Zebrafish stress-signaling peptide (PACAP2 / zfPACAP2)
A natural zebrafish brain peptide related to human stress hormones; used in lab studies of fear, anxiety, and pain, a research tool, not an approved drug.
- Class
- Neuropeptide / pituitary signaling peptide (non-mammalian isoform)
- Status
- No approved therapeutic status; research-only zebrafish peptide
- Best-supported effect
- Biological characterization in zebrafish ovary (single published study); no mammalian or human efficacy data attached
- Main caveat
- Zebrafish-specific isoform; not equivalent to mammalian PACAP-38 or PACAP-27. No mammalian animal-model, in vitro, or human data in source file.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
Pituitary adenylate cyclase-activating peptide 2 (PACAP2, also called zfPACAP2) is the zebrafish form of a highly conserved neuropeptide that signals through the PAC1 receptor — a stress-circuit protein implicated in fear, anxiety, and pain processing. It is one of two PACAP variants that teleost fish carry as a result of an ancient whole-genome duplication; the other is zebrafish PACAP1. The two isoforms arose from a common ancestral PACAP gene and share about 82% sequence identity with each other (Wang and colleagues, 2019). Mammalian PACAP — which has remained virtually unchanged across 80–100 million years of evolution — was first identified in 1989 by Arimura and colleagues from ovine hypothalamus as a novel peptide that potently activated adenylate cyclase in pituitary cells. The zebrafish PACAP2 variant was first cloned and characterized from zebrafish ovary by Wang, Wong, and Ge (Wang and colleagues, 2003).
History
The parent PACAP family was discovered in 1989 in Arimura's laboratory during a search for a follicle-stimulating hormone-releasing factor; a fraction of ovine hypothalamus was found to powerfully activate adenylate cyclase in rat anterior pituitary cells, and the responsible 38-residue peptide was isolated and sequenced. Mammals express PACAP as two biologically active forms — PACAP-27 and PACAP-38 — that share an identical N-terminal sequence but differ at the C-terminus. The zebrafish-specific PACAP2 isoform, encoded by the adcyap1b gene, was described in 2003 when Wang, Wong, and Ge cloned it from zebrafish ovary and demonstrated that it is a distinct nonmammalian vertebrate variant with only about 64% precursor homology to the other zebrafish PACAP (zfPACAP38-1) (Wang and colleagues, 2003). Subsequently, brain distribution studies confirmed that adcyap1b is the dominant PACAP gene in the zebrafish central nervous system, with expression levels roughly 15-fold higher than those of adcyap1a in whole brain, and highest in the telencephalon and diencephalon (Wang and colleagues, 2019; Nakamachi and colleagues, 2018).
What it does
PACAP2 acts on the PAC1 receptor (encoded by adcyap1r1), a class B G protein-coupled receptor that is highly expressed in brain regions controlling stress and emotion. When PACAP binds PAC1R, it boosts levels of a molecular messenger called cyclic AMP, which in turn activates protein kinase A and a cascade of downstream signaling. The receptor also drives a separate pathway through ERK kinase after the receptor is pulled inside the cell (a process called internalization), and this endosomal ERK route appears to be the primary signal controlling neuronal excitability in hippocampal dentate gyrus granule cells (Johnson and colleagues, 2020). In zebrafish, intracerebroventricular injection of PACAP2 suppresses food intake, demonstrating a conserved anorexigenic function shared with mammalian PACAP (Wang and colleagues, 2019). Brain mapping studies using immunostaining show PACAP2-positive cells and fibers distributed across the telencephalon, hypothalamus, and cerebellum, with a distinctive fiber tract projecting toward the habenula — a pattern consistent with roles in higher brain functions, feeding, and emotional regulation (Nakamachi and colleagues, 2018).
Evidence
- Human: Human studies of the PAC1 receptor system — not of zfPACAP2 itself — provide the strongest translational context. Ressler and colleagues (2011) reported in Nature that a single-nucleotide polymorphism in the PAC1 receptor gene (rs2267735, in a putative estrogen response element of ADCYAP1R1) significantly predicted PTSD diagnosis and hyperarousal symptoms in females (N=1,237 combined analysis; effect absent in males). PACAP-38 infusion has been shown to induce migraine-like attacks in patients with migraine without aura, supporting PAC1R as a target in primary headache (Liao and colleagues, 2019; reviewed in Moody and colleagues, 2016).
- Animal: In zebrafish, both PACAP1 and PACAP2 suppressed food intake when injected intracerebroventricularly, demonstrating conserved anorexigenic function (Wang and colleagues, 2019). PAC1 receptor knockout mouse models display impaired contextual fear conditioning and novel object recognition, implicating the PACAP–PAC1R axis in memory consolidation (Hashimoto, 2002). The PACAP–PAC1R system modulates fear acquisition, recall, and extinction in a sexually dimorphic manner in rodent models, with amygdala and hippocampal circuits playing key roles (King and colleagues, 2017).
- In vitro: PACAP-induced PAC1 receptor internalization and recruitment of MEK/ERK signaling was demonstrated in hippocampal dentate gyrus granule cells; blocking receptor internalization or MEK activity eliminated PACAP's enhancement of neuronal excitability (Johnson and colleagues, 2020).
Known effects
- Anorexigenic (appetite suppression) — Demonstrated in zebrafish intracerebroventricular injection studies (Wang and colleagues, 2019); analogous effects described across vertebrates.
- Neuronal excitability modulation — PAC1R-driven MEK/ERK endosomal signaling enhances dentate gyrus granule cell firing; Preclinical/mechanistic (Johnson and colleagues, 2020).
- Fear and stress circuit regulation — PAC1R activation modulates fear acquisition, generalization, and extinction; Preclinical (multiple rodent studies; King and colleagues, 2017).
- Neuroprotection — Mammalian PACAP has been studied for neuroprotective roles across multiple models; relevance of zfPACAP2 specifically is inferred by sequence conservation and receptor binding.
- Reproductive signaling — The 2003 discovery study showed zfPACAP2 stimulates oocyte maturation and follistatin expression in zebrafish follicle cells (Wang and colleagues, 2003).
Safety signals
No safety data specific to synthesized zfPACAP2 peptide is available in the published literature. The broader PACAP family is an endogenous neuropeptide conserved across vertebrates; adverse-event profiles arise in the context of pharmacological manipulation of the PAC1R system rather than from the endogenous ligand itself. PACAP-38 infusion in migraine patients produces transient vasodilatory effects and headache, consistent with PAC1R-mediated cAMP signaling in cranial vasculature (reviewed in Moody and colleagues, 2016). A monoclonal PAC1 receptor antibody (targeting the therapeutic application) failed in a proof-of-concept migraine prevention trial, suggesting that receptor-level antagonism carries its own complexity.
Regulatory status
- US: Not an approved drug. No IND or NDA. Research compound only.
- EU: Not approved. Research use only.
- WADA: Not currently listed on the WADA prohibited list as a named substance; PACAP analogs with growth factor or hormonal activity could fall under general peptide hormone categories depending on mechanism.
Mechanism
PACAP2 binds the PAC1 receptor (ADCYAP1R1), a class B secretin-family GPCR that couples to both Gαs and Gαq proteins. Gαs coupling activates adenylyl cyclase, raising intracellular cAMP and activating protein kinase A (PKA) and EPAC1/2; Gαq coupling activates phospholipase C (PLC), generating IP3 and diacylglycerol and mobilizing calcium. A third, β-arrestin–mediated pathway operates after receptor internalization: endosomal PAC1R recruits MEK/ERK signaling independently of surface cAMP or PLC cascades. In hippocampal dentate gyrus granule cells, this endosomal MEK/ERK route is the primary mechanism by which PACAP regulates intrinsic neuronal excitability (Johnson and colleagues, 2020; May and colleagues, 2021). The PAC1 receptor also activates Akt, contributing to neuroprotective and anti-apoptotic signaling (Liao and colleagues, 2019).
Zebrafish PACAP2 is a 38-residue peptide whose mature C-terminal amide form is not represented in the raw stored sequence (HSDGIFTDIYSRYRKQMAVKKYLAAVLGRRYRQRVKNK) — the active peptide carries a C-terminal amide that protects against carboxypeptidase degradation, as is standard for the PACAP family. The sequence is the zebrafish-specific adcyap1b product and differs from human/mammalian PACAP-38 at several positions, particularly in the C-terminal extension, while retaining the N-terminal pharmacophore required for PAC1R engagement.
Related peptides
- Zebrafish PACAP1 (zfPACAP1, encoded by adcyap1a) — the paralogue arising from teleost whole-genome duplication; ~82% identical to PACAP2 but expressed at lower levels in zebrafish brain and enriched in brainstem rather than telencephalon.
- Human/mammalian PACAP-38 — the evolutionarily conserved form that served as the original target of PAC1R research; virtually identical to PACAP-27 at the N-terminal pharmacophore; the reference ligand for most PAC1R mechanistic and clinical studies cited in this card.
- VIP (vasoactive intestinal peptide) — a structurally related class B GPCR neuropeptide that shares VPAC1 and VPAC2 receptors with PACAP but does not bind PAC1R with high affinity; reviewed alongside PACAP in Moody and colleagues (2016).
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Could this peptide, originally discovered in fish ovaries, help control follicle development or ovulation in mammals?
If true, it could inspire a new class of fertility medicines that act more specifically in the reproductive system, reducing neurological side effects that have held back similar peptides. Couples struggling with infertility could benefit from more targeted hormonal treatments.
Could the strongly positive-charged tail of this peptide be used to carry drug molecules directly into cells?
If the cationic tail of this natural fish peptide can sneak molecules through cell membranes, it could form the basis of a safer drug-delivery system that avoids immune problems caused by current synthetic carriers. This would help patients who need drugs delivered inside specific cells, such as those with certain cancers or genetic diseases.
▸full evidence table2 metrics
| metric | value | tool |
|---|---|---|
| ipTM | 0.754585862159729 | openfold3-mlx |
| ranking score | 0.8404658436775208 | openfold3-mlx |
▸structural qualityopenfold3
| metric | value | note |
|---|---|---|
| gpde | 0.841 | global PDE — lower = better |
| disorder | 0.196 | fraction disordered |
| chain pair ipTM (A, B) | 0.755 | interface quality |
▸3-letter notation
▸recipeopenfold3-mlx 0.3.1
| parameter | value |
|---|---|
| model | openfold3-mlx 0.3.1 |
| weights | aedd8f3eb814e392… |
| hardware | apple_m4_base_16gb |
| mlx version | 0.31.1 |
| python | 3.14.3 |
| random seed | 42 |
| msa strategy | colabfold |
| diffusion samples | 1 |
| runtime | 449s |
| predicted by | mlx@peptide |
| predicted at | 2026-04-23 |
python3 openfold3/run_openfold.py predict --query_json {query.json} --runner_yaml examples/example_runner_yamls/mlx_runner.yml --output_dir {output_dir} --num_diffusion_samples 1 ▸citationbibtex
@peptide{pep10584,
sequence = {HSDGIFTDIYSRYRKQMAVKKYLAAVLGRRYRQRVKNK},
target = {pac1r},
author = {peptidemodel},
year = {2026},
status = {synthesized}
}