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Growth hormone–releasing hormone
Growth hormone–releasing hormone
Growth hormone–releasing hormone
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Growth hormone releasing hormone
Identifiers
SymbolGHRH
Alt. symbolsGRF, GHRF
CAS number9034-39-3
NCBI gene2691
HGNC4265
OMIM139190
RefSeqNM_021081
UniProtP01286
Other data
LocusChr. 20 p12 or q11.2-q12
Search for
StructuresSwiss-model
DomainsInterPro

Growth hormone–releasing hormone (GHRH), also known as somatocrinin among other names in its endogenous form and as somatorelin (INN) in its pharmaceutical form, is a releasing hormone of growth hormone (GH). It is a 44[1]-amino acid peptide hormone produced in the arcuate nucleus of the hypothalamus.

GHRH first appears in the human hypothalamus between 18 and 29 weeks of gestation, which corresponds to the start of production of growth hormone and other somatotropes in fetuses.[1]

Nomenclature

[edit]
  • Endogenous:
    • somatocrinin
    • somatoliberin
    • growth hormone–releasing hormone (GHRH or GH-RH; HGNC symbol is GHRH)
    • growth hormone–releasing factor (GHRF or GRF)
    • somatotropin-releasing hormone (SRH)
    • somatotropin-releasing factor (SRF)
  • Pharmaceutical:

Origin

[edit]

GHRH is released from neurosecretory nerve terminals of these arcuate neurons, and is carried by the hypothalamo-hypophyseal portal system to the anterior pituitary gland, where it stimulates growth hormone (GH) secretion by stimulating the growth hormone-releasing hormone receptor. GHRH is released in a pulsatile manner,[2][3] stimulating similar pulsatile release of GH. In addition, GHRH also promotes slow-wave sleep directly.[4] Growth hormone is required for normal postnatal growth, bone growth, regulatory effects on protein, carbohydrate, and lipid metabolism.[1]

Effect

[edit]

GHRH stimulates GH production and release by binding to the GHRH receptor (GHRHR) on cells in the anterior pituitary.

Receptor

[edit]
Main article: Growth hormone-releasing hormone receptor

The GHRHR is a member of the secretin family of G protein-coupled receptors, and is located on chromosome 7 in humans. This protein is transmembranous with seven folds, and its molecular weight is approximately 44 kD.[1]

Signal transduction

[edit]

GHRH binding to GHRHR results in increased GH production mainly by the cAMP-dependent pathway,[5] but also by the phospholipase C pathway (IP3/DAG pathway),[1] and other minor pathways.[1]

The cAMP-dependent pathway is initiated by the binding of GHRH to its receptor, causing receptor conformation that activates Gs alpha subunit of the closely associated G-Protein complex on the intracellular side. This results in stimulation of membrane-bound adenylyl cyclase and increased intracellular cyclic adenosine monophosphate (cAMP). cAMP binds to and activates the regulatory subunits of protein kinase A (PKA), allowing the free catalytic subunits to translocate to the nucleus and phosphorylate the transcription factor cAMP response element-binding protein (CREB). Phosphorylated CREB, together with its coactivators, p300 and CREB-binding protein (CBP) enhances the transcription of GH by binding to CREs cAMP-response elements in the promoter region of the GH gene. It also increases transcription of the GHRHR gene, providing positive feedback.[1]

In the phospholipase C pathway, GHRH stimulates phospholipase C (PLC) through the βγ-complex of heterotrimeric G-proteins. PLC activation produces both diacylglycerol (DAG) and inositol triphosphate (IP3), the latter leading to release of intracellular Ca2+ from the endoplasmic reticulum, increasing cytosolic Ca2+ concentration, resulting in vesicle fusion and release of secretory vesicles containing premade growth hormone.[1]

Some Ca2+ influx is also a direct action of cAMP, which is distinct from the usual cAMP-dependent pathway of activating protein kinase A.[1]

Activation of GHRHRs by GHRH also conveys opening of Na+ channels by phosphatidylinositol 4,5-bisphosphate, causing cell depolarization. The resultant change in the intracellular voltage opens a voltage-dependent calcium channel, resulting in vesicle fusion and release of GH.[1]

Relationship of GHRH and somatostatin

[edit]

The actions of GHRH are opposed by somatostatin (growth-hormone-inhibiting hormone). Somatostatin is released from neurosecretory nerve terminals of periventricular somatostatin neurons, and is carried by the hypothalamo-hypophyseal portal circulation to the anterior pituitary where it inhibits GH secretion. Somatostatin and GHRH are secreted in alternation, giving rise to the markedly pulsatile secretion of GH.[6]

Other functions

[edit]

GHRH expression has been demonstrated in peripheral cells and tissues outside its main site in the hypothalamus, for example, in the pancreas, epithelial mucosa of the gastrointestinal tract and, pathologically, in tumour cells.[1]

Sequence

[edit]

The amino acid sequence (44 long) of human GHRH is:

HO - Tyr - Ala - Asp - Ala - Ile - Phe - Thr - Asn - Ser - Tyr - Arg - Lys - Val - Leu - Gly - Gln - Leu - Ser - Ala - Arg - Lys - Leu - Leu - Gln - Asp - Ile - Met - Ser - Arg - Gln - Gln - Gly - Glu - Ser - Asn - Gln - Glu - Arg - Gly - Ala - Arg - Ala - Arg - Leu - NH2

Analogs

[edit]

Growth-hormone-releasing hormone is the lead compound for a number of structural and functional analogs, such as Pro-Pro-hGHRH(1-44)-Gly-Gly-Cys,[7] CJC-1293,[8] and CJC-1295.[9]

Many GHRH analogs remain primarily research chemicals, although some have specific applications. Sermorelin, a functional peptide fragment of GHRH, has been used in the diagnosis of deficiencies in growth hormone secretion.[10] Tesamorelin,[11] under the trade name Egrifta, received U.S. Food and Drug Administration approval in 2010 for the treatment of lipodystrophy in HIV patients under highly active antiretroviral therapy,[12] and, in 2011, was investigated for effects on certain cognitive tests in the elderly.[13] As a category, the use of GHRH analogs by professional athletes may be prohibited by restrictions on doping in sport because they act as growth hormone secretagogues.[14]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Growth hormone–releasing hormone

View on Grokipedia
from Grokipedia
Growth hormone–releasing hormone (GHRH), also known as somatocrinin, is a peptide hormone consisting of 44 amino acids that is synthesized in the hypothalamus and acts primarily to stimulate the production, proliferation, and secretion of growth hormone (GH) from somatotroph cells in the anterior pituitary gland. The hormone is initially produced as a preprohormone, which is processed to yield the mature form and a C-terminal GHRH-related peptide. Encoded by the GHRH gene on chromosome 20q11.23, GHRH binds to the G protein-coupled growth hormone-releasing hormone receptor (GHRHR) on pituitary cells, activating adenylate cyclase and increasing intracellular cyclic AMP to trigger GH release. GHRH release from the occurs in a pulsatile manner, synchronized with GH secretion, and is regulated by a complex interplay of factors including negative feedback from GH and (IGF-1), inhibition by , and stimulation by . This pulsatile pattern peaks during and declines with age, contributing to age-related changes in growth and . Beyond its central role in the GH/IGF-1 axis, which governs linear growth, protein synthesis, and metabolic , GHRH exhibits extrahypothalamic expression in tissues such as the , , and gonads, where it may function in an autocrine or paracrine manner to promote and tissue repair. Clinically, GHRH and its synthetic analogues are significant for diagnosing and treating GH-related disorders; for instance, GHRH stimulation tests assess pituitary function in suspected GH deficiency, while long-acting agonists like are approved for reducing visceral fat in HIV-associated by enhancing pulsatile GH secretion. Conversely, GHRH receptor antagonists show promise in inhibiting tumor growth and in cancers such as and by blocking aberrant GHRH signaling. Dysregulation of GHRH can lead to conditions like isolated GH deficiency or, in cases of ectopic hypersecretion, with pituitary .

Discovery and Nomenclature

Discovery

The existence of a hypothalamic factor capable of stimulating (GH) release was first inferred in the mid-20th century, but direct evidence emerged in the through experiments on animal models. In studies, hypothalamic extracts were shown to potently stimulate GH secretion from pituitary glands and , distinguishing this activity from other releasing factors and supporting the of a specific GH-releasing substance. The purification and isolation of growth hormone-releasing hormone (GHRH), also known as growth hormone-releasing factor (GRF), marked a major breakthrough in 1982. and colleagues extracted GHRH from a pancreatic tumor associated with , yielding sufficient material for characterization after years of unsuccessful attempts to isolate it from hypothalamic tissue. This discovery built on Guillemin's earlier work on hypothalamic hormones, for which he shared the 1977 in Physiology or Medicine with for identifying thyrotropin-releasing hormone and luteinizing hormone-releasing hormone. Independently, Jean Rivier and Wylie Vale's group isolated an identical from the same tumor type in the same year. Early purification relied on advanced techniques tailored to the peptide's low abundance and instability. Hypothalamic or tumor extracts were fractionated using (HPLC) and gel filtration, with activity monitored via radioimmunoassays (RIA) for GH and bioassays involving dispersed rat pituitary cells to measure stimulated GH release. These methods confirmed GHRH's potency, with purified fractions eliciting dose-dependent GH secretion at nanomolar concentrations. The amino acid sequence of human GHRH was determined in 1983 through of the purified 44-residue peptide, revealing its structure and enabling subsequent synthetic production. This sequencing effort, led by Guillemin's team, identified two bioactive forms (1-40 and 1-44 amino acids) and confirmed the peptide's novelty compared to other hypothalamic factors.

Nomenclature

Growth hormone–releasing hormone (GHRH), also referred to as growth hormone-releasing factor (GRF), somatocrinin, or somatorelin, represents the primary nomenclature for this hypothalamic that stimulates pituitary secretion. In humans, the gene is designated by the symbol GHRH (HGNC:4265) and maps to 20q11.23 at genomic coordinates 20:37,251,086-37,261,814 (GRCh38). The encoded protein precursor, known as prepro-GHRH, is cataloged in under accession P01286 (108 amino acids); this distinguishes it from the intermediate pro-GHRH (after cleavage) and the bioactive mature GHRH (a 44-amino-acid ), with nomenclature reflecting their sequential processing stages in databases such as and NCBI. Initially isolated in 1982 and termed GRF due to its functional identification from pancreatic tumors, the nomenclature shifted to GHRH in subsequent years to standardize it alongside other hypothalamic regulators like (TRH) and (CRH); the synonym somatocrinin was formally proposed in 1983 following cDNA of the precursor.

Structure and Biosynthesis

Chemical Structure

Growth hormone–releasing hormone (GHRH) is a whose biologically active form in humans consists of the first 29 derived from a larger precursor, featuring an amidated essential for its stability and function. The primary sequence of human GHRH(1-29)-NH₂ is Tyr-Ala-Asp-Ala-Ile-Phe-Thr-Asn-Ser-Tyr-Arg-Lys-Val-Leu-Gly-Gln-Leu-Ser-Ala-Arg-Lys-Leu-Leu-Gln-Asp-Ile-Met-Ser-Arg-NH₂. This amidation occurs as a during processing from the prepro-GHRH precursor. The molecular weight of GHRH(1-29)-NH₂ is approximately 3,358 Da. Structurally, GHRH lacks bonds and adopts an α-helical conformation in solution, with two distinct helical regions spanning residues 6–13 and 16–29, which contribute to its amphiphilic nature and receptor interaction. The N-terminal residue is critical for , as modifications at this position significantly reduce potency in structure-activity relationship studies. While the human form is processed to a 44-amino acid peptide with full activity retained in the N-terminal 29 residues, species variations exist; for example, rat GHRH comprises 43 amino acids due to a shortened C-terminal extension. These differences primarily affect the C-terminal region beyond residue 29, with the core active sequence highly conserved across mammals.

Biosynthesis and Origin

Growth hormone-releasing hormone (GHRH) is primarily synthesized in neurons of the arcuate nucleus within the , where these neurons extend projections to the for release into the . The , located on human , is transcribed and translated into a 108-amino acid prepro-GHRH precursor protein. Posttranslational processing of prepro-GHRH occurs in the secretory pathway, beginning with cleavage of the N-terminal signal peptide to yield pro-GHRH. Subsequent proteolytic cleavages are mediated by prohormone convertases, including furin at the preproGHRH29-30 site to generate an intermediate pro-GHRH form, followed by PC1/3 cleavage primarily at the preproGHRH74 site to produce the mature GHRH peptide along with a glycine-arginine-phenylalanine-amide-related peptide (GHRH-RP). This processing results in the biologically active mature forms of GHRH, predominantly the 44-amino acid peptide, with a shorter 29-amino acid variant arising from alternative cleavage. Beyond the hypothalamus, GHRH expression has been detected in extraneural tissues, including the , , and . In the placenta, GHRH plays a role in fetal development by potentially regulating secretion during embryogenesis.

Mechanism of Action

Receptor

The growth hormone-releasing hormone receptor (GHRHR) is a (GPCR) belonging to class B ( family), characterized by seven transmembrane domains and an extracellular N-terminal domain that facilitates ligand binding. The GHRHR gene, encoding this 423-amino-acid protein, is located on the short arm of human at position 7p14. Structural insights from a 2020 cryo-electron microscopy (cryo-EM) study at 2.6 Å resolution reveal how GHRH binding induces conformational changes in the receptor's transmembrane helices, enabling interaction with the stimulatory (Gs). GHRHR is predominantly expressed in the somatotropic cells of the gland, where it mediates the primary physiological effects of GHRH. Lower expression levels occur in extrapituitary tissues, including the , , , and testis, suggesting potential paracrine or autocrine roles in these sites. The receptor exhibits high-affinity binding to GHRH, with a (Kd) of approximately 1 nM for the bioactive fragment GHRH(1-29)-NH₂, and demonstrates selectivity over related peptides such as (VIP) or pituitary adenylate cyclase-activating polypeptide (PACAP). This specific interaction upon ligand binding couples GHRHR to Gs proteins, initiating downstream signaling.

Signal Transduction

Upon binding of growth hormone-releasing hormone (GHRH) to its (GHRHR), the receptor undergoes a conformational change that facilitates coupling to the stimulatory (Gs). The activated GHRHR catalyzes the exchange of GDP for GTP on the Gs α-subunit, leading to dissociation of the α-subunit from the βγ complex; the GTP-bound Gs α-subunit then stimulates to convert ATP into (cAMP). Elevated intracellular cAMP levels bind to and activate (PKA), a heterotetrameric consisting of two regulatory and two catalytic subunits. Dissociation of the regulatory subunits allows the catalytic subunits of PKA to phosphorylate downstream targets, including the cAMP response element-binding protein (CREB) at serine 133. Phosphorylated CREB dimerizes, translocates to the nucleus, and binds to cAMP response elements (CRE) in the promoter region of the gene (GH1 in humans, Gh1 in ), thereby enhancing transcription through recruitment of coactivators such as p300 and upregulation of pituitary-specific 1 (Pit-1). In addition to the primary cAMP-dependent pathway, GHRHR activation can engage (PLC), generating inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) from (PIP2). IP3 binds to IP3 receptors on the , releasing stored calcium ions into the , while cAMP promotes calcium influx via voltage-gated calcium channels. These calcium transients contribute to further CREB phosphorylation via calcium/calmodulin-dependent pathways, supporting sustained GH1/Gh1 gene expression and replenishment of growth hormone stores in somatotroph cells. Prolonged or continuous GHRH exposure induces desensitization of the GHRHR to prevent overstimulation, primarily through phosphorylation of the receptor's C-terminal tail by kinases such as PKA and protein kinase C (PKC), leading to uncoupling from Gs proteins. This is followed by recruitment of β-arrestin proteins, which sterically hinder further G protein interaction and promote clathrin-mediated endocytosis and internalization of the receptor complex, reducing surface receptor availability.

Physiological Effects and Regulation

Effects on Growth Hormone Secretion

Growth hormone–releasing hormone (GHRH) is secreted by hypothalamic neurons in a pulsatile pattern, with bursts occurring approximately every 3–5 hours, thereby driving the episodic peaks of growth hormone (GH) release from anterior pituitary somatotroph cells. This rhythmic secretion ensures that GH levels fluctuate in a physiological manner, with higher amplitudes during sleep and periods of fasting, supporting optimal anabolic and metabolic functions. At the , GHRH exerts its primary effects by binding to G protein-coupled receptors on somatotrophs, triggering a dose-dependent increase in GH synthesis through enhanced transcription of the GH gene and promoting the of stored GH granules. This response is rapid and concentration-dependent, with effective doses as low as 0.03 nM eliciting significant GH release, and it involves elevated intracellular (cAMP) as a key signaling intermediate. A critical regulatory mechanism involves , where elevated GH levels stimulate hepatic production of insulin-like growth factor-1 (IGF-1), which acts on the to autoregulate and suppress further GHRH secretion, thereby preventing excessive GH output. This loop maintains in the somatotropic axis. In developmental physiology, GHRH is indispensable for linear growth in childhood, as its stimulation of GH secretion directly supports epiphyseal proliferation and bone elongation; deficiencies in GHRH signaling lead to profound . Beyond growth, GHRH-mediated GH release influences systemic metabolism by enhancing in to mobilize free fatty acids and by inducing transient to prioritize glucose availability for tissues.

Interaction with Somatostatin

Somatostatin (SST), also known as somatotropin release-inhibiting hormone, acts as the principal inhibitor of (GH) secretion from pituitary somatotroph cells. It exerts this effect primarily by binding to specific G protein-coupled receptors, notably subtypes SSTR2 and SSTR5, which are expressed on the surface of somatotrophs. Upon binding, SST activates inhibitory G proteins that suppress activity, leading to decreased intracellular (cAMP) levels and subsequent reduction in GH synthesis and release. This mechanism directly counteracts the stimulatory effects of growth hormone-releasing hormone (GHRH), which promotes GH secretion through increased cAMP production via its own receptor. The interaction between GHRH and SST involves reciprocal regulation at the hypothalamic level, primarily through feedback from GH and IGF-1, which inhibits GHRH neurons and stimulates SST release from neurons, contributing to the fine-tuned control of GH secretion. SST suppresses the electrical activity of GHRH neurons, often in an irregular pattern that influences population-level oscillations. This bidirectional control ensures that excessive GH stimulation by GHRH is tempered, preventing sustained hypersecretion. In the integrated model of GH regulation, GHRH and SST orchestrate an characterized by episodic GH bursts. Pulses of GHRH drive the peaks of GH secretion, while troughs in SST tone allow these bursts to occur; subsequent rises in SST terminate the pulses, reestablishing inhibition until the next cycle. This rhythmic interplay, observed prominently in male rats with approximately 3- to 4-hour intervals, maintains physiological GH pulsatility essential for growth and . Pharmacological studies further illustrate this antagonism, demonstrating that the net GH response depends on the relative balance of GHRH and SST activity. For instance, simultaneous infusions of GHRH and SST in humans and animals result in attenuated GH release compared to GHRH alone, with the magnitude of inhibition varying based on the dosage ratio; withdrawal of SST during ongoing GHRH administration, however, restores robust GH pulses. These findings underscore the dominant inhibitory role of SST when co-administered, highlighting the therapeutic potential of modulating this interaction in GH-related disorders.

Other Regulatory Influences

Ghrelin acts as a synergist to GHRH by activating its cognate receptor, GHS-R1a, which amplifies GHRH-induced (GH) release from the . When and GHRH are administered simultaneously, they exhibit a synergistic effect on GH secretion, enhancing the magnitude of the response beyond what either hormone achieves alone. This interaction occurs primarily in the arcuate nucleus of the , where stimulates GHRH neurons indirectly through GHS-R1a-expressing intermediates, potentiating the overall GH axis during states of energy need. Several inhibitory factors modulate GHRH activity, including and glucose, which suppress its stimulatory effects on GH secretion. directly inhibits GHRH-stimulated GH release from pituitary somatotrophs in short-term cultures, likely by altering intracellular signaling pathways in the pituitary. Similarly, acute suppresses GHRH-induced GH secretion independently of adrenergic mechanisms, inferentially through hypothalamic inhibition of GHRH release or enhanced somatostatin tone. In contrast, sex steroids such as and testosterone enhance hypothalamic-pituitary sensitivity to GHRH, amplifying GH responses to GHRH stimulation; for instance, supplementation in postmenopausal women selectively increases the GH secretory response to GHRH without altering basal levels. Testosterone similarly potentiates GHRH responsiveness in hypogonadal states, contributing to sexually dimorphic patterns in GH secretion. Neurotransmitters also exert modulatory influences on GHRH neurons, with (NPY) and gamma-aminobutyric acid (GABA) providing stimulatory inputs, while offers inhibitory regulation. NPY, released from arcuate nucleus neurons during energy deficit, directly stimulates GHRH expression and release, thereby facilitating GH secretion as part of the adaptive response to . inputs to GHRH neurons in the promote GH release by inhibiting neurons, with activation of GABA receptors enhancing overall pituitary responsiveness. Dopaminergic neurons in the arcuate nucleus, particularly A12 tyrosine hydroxylase-expressing cells, inhibit GH secretion primarily by stimulating release via D1 receptors. Circadian rhythms and nutritional status further influence GHRH secretion, with sleep onset providing a key boost and fasting enhancing activity via NPY. The onset of sleep, particularly slow-wave sleep, triggers a prominent pulse of GH secretion primarily through increased GHRH stimulation during this period of relative somatostatin withdrawal. Prolonged fasting elevates NPY expression in the arcuate nucleus, which in turn stimulates GHRH neurons to restore GH pulsatility and support metabolic adaptation, despite initial short-term suppression of GH output.

Clinical Significance

Role in Disorders

Growth hormone–releasing hormone (GHRH) dysregulation contributes to several pathological conditions, primarily through its effects on (GH) secretion and downstream signaling. In cases of excess GHRH, such as ectopic production by tumors, it leads to GH hypersecretion and associated disorders. Conversely, deficiencies or reduced GHRH activity result in impaired GH release, manifesting as growth deficits or metabolic alterations. Additionally, GHRH expression in certain cancers promotes tumor progression via autocrine mechanisms. Ectopic GHRH secretion is a rare cause of , accounting for less than 1% of cases, and is most commonly associated with neuroendocrine tumors like bronchial carcinoids. These tumors produce excessive GHRH, stimulating pituitary somatotroph and GH oversecretion, which results in clinical features such as enlarged hands and feet, coarsened facial features, and increased risk of and . Bronchial carcinoids are the predominant source, with reported cases showing resolution of following tumor resection. Isolated GH deficiency (IGHD) can arise from inactivating mutations in the GHRHR gene, leading to in familial forms. These loss-of-function variants disrupt GHRH signaling, preventing GH release and resulting in severe growth retardation, with affected individuals exhibiting heights below the third and delayed . Such mutations are autosomal recessive and represent approximately 10% of autosomal recessive familial IGHD cases, often identified in consanguineous families. In aging and obesity, reduced GHRH tone contributes to diminished pulsatile GH secretion, exacerbating and features of . Age-related decline in hypothalamic GHRH release, coupled with increased inhibition, leads to lower GH levels, promoting muscle loss and frailty in . In , particularly visceral adiposity, blunted GHRH responsiveness further suppresses GH, fostering , , and abdominal fat accumulation characteristic of . GHRH is aberrantly expressed in various cancers, including and tumors, where it acts as an autocrine to enhance and survival. In cell lines, GHRH and its receptor promote tumor growth through pathways involving cyclic AMP and signaling. Similarly, in , local GHRH production stimulates androgen-independent proliferation and inhibits , contributing to disease progression. Knocking down GHRH expression in these cancers significantly reduces proliferation rates. Recent studies as of 2024 also suggest GHRH antagonists may inhibit progression in neurodegenerative diseases and by blocking aberrant signaling.

Diagnostic and Measurement Methods

The primary methods for measuring growth hormone-releasing hormone (GHRH) levels in plasma rely on immunoassays, including and , which detect immunoreactive GHRH with sensitivities typically around 5-10 pg/mL. These assays are essential for quantifying low circulating GHRH concentrations, which average approximately 10 pg/mL in healthy adults, but they face challenges due to the hormone's short (less than 10 minutes) and , often requiring extraction steps to minimize interference from binding proteins. is considered the gold standard for routine clinical use owing to its high specificity, non-radioactive nature, and commercial availability, while RIA offers comparable sensitivity but is less favored due to handling radioactive materials. Stimulation tests using GHRH analogs provide an indirect assessment of pituitary function by evaluating (GH) secretion in response to GHRH administration, particularly in suspected GH deficiency. A standard protocol involves intravenous bolus infusion of GHRH (1 μg/kg body weight), followed by serial blood sampling for GH levels at baseline and intervals (e.g., 15, 30, 45, 60, and 90 minutes post-infusion) to measure peak GH response, with normal peaks exceeding 5-10 ng/mL indicating intact pituitary responsiveness. This test helps differentiate hypothalamic from pituitary causes of GH deficiency but requires careful interpretation in conditions like , which can blunt responses. Genetic testing targets mutations in the GHRH receptor (GHRHR) through to identify causes of isolated GH deficiency (IGHD), particularly type IB or IV subtypes associated with biallelic loss-of-function variants. Over 20 GHRHR mutations have been documented, often leading to severe if untreated, and sequencing is recommended for familial or consanguineous cases with clinical suspicion of IGHD. These tests confirm and guide recombinant GH decisions. For detecting ectopic GHRH-secreting tumors, which rarely cause through excessive GHRH production, imaging modalities such as (PET) with gallium-68 DOTATATE or (SPECT) using indium-111 pentetreotide (Octreoscan) target somatostatin receptors overexpressed on neuroendocrine tumors. These functional scans, combined with anatomical like CT or MRI, localize tumors (e.g., in the lungs or ) when plasma GHRH exceeds 300 pg/mL, enabling surgical intervention.

Analogs and Therapeutics

Synthetic Analogs

Synthetic analogs of growth hormone–releasing hormone (GHRH) are engineered peptides designed to mimic the endogenous 44-amino-acid structure of human GHRH while incorporating modifications to enhance stability against enzymatic degradation and extend pharmacokinetic duration. Development of these analogs began in the 1980s following the isolation of native GHRH in 1982, with early efforts focusing on truncating the peptide to the biologically active N-terminal region and adding protective groups to improve resistance to proteases like dipeptidyl peptidase-4 (DPP-4). A seminal analog is , consisting of the first 29 of GHRH with a C-terminal (GHRH(1-29)-NH2), which was FDA-approved in 1997 for the treatment of idiopathic (GHD) in children, and also used for diagnostic evaluation of pituitary function. This truncation and amidation confer greater stability compared to the full-length , reducing susceptibility to carboxypeptidase cleavage while retaining full potency in stimulating release and promoting natural pulsatile GH secretion. of achieves of approximately 60-70%, with peak plasma concentrations reached within 5-20 minutes and a of about 10-20 minutes, though it remains more resistant to rapid plasma degradation than native GHRH. Further advancements include amino acid substitutions, such as replacing alanine at position 2 with D-alanine (D-Ala2), which sterically hinders enzymatic attack and reduces metabolic clearance, thereby enhancing biological activity and in vivo stability. Another key analog, tesamorelin, is a 44-amino-acid peptide featuring trans-3-hexenoyl acylation at the N-terminus, approved by the FDA in 2010; this fatty acid-like modification protects against DPP-4 degradation, extending the half-life to approximately 26-38 minutes and improving overall pharmacokinetics relative to unmodified GHRH, while promoting natural pulsatile GH release. For prolonged action, CJC-1295 incorporates a drug affinity complex (DAC) involving maleimidopropionyl linkage to lysine residues, enabling reversible binding to albumin and dramatically prolonging the half-life to 5.8-8.1 days after subcutaneous injection, with bioavailability supporting sustained growth hormone stimulation over days.

Therapeutic Applications

Growth hormone-releasing hormone (GHRH) analogs have been developed for therapeutic purposes, primarily to stimulate endogenous (GH) secretion and modulate the GH/insulin-like growth factor-1 (IGF-1) axis. One of the primary approved applications is in the management of HIV-associated , where the GHRH analog (Egrifta) is used to reduce excess visceral . Approved by the U.S. (FDA) on November 10, 2010, is indicated for HIV-infected adults with and abdominal fat accumulation. Clinical trials demonstrated that of 2 mg daily for 26 weeks significantly reduced visceral by approximately 15-18% compared to , with sustained effects upon continued use. Additionally, tesamorelin has shown stronger evidence for increasing muscle density in various muscle groups and modestly increasing lean body mass while reducing fat mass. Another GHRH analog, , was historically approved for the treatment of (GHD) in children, particularly to address by promoting linear growth through pulsatile GH release. FDA-approved in 1997 under the brand name Geref, was administered subcutaneously at doses of 0.2-0.3 mg daily, often at to mimic physiological GH secretion patterns. It facilitated catch-up growth in a majority of GHD pediatric patients, with increases of 2-4 cm/year observed in responsive children. However, production was discontinued in 2008 by the manufacturer for commercial reasons, leading to its removal from the U.S. market as an FDA-approved therapy, though generic forms may be available through pharmacies. of has persisted in anti-aging protocols to counteract age-related declines in GH levels, aiming to improve and vitality, despite limited evidence from controlled trials supporting long-term efficacy. Investigational applications of GHRH analogs extend to conditions involving catabolic states and tissue repair, leveraging their modulation of the GH/IGF-1 pathway. In cancer , plasmid-mediated GHRH has shown promise in preclinical and early clinical models by reversing muscle wasting and improving ; for instance, in dogs with cancer (including sarcomas), GHRH supplementation increased IGF-1 levels and extended by up to 84% in responders without promoting tumor growth. For , agonistic GHRH analogs such as JI-38 and MR-409 accelerate dermal repair by enhancing proliferation, synthesis, and cell via ERK and AKT signaling pathways, with in vivo studies demonstrating faster closure of excisional wounds in . In neurodegenerative diseases, both GHRH agonists (e.g., MR-409) and antagonists (e.g., MIA-690) exhibit neuroprotective effects; agonists reduce and in models of , while antagonists inhibit aberrant GH/IGF-1 signaling to mitigate neuronal damage, suggesting potential roles in Alzheimer's and Parkinson's management. As of 2025, ongoing research explores additional analogs like the MR-series for cancer and regenerative applications. The safety profile of GHRH analogs is generally favorable for short-term use, though monitoring is essential due to their impact on GH/IGF-1 levels. Tesamorelin has a favorable safety profile among injectable GHRH analogs. Common side effects include injection-site reactions such as redness, swelling, and pain, affecting up to 40% of users, along with transient elevations in GH and IGF-1 that resolve post-treatment. For , mild effects like , flushing, and occur infrequently. More serious risks involve potential glucose intolerance or reactions, with contraindicated in patients with active due to theoretical tumor growth promotion via IGF-1 stimulation. Long-term data from 52-week trials indicate no increased incidence of or cardiovascular events in appropriately selected patients.

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