Orexin A but not orexin B rapidly enters brain from blood by simple diffusion

We determined the ability of orexin A and orexin B, recently discovered end ogenous appetite enhancers, to cross the blood-brain barrier (BBB) of mice. Multiple time-regression analysis showed that an i.v. bolus of I-125-orexi n A rapidly entered the brain from the blood, with an influx rate (K-i = 2. 5 +/- 0.3 x 10(-4) ml/g.min) many times faster than that of the Tc-99m-albu min control. This relatively rapid rate of entry was not reduced by adminis tration of excess orexin A (or leptin) or by fasting for 22 h, even when pe netration into only the hypothalamus was measured. Lack of saturability als o was shown by perfusion in blood-free buffer. HPLC revealed that most of t he injected I-125-orexin A reached the brain as intact peptide. Capillary d epletion studies showed that the administered peptide did not remain bound to the endothelial cells comprising the BBB but reached the brain parenchym a. Efflux of I-125-orexin A from the brain occurred at the same rate as Tc- 99m-albumin. The octanol/buffer partition coefficient of 0.232 showed that orexin A was highly lipophilic, whereas the value for orexin B was only 0.0 30. Orexin B, moreover, was rapidly degraded in blood, so I-125-orexin B co uld be detected in intact form in brain when no injected peripherally. Thus , although orexin B is rapidly metabolized in blood and has low lipophilici ty, orexin A rapidly crosses the BBB from blood to reach brain tissue by th e process of simple diffusion.