Cognitive decline with age is one of the most studied problems in biology, yet researchers have not identified a treatment that directly targets the underlying processes inside the brain. One candidate that keeps appearing in the scientific literature is GHK-Cu, a small peptide the body produces naturally by combining three amino acids with a copper ion. Levels of this peptide fall as people age, which has led researchers to ask whether restoring it might slow some aspects of brain aging.
A 2026 preprint from Mazzola, Jordan, and colleagues added a new layer to that question. Instead of simply asking whether GHK-Cu does something in aged mice, the team asked whether it matters how the peptide gets in. They compared two delivery routes, injection into the abdominal cavity and a spray delivered directly into the nose, in mice that were already well into middle age. What they found was striking: both approaches improved the animals' ability to navigate a spatial learning task, but the molecular changes inside the hippocampus, the brain region most associated with memory and navigation, looked almost nothing alike.
That result has broad implications for how researchers design gerotherapeutic studies, a term the authors use for treatments aimed at the biology of aging itself. It also raises a practical point that is easy to overlook: the same molecule, at the same dose, can engage completely different biological programs depending on how it reaches its target tissue.
Study design and animals
The researchers used C57BL/6J mice aged 20 to 21 months, which in mouse terms corresponds roughly to late middle age. One group received GHK-Cu by intraperitoneal injection at 15 mg/kg for five days, a short and relatively intense exposure. A second group received the same dose intranasally over eight weeks, a longer and more gradual course. Both male and female animals were included in each group, which turned out to be important because some effects were sex-dependent.
Hippocampal-dependent learning was measured using a spatial navigation task that required the mice to remember the location of an escape platform across repeated trials. The team then examined hippocampal tissue with immunohistochemistry, which uses antibodies to visualize specific proteins, and with bulk RNA sequencing, which reads out the activity levels of thousands of genes at once. Statistical significance was assessed with false discovery rate correction to reduce the chance of false positives.
Behavioral results
Intranasal delivery produced the more consistent behavioral improvement. Mice in that group showed faster escape latency, meaning they found the platform more quickly, across trials two through four, and this held for both males and females. The effect was statistically significant at a threshold below 0.05.
Intraperitoneal injection produced a narrower result. Male mice showed a transient improvement during the second trial, but the effect did not persist into later trials and was not observed in females at all. The researchers noted this as a meaningful difference, not just because one route looked better than the other, but because the same underlying behavioral outcome, improved spatial learning, appeared to emerge from very different biological starting points.
Protein-level changes in the hippocampus
Immunohistochemistry revealed that the two delivery routes left distinct molecular fingerprints at the protein level. Intranasal treatment increased synaptophysin, a marker associated with synaptic density, in female mice. Both sexes showed a reduction in GFAP, a protein that tends to accumulate in activated glial cells and is often elevated during neuroinflammation and aging.
Intraperitoneal treatment produced a different pattern. In males, TGF-beta, GFAP, and MCP-1 all declined. TGF-beta is a signaling molecule involved in inflammation and tissue remodeling, while MCP-1 is a chemokine associated with immune cell recruitment. In females receiving the intraperitoneal route, p21, a protein that promotes cell cycle arrest and is tied to cellular senescence, dropped to a statistically significant degree. Senescence is a state in which cells stop dividing and begin secreting inflammatory signals, and reducing p21 is one marker researchers use to assess changes in that process.
Transcriptomic findings
The RNA sequencing data is where the divergence between the two routes became most vivid. Gene set enrichment analysis, a method that looks at coordinated changes across whole biological pathways rather than single genes, showed nearly opposite patterns depending on how the peptide was delivered.
Intranasal GHK-Cu suppressed oxidative phosphorylation, the mitochondrial process by which cells generate energy, with strong effect sizes in both males and females. It also suppressed MYC target genes in females and attenuated PI3K-AKT-mTOR signaling, a growth-regulating pathway that many aging researchers consider a central driver of biological aging. The consistent direction of these changes, toward suppression of metabolic and growth signaling, is a pattern associated in the broader literature with slower biological aging.
Intraperitoneal GHK-Cu moved in the opposite direction. It activated oxidative phosphorylation in females, activated DNA repair pathways, and increased MYC target gene expression. The researchers interpreted this as a signature of acute stress response and repair, consistent with a short, high-intensity exposure that prompts the cell to patch damage rather than reset its metabolic programs. Both signatures were associated with cognitive improvement, which is the core puzzle the paper leaves for future research to untangle.
What the divergence suggests
The authors concluded that functional improvement in hippocampal learning can arise from at least two distinct molecular states. One state, seen after intranasal delivery, looks like a sustained recalibration of metabolic and growth signaling toward patterns more commonly associated with younger tissue. The other state, seen after intraperitoneal injection, looks like a mobilization of repair machinery in response to an acute stimulus.
Neither pathway is necessarily better in a simple sense. The intraperitoneal route produced behavioral effects within five days, which is a remarkably short window. The intranasal route produced broader and more durable behavioral effects over eight weeks. Whether those trajectories would converge or diverge further with longer follow-up is a question the study was not designed to answer.
The sex differences the team observed also deserve attention. Several effects, both behavioral and molecular, appeared only in males or only in females, or differed substantially in magnitude between sexes. This is a pattern that appears repeatedly in peptide research but is often underreported in studies that do not stratify their analyses by sex. Including both sexes and reporting results separately is a methodological strength of this work.
Limitations and open questions
The study was conducted in mice, and the degree to which these findings translate to human biology is not established. The researchers used a single strain of mouse and a specific age window, so it is not clear whether the same route-dependent divergence would appear at other life stages or in other genetic backgrounds. The intranasal and intraperitoneal protocols also differed in duration, five days versus eight weeks, which means duration and route are confounded. A future study holding one variable constant while changing the other would help separate those effects.
The paper was posted as a preprint on Research Square, meaning it had not yet completed formal peer review at the time of publication. The molecular findings are complex and come from bulk RNA sequencing rather than single-cell sequencing, so the specific cell types driving the transcriptomic changes remain unresolved. Despite these limitations, the study offers a detailed and carefully analyzed dataset that advances the understanding of how GHK-Cu interacts with the aging hippocampus and why delivery context may be as important as the molecule itself.
