Ghrelin Stimulation of Growth Hormone-Releasing Hormone Neurons Is Direct in the Arcuate Nucleus

Abstract

BackgroundGhrelin 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 FindingsHere, 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.ConclusionThus, 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.

Peptide hormone ghrelin enhances neuronal excitability by inhibition of Kv7/KCNQ channels

“…The KCNQ/M-current deactivation kinetics features KCNQ2 channels that have been shown to be expressed in nigral dopaminergic neurons 15,18 . In addition, although previous studies have shown that the signal transduction pathway for ghrelin is mediated by PLC/PKC in non-neuronal tissues and cell lines 13,27,28 , our findings first demonstrated that PKC is involved in the effects of ghrelin in native neurons. The signalling transduction pathway of how ghrelin promotes nigral neuronal firing is summarized in Supplementary Fig.…”

“…To identify the downstream effector for ghrelin-mediated effect on excitability of nigral dopaminerigc neurons, we hypothesized that ghrelin might affect neuronal Kv7/ KCNQ/M-channels that are predominantly expressed in the CNS and have a profound effect on membrane excitability 32,33 . This hypothesis was based on observations that the pathway activated by binding of ghrelin to its G-protein-coupled receptor GHS-R1a regulates the activation of downstream PLC that depletes membrane PIP 2 levels 13 and activates PKC. The depletion of PIP 2 or phosphorylation of KCNQ channels by PKC can lead to a suppression of neuronal voltage-gated Kv7/KCNQ/M-channel activity 24 .…”

“…It is mainly secreted from stomach 7 , with small amounts produced in the brain 11 . GHS-R is expressed in various brain areas including SNc, hypothalamus, ventral tegmental area and hippocampus where ghrelin directly modulates the neuronal activity 9,10,12,13 . In SNc, ghrelin electrically activates dopaminergic neurons, and increases dopamine concentration in the striatum 12 .…”

“…KCNQ channels are also negatively modulated by G-protein-coupled receptors via Gq and/ or G 11 activated phospholipase C (PLC)-mediated signal pathway [24][25][26] . It has also been reported that the effects of ghrelin are mediated by PLC that hydrolyses PIP 2 for IP 3 signalling in hypothalamus 13 , or by protein kinase C (PKC) in non-neuronal tissues and cell lines 27,28 , ultimately affecting potassium channel conductance 29 . All these observations suggest that ghrelin signalling is likely coupled with neuronal KCNQ/ M-channel function.…”

Pituitary growth hormone network responses are sexually dimorphic and regulated by gonadal steroids in adulthood

“…This has led to the conclusion that the sexually dimorphic control of GH patterns reflects sex differences in GHRH and somatostatin inputs to the pituitary gland. Acute changes in gonadal steroid environment drastically alter the patterns of GH pulsatility in adulthood (7,8); however, although they receive sexually dimorphic inputs (9,10), GHRH neurons do not display sex-specific electrical characteristics (9,11). We have previously shown that GH cells in the male mouse pituitary gland form an extensive homotypic cell network with an architecture that exhibits marked plasticity during sexual maturation and that can be altered by gonadectomy (12).…”