The Barrier at the Door

Blood vessels in the brain are not like those elsewhere. Their cells are sealed together with tight junctions, supported by neighboring cells called astrocytes, and equipped with pumps that eject unwanted molecules. The result is the blood-brain barrier.

What gets through

Small, fat-soluble molecules cross easily, which is why many psychiatric drugs are built that way. Others are carried through by dedicated transporters, such as the one that brings in glucose.

Most peptides are poor candidates. They are larger, more polar and easily broken down by enzymes in blood and in the barrier itself. For many natural peptides the amount reaching the brain after a peripheral dose is a tiny fraction of what circulates.

Ways around the wall

Researchers explore several strategies:

• Receptor-mediated transport: attaching a peptide to a molecule that uses a natural carrier, such as an antibody fragment aimed at the transferrin receptor.
• Chemical changes that make a peptide more fat-soluble or resistant to enzymes.
• Direct delivery, for example into the fluid around the spinal cord, used only in specific clinical situations.

The intranasal question

Spraying a peptide into the nose is often described as a route "straight to the brain" along nerves in the nasal lining. Some peptides, including oxytocin, are studied this way. Animal work supports a pathway, but human data on how much reaches the brain, and whether it matters, are mixed. Results vary between studies and formulations.

Reading claims

If a product says a peptide "crosses the blood-brain barrier", ask for the measurement: in which species, how much, by which route, and was an effect on behavior or brain activity shown in people? A single mouse experiment is a start, not a conclusion.