Biosynthesis, processing, and control of release of melanotropic peptides in the neurointermediate lobe of Xenopus laevis

Pituitary melanotrope cells of Xenopus laevis are of neural ridge origin and do not require induction by the infundibulum

Classical studies in amphibians have concluded that the endocrine pituitary and pars intermedia are derived from epithelial buccal epidermis and do not require the infundibulum for their induction. These studies also assumed that the pituitary is not subsequently determined by infundibular induction. Our extirpation, auto-transplantation and immunohistochemical studies with Xenopus laevis were initiated to investigate early presumptive pituitary development. These studies were conducted especially with reference to the pars intermedia melanotrope cell's induction, and its production and release of α-melanophore stimulating hormone (α-MSH) from the precursor protein proopiomelanocortin (POMC). Auto-transplantation studies demonstrated that the pituitary POMC-producing cells are determined at a stage prior to pituitary-infundibular contact. The results of experiments involving the extirpation of the presumptive infundibulum also indicated that the infundibulum is not essential for the differentiation of POMC-producing cells. We also demonstrated that early pituitary development involves adherence to the prechiasmatic area of the diencephalon with the pituitary placode growing in a posterior direction toward the infundibulum where contact occurs at Xenopus stage 39/40. Overall, our studies provide a model for early tissue relations among presumptive pituitary, suprachiasmatic nucleus, pars tuberalis and infundibulum during neurulation and later neural tube stages of development. It is hypothesized that the overlying chiasmatic area suppresses pituitary differentiation.

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.

Processing, turnover and release of corticotropins, endorphins and melanotropin in the toad pituitary intermediate lobe

The significance of glycosylation of the ACTH/alpha-MSH-endorphin precursor in the biosynthesis, processing and secretion of its peptide products was examined in the toad neurointermediate (intermediate - posterior) lobe, with the aid of a specific inhibitor of glycosylation, tunicamycin. Tunicamycin did not affect the synthesis of the precursor but prevented its glycosylation. In the presence of tunicamycin the precursor underwent rapid intracellular degradation. Precursor molecules that escaped complete degradation were processed to an ACTH molecule with approximately 19 000 molecular weight and to other atypical peptides, which were released. In vitro studies showed that trypsinization of the non-glycosylated precursor resulted in its random proteolysis while large forms of ACTH were cleaved from the glycosylated precursor. The results indicate that glycosylation of the ACTH/alpha-MSH-endorphin precursor may confer specific conformational properties upon the molecule, thus regulating its limited proteolysis. Turnover and release studies revealed two different pools of ACTH, beta-LPH and alpha-MSH-related peptides in the toad intermediate lobe. One pool contained ACTH, beta-LPH, alpha-MSH and beta-endorphin, which were rapidly synthesized and released, or degraded within 6 h of synthesis if their release was inhibited. The other pool was stored and was stable for at least 10 h, if prevented from being released. Peptides in this stored pool primarily included ACTH, alpha-MSH and beta-LPH; beta-endorphin was a minor component of this pool. The release from both pools of peptides was inhibited by dopamine, while the stored pool was selectively inhibited from release by L-isoprenaline (L-isoproterenol).

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.

Minimal model for intracellular calcium oscillations and electrical bursting in melanotrope cells of Xenopus laevis

A minimal model is presented to explain changes in frequency, shape, and amplitude of Ca2+ oscillations in the neuroendocrine melanotrope cell of Xenopus Laevis. It describes the cell as a plasma membrane oscillator with influx of extracellular Ca2+ via voltage-gated Ca2+ channels in the plasma membrane. The Ca2+ oscillations in the Xenopus melanotrope show specific features that cannot be explained by previous models for electrically bursting cells using one set of parameters. The model assumes a KCa-channel with slow Ca2+-dependent gating kinetics that initiates and terminates the bursts. The slow kinetics of this channel cause an activation of the Kca-channel with a phase shift relative to the intracellular Ca2+ concentration. The phase shift, together with the presence of a Na+ channel that has a lower threshold than the Ca2+ channel, generate the characteristic features of the Ca2+ oscillations in the Xenopus melanotrope cell.

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.

Carboxypeptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpe(fat) mice

A proposed mechanism for sorting secretory proteins into granules for release via the regulated secretory pathway in endocrine-neuroendocrine cells involves binding the proteins to a sorting receptor at the trans-Golgi network, followed by budding and granule formation. We have identified such a sorting receptor as membrane-associated carboxypeptidase E (CPE) in pituitary Golgi-enriched and secretory granule membranes. CPE specifically bound regulated secretory pathway proteins, including prohormones, but not constitutively secreted proteins. We show that in the Cpe(fat) mutant mouse lacking CPE, the pituitary prohormone, pro-opiomelanocortin, was missorted to the constitutive pathway and secreted in an unregulated manner. Thus, obliteration of CPE, the sorting receptor, leads to multiple endocrine disorders in these genetically defective mice, including hyperproinsulinemia and infertility.

Analysis of inositol phosphate metabolism in melanotrope cells of Xenopus laevis in relation to background adaptation

The present study examined inositol phosphate metabolism in melanotrope cells of Xenopus laevis to determine if inositol phosphates are involved in regulating the biosynthetic or secretory activity of these cells. No correlation could be found between inositol phosphate metabolism and the secretory activity of the melanotrope cells. Therefore, we conclude that inositol phosphate production is not directly involved in the regulation of release of alpha-MSH from these cells. However, there were dramatic differences in the capacity of the melanotrope cells to produce inositol phosphates dependent on the state of background adaptation of the animals from which the melanotropes were derived; cells from white-adapted animals had a low capacity to produce inositol phosphates, whereas melanotropes from black-adapted animals had a high capacity in this regard. During adaptation of animals from a white to a black background, the capacity of the melanotrope cells to produce inositol phosphates was only very slowly acquired, reminiscent of the slow acquisition displayed by these cells to produce POMC during background adaptations. Likewise, during black to white background adaptation, the melanotrope cells very slowly lost the capacity to phosphorylate inositol, which correlates with the slow loss of the biosynthetic capacity of melanotrope cells during such adaptations. Altogether we conclude that inositol phospholipid metabolism is likely involved in the regulation of the biosynthetic processes of melanotrope cells of Xenopus laevis.

Analysis of autofeedback mechanisms in the secretion of pro-opiomelanocortin-derived peptides by melanotrope cells of Xenopus laevis

The secretion of most pituitary hormones is under the control of feedback mechanisms. The feedback control of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells is controversial. The possible existence of an autofeedback exerted by alpha-MSH or other POMC-derived peptides on melanotrope cells of the amphibian Xenopus laevis has been investigated. alpha-MSH or its potent agonist 4-norleucine,7-D-phenylalanine-alpha-MSH has no effect on the release of radiolabeled POMC-derived peptides or immunoreactive beta-endorphin from superfused neurointermediate pituitary lobes. Melanin concentrating hormone, previously reported to have an alpha-MSH-like effect on melanophores, did not affect alpha-MSH secretion. Neurointermediate lobe superfusate, which contains a mixture of POMC-derived peptides, failed to affect the secretory activity of melanotropes. It is concluded that in X. laevis the secretory activity of melanotropes is not under the control of short-term autofeedback mechanisms involving alpha-MSH or other POMC-derived peptides.

Ovarian opioids and the reproductive cycle of the frog Rana esculenta

In mammals, proopiomelanocortin-related peptides are involved in reproductive processes both at the hypothalamo-pituitary and ovarian levels. Using immunocytochemical, biochemical and physiological "in vitro" studies, we provide here evidence for a diffuse POMC-related opioid system in the frog Rana esculenta. Ovarian beta-endorphin (beta-EP) is expressed in thecal cells and changes during the reproductive cycle in an inverse relationship with follicular development. Seasonal changes in the ovary are different to those in the brain or in the pituitary. The ratio of acetylated vs native beta-EP in the ovary also changes over the reproductive period, affecting the biological activity of the peptide. During both the reproductive spring period and the summer post-reproductive phase pMol amounts of beta-EP stimulate follicular androgen secretion in vitro, in a naloxone-reversible way. In either period, an inhibition of estradiol, possibly mediated via other factors, is the result of opioid action. In conclusion, these data demonstrate for the first time the widespread presence of beta-EP-related peptides in the frog Rana esculenta. Both immunocytochemical and biochemical evidence, as well as in vitro responses, support a physiological role for beta-EP in ovarian seasonality during the reproductive cycle of this amphibian.

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.

The novel pituitary polypeptide 7B2 is a highly-conserved protein coexpressed with proopiomelanocortin

In the amphibian intermediate pituitary gland the biosynthetic activity for production of the precursor protein proopiomelanocortin (POMC) can be physiologically manipulated; POMC synthesis is high in animals adapted to a black background and low in white-adapted animals. In order to study genes associated with POMC gene expression we applied a differential hybridization technique involving screening of a pituitary cDNA library with probes derived from RNA of inactive and physiologically activated intermediate pituitary cells of the amphibian Xenopus laevis. A differentially hybridizing Xenopus pituitary cDNA clone encoded the novel polypeptide 7B2. This Mr-21,000 secretory granule-associated protein of unknown function is shown to be highly conserved between Xenopus and human (83% amino acid sequence similarity). Conserved segments within the 7B2 structure included the N-terminal portion, three pairs of basic amino acids which are potential recognition sites for proteolytic enzymes, and three regions sharing similarity with putative GTP-binding domains. Levels of 7B2 mRNA were about 3% of POMC mRNA levels in Xenopus pituitary glands. In the intermediate pituitary the amount of both POMC and 7B2 mRNA was much higher in black-adapted toads than in white-adapted animals. These physiologically-induced changes in POMC and 7B2 mRNA levels were not found in the anterior pituitary. We conclude that the POMC and 7B2 genes are coexpressed and that modulation of the activity of these genes is tissue-specific.

Particular processing of pro-opiomelanocortin in Xenopus laevis intermediate pituitary. Sequencing of alpha- and beta-melanocyte-stimulating hormones

alpha- and beta-melanocyte-stimulating hormones (alpha-MSH and beta-MSH) have been isolated from Xenopus laevis neurointermediate pituitary and microsequenced. Intracellular alpha-MSH is not N-acetylated after proteolytic processing of pro-opiomelanocortin in contrast to mammalian alpha-MSHs. There is a high preservation of the melanotropic amino acid sequence common to all MSHs although in Xenopus beta-MSH a histidine residue replaces the glutamic acid residue found in position 8 of mammalian beta-MSHs.

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.

Regulation of cyclic-AMP synthesis in amphibian melanotrope cells through catecholamine and GABA receptors

Catecholamines and GABA are neurotransmitters involved in the regulation of release of pro-opiomelanocortin (POMC) derived peptides from the neurointermediate lobe of Xenopus laevis. The present study concerns the relation of these neurotransmitters to the adenylate cyclase system of the melanotrope cell. During in vitro incubation of isolated melanotrope cells it was found that dopamine, adrenaline and LY 171555 induced inhibition of forskolin-stimulated cAMP production and concomitantly inhibited MSH release. Activation of the GABAb receptors by baclofen also induced inhibition of cAMP production and alpha MSH secretion. Activation of the GABAa receptors evoked stimulation of cAMP production, while alpha MSH release was slightly inhibited, indicating that the GABAa mechanism may prove to be complex. A dual regulation through two subtypes of this receptor might be involved, one stimulating release through the adenylate cyclase system, while the other would inhibit secretion.

Assessment of TRH as a potential MSH release stimulating factor in Xenopus laevis

This study considers the possible involvement of the tripeptide TRH (thyrotropin releasing hormone) in the physiological regulation of melanophore stimulating hormone (MSH) secretion from the pars intermedia of the toad, Xenopus laevis. TRH was shown to stimulate release of MSH from superfused neurointermediate lobes obtained from white-background adapted animals, but had no effect on secretion from lobes of black-background adapted animals. Immunohistochemical analysis revealed a rich TRH-containing neuronal network terminating in the neural lobe of the Xenopus pituitary. Plasma levels of TRH, determined with a specific radioimmunoassay, proved to be extremely high and no significant difference in this level could be found between white- and black-adapted animals. Plasma TRH probably originates from the skin, and our results show that its concentration is within the effective concentration range established for this peptide in stimulating MSH release from the pars intermedia. Therefore, while both our superfusion and immunohistochemical results argue favourably for a function of TRH in the regulation of MSH secretion, we conclude that, in any regulatory role, it would likely have to function within the pars intermedia at concentrations exceeding the high plasma values. While TRH could be involved in short-term activation of the secretory process in white-background adapted animals or in animals undergoing the initial stages of black background adaptation, our results indicate that this peptide may have no function in the maintenance of secretion from the pars intermedia of animals fully adapted to black background.

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.

Regulation of melanotropin release from the pars intermedia of the amphibian Xenopus laevis: evaluation of the involvement of serotonergic, cholinergic, or adrenergic receptor mechanisms

Melanophore-stimulating hormone (MSH) release from the pars intermedia of the pituitary gland is probably regulated by multiple factors of hypothalamic origin. We have examined a number of potential regulatory factors for their effects on MSH release from the amphibian Xenopus laevis. Serotonin and acetylcholine have no effect on MSH release. Both adrenaline and noradrenaline inhibit release of MSH in a dose-dependent manner. Studies with specific receptor agonists and antagonists reveal that these neurotransmitters exert their in vitro effects primarily through a dopamine D-2 receptor, although an alpha-adrenergic receptor could not be excluded. We further conclude that the pars intermedia of X. laevis lacks a beta-adrenergic receptor for the regulation of MSH secretion from the pars intermedia. In mammals, this receptor activates the adenylate cyclase system. Our studies reveal that despite the lack of beta-adrenergic receptors, cyclic-AMP is likely an intracellular factor involved in the stimulation of MSH release.

Biosynthesis, processing and release of pro-opiomelanocortin related peptides in the intermediate lobe of the pituitary gland of the frog (Rana ridibunda)

The biosynthesis of pro-opiomelanocortin (POMC) and related peptides by the intermediate lobe of the pituitary gland was studied in the frog Rana ridibunda using the pulse-chase technique. Analysis of radioactive proteins by dodecyl sulfate polyacrylamide gel electrophoresis showed that during pulse incubations a 36,000 dalton (36K) glycosylated prohormone was synthesized. It disappeared slowly during chase incubations, giving rise to another glycosylated protein (Mr 18K), identified as the N-terminal fragment of POMC. This latter protein was secreted to the incubation medium. High performance liquid chromatography analysis of peptides synthesized during chase incubations revealed the biosynthesis of two peptides related to gamma-MSH, three peptides related to alpha-MSH, one endorphin-related and one CLIP-related peptides. These newly synthesized peptides were slowly secreted to the incubation medium. Among the alpha-MSH related peptides, only the des-N alpha-acetyl alpha-MSH form of the peptide was found to be present within the cells, in contrast to the incubation medium where the presence of des-N alpha-acetyl alpha-MSH and a modified alpha-MSH was demonstrated.

The development of the pars intermedia and its role in the regulation of dermal melanophores in the larvae of the amphibian Xenopus laevis

The ontogenesis of biosynthesis of pro-opiomelanocortin (POMC)-related peptides in the pars intermedia of Xenopus laevis tadpoles was studied. The results were related to the capacity of the animal to adapt to background color through regulation of pigment dispersion in dermal melanophores. Using immunocytochemical techniques with antisera to alpha-melanophore-stimulating hormone (alpha-MSH), it was revealed that this peptide first appeared at developmental stage 37/38, just prior to the animal's ability to adapt to background. It was shown that pigment dispersion in melanophores between stages 33 and 39 was not dependent on melanotropins of pituitary origin. Using in vitro biosynthetic studies it was possible to follow POMC biosynthetic activity, its processing and the release of peptides from stage 48 onward. Among the newly synthesized peptides observed were a gamma 3-MSH-like peptide, des-N-alpha-acetyl-alpha-MSH, alpha-MSH, and two endorphin-like peptides. By stage 57 a biosynthetic pattern almost identical to that of the adult pars intermedia had evolved. It was concluded that stage 39/40 is a critical stage in the simultaneous development of a number of the components involved in the neuroendocrine control of background adaptation.

Immunohistochemical localization of enkephalin- and ACTH-related substances in the pituitary of the lamprey

The distributions of ACTH-, alpha MSH-, beta LPH-, and enkephalin-related substances were determined immunohistochemically in the pituitary of the brook lamprey, Lampetra lamotenii. an antiserum directed against the middle region of ACTH reacted chiefly with cells in the pro-adenohypophysis. An antiserum specific for alpha MSH reacted with all of the cells of the meta-adenohypophysis, but did not react with any of the middle ACTH-positive cells in the pro-adenohypophysis. Several antisera which crossreact with both beta LPH and beta-endorphin did not react with any region of the lamprey pituitary. However, an antiserum directed against gamma LPH did react with a small population of cells in the meso-adenohypophysis. This reactivity could be blocked following pre-absorption with mouse beta LPH but was not blocked by synthetic beta-endorphin (1-31). Antisera directed against either met-enkephalin or leuenkephalin reacted with fibers in the anterior neurohypophysis, cells in the pro-adenohypophysis, and all the cells of the meta-adenohypophysis. This crossreactivity could be blocked following pre-absorption with the appropriate enkephalin, but not by pre-absorption with synthetic beta-endorphin (1-31) or dynorphin (1-13). In addition, the enkephalin-like reactivity in the adenohypophysis of the lamprey was coincident with middle ACTH-like immunoreactivity in the pro-adenohypophysis and with alpha MSH-like immunoreactivity in the meta-adenohypophysis. The absence of beta LPH/beta-endorphin immunoreactivity coincident with ACTH immunoreactivity, and the presence of enkephalin-like material in the adenohypophysis are unique to the lamprey.

In vitro study of frog (Rana ridibunda Pallas) neurointermediate lobe secretion by use of a simplified perifusion system. III. Effect of neuropeptides on alpha-MSH secretion

It has been previously demonstrated that thyrotropin-releasing hormone (TRH) stimulates in vitro the release of alpha-melanocyte-stimulating hormone (alpha-MSH) in frog. In the present study, the effects of various neuropeptides on spontaneous and/or TRH-induced alpha-MSH secretion were investigated, using a well-defined perifusion system technique. Vasoactive intestinal peptide, (VIP) a neurohormone which stimulates TRH target cells in mammals, was totally devoid of effect on frog melanotrophs although VIP-like material could be detected in neurointermediate lobe extracts. Somatostatin-like immunoreactive material was found in high concentrations in the frog neurointermediate lobe complex, but synthetic somatostatin (from 10(-10) to 10(-6) M) did not modify the spontaneous release of alpha-MSH. At doses of 10(-8) and 10(-6) M, synthetic somatostatin did not modify TRH-induced alpha-MSH secretion. Morphine (10(-5) M) and opioid peptides (10(-10) to 10(-6) M) had no effect on spontaneous alpha-MSH secretion. In addition, methionine enkephalin (10(-5) M) did not modify the stimulatory effect of TRH on alpha-MSH secretion. From these results we conclude that, among the neuropeptides which modulate prolactin secretion in mammals, only TRH is involved in alpha-MSH secretion in the frog.

Plasminogen activators of the pituitary gland: enzyme characterization and hormonal modulation

We studied plasminogen activator (PA) of the rat pituitary gland in organ and cell monolayer culture. Both anterior and intermediate lobes contain, synthesize and secrete a mixture consisting of the two known types of PA: urokinase and so-called tissue PA. Both enzymes were formed essentially by all PA secreting cells, and PA was identified specifically in mammotrophs, corticotrophs, and luteinizing hormone containing gonadotrophs. Pituitary PA production was modulated on exposure to a variety of biological effectors: anterior lobe PA secretion was stimulated by agents that raised intracellular cAMP concentration; his process depended on de novo enzyme synthesis. Enzyme production was repressed by androgens and glucocorticoids. When anterior lobe cultures were maintained in plasminogen-free media, the extracellular, secreted forms of ACTH consisted almost exclusively of the high molecular weight forms (31,000 and 23,000); the smaller forms (13,000 and 4,500) were also found in the extracellular medium of cultures supplemented with plasminogen. In contrast, the size distribution of intracellular ACTH species was unaffected by the presence of plasminogen. These results resemble those previously obtained with pancreatic islets and are consistent with the possibility that plasmin, generated by PA secretion, participates in prohormone processing. PA synthesis in intermediate lobe explants was stimulated by exposure to dibutyryl cAMP, and repressed by hydrocortisone. In accordance with the dopaminergic control of intermediate lobe function in some vertebrates, apomorphine strongly repressed PA synthesis in intermediate, but not anterior lobe cultures.

Isolation of the biosynthetic products of the PAS positive pars intermedia cells in the cichlid teleost Sarotherodon mossambicus

The pars intermedia of teleosts contains two types of granular cells with the predominant type being similar to the pars intermedia cells in other vertebrate groups and containing peptides derived from the pro-opiomelanocortin precursor molecule. The function and products of the second cell type, the PAS positive cells, are unknown. This study reports on the identification of biosynthetic products of the PAS positive cells of the cichlid teleost Sarotherodon mossambicus. The experimental regimen took advantage of earlier morphometric analyses which showed marked differences in metabolic activity of the PAS positive cells resulting from adaptation to different background colours and illumination. Autoradiography at the light microscopic level showed that both cell types of the pars intermedia incorporate labeled amino acids during in vitro incubation. To identify the products synthesized by the PAS positive cells, labeled products of the pars intermedia tissue were analysed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-pressure liquid chromatography. Comparison of pulse incubations of pars intermedia tissue of fish adapted to different backgrounds and conditions of illumination revealed that an increase in the number and metabolic activity of the PAS positive cells, as deduced from morphometric data, was paralleled by an increase of the amount of label incorporated into 27K and 25K molecules. Pulse--chase experiments with pars intermedia lobes of white and black background adapted fish showed that these two products, unlike the other newly synthesized products, were not involved in any precursor-product relationship. Our data, therefore, suggest that the 27K and 25K peptides were synthesized by the PAS positive cells.

In vivo biosynthesis of melanotropins and related peptides in the pars intermedia of Xenopus laevis

To study in vivo biosynthesis of pars intermedia peptides in Xenopus laevis, [3H]lysine was administered by an osmotic minipump via a cannula inserted near the pituitary gland. Following extraction of the neurointermediate lobe, high-performance liquid chromatography was used to separate the newly synthesized peptides. In black-background adapted animals, [3H]lysine was incorporated into a number of peptides. The elution characteristics of these peptides corresponded exactly with those of peptides synthesized during in vitro incubation of neurointermediate lobes, and which were identified as des-N alpha-acetyl-alpha-MSH, a gamma-MSH-like peptide, two corticotropin-like intermediate lobe peptides, and two forms of endorphin. In white-background adapted Xenopus, practically no synthesis of pars intermedia peptides occurred. Transfer of black-adapted toads to a white background at the beginning of infusion led to storage of newly synthesized peptides. When such animals were maintained on a white background for 10 days, des-N alpha-acetyl-alpha-MSH, but not alpha-MSH, was present in the pars intermedia; this supports the notion that des-N alpha-acetyl-alpha-MSH constitutes the "storage form" of alpha-MSH.

Localization of multiple forms of ACTH- and beta-endorphin-related substances in the pituitary of the reptile, Anolis carolinensis

Immunohistochemical studies on the pituitary of Anolis carolinensis detected ACTH-like, beta-endorphin-like, and 16K fragment-like immunoreactivity in distinct clusters of cells in the anterior lobe; ACTH-like, alpha MSH-like, beta-endorphin-like, and 16K fragment-like immunoreactivity was detected in all the cells of the intermediate lobe. Crude acid extracts of both lobes, when analyzed by radioimmunoassay, gave displacement curves in ACTH and beta-endorphin assays which were parallel to the appropriate synthetic standard. Only extracts of the intermediate lobe gave parallel displacement curves in an alpha MSH radioimmunoassay. Extracts of both lobes crossreacted with antiserum to 16K fragment, but the displacement curves were not parallel to that of mouse 16K fragment standard. The levels of immunoreactive ACTH and beta-endorphin in the intermediate lobe were approximately 8-fold higher than in the anterior lobe. Fractionation of anterior lobe and intermediate lobe extracts by either gel filtration on Sephadex G-75 in 10% formic acid or sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed multiple forms of ACTH-related and beta-endorphin-related substances in both lobes. In the anterior lobe the major forms of immunoreactivity were, respectively, ACTH-sized and beta-endorphin-sized. In the intermediate lobe the major forms of immunoreactivity were alpha MSH-sized, CLIP-sized, and beta-endorphin-sized. In both lobes, antisera directed against ACTH and beta-endorphin detected high molecular weight material with an apparent molecular weight slightly less than that of mouse pro-ACTH/endorphin; this material probably represents the putative common precursor for ACTH and beta-endorphin in this species.

Evidence for a common precursor for alpha MSH and beta-endorphin in the intermediate lobe of the pituitary of the reptile Anolis carolinensis

In order to investigate the biosynthesis of alpha MSH and beta-endorphin in a non-mammalian vertebrate, individual lizard intermediate pituitaries were incubated in complete medium containing a radioactive amino acid, using either a steady label or a pulse/chase protocol. Following incubation, acid extracts of the tissue were immunoprecipitated with either an NH2-terminal ACTH antiserum or a beta-endorphin antiserum and analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. After a 24 hr steady label in medium containing [3H]tyrosine, multiple molecular weight forms of beta-endorphin-related and NH2-terminal ACTH-related radioactivity were detected. The major peak of beta-endorphin-related radioactivity co-migrated with synthetic beta-endorphin(1-31); minor peaks of beta LPH-sized material and precursor-sized material were also detected. The major peak of NH2-terminal ACTH-related material co-migrated with synthetic alpha MSH; in addition, smaller amounts of material designated ACTH biosynthetic intermediate 1, ACTH biosynthetic intermediate 2, and precursor-sized material were detected. Sequential immunoprecipitation experiments revealed that the precursor-sized material had antigenic determinants for both alpha MSH and beta-endorphin. Pulse/chase experiments established that this material is the common precursor for alpha MSH and beta-endorphin. Based on gel filtration chromatography in 6 M guanidine HCl, the molecular weights of these various peptides are: common precursor, 23,300 daltons; ACTH biosynthetic intermediate 1, 12,200 daltons; ACTH biosynthetic intermediate 2, 4,200 daltons; alpha MSH, 1,500 daltons; beta LPH, 8000 daltons; beta-endorphin, 3,400 daltons. None of the peptides precipitated with either antiserum incorporated [3H]glucosamine; thus glycosylation does not appear to be involved in this biosynthetic pathway in the lizard. The results of the kinetic experiments and molecular weight determinations indicate that the major biosynthetic pathway involves the following events: common precursor is first cleaved to yield ACTH biosynthetic intermediate 1 plus beta LPH; subsequently, beta LPH is cleaved to produce beta-endorphin; ACTH biosynthetic intermediate 1 is cleaved to produce ACTH biosynthetic intermediate 2 which is subsequently cleaved to produce alpha MSH. The pulse/chase experiments indicate minor pathways exist for cleaving beta-endorphin directly from the common precursor or via a high molecular weight form intermediate in size between the common precursor and beta LPH.

Biosynthesis of pairs of peptides related to melanotropin, corticotropin and endorphin in the pars intermedia of the amphibian pituitary gland

This study concerns the biosynthesis of a number of peptides in the neurointermediate lobe of the pituitary gland of the aquatic toad, Xenopus laevis. Using pulse-chase incubations in vitro and high-performance liquid chromatographic analysis, it could be shown that these peptides are synthesized through processing of a prohormone, pro-opiomelanocortin; all peptides were released into the incubation medium. On the basis of electrophoretic analysis, selective amino acid incorporation and immunoprecipitation, as well as peptide mapping by high-performance liquid chromatography, the peptides were classified into three distinct groups: two related to melanocyte-stimulating hormone (melanotropin), two related to adrenocorticotropic hormone (corticotropin) and two endorphin-like peptides. Using tryptic and chymotryptic maps of synthetic alpha-melanotropin and des-Ac alpha N-alpha-melanotropin as references, one of the melanotropin-like peptides was identified as des-Ac alpha N-alpha-melanotropin; the other one represents neither alpha-melanotropin nor any other known melanotropic peptide. The two peptides that were immunologically related to corticotropin had characteristics consistent with a structures resembling a peptide previously named 'corticotropin-like intermediate lobe peptide', corticotropin-(18-39). The two endorphin-like peptides, although highly related, do not have the same primary structure. In view of the apparent structural differences between the two peptides in each group, the possible occurrence of two prohormones is discussed.

Gamma-melanotropin is not present in an N-terminal peptide of salmon proopiocortin

The N-terminal peptide of salmon proopiocortin has been isolated and the primary structure including two disulfide bonds elucidated. The peptide consisted of 76 amino acid residues, which is 27 residues shorter than the bovine and human peptides. The N-terminal 44 residues of the teleost peptide exhibited significant sequence homology to those of the mammalian peptides. The salmon peptide, however, is lacking in the counterpart of gamma-MSH which is located between residues 51 and 64 in the mammalian peptides.

Distribution of alpha-melanocyte-stimulating hormone in the rat brain: evidence that alpha-MSH-containing cells in the arcuate region send projections to extrahypothalamic areas

The distribution and concentration of alpha-MSH in the rodent brain has been determined by radioimmunoassay. The limbic system contained substantial quantities of alpha-MSH. Forty per cent of the alpha-MSH present in the brain was localized in the hypothalamus, with the highest concentration of alpha-MSH in the arcuate nucleus. More than 40% of the extrahypothalamic alpha-MSH in the brain was found in the following areas: midbrain (16%), preoptic area (13%), septum (7%), and thalamus (7%). To determine the source of the hypothalamic and extrahypothalamic alpha-MSH, the anterior hypothalamic preoptic area of the brain was surgically separated from more caudal diencephalic structures, and the arcuate region of the hypothalamus was surgically isolated from the remainder of the brain. Following these deafferentations, no significant reduction in hypothalamic alpha-MSH levels was observed; however, a significant reduction in extrahypothalamic alpha-MSH level was demonstrated. This dramatic decrease of alpha-MSH in extrahypothalamic areas of the rodent brain strongly suggests that the bulk of the extrahypothalamic alpha-MSH arises from neuronal perikarya in the arcuate region. These findings are consistent with the hypothesis that a population of neuronal cell bodies producing alpha-MSH originate in the arcuate region of the hypothalamus and that they send axonal projections to many areas of the limbic system and brain stem.

Immunological evidence for two common precursors to corticotropins, endorphins, and melanotropin in the neurointermediate lobe of the toad pituitary

The biosynthesis of corticotropin (ACTH1--39), beta-endorphin [beta(61--91)-lipotropin] and alpha-melanotropin in the toad intermediate lobe was studied by using immunoprecipitation procedures with antisera specific for these peptides. Intermediate lobes were pulse-incubated with [3H]phenylalanine and then chase-incubated for varying periods; the radioactive proteins were immunoprecipitated. Immunoprecipitates were separated by acidic urea or sodium dodecyl sulfate polyacrylamide gel electrophoresis. Evidence from the pulse-chase and sequential immunoprecipitation studies using antisera to ACTH and beta-endorphin suggests that the toad intermediate lobe synthesizes two common precursors (apparent Mr 32,000 and 29,500) containing both the ACTH and beta-endorphin sequences. These precursors are processed to yield several forms of immunoreactive corticotropin (apparent Mr 23,000, 21,000, 13,000, and 4300), immunoreactive endorphin (apparent Mr 11,700 and 3500), and immunoreactive alpha-melanotropin. The 4300 Mr form of corticotropin and the 11,700 and 3500 Mr forms of endorphins were found to comigrate with synthetic ACTH1--39, beta-lipotropin and beta-endorphin, respectively, on both acidic urea and sodium dodecyl sulfate gels.

Biosynthesis of MSH and related peptides in the pars intermedia of the mouse: a pulse-chase analysis

Protein biosynthesis in neurointermediate lobes of mouse pituitaries was investigated using pulse and pulse-chase techniques with [3H]lysine. Electrophoretic analysis of lobe homogenates on acid-urea gels resolved 11 labeled products. One was a large protein which was rapidly synthesized during pulse-incubations and disappeared during chase incubations. Three of the products increased during chase incubations, suggesting a precursor-product mode of biosynthesis for these chasde peptides. One of these three products co-migrated with synthetic alpha-MSH and also corresponds to the major peak of mouse neurointermediate lobe MSH bioactivity and immunoactivity on electrophoretograms. Another case of these peptides has electrophoretic properties similar to those of ACTH.

Melanotropic peptides: presence in brain of normal and hypophysectomized rats, and subcellularly localized in synaptosomes

A major and several minor trichloroacetic acid (TCA) soluble, bioassayable melanotropic peptides, as well as bioreactive and immunoreactive alpha-MSH have been found in the hypothalamus, olfactory bulb and cerebral cortex of normal and hypophysectomized rats. By employing subcellular fractionation procedures it was demonstrated that alpha-MSH and the major TCA soluble melanotropic peptide (MMPB) were localized in synaptosomes and were released by hypoosmotic shock. The analysis of MMPB by electrophoretic and chromatographic procedures reveales that it is not ACTH4-10, ACTH1-10, ACTH1-24, NAcACTH1-10 or alpha-MSH. MMPB was found to cross-react with an antiserum specific for the Lys-Pro-Val NH2 sequence in alpha-MSH, indicating that this C-terminal sequence of alpha-MSH may be present in its structure. MMPB was also shown to differ electrophoretically from the two major TCA soluble melanotropic peptides found in the neurointermediate lobe of the pituitary. In view of their synaptosomal localizations MMPB and alpha-MSH may play a role in synaptic function in the nervous system.