Blood-Brain Barrier (BBB)

The blood-brain barrier is not a membrane but a property of the brain's capillary endothelium. Those cells are joined by continuous tight junctions, have almost no fenestrations and little vesicular transport, and sit within a neurovascular unit of pericytes and astrocyte end-feet. Layered on that is an active component: efflux transporters, principally P-glycoprotein and BCRP, that pump substrates back into blood. What does cross uses defined routes, chiefly carrier-mediated transport for glucose and amino acids, or receptor-mediated transcytosis at the transferrin and insulin receptors.

For peptides the practical answer is that almost nothing crosses in useful quantity. Antibody uptake into brain is on the order of a tenth of a percent of the plasma concentration, which is why brain-targeted biologics are engineered as transferrin-receptor shuttles rather than simply dosed higher. Some peptide hormones do reach the brain through saturable transporters, or through circumventricular organs such as the area postrema, where the barrier is anatomically absent.

A plasma concentration therefore tells you nothing about brain exposure. A central claim needs evidence of its own: cerebrospinal fluid sampling, an unbound brain measurement, receptor occupancy imaging, or a pharmacodynamic readout that only a central action could produce.

The recurring error is treating a central effect as proof of crossing. Vagal afferent signalling and circumventricular access both produce genuine brain effects from a molecule that never enters the parenchyma. In the other direction, rodent radiolabel studies get read as penetration when the counts may represent tracer trapped in cerebral vasculature or a free label cleaved off the peptide, and injury or anaesthesia models with a leaky barrier exaggerate what an intact human barrier would allow.