In newborn rats, a version of the sleep‑inducing peptide DSIP can be taken up through the gut and get into the blood, and some of it even reaches the brain. A relatively high oral dose (100 µg per animal) was needed for a measurable rise, and tiny amounts were also seen when the mother rat was given the peptide.

Entry of delta sleep-inducing peptide (DSIP) into the circulation from the gastrointestinal (GI) tract was studied in unweaned rat pups. The pups were fed an analog of DSIP (N-Tyr-DSIP) or 125I-N-Tyr-DSIP and blood samples collected. Significant increases in plasma DSIP-like immunoreactivity occurred after the feeding of 100 micrograms/animal of N-Tyr-DSIP but not after vehicle (normal saline) or 1 microgram/animal. Column chromatography showed this immunoreactivity to coelute with intact DSIP and des-Trp1-DSIP. A small but statistically significant increase of immunoreactivity occurred in the plasma of pups whose nursing mothers were injected with N-Tyr-DSIP but not in those whose mothers were injected with saline. Radioactivity appeared in both the brain and blood of 1-2 and 10 day old rat pups fed 125I-N-Tyr-DSIP. Although only a small amount of the radioactivity in plasma co-eluted with intact 125I-N-Tyr-DSIP on column chromatography, almost all of the radioactivity in brain did, suggesting that the radioactivity in the brain represented crossing of the blood-brain-barrier by the peptide and not just contamination by blood. The results cannot be explained by either regurgitation of intestinal contents, or by stimulation of endogenous peptide. They show that a DSIP peptide administered orally can be absorbed through the GI tract into the systemic circulation.