Additional evidence that small amounts of a peptide can cross the blood-brain barrier

It was determined that an antiserum against delta-sleep inducing peptide (DSIP) required eight of the nine constituent amino acids for antigenic activity. Measurement by this radioimmunoassay (RIA) or DSIP-like material in the rat brain, therefore, would necessarily involve almost the entire molecule present in essentially intact form. Injection of 200 microgram DSIP into the carotid artery of rats resulted in a doubling of brain levels of peptide as measured shortly afterwards by RIA. The brain tissue to plasma ratio of radioactivity in rats injected with labeled DSIP was much higher than that in rats injected with labeled inulin; this suggests that the increased amount of material measured by RIA was not merely trapped in the blood vessels. Thus, the results indicate that a small amount of essentially intact peptide can cross the blood-brain barrier. This could represent one of the mechanisms by which central effects of peripherally injected peptides can be exerted.