How Nasal Peptides Reach Your Brain
Inhaled & Intranasal Peptide Delivery
3 min
Time for a CPP-modified GLP-2 derivative to reach the trigeminal sensory nucleus after intranasal dosing in mice, confirming rapid extracellular nerve transport.
Akita et al., Journal of Controlled Release, 2021
Akita et al., Journal of Controlled Release, 2021
View as imageA peptide sprayed into your nose can reach your brain within minutes, completely bypassing the blood-brain barrier. That single fact has reshaped how researchers think about treating neurological diseases with peptide drugs. The nose-to-brain delivery route exploits two cranial nerve pathways, the olfactory and the trigeminal, to shuttle molecules from the nasal mucosa directly into the central nervous system. For peptides that are too large, too fragile, or too polar to cross the blood-brain barrier on their own, this route offers a non-invasive alternative to direct brain injection.[1] If you are looking for a broader overview of nasal peptide formulations, our guide to intranasal peptide delivery covers the full landscape. This article focuses specifically on how peptides make the journey from nose to brain, what evidence supports each transport route, and where the science still falls short. For even broader context on pulmonary and nasal drug delivery, see our pillar on inhaled peptide drugs.
Key Takeaways
- A GLP-2 derivative conjugated with a cell-penetrating peptide reached the trigeminal sensory nucleus within 3 minutes of intranasal dosing in mice (Akita et al., Journal of Controlled Release, 2021)
- The first-in-human PET imaging study of intranasal oxytocin detected brain uptake between 25-45 minutes, but levels were low and variable across individuals (Winterdahl et al., EJNMMI Research, 2025)
- In rodents, the olfactory epithelium covers 40-50% of nasal surface area; in humans, it covers less than 10%, a major translational barrier (Maeng and Lee, Frontiers in Pharmacology, 2022)
- Intranasal nerve growth factor increased CNS concentrations 10-45 fold compared to systemic delivery in preclinical models (Meredith et al., The AAPS Journal, 2015)
- Cell-penetrating peptides boosted nose-to-brain delivery of GLP-2 enough to produce antidepressant-like effects at doses that failed when given intravenously (Akita et al., 2021)
- Nasal mucociliary clearance eliminates most deposited material within 15-20 minutes, limiting the absorption window for unprotected peptides (Khan et al., Expert Opinion on Drug Delivery, 2024)
Two Routes from Nose to Brain
The nasal cavity connects to the brain through two cranial nerve pathways. The olfactory nerve (cranial nerve I) extends from the olfactory epithelium in the upper nasal cavity through the cribriform plate to the olfactory bulb. The trigeminal nerve (cranial nerve V) innervates the respiratory epithelium across the broader nasal mucosa and connects to the brainstem via the trigeminal ganglion.[2]
These two pathways differ in speed, mechanism, and the brain regions they access. The olfactory route delivers substances to the olfactory bulb and, from there, to regions including the hippocampus and cortex. The trigeminal route delivers to the brainstem and pons. Both pathways offer transport via three distinct mechanisms: intracellular axonal transport (slow, taking hours to days), extracellular perineural transport (fast, reaching the brain within minutes), and transcellular transport through supporting cells.[1]
The extracellular pathway appears to dominate for most peptides. Molecules travel along the perineural space surrounding olfactory and trigeminal nerve bundles, which connects directly to the cerebrospinal fluid (CSF) in the subarachnoid space. This explains why some intranasally administered peptides appear in the CSF within minutes of dosing.[3]
The Olfactory Pathway: Fast but Anatomically Limited
The olfactory epithelium is the only place in the body where neurons are directly exposed to the external environment. Olfactory receptor neurons send axonal projections (called fila olfactoria) through perforations in the cribriform plate to synapse in the olfactory bulb. Substances deposited on the olfactory mucosa can hitch a ride along these axons or travel through the extracellular spaces between them.[3]
In animal models, this pathway is remarkably efficient. Meredith and colleagues (2015) documented that intranasal delivery of nerve growth factor produced 10-45 fold higher CNS concentrations compared to systemic routes in rodents. Fibroblast growth factor reached the olfactory bulb within 30 minutes and distributed to deeper brain regions over 4 hours.[1]
The problem is anatomy. In rodents, the olfactory epithelium covers 40-50% of total nasal surface area. In humans, it occupies less than 10%, and it is tucked into the uppermost recess of the nasal cavity where standard nasal sprays deposit poorly.[2] This species difference is the single biggest obstacle to translating nose-to-brain peptide delivery from animal models to human therapeutics. The vascular density in the olfactory region is lower than in the respiratory region, which paradoxically correlates with higher brain delivery efficiency because less drug is absorbed into the bloodstream and diverted away from the neural pathways.[2]
The Trigeminal Pathway: Slower, but Larger Target Area
The trigeminal nerve, the largest cranial nerve, innervates the entire respiratory epithelium of the nasal cavity. Its ophthalmic and maxillary branches form a dense network across the nasal mucosa that standard nasal sprays reach easily. Substances absorbed by or around these nerve endings travel along trigeminal axons to the trigeminal ganglion and then to the brainstem.[4]
Transit times along the trigeminal pathway are generally longer than olfactory transport for slow axonal mechanisms.[1] But the extracellular perineural route along trigeminal nerves can be far faster. Akita and colleagues (2021) demonstrated that a GLP-2 derivative conjugated with a cell-penetrating peptide and penetration accelerating sequence reached the trigeminal principal sensory nucleus within approximately 3 minutes of intranasal administration in mice.[5] The same group later showed that GLP-1 derivatives with functional sequences also transit and migrate through trigeminal neurons after nasal dosing.[6]
This matters because the trigeminal pathway may be the more clinically relevant route in humans. Given that the human respiratory epithelium vastly exceeds the olfactory region in surface area, and conventional nasal sprays preferentially deposit on respiratory mucosa, the trigeminal nerve network receives the majority of intranasally administered drug.
What Happens in Humans: The PET Imaging Evidence
Most nose-to-brain transport data comes from animal models. A landmark 2025 study from Winterdahl and colleagues changed that by using positron emission tomography (PET) with [13N]-labeled oxytocin to track intranasal peptide distribution in six healthy human volunteers in real time.[7]
PET/MRI scans showed high tracer concentration in the nasal cavity within the first 5 minutes, followed by gradual decline as the peptide was absorbed. Brain uptake was detectable between 25-45 minutes post-administration. The trigeminal ganglia showed measurable signal, consistent with trigeminal nerve transport.[7]
The results, however, were sobering. Brain uptake was low and highly variable between individuals. There was no clear dose-response relationship for trigeminal or brain regions. Systemic absorption accounted for the majority of the administered dose. This does not disprove nose-to-brain transport in humans, but it demonstrates that the efficiency observed in rodents does not translate directly.[7]
The study used standard nasal spray delivery, which predominantly deposits drug on the lower and middle nasal turbinates, far from the olfactory cleft. Targeted delivery devices that direct aerosol to the upper nasal cavity may produce different results. The short half-life of the 13N label (approximately 10 minutes) also limited the imaging window, potentially missing delayed transport.
Barriers That Degrade Peptides Before They Arrive
Even when the anatomical pathway exists, nasal peptide delivery faces three biological hurdles that reduce the amount of intact drug reaching the brain.[4]
Mucociliary clearance is the first and most unforgiving barrier. The nasal epithelium is coated in a mucus blanket propelled by cilia toward the nasopharynx at approximately 5-6 mm per minute. Most deposited material is swept to the throat and swallowed within 15-20 minutes. This sets a narrow absorption window: any peptide that has not been absorbed by then is lost.[4]
Enzymatic degradation is the second barrier. The nasal mucosa contains aminopeptidases, carboxypeptidases, and other proteases that cleave peptide bonds. Insulin, for instance, is rapidly degraded by nasal enzymes, explaining why simple intranasal insulin formulations have low bioavailability without protective excipients.[1]
Low epithelial permeability is the third. Tight junctions between nasal epithelial cells restrict paracellular transport of hydrophilic molecules larger than approximately 1 kDa. Most therapeutic peptides exceed this size, requiring active transport mechanisms or permeation-enhancing formulations to cross the epithelium.[3]
Cell-Penetrating Peptides: Solving the Transport Problem
Cell-penetrating peptides (CPPs) have emerged as one of the most effective tools for enhancing nose-to-brain peptide delivery. These short, typically cationic peptide sequences (such as TAT, penetratin, and polyarginine) facilitate the translocation of cargo molecules across cell membranes through a combination of direct penetration and endocytosis.[8]
The evidence for CPP-enhanced nose-to-brain delivery is substantial. Akita and colleagues (2021) conjugated GLP-2 with a cell-penetrating peptide and a penetration accelerating sequence (PAS). The resulting PAS-CPP-GLP-2 construct showed antidepressant-like effects within 20 minutes of intranasal administration in mice, at the same dose that was effective by direct intracerebroventricular injection. Intravenous delivery of the same dose was completely ineffective. Fluorescent imaging confirmed that the modified peptide traveled through trigeminal neurons via transcellular transport rather than the extracellular pathway used by unmodified peptides.[5]
Maeng and Lee (2022) reviewed CPP applications for nasal antidiabetic peptide delivery, documenting that CPP conjugation improved brain delivery of both insulin and exendin-4 through the nasal route. The CPP-modified peptides achieved systemic and central effects that unmodified versions could not match when given intranasally.[2]
De Martini and colleagues (2023) catalogued CPP applications specifically for nose-to-brain delivery of biological drugs, finding that CPPs address all three major barriers to nasal peptide absorption simultaneously: they protect cargo from enzymatic degradation, promote membrane translocation, and facilitate axonal transport.[8]
Hong et al. (2025) published a comprehensive update on advances in CPP-based nose-to-brain delivery systems, documenting that CPP conjugation improved brain bioavailability of various peptide therapeutics by 2- to 10-fold compared to unconjugated peptides in animal models.[9]
Nanocarriers and Advanced Formulations
Beyond CPPs, nanotechnology-based formulations offer additional strategies for nose-to-brain peptide delivery. Nanoencapsulation protects peptides from enzymatic degradation, promotes mucoadhesion (extending contact time with the nasal epithelium), and can be engineered to target specific cell types.[3]
Majie and colleagues (2026) reviewed the latest intranasal peptide delivery systems specifically designed for Alzheimer's disease management. Their analysis covered nanoparticles, liposomes, nanoemulsions, and thermosensitive gels, all formulated to deliver amyloid-targeting and neuroprotective peptides from nose to brain. The field has advanced from simple nasal solutions to sophisticated multi-component systems that address mucociliary clearance, enzymatic degradation, and epithelial permeability simultaneously.[10]
Han et al. (2025) delivered acyl-ghrelin conjugated to gold nanoparticles intranasally for neurodegenerative disease treatment. The ghrelin-gold nanoconjugates reached the brain and demonstrated neuroprotective effects in an animal model, with the nanoparticle formulation improving stability and brain uptake compared to free ghrelin peptide.[11]
Particle size matters for deposition targeting. In 3D human nasal models, particles of 8-12 micrometers showed higher olfactory region deposition than 2 micrometer particles. Nanoparticles of 520 nm demonstrated transcellular neuronal transport. Very small particles (1-2 nm) showed the highest olfactory deposition via Brownian motion diffusion. This bimodal size-deposition relationship suggests that optimal formulations may need to balance olfactory targeting with neuronal uptake.[2]
Peptides Already Tested Via the Nose-to-Brain Route
The list of therapeutic peptides investigated for intranasal nose-to-brain delivery is growing. Khan and colleagues (2024) documented that the pathway has been studied for exendin-4, leptin, orexin, vasoactive intestinal peptide, galanin-like peptide, interferon-beta, insulin-like growth factor, nerve growth factor, and brain-derived neurotrophic factor, among others.[4]
Neuropeptide Y (NPY) has shown particular promise for psychiatric applications. Sabban and Serova (2018) reviewed evidence that intranasal NPY reversed anxiety-like and depressive-like behavior in rodent models of traumatic stress, including models relevant to PTSD. NPY is a 36-amino-acid peptide too large to cross the BBB efficiently through systemic circulation, making the nose-to-brain route attractive for targeting its central receptors.[12]
The GLP-1 receptor agonist family is a major focus. Khan and colleagues (2024) reviewed the case for intranasal GLP-1 delivery to brain appetite centers, noting that subcutaneous GLP-1 drugs must cross the blood-brain barrier to reach their central targets and that nasal delivery could potentially achieve higher brain concentrations with lower systemic exposure and fewer gastrointestinal side effects.[4]
Hatakawa et al. (2025) achieved efficient nose-to-brain delivery of JAL-TA9, a nine-residue peptide with hydrolytic activity against amyloid-beta plaques. The peptide reached the brain in sufficient concentrations to demonstrate amyloid-beta cleavage activity, supporting the feasibility of intranasal peptide therapeutics for Alzheimer's disease.[13]
The Translation Gap: Animals vs. Humans
The single biggest limitation in this field is the species difference in nasal anatomy. Nearly all the compelling nose-to-brain delivery data comes from rodent studies, where the olfactory epithelium represents 40-50% of nasal surface area. In humans, that figure drops below 10%, and the olfactory region sits in a narrow, hard-to-reach cleft at the top of the nasal cavity.[2]
The Winterdahl (2025) PET study confirmed that human brain uptake after intranasal oxytocin is low and variable.[7] This does not mean the pathway is non-functional in humans, but it sets realistic expectations: the dramatic brain concentrations seen in mouse studies are unlikely to be replicated in humans without major formulation advances.
Several strategies are being tested to close this gap. Specialized delivery devices that target the olfactory cleft (such as breath-powered bidirectional devices) improve upper nasal deposition. Mucoadhesive formulations extend contact time beyond the 15-20 minute mucociliary clearance window. CPP conjugation and nanoencapsulation improve transcellular transport efficiency. Whether these approaches can collectively overcome the anatomical disadvantage remains an open question.
The Bottom Line
Nose-to-brain transport of peptides is real, well-characterized in animal models, and supported by emerging human imaging data. Two cranial nerve pathways, olfactory and trigeminal, provide direct routes from the nasal cavity to the brain that bypass the blood-brain barrier. The trigeminal pathway may be more relevant in humans given the limited olfactory surface area. Formulation advances, particularly cell-penetrating peptides and nanocarriers, have dramatically improved delivery efficiency in preclinical models. The critical unanswered question is whether these improvements can overcome the anatomical constraints of the human nasal cavity to produce clinically meaningful brain concentrations of therapeutic peptides.