ORF29-06 - Dexamethasone-Induced Glucocorticoid Resistance in the Brain Is Not Due to Blood-Brain Barrier Exclusion

Introduction:
Previously we showed that chronic dexamethasone (DEX) downregulated glucocorticoid receptor (GR)–dependent genes in brain, but not peripheral tissue (liver). Here we investigated if this apparent glucocorticoid (GC) resistance could be due to blood-brain barrier (BBB) exclusion of chronically administered DEX. To do so, we assessed whether GR-dependent gene expression could be rescued by corticosterone (CORT), a more BBB-permeable GC. We also measured DEX concentrations in brain and plasma after acute and chronic DEX.

Methods:
Part 1: 14–16-week-old C57bl6 mice (n=40, 20/sex) were randomized to 4 groups, each receiving twice daily injections for 10 days: chronic vehicle (VEH) for all 20 injections, (VEH/VEH), chronic VEH with a final (20th) injection of acute CORT (12.5 mg/kg) (VEH/CORT), chronic DEX (5 mg/kg) for all 20 injections (DEX/DEX), or chronic DEX with a final (20th) injection of CORT (DEX/CORT). Expression of Gilz, Sgk1, and Fkbp5 mRNA in brain and liver were quantified by RT-qPCR.
Part 2: A separate cohort of mice received acute (VEH or DEX: 0.05, 0.5, or 5.0 mg/kg) or chronic DEX (5 mg/kg twice daily for 10 days). DEX concentrations in plasma and brain were analyzed by LC-MS.
Data was analyzed with one-way ANOVA and Tukey’s post hoc tests.

Results:
Part 1: Treatment significantly affected expression of all three genes in liver and brain, but post-hoc testing showed that the nature of the treatment effect differed. Chronic DEX (DEX/DEX) increased expression of Sgk1 (P=0.0036), Fkbp5 (P=0.0018), and Gilz (P=0.0016) in the liver relative to VEH/VEH, but this was not seen in brain (P=0.8 for SGK1, P=1.0 for Fkbp5, and P=0.98 for Gilz). Acute CORT did increase transcription in the brain (VEH/VEH vs VEH/CORT, P=0.012 for Sgk1, P=0.0017 for Fkpb5, P=0.016 For Gilz). Acute CORT also overcame DEX resistance in the brain, with the DEX/CORT group having increased expression of Sgk1 (P=0.0082) and Fkbp5 (P=0.01) relative to VEH/VEH.
Part 2: Treatment had a significant effect on plasma DEX (F(4,20) =19.06, P< 0.0001) and brain DEX (F(4,21) =12.87, P< 0.0001). In both, an apparent dose response to acute DEX was seen where DEX levels were highest with 5 mg/kg and lowest with 0.05 mg/kg of DEX; in both tissues, only the 5 mg/kg group had DEX levels significantly higher than VEH (P < 0.0001 for plasma and brain). Chronic DEX treatment increased brain DEX levels (P < 0.0026 vs VEH), and there was no difference in brain DEX levels between acute and chronic DEX (P=1.0).

Conclusions:
CORT restored GR-dependent transcription after chronic DEX in mouse brain. The lack of GR-dependent transcription with chronic DEX is not due to BBB exclusion since DEX was detected at similar levels in brain after acute and chronic DEX exposure. Future work will explore CORT’s role on the mineralocorticoid receptor in its rescue of GR-dependent genes. Our work may help the understanding of the neuropsychiatric effects of GC excess.