Deltorphin transport across the blood-brain barrier

In vivo antinociception studies demonstrate that deltorphins are opioid peptides with an unusually high blood-brain barrier penetration rate. In vitro, isolated bovine brain microvessels can take up deltorphins through a saturable nonconcentrative permeation system, which is apparently distinct from previously described systems involved in the transport of neutral amino acids or of enkephalins. Removing Na+ ions from the incubation medium decreases the carrier affinity for deltorphins (-25%), but does not affect the Vmax value of the transport. The nonselective opiate antagonist naloxone inhibits deltorphin uptake by brain microvessels, but neither the selective delta-opioid antagonist naltrindole nor a number of opioid peptides with different affinities for delta- or mu-opioid receptors compete with deltorphins for the transport. Binding studies demonstrate that mu-, delta-, and kappa-opioid receptors are undetectable in the microvessel preparation. Preloading of the microvessels with L-glutamine results in a transient stimulation of deltorphin uptake. Glutamine-accelerated deltorphin uptake correlates to the rate of glutamine efflux from the microvessels and is abolished by naloxone.