aggregation require protein phosphorylation and dephosphorylation, respectively. Furthermore, studies using the ATP analog, ATP gamma S, suggest either that protein phosphorylation alone is sufficient for dispersion or that transport is mediated by a unique force-generating ATPase that can use ATP gamma S for hydrolyzable energy.

Action of melanin-concentrating hormone (MCH) on teleost chromatophores

The in vitro effects of synthetic salmon melanin-concentrating hormone (MCH) on chromatophores of four teleost species were studied. In the erythrophores of the platyfish (Xiphophorus maculatus) and the swordtail (Xiphophorus helleri), and in the xanthophores and amelanotic melanophores of the medaka (Oryzias latipes), pigment aggregation took place in response to MCH even in the absence of Ca2+. In contrast to this, the leucophores of the medaka responded to MCH by the pigment dispersion but only when Ca2+ was present. The motile iridophores of the blue damselfish (Chrysiptera cyanea), which play a predominant role in coloration and its changes, were not affected by the hormone. Pharmacological studies employing various blocking agents suggest that the pigment-aggregating action of MCH is probably mediated through specific receptors possessed by the erythrophores, xanthophores, or amelanotic melanophores, while the pigment-dispersing action on the leucophores might be revealed through the receptors for melanophore-stimulating hormone (MSH).

Effects of chronic administration of melanin-concentrating hormone on corticotrophin, melanotrophin, and pigmentation in the trout

This paper reports the effects of salmonid melanin-concentrating hormone (MCH), administered via an Alzet minipump, on pigmentation and secretion of pituitary melanotrophin (MSH) and corticotrophin (ACTH) by black-adapted, adult rainbow trout. The drug induced melanin concentration in the skin melanophores and prevented melanogenesis. Both the cytological appearance of the pituitary pars intermedia and determinations of plasma alpha-MSH suggest that MCH prevented the increase in secretory activity of the melanotrophic cells seen normally in black-adapted trout. Resting plasma cortisol titres were similar in all groups of fish but anterior pituitary glands taken from stressed, MCH-treated fish released less ACTH in vitro than those from corresponding saline-treated fish.

Distribution of immunoreactive melanin-concentrating hormone in the central nervous system of the rat

The distribution of melanin-concentrating hormone-like immunoreactivity (MCH-LI) in 41 microdissected brain and spinal cord regions was determined using radioimmunoassay with antibodies to salmon MCH. The highest concentration of MCH-LI was detected just ventral to the zona incerta (subzona incerta) (2923.2 fmol/mg protein). Very high concentrations of MCH-LI (greater than 1000 fmol/mg protein) were detected also in the nucleus of the diagonal band, medial forebrain bundle, posterior hypothalamic nucleus and medial mammillary nucleus. High concentrations of the peptide (between 500-1000 fmol/mg protein) were measured in 11 brain regions, including bed nucleus of stria terminalis, paraventricular nucleus, anterior hypothalamic nucleus, median eminence, parabrachial nucleus. Moderate concentrations of MCH-LI (between 250-500 fmol/mg protein) were measured in 16 brain regions, such as frontal cortex, central amygdaloid nucleus, medial septum, periventricular nucleus (preoptic) and nucleus of the solitary tract. Low concentrations of MCH-LI (less than 250 fmol/mg protein) were measured in 9 brain regions such as cortical areas, hippocampus, caudate nucleus and substantia nigra. Cervical spinal cord and neurointermediate lobe of the pituitary gland contain low concentrations of the peptide.

Melanin-concentrating hormone: unique peptide neuronal system in the rat brain and pituitary gland

A unique neuronal system was detected in the rat central nervous system by immunohistochemistry and radioimmunoassay with antibodies to salmon melanin-concentrating hormone (MCH). MCH-like immunoreactive (MCH-LI) cell bodies were confined to the hypothalamus. MCH-LI fibers were found throughout the brain but were most prevalent in hypothalamus, mesencephalon, and pons-medulla regions. High concentrations of MCH-LI were measured in the hypothalamic medial forebrain bundle (MFB), posterior hypothalamic nucleus, and nucleus of the diagonal band. Reversed-phase high-performance liquid chromatography of MFB extracts from rat brain indicate that MCH-like peptide from the rat has a different retention time than that of the salmon MCH. An osmotic stimulus (2% NaCl as drinking water for 120 hr) caused a marked increase in MCH-LI concentrations in the lateral hypothalamus and neurointermediate lobe. The present studies establish the presence of MCH-like peptide in the rat brain. The MCH-LI neuronal system is well situated to coordinate complex functions such as regulation of water intake.

Immunohistochemical localization of a melanin concentrating hormone-like peptide in the rat brain

Using antisera generated in rabbits against salmon melanin concentrating hormone (MCH) coupled to human thyroglobulin, the distribution of MCH-like immunoreactivity was mapped throughout the rat central nervous system. The distribution of MCH-like immunoreactivity in rat brain is unique and different from the distribution of other neuropeptides. MCH-like immunoreactive perikarya and fibers are predominant in the posterior hypothalamic area, mostly in the medial forebrain bundle-lateral hypothalamic area subzona incerta and the perifornical area. Cell bodies are located mainly in the medial forebrain bundle and in proximity to well defined hypothalamic nuclei. Fibers are seen throughout the rat brain in all neocortical areas, the neostriatum and the amygdala, in the diencephalon in most hypothalamic nuclei, the habenula, the mamillary body and very dense in the medial forebrain bundle and just ventral to the zona incerta ("subzona incerta"). In the mesencephalon there are fibers in the central gray; in the pons-medulla fibers are contained in the dorsal and ventral parabrachial nuclei; in the tegmental area ventral to the fourth ventricle; in the spinal trigeminal area, the substantia gelatinosa and the reticular nuclei. In the spinal cord there are more fibers in the dorsal than in the ventral horn. The posterior pituitary also contained few MCH-like fibers. It is suggested that a peptide similar, but not identical, to salmon MCH is present in the rat central nervous system.

Effect of melanin concentrating hormone on pigment and adrenal cells in vitro

Highly purified synthetic salmonid melanin concentrating hormone (MCH) and some analogs were investigated for their ability to concentrate the pigment in scale melanophores of the Chinese grass carp, Ctenopharyngodon idellus, to produce melanin dispersion in frog or lizard melanophores and to inhibit alpha-MSH in its action on mouse melanoma and rat adrenal glomerulosa cells in vitro. In the grass carp, MCH produced half-maximal pigment aggregation at 6 X 10(-11) M and its oxidized form at 7 X 10(-11) M. Replacement of the two methionines at position 3 and 6 with norvaline lowered the potency by a factor of 2.7 and with propargylglycine by a factor of about 7. Linear, Cys5,14-Acm-protected MCH was a full agonist of MCH but with a 345-fold lower potency. Iodinated MCH showed similar, low activity. In tetrapods, salmonid MCH and its analogs displayed only marginal pigment dispersion at concentrations greater than 10(-5) M. Alkali-treatment of MCH increased the pigment-dispersing potency by a factor of about 30 whereas the activity for pigment aggregation in the grass carp was destroyed. At high concentrations (10(-6), 10(-5) M) MCH also stimulated tyrosinase activity in B-16 mouse melanoma cells but did not modify the effects of alpha-MSH in this system. By contrast, when tested on rat adrenal glomerulosa cells, salmonid MCH had no effect alone but at a concentration of greater than 10(-10) M it slightly reduced corticosterone production by an alpha-MSH concentration of 10(-7) M. Aldosterone production was not affected and MCH did not influence the response to ACTH.

Melanin concentrating hormone (MCH) effects on teleost (Chrysiptera cyanea) melanophores

The in vitro biological actions of synthetic chum salmon melanin concentrating hormone (MCH) on melanophores of the blue damselfish (a teleost), Chrysiptera cyanea, were studied. This cyclic heptadecapeptide stimulated melanosome (melanin granule) aggregation (centripetal migration) within melanophores at a threshold concentration of about 10(-10) M. The action of this putative hormone was not blocked by alpha- or beta-adrenoceptor antagonists. It was concluded that the effects of MCH were direct and were not mediated indirectly through the actions of adrenergic neurotransmitters released from nerve terminals. Further evidence for this view comes from the observation that, unlike the case of neurotransmitter release, melanosome aggregation in response to MCH proceeded in the absence of calcium. The possible role of MCH in the control of color change of teleost fishes is discussed.

Effect of 5-HT on basal and stimulated secretions of acid and pepsin and on gastric volume outflow in the in vivo gastrically and intestinally perfused cod, Gadus morhua

Gastric acid and pepsin responses to 5-HT was measured, in cod, during gastric and intestinal perfusions. During basal conditions, both acid and pepsin secretions were stimulated by 0.25 mumol/kg X h of 5-HT. A higher dose, 1 mumol/kg X h inhibited acid secretion and stimulated the output of pepsin both during basal conditions and during stimulation with histamine or carbachol. Histamine and carbachol, when given alone, were powerful acid stimulators but in comparison with 5-HT poor pepsigogues. Most probably due to inhibition of gastric volume secretion, gastric outflow volume decreased during treatment with higher doses of 5-HT. However, when the intestinal perfusion was omitted and water support instead given by the intramuscular route, 5-HT induced a large increase in gastric outflow volume. Our results suggest that 5-HT may be a physiological regulator of acid and pepsin secretion in the fish. The dipsogenic effect seen in the absence of intestinal perfusion indicates that 5-HT may be involved also in the regulation of drinking.

Synthesis and biological actions of melanin concentrating hormone

A melanin (melanosome) concentrating hormone, MCH, was synthesized and the methodology for its synthesis is detailed. This heptadecapeptide, H-Asp-Thr-Met-Arg-Cys-Met-Val-Gly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-Glu-Val-OH , stimulated melanosome concentration (centripetal aggregation) within melanophores of all species of teleost fishes studied. Melanosome aggregation in response to MCH was not blocked by Dibenamine as was the response to norepinephrine (NE), demonstrating that melanosome aggregating responses to MCH and NE are mediated through separate receptors. Melanosome aggregation in response to MCH was reversed by an equimolar concentration of alpha-melanocyte stimulating hormone (alpha-MSH). In contrast, MCH stimulated melanosome dispersion (centrifugal movement) within melanophores of a frog (Rana pipiens) and a lizard (Anolis carolinensis). Therefore, MCH exhibits both melanosome concentrating and dispersing actions depending upon the species studied.

Changes in pituitary and plasma levels of MSH in teleosts during physiological colour change

Immunoreactive alpha MSH was measured in the plasma of eels, trout, and flounder, following different periods of adaptation to illuminated white or black backgrounds. In both eels and trout, plasma hormone titres changed rapidly in response to the background colour. By contrast, in the flounder, whose pituitary melanotrophs appeared cytologically unresponsive to background change, plasma hormone levels remained similar for fish from black or white tanks following a 3-week adaptation period, but a difference became apparent after nearly 7 weeks adaptation. Similarly, bioactive and immunoreactive levels of MSH in trout pituitary glands usually showed significant changes with black ground colour, whereas flounder pituitary levels were unaltered. It is concluded that MSH is involved in physiological colour change in eels and trout, but probably not in flounders.

Synthesis of a cyclic melanotropic peptide exhibiting both melanin-concentrating and -dispersing activities

A putative melanin-concentrating hormone was synthesized. This peptide, H-Asp-Thr-Met-Arg-Cys-Met-Val-Gly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-Glu-Val-OH , stimulates melanin granule aggregation within teleost melanocytes at nanomolar concentrations as does the natural purified teleost pituitary preparation. In addition, this peptide stimulates melanin granule dispersion within melanocytes of frogs and lizards. The peptide has about one six-hundredth of the activity of alpha-melanocyte-stimulating hormone on frog and lizard melanocytes and is a full agonist.

Characterization of melanin-concentrating hormone in chum salmon pituitaries

Many lower vertebrates exhibit colour change in response to the background. A dual hormonal control of colour change by two antagonistic pituitary melanophorotropic hormones was first postulated in amphibia by Hogben and Slome. It is well established that the melanotropins alpha- and beta-MSH are responsible for pigment dispersion in the integumentary melanophore of lower vertebrates and that these molecules are derived from a common precursor protein, proopiocortin, by specific processing within the intermediate lobe. No evidence has been found for an antagonistic hormone in amphibia, although the existence of such a molecule in the pituitary gland of teleost fishes has long been recognized and was termed the melanophore-concentrating hormone by Enami. Early attempts to separate the two hormones proved unsuccessful. Recently, Baker and Ball re-invoked the dual hormone concept, and it has been suggested that a melanin-concentrating hormone (MCH) is synthesized in the hypothalamus of teleosts and stored and released by the neurohyphophysis. We have now isolated a novel peptide from the pituitary of the salmon (Oncorhynchus keta) possessing an antagonistic function to MSH, and we describe here its chemical and biological characteristics.

Further observations on the distribution and properties of teleost melanin concentrating hormone

The distribution of melanocyte concentrating hormone (MCH) bioactivity was mapped in the trout brain from cryostat sections cut in several planes. Most of the bioactivity occurred in the ventral third of the hypothalamus, with about 30% of the activity in the dorsal hypothalamus. The bioactivity was rapidly lost if the hypothalami were extracted in dilute acid, with a final extraction pH of 5.2. This loss, which can be avoided if the extract is heated, is presumed to be the result of hypothalamic enzyme activity. Preliminary chemical characterisation indicates that the molecule is a small basic peptide, of less than 2000 daltons (Da) and with an isoelectric point greater than 9.5. MCH bioactivity was also found in the hypothalamus but not the pituitary of Lampetra, Rana, Xenopus, and the rat. The activity from Xenopus and Lampetra had a similar Rf value to MCH from trout during polyacrylamide gel electrophoresis. Partially purified MCH of trout origin, free from MSH bioactivity, induced melanin concentration in eel melanophores but Xenopus melanophores failed to respond.