Saturable mechanism for delta sleep-inducing peptide (DSIP) at the blood-brain barrier of the vascularly perfused guinea pig brain

Abstract

Cellular uptake of [ 125I] labelled DSIP at the luminal interface of the blood-brain barrier (BBB) was studied in the ipsilateral perfused in situ guinea pig forebrain. Regional unidirectional transfer constants (K in) calculated from the multiple-time brain uptake analysis were 0.93, 1.33 and 1.66 μl·min −1g −1 for the parietal cortex, caudate nucleus and hippocampus, respectively. In the presence of 7 μM unlabelled DSIP the brain uptake of [ 125I]-DSIP (0.3 nM) was inhibited, the values of K in being reduced to 0.23–0.38 μl·min −1g −1, values that were comparable with the K in for mannitol. The rapidly equilibrating space of brain, measured from the intercept of the line describing brain uptake versus time on the brain uptake ordinate, V i, was greater for [ 125I]-DSIP than for mannitol; in the presence of unlabelled DSIP this was reduced to that of mannitol, and it was suggested that the larger volume for [ 125I]-DSIP represented binding at specific sites on the brain capillary membrane. L-tryptophan, the N-terminal residue of DSIP, in concentrations of 7 μM and 1 mM, inhibited K in without affecting V i. A moderate inhibition of K in was obtained by vasopressin ([Arg 8]-VP), but only at a concentration as high as 0.2 mM. The results suggest the presence of a high affinity saturable mechanism for transport of DSIP across the blood-brain barrier, with subsequent uptake at brain sites that are highly sensitive to L-tryptophan, and may be modulated by [Arg 8]-VP.