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Ghrelin: A hormone regulating food intake and energy homeostasis


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Regulation of energy homeostasis requires precise coordination between peripheral nutrient-sensing molecules and central regulatory networks. Ghrelin is a twenty-eight-amino acid orexigenic peptide acylated at the serine 3 position mainly with an n-octanoic acid, which is produced mainly in the stomach. It is the endogenous ligand of the growth hormone secretagogue (GHS) receptors. Since plasma ghrelin levels are strictly dependent on recent food intake, this hormone plays an essential role in appetite and meal initiation. In addition, ghrelin is involved in the regulation of energy homeostasis. The ghrelin gene is composed of four exons and three introns and renders a diversity of orexigenic peptides as well as des-acyl ghrelin and obestatin, which exhibit anorexigenic properties. Ghrelin stimulates the synthesis of neuropeptide Y (NPY) and agouti-related protein (AgRP) in the arcuate nucleus neurons of the hypothalamus and hindbrain, which in turn enhance food intake. Ghrelin-expressing neurons modulate the action of both orexigenic NPY/AgRP and anorexigenic pro-opiomelanocortin neurons. AMP-activated protein kinase is activated by ghrelin in the hypothalamus, which contributes to lower intracellular long-chain fatty acids, and this appears to be the molecular signal for the expression of NPY and AgRP. Recent data suggest that ghrelin has an important role in the regulation of leptin and insulin secretion and vice versa. The present paper updates the effects of ghrelin on the control of energy homeostasis and reviews the molecular mechanisms of ghrelin synthesis, as well as interaction with GHS receptors and signalling. Relationships with leptin and insulin in the regulation of energy homeostasis are addressed.
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Review Article
Ghrelin: a hormone regulating food intake and energy homeostasis
Mercedes Gil-Campos
, Concepcio
n Marı
a Aguilera
, Ramo
n Can
and Angel Gil
Unit of Paediatric Endocrinology, Reina Sofia University Hospital, Cordoba, Spain
Institute of Nutrition and Food Technology, Department of Biochemistry and Molecular Biology, University of Granada, Campus
de Cartuja 18071, Granada, Spain
(Received 8 August 2005 Revised 21 February 2006 Accepted 22 February 2006)
Regulation of energy homeostasis requires precise coordination between peripheral nutrient-sensing molecules and central regulatory networks.
Ghrelin is a twenty-eight-amino acid orexigenic peptide acylated at the serine 3 position mainly with an n-octanoic acid, which is produced
mainly in the stomach. It is the endogenous ligand of the growth hormone secretagogue (GHS) receptors. Since plasma ghrelin levels are strictly
dependent on recent food intake, this hormone plays an essential role in appetite and meal initiation. In addition, ghrelin is involved in the regu-
lation of energy homeostasis. The ghrelin gene is composed of four exons and three introns and renders a diversity of orexigenic peptides as well as
des-acyl ghrelin and obestatin, which exhibit anorexigenic properties. Ghrelin stimulates the synthesis of neuropeptide Y (NPY) and agouti-related
protein (AgRP) in the arcuate nucleus neurons of the hypothalamus and hindbrain, which in turn enhance food intake. Ghrelin-expressing neurons
modulate the action of both orexigenic NPY/AgRP and anorexigenic pro-opiomelanocortin neurons. AMP-activated protein kinase is activated by
ghrelin in the hypothalamus, which contributes to lower intracellular long-chain fatty acids, and this appears to be the molecular signal for the
expression of NPY and AgRP. Recent data suggest that ghrelin has an important role in the regulation of leptin and insulin secretion and vice
versa. The present paper updates the effects of ghrelin on the control of energy homeostasis and reviews the molecular mechanisms of ghrelin
synthesis, as well as interaction with GHS receptors and signalling. Relationships with leptin and insulin in the regulation of energy homeostasis
are addressed.
Energy balance: Energy expenditure: Food intake: Ghrelin: Obesity
Over the last 20 years, growing attention has been paid to the
various elements involved in the regulation of energy balance;
research has identified the critical role of hypothalamic pep-
tide systems in the central regulation of appetite and energy
metabolism (Ahima & Osei, 2001; Nakazato et al. 2001).
The discovery of ghrelin and its influence on appetite,
utilisation of energy substrates, weight and body composition
together with its growth hormone (GH) releasing activity
adds a further component to the complex interactions involved
in regulating energy balance (Kojima et al. 1999; Takaya et al.
2000; Tschop et al. 2000; Nakazato et al. 2001; Kojima et al.
Regulation of energy homeostasis requires precise
coordination between peripheral nutrient-sensing molecules
and central regulatory networks (Hahn et al. 1998). Ghrelin,
a twenty-eight-amino acid peptide acylated at the serine 3
(Ser3) position with n-octanoic or other medium-chain fatty
acids, produced mainly in the stomach, as well as in other
gastrointestinal segments and selected areas of the brain, is
the first peripheral orexigenic hormone identified (Kojima
et al. 1999; Date et al. 2000; Tomasetto et al. 2000; Tschop
et al. 2000; Nakazato et al. 2001; Wren et al. 2001). Des-
acyl ghrelin (desacylated or unacylated ghrelin) coexists
with acylated forms both in the gastrointestinal tract and
plama (Hosoda et al. 2003; Fig. 1).
Ghrelin was initially found as an endogenous ligand of the
GH secretagogue (GHS) receptor (GHS-R; Takaya et al.
2000) and several studies have provided evidence that ghrelin
is involved in the regulation of energy homeostasis. Ghrelin
levels increase before and decrease after meals, potentially
playing a role in meal initiation and satiety in an inverse pat-
tern to that of insulin (Tschop et al. 2000; Cummings et al.
2001; Wren et al. 2001; Bacha & Arslanian, 2005). In
addition, ghrelin is involved in the regulation of energy
* Corresponding author: Professor Angel Gil, fax þ 34 958 248960, email
Abbreviations: ACC, acetyl-CoA carboxylase; AgRP, agouti-related protein; AMPK, AMP-activated protein kinase; ARC, arcuate nucleus; CART, cocaine
amphetamine-related transcript; CNS, central nervous system; GH, growth hormone; GHRH, growth hormone-releasing hormone; GHS, growth hormone
secretagogue; GHS-R, growth hormone secretagogue receptor; ICV, intracerebroventricular; LCFA, long-chain fatty acid; NPY, neuropeptide Y; POMC, pro-
opiomelanocortin; PVN, paraventricular nucleus; Ser3, serine 3; VTA, ventral tegmental area.
British Journal of Nutrition (2006), 96, 201–226 DOI: 10.1079/BJN20061787
q The Authors 2006
balance by increasing food intake and reducing fat utilisation
(Tschop et al. 2000; Nakazato et al. 2001; Wren et al. 2001).
Moreover, ghrelin regulates glucose metabolism (Patel et al.
2006) and possibly is involved in the regulation of insulin
activities in man (Murata et al. 2002). Likewise, ghrelin
appears to be related with the regulation of energy expenditure
(St-Pierre et al. 2004; Zigman et al. 2005; Maffeis et al.
Importantly, signalling by circulating ghrelin is mediated
downstream by neurons of the arcuate nucleus (ARC) of the
hypothalamus; in particular, neurons expressing neuropeptide
Y (NPY) and agouti-related protein (AgRP), two potent orexi-
genic peptides (Guan et al. 1997; Kamegai et al. 2001; Chen
et al. 2004; Gropp et al. 2005).
Ghrelin has changed our understanding of the central regu-
lation of GH secretion, appetite control and energy balance,
and the importance of ghrelin as a unique hormone regulating
energy homeostasis is supported by more than 1400 papers
published from its discovery until January 2006. In spite of
that, it is thought not to be physiologically involved in the
regulation of GH secretion, since circulating levels are not
correlated with those of GH, either in physiological or in
pathological conditions. Nevertheless, co-administration of
GH-releasing hormone (GHRH) and ghrelin has synergistic
effects on pituitary GH secretion. Ghrelin-induced GH
secretion is mostly antagonised by somatostatin via a
GHRH-dependent mechanism and also through an indepen-
dent effect mediated by GHS-R (Casanueva & Die
guez, 2002).
Injected centrally or peripherally ghrelin stimulates the
appetite more effectively than does any other molecule
except NPY, with which it is fairly equipotent, and it is the
only hormone that increases short-term food intake when
administered to mice and human subjects (Asakawa et al.
2001; Wren et al. 2001). In addition, chronic administration
of ghrelin increases body weight, due to its effects promoting
adipogenesis (Asakawa et al. 2001), and decreases energy
expenditure (Asakawa et al. 2003; Zigman et al. 2005; De
Smet et al. 2006; Maffeis et al. 2006), fat catabolism and lipo-
lysis (Tschop et al. 2000; Muccioli et al. 2004). The secretion
pattern of ghrelin is inverse to that of insulin, and its action is
opposed to that of leptin, which inhibits the synthesis of NPY
and AgRP in the hypothalamus (Rosicka
et al. 2003; Kim et al.
Although the acylated forms of ghrelin have been recog-
nised as the major active orexigenic molecules regulating
energy balance (Date et al. 2000; Hosoda et al. 2000a),
recent data provide evidence that when studying the effects
of ghrelin on energy balance, differential influences of the
acylated and non-acylated forms of the peptide must be con-
sidered. Although the correlation between the two forms is
good, they may be different hormones with opposite actions,
with non-acylated ghrelin being able to antagonise some of
the effects of the acylated form (Asakawa et al. 2005;
Chen et al. 2005a,b; Ukkola, 2005).
On the other hand, ghrelin stimulates lactotroph and cortico-
troph function, influences the pituitary gonadal axis, inhibits
pro-inflammatory cytokine expression, controls gastric moti-
lity and acid secretion and influences pancreatic exocrine
and endocrine function, as well as impacting on glucose
metabolism (van der Lely et al. 2004; Otto et al. 2005). More-
over, ghrelin also inhibits apoptosis and enhances osteoblast
differentiation (Kim et al. 2005). Ghrelin also possesses regu-
latory properties in the cardiovascular system, both in the
heart and in the vasculature (Nagaya et al. 2001a,b; Wiley
& Davenport, 2002; Kleinz et al. 2006), it mediates antiproli-
ferative effects in neoplastic cell lines, and influences sleep,
memory and anxiety-like behavioural responses (Eisenstein
& Greenberg, 2003; van der Lely et al. 2004; Ghigo et al.
2005). That is possible since GHS-R is expressed not only
in the brain but also in peripheral tissues, especially the
stomach, intestine, pancreas, thymus, gonads, thyroid and
heart (Howard et al. 1996; Sun et al. 2004). Fig. 2 summarises
the biological effects of ghrelin.
The present paper examines only the effects of ghrelin on
the control of appetite and energy balance; GHS activity and
other peripheral effects are not addressed. The structure and
role of ghrelin and its derived molecules, as well as their
receptors, are reviewed. Likewise, we have placed emphasis
on the central neuronal pathways which mediate the ghrelin
effects on energy homeostasis and its suggested molecular
mechanisms of action.
Historical background
Since the 1970s it has been known that a number of small syn-
thetic molecules termed GHS act on the pituitary gland and
the hypothalamus to stimulate and amplify pulsatile GH
release (Bowers et al. 1977) and this phenomenon was later
shown independent of GHRH (Giustina et al. 1997). This
was followed by the development of a number of both peptide
and non-peptide GHS (Bercu & Walker, 1997).
In 1996, Howard et al. (1996) cloned a heterotrimeric GTP-
binding protein-coupled receptor of the pituitary and arcuate
ventromedial and infundibular hypothalamus of swine and
man and it was shown to be the target of the GHS. In addition,
on the basis of its pharmacological and molecular characteris-
ation, they suggested that the GTP-binding protein-coupled
receptor defined a new neuroendocrine pathway for the control
of pulsatile GH release and supported the notion that the GHS
mimicked an undiscovered hormone.
-Octanoyl residue
Fig. 1. Structure of human acylated (128)-ghrelin. Ghrelin variants with
twenty-seven amino acids lack Arg at the C-terminus. The n-octanoyl residue
at the serine 3 position is essential for neuroendocrine activities of 27- and
28-ghrelin. Other medium-chain fatty acids, namely decanoic (10 : 0) and
decenoic (10 : 1) acids, can also acylate ghrelin. Des-acyl-ghrelin lacks the
n-octanoyl residue and seems to have peripheral activities but it does not
exhibit any neuroendocrine activity. Amino acid residues: A, Ala; C, Cys; D,
Asp; E, Glu; F, Phe; G, Gly; H, His; I, Ile; K, Lys; L, Leu; M, Met; N, Asn; P,
Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; W, Trp; Y, Tyr.
M. Gil-Campos et al.202
In December 1999, Kojima et al. (1999) reported for the first
time the purification and identification in rat stomach of an
endogenous ligand specific for the GHS-R. The purified ligand
was a twenty-eight-amino acid peptide named as ghrelin, in
which the Ser3 residue was acylated with a molecule of n-octa-
noic acid. The acylated peptide specifically released GH both in
vivo and in vitro and O-n-octanoylation was essential for the
activity. They designated the GH-releasing peptide as ‘ghrelin’,
a term that contains ‘ghre’ as the etymological root for ‘growth’
in many languages. ‘GH’ and ‘relin’, a suffix for releasing sub-
stances in generic names according to United States Pharmaco-
peia ‘USP Dictionary of USAN and International Drug Names’
(, also represents an abbreviation for
‘growth-hormone release’, a characteristic effect of ghrelin
(Kojima et al. 1999; Hosoda et al. 2000b).
In August 2000, Tomasetto et al. (2000) also reported the iso-
lation and sequentiation of a complementary DNA obtained
from mouse gastric epithelium and its expression was examined
at both mRNA and protein levels. The novel gene identified and
characterised encoded the termed ‘pre-pro-motilin-related pep-
tide’, had similarity sequence with pre-pro-motilin and
coincided with that of ghrelin (Del Rinco
n et al. 2001).
Takaya et al. (2000) studied GH-releasing activity and other
effects generated by ghrelin in four normal men aged 28 37
years. They demonstrated that ghrelin strongly stimulates GH
release in human subjects in a dose-dependent manner. Per
mol, ghrelin is more potent for GH release than GHRH. The
lowest dose of ghrelin used led to massive GH release, with
minimum effects on corticotropin or prolactin. Ghrelin adminis-
tration did not change serum luteinising hormone, follicle-sti-
mulating hormone or thyroid-stimulating hormone levels.
Tschop et al. (2000) showed that peripheral daily adminis-
tration of ghrelin caused adiposity by reducing fat utilisation
in mice and rats.
Intracerebroventricular (ICV) administration of ghrelin gen-
erated a dose-dependent increase in food intake and body
weight. Rat serum ghrelin concentrations were increased by
fasting and were reduced by refeeding or oral glucose admin-
istration, but not by water ingestion. Tschop et al. (2000) pro-
posed that ghrelin, in addition to its role in regulating GH
secretion, signals the hypothalamus when an efficient meta-
bolic state is necessary.
Nakazato et al. (2001) demonstrated that ghrelin is involved
in the hypothalamic regulation of energy homeostasis. ICV
injections of ghrelin strongly stimulated feeding in rats and
increased body-weight gain. Ghrelin also increased feeding
in rats that were genetically deficient in GH. Anti-ghrelin
IgG robustly suppressed feeding. After ICV ghrelin adminis-
tration, Fos protein, a marker of neuronal activation, was
found in regions of primary importance in the regulation of
feeding, including NPY/AgRP neurons. Antibodies and antag-
onists of NPY and AgRP abolished ghrelin-induced feeding.
Ghrelin augmented NPY gene expression and blocked
leptin-induced feeding reduction, implying that there is a com-
petitive interaction between ghrelin and leptin in feeding regu-
lation. These authors concluded that ghrelin is a physiological
mediator of feeding and probably has a function in growth
regulation by stimulating feeding and release of GH.
Date et al. (2001) demonstrated a role for ghrelin in the central
regulation of gastric function. Specifically, ICV administration
of ghrelin stimulated gastric acid secretion in a dose-dependent
and atropine-sensitive manner. Vagotomy abolished gastric acid
secretion. Immunohistochemistry demonstrated the induction of
Fos expression in the nucleus of the solitary tract and dorsomotor
nucleus of the rat vagus nerve.
Kojima et al. (2001) reviewed the role of ghrelin. The pep-
tide is found in the secretory granules of X/A-like cells, a dis-
tinct endocrine cell type found in the submucosal layer of the
stomach (Date et al. 2000). These cells contain round, com-
pact, electron-dense granules and are filled with ghrelin. Ghre-
lin immunoreactive cells are also found in the small and large
Fig. 2. Main biological functions of ghrelin. Ghrelin is mainly synthesised at the stomach but it can be produced in low amounts in other tissues such as the gastro-
intestinal tract, the pancreas and the hypothalamus, having endocrine, paracrine or autocrine character. Many of the biological functions of ghrelin are indirectly
mediated via the hypothalamus, brainstem and vagal nerve afferents; others, such as the effects on apoptosis and cell proliferation, and regulation of vascular
tone in the heart and vasculature, are direct.
Ghrelin and energy balance 203
Cummings et al. (2002) investigated plasma ghrelin levels
after weight loss induced by diet or by gastric bypass surgery.
They found an increase in the plasma ghrelin level with diet-
induced weight loss. Gastric bypass was associated with mark-
edly suppressed ghrelin levels, possibly contributing to the
weight-reducing effect of the procedure.
Cowley et al. (2003) discovered expression of ghrelin in a
previously uncharacterised group of neurons adjacent to the
third ventricle between the dorsal, ventral, paraventricular
and arcuate hypothalamic nuclei. These neurons send efferents
onto key hypothalamic circuits, including those producing
NPY, AgRP, pro-opiomelanocortin (POMC) products and cor-
ticotropin-releasing hormone. Within the hypothalamus, ghre-
lin is bound mostly on presynaptic terminals of NPY neurons.
Using electrophysiological recordings, these authors found
that ghrelin stimulated the activity of arcuate NPY neurons
and mimicked the effect of NPY in the paraventricular nucleus
(PVN) of the hypothalamus, and they proposed that at these
sites, release of ghrelin may stimulate the release of orexigenic
peptides and neurotransmitters, thus representing a novel regu-
latory circuit controlling energy homeostasis.
Chen et al. (2004) in a series of elegant experiments using
knockout mice demonstrated that NPYand AgRP are required
for the orexigenic effects of ghrelin, as well as the involve-
ment of the melanocortin pathway in ghrelin signalling.
Two major molecular forms exist in rats and man: ghrelin,
which is acylated at Ser3 with a medium-chain fatty acid
usually n-octanoic acid and des-acyl ghrelin (Date et al.
2000; Hosoda et al. 2000a, 2003). In man, des-octanoyl ghrelin
is the predominant circulating form of the peptide (Hosoda et al.
2003), which, until recently, was considered biologically inac-
tive. While ghrelin activates GHS-R-expressing cells, the non-
modified des-n-octanoyl form of ghrelin (des-acyl ghrelin)
does not (Hosoda et al. 2000a). In addition to those forms, mul-
tiple ghrelin-derived molecules produced by post-translational
processing of the ghrelin gene (GHRL) have been identified.
As indicated earlier in the present review, both ghrelin and the
ghrelin-derived molecules were found to be present in plasma
as well stomach tissue (Date et al. 2000; Hosoda et al. 2000a,
2003) and, to a lesser extent, in the arcuate, ventromedial and
infundibular nuclei of the hypothalamus (Howard et al. 1996;
Papotti et al. 2000; Muccioli et al. 2001).
Furthermore, the pre-pro-ghrelin peptide also produces
obestatin. Contrary to the appetite-stimulating effects of ghre-
lin, treatment of rats with obestatin suppressed food intake,
inhibited jejunal contraction, and decreased body-weight
gain (Zhang et al. 2005).
Non-acylated ghrelin, which is present in human serum in
far greater quantities than acylated ghrelin, was initially con-
sidered to be devoid of any endocrine action. However, it is
able to exert some non-endocrine actions including cardiovas-
cular and antiproliferative effects, probably by binding differ-
ent GHS-R subtypes or receptor families (Date et al. 2000;
Baldzani et al. 2002; Bedendi et al. 2003; Cassoni et al.
2004; Ariyasu et al. 2005; Kleinz et al. 2006).
In addition to roles in meal initiation, weight regulation and
gastrointestinal activity, ghrelin also regulates the pituitary
hormone axis, carbohydrate metabolism, and various functions
of the heart, kidney, pancreas, adipose tissues, gonads and cell
proliferation (Gnanapavan et al. 2002; Corbetta et al. 2003;
Dixit et al. 2004; Tena-Sempere, 2005). Cardiovascular
actions and modulation of the proliferation of neoplastic
cells, as well as of the immune system, are also actions of
ghrelin and/or other GHS (van der Lely et al. 2004). More-
over, ghrelin also inhibits TNF-a-induced apoptosis and sup-
presses caspase-3 activation as well as enhancing osteoblast
differentiation (Kim et al. 2005).
Role of ghrelin and its receptors
Structure of the ghrelin gene and diversity of ghrelin
The ghelin human gene (GHRL) is located on 3q2526 and is
predicted to be composed of four exons and three introns
(Wajnrajch et al. 2000) that encode a molecule designated
pre-pro-ghrelin (Kojima et al. 1999). The presence of a
short non-coding first exon has also been suggested to occur
in rat and mouse ghrelin genes (Tanaka et al. 2001). Fig. 3
shows the human GHRL as well as the main transcriptional
and translational products.
Using rat ghrelin cDNA, a human stomach cDNA library
was screened and analysis of several clones yielded a deduced
amino acid sequence for the designated human pre-pro-ghrelin
(a 117-amino acid precursor; Kojima et al. 1999). The putative
initiation codon ATG is located at nucleotides 3436, pre-
ceded by the consensus initiation sequence, whereas a terminal
codon TAG is found 117 codons downstream at position 385
387. A typical polyadenylation signal, AATAAA, is found at
position 494 499 (Hosoda et al. 2003) similar to that occur-
ring in the mouse and rat GHRL (Tanaka et al. 2001). The
open reading frame starting at the ATG codon encodes pre-
pro-ghrelin and the N-terminal twenty-three-residue sequence
of pre-pro-ghrelin exhibits features characteristic of a
secretory signal peptide; the ghrelin sequence starts from
Gly24, which directly follows the signal peptide, and the
last two residues of ghrelin Pro-Arg correspond to a proces-
sing signal (Kojima et al. 1999).
Functional analysis using promoterreporter constructs
containing the 5
-flanking region of the human GHRL indicate
that the sequence residing within the 2 349 to 2 193 region is
necessary for human ghrelin promoter function in a human
medullary thyroid carcinoma cell line. Within this region
existed several consensus sequences for a number of transac-
tivating regulatory proteins, including an E-box site. Destruc-
tion of this site decreased the promoter activity to 40 %. The
upstream region of the promoter has two additional putative
E-box sites, and site-directed mutagenesis suggested that
these are also involved in promoter activation (Kanamoto
et al. 2004).
Amino acid identities between rat and human pre-pro-ghre-
lins are 82·9 % and only two amino acids (Arg11 Val12) are
replaced in the twenty-eight-amino acid residue segment, indi-
cating that ghrelins are highly conserved between species
(Kojima et al. 1999; Hosoda et al. 2003), particularly in mam-
mals (van der Lely et al. 2004). Partial conservation of the
ghrelin sequence has been confirmed in other species such
as chicken, bullfrog and tilapia (Kaiya et al. 2001, 2003;
Saito et al. 2002). Nevertheless, results indicate that although
the regulatory mechanism of ghrelin to induce GH secretion is
evolutionarily conserved, the structural changes in the
different ghrelins result in species-specific receptor binding
M. Gil-Campos et al.204
(Kaiya et al. 2001; Saito et al. 2002). The sequence of human
ghrelin and acylation of the Ser3 residue are shown in Fig. 1.
Sequence comparison between ghrelin and motilin shows
that both molecules have a number of identical amino acids,
which suggest that ghrelin and motilin might have evolved
from a common ancestral peptide. However, motilin is not
modified by n-octanoic acid or by other acyl acids
(Kojima et al. 2001).
The purification of a second endogenous ligand for GHS-R
from rat stomach has also been reported (Hosoda et al. 2000b).
This ligand, named des-Gln14-ghrelin, is a twenty-seven-
amino acid peptide whose sequence is identical to ghrelin
except that one glutamine (14th Gln of ghrelin) is missing
in des-Gln14-ghrelin. This new GHS-R ligand also has an
n-octanoyl modification at Ser3 like ghrelin, which is also
essential for its activity. Furthermore, genomic sequencing
and cDNA analysis indicated that des-Gln14-ghrelin is not
encoded by a gene distinct from ghrelin but is encoded by
an mRNA created by alternative splicing of the ghrelin
gene. The analysis of the genomic structure of rat ghrelin
revealed that an intron exists between Gln13 and Gln14 for
the ghrelin sequence (Hosoda et al. 2000b; Wajnrajch et al.
2000). The 3
-end of the intron has two tandem CAG
sequences. The AG parts of these sequences may serve as spli-
cing signals (McKeown, 1992). When the first AG is used for
the splicing signal, pre-pro-ghrelin mRNA is produced and the
second CAG is translated into Gln14. On the other hand, when
the second AG is used, pre-pro-des-Gln14-ghrelin mRNA is
generated to produce pre-pro-des-Gln14-ghrelin, which con-
tains 116 amino acid residues, and then by post-translational
mRNA pre-pro-ghrelin
mRNA des-GIN
Matured mRNA
Alternative splicing
Pre-pro-ghrelin (pre-motilin-like peptide)
Initial codon (ATG)
Initial codon (ATG)
Stop codon
Poly A signal
cleavage and acylation
Ghrelin C-terminal domain
w/o Arg
w/o Arg
Non-acylated ghrelins
Acylation at Ser
With 8:0, 10 : 0 or 10 :1 With 8:0 or 10 : 0
Acylated ghrelins
Fig. 3. Structure of the human ghrelin gene, transcription, alternative splicing of high nuclear RNA (hnRNA), translation of matured RNA and post-translational pro-
cesses leading to the synthesis of acylated and non-acylated ghrelins, as well as the new discovered molecule obestatin. Alternative splicing of the intron 1 at the
ACG codon results into two different mRNA, which in turn originate two different pre-pro-ghrelin molecules; one of them is without (w/o) the Arg in position 14
(des-Gln14-pre-pro-ghrelin). This molecule is practically absent in the stomach of man but present in that of rats.
Ghrelin and energy balance 205
cleavage, des-Gln14-ghrelin missing Gln14 (Hosoda et al.
2000b). Although nearly all of the cDNA clones isolated
from human stomach encoded the pre-pro-ghrelin precursor,
a few cDNA clones encoded the pre-pro-des-Gln14-ghrelin
precursor (Hosoda et al. 2003). The ratios observed between
the two precursor populations, ghrelin and des-Gln14-ghrelin,
was 5:1 in rat stomach and 6:5 in mouse stomach (Tanaka
et al. 2001). However, the amount of des-Gln14-ghrelin
from the human stomach extracts is negligible.
Recently, a novel, exon 3-deleted pre-pro-ghrelin isoform
has been detected in breast and prostate cancer, although its
potential function is unknown (Yeh et al. 2005). Exon
4-deleted pro-ghrelin, a novel mouse pro-ghrelin isoform
with a unique C-terminal peptide sequence, is also widely
expressed in the mouse and thus may possess biological
activity (Jeffery et al. 2005).
The purification and characterisation of human ghrelin and
other minor ghrelin-derived molecules from the stomach has
been reported, and the collected ghrelins have been classified
into two groups on the basis of amino acid length and into four
groups by type of acylation at Ser3: non-acylated, octanoy-
lated, decanoylated, and possibly decenoylated. Although the
major active form of human ghrelin is a twenty-eight-amino
acid peptide with an n-octanoyl modification at Ser3, the
ghrelin-derived molecules observed include octanoyl ghrelin-
(127), decanoyl ghrelin-(1 28), decanoyl ghrelin-(1 27),
and decenoyl ghrelin-(1 28). Moreover, the non-active
forms des-acyl ghrelin and des-acyl ghrelin-(1 27) are also
present in the human stomach (Hosoda et al. 2003). Further-
more, all of these molecular forms of ghrelin were found in
human plasma as well as in the stomach. In human stomach,
the twenty-seven-amino acid ghrelin:twenty-eight-amino acid
ghrelin processing product ratio was observed to be 1:3. It is
likely that the twenty-seven- and twenty-eight-amino acid
ghrelin molecules isolated are produced through alternative
C-terminal processing of the same ghrelin precursor (Weber
et al. 1983).
Ghrelin has been the first example discovered of a bioactive
peptide modified by an n-octanoic acid moiety (Kojima et al.
1999) and all of the acyl-modified ghrelins and ghrelin-
derived molecules studied have the same potency to induce
an increase of Ca
concentration in the GHS-R-expressing
cells and stimulate GH release in anaesthetised rats (Hosoda
et al. 2003). In addition, the de novo synthesis of molecules
has demonstrated that octanoic acid is not the only Ser3 modi-
fication that will confer full activity to ghrelin (Bednarek et al.
2000; Matsumoto et al. 2001). In fact, short peptides encom-
passing the first four or five residues of ghrelin were found to
functionally activate GHS-R about as efficiently as the full-
length ghrelin and only the segment Gly-Ser-Ser-(n-octa-
noyl)-Phe appeared to constitute the ‘active core’ required
for agonist potency at human GHS-R (Bednarek et al.
2000). In addition, more stable ether or thioether bonds of
the Ser3 are capable of replacing the octanoyl ester bond in
ghrelin, advantageous for the generation of pharmaceuticals
with longer stability (Matsumoto et al. 2001).
In the human stomach, the octanoylated:decanoylated ghre-
lin ratio was found to be roughly 3:1 (Hosoda et al. 2003).
Because acylation of ghrelin is essential for its activity, the
enzyme that catalyses this modification step should be an
important regulator of ghrelin biosynthesis. However, the
mechanism by which ghrelin is acylated during post-transla-
tional processing remains to be clarified.
Very recently, Zhang et al. (2005), searching GenBank for
orthologues of the human ghrelin gene and comparing pre-
pro-ghrelin sequences from eleven mammalian species, have
found in addition to the known ghrelin mature peptide,
which immediately follows the signal peptide, another con-
served region that was flanked by potential convertase
cleavage sites. This region encodes a putative twenty-three-
amino acid peptide, with a flanking conserved glycine residue
at the C terminus, suggesting that it might be amidated. This
new ghrelin-associated peptide has been named ‘obestatin’, a
contraction of obese, from the Latin ‘obedere’ meaning ‘to
devour’, and ‘statin’ denoting ‘suppression’. Contrary to the
appetite-stimulating effects of ghrelin, treatment of rats with
obestatin suppressed food intake, inhibited jejunal contraction
and decreased body-weight gain. Obestatin is bound to the
orphan GTP-binding protein-coupled receptor GPR39 that
shares homology with GHS-R. Both receptors could have
evolved from a common ancestor but diverged in their func-
tions, thus maintaining a delicate balance of body-weight
regulation. Thus, two peptide hormones with opposing
action in weight regulation are derived from the same ghrelin
gene and activate distinct receptors (Zhang et al. 2005).
Genetic variability
Korbonits et al. (2002) studied the ghrelin gene in a group of
seventy tall and obese children. They found ten single nucleo-
tide polymorphisms. One common polymorphism of the ghre-
lin gene, Leu72 to Met (L72M), corresponding to an amino
acid change in the tail of the pre-pro-ghrelin molecule, was
significantly associated with children with a higher BMI,
and with lower insulin secretion during the first part of an
oral glucose tolerance test, although no difference in glucose
levels was noted. The authors concluded that variations in
the ghrelin gene contribute to obesity in children and may
modulate glucose-induced insulin secretion. An Arg51-to-
Gln polymorphism (R51Q) in the ghrelin gene associated
with obesity has also been reported (Ukkola et al. 2001;
Vivenza et al. 2004).
Hinney et al. (2002) screened the ghrelin coding region in
215 extremely obese German children and adolescents
(study group 1) and ninety-three normal-weight students
(study group 2) by single-strand conformation polymorphism
analysis. They found two previously described single nucleo-
tide polymorphisms, R51Q and L72M, in similar frequencies
in study groups 1 and 2. Hence, they could not confirm the
previous finding. Additionally, two novel variants were ident-
ified within the coding region. They detected a non-conserva-
tive amino acid change from Gln to Leu at codon 90. They
also detected a frameshift mutation in one healthy normal-
weight individual. The authors concluded that none of the var-
iants seem to influence weight regulation.
Recently, two novel variants in the pre-pro-ghrelin gene,
36C . T and IVS3 þ 715delC, and four known variants,
Arg51Gln, Leu72Met, Gln90Leu and IVS1 þ 169G . , have
been identified in Danish Caucasian subjects. None of the var-
iants showed any significant association with obesity or type 2
diabetes or estimates of glucose and lipid metabolism in glu-
cose-tolerant subjects. Indeed, variation in the pre-pro-ghrelin
M. Gil-Campos et al.206
gene is not associated with juvenile-onset obesity, type 2 dia-
betes or related phenotypes among the examined Danish Cau-
casian subjects (Bing et al. 2005; Larsen et al. 2005).
However, Steinle et al. (2005) have also analysed three mis-
sense polymorphisms in GHRL (Arg51Gln, Leu72Met and
Gln90Leu) in subjects of the Amish Family Diabetes Study,
and concluded that mutations in GHRL may confer risk for
the metabolic syndrome.
Tissue distribution and secretion of ghrelin and ghrelin-
derived molecules
Both ghrelin and des-acyl ghrelin are expressed in gastrointes-
tinal tissues, with the stomach having the highest concen-
trations and significant amounts in the duodenum, jejunum,
ileum and caecum (Hosoda et al. 2000a). Rat ghrelin is pre-
sent in round, compact, electron-dense granules compatible
with those of X/A-like cells whose hormonal product and
physiological functions had not previously been clarified.
These ghrelin-immunoreactive cells, which are not entero-
chromaffin-like cells, D cells or enterochromaffin cells,
account for about 20 % of the endocrine cell population in
rat and human oxyntic glands and are associated with the
capillary network running through the lamina propria
(Date et al. 2000).
In contrast to GHS-R1a receptor that is predominantly
expressed in the pituitary and at much lower levels in the thyroid
gland, pancreas, spleen, myocardium and adrenal gland
(Howard et al. 1996), ghrelin has been found not only in the
stomach and other parts of the gut but also in all the tissues stu-
died (adrenal gland, atrium, breast, buccal mucosa, oesophagus,
fallopian tube, fat tissue, gall bladder, human lymphocytes,
ileum, kidney, left colon, liver, lung, lymph node, muscle,
muscle, myocardium, ovary, pancreas, pituitary, placenta, pros-
tate, right colon, skin, spleen, testis, thyroid and vein; Gnanapa-
van et al. 2002). The biological significance of this widespread
distribution of ghrelin is unknown, although ghrelin may have
a range of still-unidentified physiological autocrine, paracrine
or endocrine effects in both neural and peripheral tissues,
using different types of receptors (van der Lely et al. 2004).
The ghrelin content of the central nervous system (CNS) is
low but ghrelin-responsive neuronal cells can be demonstrated
in a very limited region in the hypothalamic ARC (Kojima
et al. 1999; Hosoda et al. 2000b). Likewise, ghrelin mRNA
and peptide have been detected in rat and in normal and
abnormal human pituitaries (Korbonits et al. 2001).
The plasma ghrelin level can be measured by RIA using
two polyclonal rabbit antibodies raised against the N-terminal
(1 11; Gly1 Lys11) and C-terminal (13 28; Gln13 Arg28)
fragments of rat ghrelin (Kojima et al. 1999; Hosoda et al.
2000a; Ariyasu et al. 2002). Ghrelin values obtained by the
RIA system using anti-rat ghrelin (1 11) antiserum specifi-
cally represent the active, n-octanoylated ghrelin. The values
acquired by RIA with anti-rat ghrelin (13 28) represent the
total ghrelin concentration, including inactive, des-acyl
Adult human plasma has been reported to contain 100
120 fmol total inmunoreactive ghrelin/ml (Kojima et al.
2001) and, more recently, 61 293 fmol/ml (Murashita et al.
2005). Rat plasma levels of total immunoreactive ghrelin
range from 220 to 560 fmol/ml (Date et al. 2000; Hosoda
et al. 2000a). However, the concentration of active acyl ghre-
lin is about five to twenty times less, both in rat and human
plasma (Date et al. 2000; Hosoda et al. 2000a; Gauna et al.
2004; Murashita et al. 2005). In fact, acylated ghrelin ranges
from 3·6 to 23·9 fmol/ml in adult human subjects, with an
average of 9·1 fmol/ml (Murashita et al. 2005). In human
cord blood acylated ghrelin ranged from 7·7 to 38·4 fmol/ml
and the median for the non-acylated:acylated ghrelin ratio
was 13·8, and in neonatal blood acylated ghrelin concen-
trations ranged from 4·6 to 22·6 fmol/ml, with a non-acylate-
d:acylated ghrelin ratio of 12:50. In both cases, plasma
levels of active ghrelin correlated excellently with those of
total ghrelin (Yokota et al. 2005).
Acylated ghrelin is very rapidly cleared from the circulation
because hardly any of it can be found in serum in the follow-
ing hour after injection (Gauna et al. 2004). The half-life of
human ghrelin has been reported to be about 10 min
(Hosoda et al. 2003). Carboxylesterase appears to be the
enzyme responsible in rat and human serum for des-octanoy-
lation. In addition, several esterases, including butyrylcholi-
nesterase, contribute to ghrelin des-octanoylation in human
serum (De Vriese et al. 2004).
Among determinants of ghrelin secretion, the most important
appear to be insulin (Mohlig et al. 2002; Saad et al. 2002; Flana-
gan et al. 2003), glucose (Tschop et al. 2000; Nakagawa et al.
2002; Nakai et al. 2003), and somatostatin (Broglio et al.
2002; Barkan et al. 2003). Possibly, GH (Muller et al. 2002;
Tschop et al. 2002), leptin (Rosicka
et al. 2003; Tolle et al.
2003), melatonin (Mustonen et al. 2001), thyroid hormones
(Riis et al. 2003), glucagon (Kishimoto et al. 2003), and the para-
sympathetic nervous system (Masuda et al. 2000; van der Lely
et al. 2004) also play a role in ghrelin metabolism. In mice,
rats, cows and human subjects, ghrelin mRNA expression
levels or circulating ghrelin levels are increased by food depri-
vation and appear to be decreased postprandially (Tschop et al.
2000, 2001; Asakawa et al. 2001; Cummings et al. 2001, 2002;
Toshinai et al. 2001; Ariyasu et al. 2002). This phenomenon,
which has been confirmed by several study groups in the
recent past, further supports the emerging concept of ghrelin
as an endogenous regulator of energy homeostasis.
In addition to fasting, ghrelin expression can be stimulated
in rats by insulin-induced hypoglycaemia, leptin adminis-
tration, and central leptin gene therapy (Toshinai et al.
2001). Ingestion of sugar suppresses ghrelin secretion in rats
(Tschop et al. 2000). These observations indicate a direct
inhibitory effect of glucose or energy intake on ghrelin-con-
taining X/A-like cells in the oxyntic mucosa of the rat stomach
rather than an exclusively insulin-mediated effect. That insulin
is an independent determinant of the circulating ghrelin con-
centration has recently been shown by several study groups
using hyperinsulinaemic euglycaemic clamps in human sub-
jects (Mohlig et al. 2002; Saad et al. 2002). These findings
add further evidence connecting ghrelin to mechanisms gov-
erning energy balance and the regulation of glucose homeosta-
sis. Recent studies have shown that there is a novel islet cell
type, the
-cell, that it is specialised in ghrelin synthesis
(Prado et al. 2004; Wierup et al. 2004; Heller et al. 2005).
Ghrelin is also synthesised by pancreas a-cells, the gluca-
gon-producing cell population (Heller et al. 2005).
Because ghrelin is highly expressed in the fetal pancreas
(Chanoine & Wong, 2004), it may contribute to islet cell
Ghrelin and energy balance 207
development, and some evidence suggests that ghrelin may
modulate insulin secretion in human adults (Cummings et al.
2005; Soule et al. 2005) and children (Gil et al. 2004;
Gil-Campos, 2004).
Interestingly, it has been reported that ghrelin binds a
species of HDL associated with the plasma esterase, paraoxo-
nase. Both free ghrelin and paraoxon, a substrate for paraoxo-
nase, can inhibit this interaction. Some endogenous ghrelin is
found to co-purify with HDL during density-gradient centrifu-
gation. This interaction links the orexigenic peptide hormone
ghrelin to lipid transport and a plasma enzyme that breaks
down oxidised lipids in LDL. Furthermore, the interaction of
the esterified ghrelin with a species containing an esterase
suggests a possible mechanism for the conversion of ghrelin
to des-acyl ghrelin (Beaumont et al. 2003).
Growth hormone secretagogue receptor and other receptors
Two types of GHS-R cDNA, as a result of alternate processing
of pre-mRNA, have been identified, and designated type 1a
and type 1b receptors (Fig. 4). GHS-R1a consists of 366
amino acids with seven transmembrane regions and a molecu-
lar mass of approximately 41 kDa, whereas GHS-R1b consists
of 289 amino acids with only five transmembrane regions.
Unlike GHS-R1a, GHS-R1b is not activated by ghrelin or syn-
thetic GHS and it is unclear whether it is a functional receptor.
However, a recent study has reported that when GHS-R1b is
co-expressed with GHS-R1a in HEK293 cells, the signal
transduction capacity of GHS-R1a is attenuated, providing a
possible explanation for the existence and the biological sig-
nificance of the GHS-R1b transcript (Chan & Cheng, 2004).
As commented before, GHS-R1a is highly expressed in
the hypothalamus and pituitary gland, consistent with the
actions of ghrelin on the control of appetite, food intake
and energy balance (Gnanapavan et al. 2002). Interestingly,
however, GHS-R1a expression has also been reported in
other areas of the CNS that affect biological rhythms,
mood, cognition, memory and learning, such as the
hippocampus, pars compacta of the substantia nigra, ventral
tegmental area (VTA), dorsal and medial raphe nuclei, Edin-
gerWestphal nucleus and pyriform cortex (van der Lely
et al. 2004). In addition, multiple peripheral organs, such
as the stomach, intestine, pancreas, thyroid, gonads, adrenal,
kidney, heart and vasculature as well as several endocrine
and endocrine tumours and cell lines, have been found to
express GHS-R1a (Gnanapavan et al. 2002; Gaytan et al.
2004). In contrast, negligible binding was found in the para-
thyroid, pancreas, placenta, mammary gland, prostate, sali-
vary gland, stomach, colon and spleen (Papotti et al.
2000). All these data are in agreement with previous reports
indicating that ghrelin and synthetic GHS possess broader
functions beyond the control of GH release and food
intake (Tschop et al. 2000; Broglio et al. 2001) and suggest
that a still-unknown receptor subtype may exist in the heart
and in other tissues (Smith et al. 2005).
Concerning cellular signalling, GHS-R are G-protein
coupled and show strong homology across species. GHS-R
ghrelin binding activates the phospholipase C signalling path-
way through Gq, leading to protein kinase C activation, fol-
lowed by release of Ca from intercellular stores and
diacylglycerol production (Kojima et al. 2001; Kohno et al.
2003). Similarly, a Gs/protein kinase A pathway appears to
be involved in ghrelin action (Casanueva & Die
guez, 2002).
The ghrelin receptor also co-couples to a Gs protein, acti-
vating the cAMP/cAMP responsive element binding protein
pathway (Holst et al. 2003). This secondary pathway could
augment signalling by dopaminergic activation of Gs-coupled
D1 receptors in the nucleus accumbens and the VTA, both
located in the mesolimbic area of the brain (Kelley &
Berridge, 2002). In the VTA, since activation of Gq or Gs pro-
teins activates neurons (Malagon et al. 2003), the ghrelin
receptors would probably be found on dopaminergic neurons,
rather than on inhibitory neurons expressing g-aminobutyric
acid. Stimulation of g-aminobutyric acid neurons in the
VTA leads to inhibition of dopamine release in target sites
(Koob, 1992), decreasing motivated behaviour; however,
stimulation of the dopaminergic neurons would enhance dopa-
mine release in these sites, increasing food intake.
Stop 2
Stop 1
Ghrelin receptor 1a
Ghrelin receptor
Ghrelin receptor 1b
Fig. 4. Structure and translation of the growth hormone secretagogue receptors (ghrelin receptors) 1a and 1b. The former is the known active form, which binds
to ghrelin in the hypothalamus and in other peripheral tissues.
M. Gil-Campos et al.208
Although GHRH and GHS use different receptors and
second messengers, co-activation of both receptors induces a
cAMP response which is dose-dependent, and is twice that
observed after activation of the GHRH receptor alone
(Ghigo et al. 2001). As we indicated earlier, the acyl group
at Ser3 of the ghrelin molecule is essential for binding and
activating GHS-R1a. Some carboxy-terminally truncated ghre-
lin analogues have been found to be able to bind and activate
GHS-R1a. These findings indicate that not only the acyl group
but also the first seven amino acids of the ghrelin sequence are
also essential for GHS-R1a activation (Silva-Elipe et al.
2001). Short peptides encompassing the first four or five
amino acids and the octanoyl residue are as functional as
full-length ghrelin; thus, the Gly-Ser-Ser-(n-octanoyl)-Phe
segment is considered to be the active core of the molecule
(Bednarek et al. 2000). However, although these peptides
show high receptor affinity, they are unable to stimulate GH
secretion from somatotroph cells (Ghigo et al. 2001). This
sequence of seven amino acids has been universally conserved
among different species from fish and reptiles to birds and
mammals (Palyha et al. 2000), which suggests that ghrelin
has an important physiological significance.
The ghrelin receptor is highly constitutively active and this
activity could be of physiological importance in its role as a
regulator of both GH secretion and appetite control (Howard
et al. 1996). By measuring inositol phosphate turnover or by
using a reporter assay for transcriptional activity controlled
by cAMP-responsive elements, the ghrelin receptor showed
strong, ligand-independent signalling in transfected COS-7
or human embryonic kidney 293 cells. Ghrelin and a
number of the known non-peptide GHS act as agonists
stimulating inositol phosphate turnover further. In contrast,
the low-potency ghrelin antagonist (
Trp7,9,Leu11)-substance P was surprisingly found to be a
high-potency full inverse agonist, as it decreased the constitu-
tive signalling of the ghrelin receptor down to that observed in
untransfected cells (Holst et al. 2003). These results suggest
that inverse agonists for the ghrelin receptor might be particu-
larly interesting for the treatment of obesity.
The GHS-R1a is unlikely to be the only GHS-R (van der Lely
et al. 2004). It has already been demonstrated that a GHS-R sub-
type able to bind non-acylated as well as acylated ghrelin exists
and probably mediates biological activities (Cassoni et al.
2004). Another GHS-R subtype probably mediates the influence
of ghrelin on insulin secretion and glucose metabolism, because
this effect is not shared by synthetic peptidyl GHS that generally
mimic ghrelin actions (Broglio et al. 2001).
Recently, the presence of an unknown ghrelin receptor has
been suggested that modulates ghrelin actions on weight gain.
This assumption is based on the discovery of an analogue of
full-length human ghrelin, BIM-28163, which fully antagon-
ises GHS-1a by binding to but not activating the receptor.
BIM-28 163 blocks ghrelin activation of the GHS-1a receptor,
and inhibits ghrelin-induced GH secretion in vivo (Halem et al.
2004). However, unexpectedly, BIM-28163 acts as an agonist
with regard to stimulating weight gain (Halem et al. 2005).
Additionally, Gauna et al. (2005) have reported that glucose
output by primary hepatocytes is time- and dose-dependently
stimulated by acylated ghrelin and inhibited by des-acyl ghre-
lin. Moreover, the latter counteracts the stimulatory effect of
acylated ghrelin on glucose release. These actions might be
mediated by a different receptor than GHS-R1a and, appar-
ently, we should consider acylated ghrelin and desacyl-ghrelin
as separate hormones that can modify each other’s actions on
glucose handling, at least in the liver.
Ghrelin and central pathways involved in the regulation
of energy homeostasis
There are two systems that operate in the regulation of the
quantity of food intake: short-term regulation, that is con-
cerned primarily with preventing overeating at each meal,
and long-term regulation, which is primarily related with the
maintenance of normal quantities of energy stores in the
form of fat in the body (Konturek et al. 2005; Fig. 5).
The hypothalamus is considered the key region in the CNS
involved in feedback control of appetite and food intake,
though other brain regions have also been implicated. Nucleus
tractus solitarius in the brain stem serves as a gateway for
neural signals from the gastrointestinal tract to the hypothala-
mic centres. Also the amygadala, the cortex prefrontalis and
the area postrema have been held responsible for feeding dis-
orders and inadequate conservation or storage of energy
(Cone, 2005; Horvath, 2005). Additionally, the mesolimbic
region, particularly the VTA, which is involved in the
reward of feeding, the hippocampus, substantia nigra, and
dorsal and medial raphe nuclei have also been implicated
(Guan et al. 1997; Gnanapavan et al. 2002).
Early animal experiments with hypothalamic lesions and
post mortem examinations with morbid obesity led to a propo-
sal for the ‘dual centre’ hypothesis, postulating that ventro-
medial nuclei serve as the ‘satiety centre’ and the lateral
hypothalamic area as the ‘feeding or hunger centre’ (Druce
et al. 2004). The ARC and the PVN of the hypothalamus
are also considered to be a part of the hunger centre. It appears
that the hunger centre is chronically active and that its activity
may be transiently inhibited by activity of the satiety centre
occurring just after the ingestion of food (Cone, 2005; Kon-
turek et al. 2005).
In the short-term regulation of energy intake, the structures in
the brain control the intake of a single meal regarding its volume,
energy content and duration. Following food ingestion the sig-
nals from the oro-pharyngeal and gastric area are conveyed to
the nucleus tractus solitarius in the brain stem through afferent
vagal nerves and other sympathetic nervous system afferents. In
addition to mechanical distension, the chemical stimulation of
receptors in the gastrointestinal mucosa and various hormones
released from the gastrointestinal tract by nutrients contribute
to the peripheral signalling with orexigenic and anorexigenic
properties (Dockray, 2003; Konturek et al. 2004). In the long-
term coordination of dietary intake and energy expenditure the
CNS receives numerous impulses and peripheral signals,
especially from the gastrointestinal tract and fat tissue in
response to constantly altered (Halem et al. 2004) balance
(Druce et al. 2004; Horvath, 2005).
In the brain, the mammalian central melanocortin system is
defined as a collection of CNS circuits that include: (i) neur-
ons that express hypothalamic NPY and agouti gene-related
protein (NPY/AgRP) or POMC and that originate in the
ARC; (ii) brainstem POMC neurons originating in the
commissural nucleus tractus solitarius; (iii) downstream tar-
gets of these POMC and AgRP neurons expressing the
Ghrelin and energy balance 209
melanocortin-3 and melanocortin-4 receptors. In the CNS,
melanocortin peptides, namely a-melanocyte-stimulating hor-
mone, are agonists of the melanocortin-3 and melanocortin-4
receptors, whereas AgRP is a high-affinity antagonist of
both these receptors. The melanocortin system is unique in
that many melanocortin receptor sites in the CNS receive pro-
jections from both agonist-expressing POMC fibres and antag-
onist-expressing AgRP fibres, whereas some seem to receive
only agonist innervation (Cone, 2005). Another unique feature
of the central melanocortin system is that it acts like a rheostat
on energy storage (Huszar et al. 1997).
The major site of POMC expression in the CNS originates
in neurons of the ARC; most of POMC-positive cells also
express the anorectic peptide cocaine amphetamine-related
transcript (CART). The POMC- and CART-positive (POMC/
CART) cell bodies are found throughout the rostrocaudal
extent of the ARC and periarcuate area of the hypothalamus
(Jacobowitz & O’Donohue, 1978; Watson et al. 1978). Arcu-
ate POMC cells send a dense bundle of fibres ventral to other
brain regions such as the thalamus and the mesolimbic area.
As the arcuate NPY/AgRP-expressing neurons express the
potent melanocortin-3 receptor and melanocortin-4 receptor
antagonist AgRP, they are also a critical component of the
central melanocortin system since they are the target of var-
ious peripheral hormonal signals such as ghrelin, leptin and
insulin (Chen et al. 2004). AgRP-immunoreactive fibres
appear primarily in a subset of the same hypothalamic and
septal brain regions containing dense POMC innervation,
with the densest fibres found innervating the PVN, dorsome-
dial hypothalamus, posterior hypothalamus and septal regions
around the anterior commissure (Broberger et al. 1997;
Haskell-Luevano et al. 1999). The interaction of these two
POMC/CART and NPY/AgRP systems seems to be the pri-
mary driving force in the regulation of energy homeostasis
(Huszar et al. 1997; Cone, 1999, 2005).
Le Roux et al. (2005a) have shown that an intact vagus
nerve may be required for exogenous ghrelin to increase appe-
tite and food intake in man. In six men and one woman who
all had a previous complete truncal vagotomy with lower
oesophageal or gastric surgery, ghrelin stimulated GH release
in a dose-dependent manner, confirming bioactivity. However,
no change in energy intake was observed with either dose of
ghrelin (1 and 5 pmol/kg £ min).
The melanocortin system is not the sole regulator of energy
balance within the hypothalamus. The lateral hypothalamic
hypocretin orexin system is important in establishing orexi-
genic drive associated with arousal (Yamanaka, 2003).
Additionally, although ghrelin’s effects are mainly mediated
at the hypothalamic ARC, the ghrelin receptor is expressed
also in other brain sites. These areas are the caudal brainstem,
where some gustatory information is processed (Horvath,
2005), the mesolimbic region, particularly the VTA, nucleus
accumbens, hippocampus, substantia nigra, and dorsal and
medial raphe nuclei (Guan et al. 1997; Gananapavan et al.
Reward for feeding is governed by the mesolimbic dopa-
mine pathway, which consists of dopaminergic cell bodies
that reside in the VTA and project to multiple nodes, including
the nucleus accumbens, amygdala, prefrontal cortex and hip-
pocampus (Kelley & Berridge, 2002). Expression of the ghre-
lin receptor in the VTA and hippocampus might indicate that
ghrelin, normally considered a homeostatic factor, can also act
on reward circuitry, perhaps to modulate feeding behaviour.
Recently, it has been found that administration of ghrelin
into the VTA or nucleus accumbens of rats significantly
increased feeding (Naleid et al. 2005). Intake stimulated
from the VTA was at least twice as great as that stimulated
from the nucleus accumbens. When ghrelin is injected into
the VTA, it does not come into contact with known NPY neur-
ons. Therefore, in this region, ghrelin’s orexigenic effects are
Food intake
signals (NPY, AgRP,
α-MSH, etc)
Short-term regulators
of energy homeostasis
(ghrelin, CCK,PYY,
Long-term regulators
of energy homeostasis
Fig. 5. Schematic view of the short- and long-term regulation of food intake mediated by ghrelin, insulin and leptin. NPY, neuropeptide Y; AgRP, agouti-related
protein; a-MSH, melanocyte-stimulating hormone; CCK, cholecystokinin; PYY, peptide YY; GLP, glucagon-like peptide.
M. Gil-Campos et al.210
probably independent of NPY signalling. The most likely
mechanism is that ghrelin activates its own receptors in the
VTA (Guan et al. 1997), thereby increasing dopamine release
in target sites.
Fig. 6 shows the main pathways involved in the anorexi-
genic properties of ghrelin, as well as the opposite effects of
leptin and insulin.
Evidence of ghrelin’s effects on appetite and regulation
of energy balance
The first evidence of ghrelin’s involvement in regulating appe-
tite was obtained by Arvat et al. (2000) who, in a study of GH
release, found that three out of four healthy volunteers injected
with ghrelin reported hunger as a ‘collateral effect’. These
findings were confirmed in subsequent studies (Horvath et al.
2001; Nakazato et al. 2001; Wren et al. 2001; Eisenstein &
Greenberg, 2003). At the same time, numerous animal and
human studies have added strength to the argument that ghre-
lin is involved in the regulation of energy balance (Tschop
et al. 2000; Asakawa et al. 2001; Shintani et al. 2001;
Eisenstein & Greenberg, 2003).
Roles of ghrelin in appetite and short-term food intake
Plasma ghrelin levels are dependent on recent food intake;
they are increased by fasting and decline after eating
(Tschop et al. 2001). For that reason, it has been suggested
that they may play a major role in meal initiation (Cummings
et al. 2001), although the link between the rise in circulating
ghrelin and meal initiation has not been proven. Daytime
secretion of stomach ghrelin appears to be suppressed by
food intake, whilst it is augmented by night-time fasting. By
contrast, the daytime secretion of hypothalamic ghrelin is
increased, and nocturnal secretion decreased, thus regulating
food intake (Bowers, 2001). In fact, plasma ghrelin concen-
tration shows a nocturnal rise that exceeds the meal-associated
increase in lean subjects, although this rise is blunted in the
obese (Yildiz et al. 2004). In addition, circulating ghrelin
levels decrease in normal-weight subjects after mixed meals.
However, obese subjects demonstrate a much reduced ghrelin
postprandial suppression both in adults and children (English
et al. 2002; Gil et al. 2004; Gil-Campos, 2004; Le Roux
et al. 2005b).
The main factors promoting ghrelin production are fasting,
hypoglycaemia and leptin, whilst the main inhibiting factors
are food intake, hyperglycaemia and obesity (Cummings
et al. 2001; Toshinai et al. 2001; Tschop et al. 2001; Shiiya
et al. 2002). High glucose levels and consequently hyperinsu-
linaemia reduce ghrelin secretion; however, stomach expan-
sion does not display this effect (Shiiya et al. 2002). The
depth and duration of prandial ghrelin suppression are dose-
dependently related to the energy intake (Callahan et al.
2004). That means that large meals suppress both ghrelin
and hunger more thoroughly than do small meals.
There are conflicting results regarding the specific effects of
nutrients on ghrelin postprandial response. Initially, it was
proposed that a fat-rich diet decreases plasma ghrelin levels,
whereas a protein-rich diet increases them (Lee et al. 2002).
However, it is now well established that ingested energy
Endocrine axes Behavioural changes
(+) (–) (–) (+) (+)
Appetite Satiety
Effector neurons
Arcuate nucleus
Signals from GI tract
Leptin Insulin
Fig. 6. Regulation of food intake in the hypothalamus by ghrelin and counteracting effects of leptin and insulin. Neuropeptide Y (NPY)/agouti-related protein
(AgRP)-expressing neurons, located mainly in the arcuate nucleus of the hypothalamus, are stimulated by ghrelin, which in turn stimulate effector pro-opiomelono-
cortin (POMC) neurons in the paraventricular nucleus (PVN). These neurons modulate a resulting efferent message. The nucleus tractus solitarius (NTS) also
receives neuroendocrine signals from vagal afferents, which are activated by a number of factors including mechanical distension of the gastrointestinal (GI) tract,
enterohormones and nutrients. The NTS also mediates activation of PVN neurons. Other regions of the brain also have a role in the control of food intake, namely
the caudal brainstem and the mesolimbic region, particularly the ventral tegmental area (VTA), nucleus accumbens, hippocampus, substantia nigra, and dorsal
and medial raphe nuclei. PSNS, parasympathetic nervous system; HCRT, hypocretin; CART, cocaine amphetamine-related transcript; CCK, cholecystokinin;
PYY, peptide YY; GLP, glucagon-like peptide.
Ghrelin and energy balance 211
suppresses ghrelin with an efficacy order in rat and human
subjects of: carbohydrates . proteins . lipids (Cummings
et al. 2005). Glucose and amino acids suppress ghrelin more
rapidly and strongly (by approximately 70 %) than do lipids
(by approximately 50 %); the relatively weak suppression of
ghrelin by lipids compared with glucose or amino acids
could represent one mechanism promoting high-fat dietary
weight gain (Overduin et al. 2005). On the other hand, a
high-fat diet decreases ghrelin pulse amplitude in obesity-
prone rats relative to obesity-resistant rats, which precedes
the weight increase (Otukonyong et al. 2005). Increasing diet-
ary protein relative to carbohydrate and fat enhances weight
loss, at least in part by increasing satiety. Nevertheless, the
satiating effect of dietary protein appears to be unrelated to
postprandial ghrelin secretion (Moran et al. 2005).
Plasma ghrelin was determined together with hunger and
satiety ratings and with insulin and glucose concentrations
after the ingestion of satiating quantities of carbohydrate-,
fat-, protein-, fruit- and vegetable-rich meals (Erdmann et al.
2004). Standardised sandwiches were consumed 4 h later.
After carbohydrate, ghrelin decreased, whereas fat, protein,
fruit, and vegetable ingestion significantly increased ghrelin
levels. Considering all test meals, no significant correlation
existed between changes of ghrelin levels and satiety ratings,
whereas a significant inverse relationship was observed
between plasma ghrelin and insulin levels. During the
second meal, sandwich consumption was significantly greater
after the preceding fruit and vegetable meals, which was sig-
nificantly correlated with the 4th hour increase of ghrelin.
These results suggest that after an overnight fast, ghrelin
release appears to depend on the ingested macronutrients
more than being a major regulator of acute food intake,
although it is of greater importance for the recurrence of
hunger and subsequent meal size.
Increasing dietary protein relative to carbohydrate and fat
enhances weight loss, at least in part by increasing satiety,
although the mechanism for this is unclear. Recently, the
effects of isoenergetic test meals with differing protein:fat
ratios on fasting and postprandial ghrelin, insulin, glucose,
appetite, and energy expenditure before and after weight
loss on the respective dietary patterns were compared. Fasting
ghrelin increased, and the postprandial ghrelin response
improved with weight loss independently of diet composition,
suggesting that the reduced appetite observed with increased
dietary protein appears not to be mediated by ghrelin homeo-
stasis (Moran et al. 2005). On the other hand, before weight
loss, there was no significant difference in postprandial ghrelin
response between test meals rich in fat or carbohydrates. How-
ever, after weight reduction, the ghrelin response was more
pronounced after the carbohydrate test meal than after the
fat test meal (Romon et al. 2006). In obese children, low-fat
high-carbohydrate diet-induced weight loss does not change
ghrelin secretion, but significantly decreases leptin levels,
increases adiponectin levels and improves insulin resistance
determined by significantly decreased insulin resistance indi-
ces as well as lowered serum insulin levels (Reinehr et al.
Additionally, the postprandial effect of diet composition on
circulating acylated ghrelin levels in healthy women has been
recently investigated (Al Awar et al. 2005). Acylated ghrelin
fell significantly after ingestion of both balanced and
high-protein meals and ghrelin persisted at significantly
lower levels than baseline for a longer duration following
the high-protein meal compared with the balanced meal.
Again, in this study, postprandial changes in acylated
plasma ghrelin appear to depend on the macronutrient compo-
sition of the meal.
On the other hand, a soluble fibre such as arabinoxylan in an
enriched meal increases serum ghrelin levels by an unknown
mechanism (Mohlig et al. 2005). Additionally, high-fructose
sweetened beverages suppress ghrelin less well than iso-
energetic glucose beverages, probably because of the lower
capacity of fructose to increase insulin plasma concentrations
compared with glucose. Given that insulin, leptin, and ghrelin
function as key signals to the CNS in the long-term regulation
of energy balance (see later), decreases of circulating insulin
and leptin and increased ghrelin concentrations mediated by
chronic consumption of enriched fructose beverages could
lead to increased energy intake and ultimately contribute to
weight gain and obesity (Teff et al. 2004).
Regardless of the effect of ghrelin on appetite in healthy
human subjects, this hormone appears to enhance the per-
ceived palatability of the food offered (Druce et al. 2006).
Table 1 summarises the observations which support the role
of ghrelin in appetite and shot-term food intake.
Roles of ghrelin in the regulation of body weight
and energy balance
The regulation of body weight is achieved through complex hor-
monal and neuroendocrine pathways, which result in energy
homeostasis whereby energy balance is closely matched over
long periods of time (Cummings et al. 2005). Critical elements
of this control system are hormones secreted in proportion to
body adiposity, including leptin, insulin and adiponectin, and
the CNS and other peripheral targets upon which they act (Gil-
Campos et al. 2004a,b). Some of these CNS targets stimulate
food intake and anabolic pathways promoting weight gain,
whereas others reduce food intake and catabolic pathways
promoting weight loss (Batterham et al. 2002; Marx, 2003).
Moreover, a number of hormones, including leptin, insulin, adi-
ponectin and catecholamines, regulate fuel metabolism, i.e. lipid
metabolism, in a number of peripheral tissues, independently
Table 1. Features which support the role of ghrelin in appetite and
short-term food intake
1 Most ghrelin is synthesised in the stomach, a well-positioned organ
to detect recent intake of food
2 The main effects of intraventricular and blood system ghrelin
injection at times of minimal spontaneous intake is to
trigger eating and to decrease the latency of feeding
3 Human ghrelin secretion is suppressed immediately after a meal,
the depth and duration of the suppression being proportional to
the energy intake
4 Ghrelin stimulates gastrointestinal motility, and gastric and
exocrine pancreatic secretions
5 Ghrelin stimulates the secretion of neuropeptide Y and
agouti-related protein, two well-known orexigens, in the
arcuate nucleus of the hypothalamus
6 Some ghrelin gene polymorphisms are associated with alterations
in eating patterns
M. Gil-Campos et al.212
of the regulation of food intake (Baile et al. 2000; Yamauchi
et al. 2002; Gil-Campos et al. 2004a,b).
Ghrelin also appears to be involved in the regulation of
feeding behaviour and energy homeostasis (Ariyasu et al.
2002; Shiiya et al. 2002). Thus, serum ghrelin levels are inver-
sely correlated with BMI, age and insulin concentrations in
normal children and are markedly increased in the Prader
Willi syndrome (Haqq et al. 2003; Park et al. 2005), whereas
they correlate positively with leptin (Park et al. 2005).
Nutritional status also influences ghrelin levels in man. In
fact, ghrelin levels increase in response to weight loss result-
ing from low-energy diets, mixed life-style modifications,
cancer anorexia and cachexia, Huntington’s disease, anorexia
and bulimia nervosa, and chronic failure of the heart, lungs,
liver, or kidneys (Horvath et al. 2001; Tanaka et al. 2002;
Tolle et al. 2003; Soriano-Guillen et al. 2004; Cummings
et al. 2005). This has led to the suggestion that ghrelin signals
the need to conserve energy, and that its secretion is the
‘trigger’ that counteracts a subsequent energy deficit, prevent-
ing cachexia (Horvath et al. 2001; Cummings et al. 2005).
In contrast to healthy individuals, obese subjects display
reduced plasma ghrelin levels, together with low plasma GH
and high plasma leptin levels (Tschop et al. 2001; Shiiya et al.
2002; Rosicka
et al. 2003). Plasma ghrelin concentrations rise
rapidly after fasting in normal-weight animals, but this increase
is delayed in obese ob/ob and db/db mice and in fatty Zucker rats
( fa/fa), suggesting that short-term regulation is modified by
excess energy deposit (Ariyasu et al. 2002). After eating a test
meal, obese human adults do not exhibit the decline in ghrelin
levels observed in lean subjects. Fasting ghrelin levels fall by
39·5 % in lean subjects 30 min after eating, before returning
rapidly towards baseline levels. There was no change in circulat-
ing ghrelin in the obese group (English et al. 2002).
It is also reported that ghrelin levels are lower in prepuber-
tal obese children than in healthy controls (Gil et al. 2004;
Gil-Campos, 2004; Bacha & Arslanian, 2005), and that a
reduction of up to 50 % in the standard deviation of the
BMI prompts a significant increase (Soriano-Guillen et al.
2004). Moreover, in obese subjects there appears to be a
short-term delay in the regulation of ghrelin secretion both
in animals and in human subjects (Ariyasu et al. 2002). The
decline in plasma ghrelin levels recorded in lean subjects
after eating a test meal is not observed in adult obese subjects
(English et al. 2002). Likewise, after the intake of a standar-
dised breakfast, obese children recover plasma fasting ghrelin
levels more rapidly than healthy children, which suggests that
this biased pattern may have an impact in the increased con-
sumption of foods (Gil et al. 2004). Moreover, in a recent
study oral glucose tolerance test-induced absolute suppression
in ghrelin was about 50 % less in overweight v. normal-weight
children (Bacha & Arslanian, 2005). Also, bariatric surgery,
the most effective method to sustain weight in morbid obesity,
is associated with reduced ghrelin plasma levels (Cummings
& Shannon, 2003; Holdstock et al. 2003).
Regardless, ghrelin circulates in proportion to body-energy
stores and exhibits compensatory changes in response to
fluctuation in fat mass; an adiposity hormone should influence
neuronal activity in brain centres known to regulate body
weight, and ghrelin’s exogenous administration or blockade
should alter food intake and energy expenditure (Cummings
et al. 2005).
Ghrelin enhances food intake by stimulation of NPY and
AgRP synthesis in the ARC of the hypothalamus and hindbrain
(Bagnasco et al. 2003). Likewise, peripheral or central chronic
administration of ghrelin increases body weight (Tschop et al.
2000). Moreover, the chronic central administration of ghrelin
reverses the effects of leptin, primarily by altering food intake,
but it also shows regulatory effects on adiposity and plasma
insulin levels independent of feeding effect (Kim et al.
2004). It can also decrease energy expenditure (Asakawa
et al. 2001; Kamegai et al. 2001), fat catabolism and lipolysis
(Tschop et al. 2000; Muccioli et al. 2004; Barazzoni et al.
2005) and adipocyte apoptosis (Kim et al. 2004). Des-acyl
ghrelin, as well as ghrelin, and short ghrelin fragments act
directly as antilipolytic factors on the adipose tissue through
binding to a specific receptor, which is apparently distinct
from GHS-R1a (Muccioli et al. 2004).
Acute ghrelin blockade in animals using anti-ghrelin anti-
bodies, ghrelin receptor antagonists or antisense oligonucleotides
has demonstrated decreased food intake and weight loss
(Nakazato et al. 2001; Asakawa et al. 2003; Bagnasco et al.
2003). However, knockout ghrelin gene animal models exhibit
a subtle increased fat catabolism and leanness (Sun et al. 2004;
Wortley et al. 2004). However, analyses of ghrelin
(2 /2 )
demonstrate that endogenous ghrelin plays a prominent role in
determining the type of metabolic substrate (i.e. fat v. carbo-
hydrate) that is used for maintenance of energy balance, particu-
larly under conditions of high fat intake (Wortley et al. 2004).
Table 2 summarises the observations which support the role
of ghrelin in the control of body weight and energy
Recently, studies in ghrelin knockout mice have suggested
that this hormone is not an essential regulator of food intake
and gastric emptying since ghrelin
(2 /2 )
mice showed similar
body-weight gain and 24 h food intake to that of wild-type ani-
mals (Sun et al. 2003; Wortley et al. 2004). In addition,
exogenous ghrelin increased food intake in both wild and
(2 /2 )
mice (De Smet et al. 2006). Furthermore, adult
mice with deletion of the ghrelin gene display a normal sensi-
tivity to high-fat-diet-induced obesity (Grove & Cowley,
2005). However, the disruption of the ghrelin gene may
affect not only the synthesis of ghrelin but also obestatin,
which is involved in the suppression of food intake (Zhang
et al. 2005). On the other hand, recent studies by Wortley
Table 2. Features which support the role of ghrelin in the regulation of
energy balance
1 Serum ghrelin levels are inversely correlated with BMI, age
and insulin concentrations
2 Ghrelin levels are influenced by nutritional status, increasing
in response to weight loss resulting from low-energy diets,
life-style modifications and diseases leading to malnutrition
3 Obese children and adults have lower ghrelin plasma levels
than lean subjects, and they exhibit a lower postprandial
decline and a more rapid returning towards baseline levels
4 Bariatric surgery is associated with reduced ghrelin plasma levels
5 Peripheral or central chronic administration of ghrelin increases
body weight and reverses the effects of leptin
6 Ghrelin decreases energy expenditure limiting fat catabolism,
lipolysis and adipocyte apoptosis
7 Acute ghrelin blockade in animals using anti-ghrelin antibodies,
ghrelin receptor antagonists or antisense oligonucleotides
decreases food intake and weight loss
Ghrelin and energy balance 213
et al. (2005) and Zigman et al. (2005) demonstrate that both
(2 /2 )
and GHS-R
(2 /2 )
mice are resistant to diet-
induced obesity when fed a high-fat diet during the early
post-weaning period, whereas those mice in the adult period
become obese on a high-fat diet. These results suggest a
role for ghrelin in the development of metabolic pathways,
both central and peripheral, as well as the development of
compensatory signals that allow for the maintenance of
normal body weight and sensitivity to high-fat diets in the
absence of potent orexigenic systems (Grove & Cowley,
Des-acyl ghrelin in the regulation of of food appetite
and food intake
Although des-acyl ghrelin was initially considered as a non-
functional peptide in the regulation of appetite and food
intake, recent studies have shown that it decreases food
intake and gastric emptying in mice and rats. In fact, adminis-
tration of des-acyl ghrelin to mice decreased food intake and
gastric emptying rate through an action on the PVN and the
ARC in the hypothalamus (Asakawa et al. 2005). Moreover,
intracisternal administration of des-acyl ghrelin decreases
food intake in food-deprived rats and inhibits gastric emptying
without altering small-intestinal transit (Chen et al. 2005a). In
addition, intraperitoneal injection of des-acyl ghrelin has
recently been reported to decrease food intake in conscious
rats (Chen et al. 2005b). Intraperitoneal injection of des-acyl
ghrelin enhanced c-Fos expression in the ARC and PVN but
not in the nucleus of the solitary tract. Both ICV and intrave-
nous injection of des-acyl ghrelin disrupted fasted motor
activity in the antrum but not in the duodenum. Changes in
gastric motility induced by intravenous injection of des-acyl
ghrelin were completely antagonised by ICV injection of a
selective corticotropin-releasing factor 2 receptor antagonist;
however, the corticotropin-releasing factor 1 receptor antagon-
ist had no effects. Thus, these results suggest that peripheral
des-acyl ghrelin may disrupt fasted motor activity in the
stomach by direct activation of corticotropin-releasing factor
2 receptor in the brain and that corticotropin-releasing factor
1 receptor is not involved in this action.
These findings indicate that, in contrast to acylated ghrelin,
des-acyl ghrelin induces a negative energy balance by decreas-
ing food intake and delaying gastric emptying; the effect is
mediated via the hypothalamus. Although derived from the
same precursor, the inverse effects of these two peptides
suggest that the stomach might be involved as an endocrine
organ in the regulation of the energy balance (Broglio et al.
2004; Asakawa et al. 2005). This idea is also supported by
the new discovery of obestatin (Zhang et al. 2005).
Ghrelin and energy expenditure
Although speculative, serum ghrelin may play a role in the
regulation of energy expenditure through the induction of
metabolic changes that would lead to an efficient metabolic
state resulting in increased body weight and fat mass.
Thus, despite the known stimulatory effects on appetite and
eating behaviour of ghrelin, little information is available
regarding its relationship with energy expenditure in both
animals and man.
Tschop et al. (2000) showed that single subcutaneous ghre-
lin injections induced an increase of the RQ in mice and that
such increased utilisation of carbohydrate and reduced utilis-
ation of fat was congruent with the observed increase in
body fat. A selective RQ increase without concomitant
increases in carbohydrate intake is unusual and may reflect
reduced sympathetic nervous system activity (Snitker et al.
1998). In addition, direct stimulation of hypothalamic areas
can induce a selective change in RQ (Atrens et al. 1985).
The chronic ICV injection of ghrelin increased cumulative
food intake and decreased energy expenditure, resulting in
body-weight gain (Kamegai et al. 2001). On the other hand,
in human adults ghrelin enhances the sensation of hunger
and reduces fat depot utilisation, increasing carbohydrate con-
sumption through the mediation of g-aminobutyric acid and
the inhibition of anorexigenic substances such as a-melano-
cyte-stimulating hormone (Casanueva & Die
guez, 2002).
Additionally, St-Pierre et al. (2004) examined the relationship
between serum ghrelin and RMR, the thermic effect of food,
fasting and postprandial RQ, physical activity level, peak
aerobic capacity (VO
), energy intake, and psychological
measures of feeding behaviour in young healthy women.
They noted significant inverse correlations between ghrelin
and RMR and the thermic effect of food and these inverse cor-
relations persisted after statistical control for both fat-free
mass and fat mass, which suggest that higher levels of ghrelin
are associated with low levels of resting and postprandial
To elucidate the role of endogenous ghrelin in the regu-
lation of energy homeostasis and gastric emptying, ghrelin
knockout mice (ghrelin
(2 /2 )
) were generated (De Smet et al.
2006). Although body-weight gain and 24 h food intake were
not affected, during the dark period young ghrelin
(2 /2 )
had a lower RQ, whereas their heat production was higher
than that of the wild-type littermates, inferring a role of ghre-
lin in the regulation of energy expenditure.
Very recently, changes of ghrelin circulating levels induced
by a mixed meal and their relationship with postprandial sub-
strate oxidation rates in overweight and obese children with
different levels of insulin sensitivity have been reported
(Maffeis et al. 2006). Insulin sensitivity and postprandial
maximal decrease of ghrelin concentration showed a
significant correlation. Moreover, the postprandial carbo-
hydrate oxidation rate correlated with the area under the
curve for both insulin and ghrelin. These results suggest that
the oxidation rate of glucose is affected not only by insulin
but also by ghrelin, which in turn will influence the energy
On the other hand, Zigman et al. (2005) have recently
shown that when fed a high-fat diet, both female and male
GHS-R-null mice eat less food, store less of their consumed
energy, preferentially utilise fat as an energy substrate, and
accumulate less body weight and adiposity than control
mice. This suggests that ghrelin signalling is required for the
control of energy expenditure.
Mechanisms of action of ghrelin as an orexigenic peptide
One of the modes by which ghrelin can influence dietary
intake is based on the feature that it acts as a hormone,
secreted primarily by the stomach and small intestine into
M. Gil-Campos et al.214
the bloodstream, from which it gains access to NPY/AgRP
neurons in the medial ARC across an incomplete blood
brain barrier at that site. Additional effects may result from
circulating ghrelin accessing its receptor at circumventricular
sites in the hindbrain, which may subsequently affect ARC
neuronal activity via ascending projections.
Evidence for the effects of ghrelin on food intake being
mediated by NPY and AgRP has been supported by a
number of experimental approaches (Hewson & Dickson,
2000; Nakazato et al. 2001; Kamegai et al. 2001; Shintani
et al. 2001; Wang et al. 2002), including blockade of ghre-
lin-induced food intake by either ICV injection of antibodies
against NPY and AgRP (Nakazato et al. 2001), NPY Y1
receptor antagonists (Nakazato et al. 2001; Shintani et al.
2001), or peripheral administration of the melanocortin ago-
nist melanotan-II (Tschop et al. 2002). Additionally, periph-
eral administration of ghrelin activates c-Fos expression only
in arcuate NPY/AgRP neurons, but not in other hypothalamic
or brainstem sites (Wang et al. 2002), and ablation of the ARC
blocks the actions of ghrelin administration on feeding but not
elevation of GH (Tamura et al. 2002). Finally, electrophysio-
logical approaches have demonstrated that ghrelin can
activate NPY/AgRP neurons and simultaneously reduce the
activity of POMC neurons (Cowley et al. 2003). Orexigenic
activity results from activation of the hypothalamic NPY/Y1
receptor, which antagonises the effects of leptin, an appetite-
suppressing hormone from adipose tissues (Shintani et al.
2001; Bagnasco et al. 2002; Fig. 6).
Surprisingly, disruption of the AgRP and NPY genes either
alone or in combination does not alter energy homeostasis to
the extent that is predicted from the proposed role of NPY/
AgRP neurons in the ARC (Hewson & Dickson, 2000). How-
ever, a recent study indicates that induced selective ablation of
AgRP-expressing neurons in adult mice by cell type-specific
expression of a diphtheria toxin receptor and the adminis-
tration of diphtheria toxin results in acute reduction of feeding,
demonstrating direct evidence for a critical role of these neur-
ons in the regulation of energy homeostasis (Gropp et al.
2005). In addition, co-administration of ghrelin with AgRP
into the hypothalamic PVN during the light and dark phases
of feeding in rats did not produce a synergistic effect on
food intake, suggesting that ghrelin induction of feeding
occurs by recruiting AgRP as one of the obligatory mediators
of its orexigenic effect (Shrestha et al. 2006).
It has been suggested that two ghrelin-mediated networks
exist for energy balance: the NPY network exerting an acute
effect and the AgRP network a chronic effect (Muccioli
et al. 2004). A number of studies have shown that ghrelin
acts, at least partially, by activating genes coding for these
powerful promoters of food intake, NPY and AgRP, and its
action is mediated by receptors different from those causing
the GHS effect (Seoane et al. 2003; Thompson et al. 2004;
Cone, 2005; Horvath, 2005).
Another model to explain activation of NPY/AgRP neurons
by ghrelin is the vagal pathway. It is based on the assumption
that ghrelin, produced mostly in the stomach, acts on the
hunger centre and does not cross the blood brain barrier.
Thus, there should be an indirect pathway through which per-
ipherally administered ghrelin can activate the hypothalamic
appetite-regulatory neurons. This pathway may be via the
vagus nerve system, since the appetite-stimulating effect of
ghrelin is suppressed by vagotomy (Date et al. 2002). More-
over, ghrelin administration at low doses in mice improves
the efferent activity of this nerve, which stimulates stomach
contraction, secretion and filling (Asakawa et al. 2001).
Despite activating c-Fos only in ARC NPY neurons, periph-
eral ghrelin may access the ARC through vagal afferents
(Horvath, 2005).
Although models by which ghrelin stimulates NPY/AgRP
neurons are universally accepted, one additional model has
been proposed in the scientific literature. Ghrelin could act
as a neuropeptide, released from hypothalamic ghrelinergic
neurons that synapse with NPY/AgRP cells and other neurons
involved in energy homeostasis (Cowley et al. 2003; Toshinai
et al. 2003; Cone, 2005; Horvath, 2005). However, determin-
ing what pathways are the most critical mediators of ghrelin’s
anabolic actions will require tissue and/or cell-specific elimin-
ation of ghrelin signalling.
Fig. 7 exhibits the neurons and pathways involved in ghrelin
hypothalamic action.
Molecular mechanisms of action
To control energy homeostasis, hypothalamic energy centres
must gather nutritional information from multiple signals
that were initially included within the glucostatic and adipo-
statlipostatic hypotheses (Mayer, 1955; Lam et al. 2005).
However, how the hypothalamus converts these diverse sig-
nals into a cogent and coordinated response to changes in
nutrient availability is mostly unknown. Recently, it has
been proposed that the hypothalamic sensing of fatty acids
provides a viable biochemical explanatory framework (Lam
et al. 2005) and that 5
-AMP-activated protein kinase
(AMPK) is an important signalling molecule that integrates
nutritional and hormonal signals and modulates feeding
behaviour and energy metabolism (Andersson et al. 2004;
Minokoshi et al. 2004; Kim & Lee, 2005; Lam et al. 2005).
The hypothesis advanced is that circulating lipids such as
long-chain fatty acids (LCFA) regulate feeding behaviour and
glucose production by generating an increase in the cellular
LCFA-CoA pool in the hypothalamus. In turn, LCFA-CoA
signal an energy ‘surfeit’ within the hypothalamus, which acti-
vates neural pathways designed to curtail both food intake and
liver glucose production (Lam et al. 2005). Consistent with
this hypothesis, intravenous infusion of a lipid emulsion is suffi-
cient to suppress food intake in baboons (Woods et al. 1984).
Thus, circulating lipids (triacylglycerol, glycerol and LCFA)
seem to generate a signal of nutrient ‘surfeit’ (Lam et al.
2005). This signal is independent of measurable changes in
plasma insulin and does not require gastrointestinal nutrient
absorption (Woods et al. 1984; Matzinger et al. 2000). Likewise,
sustained ICV infusion of brain fuels, such as glucose, glycerol
and b-hydroxybutylate, causes a decrease in body weight and
food intake (Davis et al. 1981). The well-established biochemi-
cal link between cellular carbohydrate and lipid metabolism
must be important in modulating the hypothalamic sensing of
fatty acids. Thus, it is likely that this central nutrient-sensing
mechanism is able to respond to increased availability of
lipids, carbohydrates or both (Lam et al. 2005).
On the other hand, in vivo administration of leptin, which
leads to a reduction in food intake, decreases hypothalamic
AMPK activity (Andersson et al. 2004; Minokoshi et al.
Ghrelin and energy balance 215
2004). By contrast, injection of ghrelin in vivo, which
increases food intake, stimulates AMPK activity in the hypo-
thalamus. Consistent with the effect of ghrelin, injection of
5-amino-4-imidazole carboxamide riboside, a pharmacologi-
cal activator of AMPK, into either the third cerebral ventricle
or directly into the PVN of the hypothalamus significantly
increased food intake (Andersson et al. 2004).
-AMPK is known as a cellular ‘energy sensor’, as its
activity is sensitively changed by the cellular energy state.
AMPK controls a number of metabolic processes to help the
restoration of energy depletion in the peripheral tissues
(Hardie et al. 1999; Kemp et al. 2003) and is also expressed
in the neurons of the CNS (Turnley et al. 1999; Culmsee
et al. 2001). AMPK was first described as an enzyme capable
of phosphorylating and inactivating hydroxymethylglutaryl-
CoA reductase and acetyl-CoA carboxylase (ACC), key
enzymes in the synthesis of cholesterol and fatty acids
(Beg et al. 1973; Carlson & Kim, 1973). It was later named
AMPK, because its activity is highly dependent on the pre-
sence of 5
-AMP (Yeh et al. 1980). In fact, AMPK activity
is regulated by the AMP:ATP ratio (Hardie et al. 1999).
Thus, an increase in the AMP:ATP ratio activates AMPK
and inactivates ACC (Carlson & Kim, 1973), causing a
decrease in intracellular levels of malonyl-CoA, an inhibitor
of carnitine palmitoyl transferase-1 (Ruderman et al. 2003).
As a result, AMPK activation causes the stimulation of mito-
chondrial fatty acid oxidation (Ruderman et al. 2003).
Recent studies have demonstrated that adipocyte-derived
hormones, leptin and adiponectin, increase fatty acid oxidation
through AMPK activation in skeletal muscle (Tomas et al.
2002; Yamauchi et al. 2002; Minokoshi et al. 2004). AMPK
is also required for mitochondrial biogenesis in response to
chronic energy deprivation in skeletal muscle (Zong et al.
2002). AMPK activation in hepatocytes inhibits fatty acid
and cholesterol synthesis by inactivating ACC and hydroxy-
methylglutaryl-CoA reductase (Beg et al. 1973; Carlson &
Kim, 1973). In contrast to its action in muscle cells, AMPK
activation in 3T3-L1 adipocytes has little effect on glucose
transport at the basal state, but inhibits insulin-stimulated glu-
cose transport. However, the role of AMPK in the regulation
of lipolysis is not presently settled (Kim & Lee, 2005).
The mechanisms by which AMPK activity in hypothalamic
neurons affects feeding behaviour are still not fully under-
stood. However, hypothalamic AMPK controls feeding beha-
viour, at least in part, through the regulation of NPY and
AgRP expression. Over-expression of AMPK in the medial
hypothalamus decreases NPY and AgRP mRNA expression
in ad libitum-fed rats, whereas over-expression of constitu-
tively active AMPK augments the fast-induced increase in
NPYand AgRP expression (Minokoshi et al. 2004). It is
likely that increased AMPK activity associated with high
ghrelin levels at fasting would decrease hypothalamic malo-
nyl-CoA levels by lowering the activity of ACC. Consistent
with this idea, the decreased cellular levels of malonyl-CoA
would in turn activate carnitine palmitoyl transferase-1
activity and increase food intake through a cellular decrease
of LCFA-CoA (Yeh et al. 1980). On the other hand, AMPK
could also modulate the transcription of hypothalamic neuro-
peptides independently of its effects on ACC or malonyl-
CoA (Minokoshi et al. 2004).
Increased food
intake and
decreased energy
effector neurons
effector neurons
GSH-R1 a
GSH-R1 a
Arcuate nucleus
a -MSH
Fig. 7. Integrative regulatory circuits in the regulation of appetite and food intake mediated by ghrelin in the central nervous system. Neuropeptide Y (NPY)/agouti-
related protein (AgRP) neurons are activated by the binding of ghrelin to its receptors. These neurons co-express NPY, which activates effector neurons in the
paraventricular nucleus (PVN) of the hypothalamus, and AgRP, which inhibits the binding of melanocyte-stimulating hormone (a-MSH) to melanocortin receptor 4
(MC4R). All these signals are integrated in the PVN and modulate a resulting efferent message, which is believed to be mediated in part by thyrotropin-releasing
hormone (TRH) and corticotropin-releasing hormone (CRH). Ghrelin-expressing neurons are adjacent to the third ventricle between the ventromedial hypothala-
mus, the dorsal hypothalamus, the PVN and the arcuate nucleus. Efferents of ghrelin-expressing neurons project to key circuits of central energy balance regu-
lation and may contribute to balance the activity of orexigenic NPY/AgRP with anorexigenic pro-opiomelanocortin (POMC) neurons. YR1, GHS-R1a, ghrelin
receptor; CART, cocaine amphetamine-related transcript.
M. Gil-Campos et al.216
Fig. 8 shows a potential mechanism by which ghrelin may
signal on hypothalamic LCFA-CoA and indeed on energy
Interactions between ghrelin, leptin and insulin
The regulation of ghrelin secretion and its biological effects
appear to be opposed to those of leptin (Shintani et al.
2001; Williams & Mobarhan, 2003). However, they are comp-
lementary molecules within the same regulating system for
informing the CNS about the current acute and chronic
energy balance (Tschop et al. 2000, 2001). It has been
suggested that circadian rhythms in the afferent signal of
leptin from adipocytes and from ghrelin in the stomach code
for a corresponding discharge pattern in the NPY-dependent
hypothalamic system (Kalra & Kalra, 2003; Kalra et al. 2003).
Leptin does not appear to be a major regulatory factor of
ghrelin levels, at least in animals (Ariyasu et al. 2002). In
fact, no correlation has been found between ghrelin and
leptin levels in obese children and adolescents (Ikezaki et al.
2002). However, other studies suggest that there is a complex
interaction between insulin, leptin and ghrelin, and that leptin
may regulate ghrelin levels and affect changes in body weight
(Williams & Mobarhan, 2003). In a recent report, it has been
shown that leptin inhibits both the secretion of gastric ghrelin
and the stimulation of feeding by ghrelin (Kalra et al. 2005).
A reciprocal rhythmic pattern of the two afferent hormonal
signals, anorexigenic leptin and orexigenic ghrelin, imparts
rhythmicity to the NPY system, the final common pathway
for appetite expression in the hypothalamus. It has been
shown that leptin inhibits both the secretion of gastric ghrelin
and the stimulation of feeding by ghrelin. It has been proposed
that this dual leptin restraint is the major regulatory arm of the
feedback communication between the periphery and the hypo-
thalamus for weight homeostasis (Barazzoni et al. 2003;
Cummings & Foster, 2003; Kontureck et al. 2004), and dis-
ruption in the rhythmic communication at any locus in the
leptin ghrelin NPY feedback loop impels loss of hypothala-
mic control, leading to abnormal weight gain and obesity
(Kalra et al. 2005).
Barazzoni et al. (2003) tested the hypothesis that increased
plasma leptin prevents the potential increase in plasma ghrelin
concentration during moderate energy restriction in lean rats.
These authors found that moderate hyperleptinaemia prevents
an increase of plasma ghrelin during moderate short-term
energy restriction and satiety-inducing effects of leptin include
suppression of gastric orexigenic signals and disruption of a
potential feedback mechanism between body-weight changes
and plasma ghrelin in lean adult rats. This cross-talk between
leptin and ghrelin has been termed as the ‘ghrelin leptin
tango’ in body-weight regulation (Cummings & Foster,
2003; Konturek et al. 2005). Moreover, increased plasma
ghrelin concentration was reported during diet-induced
weight loss in obese human subjects, suggesting that ghrelin
contributes to adaptive increment in appetite associated with
energy restriction and lower leptin levels (Moran et al. 2005).
The ‘ghrelin leptin tango’ hypothesis implies that weight-
reducing effects of leptin are mediated not only by its direct
Food intake
Fig. 8. Role of the 5
-AMP-activated protein kinase (AMPK) in the maintenance of energy homeostasis mediated by ghrelin (Ghrl) in the hypothalamus. It is likely
that increased AMPK activity, associated with high Ghrl levels at fasting, would decrease hypothalamic malonyl-CoA levels by lowering the activity of acetyl-CoA
carboxylase (ACC). Consistent with this idea, the decreased cellular levels of malonyl-CoA would in turn activate carnitine palmitoyl transferase-1 (CPT1) activity,
increasing the transport of long-chain fatty acids (LC-acyl-CoA) to the mitochondria as well as their oxidation. Decreased intracellular levels of LC-acyl-CoA would
result in enhanced food intake mediated by a higher expression of neuropeptide Y.
Ghrelin and energy balance 217
central action on the hypothalamus but also through its periph-
eral inhibitory effect on the release and action of ghrelin.
According to that, parenteral administration of ghrelin in
rats at a dose that raises plasma hormone to the level observed
under fasting conditions significantly attenuates plasma levels
of leptin, while markedly increasing food intake (Konturek
et al. 2003). Moreover, immunoneutralisation of circulating
plasma ghrelin with specific IgG anti-ghrelin antibodies
causes a marked increase in plasma leptin and decrease in
food intake. In contrast, exogenous leptin, at the dose that
raises plasma leptin to the level occurring postprandially,
markedly reduced plasma levels of ghrelin and attenuated
food intake; these effects can be reversed by the adminis-
tration of specific IgG anti-leptin antibodies (Konturek et al.
It has long been thought that glucose homeostasis is finely
regulated by the brain (Mayer, 1955; Lam et al. 2005).
The brain receives nutritional and hormonal signals from
peripheral tissues, giving rise to efferent signals that control
peripheral glucose metabolism, including insulin secretion,
hepatic glucose production, and glucose uptake and glycogen
synthesis in skeletal muscle (Minokoshi et al. 1999; Obici
et al. 2001, 2002).
The observations that insulin and ghrelin have reciprocal
24 h profiles and that both are involved in the physiological
response to food intake has led to hypothesise that insulin
regulates ghrelin secretion and vice versa (Cummings et al.
2005). Ghrelin’s action on energy metabolism appears to be
associated with a reduction in insulin synthesis (Broglio
et al. 2001) and it has been suggested that postprandial insulin
is responsible for reducing plasma ghrelin levels following
food intake (Lucidi et al. 2002). However, most studies
designed to evaluate whether insulin inhibits ghrelin have con-
cluded that insulin can suppress ghrelin only at doses higher
than those observed physiologically and that insulin is not
absolutely required for postprandial ghrelin suppression,
although it allows the duration of the response, at least in ani-
mals (Cummings et al. 2005).
How ghrelin regulates insulin is also controversial. At least
three different models have been proposed to explain the
action of ghrelin in decreasing insulin secretion. First, several
observations suggest a potential model of ghrelin as a counter-
regulatory hormone that blocks insulin secretion and action to
maintain blood glucose levels. In fact, a variation in the ghre-
lin gene increases weight and decreases insulin secretion in
obese children (Korbonits et al. 2002). Moreover, a recent
work has shown that ghrelin is suppressed by glucagon,
although it does not mediate glucagon-related GH release
(Hirsh et al. 2005). Ghrelin can also stimulate GH, cortisol
and adrenaline (Nagaya et al. 2001 a,b). In addition, ghrelin
has been demonstrated to down-regulate the expression of
adiponectin in differentiating adypocytes (Ott et al. 2002),
an adipocytokine involved in the sensitisation of tissues to
insulin, which enhances fatty acid oxidation and that has
been involved in the pathogenesis of obesity and insulin resist-
ance (Gil-Campos et al. 2004b). Furthermore, it has been
shown that GHS-R is found in hepatoma cells, raising the
possibility that ghrelin modulates insulin activities in man.
In fact, it has been demonstrated that exposure of those
cells to ghrelin caused up-regulation of several insulin-induced
activities including tyrosine phosphorylation of insulin
receptor substrate-1, association of the adapter molecule
growth factor receptor-bound protein 2 with insulin receptor
substrate-1, mitogen-activated protein kinase activity, and
cell proliferation. Unlike insulin, ghrelin inhibited Akt
kinase activity as well as up-regulated phosphoenolpyruvate
carboxykinase gene expression, a key enzyme in gluconeogen-
esis (Murata et al. 2002).
A recent study in human volunteers has added support to the
hypothesis that insulin is a negative regulator of ghrelin
secretion in the postprandial state (Soule et al. 2005). Thus,
basal ghrelin was decreased significantly by 15 min after glu-
cagon administration, then remained suppressed relative to the
basal value until 240 min after glucagon, and there was an
inverse statistical relationship between the increase in insulin
over the first 120 min and the decrease in ghrelin.
Gauna et al. (2004) investigated the metabolic actions of
ghrelin in human subjects by examining the effects of acute
administration of acylated ghrelin, des-acyl ghrelin, and the
combination in eight adult-onset GH-deficient patients. They
followed glucose, insulin and NEFA concentrations before
and after lunch and with or without the presence of GH in
the circulation. They found that acylated ghrelin, which is
rapidly cleared from the circulation, induced a rapid rise in
glucose and insulin levels. Des-acyl ghrelin, however, pre-
vented the acylated ghrelin-induced rise in insulin and glucose
when it was co-administered with acylated ghrelin. Finally,
acylated ghrelin decreased insulin sensitivity up to the end
of a period of 6 h after administration. This decrease in insulin
sensitivity was prevented by coinjection of unacylated ghrelin.
This combined administration of acylated and unacylated
ghrelin even significantly improved insulin sensitivity, com-
pared with placebo, for at least 6 h.
The role of AMPK in the regulation of glucose homeostasis
has been suggested by a study that demonstrated glucose intol-
erance and insulin resistance in a2-AMPK knockout mice
(Viollet et al. 2003). Since the pancreas, skeletal muscle and
adipose tissue express GHS-R1a (Howard et al. 1996) and
ghrelin has been shown to activate AMPK in the ARC, it
might be speculated that ghrelin would influence glucose
homeostasis not only at the hypothalamic level but also
through a direct interaction with peripheral tissues.
One of the most important features of the metabolic syndrome
is insulin resistance (Reaven, 2005). An inverse association
between endogenous ghrelin and the metabolic syndrome has
been recently reported in older adults, which is largely explained
by the strong ghrelin BMI association. Nevertheless, a signifi-
cant association independent of BMI was also observed between
insulin and ghrelin (Langenberg et al. 2005).
The associations between plasma ghrelin concentration and
metabolic parameters in children and adolescents have also
been reported (Park et al. 2005). Fasting plasma ghrelin con-
centration was negatively correlated with height, weight, BMI,
percentage body fat, waist circumference and hip circumfer-
ence in both boys and girls. Fasting plasma ghrelin levels
were significantly negatively correlated with triacylglycerols
and fasting insulin levels and positively correlated with
HDL-cholesterol in boys, but not in girls. All these results
suggest that higher plasma ghrelin levels have beneficial
effects on metabolic parameters in boys and that the relation-
ships between fasting plasma ghrelin levels and metabolic
parameters differ according to sex.
M. Gil-Campos et al.218
The role of ghrelin in childhood obesity, a state associated
with hyperinsulinaemia and insulin resistance, is not fully under-
stood. Recently, Bacha & Arslanian (2005) have investigated the
dynamics of ghrelin suppression after an oral glucose tolerance
test in normal-weight v. overweight children and the relationship
of ghrelin suppression to insulin sensitivity. Fasting ghrelin
levels were significantly lower in overweight v. normal-weight
youth and were mainly influenced by insulin sensitivity indepen-
dently of adiposity. Oral-glucose-tolerance-test-induced absol-
ute suppression in ghrelin was approximately 50 % less in
overweight v. normal-weight children and the suppression of
ghrelin correlated positively with the whole-body insulin sensi-
tivity index and negatively with the change in insulin at 30 min.
Fasting ghrelin, the change in insulin, and the change in glucose
during the oral glucose tolerance test were the significant inde-
pendent variables contributing to the variance in absolute sup-
pression of ghrelin. Thus, alterations in ghrelin suppression in
overweight children may be yet another manifestation of the
insulin resistance of obesity. However, whether this is respon-
sible for differences in satiety in overweight individuals merits
additional investigation.
Rat adipose tissue expresses GHS-R1a mRNA, suggesting
ghrelin may directly influence adipocyte function. Indeed,
the effects of ghrelin on insulin-stimulated glucose uptake in
isolated white adipocytes in vitro have been investigated
(Patel et al. 2006). Ghrelin increased insulin-stimulated
deoxyglucose uptake by 55 % in isolated white adipocytes
extracted from the epididymal fat pads of male Wistar rats.
However, ghrelin administration in the absence of insulin
had no effect on adipocyte deoxyglucose uptake, suggesting
that ghrelin acts synergistically with insulin. Des-acyl ghrelin
had no effect on insulin-stimulated glucose uptake. All these
results suggest that ghrelin may play a role in adipocyte regu-
lation of glucose homeostasis.
The inhibitory effect of ghrelin on insulin secretion was
suggested to be due to a tonic regulation of pancreatic
b-cells, prompting inhibition of both insulin and pancreatic
somatostatin secretion (Egido et al. 2002). However, more
recently, it has been shown that ghrelin is produced in a
new type of islet cell, the
-cell, and that the production of
the hormone in those cells would affect b-cells via a paracrine
mechanism that may require higher local levels of ghrelin than
those found in plasma (Prado et al. 2004; Wierup et al. 2004).
In fact, it has been proposed that that insulin and ghrelin cells
share a common progenitor and that Nkx2·2 and Pax4, two
homeodomain proteins, are required to specify or maintain
differentiation of the b-cell fate. This finding also suggests
that there is a genetic component underlying the balance
between insulin and ghrelin in regulating glucose metabolism
(Prado et al. 2004). Recently, definitive evidence has proven
that the ghrelin hormone is also synthesised by pancreas
a-cells, the glucagon-producing cell population, and con-
firmed the presence of single-hormone ghrelin-producing
cells devoid of any of the four classical islet hormones,
-cells (Heller et al. 2005). All these findings taken
together raise the possibility that ghrelin modulates insulin
activities in man.
In conclusion, some evidence suggests that ghrelin has a
key role in the control of appetite and food intake and most
probably in energy expenditure. The mechanism of action
for ghrelin involves several pathways including hormonal
actions on hypothalamus and hindbrain neurons. The inter-
action of ghrelin with its receptor GHS-R1a appears to be
the most important mechanism by which ghrelin mediates
the synthesis of the orexigenic NPY and AgRP in the hypo-
thalamus. In addition, a neuroendocrine vagal pathway and a
neuropeptide system, working in the brain have also been
reported, although their biological significance is practically
unknown. Moreover, new receptors for ghrelin and desacyl-
ghrelin seem to contribute to diverse central and peripheral
actions of ghrelin. A role for LCFA acyl-CoA and AMPK in
the regulation of hypothalamic signalling associated with
food intake and energy expenditure has been recently
described. Both LCFA acyl-CoA and AMPK would act as
‘fuel sensors’ within the hypothalamus and will inform
about the current energy body status.
Leptin inhibits both the secretion of gastric ghrelin and the
stimulation of feeding by ghrelin. In turn, ghrelin suppresses
the secretion of leptin in the stomach and it has been proposed
that this dual leptin restraint is the major regulatory arm of the
feedback communication between the periphery and the hypo-
thalamus for weight homeostasis.
How ghrelin regulates insulin is a matter of controversy and
three different models have been proposed to explain the
action of ghrelin in decreasing insulin secretion. Furthermore,
recent data suggest that ghrelin may play a role in the regu-
lation of glucose homeostasis in adipocytes and new findings
claim that ghrelin may act as an anti-hypoglycaemic hormone
modulating insulin tissue activities.
Perspectives and future research
Given the wide spectrum of biological activities of ghrelin and
related peptides, it is evident that the discovery of ghrelin has
opened many new perspectives within neuroendocrine and
metabolic research and even has an influence on fields of
internal medicine such as gastroenterology, immunology,
oncology and cardiology. It is therefore increasingly likely
that ghrelin and its GHS analogues, acting as either agonists
or antagonists on different physiological and pathophysiologi-
cal processes, might have clinical impact and therapeutic
potential (van der Lely et al. 2004). However, a better under-
standing of the roles of the various different forms of ghrelin
and ghrelin-related peptides, i.e. des-acyl ghrelin and obesta-
tin, in the intricate balance of energy homeostasis and body-
weight control may be essential for the successful treatment
of obesity.
In addition, due to the complexity of GHS-R, it is clear that
further studies are required to clarify whether ghrelin is the
sole ligand or one of a number of ligands activating the
GHS-R1a and whether that receptor used for ghrelin isolation
is the sole receptor or one of a group of receptors for such
ligands. Likewise, much work needs to be done to establish
the physiological role of ghrelin in meal initiation and
body-weight regulation and to establish its mechanism or
mechanisms of action.
The present study was supported by the Plan Nacional de
n Cientı
fica, Desarrollo e Innovacio
n Tecnolo
Ghrelin and energy balance 219
(I þ D þ I), Instituto de Salud Carlos III-Fondo de Investiga-
n Sanitaria project no. PI 020826 from the Spanish Ministry
of Health and Consumption, and co-financed by FEDER,
n Salud 2000, and Hero Espan
a, S.A., Spain.
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