Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide originally isolated from ovine hypothalamic extracts by Miyata et al. in 1989, identified by its ability to stimulate adenylate cyclase in rat anterior pituitary cells Miyata et al. (1989). PACAP belongs to the vasoactive intestinal peptide (VIP)/secretin/glucagon superfamily and exists in two bioactive forms — PACAP-38 (38 amino acids, the dominant form constituting >90% of endogenous PACAP) and PACAP-27 (the N-terminal 27 amino acids).

Overview

PACAP is encoded by the ADCYAP1 gene and is widely distributed throughout the central and peripheral nervous systems, with particularly high expression in the hypothalamus, hippocampus, amygdala, brainstem, trigeminal ganglia, adrenal medulla, and enteric nervous system. PACAP signals through three G protein-coupled receptors: PAC1 (highly selective for PACAP over VIP, ~1000-fold preference), VPAC1 (shared equally with VIP), and VPAC2 (shared equally with VIP) Vaudry et al. (2009).

PAC1 exists in multiple splice variants (PAC1-null, PAC1-hop, PAC1-hip, PAC1-hiphop, and others) with distinct signaling properties — PAC1-null couples primarily to adenylyl cyclase/cAMP, while PAC1-hop and PAC1-hiphop additionally activate phospholipase C, generating IP3 and diacylglycerol. This receptor diversity allows PACAP to engage different intracellular signaling cascades depending on tissue context and PAC1 splice variant expression.

PACAP-38 and PACAP-27 share identical N-terminal sequences and both activate all three receptors, but PACAP-38 predominates in most tissues and has a distinct C-terminal domain (residues 28-38) that influences receptor binding kinetics, peptide stability, and membrane interactions. The two isoforms may have partially distinct functional profiles in vivo.

Mechanism of Action

PACAP engages multiple signaling cascades depending on receptor subtype and splice variant:

PAC1/Gs/cAMP/PKA/CREB Pathway (Neuroprotection): PAC1 activation stimulates adenylyl cyclase, increasing cAMP and activating PKA. PKA phosphorylates CREB at Ser133, driving transcription of pro-survival genes including Bcl-2, BDNF, and c-fos. This pathway underlies PACAP's potent neuroprotective effects against excitotoxicity, oxidative stress, and apoptosis. PACAP is among the most potent known activators of adenylyl cyclase, producing cAMP increases at subnanomolar concentrations.

PAC1-hop/PLC/IP3/Ca2+ Pathway: The hop splice variant of PAC1 couples to Gq/PLC, generating IP3 and diacylglycerol. IP3-mediated calcium release from intracellular stores activates CaMKII and calcineurin, while diacylglycerol activates PKC. This pathway contributes to neurotransmitter release modulation, synaptic plasticity, and some neuroprotective effects.

PI3K/Akt Anti-apoptotic Signaling: PACAP activates PI3K/Akt through PAC1, phosphorylating and inactivating pro-apoptotic factors Bad and caspase-9, while activating NF-kappaB survival signaling. This pathway is particularly important for PACAP's protection against mitochondrial apoptosis in neurons.

MAPK/ERK Neurotrophic Signaling: PAC1 activation engages Ras/Raf/MEK/ERK signaling, promoting neuronal differentiation, neurite outgrowth, and synaptic plasticity. ERK activation by PACAP is sustained relative to transient growth factor-mediated ERK signaling, which is important for its neurotrophic effects.

Immune Modulation: Through VPAC1 and VPAC2 on immune cells, PACAP shifts cytokine profiles from pro-inflammatory (TNF-alpha, IL-6, IL-12) toward anti-inflammatory (IL-10, TGF-beta), suppresses macrophage activation, and promotes regulatory T-cell differentiation Delgado et al. (2004).

Reconstitution Calculator

Research

Safety Profile

PACAP has been administered to humans in controlled research settings, primarily for migraine provocation studies and cardiovascular investigations:

• Migraine provocation: IV PACAP-38 infusion (10 pmol/kg/min for 20 minutes) produces transient facial flushing, sensation of warmth, and palpitations during infusion. A delayed headache/migraine develops 2-7 hours post-infusion in migraine-susceptible individuals. These effects are pharmacologically predictable and self-limiting
• Cardiovascular effects: PACAP is a potent vasodilator — IV infusion produces dose-dependent reductions in blood pressure and reflex tachycardia. Cranial and meningeal arterial dilation is particularly pronounced due to high PAC1 expression in cranial vasculature
• Gastrointestinal effects: PACAP stimulates pancreatic secretion and gastrointestinal motility. Mild nausea may occur with systemic administration
• Endocrine effects: PACAP stimulates catecholamine release from the adrenal medulla and modulates insulin secretion from pancreatic beta cells
• Short half-life: Rapid enzymatic degradation (DPP-IV, NEP) limits duration of adverse effects with native PACAP peptides
• PACAP-deficient phenotypes: ADCYAP1 knockout mice show impaired stress responses, reduced fertility, increased vulnerability to brain injury, and circadian disruption — indicating essential physiological roles

Clinical Research Protocols

• Migraine provocation model: PACAP-38 at 10 pmol/kg/min IV infusion over 20 minutes, with headache diary monitoring for 24 hours post-infusion. Exclusion criteria include cardiovascular disease, uncontrolled hypertension, and pregnancy.
• Anti-PACAP antibodies (migraine prevention): Lu AG09222 (Lundbeck) — anti-PACAP monoclonal antibody in Phase 2 trials for episodic migraine. AMG 301 (Amgen) — anti-PAC1 receptor antibody tested in Phase 2 for migraine prevention.
• Biomarker measurement: Plasma PACAP measured by RIA or ELISA; reference range approximately 20-100 pg/mL (assay-dependent). CSF PACAP levels are higher and more stable than plasma levels.
• Genetic studies: ADCYAP1R1 rs2267735 genotyping for PTSD risk stratification (primarily validated in female cohorts).

Pharmacokinetic Profile

Ongoing & Future Research

• Anti-PACAP/PAC1 antibodies for migraine: Clinical trials of Lu AG09222 and successor anti-PACAP antibodies for episodic and chronic migraine prevention, particularly in anti-CGRP non-responders.
• PACAP for neuroprotection (stroke/TBI): Development of metabolically stable PACAP analogs and intranasal PACAP delivery for acute neuroprotection following cerebral ischemia and traumatic brain injury.
• PACAP/PAC1 in PTSD therapeutics: Investigation of PAC1 modulation as a sex-specific therapeutic strategy for PTSD, building on the ADCYAP1R1 genetic findings.
• PACAP in neurodegenerative disease: Preclinical studies of PACAP in Alzheimer's and Parkinson's disease models, where PACAP's neurotrophic and anti-apoptotic properties may slow neurodegeneration.
• Circadian medicine: PACAP-based interventions for circadian rhythm disorders, shift work adaptation, and jet lag, leveraging PACAP's role in SCN photic entrainment.
• PACAP in metabolic disease: Investigation of PACAP's role in glucose homeostasis, insulin secretion, and energy balance, with potential applications in type 2 diabetes.