Orexin A but Not Orexin B Rapidly Enters Brain from Blood by Simple Diffusion

• Published in: The Journal of pharmacology and experimental therapeutics 1999-04, Vol.289 (1), p.219-223
• Main Authors: Kastin, A J, Akerstrom, V
• Format: Article
• Language: English
• Subjects: Animals; Blood-Brain Barrier; Brain - metabolism; Buffers; Capillaries - metabolism; Carrier Proteins - blood; Carrier Proteins - chemistry; Carrier Proteins - pharmacokinetics; Chromatography, High Pressure Liquid; Diffusion; Injections, Intravenous; Intracellular Signaling Peptides and Proteins; Iodine Radioisotopes; Male; Mice; Mice, Inbred ICR; Neuropeptides - blood; Neuropeptides - chemistry; Neuropeptides - pharmacokinetics; Octanols; Orexins; Solubility; Technetium Tc 99m Aggregated Albumin
• ISSN: 0022-3565; 1521-0103

We determined the ability of orexin A and orexin B, recently discovered endogenous appetite enhancers, to cross the blood-brain barrier (BBB) of mice. Multiple time-regression analysis showed that an i.v. bolus of 125 I-orexin A rapidly entered the brain from the blood, with an influx rate ( K i = 2.5 ± 0.3 × 10 −4 ml/g·min) many times faster than that of the 99m Tc-albumin control. This relatively rapid rate of entry was not reduced by administration of excess orexin A (or leptin) or by fasting for 22 h, even when penetration into only the hypothalamus was measured. Lack of saturability also was shown by perfusion in blood-free buffer. HPLC revealed that most of the injected 125 I-orexin A reached the brain as intact peptide. Capillary depletion studies showed that the administered peptide did not remain bound to the endothelial cells comprising the BBB but reached the brain parenchyma. Efflux of 125 I-orexin A from the brain occurred at the same rate as 99m Tc-albumin. The octanol/buffer partition coefficient of 0.232 showed that orexin A was highly lipophilic, whereas the value for orexin B was only 0.030. Orexin B, moreover, was rapidly degraded in blood, so no 125 I-orexin B could be detected in intact form in brain when injected peripherally. Thus, although orexin B is rapidly metabolized in blood and has low lipophilicity, orexin A rapidly crosses the BBB from blood to reach brain tissue by the process of simple diffusion.