VIP (Vasoactive Intestinal Peptide) Research Overview

Background and Discovery

Vasoactive Intestinal Peptide (VIP) was first isolated from porcine small intestine in 1970 by Said and Mutt, initially characterised by its potent vasodilatory and intestinal smooth muscle relaxing effects — hence "vasoactive intestinal." Subsequent decades of research revealed a biological profile far more complex and consequential than its name suggests: VIP is a broadly expressed neuropeptide acting as a neurotransmitter, neuromodulator, and immunomodulator with documented effects on immune regulation, circadian timing, pulmonary vasculature, neuroendocrine signalling, and cell survival across multiple organ systems.

VIP belongs to the glucagon/secretin superfamily of peptides and shares approximately 70% sequence homology with PACAP (pituitary adenylate cyclase-activating polypeptide) — a relationship reflected in their shared receptor use and overlapping biological effects. VIP is encoded by the VIP gene on chromosome 6q25 and is processed from a 170-amino acid prepro-VIP precursor that also encodes PHM-27 (peptide histidine methionine), a related peptide with overlapping actions.

• Sequence: His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Thr-Arg-Leu-Arg-Lys-Gln-Met-Ala-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Ile-Leu-Asn-NH₂

• Molecular Weight: 3,326 Da (28 amino acids)

• Primary Receptors: VPAC1 (VIPR1); VPAC2 (VIPR2) — both Gs/cAMP-coupled

• Superfamily: Glucagon/secretin/PACAP peptide superfamily

• Plasma Half-life: ~1–2 minutes (rapidly degraded by endopeptidases)

• Primary Expression: Enteric nervous system, lung, hypothalamus (SCN), immune cells

Receptor System: VPAC1 and VPAC2

• Receptor: VPAC1 (VIPR1) — Distribution: Lung, liver, intestine, T-cells, brain (widely) — Primary Research Function: Anti-inflammatory; immunomodulation; smooth muscle relaxation — Signalling: Gs → cAMP → PKA; also PI3K in some cells

• Receptor: VPAC2 (VIPR2) — Distribution: SCN (circadian), pancreas, smooth muscle, brain — Primary Research Function: Circadian rhythm synchronisation; insulin secretion; vasodilation — Signalling: Gs → cAMP → PKA; MAPK in smooth muscle

Both VPAC1 and VPAC2 are Gs-coupled GPCRs that elevate intracellular cAMP and activate PKA — the primary signalling mechanism for VIP's anti-inflammatory and smooth muscle relaxing effects. cAMP elevation in immune cells suppresses NF-κB activation and pro-inflammatory cytokine production, while activating CREB-mediated transcription of anti-inflammatory genes. In smooth muscle cells, PKA phosphorylates myosin light chain kinase (MLCK), reducing contractility and producing vasodilation and bronchodilation.

Research Domains

Anti-Inflammatory

Potent NF-κB suppressor in macrophages, DCs, and T-cells. Reduces TNF-α, IL-6, IL-12. Promotes IL-10 and TGF-β anti-inflammatory cytokines. One of the most potent endogenous immunosuppressants identified.

Autoimmune Disease

Attenuates experimental arthritis, IBD, multiple sclerosis, and type 1 diabetes models. Induces Treg expansion. Promotes tolerogenic dendritic cell phenotype. Phase II trials in RA and septic shock completed.

Pulmonary Hypertension

Pulmonary arterial vasodilator via smooth muscle VPAC1/2. Phase II trial (inhaled VIP): significant reduction in pulmonary vascular resistance. VIP deficiency documented in PAH patient lung tissue.

Circadian Regulation

VPAC2 is the primary synchronisation receptor in the SCN (suprachiasmatic nucleus) — the master circadian clock. VIP/VPAC2 signalling coordinates inter-neuronal coupling within the SCN. Critical for circadian rhythm maintenance.

Neuroprotection

Reduces neuroinflammation. Protects against glutamate excitotoxicity. Promotes neurotrophic factor expression (BDNF, NGF). Studied in Parkinson's, Alzheimer's, and stroke models.

Acute Lung Injury

Potent anti-inflammatory in ALI/ARDS models. Reduces mechanical ventilation-induced injury. Phase I/II data for inhaled VIP in COVID-19-associated ARDS showed safety and biomarker improvement signals.

Anti-Inflammatory Immunomodulation: The Defining Research Property

VIP's most extensively documented and potentially therapeutically significant property is its profound anti-inflammatory immunomodulatory activity. In macrophages and monocytes, VIP activates VPAC1-mediated cAMP/PKA signalling that phosphorylates and inactivates IKKβ — preventing IκB degradation and blocking NF-κB nuclear translocation. This shuts down transcription of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-12) and chemokines (CXCL8, CCL2) at the gene level.

Simultaneously, VIP promotes alternative macrophage activation (M2 polarisation) through cAMP-mediated upregulation of IL-10 and TGF-β — anti-inflammatory and immunosuppressive cytokines that attenuate adaptive immune activation and promote tissue repair. In T-cell compartments, VIP suppresses Th1 and Th17 differentiation (key drivers of autoimmune inflammation) while selectively expanding regulatory T-cell (Treg) populations — a particularly valuable profile for autoimmune disease research where restoring self-tolerance is the therapeutic goal.

Pulmonary Arterial Hypertension Research

VIP's vasodilatory and anti-proliferative effects in pulmonary vascular smooth muscle have made it a compelling research target for pulmonary arterial hypertension (PAH) — a progressive fatal disease characterised by pulmonary vasoconstriction, vascular remodelling, and right heart failure. Notably, VIP tissue levels are significantly reduced in the pulmonary vasculature of PAH patients compared to controls — suggesting VIP deficiency may contribute to PAH pathogenesis rather than simply being a therapeutic opportunity.

A Phase II randomised controlled trial by Petkov et al. evaluated inhaled VIP (200 µg 4× daily) in PAH patients over 12 weeks. The treated group showed significant reductions in pulmonary vascular resistance (PVR) and improvements in 6-minute walk distance compared to placebo — clinically meaningful endpoints in PAH research. The inhaled delivery route was selected to maximise local pulmonary concentration while minimising systemic vasodilation. This trial remains the most rigorous human evidence for VIP's clinical potential.

Circadian Biology: VIP as the SCN Synchroniser

The suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian clock in mammals, coordinating circadian rhythms throughout the body. Within the SCN, individual neurons oscillate with approximately 24-hour periodicity — but these individual cellular clocks must be synchronised with each other (and with the environmental light-dark cycle) to produce a coherent, unified circadian signal. VIP, acting through VPAC2 receptors on SCN neurons, is the primary inter-cellular synchronisation signal within the SCN network. VPAC2 knockout mice display severely fragmented and arrhythmic behaviour patterns, confirming VIP/VPAC2 signalling as essential for normal circadian function — not merely modulatory.

SARS-CoV-2 and Acute Lung Injury Research

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COVID-19–associated respiratory failure involves catastrophic lung inflammation driven by cytokine storm, neutrophil infiltration, and NF-κB–driven pneumocyte injury. VIP's potent pulmonary anti-inflammatory profile generated research interest in its potential application to COVID-19 ARDS in 2020–2021. A Phase I/II trial of inhaled VIP in severe COVID-19 (ARDS) demonstrated acceptable safety and signals of reduced inflammatory markers (IL-6, ferritin) and improved oxygenation in treated patients. While preliminary, these data are consistent with VIP's established mechanisms and may inform inhaled anti-inflammatory peptide strategies for future respiratory inflammatory emergencies.

Autoimmune Disease Research

VIP's dual capacity to suppress effector T-cell activation while expanding Tregs makes it mechanistically ideal for autoimmune disease research. In collagen-induced arthritis (CIA) — the standard rheumatoid arthritis model — systemic VIP significantly reduces joint inflammation scores, synovial cytokine levels, and structural joint damage compared to vehicle controls. In experimental autoimmune encephalomyelitis (EAE, the MS model), VIP reduces disease severity, demyelination, and CNS infiltrating T-cell populations. In NOD mice (type 1 diabetes model), VIP delays insulitis and diabetes onset. Across all three major autoimmune models, the mechanism involves VPAC1-mediated Treg induction and Th1/Th17 suppression.

Research Use Only — Disclaimer

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This document is prepared for laboratory and research reference purposes only. VIP is not approved by the FDA for any therapeutic indication. Phase II clinical trial data is investigational. This content does not constitute medical advice, diagnosis, or treatment recommendation. Researchers must comply with all applicable institutional and jurisdictional regulations.

References

1. Said SI, Mutt V. "Polypeptide with broad biological activity: isolation from small intestine." *Science*. 1970;169(3951):1217–1218.

1. Delgado M, et al. "The neuropeptide VIP is a potent inhibitor of the adaptive immune response." *Trends Pharmacol Sci*. 2004;25(3):136–142.

1. Petkov V, et al. "Vasoactive intestinal peptide as a new drug for treatment of primary pulmonary hypertension." *J Clin Invest*. 2003;111(9):1339–1346.

1. Harmar AJ, et al. "The VPAC2 receptor is essential for circadian function in the mouse suprachiasmatic nuclei." *Cell*. 2002;109(4):497–508.

1. Gomariz RP, et al. "VIP and PACAP as therapeutic targets in inflammatory diseases." *Curr Pharm Des*. 2001;7(2):89–111.

1. Daynes RA, Araneo BA. "Vasoactive intestinal peptide as an endogenous immunomodulator." *Prog Neuroendocrinoimmunol*. 1989;2:29–38.