Remote secondary injury in the thalamus has been observed following rodent and human stroke and is associated with poorer recovery. However the mechanisms and resultant molecular changes whereby cortical infarction leads to remote injury in distant regions of brain are not well defined. The present studies utilize a mouse model of permanent distal MCAO (pdMCAO) with delayed secondary thalamic injury. We have measured time dependent molecular and cellular events, focusing on glial activation and neuronal death in the thalamus. These studies provide an important foundation for future studies aimed at reducing secondary brain injury.
Methods: Cortical infarction by pdMCAO was induced by microcauterization of the distal MCA in male C57BL/6J mice (11-13 wks). Cresyl violet (CV), fluoro-jade C (FJC) and immunochemical staining were performed at 24 hours, 3 days, 1 week, and 2 weeks after stroke (n=5, each time point) and in shams (n=3).
Results: First, we confirmed with TTC staining that the primary infarction induced by pdMCAO was restricted to the cortex of the MCA territory at 72 hours, and that no infarct was seen in the thalamus. Microglia and astrocyte activation (IBA-1 and GFAP, respectively) was first detectable in the ipsilateral ventral posteromedial nucleus (VPN) of the thalamus at 3 days after stroke, and progressively increased at 1 and 2 weeks. However, thalamic neuronal injury (by FJC staining) was not evident until 1 week after stroke. At 2 weeks following stroke, the number of FJC positive neurons further increased and significant morphological changes were evident by CV staining, including shrunken cytoplasm and condensed pyknotic nuclei.
Conclusions: We present a mouse stroke model for studying the mechanisms of secondary thalamic injury. In this model, primary cortical injury results in delayed cellular events in the ipsilateral thalamus, including an initial activation of microglia and astrocytes with subsequent neuronal injury and death. These studies provide necessary foundation for designing future interventional strategies aimed at reducing secondary injury following stroke.