Ghrelin is synthesised from prepro-ghrelin Ghrelin is rendered biologically active by post-translational acylation with octanoic acid at its serine residue Nature. 1999 Dec 9;402(6762):656-60 along with an L-configuration of the third residue. Biochem Biophys Res Commun. 2001 Sep 14;287(1):142-6 Ghrelin can thus be one of four derivatives, namely non-acylated, octanoylated, decanoylated or decenoylated. J Biol Chem. 2003 Jan 3;278(1):64-70. Epub 2002 Oct 31. Acylation of Ghrelin is crucial to facilitate the highly regulated bidirectional transport across the blood brain barrier and to bind to its receptor. J Pharmacol Exp Ther. 2002 Aug;302(2):822-7 Non-acylated Ghrelin is present in greater proportion in circulation and may exert cardiovascular and anti- proliferative effects.
Ghrelin receptors GHS-R1a and 1b have been identified. The GHS-R is a member of the G protein coupled receptor superfamily and is encoded by a gene whose chromosomal location is close to the position of the gene for Brachmann-de-Lange syndrome which has phenotypic disturbances in the form of pre and postnatal growth retardation and other anomalies. Both central and peripheral Ghrelin receptors have been described (peripheral receptor distribution outside the CNS is a bit controversial) with mRNA expression of both GHSR1a and GHSR1b being detected in peripheral tissues; GHS-R Ia is expressed mainly in the pituitary and hypothalamus but also in lower levels in other brain areas including substantia nigra, NTS, and hippocampus and outside the nervous system in the pancreas, thyroid, spleen, myocardium and the adrenal cortex. J Clin Endocrinol Metab. 2002 Jun;87(6):2988 GHS-R1b is almost ubiquitous and demonstrated in stomach, oesophagus, intestine, liver, lung, muscle, testis. Ghrelin receptors have been identified in the myenteric plexus of the guinea pig, rat leydig cells, and pancreatic islets, Pancreas. 2003 Aug;27(2):161-6 and human hypothalamus, pituitary, myocardium, adrenal cortex and testis. An analogue of Ghrelin, BIM-28163 has been shown to block Ghrelin induced GH secretion, while facilitating increase in appetite and weight gain. Eur J Endocrinol. 2004 Aug;151 Suppl 1:S71-5 This may suggest that Ghrelin may produce its GH secretagogue and orexigenic actions through separate receptors. GHS-R1a transduces the GH-releasing effects of GH secretagogues, while GHS-R1b is a non functional receptor mRNA variant. GHS-R1a activates the phospholipase-IP3 pathway while inhibiting the potassium channels leading to a rise in intracellular calcium levels.
The primary site of action of Ghrelin seems to be the Arcuate nucleus (ARC). Destruction of the NPY and AgRP neurons in the ARC by monosodium glutamate results in a marked decrease in Ghrelin-stimulated Growth hormone release as well as an inability to stimulate food intake by Ghrelin. Endocrinology. 2002 Sep;143(9):3268-75
Ghrelin is thought to act via NPY and AGRP neurones (as evidenced by mRNA expression in the ARC in mice after Ghrelin injection) Neurosci Lett. 2002 May 31;325(1):47-51 with some effect on the orexin pathway (as evidenced by studies of orexin antagonism with antibodies and knockout mice). Arcuate nucleus ablation in rats blocked Ghrelin induced feeding stimulation, further suggesting the mediation of this action through the NPY/AGRP pathway. Endocrinology. 2002 Sep;143(9):3268-75.
Ghrelin's actions are partly mediated through NPY neurons. NPY neurons in the arcuate nucleus express the GHS (Growth hormone secretagogue) receptor. Neuronal cells activated by growth hormone secretagogues show NPY mRNA. Endocrinology. 1997 Feb;138(2):771-7. Ghrelin's orexigenic effect in the rat is abolished dose-dependently by ICV co-injection of NPY-Y1 receptor antagonist. Nature. 2001 Jan 11;409(6817):194-8. At the same time, Leptin-induced reduction in hypothalamic NPY mRNA expression was abolished by ICV Ghrelin. Leptin-induced inhibition of food intake was also reversed by ICV injection of Ghrelin. Diabetes. 2001 Feb;50(2):227-32.
But, NPY Knock out mice still respond to Ghrelin and hence AGRP is thought to be probably the more important pathway. AgRP inhibitor and anti-AgRP IgG inhibit the appetite stimulating effects of Ghrelin.
Ghrelin actions on the NPY/AgRP system results in further activation of other systems, notably the Orexins. Ghrelin-immunoreactive axonal terminals made direct synaptic contacts with orexin-producing neurons. Central Ghrelin administration activates Orexin containing neurons (but not MCH neurons). Endocrinology. 2003 Apr;144(4):1506-12. On a similar note, pre-treatment with anti-Orexin A and anti-Orexin B IgG attenuates Ghrelin-induced feeding, while anti-MCH IgG pre-treatment does not. Endocrinology. 2003 Apr;144(4):1506-12. see below
While a fall in leptin increases arcuate NPY mRNA expression during fasting, Ghrelin up regulates NPY expression, the final effect being stimulation of feeding. Nature. 2001 Jan 11;409(6817):194-8 Ghrelin and Leptin are thus complementary and antagonistic. Ghrelin may inhibit leptin, thus facilitating increased feeding. Diabetes. 2001 Feb;50(2):227-32. Central leptin gene therapy using recombinant adeno-associated virus encoding rat leptin (rAAV-lep), produces hyperleptinaemia and increases circulating Ghrelin levels.
The role of VAGUS:
Ghrelin receptors have been shown in rat nodose ganglion. Gastroenterology. 2002 Oct;123(4):1120-8 Peripherally injected (IV) Ghrelin fails to be effective in vagotomised or vagally blocked (capsaicin) rats while ICV Ghrelin remained effective. These suggest a peripheral pathway operating via the vagus through the nodose ganglion talking to the nucleus of tractus solitarius which then communicates with hypothalamic nuclei; specifically ARC, and VPM nuclei.
Vagal stimulation by modified sham feeding in 8 healthy people 1 hour prior to a 50 gram fat load resulted in greater Ghrelin suppression than the fat load alone. J Endocrinol. 2004 Feb;180(2):273-81. Another study evaluated the effect of sham feeding on Ghrelin levels in healthy volunteers. J Clin Endocrinol Metab. 2004 Oct;89(10):5101-4. Ghrelin levels rose pre-meals in sham fed and actually fed subjects to equal levels, and decreased in response to both sham feeding and actual feeding to levels that were not significantly different suggesting the major role of the cephalic phase (vagally mediated) in Ghrelin suppression with meals.
Intraperitoneal administration of Ghrelin increased c-Fos activation in the paraventricular nucleus of the hypothalamus (PVN) and in the arcuate nucleus of the hypothalamus (ARC) but no significant c-Fos activation occurred in the nucleus of the solitary tract (NTS) or the area postrema (AP) questioning the role of vagus in facilitating the actions of peripherally administered Ghrelin. Brain Res. 2003 Nov 21;991(1-2):26-33.
The role of Orexins
Intracerebroventricular administration of Ghrelin induced Fos expression, a marker of neuronal activation, in Orexin-producing neurons. Pre-treatment with anti-orexin-A IgG and anti-orexin-B IgG attenuated Ghrelin-induced feeding. Even after pre-treatment with anti-NPY IgG, Ghrelin induced Fos expression in orexin-producing neurons. Endocrinology. 2003 Apr;144(4):1506-12 On a similar note, in the same study, administration of NPY receptor antagonist further attenuated Ghrelin-induced feeding in rats treated with anti-orexin-IgGs. Ghrelin-induced feeding was also suppressed in orexin knockout mice. Read more about orexins
Ghrelin reduces fat utilisation and reduces oxygen consumption. Nature. 2000 Oct 19;407(6806):908-13 Ghrelin also lowers core body temperature.Endocrinology. 2002 Jan;143(1):155-62.
Ghrelin decreases TSH levels Endocrinology. 2001 Oct;142(10):4163-9 while stimulating the hypothalamo-pituitary-adrenal axis as evidenced by studies in hypophysectomised rats. Obes Res. 2002 Oct;10(10):991-9. Both of these actions contribute to inducing a positive energy balance.
Ghrelin and Ghrelin receptor-deficient mice exhibited both normal growth rates and food intake patterns! Mol Cell Biol. 2003 Nov;23(22):7973-81. On a standard diet, Ghrelin(-/-) mice showed no differences in their body composition, basal metabolic rate, respiratory quotient, or various serum parameters. When challenged with a high-fat diet, however, Ghrelin-deficient mice gained an equal amount of body weight, but showed a significant reduction in respiratory quotient, suggesting a preferential burning of fat. pDEXA analysis showed a trend for lower body fat mass in these rats. This seems to suggest that Ghrelin is not a critically required orexigenic factor. Ghrelin-null mice had no difference in size, growth rate, food intake, body composition, reproduction, gross behaviour, and tissue pathology compared to their wild-type littermates, Mol Cell Biol. 2003 Nov;23(22):7973-81 but Ghrelin administration in these rats stimulated appetite. Overall, these results are depressing in that it makes it unlikely that Ghrelin antagonism will be an effective treatment for diet-induced obesity.
Ghrelin receptors GHS-R1a and 1b have been identified. The GHS-R is a member of the G protein coupled receptor superfamily and is encoded by a gene whose chromosomal location is close to the position of the gene for Brachmann-de-Lange syndrome which has phenotypic disturbances in the form of pre and postnatal growth retardation and other anomalies. Both central and peripheral Ghrelin receptors have been described (peripheral receptor distribution outside the CNS is a bit controversial) with mRNA expression of both GHSR1a and GHSR1b being detected in peripheral tissues; GHS-R Ia is expressed mainly in the pituitary and hypothalamus but also in lower levels in other brain areas including substantia nigra, NTS, and hippocampus and outside the nervous system in the pancreas, thyroid, spleen, myocardium and the adrenal cortex. J Clin Endocrinol Metab. 2002 Jun;87(6):2988 GHS-R1b is almost ubiquitous and demonstrated in stomach, oesophagus, intestine, liver, lung, muscle, testis. Ghrelin receptors have been identified in the myenteric plexus of the guinea pig, rat leydig cells, and pancreatic islets, Pancreas. 2003 Aug;27(2):161-6 and human hypothalamus, pituitary, myocardium, adrenal cortex and testis. An analogue of Ghrelin, BIM-28163 has been shown to block Ghrelin induced GH secretion, while facilitating increase in appetite and weight gain. Eur J Endocrinol. 2004 Aug;151 Suppl 1:S71-5 This may suggest that Ghrelin may produce its GH secretagogue and orexigenic actions through separate receptors. GHS-R1a transduces the GH-releasing effects of GH secretagogues, while GHS-R1b is a non functional receptor mRNA variant. GHS-R1a activates the phospholipase-IP3 pathway while inhibiting the potassium channels leading to a rise in intracellular calcium levels.
The primary site of action of Ghrelin seems to be the Arcuate nucleus (ARC). Destruction of the NPY and AgRP neurons in the ARC by monosodium glutamate results in a marked decrease in Ghrelin-stimulated Growth hormone release as well as an inability to stimulate food intake by Ghrelin. Endocrinology. 2002 Sep;143(9):3268-75
Ghrelin is thought to act via NPY and AGRP neurones (as evidenced by mRNA expression in the ARC in mice after Ghrelin injection) Neurosci Lett. 2002 May 31;325(1):47-51 with some effect on the orexin pathway (as evidenced by studies of orexin antagonism with antibodies and knockout mice). Arcuate nucleus ablation in rats blocked Ghrelin induced feeding stimulation, further suggesting the mediation of this action through the NPY/AGRP pathway. Endocrinology. 2002 Sep;143(9):3268-75.
Ghrelin's actions are partly mediated through NPY neurons. NPY neurons in the arcuate nucleus express the GHS (Growth hormone secretagogue) receptor. Neuronal cells activated by growth hormone secretagogues show NPY mRNA. Endocrinology. 1997 Feb;138(2):771-7. Ghrelin's orexigenic effect in the rat is abolished dose-dependently by ICV co-injection of NPY-Y1 receptor antagonist. Nature. 2001 Jan 11;409(6817):194-8. At the same time, Leptin-induced reduction in hypothalamic NPY mRNA expression was abolished by ICV Ghrelin. Leptin-induced inhibition of food intake was also reversed by ICV injection of Ghrelin. Diabetes. 2001 Feb;50(2):227-32.
But, NPY Knock out mice still respond to Ghrelin and hence AGRP is thought to be probably the more important pathway. AgRP inhibitor and anti-AgRP IgG inhibit the appetite stimulating effects of Ghrelin.
Ghrelin actions on the NPY/AgRP system results in further activation of other systems, notably the Orexins. Ghrelin-immunoreactive axonal terminals made direct synaptic contacts with orexin-producing neurons. Central Ghrelin administration activates Orexin containing neurons (but not MCH neurons). Endocrinology. 2003 Apr;144(4):1506-12. On a similar note, pre-treatment with anti-Orexin A and anti-Orexin B IgG attenuates Ghrelin-induced feeding, while anti-MCH IgG pre-treatment does not. Endocrinology. 2003 Apr;144(4):1506-12. see below
While a fall in leptin increases arcuate NPY mRNA expression during fasting, Ghrelin up regulates NPY expression, the final effect being stimulation of feeding. Nature. 2001 Jan 11;409(6817):194-8 Ghrelin and Leptin are thus complementary and antagonistic. Ghrelin may inhibit leptin, thus facilitating increased feeding. Diabetes. 2001 Feb;50(2):227-32. Central leptin gene therapy using recombinant adeno-associated virus encoding rat leptin (rAAV-lep), produces hyperleptinaemia and increases circulating Ghrelin levels.
The role of VAGUS:
Ghrelin receptors have been shown in rat nodose ganglion. Gastroenterology. 2002 Oct;123(4):1120-8 Peripherally injected (IV) Ghrelin fails to be effective in vagotomised or vagally blocked (capsaicin) rats while ICV Ghrelin remained effective. These suggest a peripheral pathway operating via the vagus through the nodose ganglion talking to the nucleus of tractus solitarius which then communicates with hypothalamic nuclei; specifically ARC, and VPM nuclei.
Vagal stimulation by modified sham feeding in 8 healthy people 1 hour prior to a 50 gram fat load resulted in greater Ghrelin suppression than the fat load alone. J Endocrinol. 2004 Feb;180(2):273-81. Another study evaluated the effect of sham feeding on Ghrelin levels in healthy volunteers. J Clin Endocrinol Metab. 2004 Oct;89(10):5101-4. Ghrelin levels rose pre-meals in sham fed and actually fed subjects to equal levels, and decreased in response to both sham feeding and actual feeding to levels that were not significantly different suggesting the major role of the cephalic phase (vagally mediated) in Ghrelin suppression with meals.
Intraperitoneal administration of Ghrelin increased c-Fos activation in the paraventricular nucleus of the hypothalamus (PVN) and in the arcuate nucleus of the hypothalamus (ARC) but no significant c-Fos activation occurred in the nucleus of the solitary tract (NTS) or the area postrema (AP) questioning the role of vagus in facilitating the actions of peripherally administered Ghrelin. Brain Res. 2003 Nov 21;991(1-2):26-33.
The role of Orexins
Intracerebroventricular administration of Ghrelin induced Fos expression, a marker of neuronal activation, in Orexin-producing neurons. Pre-treatment with anti-orexin-A IgG and anti-orexin-B IgG attenuated Ghrelin-induced feeding. Even after pre-treatment with anti-NPY IgG, Ghrelin induced Fos expression in orexin-producing neurons. Endocrinology. 2003 Apr;144(4):1506-12 On a similar note, in the same study, administration of NPY receptor antagonist further attenuated Ghrelin-induced feeding in rats treated with anti-orexin-IgGs. Ghrelin-induced feeding was also suppressed in orexin knockout mice. Read more about orexins
Ghrelin reduces fat utilisation and reduces oxygen consumption. Nature. 2000 Oct 19;407(6806):908-13 Ghrelin also lowers core body temperature.Endocrinology. 2002 Jan;143(1):155-62.
Ghrelin decreases TSH levels Endocrinology. 2001 Oct;142(10):4163-9 while stimulating the hypothalamo-pituitary-adrenal axis as evidenced by studies in hypophysectomised rats. Obes Res. 2002 Oct;10(10):991-9. Both of these actions contribute to inducing a positive energy balance.
Ghrelin and Ghrelin receptor-deficient mice exhibited both normal growth rates and food intake patterns! Mol Cell Biol. 2003 Nov;23(22):7973-81. On a standard diet, Ghrelin(-/-) mice showed no differences in their body composition, basal metabolic rate, respiratory quotient, or various serum parameters. When challenged with a high-fat diet, however, Ghrelin-deficient mice gained an equal amount of body weight, but showed a significant reduction in respiratory quotient, suggesting a preferential burning of fat. pDEXA analysis showed a trend for lower body fat mass in these rats. This seems to suggest that Ghrelin is not a critically required orexigenic factor. Ghrelin-null mice had no difference in size, growth rate, food intake, body composition, reproduction, gross behaviour, and tissue pathology compared to their wild-type littermates, Mol Cell Biol. 2003 Nov;23(22):7973-81 but Ghrelin administration in these rats stimulated appetite. Overall, these results are depressing in that it makes it unlikely that Ghrelin antagonism will be an effective treatment for diet-induced obesity.