Mason Brittany L, Pariante Carmine M, Thomas Sarah A
Pharmaceutical Science Research Division, King's College London, London, UK.
Endocrinology. 2008 Oct;149(10):5244-53. doi: 10.1210/en.2008-0041. Epub 2008 Jun 12.
The ABCB1-type multidrug resistance efflux transporter P-glycoprotein (P-gp) has been hypothesized to regulate hypothalamic-pituitary-adrenal axis activity by limiting the access of glucocorticoids to the brain. In vivo systemic administration studies using P-gp-deficient mice have shown increased glucocorticoid entry to the brain compared with wild-type controls. However, these studies did not control for the presence of radiolabeled drug in the capillaries, verify an intact blood-brain barrier, or confirm stability of the glucocorticoids used. In the present study, an in situ brain perfusion method, coupled with capillary depletion and HPLC analyses, was used to quantify brain uptake of [3H]dexa-methasone, [3H]cortisol, and [3H]corticosterone in P-gp-deficient and control mice. A vascular marker was included in these experiments. The results show that brain uptake of [3H]dexamethasone was increased in the frontal cortex, hippocampus, hypothalamus, and cerebellum of P-gp-deficient mice compared with wild-type controls. Brain uptake of [3H]cortisol was increased in the hypothalamus of P-gp-deficient mice compared with wild-type controls, but no differences were detected in other regions. Brain uptake of [3H]corticosterone was not increased in P-gp-deficient mice compared with wild-type controls in any brain areas. After our systemic administration of the same radiolabeled glucocorticoids, HPLC analysis of plasma samples identified additional radiolabeled components, likely to be metabolites. This could explain previous findings from systemic administration studies, showing an effect of P-gp not only for dexamethasone and cortisol, but also for corticosterone. This in situ study highlights the different affinities of dexamethasone, cortisol, and corticosterone for P-gp, and suggests that the entry of the endogenous glucocorticoids into the mouse brain is not tightly regulated by P-gp. Therefore, our current understanding of the role of P-gp in hypothalamic-pituitary-adrenal regulation in mice requires revision.
ABCB1型多药耐药外排转运蛋白P-糖蛋白（P-gp）被推测可通过限制糖皮质激素进入大脑来调节下丘脑-垂体-肾上腺轴的活动。使用P-糖蛋白缺陷小鼠的体内全身给药研究表明，与野生型对照相比，糖皮质激素进入大脑的量增加。然而，这些研究没有控制毛细血管中放射性标记药物的存在，没有验证血脑屏障的完整性，也没有确认所用糖皮质激素的稳定性。在本研究中，采用原位脑灌注方法，结合毛细血管清除和高效液相色谱分析，来量化P-糖蛋白缺陷小鼠和对照小鼠脑中[3H]地塞米松、[3H]皮质醇和[3H]皮质酮的摄取量。这些实验中加入了一种血管标记物。结果显示，与野生型对照相比，P-糖蛋白缺陷小鼠额叶皮质、海马体、下丘脑和小脑中[3H]地塞米松的脑摄取量增加。与野生型对照相比，P-糖蛋白缺陷小鼠下丘脑中[3H]皮质醇的脑摄取量增加，但在其他区域未检测到差异。与野生型对照相比，P-糖蛋白缺陷小鼠任何脑区中[3H]皮质酮的脑摄取量均未增加。在我们全身给予相同的放射性标记糖皮质激素后，血浆样本的高效液相色谱分析鉴定出了其他放射性标记成分，可能是代谢产物。这可以解释全身给药研究之前的发现，表明P-糖蛋白不仅对地塞米松和皮质醇有作用，对皮质酮也有作用。这项原位研究突出了地塞米松皮质醇和皮质酮对P-糖蛋白的不同亲和力，并表明内源性糖皮质激素进入小鼠大脑不受P-糖蛋白的严格调控。因此，我们目前对P-糖蛋白在小鼠下丘脑-垂体-肾上腺调节中作用的理解需要修正。
