Age related changes in microglial phenotype vary between CNS regions: grey versus white matter differences.

Subtle regional differences in microglial phenotype exist in the adult mouse brain. We investigated whether these differences were amplified during ageing and following systemic challenge with lipopolysaccharide (LPS). We studied microglial morphology and phenotype in young (4mo) and aged (21mo) C57/BL6 mice using immunohistochemistry and quantified the expression levels of surface molecules on microglia in white and grey matter along the rostral-caudal neuraxis. We detected significant regional, age dependent differences in microglial phenotypes, with the microglia of white matter and caudal areas of the CNS exhibiting greater upregulation of CD11b, CD68, CD11c, F4/80 and FcγRI than grey matter and rostral CNS areas. Upregulation of CD11c with age was restricted to the white matter, as was the appearance of multinucleated giant cells. Systemic LPS caused a subtle upregulation of FcγRI after 24 h, but the other markers examined were not affected. Burrowing behaviour and static rod assays were used to assess hippocampal and cerebellar integrity. Aged mice exhibited exaggerated and prolonged burrowing deficits following systemic LPS injection, while in the absence of an inflammatory challenge aged mice performed significantly worse than young mice in the static rod test. Taken together, these findings show that the effects of age on microglial phenotype and functional integrity vary significantly between CNS compartments, as do, albeit to a lesser extent, the effects of systemic LPS.

In vitro characterisation of the duration of action of the histamine-1 receptor antagonist azelastine.

Azelastine is a selective antagonist at the human histamine-1 receptor and is used clinically in the treatment of allergic rhinitis. In this study we have investigated its duration of action in vitro in an effort to characterise the receptor and tissue components involved. Chinese hamster ovary cell membrane fragments were used to determine the kinetics of azelastine at the H1 receptor in a radioligand binding assay. Further duration of action studies were completed in tissue preparations using guinea-pig trachea and human bronchus. In radioligand binding studies, azelastine reached steady state at the H1 receptor after approximately 41 min and exhibited a significantly slower dissociation rate constant from the receptor than the first generation antihistamine, diphenhydramine. In washout studies completed in guinea-pig and human airway in vitro tissue preparations, azelastine continued to antagonise the effects of histamine at the H1 receptor for at least 18 h post-washout of the antagonist. This outcome was reversed following removal of the epithelium from guinea-pig isolated tracheal strips. These studies indicate there is a tissue component contributing to azelastine's duration of action, in addition to its direct H1 receptor binding, with evidence suggesting a role for the epithelial layer.