PACAP: The Neuropeptide Protecting Retinas
Neuroprotective Peptides for Retinal Disease
100 pmol
A single intravitreal injection of 100 pmol PACAP protected cone photoreceptors and ganglion cells in diabetic rat retinas over three weeks.
Szabadfi et al., Cell and Tissue Research, 2012
Szabadfi et al., Cell and Tissue Research, 2012
View as imagePituitary adenylate cyclase-activating polypeptide (PACAP) is one of the most studied neuroprotective peptides in retinal research. Since its discovery in 1989, PACAP has demonstrated the ability to protect retinal neurons from diabetic damage, ischemia-reperfusion injury, excitotoxicity, and elevated intraocular pressure in animal models. The peptide exists naturally in the retina, where it acts through three receptor subtypes to activate antiapoptotic signaling cascades. Despite over two decades of animal data, PACAP retina neuroprotection has not yet reached human clinical trials. This article covers the full evidence landscape: what PACAP does in the retina, which disease models show protection, how it can be delivered as eye drops, and why clinical translation has stalled. For a broader look at other peptides being studied for retinal conditions, see Neuroprotective Peptides for Retinal Degeneration: Research Frontiers.
Key Takeaways
- PACAP (100 pmol intravitreal) preserved cone photoreceptors, dopaminergic amacrine cells, and ganglion cell layer neurons in streptozotocin-diabetic rat retinas (Szabadfi et al., 2012)
- Mice with targeted PAC1 receptor deletion in retinal neurons showed baseline ganglion cell deficits and increased neuron loss in an MS/optic neuritis model (Van et al., 2021)
- PACAP1-38 eye drops crossed ocular barriers and reached the retina in rodent models, with 80-90% peptide stability after two weeks in saline at 4 degrees C (Kovacs et al., 2021)
- PACAP activates antiapoptotic pathways (ERK1/2 phosphorylation) while inhibiting proapoptotic signals (JNK, p38 MAPK, caspase-3) in retinal tissue
- PACAP-deficient mice showed accelerated retinal aging and increased vulnerability to ischemic injury compared to wild-type controls
- No human clinical trials of PACAP for any retinal condition have been completed as of 2026
What Is PACAP?
PACAP belongs to the secretin/glucagon/vasoactive intestinal peptide (VIP) superfamily. It was first isolated from ovine hypothalamic extracts in 1989 by Arimura and colleagues. The peptide exists in two biologically active forms: PACAP1-27 (27 amino acids) and PACAP1-38 (38 amino acids), with PACAP1-38 being the predominant form in most tissues.[1]
PACAP is one of the most evolutionarily conserved peptides known. Its amino acid sequence is 97% identical between mammals, birds, and amphibians, suggesting strong selective pressure to maintain its structure over roughly 500 million years. This conservation spans its neuroprotective, neurodevelopmental, and immunomodulatory functions.
Three receptor subtypes mediate PACAP's actions. PAC1 is highly selective for PACAP over VIP and mediates most neuroprotective effects. VPAC1 and VPAC2 bind both PACAP and VIP with similar affinity and are more involved in immune regulation and vasodilation.[2] All three receptor types are G protein-coupled receptors linked to the adenylate cyclase-cAMP pathway, though PAC1 also couples to phospholipase C signaling through specific splice variants.
PACAP shares 68% amino acid sequence identity with VIP but differs at the PAC1 receptor, where it binds with far higher affinity. This receptor selectivity is the foundation of PACAP's distinctive neuroprotective potency compared to VIP and other family members.[3]
PACAP Receptors in the Retina
PACAP and its receptors are expressed throughout the mammalian retina. Immunohistochemistry studies have identified PACAP in retinal ganglion cells and in nerve fibers of the inner plexiform layer. The peptide is also present in the iris, cornea, and ciliary body.[4]
The PAC1 receptor shows distinct expression patterns during retinal development. Four PAC1 splice variants (Null, Hip, Hop1, and HipHop1) have been identified in the developing rat retina. The Hop1 isoform becomes dominant in adult tissue, with expression peaks coinciding with eye opening around postnatal day 10. This developmental timing suggests PACAP signaling is involved in retinal maturation, not just injury response.
PACAP also has a specialized function in the retinohypothalamic tract, where retinal ganglion cells project to the suprachiasmatic nucleus to regulate circadian rhythms. PACAP and glutamate are co-stored in these projections. PACAP attenuates glutamate-induced neurotoxicity in cultured retinal neurons, suggesting a built-in protective mechanism against excitotoxic damage.[4]
The functional importance of these receptors was demonstrated directly by Van et al. (2021), who used adeno-associated virus (AAV2) to delete PAC1 receptors specifically from retinal neurons in mice. Even without any injury, PAC1 deletion caused a measurable deficit in retinal ganglion neurons and their dendrites.[5] This finding established that PAC1 signaling is not merely protective during injury; it plays a homeostatic role in maintaining normal retinal neuron populations.
PACAP in Diabetic Retinopathy
Diabetic retinopathy affects approximately 75% of people with type 1 diabetes and 50% of those with type 2 diabetes over time, progressing to blindness in roughly 5% of cases. The neuronal component of diabetic retinal damage, including loss of ganglion cells and photoreceptors, often precedes the vascular changes that dominate clinical attention.
Szabadfi et al. (2012) conducted the most detailed study of PACAP in diabetic retinopathy. Using streptozotocin-induced diabetic rats, they administered three intravitreal injections of 100 pmol PACAP during the final week of a three-week survival period. The findings were striking across multiple cell types.[6]
In diabetic retinas without PACAP treatment, cone photoreceptors degenerated, their outer segment length decreased, and the number of cells in the ganglion cell layer dropped. Dopaminergic amacrine cells, which are critical for light adaptation, also deteriorated. Glial fibrillary acidic protein (GFAP) was upregulated in Muller glial cells, a marker of retinal stress and reactive gliosis.
PACAP treatment ameliorated all of these changes. Cone photoreceptor numbers and outer segment length were preserved. Ganglion cell layer cell counts improved. PACAP also increased PAC1 receptor expression and tyrosine hydroxylase levels, the rate-limiting enzyme for dopamine synthesis, suggesting active upregulation of protective signaling rather than passive prevention of damage.[6]
These findings place PACAP as a candidate for neuroprotective therapy in the early, pre-vascular stages of diabetic retinopathy. The clinical relevance is substantial: diabetic retinopathy affects approximately 100 million people worldwide, and current treatments (anti-VEGF injections, laser photocoagulation) address vascular complications but do not target the underlying neurodegeneration. All evidence comes from rodent models with streptozotocin-induced diabetes, which produces an acute metabolic state that does not perfectly recapitulate the chronic, slowly progressive disease seen in human type 2 diabetes.
PACAP in Excitotoxic and UV-Induced Retinal Damage
Beyond diabetic models, PACAP has been tested in other injury paradigms that target different retinal cell populations through different damage mechanisms.
Excitotoxic retinal injury, caused by excessive glutamate or its analog monosodium glutamate (MSG), selectively damages retinal ganglion cells and inner retinal neurons. Intravitreal PACAP1-38 reduced ganglion cell loss and preserved inner plexiform layer thickness in glutamate excitotoxicity models. The protection was mediated through PAC1 receptor activation, as PAC1 antagonists (PACAP6-38) blocked the neuroprotective effect. PACAP also modulated the microglia/macrophage response in excitotoxic injury, shifting these immune cells toward a repair-promoting "acquired deactivation" phenotype rather than a cytotoxic state.
UV-A light exposure causes oxidative damage to photoreceptors and retinal pigment epithelial (RPE) cells. PACAP1-38 protected against UV-A-induced retinal degeneration in rat models, reducing photoreceptor cell death and preserving outer nuclear layer thickness. The mechanism involved suppression of reactive oxygen species production and upregulation of antioxidant defense enzymes. PACAP also protected RPE cells against oxidative stress in vitro, maintaining mitochondrial membrane integrity, which is relevant because RPE dysfunction contributes to both diabetic retinopathy and age-related macular degeneration.
Optic nerve transection, a model for traumatic optic neuropathy, causes retrograde degeneration of retinal ganglion cells. PACAP delayed ganglion cell death after optic nerve transection, though it could not prevent eventual degeneration. This finding suggests PACAP provides a protective time window rather than permanent rescue, a distinction relevant for clinical design: a drug that delays cell death by weeks could provide time for other interventions to act.
The consistency of protection across these diverse injury mechanisms, each targeting different retinal cell types through different pathological processes, strengthens the case that PAC1 receptor activation engages a fundamental neuroprotective program rather than counteracting a single specific insult.
PACAP and Retinal Ischemia
Retinal ischemia occurs in conditions ranging from central retinal artery occlusion to diabetic retinopathy. Animal models of bilateral carotid occlusion and transient high intraocular pressure have been the primary testing grounds for PACAP's protective effects in this context.
In ischemia-reperfusion models, intravitreal PACAP administration produced near-complete preservation of retinal layer architecture. PACAP decreased apoptosis and glutamate accumulation, reduced levels of peroxidized lipids (markers of oxidative damage), and suppressed inflammatory mediators in ischemic retinas. Electroretinographic measurements confirmed that functional preservation accompanied the structural protection. Chronic retinal ischemia models, induced by bilateral carotid occlusion to mimic the reduced blood flow seen in cerebrovascular disease, also showed improved functional outcomes with PACAP treatment measured by ERG recordings.
PACAP modulates the microglia/macrophage response during ischemic injury. Rather than simply blocking inflammation, PACAP shifted microglial cells toward an "acquired deactivation" phenotype that favors tissue repair over cytotoxic responses. This nuanced immune modulation distinguishes PACAP from simple anti-inflammatory agents.[7]
PACAP-deficient mice showed markedly worse outcomes after retinal ischemia than wild-type animals. Their retinas were more vulnerable to ischemic injury and showed accelerated aging-related changes, with reduced retinal layer thickness and increased ganglion cell loss over time even without experimental injury. This genetic evidence confirmed that endogenous PACAP provides constitutive protection to the retina, not just pharmacological protection when administered at high doses.[5]
PACAP and Glaucoma
Glaucoma is the leading cause of irreversible blindness worldwide, involving progressive loss of retinal ganglion cells typically associated with elevated intraocular pressure (IOP). Current treatments focus on reducing IOP, but ganglion cell death can continue even when pressure is normalized. This has driven interest in neuroprotective agents that could preserve ganglion cells independently of IOP control.
PACAP1-38 eye drops have been tested in a microbead-induced glaucoma model in rats. When dosed at 1 microgram per drop three times daily for four weeks, PACAP treatment lowered IOP to approximately 10 mmHg (normal range), compared to persistently elevated pressure in untreated glaucoma animals. Retinal morphology and electrophysiological function in treated animals were close to normal values. For context on how other peptides, particularly endothelin, contribute to glaucoma pathology, see Endothelin and Glaucoma: The Vasoconstrictor Peptide's Role.
The Van et al. (2021) PAC1 knockout study provided genetic evidence for this connection. When PAC1 receptors were deleted from retinal neurons and the mice were subjected to experimental autoimmune encephalomyelitis (a model that includes optic neuritis), a specific subpopulation of retinal ganglion neurons, the same subtype known to be vulnerable in glaucoma models, showed increased death. Optic nerve axonopathy and secondary microglial infiltration were also more severe.[5]
These findings from separate research groups converge on the same conclusion: PACAP signaling through PAC1 receptors is specifically protective for retinal ganglion cells, the cell type that dies in glaucoma.
PACAP Eye Drops: Crossing Ocular Barriers
A major advantage of PACAP for retinal therapy is that it can be delivered as eye drops rather than requiring intravitreal injection. Radioactive labeling studies demonstrated that PACAP1-38 applied to the corneal surface penetrated the ocular barriers and reached the retina in measurable concentrations in rodent models.
The practical viability of this delivery route depends on peptide stability. Kovacs et al. (2021) conducted systematic stability testing of both PACAP1-27 and PACAP1-38 in four common ophthalmic media and a commercial artificial tear solution. Mass spectrometry results showed that PACAP1-38 achieved 80-90% peptide persistence after two weeks in water and 0.9% saline at 4 degrees Celsius.[8]
PACAP1-38 showed better stability than PACAP1-27 across all conditions. In artificial tear solution at room temperature, both peptides degraded rapidly, but stability was maintained at refrigerated temperatures. This suggests that PACAP eye drops would need cold-chain storage but could maintain therapeutic concentrations for clinically practical timeframes.
The challenge of systemic PACAP delivery, rapid degradation by dipeptidyl peptidase IV (DPP-IV) in serum, does not apply to topical ocular delivery. Eye drops bypass the systemic circulation entirely, delivering the peptide directly to the target tissue through trans-corneal and trans-scleral absorption routes.
How PACAP Protects Neurons: The Signaling Cascade
PACAP's neuroprotective mechanism involves simultaneous activation of survival pathways and suppression of death pathways. Upon binding PAC1, PACAP activates adenylate cyclase, increasing intracellular cAMP, which in turn activates protein kinase A (PKA). This cascade leads to phosphorylation of ERK1/2, promoting cell survival, and phosphorylation of Bad, a proapoptotic protein, which sequesters it away from the mitochondrial membrane.[1]
On the proapoptotic side, PACAP inhibits JNK/SAPK and p38 MAPK, stress-activated kinases that drive cell death in response to ischemia, excitotoxicity, and oxidative damage. PACAP also prevents the mitochondrial release of cytochrome c, blocking the intrinsic apoptosis pathway at its source.
In astrocytes, PACAP stimulates IL-6 release, which provides additional paracrine neuroprotection to neighboring neurons. After ischemia-reperfusion injury, both retinal pyramidal cells and astrocytes upregulate PAC1 receptor expression, suggesting an endogenous amplification loop: injury increases receptor density, which increases sensitivity to PACAP's protective effects.[1]
This dual action, promoting survival signals while suppressing death signals, makes PACAP unusually effective compared to neuroprotective agents that target only one pathway. The coupling to phospholipase C through specific PAC1 splice variants adds another layer of signaling complexity, activating protein kinase C and calcium mobilization that contribute to differentiation signaling during development and may support regenerative processes after injury.
PACAP as an Immunomodulatory Neuropeptide
PACAP's protective effects extend beyond direct neuron-to-neuron signaling. The peptide belongs to a class of endogenous anti-inflammatory neuropeptides that includes VIP, alpha-MSH, CGRP, and cortistatin. These neuropeptides generate regulatory T cells, suppress pathogenic Th1 and Th17 responses, and promote inflammation resolution through specific receptor pathways.[2]
Tan et al. (2009) demonstrated this immunoregulatory role directly by subjecting PACAP-deficient mice to experimental autoimmune encephalomyelitis. PACAP knockout mice developed worse disease with heightened expression of proinflammatory cytokines (TNF-alpha, IL-6, IFN-gamma, IL-12, IL-23, IL-17) and chemokines (MCP-1, MIP-1alpha, RANTES) in the spinal cord. Anti-inflammatory cytokines (IL-4, IL-10, TGF-beta) were downregulated. The abundance of CD4+CD25+FoxP3+ regulatory T cells in lymph nodes was diminished.[9]
This immune dimension is relevant to retinal disease because neuroinflammation contributes to ganglion cell death in glaucoma, diabetic retinopathy, and age-related macular degeneration. The retina exists behind the blood-retinal barrier, an immune-privileged site where neuropeptides help maintain tolerance. Staines (2008) proposed that autoimmune targeting of VIP and PACAP in retinal vessels could impair local blood flow regulation and contribute to vascular retinal diseases.[4]
The therapeutic implications extend beyond the retina. Therapies targeting the VIP/PACAP receptor system, including monoclonal antibodies now in development for migraine, could have effects on retinal neuroprotection that warrant investigation.[3] For how the VIP/PACAP system connects to other neuropeptide functions, see Neuropeptide Y: The Stress Resilience Peptide.
PACAP Beyond the Retina
While this article focuses on retinal applications, PACAP's neuroprotective properties extend throughout the central nervous system. The peptide protects hippocampal neurons from ischemic delayed death, reduces infarct volume in stroke models, and shows protective effects in neurodegeneration models.[1]
PACAP has also entered clinical relevance in headache medicine. PACAP-38 infusion triggers migraine-like attacks in susceptible individuals, establishing it as a migraine mediator alongside CGRP. Anti-PACAP monoclonal antibodies are now in clinical development for migraine prevention, following the successful model of anti-CGRP therapies.[3] Whether blocking PACAP systemically for migraine could affect its protective functions in the retina remains an open question. For context on how neuropeptides support brain function through growth factor upregulation, see How Semax Upregulates BDNF: The Neuroplasticity Mechanism.
PACAP-deficient mice show behavioral abnormalities including increased anxiety, impaired learning, and altered circadian rhythms. In the context of sensory function, PACAP knockout mice do not develop neuropathic pain after nerve injury, positioning PACAP as a key peptide in neuropathic pain signaling, a distinct role from its neuroprotective function.[10]
The Clinical Translation Gap
Over 4,500 PACAP studies have been published since 1989, with roughly 1,400 specifically addressing neuroprotection. Research output on PACAP neuroprotection has averaged approximately 40 publications per year. Despite this volume of preclinical evidence, no human clinical trial of PACAP for any retinal condition has been completed.
Several factors explain this gap. PACAP's rapid degradation by DPP-IV gives it a serum half-life measured in minutes, making systemic delivery impractical for chronic retinal conditions. The eye drop formulation partially solves this problem but requires cold storage and frequent dosing.[8] PACAP is also a pleiotropic peptide affecting immune function, vasodilation, glucose metabolism, pain signaling, and circadian rhythms. This breadth of action raises safety concerns that single-target drugs do not face.
Pharmaceutical interest in PACAP has been directed toward migraine, where anti-PACAP antibodies represent a clearer commercial opportunity than neuroprotective eye drops. The retinal application, while scientifically compelling, serves smaller patient populations and faces competition from established anti-VEGF therapies that address the vascular (though not the neurodegenerative) component of diseases like diabetic retinopathy.
Most PACAP retinal studies use acute injury models (ischemia-reperfusion, excitotoxic injection) that do not perfectly recapitulate the chronic, slowly progressive neurodegeneration seen in human glaucoma or diabetic retinopathy. The streptozotocin diabetic rat develops retinal changes over weeks, while human diabetic retinopathy evolves over years to decades. Bridging from acute protection to chronic disease prevention requires fundamentally different study designs, longer observation periods, and sustained drug delivery.
The field has accumulated sufficient preclinical data to support a phase I safety trial of PACAP eye drops in a retinal condition. The stability data from Kovacs et al. (2021) provide formulation groundwork.[8] The Van et al. (2021) genetic evidence demonstrates cell-autonomous neuroprotection through a specific receptor.[5] What remains absent is the clinical investment to bridge from animal models to human patients.
Other peptide approaches to retinal neuroprotection have made more clinical progress. CNTF (ciliary neurotrophic factor) has been tested in sustained-release intravitreal implants in human trials for retinitis pigmentosa and geographic atrophy. GLP-1 receptor agonists, already approved for diabetes, have shown retinal neuroprotective effects in preclinical models that may eventually justify ophthalmologic investigation. Somatostatin eye drops have been tested in a European clinical trial for diabetic retinopathy. PACAP's path to the clinic will likely depend on whether the migraine-focused investment in PACAP pharmacology produces delivery tools and safety data that can be repurposed for ocular applications.
For a broader perspective on how different peptides are being studied for retinal diseases, including CNTF and other neurotrophic factors, see CNTF and the Retina: A Neurotrophic Peptide for Degenerative Eye Disease.
The Bottom Line
PACAP is among the most thoroughly studied neuroprotective peptides in retinal research, with consistent evidence of protection against diabetic, ischemic, excitotoxic, and pressure-related retinal damage across animal models. The peptide can be delivered as eye drops that cross ocular barriers, and genetic studies confirm that its PAC1 receptor plays a homeostatic role in maintaining retinal ganglion cell health. The gap between preclinical promise and clinical reality remains wide: no human trial has tested PACAP for retinal disease, and pharmaceutical attention has been directed toward migraine rather than ophthalmology.