gms | German Medical Science

Objective: Cerebral inflammation and neuronal cell death play an important role in the development of brain injury after SAH. In previous studies we have demonstrated an array of SAH-induced inflammatory processes in the injured brain including: microglia accumulation, release of proinflammatory cytokines and neuronal cell death. Termination of these inflammatory processes is of utmost importance, as it is required for restoring tissue homeostasis post SAH. Therefore, we evaluate the inflammatory resolution phase, while signs for a possible tertiary brain damage in case of incomplete resolution were also considered.

Methods: Experimental SAH was induced through endovascular filament perforation in mice. Animals were sacrificed on days 1, 7, 14 and after 1, 2 and 3 months post SAH. Brain cryosections were immunolabeled for Iba-1 to detect microglia, NeuN to visualize neurons and by TUNEL staining for apoptotic cells. The numbers of microglia and neurons were counted in regions of interest in all experimental groups at all time points. Additionally, the gene expression of various proinflammatory cytokines in whole brain samples was examined using qPCR, as well as the IL6 protein quantity by using ELISA. Sham-operated mice served as controls.

Results: The number of microglia cells increased from the baseline level of sham-operated mice up to 5.4 ± 1.8-fold on day 1, up to 15.5 ± 5.3-fold on day 7 and up to 16.4 ± 9-fold on day 14. At the same time the amount of apoptotic neurons increased by 11.1 ± 5.2-fold on day 1, by 28.5 ± 8.5-fold on day 7 and by 25.4 ± 12.1-fold on day 14 post SAH. After month 1 there were no significant differences between the investigated groups and the sham-operated mice, showing a strong decrease of microglia accumulation and neuronal apoptosis between day 14 and month 1. In contrast to these results, gene expression of the investigated cytokines IL6, IL1β and TNFα was still on a high level after 1 month. The IL6 protein concentration was increased on day 1 to 1.7 ± 1.2-fold and on day 14 to 2.4 ± 0.5-fold in comparison to control levels, respectively. By passing month 1 no significant differences of the IL6 protein concentration could be shown.

Conclusion: In our study we observed restoration of tissue homeostasis regarding microglia accumulation, neuronal apoptosis and IL6 protein concentration one month post SAH. However, the gene expression of the investigated proinflammatory cytokines was still on a high level at this time point, whereby an exclusion of a possible tertiary brain damage by incomplete resolution could not be assured. Inflammatory resolution and tissue homeostasis appear to be important parts of the disease pathology and future studies should also examine later time points after the bleeding to define the impact on brain damage and outcome.