The study shows that the sleep‑inducing peptide DSIP can move from the bloodstream into the brain in rats, especially a synthetic version called [D‑Ala3]‑DSIP, which gets into the brain even better. This means that taking DSIP by injection or possibly other peripheral routes could still affect brain function, not just stay in the blood.

An improved radioimmunoassay (RIA) for delta sleep-inducing peptide (DSIP) was used to study the permeability of the blood-brain barrier (BBB) to DSIP peptides. Although the synthetic analog [D-Ala3]-DSIP reacts fully in the assay, at least 8 of the 9 amino acids of the naturally occurring DSIP sequence are required for recognition by the antibody. A significant increase in DSIP-like immunoreactivity found in rat brain in one part of the study after peripheral injection of DSIP indicated, therefore, that some peptide crossed the BBB. The much higher levels found in brain tissue after intracarotid administration of [D-Ala3]-DSIP as compared with DSIP made it unlikely that non-specific leakage of peptide across the BBB was the only explanation. Furthermore, the findings after infusion with saline for one minute tended to rule out contamination of brain tissue by peptide remaining in blood. Thus, the results provide additional evidence that one of the ways peptides administered peripherally can influence the brain is by transport across the BBB essentially intact.