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Ghrelin stimulation of growth hormone-releasing hormone neurons is direct in the arcuate nucleus.

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 gamma-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.

SUBMITTER: Osterstock G

PROVIDER: S-EPMC2820089 | biostudies-literature | 2010 Feb

REPOSITORIES: biostudies-literature

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Ghrelin stimulation of growth hormone-releasing hormone neurons is direct in the arcuate nucleus.

Osterstock Guillaume G Escobar Pauline P Mitutsova Violeta V Gouty-Colomer Laurie-Anne LA Fontanaud Pierre P Molino François F Fehrentz Jean-Alain JA Carmignac Danielle D Martinez Jean J Guerineau Nathalie C NC Robinson Iain C A F IC Mollard Patrice P Méry Pierre-François PF

PloS one 20100211 2

<h4>Background</h4>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.<h4>Principal findings</h4>Here, we show that ghrelin ...[more]

PMID: 20161791

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Project description:It is known that growth hormone (GH) is expressed in immune cells, where it exerts immunomodulatory effects. However, the mechanisms of expression and release of GH in the immune system remain unclear. We analyzed the effect of growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), ghrelin (GHRL), and somatostatin (SST) upon GH mRNA expression, intracellular and released GH, Ser133-phosphorylation of CREB (pCREBS133), intracellular Ca2+ levels, as well as B-cell activating factor (BAFF) mRNA expression in bursal B-lymphocytes (BBLs) cell cultures since several GH secretagogues, as well as their corresponding receptors (-R), are expressed in B-lymphocytes of several species. The expression of TRH/TRH-R, ghrelin/GHS-R1a, and SST/SST-Rs (Subtypes 1 to 5) was observed in BBLs by RT-PCR and immunocytochemistry (ICC), whereas GHRH/GHRH-R were absent in these cells. We found that TRH treatment significantly increased local GH mRNA expression and CREB phosphorylation. Conversely, SST decreased GH mRNA expression. Additionally, when added together, SST prevented TRH-induced GH mRNA expression, but no changes were observed in pCREBS133 levels. Furthermore, TRH stimulated GH release to the culture media, while SST increased the intracellular content of this hormone. Interestingly, SST inhibited TRH-induced GH release in a dose-dependent manner. The coaddition of TRH and SST decreased the intracellular content of GH. After 10 min. of incubation with either TRH or SST, the intracellular calcium levels significantly decreased, but they were increased at 60 min. However, the combined treatment with both peptides maintained the Ca2+ levels reduced up to 60-min. of incubation. On the other hand, BAFF cytokine mRNA expression was significantly increased by TRH administration. Altogether, our results suggest that TRH and SST are implicated in the regulation of GH expression and release in BBL cultures, which also involve changes in pCREBS133 and intracellular Ca2+ concentration. It is likely that TRH, SST, and GH exert autocrine/paracrine immunomodulatory actions and participate in the maturation of chicken BBLs.

Project description:BackgroundEnhanced GABA activity in the brain and a hyperactive hypothalamic-pituitary-gonadal axis are associated with polycystic ovary syndrome (PCOS), the most common form of anovulatory infertility. Women with PCOS exhibit elevated cerebrospinal fluid GABA levels and preclinical models of PCOS exhibit increased GABAergic input to GnRH neurons, the central regulators of reproduction. The arcuate nucleus (ARN) is postulated as the anatomical origin of elevated GABAergic innervation; however, the functional role of this circuit is undefined.MethodsWe employed a combination of targeted optogenetic and chemogenetic approaches to assess the impact of acute and chronic ARN GABA neuron activation. Selective acute activation of ARN GABA neurons and their fiber projections was coupled with serial blood sampling for luteinizing hormone secretion in anesthetized male, female and prenatally androgenised (PNA) mice modelling PCOS. In addition, GnRH neuron responses to ARN GABA fiber stimulation were recorded in ex vivo brain slices. Chronic activation of ARN GABA neurons in healthy female mice was coupled with reproductive phenotyping for PCOS-like features.FindingsAcute stimulation of ARN GABA fibers adjacent to GnRH neurons resulted in a significant and long-lasting increase in LH secretion in male and female mice. The amplitude of this response was blunted in PNA mice, which also exhibited a blunted LH response to GnRH administration. Infrequent and variable GABAA-dependent changes in GnRH neuron firing were observed in brain slices. Chronic activation of ARN GABA neurons in healthy females impaired estrous cyclicity, decreased corpora lutea number and increased circulating testosterone levels.InterpretationARN GABA neurons can stimulate the hypothalamic-pituitary axis and chronic activation of ARN GABA neurons can mimic the reproductive deficits of PCOS in healthy females. Unexpectedly blunted HPG axis responses in PNA mice may reflect a history of high frequency GnRH/LH secretion and reduced LH stores, but also raise questions about impaired function within the ARN GABA population and the involvement of other circuits.

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Ghrelin stimulation of growth hormone-releasing hormone neurons is direct in the arcuate nucleus.

Publications

Ghrelin stimulation of growth hormone-releasing hormone neurons is direct in the arcuate nucleus.

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