How Ghrelin Stimulates GH and Appetite
Ghrelin
2 Distinct Brain Targets
Ghrelin activates GHS-R1a receptors in both the pituitary (releasing growth hormone) and the hypothalamic arcuate nucleus (triggering hunger) at the same time.
Kojima et al., Nature, 1999
Kojima et al., Nature, 1999
View as imageGhrelin is the only circulating hormone that increases both growth hormone release and appetite. These two effects seem unrelated until you trace them to the same receptor expressed in two different brain regions. In the anterior pituitary, GHS-R1a activation releases growth hormone into the bloodstream. In the hypothalamic arcuate nucleus, the same receptor triggers the NPY/AgRP neurons that drive hunger. Kojima et al. discovered ghrelin in 1999 as a 28-amino acid peptide secreted primarily by the stomach, identifying it as the endogenous ligand for the growth hormone secretagogue receptor that had been known since the 1970s.[1] For an overview of ghrelin's role as the hunger hormone, see ghrelin: the hunger hormone that rises before meals.
Key Takeaways
- Ghrelin activates GHS-R1a receptors simultaneously in the pituitary (GH release) and hypothalamic arcuate nucleus (appetite), producing both effects through one receptor[1]
- The octanoyl (fatty acid) modification on serine-3 is required for receptor binding; unacylated ghrelin does not activate GHS-R1a[2]
- Hashiguchi et al. (2017) showed ghrelin activates hypothalamic NPY neurons both directly (through GHS-R1a on NPY cells) and indirectly (through presynaptic disinhibition)[3]
- Shankar et al. (2021) demonstrated that deleting LEAP2 (ghrelin's endogenous antagonist) enhanced both orexigenic and GH-releasing effects of ghrelin in mice[4]
- Thomas et al. (2016) found central ghrelin administration increased both immediate food intake for 24 hours and food hoarding behavior in hamsters[5]
- Ghrelin-GHS-R1a signaling also extends to dopamine reward circuits, linking hunger to the motivational drive for food[6]
The Receptor: GHS-R1a
The growth hormone secretagogue receptor type 1a (GHS-R1a) is a G protein-coupled receptor (GPCR) that sits at the center of ghrelin biology. Before ghrelin was even discovered, synthetic compounds called growth hormone secretagogues (like GHRP-6 and MK-677) had been developed to stimulate growth hormone release by activating this receptor. Kojima et al. (1999) identified ghrelin as the natural hormone that the receptor had been waiting for.[1]
GHS-R1a has several unusual properties:
Constitutive activity: Unlike most GPCRs, GHS-R1a signals even without ghrelin bound to it. Lopez Soto et al. (2015) showed that this constitutive activity impairs CaV2.1 and CaV2.2 calcium channel currents, affecting neurotransmitter release even in the absence of ghrelin.[7] This means the receptor has a baseline "tone" that ghrelin amplifies rather than initiates.
Dual distribution: GHS-R1a is expressed at high density in both the anterior pituitary gland and the hypothalamic arcuate nucleus. It is also found on vagal afferent neurons, in the ventral tegmental area (VTA) of the dopamine reward system, and throughout the gastrointestinal tract. This widespread distribution is why ghrelin affects so many systems simultaneously.
Heterodimerization: Navarro et al. (2022) discovered that GHS-R1a forms receptor complexes (oligomers) with dopamine D1 receptors in the ventral tegmental area.[6] These GHS-R1a/D1R oligomers mediate ghrelin's effects on dopamine signaling and food reward. This receptor-receptor interaction explains why hunger makes food more rewarding, a phenomenon explored in depth in ghrelin and reward: why hunger makes food taste better.
How Ghrelin Releases Growth Hormone
When ghrelin binds GHS-R1a on somatotroph cells in the anterior pituitary, it activates the Gq/11 signaling cascade:
- Phospholipase C (PLC) is activated
- PLC cleaves PIP2 into IP3 and DAG
- IP3 triggers calcium release from intracellular stores
- Elevated intracellular calcium triggers growth hormone vesicle exocytosis
- GH is released into the bloodstream
This pituitary mechanism works synergistically with growth hormone-releasing hormone (GHRH) from the hypothalamus. Ghrelin amplifies the GH pulse that GHRH initiates, producing larger GH surges than either hormone alone. When GHRH and ghrelin are administered together, the resulting GH release is substantially greater than the sum of each hormone's individual effect, indicating true synergy rather than simple addition. At the same time, ghrelin acts in the hypothalamic arcuate nucleus to increase GHRH release and suppress somatostatin (the GH inhibitor), further boosting the GH response from both the pituitary and hypothalamic levels.
Magdaleno-Mendez et al. (2015) demonstrated that chronic ghrelin treatment increased both GH production and the functional expression of voltage-gated sodium channels (NaV1.1 and NaV1.2) in bovine somatotrophs, suggesting ghrelin enhances the electrical excitability of GH-producing cells as well as their secretory capacity.[8]
How Ghrelin Drives Appetite
The appetite pathway operates through a different cell population in a different brain region, but uses the same receptor.
In the hypothalamic arcuate nucleus, GHS-R1a is expressed on NPY/AgRP neurons, the primary orexigenic (appetite-stimulating) neurons in the brain. When ghrelin activates these receptors:
- NPY (neuropeptide Y) and AgRP (agouti-related protein) are released
- NPY stimulates food intake through Y1 and Y5 receptors
- AgRP blocks melanocortin-4 receptors (MC4R), removing the satiety brake
- The combined signal produces a powerful drive to eat
Hashiguchi et al. (2017) revealed that ghrelin's activation of NPY neurons is more complex than simple receptor binding.[3] Ghrelin acts both directly on NPY neurons (through GHS-R1a expressed on those cells) and indirectly by reducing inhibitory GABAergic input from surrounding neurons. This presynaptic disinhibition amplifies the orexigenic signal beyond what direct receptor activation alone would produce.
Xu et al. (2024) identified the specific intracellular pathway: ghrelin activates AMPK (AMP-activated protein kinase) in hypothalamic neurons, which signals through the mTOR pathway to increase NPY production.[9] This AMPK-mTOR-NPY cascade provides a molecular explanation for how an energy-sensing pathway (AMPK) connects to appetite regulation.
Thomas et al. (2016) demonstrated that central ghrelin does more than stimulate immediate food intake. In hamsters, third ventricle ghrelin administration increased food intake for 24 hours and triggered food hoarding behavior, indicating ghrelin activates both the consummatory drive (eating now) and the appetitive drive (securing food for later).[5] Both effects were blocked by GHS-R antagonism, confirming they operate through the same receptor.
Why One Receptor Does Two Jobs
The evolutionary logic for coupling GH release with appetite is straightforward. Growth requires calories. An organism that releases growth hormone during fasting without also driving food-seeking behavior would waste the anabolic signal. By linking both responses to a single hormone secreted when the stomach is empty, the system ensures that growth signals are accompanied by the behavioral drive to obtain the nutrients that growth requires.
This coupling also explains the metabolic role of ghrelin during fasting. When blood glucose drops and the stomach empties, ghrelin rises. The GH released in response mobilizes fatty acids from adipose tissue (lipolysis) and opposes insulin's glucose-lowering effects, protecting blood sugar during the fasted state. Simultaneously, the appetite signal motivates food-seeking to end the fast. The two effects are complementary survival mechanisms, not coincidental side effects of a single receptor.
The timing reinforces this interpretation. Ghrelin follows a pulsatile pattern that peaks before anticipated meals and drops sharply after eating. GH secretion follows a similar pre-meal surge pattern. In nocturnal fasting, ghrelin rises through the night and contributes to the large GH pulse that occurs during the first hours of sleep, a pulse associated with tissue repair and protein synthesis. The biological logic is consistent: the fasting signal primes both the metabolic machinery (GH) and the behavioral response (hunger) needed to end the fast.
The Acylation Requirement
Not all ghrelin activates GHS-R1a. The peptide exists in two forms: acyl-ghrelin (with an octanoyl fatty acid group attached to serine-3) and desacyl-ghrelin (without the modification). Only acyl-ghrelin binds and activates GHS-R1a. The enzyme ghrelin O-acyltransferase (GOAT) catalyzes the attachment of the octanoyl group, and GOAT activity is itself regulated by nutrient availability, particularly medium-chain fatty acid supply.
This acylation gate adds a layer of metabolic regulation. When dietary fat intake is low, GOAT activity decreases, reducing the proportion of circulating ghrelin that can activate GHS-R1a. The system effectively links macronutrient availability to the strength of the ghrelin signal: low-fat diets produce less active ghrelin, potentially reducing both GH pulses and hunger drive. Circulating desacyl-ghrelin is 3 to 4 times more abundant than acyl-ghrelin, but its biological role through GHS-R1a is negligible.
Chebani et al. (2016) provided genetic evidence for this coupling. Rats carrying a gain-of-function GHS-R mutation (Q343X) showed enhanced G protein-dependent signaling in response to ghrelin, resulting in both increased adiposity and enhanced GH responsiveness.[10] The mutation amplified both arms of ghrelin signaling simultaneously, consistent with a single receptor driving both outputs.
LEAP2: The Endogenous Brake
The ghrelin system has a built-in antagonist. LEAP2 (liver-expressed antimicrobial peptide 2) blocks GHS-R1a, reducing both ghrelin's GH-releasing and appetite-stimulating effects. Perello et al. (2023) helped standardize the nomenclature for this system through an expert consensus survey.[2]
Shankar et al. (2021) deleted the LEAP2 gene in mice and found that both orexigenic and GH-releasing effects of ghrelin were enhanced.[4] LEAP2-knockout females on a high-fat diet showed increased sensitivity to ghrelin's appetite effects, confirming that LEAP2 normally restrains ghrelin signaling. The fact that removing one antagonist amplifies both the GH and appetite arms simultaneously provides further evidence that these effects run through the same receptor.
LEAP2 levels rise after eating and fall during fasting, creating an inverse pattern to ghrelin. This push-pull system fine-tunes GHS-R1a activation throughout the day: ghrelin dominates before meals (driving hunger and GH release), while LEAP2 dominates after meals (suppressing both).
Clinical and Pharmacological Implications
Understanding the dual mechanism has direct consequences for drug development. Any compound that targets GHS-R1a will inevitably affect both GH release and appetite, a challenge for drugs designed to do one without the other.
Growth hormone secretagogues like MK-677, GHRP-2, and GHRP-6 all increase appetite alongside GH, precisely because they activate the same receptor ghrelin uses. The appetite increase is not a side effect; it is the receptor doing exactly what it evolved to do. For patients with cachexia, this dual effect is a feature rather than a problem. For athletes or bodybuilders who want GH elevation without increased hunger, the coupling creates an unavoidable trade-off.
Conversely, drugs that block GHS-R1a to reduce appetite (ghrelin resistance in obesity) will simultaneously suppress GH release, which carries its own metabolic consequences. The receptor's cardiovascular protective effects add another dimension: ghrelin has cardioprotective properties that operate partly through the same receptor.
Howick et al. (2017) reviewed the complexity of targeting the ghrelin receptor for appetite regulation, concluding that multiple redundant pathways (vagal, central, and reward-circuit mediated) make it difficult to block appetite through GHS-R1a antagonism alone.[11] The receptor's constitutive activity, heterodimerization with dopamine receptors, and presence at multiple sites in the brain create a pharmacological challenge that no single-target drug has cleanly solved.
The Vagal Pathway: A Third Route
Beyond the pituitary and hypothalamic pathways, ghrelin also signals through the vagus nerve. GHS-R1a receptors are expressed on vagal afferent neurons that connect the gut to the brainstem. When ghrelin activates these receptors in the stomach and intestine, the signal travels through the vagus nerve to the nucleus tractus solitarius (NTS) in the brainstem, which relays it to the hypothalamus.
This vagal route creates a gut-to-brain communication channel that operates independently of ghrelin's direct effects on hypothalamic neurons after crossing the blood-brain barrier. Vagotomy (cutting the vagus nerve) partially attenuates ghrelin's appetite-stimulating effects in animal models, indicating that the vagal pathway contributes meaningfully to ghrelin-driven hunger. The fact that vagotomy only partially blocks ghrelin's effects confirms that both the vagal and direct hypothalamic routes are active, creating redundancy in the hunger signal.
The vagal pathway also provides a mechanism for rapid signaling. Circulating ghrelin must cross the blood-brain barrier to reach the arcuate nucleus directly, which takes time and depends on transport mechanisms. Vagal afferent signaling is faster, transmitting ghrelin's gut-empty signal to the brainstem within seconds. This speed may explain why hunger can onset rapidly when a meal is delayed, before circulating ghrelin levels have fully peaked.
The Bottom Line
Ghrelin stimulates both growth hormone release and appetite through a single receptor, GHS-R1a, expressed in the pituitary gland and hypothalamic arcuate nucleus. In the pituitary, receptor activation triggers calcium-dependent GH exocytosis. In the arcuate nucleus, the same receptor activates NPY/AgRP neurons through both direct and indirect mechanisms to drive hunger. This coupling reflects an evolutionary link between growth signaling and nutrient acquisition. The endogenous antagonist LEAP2 restrains both effects simultaneously, and pharmacological manipulation of GHS-R1a inevitably affects both arms of the system.