Pathophysiology of subarachnoid hemorrhage in the rat

Abstract

Subarachnoid hemorrhage (SAH) causes brain damage, but the underlying mechanisms are poorly understood. An obstacle in SAH research is the lack of an adequate animal model. In this thesis, we developed a new approach to simulate SAH that involves the injection of blood into the prechiasmatic cistern of the rat. This model had several advantages when it was compared to the two commonest SAH rat models in the literature: unlike the endovascular perforation of the internal carotid artery model, it was reproducible, easy to perform and the mortality rate was acceptable, while when compared to the injection of blood into the cisterna magna model, the blood distribution and histological, hemodynamic and metabolic changes more closely resembled that found in patients with SAH.

We studied various mechanisms that can cause brain damage after SAH. Among the many factors occurring during the first minutes-hours after SAH, the global reduction in cerebral blood flow (CBF), but not the changes in intracranial and perfusion pressure, seemed to be a main determinant of brain injury: the degree of its reduction was correlated with the amount of subarachnoid blood, the acute changes in the expression of the N-methyl-D-aspartate (NMDA) receptor subunits, the severity of subsequent inflammation, and most importantly, with delayed cell death and the mortality rate. Signs of acute metabolic derangements after severe SAH were detected by a reduction in cerebral oxygen extraction and altered levels of extracellular glucose, lactate and pyruvate. These changes in metabolism could not always be explained by ischemic episodes.

Dying cells, mainly neurons, were seen in various areas of the brain in a large percentage of the surviving animals at 2 and 7 days after SAH. The involvement of apoptotic pathways in the brain damage after SAH was evidenced by the morphological (chromatin condensation and/or apoptotic bodies) and molecular features (upregulation of Bax and active caspase 3) of the damaged cells. Activation of an inflammatory cascade, comprising both parenchymal and vascular tissue, was also detected in the brain by the induction of intercellular adhesion molecule 1, OX6, ED1, tumor necrosis factor α, inducible nitric oxide synthase and nestin.

In accordance with the view that inflammation caused brain damage, we found a marked overlapping between areas with dying cells and those with inflammation. Using in situ hybridization, a CBF-dependent early downregulation of the hippocampal NR2A, NR2B and NR3B subunit mRNA levels after SAH was observed. Although these changes may have played a pathogenic role following SAH, their causal relationship to subsequent cell death could not be established.

In conclusion, this thesis describes a new and suitable SAH model in the rat. We showed that acute ischemic episodes, early metabolic derangements, a secondary inflammatory reaction and apoptosis are probable mechanisms which contribute to brain damage after SAH. The fact that cells were still dying at least 7 days later indicates that there is a temporal window during which adequate treatment may improve the final outcome after SAH.