VIP

Vasoactive Intestinal Peptide (VIP) is a 28-amino acid neuropeptide belonging to the glucagon/secretin/PACAP superfamily — a structural family of peptides sharing a common helical scaffold and related receptor systems. VIP was first isolated in 1970 from porcine intestinal tissue by Said and Mutt, who characterised it by its potent vasodilatory action on intestinal smooth muscle vasculature — hence the name. However, the "vasoactive intestinal" nomenclature dramatically understates VIP's biological scope. Subsequent decades of research have established VIP as one of the most biologically multifunctional neuropeptides in the vertebrate nervous system, with critical roles in immune modulation, pulmonary physiology, circadian timekeeping, GI motility, neuroprotection, and exocrine secretion — a breadth rivalled by few other signalling molecules.

VIP is produced and secreted by neurons throughout the central nervous system, peripheral autonomic nervous system (particularly parasympathetic neurons), and enteric nervous system (the intrinsic neural network of the GI tract). In the CNS, VIP is found in the cerebral cortex, hippocampus, hypothalamus, limbic system, and many other regions. In the periphery, VIPergic neurons innervate smooth muscle, glands, and immune organs throughout the body. VIP is also produced by non-neuronal cells including immune cells (T cells, mast cells, macrophages), endothelial cells, and epithelial cells under stress conditions — making it both a classical neuropeptide and a cytokine-like immunomodulatory mediator.

VIP signals through two structurally related G-protein-coupled receptors: VPAC1 (VIP/PACAP receptor type 1) and VPAC2 (VIP/PACAP receptor type 2), both of which couple to Gs and stimulate adenylate cyclase, increasing intracellular cAMP and activating protein kinase A. VPAC1 is expressed broadly in lymphoid tissue, liver, lung, intestine, and many regions of the brain. VPAC2 is more restricted, with high expression in the suprachiasmatic nucleus (the brain's master circadian clock), pancreatic islets, lymphocytes, and certain brain regions. Both receptors have anti-inflammatory and cytoprotective downstream signalling when activated, though through overlapping but partially distinct pathways.

The immune-modulatory profile of VIP is one of the most extensively documented and clinically relevant aspects of its pharmacology. VIP's VPAC1-mediated signalling in macrophages, monocytes, dendritic cells, and T cells produces potent inhibition of pro-inflammatory cytokine production — particularly TNF-α, IL-6, IL-12, IL-1β, and IL-18 — while simultaneously upregulating the anti-inflammatory cytokines IL-10 and TGF-β. VIP also promotes the development of Treg (regulatory T cell) populations, which suppress autoimmune responses and maintain peripheral tolerance. In dendritic cells, VIP signalling produces a tolerogenic rather than immunostimulatory phenotype — reducing the capacity of dendritic cells to activate effector T cells while promoting Treg induction. These anti-inflammatory and tolerogenic effects have been demonstrated across a remarkably broad range of inflammatory disease models: sepsis, inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis, systemic lupus erythematosus, uveitis, and others — establishing VIP as one of the most broadly applicable endogenous anti-inflammatory peptides known.

The pulmonary vasodilatory action of VIP is among the most potent of any endogenous substance tested in the pulmonary circulation. VPAC1 and VPAC2 receptors are expressed throughout the pulmonary vascular endothelium and smooth muscle, and VIP activation of these receptors produces direct relaxation of pulmonary arteriolar smooth muscle, reducing pulmonary vascular resistance. This is medically important because pulmonary arterial hypertension (PAH) — a progressive, life-threatening disease characterised by pathological increase in pulmonary vascular resistance and right heart failure — was found to be associated with deficiency of VIP in the pulmonary circulation. Lung biopsies from PAH patients show dramatically reduced VIPergic nerve density and VIP expression compared to normal controls, and circulating VIP levels are reduced in PAH patients. Inhaled synthetic VIP (aviptadil) was tested in a small clinical trial for PAH and produced significant improvements in pulmonary haemodynamics and exercise capacity — establishing proof-of-concept for VIP-based pulmonary vasodilator therapy.

During the COVID-19 pandemic, aviptadil (synthetic VIP) was studied in mechanically ventilated COVID-19 patients with acute respiratory failure. A Phase 2b trial (NCT04311697) published preliminary results suggesting that inhaled aviptadil significantly reduced mortality in mechanically ventilated patients compared to placebo — a striking result in a population with near-universal mortality prior to vaccination. The mechanism proposed combines VIP's pulmonary vasodilatory action (reducing ARDS-associated pulmonary hypertension), its anti-inflammatory actions (reducing the cytokine storm component of severe COVID-19), and the finding that VPAC2 receptors on type II alveolar pneumocytes (the cells that SARS-CoV-2 targets) may confer direct cytoprotective effects on the cells most critical for gas exchange.

The circadian biology role of VIP is essential and mechanistically elegant. The suprachiasmatic nucleus (SCN) — a bilateral pair of tiny nuclei in the hypothalamus containing approximately 20,000 neurons that function as the master mammalian circadian clock — expresses VPAC2 receptors at extremely high density. VIP is produced by a subset of SCN neurons and acts as a synchronising signal between SCN neurons: VIPergic interneuron signalling via VPAC2 coordinates the molecular oscillators in individual SCN cells into a coherent, synchronised population rhythm. Mice genetically lacking VIP or VPAC2 show profound disruption of circadian behaviour — markedly reduced amplitude of circadian locomotor rhythms and desynchronised individual SCN neuron oscillators — establishing VIP as physiologically essential for the coherence of the circadian pacemaker. The relevance to human circadian health, sleep disorders, and jet lag remains an active research area.