Article

Peptides and the blood–brain barrier: how research measures what gets in

Published 24 October 2026 · CognitivePepti Research Desk

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Prepared by: Research Desk. Sources are linked for verification.

Published 24 October 2026

At a glance

The question: whether peptides in the bloodstream can reach the brain, a point that was disputed until the 1980s
Current view: a review by Banks (2023) in Peptides describes three main routes: transmembrane diffusion, carriers and adsorptive transcytosis
Selectivity: transport systems can be highly specific, carrying one peptide but not a close relative (Kastin et al., 1999)
Two directions: some peptides are carried out of the brain as well as in, which helps control their levels in brain tissue (Banks, 2015)
Relevance to this site: stability in biological media, and whether a peptide reaches the brain at all, are open questions for Semax, Selank and DSIP research

Why the question was controversial

The blood–brain barrier (BBB) is formed mainly by the cells lining brain capillaries, which are sealed by tight junctions and restrict what passes from blood into brain tissue. Peptides are usually water-soluble, often charged and quickly broken down by enzymes, so for many years it was assumed that they could not cross it in meaningful amounts.

In a 2015 tribute in Peptides, Banks credits Abba J. Kastin as the first to propose that peptides could cross the BBB, the first to show that an endogenous peptide did so, and the first to describe a saturable transport system for peptides at the barrier. In a later lecture published in 2023, Banks describes the idea as "initially very controversial" before the transport work was accepted.

Three routes across the barrier

Banks (2023) groups the evidence into three main mechanisms.

Route	How it works	What decides it
Transmembrane diffusion	The peptide passes through the lipid membranes of the barrier cells without a carrier	Mainly lipid solubility, size, charge and hydrogen bonding
Saturable transport	A specific carrier moves the peptide across; the system can be saturated at high concentrations	The carrier's selectivity and how many carriers are present
Adsorptive transcytosis	The peptide binds to the cell surface, often through charge, is taken up in vesicles and released on the other side	Surface charge and binding to the barrier cells

The same review notes that a peripheral peptide can also influence brain function without crossing, for example by acting on nerves or on brain regions where the barrier is less tight.

Selectivity: small changes, different results

A 1999 paper in Brain Research by Kastin, Pan, Maness and Banks collected examples of how selective the transport systems are:

MIF-1 is transported, but Tyr-MIF-1 is not.
PACAP38 is transported, but the shorter PACAP27 is not.
Leptin enters by a saturable system, while several smaller feeding-related peptides tested (NPY, orexin A and B, MCH and others) did not.
EGF and TGF-alpha bind the same receptor, but only EGF entered by a rapid saturable system, which suggests receptors and transporters can be different proteins.

The authors also reported that many peptides cross by simple diffusion, explained mainly by their lipophilicity and other physicochemical properties. The practical point for researchers is that BBB passage cannot be inferred from sequence similarity or receptor binding; it has to be measured for each peptide.

Out of the brain as well as in

Transport runs both ways. Banks (2015) describes brain-to-blood transport as a way of controlling peptide levels within the brain, and as a route by which the brain can send signals to the periphery. The same review reports that these transporters are not fixed: they change with development and physiological state and can be altered in disease models. Kastin's 1999 paper gives an example from animal work, where saturable transport of TNF-alpha was increased in a model of multiple sclerosis and after spinal cord injury.

What this means for cognition-peptide research

The peptides covered on this site illustrate why BBB data matter.

Stability first. A peptide must survive long enough to reach the barrier. In a 2005 review, Ashmarin et al. argued that peptides ending in the Pro-Gly-Pro (glyproline) motif, including Semax and Selank, are considerably more stable in biological media than most regulatory peptides. A 2004 study by Dolotov et al. reported a half-life of more than one hour for Semax in the presence of rat brain membranes, with stepwise cleavage to shorter fragments.
Fragments complicate the picture. When a peptide is broken down into smaller pieces, a brain effect seen in an animal study may come from a fragment rather than the parent peptide. This is an open question in the Semax research record.
Early transport work on DSIP. One of the 1977 papers on delta sleep-inducing peptide examined transport of synthetic DSIP across the blood–brain barrier in rabbits (Monnier et al., 1977). See the DSIP research overview for the wider record.
Route of exposure in animal studies. Several rodent studies of Semax and Selank used intranasal application. The studies summarised on this site measured markers in brain tissue; they did not quantify how much intact peptide reached the brain.

For a side-by-side comparison of these compounds, see the cognition peptide research guide.

How researchers measure BBB passage

Transport studies in this field typically follow a labelled peptide. Labelled peptide (for example with radioactive iodine or tritium) is introduced into the blood of an animal, and its appearance in brain tissue is measured over time and compared with a marker that does not cross. Adding an excess of unlabelled peptide tests whether the route is saturable. Brain-to-blood transport is measured in the opposite direction, by introducing labelled peptide into the brain and following its loss. Each method has limits: radioactivity in brain tissue can come from breakdown products rather than intact peptide, so researchers check what form the label is in.

Open research questions

Intact peptide versus fragments. For short peptides such as Semax and Selank, how much of any brain signal comes from the parent sequence is unresolved.
Species differences. Most transport data come from mice, rats and rabbits, and carriers may differ between species.
Changes with disease models. Because transporters adapt, passage measured in healthy animals may not match passage in disease models.
Independent data for each compound. For the cognition-related peptides on this site, direct BBB transport measurements are sparse.

Limitations of the evidence

The main sources are reviews by researchers from one research school, written partly as historical tributes.
Transport studies are largely in animals, and results depend on the method and label used.
The examples of selective transport come from specific peptides and cannot be generalised to unrelated sequences.
None of the sources summarised here measured BBB passage of Semax or Selank directly.

Key terms

Blood–brain barrier (BBB). The barrier formed by brain capillary cells and their tight junctions, which limits what passes from blood to brain.
Saturable transport. Carrier-mediated movement that reaches a maximum rate when the carriers are fully occupied.
Adsorptive transcytosis. Uptake of a molecule into vesicles after it binds to the cell surface, followed by release on the far side of the cell.
Efflux. Transport from brain back into the blood.
Glyproline. A proline- and glycine-rich motif such as Pro-Gly-Pro.

Where researchers source it

Purita is a store that lists a Semax and Selank combination for laboratory research use only. Its product page labels the item for research use only and not for human or animal administration.

Purita product page

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