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B G Jenks

Publications and source records attributed to B G Jenks.

At least 55 records · Page 3Linked to original sources
PubMed1996

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

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

Analysis of Variance↗
PubMed1995

Inhibition of alpha-MSH secretion is associated with increased cyclic-AMP egress from the neurointermediate lobe of Xenopus laevis.

Cyclic-AMP is known to be released from cells and tissues and the amounts released have been reported to reflect intracellular cAMP levels. To measure cAMP release the phosphodiesterase inhibitor IBMX is often used to increase the amount of cAMP to a detectable level. Using this method to follow cAMP dynamics of melanotrope cells in the neurointermediate pituitary lobe of the amphibian Xenopus laevis we show that the alpha-MSH secreto-inhibitors baclofen (GABAB receptor agonist) and dopamine inhibit cAMP release, confirming the idea that these factors inhibit alpha-MSH secretion by reducing adenylyl cyclase activity. Using a sensitive cAMP radioimmunoassay we were able to measure cAMP release from Xenopus neurointermediate lobes in the absence of IBMX. Both baclofen and dopamine appeared to inhibit alpha-MSH secretion but strongly stimulated the release of cAMP. This indicates that the extracellular cAMP level is not a reliable parameter to measure the intracellular cAMP level in the absence of IBMX. The data furthermore suggest that cAMP release is a physiologically regulated process, which might be involved in lowering intracellular cAMP levels associated with a cellular secretory compartment. No apparent differences could be found in the lobe content of cAMP at the termination of secreto-inhibitor treatment, leading to the idea that the cAMP compartment associated with secretion in small relative to the total amount of cAMP present in the lobe.

1-Methyl-3-isobutylxanthine↗
PubMed1995

Neuropeptide Y inhibits Ca2+ oscillations, cyclic AMP, and secretion in melanotrope cells of Xenopus laevis via a Y1 receptor.

The melanotrope cells in the pituitary gland of Xenopus laevis are innervated by neurons containing neuropeptide Y (NPY). In the present study, the mechanism of action of NPY on the melanotropes has been investigated. NPY inhibited in vitro secretion from melanotropes in intact neurointermediate lobes as well as from isolated, single melanotropes. Inhibition of secretion from neurointermediate lobes was mimicked by the NPY analogues PYY and [Leu31,Pro34]NPY, whereas NPY(13-36) was inactive. Secretion from isolated melanotropes was inhibited by [Leu31,Pro34]NPY and NPY(13-36), but NPY(13-36) was 10-fold less potent than [Leu31,Pro34]NPY. Studies on isolated cells revealed that NPY and its analogues inhibited the occurrence of intracellular Ca2+ oscillations with the same potency as they inhibited secretion from isolated cells. In addition to inhibiting basal secretion and spontaneous Ca2+ oscillations, NPY inhibited the basal production of cyclic AMP. On the basis of these results it is proposed that NPY inhibits secretion from Xenopus melanotropes by inhibiting cyclic AMP-dependent spontaneous Ca2+ oscillations through a Y1-like receptor.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1995

Biosynthesis and processing of the N-terminal part of proopiomelanocortin in Xenopus laevis: characterization of gamma-MSH peptides.

The aim of this study was to determine the terminal products of processing of the N-terminal part of proopiomelanocortin (POMC) in pituitary melanotrope cells of Xenopus laevis. Biosynthetic in vitro labelling studies showed that POMC is rapidly processed to form N-terminal peptides with an estimated molecular mass of 18 kDa, 9 kDa and 4 kDa. All peptides were released into the medium, indicating that they are processing end products. An antiserum was raised against the synthetic N-terminal eight amino acids of the putative Xenopus gamma-MSH which is present in the N-terminal part of POMC. With immunocytochemistry we demonstrated that gamma-MSH-immunoreactive material in the pituitary gland is restricted to the pars intermedia. A radioimmunoassay in combination with reversed-phase HPLC revealed the presence of at least two gamma-MSH-like peptides. Complete purification followed by electrospray ionization mass spectrometry and amino acid sequence determination showed that these peptides are gamma 1-MSH and glycosylated gamma 3-MSH. The amounts of these gamma-MSH peptides were low compared to the other POMC-derived peptides, alpha-MSH and beta-endorphin. Only 10% of POMC is processed into gamma-MSH peptides and the 4 kDa peptide, leaving the 18 kDa and 9 kDa peptides as the major end products.

Adaptation, Physiological↗
PubMed1995

Differential acetylation of pro-opiomelanocortin-derived peptides in the pituitary gland of Xenopus laevis in relation to background adaptation.

Immunocytochemical analysis revealed the presence of acetylated endorphins in both melanotropes and corticotropes of the pituitary gland of Xenopus laevis. Chemical acetylation studies to determine the steady-state level of acetylated versus non-acetylated endorphins showed that virtually all endorphins are acetylated in both melanotropes and corticotropes. Apparently Xenopus is unique among vertebrates as non-acetylated endorphins are major end-products in the distal lobe of all other vertebrate species studied thus far. The dynamics of endorphin biosynthesis in melanotrope cells using pulse-chase analysis coupled to immunoaffinity chromatography revealed that processing of pro-opiomelanocortin to produce N-terminal-acetylated endorphins is very rapid. To determine the effect of long-term background adaptation on acetylation status of endorphins and alpha-MSH-related peptides, Xenopus laevis were adapted for 3 or 6 weeks to either a black or a white background. In both physiological states the major intracellular form of alpha-MSH-related peptides in melanotropes was desacetyl alpha-MSH while the major endorphin-related peptide was alpha, N-acetyl-beta-endorphin[1-8]. In the medium of superfused neuro-intermediate lobes of black background-adapted animals the major form of secreted melanotropins and endorphins was alpha-MSH and alpha, N-acetyl-beta-endorphin[1-8] respectively. We conclude that there is a marked spatio-temporal difference in acetylation of melanotropin and endorphins, with rapid intracellular acetylation of endorphins while melanotropin is acetylated at the time of its exocytosis. In the medium of superfused neurointermediate lobes of white background-adapted animals the amount of desacetyl alpha-MSH was much higher than in the medium of lobes of black-adapted animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylation↗
PubMed1994

Action of stimulatory and inhibitory alpha-MSH secretagogues on spontaneous calcium oscillations in melanotrope cells of Xenopus laevis.

The secretion of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells in the pituitary gland of Xenopus laevis is regulated by various neural factors, both classical neurotransmitters and neuropeptides. The majority of these cells (80%) display spontaneous Ca2+ oscillations. In order to gain a better understanding of the external regulation of intracellular Ca2+ ([Ca2+]i) in the melanotrope cell, we have examined the action of well known alpha-MSH secretagogues on the Ca2+ oscillations. It is shown that all secretagogues tested also control the oscillatory state of Xenopus melanotropes, that is, the secreto-inhibitors dopamine, isoguvacine (gamma-aminobutyric acid, GABAA agonist), baclofen (GABAB agonist) and neuropeptide Y evoked a rapid quenching of the spontaneous Ca2+ oscillations, whereas the secreto-stimulant sauvagine, an amphibian peptide related to corticotropin releasing hormone, induced oscillatory activity in non-oscillating cells. Supporting argument is given for the idea that the regulation of Ca2+ oscillations is a focal point in the regulation of secretory activity of melanotrope cells. There was considerable heterogeneity among melanotrope cells in the threshold of their Ca2+ response to secretagogue treatment. This heterogeneity may be the basis for melanotrope cell recruitment observed during physiological adaptations of the animal to the light intensity of its background.

Amphibian Proteins↗
PubMed1994

Spontaneous calcium oscillations in Xenopus laevis melanotrope cells are mediated by omega-conotoxin sensitive calcium channels.

The dynamics of intracellular Ca2+ signalling in single melanotrope cells of the pituitary gland of the amphibian Xenopus laevis have been studied by means of a digital imaging technique using the fluorescent dye Fura-2. When placed in vitro, the majority of the cells (77%) displayed spontaneous oscillatory changes in the free cytosolic Ca2+ concentration with a frequency of 1 +/- 0.25 (SD) min-1. The oscillations rapidly stopped when extracellular Ca2+ was reduced to nanomolar concentrations, revealing their complete dependence on Ca2+ influx. The fact that the Ca2+ oscillations were blocked by 1 microM omega-conotoxin, but not by nifedipine, at concentrations up to 50 microM, indicated that Ca2+ entered the cell via N-type rather than L-type voltage operated Ca2+ channels. Thapsigargin, a putative inhibitor of intracellular Ca(2+)-ATPase activity, elevated the baseline Ca2+ concentration but had no effect on the occurrence of the spontaneous oscillations. This suggests that intracellular Ca2+ pools are not involved in the mechanism underlying spontaneous Ca2+ oscillations. This is the first report showing spontaneous Ca2+ oscillations mediated by N-type Ca2+ channels in melanotrope cells.

Animals↗
PubMed1994

The secretion of alpha-MSH from xenopus melanotropes involves calcium influx through omega-conotoxin-sensitive voltage-operated calcium channels.

The secretory activity of endocrine cells largely depends on the concentration of free cytosolic calcium. We have studied the mechanisms that are involved in supplying the calcium necessary for the secretion of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells in the pituitary intermediate lobe of the amphibian Xenopus laevis. Using whole-cell voltage clamp, high-voltage activated calcium currents were observed, with a peak current between 0 and +20 mV. Two types of Ca(2+)-currents appeared, depending on the experimental setup. An inactivating current, which was observed after a 10 msec depolarizing prepulse, resembled currents through N-type channels as it was clearly inhibited by 1 microM omega-conotoxin. The second type was a non-inactivating current, which was blocked up to 50% by 1 microM nifedipine, indicating its L-type nature. Only a small component of this inactivating current could be blocked by omega-conotoxin. No evidence was found for the presence of transient, low-voltage activated currents. The spontaneous secretion of alpha-MSH from superfused neurointermediate lobes was dependent on extracellular calcium, as low calcium conditions (10(-4)-10(-8) M) rapidly inhibited this process. Under these conditions, secretion was not affected by depolarizing concentrations of potassium chloride. The calcium ionophore A23187 increased secretion under low calcium conditions, but had no effect on spontaneous alpha-MSH release. These results suggest that spontaneous alpha-MSH release depends on influx of calcium through voltage-operated calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Development of the ectopically transplanted primordium of epithelial hypophysis (anterior neural ridge) in Bufo japonicus embryos.

It has recently been demonstrated that the epithelial pituitary of the toad is not stomodeal, but placodal, in origin. The placodal cells in the anterior part of the neural ridge (ANR) of the open neurulae are the exclusive source of the epithelial pituitary gland. The present study was undertaken to see the self-differentiating ability of these cells in an ectopic environment. Bufo japonicus embryos at the tailbud stage received implants of either the ANR from open-neurula-stage embryos, or the pituitary primordium from tailbud-stage embryos (the ANR derivative beneath the forebrain floor) into the tail. Development of the pars intermedia and the pars distalis was monitored immunohistochemically using antisera against both synthetic alpha melanophore-stimulating hormone (alpha MSH) and bullfrog prolactin (PRL). Neither the immunoreactive alpha MSH cells nor the immunoreactive PRL cells differentiated from the neural ridge when it was dislocated from the original site at the open neurula stage. On the other hand, in grafts of the pituitary primordium transplanted from the tailbud-stage embryos, immunoreactive PRL cells developed invariably and immunoreactive alpha MSH cells were detected at an incidence of 72%. The significance of the role of brain tissue surrounding the pituitary anlage in differentiation of the pars intermedia and pars distalis is discussed.

Animals↗
PubMed1993

Apomorphine-susceptible and apomorphine-unsusceptible Wistar rats differ in novelty-induced changes in hippocampal dynorphin B expression and two-way active avoidance: a new key in the search for the role of the hippocampal-accumbens axis.

The present study examines two characteristic traits of the hippocampus in apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible (APO-UNSUS) Wistar rat lines. Since hippocampal mossy fibers contain among others dynorphin B as transmitter, a radioimmunoassay was used to analyze the hippocampal dynorphin B expression in response to novelty in these lines. Dynorphin B expression at the end of the baseline condition was greater in APO-SUS rats than in APO-UNSUS rats, while exposure to novelty decreased and increased the dynorphin B expression in APO-SUS and APO-UNSUS rats, respectively. These interline differences in dynorphin B expression could be due to (a) an interline difference in the size of the mossy fiber terminal fields, (b) an interline difference in the regulation of the firing rate of mossy fibers by corticosteroids, and/or (c) an interline difference in the release of corticosteroids in response to novelty. Since the size of the mossy fiber infra/intrapyramidal terminal field is inversely related to two-way active avoidance performance, APO-SUS and APO-UNSUS rats (n = 9 per line) were given this task: APO-UNSUS rats performed much better than APO-SUS rats. It is concluded that the neurochemical and behavioural function of the hippocampus significantly differs between lines. Given the already known interline differences in the function of the nucleus accumbens, the present results provide a new avenue in search for the functional relationship between the hippocampus and the nucleus accumbens.

Animals↗
PubMed1993

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.

Acclimatization↗
PubMed1993

Alpha,N-acetyl beta-endorphin [1-8] is the terminal product of processing of endorphins in the melanotrope cells of Xenopus laevis, as demonstrated by FAB tandem mass spectrometry.

The major N-terminal acetylated endorphin of the pars intermedia of Xenopus laevis was purified and submitted to fast-atom bombardment tandem mass spectroscopy. The collisionally induced dissociation MS/MS spectrum of the [M+H]+ ion revealed sufficient fragment ions to determine unambiguously the identity of the peptide as alpha,N-acetyl beta-endorphin [1-8], the sequence of which was predicted on the basis of the nucleotide sequence of Xenopus POMC cDNA. The determination was confirmed by showing that the synthetic peptide of this structure had identical FAB tandem mass spectrometric characteristics as the endogenous endorphin. We conclude that alpha,N-acetyl beta-endorphin [1-8] is the terminal product of processing of endorphins in the melanotrope cell of Xenopus laevis.

Animals↗
PubMed1993

Differential effects of coexisting dopamine, GABA and NPY on alpha-MSH secretion from melanotrope cells of Xenopus laevis.

The secretion of alpha-MSH from the intermediate lobe of the pituitary gland of the amphibian Xenopus laevis is under complex neural control. Three neurotransmitters, dopamine, GABA and NPY, coexist in nerve terminals that contact the melanotrope cells. All three neurotransmitters inhibit alpha-MSH release. We have investigated the significance of this neurotransmitter coexistence for the regulation of alpha-MSH release, using an in vitro superfusion system. From experiments where lobes were treated with various combinations of receptor agonists we conclude that the transmitters act in an additive way but have clear, differential actions. Inhibition of secretion by either dopamine, isoguvacine (GABAA receptor agonist) or baclofen (GABAB receptor agonist) occurs rapidly and alpha-MSH secretion rapidly returns when treatment is terminated (recovery from baclofen being relatively fast, that from dopamine relatively slow); in contrast, inhibition by NPY and recovery from NPY-induced inhibition occurs only very slowly. Differential effects of the transmitters were also seen in experiments with 8-bromo-cyclic AMP, which strongly stimulates alpha-MSH secretion from isoguvacine- or baclofen-treated lobes, but is relatively ineffective in stimulating secretion from lobes treated with dopamine or NPY. NPY, furthermore, enables a short phasic stimulation of secretion by isoguvacine and attenuates the inhibitory action of dopamine and baclofen. Altogether it is concluded that the coexisting factors differentially affect the secretory process of the melanotrope cells of Xenopus laevis. NPY has a slow, sustained action whereas dopamine and GABA act fast.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1993

Dual action of GABAA receptors on the secretory process of melanotrophs of Xenopus laevis.

The effects of GABAa receptor activation on the secretion of alpha-melanocyte-stimulating hormone (alpha-MSH) from the superfused pars intermedia of the amphibian Xenopus laevis were examined. The GABAa receptor agonist isoguvacine inhibited secretion of alpha-MSH from the pars intermedia, an action completely antagonized by the chloride channel blocker picrotoxin. Isoguvacine stimulated secretion from picrotoxin-treated tissue. Both effects were blocked by the GABAA receptor antagonist bicuculline, indicating that it is a specific action via the GABAA receptor. We conclude that, besides the inhibitory chloride channel, there is a stimulatory signaling property associated with the GABAA receptor. Isoguvacine stimulated the production of c-AMP, an action that was not blocked by picrotoxin. This suggests that the stimulatory mechanism of the GABAA receptor involves, directly or indirectly, the generation of c-AMP.

Animals↗
PubMed1993

Analysis of gamma-aminobutyric acidB receptor function in the in vitro and in vivo regulation of alpha-melanotropin-stimulating hormone secretion from melanotrope cells of Xenopus laevis.

The activity of many endocrine cells is regulated by gamma-aminobutyric acid (GABA). The effects of GABA are mediated by GABAA and/or GABAB receptors. While GABAB receptors in the central nervous system have now been extensively characterized, little is known of the function and pharmacology of GABAB receptors on endocrine cells. In the amphibian Xenopus laevis, GABA inhibits the release of alpha MSH from the endocrine melanotrope cells through both GABAA and GABAB receptors. We have investigated the following aspects of the GABAB receptor of the melanotrope cells of X. laevis: 1) the pharmacology of this receptor, using antagonists previously established to demonstrate GABAB receptors in the mammalian central nervous system; 2) the relative contribution to the regulation of hormone secretion by the GABAA and GABAB receptors on melanotrope cells in vitro; and 3) the role of the GABAB receptor with respect to the physiological function of the melanotrope cell in vivo, i.e. regulation of pigment dispersion in skin melanophores in relation to background color. Our results demonstrate that phaclofen, 2-hydroxysaclofen, and 4-aminobutylphosphonic acid dose-dependently blocked the inhibition of alpha MSH release by GABAB receptor activation, but not by GABAA receptor activation. The GABAB receptor antagonist delta-aminovaleric acid appeared to be a selective agonist on the GABAB receptor of melanotrope cells. The inhibitory secretory response to a low dose of GABA (10(-5) M) was not affected by bicuculline, but was significantly reduced by phaclofen, indicating that at a low GABA concentration, the GABAB receptor mechanism would dominate in inhibiting the melanotrope cells. Different thresholds of activation may form the basis for differential action of GABA through both GABA receptor types. The tonic inhibition of alpha MSH release in animals adapted to a white background was not affected by 4-aminobutylphosphonic acid, indicating that the GABAB receptor is not (solely) involved in the in vivo inhibition of alpha MSH release in animals on a white background.

Animals↗