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

Osterstock G, Escobar P, Mitutsova V, Gouty-Colomer LA, Fontanaud P, Molino F, Fehrentz JA, Carmignac D, Martinez J, Guerineau NC, Robinson IC, Mollard P, Méry PF - PLoS ONE (2010)

Bottom Line: 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.These results support the view that ghrelin related therapeutic approaches could be directed separately towards GH deficiency or feeding disorders.

View Article: PubMed Central - PubMed

Affiliation: Inserm U-661, Montpellier, France.

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.

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Ghrelin enhanced the activity of GHRH neurons.A, time course of an experiment where the superfusion of a sagittal brain slice with 10 nM ghrelin increased, in a reversible manner, the rate of spontaneous action potentials of a GHRH neuron (individual traces shown on the top). B, summary of the effects of ghrelin (10 nM) on the cumulative distributions of action potential frequencies in GHRH neurons from adult males; C, mean effects of 0.3 to 10 nM ghrelin on the rate of spontaneous action potentials in GHRH neurons: the action potential frequencies observed at the half maximal values of the cumulated histograms were collected in each experiment in the absence and presence of ghrelin (see Methods for details). D, the proportion of stimulatory effects induced by ghrelin increased in a dose-dependent manner in GHRH neurons. E, summary of the effects of ghrelin (10 nM) on the distributions of action potential frequencies in GHRH neurons from adult females. In B & E, the symbols and lines are the means and sem. Statistical differences (p<0.05, paired student-t test) between curves are framed by the grey areas. In D, the bars and lines are the means and sem of the numbers of experiments indicated. ***, statistical difference from control values (p<0.001, paired student-t test).
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pone-0009159-g001: Ghrelin enhanced the activity of GHRH neurons.A, time course of an experiment where the superfusion of a sagittal brain slice with 10 nM ghrelin increased, in a reversible manner, the rate of spontaneous action potentials of a GHRH neuron (individual traces shown on the top). B, summary of the effects of ghrelin (10 nM) on the cumulative distributions of action potential frequencies in GHRH neurons from adult males; C, mean effects of 0.3 to 10 nM ghrelin on the rate of spontaneous action potentials in GHRH neurons: the action potential frequencies observed at the half maximal values of the cumulated histograms were collected in each experiment in the absence and presence of ghrelin (see Methods for details). D, the proportion of stimulatory effects induced by ghrelin increased in a dose-dependent manner in GHRH neurons. E, summary of the effects of ghrelin (10 nM) on the distributions of action potential frequencies in GHRH neurons from adult females. In B & E, the symbols and lines are the means and sem. Statistical differences (p<0.05, paired student-t test) between curves are framed by the grey areas. In D, the bars and lines are the means and sem of the numbers of experiments indicated. ***, statistical difference from control values (p<0.001, paired student-t test).

Mentions: We examined the effects of ghrelin on the electrical activity of identified GHRH neurons in brain slices from GHRH-GFP mice. In the experiment of Fig. 1A, spontaneous action potentials were first recorded under control conditions. Addition of 10 nM ghrelin to the external solution increased the firing rate from ∼0.2 to 0.9 Hz, and this stimulation disappeared during the washout of the peptide. The cumulative histograms of Fig. 1B summarize the results from similar experiments where the instantaneous frequencies of the spontaneous action potentials of GHRH neurons were compared under steady-state conditions in the absence and presence of 10 nM ghrelin (see Methods for additional information). The mean distribution under control conditions was shifted to the right (into the 0–18.5 Hz range) in the presence of ghrelin (grey area, n = 28, paired student's t-test, p<0.05). This increase in firing rate was also well described as an increase in the mean frequency at the half maximal values of the cumulated histograms (Fig. 1C). Lower concentrations of ghrelin (0.3–3 nM, n = 5 to 10) did not significantly change this parameter (Fig. 1C), and did not significantly shift the cumulative distribution of GHRH neuron action potentials (data not shown). However, 0.3–3 nM ghrelin occasionally enhanced the firing rate of GHRH neurons, and the proportion of responses increased in a concentration-dependent manner (Fig. 1D). Since 10 nM ghrelin always enhanced the electrical activity of GHRH neurons, the other effects of ghrelin were studied at this concentration.


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

Osterstock G, Escobar P, Mitutsova V, Gouty-Colomer LA, Fontanaud P, Molino F, Fehrentz JA, Carmignac D, Martinez J, Guerineau NC, Robinson IC, Mollard P, Méry PF - PLoS ONE (2010)

Ghrelin enhanced the activity of GHRH neurons.A, time course of an experiment where the superfusion of a sagittal brain slice with 10 nM ghrelin increased, in a reversible manner, the rate of spontaneous action potentials of a GHRH neuron (individual traces shown on the top). B, summary of the effects of ghrelin (10 nM) on the cumulative distributions of action potential frequencies in GHRH neurons from adult males; C, mean effects of 0.3 to 10 nM ghrelin on the rate of spontaneous action potentials in GHRH neurons: the action potential frequencies observed at the half maximal values of the cumulated histograms were collected in each experiment in the absence and presence of ghrelin (see Methods for details). D, the proportion of stimulatory effects induced by ghrelin increased in a dose-dependent manner in GHRH neurons. E, summary of the effects of ghrelin (10 nM) on the distributions of action potential frequencies in GHRH neurons from adult females. In B & E, the symbols and lines are the means and sem. Statistical differences (p<0.05, paired student-t test) between curves are framed by the grey areas. In D, the bars and lines are the means and sem of the numbers of experiments indicated. ***, statistical difference from control values (p<0.001, paired student-t test).
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2820089&req=5

pone-0009159-g001: Ghrelin enhanced the activity of GHRH neurons.A, time course of an experiment where the superfusion of a sagittal brain slice with 10 nM ghrelin increased, in a reversible manner, the rate of spontaneous action potentials of a GHRH neuron (individual traces shown on the top). B, summary of the effects of ghrelin (10 nM) on the cumulative distributions of action potential frequencies in GHRH neurons from adult males; C, mean effects of 0.3 to 10 nM ghrelin on the rate of spontaneous action potentials in GHRH neurons: the action potential frequencies observed at the half maximal values of the cumulated histograms were collected in each experiment in the absence and presence of ghrelin (see Methods for details). D, the proportion of stimulatory effects induced by ghrelin increased in a dose-dependent manner in GHRH neurons. E, summary of the effects of ghrelin (10 nM) on the distributions of action potential frequencies in GHRH neurons from adult females. In B & E, the symbols and lines are the means and sem. Statistical differences (p<0.05, paired student-t test) between curves are framed by the grey areas. In D, the bars and lines are the means and sem of the numbers of experiments indicated. ***, statistical difference from control values (p<0.001, paired student-t test).
Mentions: We examined the effects of ghrelin on the electrical activity of identified GHRH neurons in brain slices from GHRH-GFP mice. In the experiment of Fig. 1A, spontaneous action potentials were first recorded under control conditions. Addition of 10 nM ghrelin to the external solution increased the firing rate from ∼0.2 to 0.9 Hz, and this stimulation disappeared during the washout of the peptide. The cumulative histograms of Fig. 1B summarize the results from similar experiments where the instantaneous frequencies of the spontaneous action potentials of GHRH neurons were compared under steady-state conditions in the absence and presence of 10 nM ghrelin (see Methods for additional information). The mean distribution under control conditions was shifted to the right (into the 0–18.5 Hz range) in the presence of ghrelin (grey area, n = 28, paired student's t-test, p<0.05). This increase in firing rate was also well described as an increase in the mean frequency at the half maximal values of the cumulated histograms (Fig. 1C). Lower concentrations of ghrelin (0.3–3 nM, n = 5 to 10) did not significantly change this parameter (Fig. 1C), and did not significantly shift the cumulative distribution of GHRH neuron action potentials (data not shown). However, 0.3–3 nM ghrelin occasionally enhanced the firing rate of GHRH neurons, and the proportion of responses increased in a concentration-dependent manner (Fig. 1D). Since 10 nM ghrelin always enhanced the electrical activity of GHRH neurons, the other effects of ghrelin were studied at this concentration.

Bottom Line: 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.These results support the view that ghrelin related therapeutic approaches could be directed separately towards GH deficiency or feeding disorders.

View Article: PubMed Central - PubMed

Affiliation: Inserm U-661, Montpellier, France.

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.

Show MeSH
Related in: MedlinePlus