Similarly LTD concentration dependently enhanced AQP express
Similarly, LTD4 concentration-dependently enhanced AQP4 expression in astrocytes (Fig. 3). The LTD4-enhanced AQP4 expression was not affected by pranlukast but inhibited by Bay u9773, a non-selective CysLT1/CysLT2 receptor antagonist . Because no selective CysLT2 receptor antagonists are currently available, the effect of Bay u9773 may represent the blockade of CysLT2 receptor in comparison with the effect of pranlukast. Furthermore, we found that the mRNA expression of CysLT2 receptor was weak in normal astrocytes but significantly increased by LTD4; while the relatively higher mRNA expression of CysLT1 receptor was not further increased by LTD4 (Fig. 4). The mRNA expression of CysLT2 receptor was also found to be much weaker than that of CysLT1 receptor in rat astrocytes . Together with the results of the receptor antagonism and expression, LTD4-induced AQP4 expression may be mediated by CysLT2 receptor activation.
However, we have previously reported that pranlukast attenuated both BBB disruption and CGRP 8-37 (rat) edema after focal cerebral ischemia in rats and mice , . Since the contents of CysLTs in the brain are increased after ischemia , the effect of exogenous LTD4 partly mimics the post-ischemic pathological changes. The effect of pranlukast on LTD4-induced BBB disruption is consistent with that we previously reported, but its effect on brain edema (increase in water content) is different. One of the possibilities for the difference is that inhibition of vasogenic edema by pranlukast may partially reduce post-ischemic edema. Another possibility is that ischemia-induced brain edema results from much more complex responses. Therefore, other anti-inflammatory abilities of pranlukast, such as inhibition of inflammatory cell infiltration ,  and cytokine production , may be also involved in the effect on post-ischemic edema. The present results suggest that CysLT1 receptor antagonist(s) may not substantially attenuate the AQP4-related cytotoxic edema.
In summary, we found that LTD4 modulates brain edema; CysLT1 receptor mediates vasogenic edema while CysLT2 receptor may mediate cytotoxic edema via up-regulating AQP4 expression. These findings indicate the distinct roles of CysLT1 and CysLT2 receptors in the pathophysiological processes in the brain.
Cysteinyl leukotrienes (CysLTs, including LTC, LTD, and LTE), 5-lipoxygenase metabolites of arachidonic acid, are potent inflammatory mediators . The actions of CysLTs are mediated by activating their receptors, cysteinyl leukotriene receptor 1 (CysLT) and receptor 2 (CysLT) . In peripheral tissues, CysLT and CysLT receptors modulate at least four responses: vascular and smooth muscle cell function, immune, inflammation, and tissue repair . In the central nervous system, the roles of CysLTs and 5-lipoxygenase in cerebral ischemic injury have been indicated by a line of evidence , , , , , , ; however, whether and how CysLT and CysLT receptors mediate the ischemic neuronal injury remains unknown. Recently, we found that the expressions of CysLT and CysLT receptors are induced in the neuron-appearing cells in human brain specimens from patients with traumatic brain injury and brain tumors , . We also reported that CysLT receptor antagonists, pranlukast and montelukast, protect against ischemic brain injury in rats and mice in vivo , , , . These findings indicate that CysLT and CysLT receptors may modulate brain injuries, including ischemic injury. However, since we have not found any protective effects of CysLT receptor antagonists on the in vitro ischemic-like injury in the primary cultured neurons (unpublished observations), the roles of CysLT and CysLT receptors in neuronal injury need to be further studied. Both CysLT and CysLT receptors are Gαq protein coupled receptors; but they may play distinct roles. For example, CysLT receptor plays an inhibiting role while CysLT receptor plays a facilitating role in bleomycin-induced pulmonary fibrosis in the receptor gene-knockout mice , . Whether the two receptors also play distinct roles in ischemic brain/neuron injury is unknown. To explore the possibly distinct roles, we investigated the effects of CysLT and CysLT receptor gene over-expression and antagonists on oxygen glucose deprivation (OGD)-induced in vitro ischemic injury in a neural cell line, rat pheochromocytoma cells (PC12 cells) , , .