EXPRESSION OF INDUCIBLE NITRIC-OXIDE SYNTHASE DURING RAT-BRAIN INFLAMMATION - REGULATION BY 1,25-DIHYDROXYVITAMIN D-3

This study, based on in situ hybridization and immunolabeling experime nts, presents the time-course and cellular distribution of inducible N O synthase (iNOS) expression in a rat model of brain inflammation. Bot h intrahippocampal injection of lipopolysaccharide (LPS) or of buffer (stab lesion) induce an early, transient, and restricted expression of iNOS mRNA and immunoreactivity in the rat CNS. The topographic and ph enotypic characteristics of iNOS-producing cells are distinct. After s tab lesion, iNOS mRNAs, expressed at 5 h mainly in cells in the interv entricular junction and in a few cells in brain parenchyma, were no mo re detectable from 15 h onwards, whereas the protein was faintly expre ssed in parenchymal cells at 15 h and 24 h. In contrast, after LPS inj ection, iNOS-mRNAs were detected from 5 to 24 h. iNOS-immunoreactivity was highly induced and sequentially observed first in choroid plexus and ependymal cells at 5 h, in monocytes and activated/reactive microg lia at 15 h and 24 h, and finally in astrocytes at 72 h. In order to i nvestigate potential regulatory effects of 1,25-dihydroxyvitamin D-3 ( 1,25-D-3) on iNOS expression, we have delivered this hormone with LPS or buffer into the rat hippocampus. 1,25-D-3 significantly inhibits iN OS expression, at both the mRNA and immunoreactive protein levels, 15 h and 24 h after LPS injection, in the cells of the monocyte lineage. Moreover, 72 h after LPS injection, the addition of 1,25-D-3 leads to a 6-fold increase in the number of macrophages around the lesion site, that correlates with a decrease in the proportion of apoptotic cells. Since 1,25-D-3 can be produced by activated macrophages/microglia and since NO stimulates 1,25-D-3 synthesis by macrophages, our results su pport the hypothesis that this hormone might be synthesized endogenous ly during CNS inflammatory reactions, thus explaining the transient an d restricted iNOS expression observed after LPS intracerebral injectio n. (C) 1998 Wiley-Liss, Inc.