PACAP — the neuroprotective peptide also implicated in migraine

The compound they found was not a minor modulator in an obscure pathway. PACAP turned out to be expressed throughout the nervous system, the endocrine system, and the immune system, acting on three distinct receptor subtypes, producing different effects in virtually every tissue where it operates. It also, they would eventually learn, does two things that appear to contradict each other: it protects the brain from serious injury, and it triggers migraine.

Both of those things are true. Understanding why requires sitting with a level of biological complexity that most peptide summaries don't attempt.

PACAP exists in two forms. The primary form is PACAP-38, the thirty-eight amino acid peptide Arimura isolated. Processing of the same precursor protein produces a truncated version, PACAP-27, which shares the first twenty-seven amino acids and retains biological activity through the same receptors. The two forms have overlapping but not identical receptor affinities and tissue distributions, which matters for understanding their distinct physiological roles. PACAP is structurally related to vasoactive intestinal peptide, or VIP — the two share roughly sixty-eight percent sequence homology in their first twenty-eight residues — and they also share two of their three receptors. The three receptors are VPAC1, VPAC2, and PAC1. VPAC1 and VPAC2 bind both PACAP and VIP with roughly equal affinity. PAC1 is highly selective for PACAP, binding it with a thousand-fold greater affinity than VIP, and PAC1 is the receptor most heavily expressed in the brain. The differential receptor distribution is part of why PACAP and VIP, despite their structural similarity, have different functional profiles in the central nervous system.

PACAP's neuroprotective effects have been extensively documented in preclinical literature — animal models of stroke, traumatic brain injury, excitotoxicity, and neurodegenerative disease. In models of ischemic stroke, exogenous PACAP reduces infarct volume, preserves neurological function, and promotes cell survival in penumbral tissue — the region surrounding the core of an infarct where neurons are stressed but potentially salvageable. The mechanisms are multiple and they operate at several levels simultaneously. PACAP activates adenylate cyclase, raising cyclic AMP, which activates protein kinase A, which in turn activates CREB — a transcription factor that drives the expression of survival genes including Bcl-2 family members and BDNF. It reduces the inflammatory cytokine cascade following injury. It modulates glutamate signaling in ways that reduce excitotoxic damage. It supports mitochondrial function under metabolic stress. And it promotes the differentiation and survival of neurons in the developing and adult brain through direct effects on progenitor cell biology.

In models of traumatic brain injury, PACAP shows similar effects: reduced secondary damage, better behavioral outcomes, preservation of white matter integrity. In models of Parkinson's disease, PACAP protects dopaminergic neurons from the specific cellular stresses that kill them — oxidative stress, mitochondrial dysfunction, alpha-synuclein aggregation. In Alzheimer's models, PACAP reduces amyloid burden and supports the survival of cholinergic neurons. The preclinical neuroprotection literature for PACAP is among the most extensive for any endogenous neuropeptide. These are animal studies with significant translational uncertainty, but the consistency of the neuroprotective signal across different injury models and different mechanistic pathways is striking.

The cognitive enhancement angle is more directly relevant for readers thinking about PACAP in a nootropic context, and here the evidence is thinner. Rodent studies have shown that intracerebroventricular or intranasal PACAP improves performance in spatial memory tasks, fear conditioning, and object recognition — suggesting effects on hippocampal function and synaptic plasticity. The proposed mechanisms connect to PACAP's CREB activation and BDNF upregulation, which are canonical pathways for long-term potentiation and memory consolidation. Intranasal delivery is particularly interesting from a translational standpoint because it offers a non-invasive route that bypasses the blood-brain barrier problem that limits many peptide therapeutics: the olfactory pathway provides direct access from nasal epithelium to brain tissue, and PACAP is among the neuropeptides that have been shown in animal studies to reach the central nervous system via this route with meaningful efficiency. The research here is earlier stage and less replicated than the neuroprotection literature, and the jump from rodent spatial memory to human cognitive enhancement has not been made in controlled human trials. Not FDA-approved as a cognitive therapeutic.

Now for the contradiction.

PACAP triggers migraine. This is not a theoretical concern or an incidental finding — it's one of the most robustly replicated and mechanistically interesting observations in migraine research of the past decade. When researchers infuse PACAP-38 intravenously into people with a history of migraine, most of them develop a headache that meets International Headache Society criteria for migraine, with the characteristic throbbing quality, photophobia, phonophobia, and sometimes nausea. The same infusion in people without migraine history produces a headache in fewer subjects, and it tends to be milder and less characteristic. PACAP is, in the language of the field, a migraine trigger — a compound whose administration provokes the condition in people susceptible to it.

Why would a neuroprotective peptide trigger migraine? The answer lies in which receptors are activated where, and what the consequences are in migraine-relevant tissue versus brain parenchyma. PACAP's migraine-triggering effects appear to be mediated primarily through PAC1 receptors — the highly PACAP-selective receptor — on structures involved in migraine pathophysiology: the trigeminal nerve and its terminals, the meningeal blood vessels, the trigeminal nucleus caudalis in the brainstem that processes pain signals from the head, and possibly the dura mater itself. When PACAP activates PAC1 on these structures, it causes vasodilation of meningeal and dural vessels, activates trigeminovascular neurons, promotes the release of CGRP and other nociceptive peptides from trigeminal terminals, and initiates the sensitization cascade that underlies migraine pain. The same PACAP molecule that rescues neurons from ischemic death in the brain parenchyma activates pain-generating pathways in the trigeminovascular system — because the same peptide, binding the same receptor, produces dramatically different effects depending on the cell type and tissue it's acting on.

This finding validated PACAP as a therapeutic target for migraine prevention — specifically, the idea that blocking PACAP or its PAC1 receptor might prevent migraine in people who are susceptible to it. Lundbeck pursued this logic directly. Their compound Lu AG09222, a monoclonal antibody against PACAP-38, was developed on the hypothesis that neutralizing circulating PACAP would reduce the frequency of migraine attacks. The antibody binds PACAP-38 directly and prevents it from activating its receptors. Clinical trial data published in 2023 showed that monthly subcutaneous Lu AG09222 reduced monthly migraine days compared to placebo in people with episodic and chronic migraine. The effect size was in the range seen for the CGRP-targeting antibodies — erenumab, fremanezumab, galcanezumab — which are currently the most effective preventive migraine biologics available.

The relationship between PACAP and CGRP in migraine is parallel rather than redundant. CGRP and PACAP are both released from trigeminal terminals and both contribute to the vasodilatory and sensitizing cascade that drives migraine, but they appear to work through different mechanisms and in different temporal windows. Patients who don't respond to CGRP-targeting therapies — and a meaningful fraction of migraineurs don't — may respond to PACAP targeting. This is the therapeutic rationale for the PACAP antibody program as a complementary option rather than a redundant one.

The cognitive enhancement literature and the migraine literature involve the same molecule doing opposite things in different contexts. In the brain, PACAP at PAC1 receptors on neurons and astrocytes in regions like the hippocampus and prefrontal cortex — where it has trophic and plasticity-supporting functions — looks protective and cognitively beneficial. In the trigeminovascular system — where the same receptor on a different cell population drives pain and inflammation — it looks damaging. A drug that enhances PACAP signaling for cognitive benefit would presumably also risk activating the trigeminovascular pathway. A drug that blocks PACAP for migraine prevention would presumably reduce whatever beneficial PACAP signaling is occurring in the brain. This is a genuine tension that the field has not resolved, and it's one of the reasons that PACAP-based cognitive enhancement remains in early investigational stages while the antibody programs move forward.

What PACAP represents, at minimum, is a window into the complexity of neuropeptide biology — the way the same molecule can be simultaneously healing and harmful, depending entirely on which receptor, on which cell, in which tissue, receives the signal. The tendency to think of neuropeptides as having single functions, or as being simply good or bad for the brain, doesn't survive contact with PACAP. Its biology is a reminder that the nervous system's signaling systems are older and more context-dependent than the frameworks we use to describe them.