Delta sleep-inducing peptide (DSIP)-like material is absorbed by the gastrointestinal tract of the neonatal rat

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 ωg/animal of N-Tyr-DSIP but not after vehicle (normal saline) or 1 ωg/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 1251I-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.