Endomorphins, Met-enkephalin, Tyr-MIF-1, and the P-glycoprotein efflux system.

The P-glycoprotein (P-gp) transport system, responsible for the efflux of many therapeutic drugs out of the brain, recently has been shown to transport the endogenous brain opiate endorphin. We used P-gp knockout mice (Mdr1a) and their controls to determine where P-gp is involved in the saturable efflux systems of four other endogenous opiate-modulating peptides across the blood-brain barrier (BBB). After injection of endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2)), endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2)), Met-enkephalin (Tyr-Gly-Gly-Phe-Met-OH), and Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH(2)) into the lateral ventricle of the mouse brain, residual radioactivity was measured at 0, 2, 5, 10, and 20 min later. The results showed no difference in the disappearance of any of these peptides from the brains of knockout mice compared with their controls. This demonstrates that unlike endorphin and morphine, P-gp does not seem to be required for the brain-to-blood transport of the endomorphins, Met-enkephalin, or Tyr-MIF-1 across the BBB.