Possible roles of histamine receptors and RhoA signalling pathways in blood-brain barrier

Blood-brain barrier (BBB) which creates a major obstacle for the transport of molecules into the brain can be opened by various treatments for effective drug delivery into the brain parenchyma, however its subsequent closure is also crucial to limit the detrimental alterations that can potentially occur in neuronal microenvironment by circulatory substances. In this study, we tested the efficacy of blocking histamine receptors (HRs) and RhoA in reversing histamine-induced BBB opening. Following intravenous histamine injection (10 mg/kg), mice were treated with HR antagonists, hydroxyzine (H1R, 3 mg/kg), cimetidine (H2R, 10 mg/kg), ciproxifan (H3R, 3 mg/kg) and JNJ-7777120 (H4R, 20 mg/kg), and Rho A inhibitor fasudil (10 mg/kg). Blood-brain barrier permeability to molecules with large and small molecular weights was evaluated by fluorescent detection of intravenously administered albumin-Alexa Fluor-594 (1%) and cadaverine Alexa-Flour-488 (1%), respectively in brain parenchyma. To analyze changes in the immunoreactivity of claudin-5 and caveolin-1 proteins, immunofluorescence staining was performed. The alterations in HR expression were detected by RT-PCR. Treatment with hydroxyzine significantly decreased fluorescence intensity of albumin-Alexa Fluor-594 in brain parenchyma following histamine-induced BBB opening (p<0.01), while there were no significant changes in the fluorescence intensity of cadaverine Alexa-Flour-488 by HR antagonists and fasudil in this setting. No significant changes were noted in HR mRNA and protein levels following administration of HR antagonists and fasudil to histamine-treated mice. Our results suggest that treatment with H1R antagonist hydroxyzine restores histamine-induced BBB disruption which emphasizes its potential use as a novel means of closing transiently opened BBB to protect the neuronal microenvironment against potentially toxic blood-borne substances in brain drug delivery applications.