4.3 Materials and methods
4.5.2 Dexamethasone treatment significantly reduces blood-brain barrier leakage
We found a significant reduction of BBB leakage after dexamethasone treatment in typically epilepsy associated brain regions during early epileptogenesis. SE-KA treated and SE-dex mice were scanned 2 d and 4 d following intrahippocampal kainate injection. Analysis revealed that mice being treated with dexamethasone after SE showed significantly less BBB leakage in the ipsilateral dorsal hippocampus, ventral hippocampus and thalamus on day 2 compared with saline treated animals.
BBB leakage was reduced by dexamethasone treatment in the ipsilateral dorsal hippocampus and thalamus on day 4, too. Thus, we conclude that dexamethasone protects BBB integrity in the most affected brain regions after KA-induced SE. T1 hyperintensity in the cortex was not reduced or modified by dexamethasone treatment. It is considered that the increase of T1 intensities is caused by the lesion of the injection cannula along the injection track rather than by excitotoxic effects of kainate or by epileptogenesis-associated BBB leakage. The cortex, therefore, may not be suitable to assess BBB leakage in the focal kainate model as discussed above.
Dexamethasone is a broad-spectrum anti-inflammatory drug which acts on glucocorticoid receptors and reduces the expression of adhesion molecules, inflammatory chemokines and cytokines (Barnes, 2006; Laurila et al., 2009). The molecular mechanisms, underlying possible therapeutic effects on the BBB, are not known in detail. In a recently published study, BBB-restorative effects were found to be mediated by upregulation of ZO-1 tight junction proteins and occurred via binding of dexamethasone to glucocorticoid receptors in an vitro model of primary blast injury (Hue et al., 2015). The mechanism of action was confirmed by application of mifepristone (RU-486), a steroid antagonist which prevented BBB-restorative effects of dexamethasone. Another suggested mechanism of action is the inhibition of NF-kappa-B activity by dexamethasone (Auphan et al., 1995). NF-NF-kappa-B can impair BBB function by alteration of tight junction protein expression (Brown et al., 2003).
Dexamethasone significantly reduced TNF production and reduced the infarct size by 50 % in a rat model of focal cerebral ischemia (Bertorelli et al., 1998).
However, as inflammatory processes themselves play a role in restoring brain homeostasis following brain insults (Abbott, 2002; Serrano et al., 2011), finding the right balance between aggravation and rehabilitation of inflammatory processes is important (Gröhn & Pitkänen, 2007). Disturbing BBB homeostasis could entail side effects (Abbott, 2002). Glucocorticoids are widely used in the clinic, for example for treatment of the Landau-Kleffner syndrome, a specific form of pediatric epilepsy (Vezzani et al., 2011). However, glucocorticoids can be harmful in patients with acute ischemic stroke (Kleinschnitz et al., 2011). Moreover, deterioration of brain injury and survival rate were the outcome of dexamethasone treatment in a recent study in the lithium-pilocarpine post SE rat model (Duffy et al., 2014). These findings suggest that the efficacy of glucocorticoids can be very different dependent on the underlying disease. Hence, effects of dexamethasone on specific alterations occurring in the process of epileptogenesis need to be further evaluated.
4.6 Conclusion
We demonstrate for the first time that dexamethasone protects BBB integrity in the intrahippocampal kainate mouse model. Moreover, longitudinal in vivo MRI detects changes in BBB leakage during epileptogenesis in this model. Restoration of impaired BBB integrity may contribute to prevention of epilepsies following brain insults as BBB leakage is strongly associated with the process of epileptogenesis (Gorter et al., 2015). We hence hypothesize that BBB-protective dexamethasone treatment may reveal antiepileptogenic or epileptogenesis-modifying effects in future studies that include evaluation of the chronic epileptic phase.
ACKNOWLEDGEMENTS: We thank Christian Bergen and Friederike Twele for skillful technical assistance. The study is funded by the European Union’s Seventh Framework Programme (FP7) under grant agreement n°602102 (EPITARGET). H.B. was supported by a scholarship from the Studienstiftung des Deutschen Volkes.
CONFLICTS OF INTEREST: none
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