VIP (Vasoactive Intestinal Peptide): Research Guide & Chemical Profile

Vasoactive Intestinal Peptide (VIP) is a linear 28-amino-acid neuropeptide with the 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. The molecular formula is C147H237N43O43S with a molecular weight of approximately 3326.26 Da. The C-terminus is amidated, a post-translational modification essential for full biological activity. VIP was first isolated from porcine duodenum by Said and Mutt in 1970 and subsequently identified in neural tissue throughout the central and peripheral nervous systems.

VIP belongs to the secretin/glucagon peptide superfamily, which includes pituitary adenylate cyclase-activating polypeptide (PACAP), secretin, glucagon, growth hormone-releasing hormone (GHRH), and glucose-dependent insulinotropic polypeptide (GIP). Within this family, VIP shares 68% sequence homology with PACAP-27 and significant structural similarity in the N-terminal alpha-helical domain critical for receptor activation. The high degree of conservation across vertebrate species (human, rat, and porcine VIP differ by only 1-2 residues) underscores its fundamental biological importance.

Structurally, VIP adopts an amphipathic alpha-helical conformation in membrane-mimetic environments (detergent micelles, lipid vesicles) spanning approximately residues 7-28, while the N-terminal hexapeptide region is more flexible. This helical structure is critical for receptor binding, with the C-terminal helix (residues 14-28) primarily responsible for high-affinity receptor recognition and the N-terminal region (residues 1-7) required for receptor activation and signaling. The amphipathic nature, with hydrophobic residues on one helical face and hydrophilic residues on the opposite face, facilitates interaction with lipid membranes and the transmembrane receptor domains.

• Sequence: HSDAVFTDNYTRLRKQMAVKKYLNSILN-NH2 (28 amino acids)
• Molecular weight: ~3326.26 Da
• Molecular formula: C147H237N43O43S
• Gene: VIP (human chromosome 6q25)
• Receptor family: Class B GPCRs (VPAC1, VPAC2)
• Post-translational modification: C-terminal amidation (essential)
• Structure: Amphipathic alpha-helix (residues 7-28)
• Superfamily: Secretin/glucagon peptide family
• Discovery: Said & Mutt, 1970 (porcine duodenum)

VIP signals through two receptor subtypes: VPAC1 (also designated VIPR1) and VPAC2 (VIPR2), both members of the class B (secretin family) G-protein-coupled receptor superfamily. These receptors share approximately 50% amino acid sequence identity and both couple primarily to Gs proteins, activating adenylyl cyclase and elevating intracellular cAMP. VIP binds both receptors with similar high affinity (Kd approximately 0.5-2 nM), distinguishing it from PACAP, which shows preferential affinity for VPAC1 and the PACAP-specific PAC1 receptor.

VPAC1 is widely expressed in the cerebral cortex, hippocampus, lung, liver, small intestine, and T lymphocytes. It plays dominant roles in smooth muscle relaxation, epithelial secretion, and T cell regulation. VPAC2 is the predominant subtype in the suprachiasmatic nucleus (SCN), pancreatic beta cells, smooth muscle, and mast cells, with major roles in circadian rhythm synchronization, glucose homeostasis, and mast cell degranulation inhibition. The differential tissue distribution of VPAC1 and VPAC2 underlies the diverse physiological functions of VIP across organ systems.

Selective pharmacological tools have been developed to dissect VPAC1- versus VPAC2-mediated effects. [Ala11,22,28]VIP and [Lys15,Arg16,Leu27]VIP(1-7)/GRF(8-27) serve as VPAC1-selective agonists, while Ro 25-1553 and BAY 55-9837 function as VPAC2-selective agonists. PG 97-269 is a VPAC1-selective antagonist, while PG 99-465 shows VPAC2 selectivity. These tools, along with VPAC1-knockout and VPAC2-knockout mice, have been instrumental in delineating receptor-specific contributions to VIP biology.

The primary signaling cascade downstream of VPAC1/VPAC2 involves Gs-mediated adenylyl cyclase activation, generating cAMP from ATP. Elevated cAMP activates protein kinase A (PKA), which phosphorylates CREB (cAMP response element-binding protein) at Ser133, enabling its interaction with the transcriptional coactivator CBP/p300 and driving CRE-dependent gene transcription. This cAMP/PKA/CREB axis is the canonical VIP signaling pathway and mediates the majority of VIP effects on gene expression, cell survival, and differentiation.

Beyond the canonical pathway, VPAC receptors engage additional signaling mechanisms. Coupling to Gq activates phospholipase C (PLC), generating inositol trisphosphate (IP3) and diacylglycerol (DAG), leading to intracellular calcium mobilization and protein kinase C (PKC) activation. Exchange proteins activated by cAMP (Epac1/2) provide PKA-independent cAMP signaling, particularly important for cell adhesion, secretion, and cytoskeletal dynamics. PI3K/Akt pathway activation downstream of VIP receptor signaling contributes to anti-apoptotic and pro-survival effects in neurons and immune cells.

VIP signaling is subject to multiple levels of regulation. Receptor desensitization occurs through GRK (G-protein-coupled receptor kinase) phosphorylation followed by beta-arrestin recruitment, leading to receptor internalization via clathrin-coated pits. Internalized receptors are either recycled to the plasma membrane or targeted for lysosomal degradation. Phosphodiesterases (PDEs), particularly PDE4 and PDE7, hydrolyze cAMP to limit signal duration. The peptide itself is rapidly degraded by dipeptidyl peptidase IV (DPP-IV) and neutral endopeptidase (NEP/neprilysin), conferring a short in vivo half-life of approximately 1-2 minutes.

VIP is one of the most potent endogenous anti-inflammatory peptides identified to date. Research has demonstrated broad immunomodulatory effects across both innate and adaptive immune systems. In macrophages and microglia, VIP inhibits the production of pro-inflammatory mediators including TNF-alpha, IL-6, IL-12, and nitric oxide (NO) by suppressing NF-kappaB nuclear translocation and AP-1 activation. Simultaneously, VIP promotes production of the anti-inflammatory cytokine IL-10 and upregulates the regulatory T cell transcription factor Foxp3.

In adaptive immunity, VIP promotes a shift from Th1 to Th2 immune responses by suppressing IFN-gamma and IL-2 production while enhancing IL-4 and IL-5 expression in CD4+ T cells. VIP also promotes the generation and expansion of regulatory T cells (Tregs) and tolerogenic dendritic cells, contributing to immune tolerance. These effects are primarily mediated through VPAC1 on T lymphocytes and VPAC2 on macrophages, though there is significant cross-talk between receptor subtypes.

Preclinical studies in animal models of autoimmune and inflammatory diseases have demonstrated robust anti-inflammatory effects of VIP. In murine models of rheumatoid arthritis (collagen-induced arthritis), experimental autoimmune encephalomyelitis (EAE, a multiple sclerosis model), inflammatory bowel disease (TNBS-colitis), and septic shock (LPS or cecal ligation and puncture), VIP treatment reduced disease severity, inflammatory infiltration, and tissue damage. These effects were reproduced with both native VIP and metabolically stable analogs, supporting therapeutic investigation.

VIP plays an essential role in circadian timekeeping within the suprachiasmatic nucleus (SCN), the master pacemaker of the mammalian circadian system. VIPergic neurons constitute approximately 10-25% of SCN neurons and are concentrated in the ventrolateral (core) subdivision of the SCN. These neurons receive direct retinal input via the retinohypothalamic tract and serve as critical relay neurons that distribute photic timing information throughout the SCN network.

VIP acts through VPAC2 receptors on SCN neurons to synchronize the circadian oscillations of individual clock cells. Without VIP or VPAC2 signaling, individual SCN neurons continue to oscillate but lose synchrony with each other, resulting in arrhythmic behavioral output. Studies using VIP-knockout and VPAC2-knockout mice demonstrate loss of coherent circadian rhythms in locomotor activity, body temperature, and hormone secretion, establishing VIP-VPAC2 signaling as essential for SCN network synchronization.

The mechanism of circadian synchronization involves VIP-induced cAMP elevation in target SCN neurons, which activates PKA and subsequently CREB, driving transcription of the core clock gene Per1 (Period 1) and Per2. This VIP-cAMP-CREB-Per pathway provides the signaling link between the VIPergic pacemaker network and the molecular clock machinery. VIP also modulates GABA signaling within the SCN and influences the expression of clock-controlled output genes that drive rhythmic physiological processes including melatonin synthesis in the pineal gland and cortisol secretion from the adrenals.

VIP demonstrates potent neuroprotective properties across multiple neurotoxic paradigms. In vitro studies using primary neuronal cultures have shown that VIP protects against glutamate excitotoxicity, oxidative stress (hydrogen peroxide, 6-hydroxydopamine), beta-amyloid peptide toxicity, and pro-inflammatory cytokine-induced neurotoxicity. The neuroprotective mechanisms involve multiple pathways: activation of the cAMP/PKA/CREB survival cascade, upregulation of Bcl-2 anti-apoptotic proteins, suppression of caspase-3 activation, and induction of neurotrophic factor expression (BDNF, ADNP/activity-dependent neuroprotective protein).

VIP serves as a neurotrophic factor during brain development. It promotes neuronal differentiation, axonal growth, and synaptogenesis during critical periods of CNS maturation. VIP stimulates the release of glial-derived neurotrophic factors including ADNP and activity-dependent neurotrophic factor (ADNF) from astrocytes, amplifying its neuroprotective effects through indirect paracrine mechanisms. Blockade of VIP signaling during embryonic development in rodent models leads to microcephaly and impaired learning behavior in offspring.

In preclinical models of neurodegeneration, VIP and stable analogs have shown protective effects in Parkinson disease models (MPTP-induced and 6-OHDA-lesioned rodents), Alzheimer disease models (amyloid-beta-infused and transgenic mice), and cerebral ischemia models (MCAO). The anti-neuroinflammatory component of VIP action is particularly relevant, as chronic neuroinflammation driven by activated microglia is recognized as a key contributor to neurodegenerative disease progression.

VIP was originally discovered as a gastrointestinal peptide and remains one of the most abundant neuropeptides in the enteric nervous system. VIPergic neurons in the submucosal and myenteric plexuses regulate intestinal secretion, blood flow, and smooth muscle tone. VIP stimulates electrogenic chloride and water secretion from intestinal epithelial cells through VPAC1-mediated cAMP elevation and subsequent activation of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel and basolateral potassium channels.

VIP is the primary non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitter in the gastrointestinal tract, mediating smooth muscle relaxation in the lower esophageal sphincter, stomach (receptive relaxation), and intestine (descending relaxation during peristalsis). The relaxant effect is mediated through cAMP-dependent inhibition of myosin light chain kinase and activation of large-conductance calcium-activated potassium (BKCa) channels, leading to smooth muscle hyperpolarization and reduced contractility.

VIPoma, a rare neuroendocrine tumor that secretes excessive VIP, produces a clinical syndrome (Verner-Morrison syndrome or WDHA syndrome) characterized by profuse watery diarrhea, hypokalemia, and achlorhydria. This clinical condition provides direct evidence for the secretory and motility effects of VIP in humans. Research on VIPoma has informed the understanding of VIP receptor signaling in intestinal epithelium and has led to the development of VIP receptor-targeted diagnostic and therapeutic approaches for neuroendocrine tumors.

VIPergic nerve fibers are abundant in the lungs, where they innervate airway smooth muscle, bronchial glands, and pulmonary vasculature. VIP is the most potent endogenous bronchodilator identified, relaxing airway smooth muscle through cAMP-mediated mechanisms. It also inhibits mucus hypersecretion from submucosal glands, modulates pulmonary vascular tone (producing vasodilation), and suppresses airway inflammation through effects on alveolar macrophages, mast cells, and eosinophils.

Research has identified VIP system dysregulation in several pulmonary conditions. Reduced VIP immunoreactivity in lung tissue and decreased VIP plasma levels have been reported in asthma research, with the degree of VIP reduction correlating with airway hyperresponsiveness severity. In pulmonary arterial hypertension (PAH) research, VIP deficiency in pulmonary vascular smooth muscle has been documented, and VIP-knockout mice develop spontaneous PAH features including right ventricular hypertrophy and pulmonary vascular remodeling.

Preclinical studies have explored inhaled VIP as a potential approach for pulmonary conditions. Aerosolized VIP delivery provides direct access to airway target cells while minimizing systemic exposure and the rapid degradation that limits intravenous VIP efficacy. Stable VIP analogs resistant to DPP-IV and NEP degradation have been designed for improved pulmonary bioavailability. Early-phase clinical investigations have evaluated inhaled VIP in sarcoidosis and PAH, with preliminary findings suggesting improvements in pulmonary hemodynamics and exercise capacity.

Native VIP has a plasma half-life of approximately 1-2 minutes due to rapid enzymatic degradation by dipeptidyl peptidase IV (DPP-IV, cleaving the His1-Ser2 bond), neutral endopeptidase (NEP/neprilysin, cleaving at multiple internal sites), and trypsin-like serine proteases. This extremely short half-life severely limits the utility of native VIP for in vivo research and has driven extensive efforts to develop stabilized analogs.

Key stabilized VIP analogs include [Stearyl-Nle17]VIP, which incorporates a fatty acid for albumin binding and plasma half-life extension; [Ac-His1,D-Phe2,Lys15,Arg16,Leu27]VIP(1-7)/GRF(8-27) (a chimeric peptide with enhanced VPAC1 selectivity and stability); and Ro 25-1553, a cyclic VIP analog with VPAC2 selectivity and markedly improved metabolic stability. PEGylation of VIP at Lys15 or Lys20 provides enhanced pharmacokinetic profiles while maintaining receptor binding affinity, extending plasma half-life from minutes to several hours.

For research applications, VIP is typically supplied as a trifluoroacetate (TFA) or acetate salt lyophilized powder. Reconstitution in sterile water or mildly acidic buffer (10 mM acetic acid, pH 4.5-5.0) is recommended, as VIP aggregation and adsorption to surfaces increase at neutral pH. Stock solutions should be aliquoted into siliconized or low-binding tubes and stored at -80°C. The methionine residue at position 17 is susceptible to oxidation; handling under nitrogen atmosphere and inclusion of antioxidants (0.1 mM DTT or methionine) in storage buffers is recommended for quantitative studies.

The following publications represent key research on VIP across its major investigational areas. Researchers are directed to these primary sources for experimental protocols and detailed data.

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