1
|
Abstract
Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.
Collapse
Affiliation(s)
- Stanko S Stojilkovic
- Program in Developmental Neuroscience, National Institute of Child Health and Human Development, National Institutes of Health, Building 49, Room 6A-36, 49 Convent Drive, Bethesda, Maryland 20892-4510, USA.
| | | | | |
Collapse
|
2
|
Osterstock G, Escobar P, Mitutsova V, Gouty-Colomer LA, Fontanaud P, Molino F, Fehrentz JA, Carmignac D, Martinez J, Guerineau NC, Robinson ICAF, Mollard P, Méry PF. Ghrelin stimulation of growth hormone-releasing hormone neurons is direct in the arcuate nucleus. PLoS One 2010; 5:e9159. [PMID: 20161791 PMCID: PMC2820089 DOI: 10.1371/journal.pone.0009159] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2009] [Accepted: 01/08/2010] [Indexed: 11/26/2022] Open
Abstract
Background Ghrelin targets the arcuate nucleus, from where growth hormone releasing hormone (GHRH) neurones trigger GH secretion. This hypothalamic nucleus also contains neuropeptide Y (NPY) neurons which play a master role in the effect of ghrelin on feeding. Interestingly, connections between NPY and GHRH neurons have been reported, leading to the hypothesis that the GH axis and the feeding circuits might be co-regulated by ghrelin. Principal Findings Here, we show that ghrelin stimulates the firing rate of identified GHRH neurons, in transgenic GHRH-GFP mice. This stimulation is prevented by growth hormone secretagogue receptor-1 antagonism as well as by U-73122, a phospholipase C inhibitor and by calcium channels blockers. The effect of ghrelin does not require synaptic transmission, as it is not antagonized by γ-aminobutyric acid, glutamate and NPY receptor antagonists. In addition, this hypothalamic effect of ghrelin is independent of somatostatin, the inhibitor of the GH axis, since it is also found in somatostatin knockout mice. Indeed, ghrelin does not modify synaptic currents of GHRH neurons. However, ghrelin exerts a strong and direct depolarizing effect on GHRH neurons, which supports their increased firing rate. Conclusion Thus, GHRH neurons are a specific target for ghrelin within the brain, and not activated secondary to altered activity in feeding circuits. These results support the view that ghrelin related therapeutic approaches could be directed separately towards GH deficiency or feeding disorders.
Collapse
Affiliation(s)
- Guillaume Osterstock
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - Pauline Escobar
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - Violeta Mitutsova
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - Laurie-Anne Gouty-Colomer
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - Pierre Fontanaud
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - François Molino
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - Jean-Alain Fehrentz
- Université Montpellier 1, 2, Montpellier, France
- CNRS UMR 5247, Institut des Biomolécules Max Mousseron, Montpellier, France
| | - Danielle Carmignac
- Division of Molecular Neuroendocrinology, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London, United Kingdom
| | - Jean Martinez
- Université Montpellier 1, 2, Montpellier, France
- CNRS UMR 5247, Institut des Biomolécules Max Mousseron, Montpellier, France
| | - Nathalie C. Guerineau
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - Iain C. A. F. Robinson
- Division of Molecular Neuroendocrinology, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London, United Kingdom
| | - Patrice Mollard
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
| | - Pierre-François Méry
- Inserm U-661, Montpellier, France
- CNRS UMR 5203, Institut de Génomique Fonctionnelle, Montpellier, France
- Université Montpellier 1, 2, Montpellier, France
- * E-mail:
| |
Collapse
|
5
|
Muccioli G, Baragli A, Granata R, Papotti M, Ghigo E. Heterogeneity of ghrelin/growth hormone secretagogue receptors. Toward the understanding of the molecular identity of novel ghrelin/GHS receptors. Neuroendocrinology 2007; 86:147-64. [PMID: 17622734 DOI: 10.1159/000105141] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/05/2007] [Accepted: 05/21/2007] [Indexed: 12/23/2022]
Abstract
Ghrelin is a gastric polypeptide displaying strong GH-releasing activity by activation of the type 1a GH secretagogue receptor (GHS-R1a) located in the hypothalamus-pituitary axis. GHS-R1a is a G-protein-coupled receptor that, upon the binding of ghrelin or synthetic peptidyl and non-peptidyl ghrelin-mimetic agents known as GHS, preferentially couples to G(q), ultimately leading to increased intracellular calcium content. Beside the potent GH-releasing action, ghrelin and GHS influence food intake, gut motility, sleep, memory and behavior, glucose and lipid metabolism, cardiovascular performances, cell proliferation, immunological responses and reproduction. A growing body of evidence suggests that the cloned GHS-R1a alone cannot be the responsible for all these effects. The cloned GHS-R1b splice variant is apparently non-ghrelin/GHS-responsive, despite demonstration of expression in neoplastic tissues responsive to ghrelin not expressing GHS-R1a; GHS-R1a homologues sensitive to ghrelin are capable of interaction with GHS-R1b, forming heterodimeric species. Furthermore, GHS-R1a-deficient mice do not show evident abnormalities in growth and diet-induced obesity, suggesting the involvement of another receptor. Additional evidence of the existence of another receptor is that ghrelin and GHS do not always share the same biological activities and activate a variety of intracellular signalling systems besides G(q). The biological actions on the heart, adipose tissue, pancreas, cancer cells and brain shared by ghrelin and the non-acylated form of ghrelin (des-octanoyl ghrelin), which does not bind GHS-R1a, represent the best evidence for the existence of a still unknown, functionally active binding site for this family of molecules. Finally, located in the heart and blood vessels is the scavenger receptor CD36, involved in the endocytosis of the pro-atherogenic oxidized low-density lipoproteins, which is a pharmacologically and structurally distinct receptor for peptidyl GHS and not for ghrelin. This review highlights the most recently discovered features of GHS-R1a and the emerging evidence for a novel group of receptors that are not of the GHS1a type; these appear involved in the transduction of the multiple levels of information provided by GHS and ghrelin.
Collapse
Affiliation(s)
- Giampiero Muccioli
- Division of Pharmacology, Department of Anatomy, Pharmacology and Forensic Medicine, University of Turin, Turin, Italy
| | | | | | | | | |
Collapse
|
6
|
Abstract
Ghrelin, a gastric peptide involved in growth hormone release and energy homeostasis, is the endogenous ligand of the growth hormone secretagogue receptor type 1a (GHS-R1a), a G-protein coupled receptor mainly expressed in the pituitary and hypothalamus. This receptor mediates the main ghrelin-stimulated endocrine actions and some of the nonendocrine actions. However, a number of nonendocrine actions associated with ghrelin appear to be mediated by various GHS-R1a-related receptor subtypes, which are widely distributed in the central and peripheral tissues. This review summarises data concerning the localisation, regulation and function of GHS-R1a, as well as related receptors.
Collapse
Affiliation(s)
- J P Camiña
- Laboratory of Molecular Endocrinology, Research Area, Complexo Hospitalario Universitario de Santiago (CHUS), PO Box 563, E-15780 Santiago de Compostela, Spain.
| |
Collapse
|
7
|
Xu R, Zhao Y, Chen C. Growth hormone-releasing peptide-2 reduces inward rectifying K+ currents via a PKA-cAMP-mediated signalling pathway in ovine somatotropes. J Physiol 2002; 545:421-33. [PMID: 12456822 PMCID: PMC2290704 DOI: 10.1113/jphysiol.2002.030916] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Inward-rectifying potassium (Kir) channels are essential for maintaining the resting membrane potential near the K(+) equilibrium and they are responsible for hyperpolarisation-induced K(+) influx. We characterised the Kir current in primary cultured ovine somatotropes and examined the effect of growth hormone-releasing peptide-2 (GHRP-2) on this current and its related intracellular signalling pathways. The Kir current was, in most cases, isolated using nystatin-perforated patch-clamp techniques. In bath solution containing 5 mM K(+), the Kir current was composed of both transient (fast activated) and delayed (slowly activated) components. An increase in the external K(+) concentration from 5 to 25 mM induced an augmentation of approximately 4-fold in the delayed part of the Kir current and both BaCl(2) and CsCl dose-dependently inhibited this current, confirming the presence of the Kir current in ovine somatotropes. Moreover, this specific effect of high K(+) on the Kir current was only observed in the cells that showed positive staining with anti-growth hormone (GH) antibodies, or in GC cells that belong to a rat somatotrope cell line. Application of GHRP-2 (100 nM) reversibly and significantly reduced the Kir current in bath solutions with 5 or 25 mM K(+) in ovine somatotropes. In addition, we found that the reduction in the Kir current mediated by GHRP-2 was totally abolished by the pretreatments with H89 (1 microM) or Rp-cAMP (100 microM) or by intracellular dialysis of a specific protein kinase A (PKA) inhibitory peptide PKI (10 microM). The specific PKC blocker chelerythrine (1 microM) or inhibitory peptide PKC(19-36) (10 microM) did not show any effects on the GHRP-2-induced decrease in the Kir current. These results suggest that the inhibition of Kir current through PKA-cAMP pathways may play an integral role in GHRP-2-induced depolarisation and GH release in ovine somatotropes.
Collapse
Affiliation(s)
- Ruwei Xu
- Prince Henry's Institute of Medical Research, Clayton, Victoria 3168, Australia
| | | | | |
Collapse
|
8
|
Kaneishi K, Sakuma Y, Kobayashi H, Kato M. 3',5'-cyclic adenosine monophosphate augments intracellular Ca2+ concentration and gonadotropin-releasing hormone (GnRH) release in immortalized GnRH neurons in an Na+ -dependent manner. Endocrinology 2002; 143:4210-7. [PMID: 12399414 DOI: 10.1210/en.2002-220508] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
In GT1-7 cells, cAMP increases the intracellular Ca2+ concentration ([Ca2+](i)) through activation of the voltage-gated Ca2+ channels, thereby facilitating GnRH release. To activate these channels, the membrane potential must be depolarized. In the present study we hypothesize that cAMP depolarizes the cells by increasing the membrane Na+ permeability, as in the case of somatotrophs and pancreatic beta-cells. To examine this, we analyzed [Ca2+](i) and [Na+](i) in GT1-7 cells by an intracellular ion-imaging technique along with cAMP assay by RIA. Forskolin, a direct activator of adenylyl cyclase, increased [Ca2+](i) and [Na+](i) via cAMP formation. The forskolin-induced increase in [Ca2+](i) depended on the presence of Ca2+ and Na+ in the extracellular solution. This response was blocked by the voltage-gated Ca2+ channel blocker, nifedipine; the nonselective cation channel blocker, gadolinium (Gd3+); and the cyclic nucleotide-gated channel blocker, l-cis-diltiazem. In contrast, the forskolin-induced increase in [Na+](i) depended only on extracellular Na+, not on Ca2+. Gd3+ and l-cis-diltiazem also blocked the increase in [Na+](i). Furthermore, the forskolin-induced increase in GnRH release was blunted in both low Ca2+ and low Na+ media. The results indicate that cAMP increases the membrane Na+ permeability, probably through nonselective cation channels on GT1-7 cells, thereby promoting GnRH release.
Collapse
Affiliation(s)
- Keisuke Kaneishi
- Department of Physiology, Nippon Medical School, Tokyo 113-8602, Japan
| | | | | | | |
Collapse
|
9
|
Gracia-Navarro F, Castaño JP, Malagon MM, Sánchez-Hormigo A, Luque RM, Hickey GJ, Peinado JR, Delgado E, Martínez-Fuentes AJ. Research progress in the stimulatory inputs regulating growth hormone (GH) secretion. Comp Biochem Physiol B Biochem Mol Biol 2002; 132:141-50. [PMID: 11997217 DOI: 10.1016/s1096-4959(01)00544-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
A review is presented on progress in the research of stimulatory inputs that regulate growth hormone secretion, including recent results on the action of the hypothalamic peptides growth-hormone releasing factor (GHRH) and pituitary adenylate cyclase-activating polypeptide (PACAP), as well as that of both peptidic (growth hormone-releasing hexapeptide; GHRP-6) and non-peptidyl (L-163,255) synthetic GHSs on somatotrope cell function.
Collapse
Affiliation(s)
- F Gracia-Navarro
- Department of Cell Biology, Edificio C-6, 3a Planta, Campus Universitario de Rabanales, University of Córdoba, E-14071 Córdoba, Spain.
| | | | | | | | | | | | | | | | | |
Collapse
|