Intranasal peptide delivery — why the nose-to-brain route matters
The published case for intranasal administration as the dominant route in nootropic peptide research, and the limits of the nose-to-brain pathway.
Small peptides like Semax, Selank, and DSIP face two obstacles to central nervous system action: rapid plasma proteolysis after parenteral administration, and the difficulty of crossing the blood-brain barrier. The intranasal route addresses both at once. Peptides applied to the nasal mucosa can be transported directly to the CNS via the olfactory and trigeminal nerve pathways, bypassing the bloodstream almost entirely.
The mechanism is anatomically well-characterised. Olfactory receptor neurons project directly from the nasal epithelium into the olfactory bulb, providing a continuous extracellular pathway from the nose to the forebrain. Trigeminal nerve endings in the nasal mucosa provide a parallel route to the brainstem. Both pathways have been visualised in animal studies using radiolabelled peptides.
The route is not without limits. Bioavailability is sub-parenteral — most studies estimate only a small fraction of the applied dose reaches the brain. Inter-individual variability is high, depending on nasal mucosa condition, vehicle formulation, and head positioning during administration. Volume per dose is constrained by the small surface area of the olfactory epithelium.
These limitations explain why intranasal protocols typically use small, repeated doses rather than single large administrations, and why peptide stability in the vehicle (saline pH, preservative choice, refrigeration) matters as much as the dose itself for reproducibility of research effects.