PACAP Peptide Research: PAC1 Receptor Pharmacology, Neuroprotection Mechanisms, and Preclinical Evidence

PACAP (pituitary adenylate cyclase-activating polypeptide) is a 38-residue, C-terminally amidated neuropeptide of the VIP-secretin-glucagon superfamily that signals through three class B receptors, PAC1, VPAC1 and VPAC2, and PACAP peptide research is dominated by neuroprotection in ischaemia, retinal injury and Parkinson models, with a secondary literature in cardioprotection and anti-inflammatory signalling. It was isolated from ovine hypothalamus in 1989 on the basis of its ability to stimulate cAMP formation in pituitary cells, it shares 68 percent N-terminal homology with VIP yet is at least a thousand times more potent at stimulating adenylate cyclase, and its sequence has been conserved from protochordates to mammals. This article summarises the receptor pharmacology and the preclinical evidence from the primary papers and removes several figures that circulated in the earlier version of this page. PACAP-38 is not a compound in the Maple Research Labs catalogue.

Disclaimer: For research purposes only. Not for human consumption. Not for diagnostic or therapeutic use. The information presented here is drawn from published scientific literature and is intended solely for educational reference.

Discovery, Structure and Forms

Miyata, Arimura and colleagues isolated PACAP from ovine hypothalamic tissue as a peptide that stimulated adenylate cyclase in rat anterior pituitary cell cultures, and reported the sequence His-Ser-Asp-Gly-Ile-Phe-Thr-Asp-Ser-Tyr-Ser-Arg-Tyr-Arg-Lys-Gln-Met-Ala-Val-Lys-Lys-Tyr-Leu-Ala-Ala-Val-Leu-Gly-Lys-Arg-Tyr-Lys-Gln-Arg-Val-Lys-Asn-Lys-NH2. The N-terminal region showed 68 percent homology with VIP, the adenylate cyclase stimulating activity was at least 1000 times greater than that of VIP, and the peptide released growth hormone, prolactin and ACTH from superfused rat pituitary cells at 0.1 nM and luteinising hormone at 1 nM (Miyata et al., 1989, Biochem Biophys Res Commun 164:567-574; PMID 2803320). A C-terminally truncated 27-residue form, PACAP-27, was isolated from the same source shortly afterwards; both are alpha-amidated and biologically active, and PACAP-38 is the dominant form in most tissues. The precursor also encodes a PACAP-related peptide of unknown function. The 2009 Pharmacological Reviews article by Vaudry and colleagues is the comprehensive reference for distribution, receptor pharmacology and physiology (Vaudry et al., 2009, Pharmacol Rev 61:283-357; PMID 19805477). The molecular formula of PACAP-38 is C203H331N63O53S and the molecular weight about 4534 Da; the single methionine at position 17 is its oxidisable residue.

Receptor Pharmacology: PAC1, VPAC1 and VPAC2

Two classes of binding site were characterised early: type I sites with high affinity for PACAP and much lower affinity for VIP, and type II sites with similar affinity for both (Vaudry 2009). The type I site is the PAC1 receptor. Spengler and colleagues cloned it in 1993 by an expression strategy based on cAMP induction of a reporter gene and found that a single gene gives rise to five splice variants differing in the third intracellular loop, the so-called hip and hop cassettes, which couple differently to adenylyl cyclase and phospholipase C, so that the isoform a cell expresses determines the qualitative nature of its response to PACAP (Spengler et al., 1993, Nature 365:170-175; PMID 8396727). That paper is from 1993, not 2003 as the earlier version of this page stated. The type II sites are VPAC1 and VPAC2, which bind PACAP and VIP with similar affinity and couple principally to adenylyl cyclase; the current IUPHAR classification and nomenclature are set out by Harmar and colleagues (2012, Br J Pharmacol 166:4-17; PMID 22289055).

The structure-activity work that produced the standard antagonist was done on human NB-OK-1 neuroblastoma membranes. Residues 28 to 38 of PACAP-38 favour receptor recognition, so PACAP(1-38) analogues are generally more potent than the corresponding PACAP(1-27) analogues; residues 1 to 3 are all important for adenylate cyclase activation; and N-terminal truncation to PACAP(6-38), unlike PACAP(6-27), is still well recognised by the receptor and acts as a potent competitive antagonist with a Ki of 1.5 nM (Robberecht et al., 1992, Eur J Biochem 207:239-246; PMID 1321043). PACAP(6-38) is therefore the reagent used throughout the literature below to confirm PAC1 involvement, with the caveat that it also has activity at VPAC2. Downstream, PAC1 activation raises cAMP and activates protein kinase A and CREB, and in neurons also engages PI3K-Akt and ERK pathways, which is the signalling basis of the survival effects.

Neuroprotection in Cerebral Ischaemia

The most cited stroke experiment is from Arimura’s group. Rats underwent two hours of transient middle cerebral artery occlusion with a filament, and PACAP-38 was given as an intravenous bolus of 20 nmol/kg followed by a 48-hour intravenous infusion by micro-osmotic pump beginning 4, 8 or 12 hours after occlusion. Treatment beginning at 4 hours reduced infarct size by 50.88 percent on TTC-stained sections; treatment beginning at 8 or 12 hours did not produce a significant reduction, although infarcts tended to be smaller (Reglodi et al., 2000, Stroke 31:1411-1417; PMID 10835464). In mice, PACAP-38 given intravenously or intracerebroventricularly one hour after MCAO significantly reduced infarct volume and ameliorated motor deficits at 24 hours; PACAP-deficient mice had larger infarcts and worse deficits than wild type, and PACAP treatment brought them to the same level of improvement, with a 35,546-clone microarray identifying transcripts regulated by ischaemia and by PACAP (Chen et al., 2006, Regul Pept 137:4-19; PMID 17027094).

The mechanism in the mouse involves IL-6. Ohtaki and colleagues found that infarct volume, neurological deficit and cytoplasmic cytochrome c after focal ischaemia were higher in PACAP heterozygous and null mice than in wild type and were ameliorated by PACAP-38 injection, that PAC1 receptor immunoreactivity co-localised with IL-6 in neurons, that PACAP-38 raised IL-6 in a manner blocked by a PAC1 antagonist, and that PACAP-38 did not reduce infarction in IL-6 null mice, placing IL-6 downstream of PAC1 in the protective pathway (Ohtaki et al., 2006, PNAS 103:7488-7493; PMID 16651528). The “Uchida 2011 intranasal 2 micrograms, n=12”, “Reglodi 2009 Regulatory Peptides 40 percent calcium overload” and “Toth 2012 52 percent TUNEL reduction” citations in the earlier version of this page could not be located and have been removed; the intranasal route for PACAP has been studied, but the specific figures had no source.

Parkinson Models and Retinal Injury

In the rat 6-hydroxydopamine model, unilateral injection into the substantia nigra produced severe hypokinesia at 1 and 10 days in untreated animals. Rats pretreated with 1, 0.1 or 0.01 µg PACAP showed less severe acute hypokinesia, complete recovery by 10 days, resolution of asymmetrical signs and markedly fewer apomorphine-induced rotations, with 0.1 µg giving the best behavioural outcome, and tyrosine hydroxylase immunohistochemistry showed more surviving dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area in all PACAP-treated groups (Reglodi et al., 2004, Regul Pept 123:85-94; PMID 15518897).

The retina is one of the richest PAC1 sites and the Pécs group has built a long series of retinal degeneration studies on it. In the monosodium glutamate model of excitotoxic inner retinal degeneration, three neonatal MSG treatments destroy the entire inner retina, and PACAP protection was demonstrable when at least two MSG treatments were used, the authors concluding that PACAP protects against excitotoxicity in the retina (Babai et al., 2006, Ann N Y Acad Sci 1070:149-155; PMID 16888157). The “Atlasz 2010, 3.8-fold photoreceptor survival, n=8” figure and the “Silveira 2002 IOVS, 60 to 75 percent RGC rescue” figure from the earlier version of this page could not be verified against any abstract and have been removed; the retinal literature supports a protective effect of PACAP against excitotoxic, ischaemic and light-induced damage, but not those specific numbers.

Cardiovascular and Anti-Inflammatory Signalling

PACAP is a vasodilator and has direct effects on cardiomyocytes. In neonatal rat cardiomyocytes exposed to hydrogen peroxide, PACAP decreased the proportion of apoptotic cells and diminished the activity of apoptosis signal-regulating kinase, and both effects were counteracted by PACAP(6-38), which the authors interpret as cardioprotection through inhibition of the MAP kinase-dependent apoptotic pathway (Gasz et al., 2006, Ann N Y Acad Sci 1070:293-297; PMID 16891268). The “Szabo 2012 Peptides, 38 percent infarct reduction, n=10” myocardial ischaemia study cited previously could not be located and has been removed.

On the immune side, VIP and PACAP inhibit TNF-alpha production by LPS-stimulated macrophages, and Delgado and colleagues showed that this involves the VPAC1 receptor and cAMP-dependent inhibition of NF-kappaB and other transcription factors (Delgado et al., 1999, J Immunol 162:2358-2367; PMID 9973516). The broader immunoregulatory literature for the two peptides, covering macrophages, microglia, dendritic cells and T cells, is reviewed by Delgado, Pozo and Ganea (2004, Pharmacol Rev 56:249-290; PMID 15169929). The NF-kappaB mechanism overlaps with that described for KPV, an alpha-MSH fragment. The “Deng 2010 European Journal of Pharmacology” macrophage polarisation citation could not be located and has been removed.

Migraine and the PAC1 Receptor

PACAP-38 is found in trigeminal sensory and parasympathetic perivascular nerve fibres, and in a randomised, double-blind, placebo-controlled crossover study of twelve healthy subjects and twelve patients with migraine without aura, intravenous PACAP-38 infusion induced headache in healthy subjects and migraine-like attacks in the patients, with transcranial Doppler and superficial temporal artery ultrasonography documenting the vascular response (Schytz et al., 2009, Brain 132:16-25; PMID 19052139). Because VIP activates the same VPAC receptors but does not provoke attacks, the PAC1 receptor became the pharmacological target of interest, and this is the basis of the PACAP and PAC1 antagonist programmes in migraine.

Degradation, Stability and Analytical Considerations

PACAP-38 is a DPP-IV substrate. Using a mass-spectrometry-based protease assay, Zhu and colleagues found PACAP-38 to be efficiently processed by DPP-IV in vitro, and in mice the exogenously administered peptide was cleared significantly more slowly in DPP-IV-deficient animals, with virtually complete suppression of the inactive PACAP(3-38) metabolite, the same His1-Ser2 cleavage that inactivates GLP-1 and GHRH (Zhu et al., 2003, J Biol Chem 278:22418-22423; PMID 12690116). For in vivo work this means the intact peptide has a short circulating half-life and that plasma measurements need to distinguish PACAP(1-38) from PACAP(3-38).

Analytically, PACAP-38 is a basic, largely helical peptide with one methionine, so the points to read on a certificate of analysis are the reversed-phase HPLC purity, a mass spectrometry identity result for the 4534 Da parent and the +16 Da Met17 sulfoxide, and confirmation of the C-terminal amide, which is required for activity. Highly basic helical peptides of this length can aggregate at high concentration, and the general storage rules apply: lyophilised at -20 °C or below, protected from light, single-use aliquots after reconstitution. Specific concentration, solvent and purity thresholds stated in the earlier version of this page (“below 1 mg/mL”, “0.1 percent acetic acid”, “95 percent or greater”) were not sourced and have been dropped.

Key Research Findings

PACAP-38 was isolated in 1989 as a hypothalamic peptide at least 1000 times more potent than VIP at stimulating pituitary adenylate cyclase (Miyata 1989). PAC1 is PACAP-selective and exists as five splice variants with distinct adenylyl cyclase and phospholipase C coupling; VPAC1 and VPAC2 bind PACAP and VIP equally (Spengler 1993; Harmar 2012). PACAP(6-38) is a competitive PAC1 antagonist with a Ki of 1.5 nM (Robberecht 1992). Intravenous PACAP-38 beginning four hours after two-hour MCAO in rats reduced infarct size by 50.88 percent, with no significant effect when started at eight or twelve hours (Reglodi 2000); in mice, PACAP-38 at one hour reduced infarct volume and deficits, PACAP-deficient mice fared worse (Chen 2006), and the protection required IL-6 downstream of PAC1 (Ohtaki 2006). PACAP pretreatment preserved nigral dopaminergic neurons and motor function after 6-OHDA in rats (Reglodi 2004) and protected the inner retina against MSG excitotoxicity (Babai 2006). PACAP reduced oxidative-stress apoptosis in cardiomyocytes via PAC1 (Gasz 2006) and inhibits macrophage TNF-alpha through VPAC1 and NF-kappaB (Delgado 1999). Intravenous PACAP-38 provokes migraine-like attacks in migraineurs where VIP does not (Schytz 2009). DPP-IV cleaves PACAP-38 to inactive PACAP(3-38) in vivo (Zhu 2003).

Note on sources. This article was rewritten in September 2026 after a citation audit. The earlier version dated Spengler’s PAC1 splice-variant paper to 2003 rather than 1993, and cited “Uchida 2011 J Neurochem”, “Reglodi 2009 Regul Pept”, “Toth 2012 J Mol Neurosci”, “Atlasz 2010 J Mol Neurosci”, “Silveira 2002 IOVS”, “Szabo 2012 Peptides” and “Deng 2010 Eur J Pharmacol” with sample sizes, p-values and percentage effects that could not be matched to any indexed abstract. The verifiable studies from the same groups (Reglodi 2000 and 2004, Babai 2006, Gasz 2006) are cited above with their actual findings. Every citation was checked against the indexed abstract on 18 September 2026.

PACAP is one of three neuropeptides that dominate preclinical neurogenic inflammation research. CGRP and the CLR/RAMP1 receptor complex account for the vasodilatory arm, and substance P at the NK1 receptor for plasma extravasation, so the three literatures are best read together. For the related peptides in the catalogue, the VIP product page and the BPC-157, GHK-Cu and TB-500 pages carry their own specifications and batch documentation.

Maple Research Labs does not list PACAP-38. For the compounds it does list, it submits batches to an independent laboratory for HPLC purity analysis and publishes the measured result for each tested batch on its Certificates of Analysis page, which also lists batches still awaiting a report. Browse the full peptide catalog for the compounds currently available.

For research purposes only. Not for human consumption. Not for diagnostic or therapeutic use.