Hormone & Biological Systems

ACTH (39 aa) is cleaved from POMC by prohormone convertase 1 in anterior pituitary corticotrophs. The N-terminal 24 amino acids contain the full biological activity — cosyntropin (ACTH 1-24, tetracosactide) retains complete adrenal-stimulating activity. ACTH binds MC2R on adrenal cortical cells, activating cAMP/PKA signalling to stimulate steroidogenesis (cortisol, aldosterone, adrenal androgens). ACTH secretion follows a circadian rhythm (peaking at 6-8 AM), is stimulated by CRH and vasopressin (synergistically), and is suppressed by cortisol (negative feedback). Corticotropin (therapeutic ACTH, repository formulation) is used for infantile spasms, multiple sclerosis flares, and other inflammatory conditions. Its mechanism involves both direct adrenal stimulation and extra-adrenal immunomodulatory effects through melanocortin receptors.

Aldosterone is synthesised in the adrenal zona glomerulosa, regulated primarily by the renin-angiotensin system (angiotensin II stimulates aldosterone release) and serum potassium levels. It acts on mineralocorticoid receptors in the renal collecting duct to promote sodium reabsorption and potassium excretion, increasing blood volume and pressure. While aldosterone itself is not a peptide, the renin-angiotensin-aldosterone system involves the peptide angiotensinogen and its processing into angiotensin I and II. ACTH provides a secondary stimulus to aldosterone — this is why corticotropin/cosyntropin administration causes both cortisol and (to a lesser extent) aldosterone responses.

α-MSH (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2, 13 aa) is derived from POMC processing (the same precursor that produces ACTH and beta-endorphin — ACTH contains the α-MSH sequence within its first 13 residues). The His-Phe-Arg-Trp tetrapeptide (positions 6-9) is the core pharmacophore for melanocortin receptor activation. Afamelanotide (Scenesse) is [Nle4, D-Phe7]-α-MSH — an α-MSH analogue with norleucine replacing methionine at position 4 (preventing oxidation) and D-phenylalanine replacing L-phenylalanine at position 7 (improving stability and MC1R selectivity). Setmelanotide uses a cyclic peptide design for MC4R selectivity. The melanocortin system represents one of the most extensively drugged peptide receptor families.

Amylin (islet amyloid polypeptide, IAPP, 37 aa) is co-secreted with insulin from pancreatic beta cells in a molar ratio of approximately 1:100. It acts on calcitonin/amylin receptors (complexes of the calcitonin receptor with receptor activity-modifying proteins, RAMPs) in the area postrema and other brain regions. Amylin's actions complement insulin: it slows gastric emptying, suppresses postprandial glucagon secretion, and promotes satiety. In type 2 diabetes, amylin secretion is reduced alongside insulin. Pramlintide (Symlin) is a synthetic amylin analogue with three proline substitutions (positions 25, 28, 29) that prevent the amyloid fibril formation (aggregation) that makes native human amylin unsuitable for pharmaceutical use. Pramlintide is injected before meals alongside insulin.

Angiogenesis is regulated by a balance of pro-angiogenic factors (VEGF, FGF-2, angiopoietins) and anti-angiogenic factors (endostatin, angiostatin, thrombospondin). In wound healing, hypoxia triggers HIF-1α (hypoxia-inducible factor), which upregulates VEGF expression, driving new vessel sprouting from existing vasculature. The process involves endothelial cell proliferation, migration, tube formation, and vessel maturation with pericyte recruitment. Tumour angiogenesis follows similar molecular pathways — neuroendocrine tumours are often highly vascular, which enables somatostatin receptor-targeted imaging and radionuclide therapy. Lutetium Lu-177 dotatate targets somatostatin receptor-positive NET vasculature and tumour cells.

The angiotensin family includes: angiotensinogen (precursor, 452 aa), angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu, 10 aa — inactive), angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe, 8 aa — the primary active peptide), angiotensin III (angiotensin II minus N-terminal Asp — stimulates aldosterone), and angiotensin(1-7) (produced by ACE2 — vasodilatory, anti-inflammatory, counterbalancing angiotensin II). Angiotensin II is one of the most potent vasoconstrictors in the body. ACE2 gained prominence during COVID-19 as the SARS-CoV-2 viral entry receptor. The complex angiotensin processing system illustrates how peptide pro-hormones can generate multiple active peptides with distinct or opposing biological effects.

The anterior pituitary (adenohypophysis) develops embryologically from Rathke's pouch (oral ectoderm) and produces six major hormones: GH (from somatotrophs, stimulated by GHRH, inhibited by somatostatin), LH and FSH (from gonadotrophs, stimulated by pulsatile GnRH), ACTH (from corticotrophs, stimulated by CRH), TSH (from thyrotrophs, stimulated by TRH), and prolactin (from lactotrophs, tonically inhibited by dopamine). Each hormone's secretion is regulated by hypothalamic releasing/inhibiting hormones and peripheral feedback. Peptide therapeutics target multiple anterior pituitary axes: GHRH analogues act on somatotrophs, somatostatin analogues inhibit multiple cell types, and GnRH compounds modulate gonadotrophs.

Apoptosis proceeds through two main pathways: the intrinsic (mitochondrial) pathway, triggered by intracellular stress signals that release cytochrome c from mitochondria, activating caspase-9 and then effector caspases (caspase-3, -6, -7); and the extrinsic (death receptor) pathway, triggered by extracellular death ligands (TNF, FasL) activating caspase-8. Proteasome inhibitors (bortezomib, carfilzomib) trigger apoptosis primarily through the intrinsic pathway: proteasome inhibition causes accumulation of pro-apoptotic proteins (BH3-only proteins, p53) and misfolded proteins that trigger ER stress → unfolded protein response → caspase activation. Cancer cells are particularly sensitive because their high protein synthesis rate generates more proteotoxic stress. Apoptosis is morphologically distinct from necrosis — cells shrink, fragment, and are cleanly phagocytosed.

ANP (28 aa) is stored in atrial granules and released in response to atrial wall stretch (increased blood volume/pressure). Its structure includes a 17 amino acid ring formed by a Cys7-Cys23 disulphide bond. ANP acts on NPR-A in kidneys (promoting sodium and water excretion), blood vessels (vasodilation), and adrenal glands (suppressing aldosterone). ANP clearance occurs through NPR-C receptor-mediated internalisation and enzymatic degradation by neutral endopeptidase (neprilysin). The combination neprilysin inhibitor/angiotensin receptor blocker sacubitril/valsartan raises endogenous ANP and BNP levels — demonstrating the therapeutic value of enhancing endogenous natriuretic peptide signalling.

Autophagy involves formation of double-membrane autophagosomes that engulf cytoplasmic cargo (damaged mitochondria — mitophagy; protein aggregates — aggrephagy; invading pathogens — xenophagy), then fuse with lysosomes for degradation. The process is regulated by over 30 autophagy-related (ATG) proteins and the mTOR (mechanistic target of rapamycin) signalling pathway — mTOR inhibition activates autophagy. Nutrient deprivation, caloric restriction, and exercise are natural autophagy inducers. Autophagy intersects with peptide therapeutics in several contexts: metabolic disease (autophagy defects contribute to insulin resistance), neurodegeneration (impaired clearance of protein aggregates), and cancer (autophagy can be either tumour-suppressive or tumour-promoting depending on context).

Despite its name, BNP's primary clinical significance relates to the heart. ProBNP (108 aa) is constitutively produced by ventricular cardiomyocytes and cleaved upon release to active BNP (32 aa, half-life ~20 minutes) and inactive NT-proBNP (76 aa, half-life ~120 minutes). Both are elevated in heart failure proportional to wall stress/severity. NT-proBNP is preferred for clinical testing due to its longer half-life and greater stability. Clinical cutoffs: NT-proBNP <300 pg/mL effectively rules out acute heart failure (high negative predictive value). Age-stratified cutoffs are used for diagnosis. BNP/NT-proBNP levels are used in heart failure diagnosis, prognosis, and treatment monitoring, and are sometimes included as secondary endpoints in cardiovascular outcomes trials for metabolic peptide drugs.

CNP exists in two forms: CNP-22 (22 aa) and CNP-53 (53 aa, N-terminally extended). CNP activates NPR-B (not NPR-A like ANP/BNP), generating cGMP that activates PKG-II in growth plate chondrocytes. PKG-II inhibits the RAF-MEK-ERK (MAPK) signalling pathway downstream of FGFR3 — the constitutively active receptor in achondroplasia. By counteracting FGFR3 signalling, CNP promotes chondrocyte proliferation and hypertrophy, enabling longitudinal bone growth. Vosoritide (BMN 111) is a 39 amino acid CNP analogue with improved stability (half-life ~30 minutes vs ~3 minutes for CNP-22). Daily subcutaneous vosoritide injection produced sustained increases in annualised growth velocity (mean gain ~1.5 cm/year over placebo) in children with achondroplasia.

Calcitonin is produced by thyroid C-cells (parafollicular cells), derived from the calcitonin gene (CALCA) that also encodes CGRP through alternative splicing. Calcitonin binds to the calcitonin receptor (CTR, a class B GPCR) on osteoclasts, rapidly inhibiting their bone-resorbing activity by disrupting the ruffled border and reducing acid secretion. This produces an acute decrease in serum calcium. Salmon calcitonin has approximately 40-50 times the potency of human calcitonin due to higher receptor binding affinity, and a longer half-life (approximately 43 minutes for SC salmon calcitonin vs 10 minutes for human calcitonin). Calcitonin also serves as a tumour marker for medullary thyroid carcinoma — this is why calcitonin monitoring is discussed in the context of GLP-1 RA safety.

CGRP (37 aa) is produced by alternative splicing of the CALCA gene — in C-cells, the gene produces calcitonin; in sensory neurons, it produces CGRP (alpha-CGRP). A related peptide (beta-CGRP) is encoded by the CALCB gene. CGRP is one of the most potent vasodilators known and is released from trigeminal sensory neurons during migraine attacks. It acts through a heterodimeric receptor complex of CLR (calcitonin receptor-like receptor) and RAMP1 (receptor activity-modifying protein 1). Anti-CGRP therapies (monoclonal antibodies erenumab, fremanezumab, galcanezumab; small molecule antagonists) have transformed migraine prevention. While these are primarily antibody-based rather than peptide drugs, understanding CGRP biology provides context for peptide signalling in neurovascular regulation.

CCK exists in multiple forms (CCK-8, CCK-33, CCK-58) produced by intestinal I-cells and neurons. It acts on CCK-A (alimentary, primarily peripheral) and CCK-B (brain) receptors. CCK-A activation stimulates gallbladder contraction, pancreatic enzyme secretion, and satiety. CCK-B activation modulates anxiety and pain pathways. Sincalide (the C-terminal octapeptide of CCK, CCK-8) is used diagnostically to stimulate gallbladder contraction for hepatobiliary imaging (cholescintigraphy) — injection of sincalide causes gallbladder ejection, and the ejection fraction is measured to diagnose biliary dyskinesia. CCK is also relevant to understanding the complex gut hormone network that regulates appetite alongside GLP-1, GIP, PYY, and ghrelin.

Corticotrophs constitute approximately 15% of anterior pituitary cells and are the source of ACTH (and other POMC-derived peptides including β-MSH, β-endorphin, and β-lipotropin). CRH receptors (CRH-R1) on corticotrophs are Gαs-coupled, activating cAMP/PKA signalling. Vasopressin (via V1b/V3 receptors) synergistically enhances CRH-stimulated ACTH release. Corticotroph adenomas that autonomously secrete ACTH cause Cushing's disease. These tumours often express somatostatin receptors (particularly SSTR5, and to a lesser extent SSTR2) — pasireotide's broader binding profile (including high SSTR5 affinity) explains its efficacy in Cushing's disease where SSTR2-selective analogues (octreotide, lanreotide) are less effective.

CRH (41 aa) is the primary activator of the HPA stress axis. It is produced in the paraventricular nucleus of the hypothalamus and released into the portal blood system. CRH binds CRH-R1 (Gαs-coupled) on pituitary corticotrophs, stimulating POMC transcription and ACTH secretion. CRH also binds CRH-R2 in the brain and periphery, involved in appetite suppression, anxiety, and immune modulation. CRH secretion follows a circadian rhythm (peaking in early morning) and is stimulated by physical and psychological stress. Cortisol provides negative feedback inhibition of CRH release. Ovine CRH is used diagnostically (CRH stimulation test) to differentiate pituitary from ectopic ACTH sources in Cushing's syndrome evaluation.

Cortisol is synthesised from cholesterol in the adrenal zona fasciculata under ACTH control. It acts through the glucocorticoid receptor (GR, a nuclear receptor), affecting gene transcription in virtually all tissues. Cortisol's physiological actions include: glucose mobilisation (gluconeogenesis, glycogenolysis), protein catabolism, fat redistribution, immunosuppression, and anti-inflammatory effects. In Cushing's disease, an ACTH-secreting pituitary adenoma drives excessive cortisol production. Cortisol measurement (24-hour urinary free cortisol, midnight salivary cortisol, low-dose dexamethasone suppression test) is used to diagnose Cushing's syndrome. The ACTH stimulation test (cosyntropin) assesses the adrenal gland's ability to produce cortisol in suspected adrenal insufficiency.

Dynorphins are derived from the prodynorphin precursor and include dynorphin A (17 aa), dynorphin B (13 aa), and alpha-neoendorphin. They preferentially bind kappa opioid receptors (KOR) — the same receptors targeted by difelikefalin. Dynorphins are found in the hypothalamus (appetite regulation), hippocampus (memory), striatum (reward/aversion), spinal cord (pain modulation), and peripheral tissues. KOR activation by dynorphins produces analgesia, dysphoria (opposite of euphoria), and anti-reward effects, and modulates itch sensation. Difelikefalin mimics dynorphin's peripheral KOR effects while being excluded from central KOR effects by its inability to cross the blood-brain barrier, thereby avoiding dysphoria.

Beta-endorphin (31 aa) is the most pharmacologically significant endorphin, derived from POMC processing in the anterior pituitary (corticotrophs) and hypothalamus. It is co-released with ACTH during stress (explaining the analgesic and euphoric effects of the stress response). Beta-endorphin is a full agonist at mu opioid receptors with 18-33 times the analgesic potency of morphine. Other endorphins (alpha, gamma, sigma) are shorter fragments with different activities. The endorphin system illustrates the concept of endogenous peptide analgesia — the body's own pain management system that therapeutic opioids and peptide drugs like difelikefalin interact with (albeit difelikefalin through the kappa rather than mu pathway).

Met-enkephalin (Tyr-Gly-Gly-Phe-Met) and leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) were the first endogenous opioid peptides discovered (1975). They are produced from the proenkephalin precursor and are widely distributed in the CNS (dorsal horn, periaqueductal grey, limbic system) and periphery (adrenal medulla, GI tract). Enkephalins preferentially bind delta opioid receptors but also activate mu receptors. They are rapidly degraded by enkephalinases (aminopeptidase N, neutral endopeptidase/neprilysin) with half-lives of seconds to minutes. Enkephalin degradation inhibitors (racecadotril) have been developed as antidiarrhoeals, demonstrating the therapeutic potential of modulating endogenous opioid peptide levels rather than using exogenous receptor agonists.

FSH is a heterodimeric glycoprotein that drives ovarian follicle growth and oestrogen production in women, and supports Sertoli cell function and spermatogenesis in men. FSH glycosylation patterns affect its biological activity and half-life. The GnRH pulse frequency-dependent regulation of FSH means that very slow GnRH pulses or continuous GnRH paradoxically preserve some FSH production even while LH is suppressed — this differential can be clinically relevant. In IVF protocols, GnRH antagonists (cetrorelix, ganirelix) are used to prevent the premature LH surge while exogenous FSH is administered to stimulate multiple follicle development (controlled ovarian hyperstimulation).

GIP (42 aa) is secreted from K-cells in the duodenum and proximal jejunum in response to glucose and fat ingestion. The GIP receptor (GIPR) is a class B GPCR expressed on pancreatic beta cells, adipocytes (where it promotes lipogenesis and adipocyte differentiation), osteoblasts (promoting bone formation), and brain neurons (where it may regulate appetite). In type 2 diabetes, GIP's insulinotropic effect is impaired despite normal or increased GIP secretion — this GIP resistance partly explains why DPP-4 inhibitors (which raise both GLP-1 and GIP) produce smaller insulin responses than GLP-1 RAs. Tirzepatide's GIP agonism may work through different mechanisms than native GIP, potentially resensitising the GIP signalling pathway. Research into GIP receptor biology is evolving rapidly.

Ghrelin is unique as the only known orexigenic (appetite-stimulating) circulating hormone. It requires octanoylation (attachment of an 8-carbon fatty acid to Ser3 by the enzyme GOAT — ghrelin O-acyltransferase) for GHS-R1a activation. Plasma ghrelin rises before meals and falls after eating, creating a hormonal hunger signal. GHS-R1a is expressed on pituitary somatotrophs (mediating GH release), hypothalamic neurons (mediating appetite stimulation), and vagal afferent neurons (gastrointestinal signalling). Research GHRPs (GHRP-2, GHRP-6, ipamorelin, hexarelin) are synthetic peptides that activate GHS-R1a but have distinct selectivity and side effect profiles. Ipamorelin is noted for more selective GH release with less effect on cortisol and prolactin than GHRP-2 or GHRP-6.

Glucagon (29 aa) is produced by pancreatic alpha cells from the same proglucagon gene that produces GLP-1 and GLP-2 in L-cells (tissue-specific processing by different prohormone convertases). The glucagon receptor (GCGR) is a class B GPCR coupled to Gαs, activating cAMP-mediated hepatic glycogenolysis and gluconeogenesis to raise blood glucose. Glucagon is the primary counter-regulatory hormone to insulin. Therapeutic applications include: emergency hypoglycaemia treatment (1mg IM/SC or nasal powder), and as a diagnostic agent for GI imaging (smooth muscle relaxation). In the context of next-generation metabolic drugs, glucagon receptor agonism (as part of triple agonists) is being explored for its potential to increase energy expenditure and promote hepatic fat oxidation.

GLP-1 is produced by post-translational processing of the proglucagon gene in intestinal L-cells. Proglucagon processing is tissue-specific: in gut L-cells, prohormone convertase 1/3 produces GLP-1 and GLP-2; in pancreatic alpha cells, prohormone convertase 2 produces glucagon. Active GLP-1 exists in two forms: GLP-1(7-36) amide (the major circulating form) and GLP-1(7-37). DPP-4 rapidly cleaves both to inactive GLP-1(9-36) amide or GLP-1(9-37). GLP-1 receptor activation on beta cells increases cAMP → PKA/Epac2 → potentiation of glucose-stimulated insulin secretion (glucose dependence is conferred by requiring glucose-mediated ATP production and KATP channel closure). Additional GLP-1R effects include beta cell proliferation and anti-apoptotic signalling (preclinical), glucagon suppression, delayed gastric emptying, and central appetite suppression.

GLP-2 (33 aa) is co-secreted with GLP-1 from L-cells and acts on the GLP-2 receptor (a class B GPCR) expressed on enteric neurons, subepithelial myofibroblasts, and intestinal endocrine cells (not directly on enterocytes). GLP-2R activation stimulates intestinal crypt cell proliferation, inhibits enterocyte apoptosis, increases villus height and crypt depth, enhances intestinal blood flow, and upregulates nutrient transporter expression. Native GLP-2 has a half-life of approximately 7 minutes (also a DPP-4 substrate). Teduglutide (Gattex/Revestive) has an Ala2→Gly substitution conferring DPP-4 resistance, extending its half-life to approximately 2-3 hours, enabling once-daily SC injection. Clinical trials showed significant reduction in parenteral nutrition requirements in short bowel syndrome patients.

Gonadotrophs constitute approximately 10% of anterior pituitary cells and produce both LH and FSH (the same cell can produce both). GnRH receptors on gonadotrophs are Gαq-coupled GPCRs — activation triggers the IP3/DAG/calcium pathway leading to gonadotropin synthesis and release. Crucially, the GnRH receptor has no cytoplasmic tail for beta-arrestin-mediated desensitisation — instead, desensitisation relies on receptor internalisation and reduced expression (downregulation). This unique receptor biology explains the mechanism of GnRH agonist therapy: continuous agonist exposure drives receptor internalisation and transcriptional downregulation over 1-2 weeks, ultimately suppressing gonadotropin output despite persistent agonist presence.

GH is released in pulsatile bursts from anterior pituitary somatotrophs, primarily during deep sleep (stages 3-4). Release is stimulated by GHRH and ghrelin, and inhibited by somatostatin and IGF-1 (negative feedback). GH acts directly on tissues (lipolysis, glucose counterregulation) and indirectly through hepatic IGF-1 production (growth, anabolic effects). GH deficiency in children causes growth failure; in adults it produces decreased bone density, increased visceral fat, reduced lean mass, and impaired quality of life. Recombinant somatropin (identical to 22 kDa human GH) requires daily injection; next-generation long-acting GH products include somapacitan (weekly, albumin-binding) and somatrogon (weekly, using CTP fusion technology).

The HPA axis cascade: hypothalamic CRH (41 aa peptide) → anterior pituitary corticotrophs release ACTH (39 aa, derived from POMC processing) → adrenal cortex produces cortisol. Cortisol feeds back at both hypothalamic and pituitary levels (negative feedback). The HPA axis follows a circadian rhythm (cortisol peaks in early morning) and is activated by physical and psychological stress. Corticotropin (therapeutic ACTH) directly stimulates adrenal cortisol production. Cosyntropin (ACTH 1-24, the biologically active N-terminal fragment) is used in the ACTH stimulation test — 250μg IV/IM with cortisol measured at 30 and 60 minutes; cortisol >18-20 μg/dL indicates normal adrenal function. Pasireotide suppresses ACTH from pituitary corticotroph adenomas in Cushing's disease.

The HPG axis requires pulsatile GnRH release (approximately every 60-90 minutes) from hypothalamic neurons to maintain normal gonadotropin secretion. Pulse frequency modulates the LH:FSH ratio — faster pulses favour LH, slower pulses favour FSH. This pulsatility dependence is therapeutically exploited: continuous GnRH (from depot/implant formulations) disrupts pulsatile signalling → desensitisation → downregulation → suppression of LH, FSH, and sex steroids. GnRH antagonists achieve the same endpoint (gonadotropin suppression) through direct receptor blockade, but without the initial flare phase. The HPG axis is the target for prostate cancer (testosterone suppression), endometriosis/fibroids (oestrogen suppression), precocious puberty (halting premature activation), and IVF (controlled ovarian stimulation).

The hypothalamus communicates with the anterior pituitary via the hypothalamic-hypophyseal portal system — a specialised vascular network that delivers releasing and inhibiting hormones directly to pituitary cells. This portal system allows low concentrations of hypothalamic peptides to reach the pituitary without systemic dilution. The posterior pituitary receives direct neural connections (axons) from hypothalamic neurons that produce oxytocin and vasopressin. Multiple peptide drug classes exploit this axis: GHRH analogues stimulate GH via the GH axis, GnRH compounds modulate LH/FSH via the reproductive axis, somatostatin analogues suppress GH/TSH/other hormones, and corticotropin acts on the adrenal axis. Pituitary tumours that disrupt this axis cause various endocrine disorders treated by peptide drugs.

The hypothalamus (approximately 4g, located below the thalamus) integrates neural and endocrine signals to regulate homeostasis. Key peptide-producing nuclei include: arcuate nucleus (GHRH, dopamine, kisspeptin, AgRP/NPY and POMC/CART neurons for appetite regulation), paraventricular nucleus (CRH, TRH, oxytocin, vasopressin), supraoptic nucleus (oxytocin, vasopressin), and preoptic area (GnRH). Hypothalamic peptides reach the anterior pituitary via the portal blood system and the posterior pituitary via direct axonal transport. The hypothalamus also contains GLP-1 receptors involved in appetite regulation — this is one mechanism by which GLP-1 RAs suppress appetite. Multiple peptide drug classes are derived from or target hypothalamic peptides.

Six IGFBPs (IGFBP-1 through IGFBP-6) regulate IGF bioavailability. IGFBP-3 is the most abundant, carrying approximately 75-90% of circulating IGF-1 in a ternary complex with ALS. This complex extends IGF-1's half-life from minutes to approximately 12-16 hours and regulates its delivery to tissues. IGFBP levels change with nutritional status, GH activity, and various diseases. IGFBP-3 is sometimes measured alongside IGF-1 in diagnostic evaluation of GH disorders. Proteolysis of IGFBPs by specific proteases (including pregnancy-associated plasma protein-A, PAPP-A) releases free IGF-1 for receptor binding, adding another layer of regulation to the IGF system.

Insulin consists of two polypeptide chains (A-chain, 21 aa; B-chain, 30 aa) connected by two interchain disulphide bonds plus one intrachain disulphide bond. It is synthesised as preproinsulin → proinsulin (with connecting C-peptide) → mature insulin + C-peptide (released in equimolar amounts). Insulin binds the insulin receptor (a tyrosine kinase receptor), activating PI3K/Akt and Ras/MAPK signalling pathways that promote glucose uptake (via GLUT4 translocation), glycogen synthesis, lipogenesis, and protein synthesis. Insulin biology is fundamental to understanding GLP-1 RA therapy — GLP-1 RAs potentiate glucose-stimulated insulin secretion (they amplify the beta cell response to glucose rather than directly causing insulin release, which is why hypoglycaemia risk is low).

IGF-1 is a 70 amino acid polypeptide with approximately 50% structural homology to insulin. It circulates primarily bound to IGFBP-3 and the acid-labile subunit (ALS), forming a ternary complex with a half-life of approximately 12-16 hours (compared to minutes for free IGF-1). Serum IGF-1 levels are age- and sex-dependent, peaking during puberty and declining thereafter. Clinically, IGF-1 measurement is used to: diagnose GH deficiency (low IGF-1) and acromegaly (elevated IGF-1), titrate GH replacement therapy to age-appropriate levels, and monitor response to somatostatin analogues in acromegaly (target: normalised IGF-1). IGF-1 acts through the IGF-1 receptor (a tyrosine kinase receptor distinct from GPCRs) to promote cell proliferation, differentiation, and survival.

LH is a heterodimeric glycoprotein (α subunit shared with FSH, TSH, hCG; unique β subunit). In women, the LH surge triggers ovulation — a rapid rise in LH causes follicular rupture and oocyte release approximately 36 hours after the surge begins. In men, LH stimulates Leydig cells to produce testosterone. The GnRH pulse frequency differentially regulates LH vs FSH synthesis: high-frequency pulses (every 60 minutes) favour LH-β transcription, while low-frequency pulses (every 2-4 hours) favour FSH-β. Continuous GnRH exposure suppresses LH within 1-2 weeks through gonadotroph GnRH receptor downregulation. GnRH antagonists produce immediate LH suppression. LH measurement is clinically important for confirming suppression during GnRH-based therapies.

The natriuretic peptide family acts through guanylyl cyclase receptors (not GPCRs): ANP and BNP activate NPR-A (natriuretic peptide receptor A), which generates cGMP; CNP activates NPR-B; and NPR-C (clearance receptor) removes all three peptides from circulation. cGMP signalling promotes vasodilation, natriuresis, diuresis, and suppression of RAAS and sympathetic nervous system. ANP (28 aa) is released from atrial cardiomyocytes in response to atrial stretch. BNP (32 aa) is released from ventricular cardiomyocytes in response to wall stress (its precursor proBNP is cleaved to active BNP and inactive NT-proBNP). CNP (22 aa or 53 aa) is primarily produced by vascular endothelium and cartilage growth plate cells — vosoritide exploits CNP's growth plate activity for achondroplasia treatment.

NPY (36 aa) acts on Y1-Y5 receptors (GPCRs). In the hypothalamic arcuate nucleus, NPY/AgRP neurons are the primary orexigenic (appetite-stimulating) pathway, counterbalanced by anorexigenic POMC/CART neurons. NPY is the most potent known appetite stimulant — intracerebroventricular injection causes intense feeding behaviour. Y1 and Y5 receptors mediate the appetite-stimulating effect. NPY is also involved in stress response, circadian rhythms, vasoconstriction, and anxiety. The melanocortin agonist setmelanotide works downstream of NPY/AgRP signalling — it activates MC4R on second-order neurons that NPY/AgRP neurons normally inhibit. Ghrelin stimulates NPY neurons, linking peripheral hunger signals to central appetite circuits.

Nociceptive signals begin when nociceptors (free nerve endings expressing transient receptor potential channels — TRPV1, TRPA1 — and voltage-gated sodium channels) detect harmful stimuli. Signals travel via Aδ fibres (fast, sharp pain) and C fibres (slow, burning pain) to the dorsal horn of the spinal cord, where they synapse with second-order neurons. Ascending pathways relay signals to the thalamus and cortex (pain perception) and brainstem (autonomic responses). Descending modulatory pathways from the periaqueductal grey and rostroventromedial medulla can inhibit or facilitate nociception using endogenous opioid peptides, serotonin, and norepinephrine. Difelikefalin acts on peripheral kappa opioid receptors on primary afferent neurons and immune cells in the skin, reducing pruritic and nociceptive signalling at its source.

Estradiol (E2) is the primary oestrogen in premenopausal women, produced by granulosa cells of ovarian follicles under FSH and LH stimulation via the aromatase enzyme (converting androgens to oestrogens). Oestrogen drives endometrial proliferation, breast development, and bone mineralisation. In endometriosis, ectopic endometrial tissue responds to oestrogen, growing and bleeding cyclically. GnRH-based therapies suppress oestrogen to reduce endometrial tissue activity. Full oestrogen suppression causes menopausal symptoms and bone loss, which is why oral GnRH antagonists (elagolix, relugolix combinations) allow dose-dependent partial suppression to balance symptom relief against side effects.

Oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, differing from vasopressin at positions 3 and 8) acts through the oxytocin receptor (OXTR), a Gαq-coupled GPCR. In the uterus, OXTR expression increases dramatically near term (up to 200-fold) due to oestrogen priming. Oxytocin stimulates rhythmic uterine contractions through IP3-mediated calcium release in myometrial smooth muscle. A positive feedback loop (Ferguson reflex) amplifies the signal during labour: cervical stretch → afferent nerve signals to hypothalamus → more oxytocin release → stronger contractions → more cervical stretch. In the mammary gland, oxytocin causes myoepithelial cell contraction for milk ejection. Synthetic oxytocin (Pitocin/Syntocinon) is administered IV for labour induction/augmentation and IM for postpartum haemorrhage prevention.

PTH (84 aa) is secreted from parathyroid glands in response to low serum calcium (detected by calcium-sensing receptor, CaSR). PTH acts on PTH1R (a class B GPCR) in bone and kidney: it mobilises calcium from bone (stimulating both osteoblasts and osteoclasts), increases renal calcium reabsorption, and stimulates renal 1,25-dihydroxyvitamin D production (which increases intestinal calcium absorption). The paradoxical bone-forming effect of intermittent PTH — the basis for teriparatide therapy — occurs because brief exposure preferentially activates osteoblastic (bone-building) pathways (via Wnt/β-catenin signalling) over osteoclastic (bone-resorbing) pathways. Continuous exposure, conversely, favours net bone resorption. Teriparatide (PTH 1-34) contains the biologically active N-terminal fragment.

PYY is released from intestinal L-cells (the same cells that produce GLP-1) in proportion to caloric intake. PYY(3-36) (the DPP-4 cleaved form, which is the major circulating species) acts on Y2 receptors in the hypothalamic arcuate nucleus to suppress appetite by inhibiting NPY/AgRP neurons and activating POMC neurons. PYY also slows gastric emptying and intestinal transit (the ileal brake mechanism). GLP-1 and PYY are co-secreted and have synergistic appetite-suppressing effects. GLP-1 RA treatment may increase endogenous PYY secretion, potentially contributing to appetite suppression beyond direct GLP-1R activation. PYY analogues have been explored as potential obesity therapeutics but have not reached approval.

The pituitary weighs approximately 0.5g and sits in the sella turcica of the sphenoid bone, connected to the hypothalamus by the pituitary stalk (infundibulum). The anterior lobe contains five major cell types: somatotrophs (GH, ~50%), lactotrophs (prolactin, ~20%), corticotrophs (ACTH, ~15%), gonadotrophs (LH/FSH, ~10%), and thyrotrophs (TSH, ~5%). Each cell type responds to specific hypothalamic releasing hormones. Pituitary adenomas (benign tumours) can oversecrete hormones — GH-producing adenomas cause acromegaly (treated with somatostatin analogues), ACTH-producing adenomas cause Cushing's disease (treated with pasireotide), and prolactinomas cause hyperprolactinaemia. Hypopituitarism (underfunction) may require replacement of multiple hormones including GH (somatropin).

The posterior pituitary (neurohypophysis) develops embryologically from neural tissue and is functionally an extension of the hypothalamus. Magnocellular neurons in the supraoptic and paraventricular nuclei of the hypothalamus synthesise oxytocin and vasopressin, which are transported along axons to the posterior pituitary for storage in nerve terminals. Release is triggered by neuronal action potentials. Oxytocin release is stimulated by cervical dilation during labour and infant suckling (positive feedback — the Ferguson reflex). Vasopressin release is stimulated by increased plasma osmolality (detected by osmoreceptors) and decreased blood volume (detected by baroreceptors). Both hormones are available as approved therapeutic peptides.

Progesterone is produced by the corpus luteum after ovulation and by the placenta during pregnancy. It transforms the endometrium from proliferative to secretory phase, preparing for implantation, and maintains pregnancy. Progesterone is regulated by LH and is suppressed by GnRH-based therapies. In IVF protocols, GnRH agonist triggers (administering a single GnRH agonist dose to trigger the LH surge for final oocyte maturation instead of hCG) reduce the risk of ovarian hyperstimulation syndrome. Progesterone supplementation is typically required after GnRH agonist trigger because the resulting LH surge is shorter than an hCG trigger.

Prolactin (199 aa) is unique among anterior pituitary hormones in being tonically inhibited by hypothalamic dopamine (via D2 receptors on lactotrophs) rather than stimulated by a releasing hormone. TRH and VIP stimulate prolactin release. Somatostatin analogues can suppress prolactin secretion through SSTR5. Some GnRH compounds can transiently alter prolactin levels. Hyperprolactinaemia can be caused by prolactinomas (treated primarily with dopamine agonists, not peptide drugs) or as a side effect of drugs that reduce dopamine activity. Prolactin monitoring may be relevant during treatment with peptide drugs that affect hypothalamic-pituitary function.

The RAAS cascade: renin (an aspartyl protease from renal juxtaglomerular cells) cleaves angiotensinogen (452 aa liver-derived protein) to angiotensin I (10 aa). ACE (angiotensin-converting enzyme, a metalloprotease on pulmonary endothelium) cleaves angiotensin I to angiotensin II (8 aa). Angiotensin II acts on AT1 receptors (vasoconstriction, aldosterone release, sodium retention, sympathetic activation) and AT2 receptors (vasodilation, anti-proliferative). ACE also degrades bradykinin, connecting the RAAS and kinin systems. ACE inhibitors and ARBs (angiotensin receptor blockers) are major cardiovascular drug classes. The RAAS demonstrates how a peptide signalling cascade can be therapeutically targeted at multiple points.

Somatostatin exists in two bioactive forms: SRIF-14 (14 aa) and SRIF-28 (28 aa, N-terminally extended). Both are cyclic peptides with a Cys3-Cys14 disulphide bridge. Somatostatin has remarkably broad inhibitory effects: suppresses GH, TSH, and prolactin from the pituitary; insulin, glucagon, and VIP from the pancreas; gastrin, secretin, CCK, and gastric acid from the GI tract; and inhibits angiogenesis and cell proliferation. This breadth is due to expression of five somatostatin receptor subtypes (SSTR1-5) across diverse tissues. The half-life of 1-3 minutes renders native somatostatin therapeutically impractical, driving development of metabolically stable analogues that selectively engage specific SSTR subtypes.

Somatotrophs constitute approximately 50% of anterior pituitary cells and are located mainly in the lateral wings of the gland. They contain secretory granules (300-600nm diameter) storing GH that is released in bursts upon GHRH receptor (GHRHR) activation. GHRHR is a Gαs-coupled GPCR that increases intracellular cAMP → PKA → GH gene transcription and granule exocytosis. Somatostatin receptors (mainly SSTR2 and SSTR5) are also expressed on somatotrophs — their activation (Gαi-coupled, decreasing cAMP) inhibits GH release. The ghrelin receptor (GHS-R1a) provides a third regulatory input. Somatotroph adenomas cause acromegaly; their high expression of SSTR2 explains the effectiveness of octreotide and lanreotide.

Substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2) belongs to the tachykinin family and acts primarily through the neurokinin-1 (NK1) receptor, a Gαq-coupled GPCR. It is released from small-diameter sensory neurons (C-fibres) in the dorsal horn and from peripheral nerve endings, transmitting nociceptive and pruritic signals. Substance P also mediates neurogenic inflammation by promoting vasodilation, plasma extravasation, and immune cell recruitment. Its role in pain pathways is relevant to understanding the endogenous context in which peripherally restricted analgesics like difelikefalin operate. NK1 receptor antagonists (aprepitant) are approved as antiemetics, demonstrating the therapeutic potential of targeting tachykinin signalling.

Testosterone is synthesised from cholesterol in testicular Leydig cells through a multistep enzymatic pathway (cholesterol → pregnenolone → DHEA → androstenedione → testosterone). LH controls the rate-limiting step (cholesterol side-chain cleavage). In the prostate, testosterone is converted to the more potent dihydrotestosterone (DHT) by 5α-reductase. Androgen deprivation therapy for prostate cancer aims to reduce testosterone to castrate levels (<50 ng/dL or 1.7 nmol/L). GnRH agonists achieve this in 2-4 weeks (after initial flare), while GnRH antagonists achieve it within 1-3 days. Testosterone monitoring is essential for confirming adequate suppression — failure to achieve castrate levels indicates treatment inadequacy.

TSH is a heterodimeric glycoprotein (shared α subunit with LH/FSH/hCG; unique β subunit) that stimulates the thyroid gland to produce T3 and T4 via the TSH receptor (a GPCR). TSH is relevant to peptide therapeutics because: somatostatin analogues (particularly octreotide and pasireotide) suppress TSH secretion and can be used for TSH-secreting pituitary adenomas; thyroid function monitoring is recommended during some peptide drug therapies; and the CALCA gene (encoding both calcitonin and CGRP) is regulated by thyroid status. Additionally, recombinant TSH (thyrotropin alfa) is itself a therapeutic peptide used in thyroid cancer management.

TRH (pGlu-His-Pro-NH2, where pGlu is pyroglutamate) is the smallest hypothalamic releasing hormone at just 3 amino acids. It is produced in the paraventricular nucleus and acts on TRH-R1 (Gαq-coupled) on pituitary thyrotrophs to stimulate TSH release, and on lactotrophs to stimulate prolactin release. TRH demonstrates that even very short peptides can have potent and specific biological effects — a concept relevant to peptide drug design. TRH also illustrates protective modifications: the N-terminal pyroglutamate and C-terminal amidation protect this tiny peptide from aminopeptidases and carboxypeptidases respectively. The TRH stimulation test (formerly used to evaluate thyroid function) has been largely replaced by sensitive TSH assays.

VIP (28 aa) belongs to the secretin/glucagon peptide superfamily. It acts through VPAC1 and VPAC2 receptors (class B GPCRs, Gαs-coupled) expressed widely in the GI tract, respiratory system, immune cells, and nervous system. VIP stimulates intestinal water and electrolyte secretion, relaxes smooth muscle (vasodilation, bronchodilation), inhibits gastric acid secretion, and has immunomodulatory effects (promoting Th2 over Th1 responses). VIPomas (VIP-secreting neuroendocrine tumours) cause watery diarrhoea syndrome (Verner-Morrison syndrome), which is effectively treated with somatostatin analogues (octreotide, lanreotide) that suppress VIP secretion and its intestinal effects.

Vasopressin (ADH, argipressin) is a 9 amino acid cyclic peptide (Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2, with a 1-6 disulphide bridge) with three receptor subtypes: V1a (vascular smooth muscle, liver — vasoconstriction, glycogenolysis), V1b/V3 (anterior pituitary — ACTH co-regulation), and V2 (renal collecting duct — water reabsorption via aquaporin-2 insertion). Synthetic vasopressin is used in vasodilatory shock (V1a-mediated vasoconstriction) and cardiac arrest. Desmopressin (1-deamino-8-D-arginine vasopressin) has enhanced V2 selectivity (antidiuretic:pressor ratio approximately 3000:1 vs 1:1 for vasopressin) and longer half-life (~3 hours vs 6-20 minutes), making it suitable for diabetes insipidus, nocturnal enuresis, and von Willebrand disease (V2-mediated release of stored vWF).