HEME-OXYGENASE-1 INDUCTION IN GLIA THROUGHOUT RAT-BRAIN FOLLOWING EXPERIMENTAL SUBARACHNOID HEMORRHAGE

The heme released following subarachnoid hemorrhage is metabolized by heme-oxygenase (HO) to biliverdin and carbon monoxide (CO) with the re lease of iron. The HO reaction is important since heme may contribute to vasospasm and increase oxidative stress in cells. HO is comprised o f at least two isozymes, HO-2 and HO-1. HO-1, also known as heat shock protein HSP32, is inducible by many factors including heme and heat s hock. HO-2 does not respond to these stresses. To begin to examine HO activity following subarachnoid hemorrhage (SAH), the expression of HO -1 and HO-2 was investigated after experimental SAH in adult rats. Imm unocytochemistry for HO-1, HO-2 and HSP70 proteins was performed at 1, 2, 3 and 4 days after injections of lysed blood, whole blood, oxyhemo globin and saline into the cisterna magna. A large increase in HO-1 im munoreactivity was seen in cells throughout brain following injections of lysed blood, whole blood, and oxyhemoglobin but not saline. Lysed blood, whole blood and oxyhemoglobin induced HO-1 in all of the cortex , hippocampus, striatum, thalamus, forebrain white matter and in cereb ellar cortex. HO-1 immunoreactivity was greatest in those regions adja cent to the basal subarachnoid cisterns where blood and oxyhemoglobin concentrations were likely highest. Double immunofluorescence studies showed the HO-1 positive cells to be predominately microglia: though H O-1 was induced in some astrocytes. HO-1 expression resolved by 48 h. HO-2 immunoreactivity was abundant but did not change following inject ions of blood. A generalized induction of HSP70 heat shock protein was not observed following injections of lysed blood, whole blood, oxyhem oglobin, or saline. These results suggest that HO-1 is induced in micr oglia throughout rat brain as a general, parenchymal response to the p resence of oxyhemoglobin in the subarachnoid space and not as a stress response. This microglial HO-1 response could be protective against t he lipid peroxidation and vasospasm induced by hemoglobin, by increasi ng heme clearance and iron sequestration, and enhancing the production of the antioxidant bilirubin.