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Ultrastructural changes in the hippocampal CA1 region following transient cerebral ischemia: evidence against
J Deshpande1, K Bergstedt, T Lindén
1Department of Anesthesiology and Critical Care Medicine, Johns Hopkins Hospital, Baltimore, MD.
Insights
Delayed neuronal death (DND) in the hippocampus involves early polysome disaggregation and dark material accumulation, not apoptosis. This suggests protein aggregates, not programmed cell death, cause neuronal damage after ischemia.
Area of Science:
- Neuroscience
- Cell Biology
- Pathology
Background:
- Cerebral ischemia can lead to delayed neuronal death (DND) in the hippocampus.
- The precise mechanisms and cellular events underlying DND remain incompletely understood.
Purpose of the Study:
- To investigate the ultrastructural changes in CA1 pyramidal neurons following transient cerebral ischemia.
- To elucidate the cellular processes involved in delayed neuronal death and its potential relation to programmed cell death.
Main Methods:
- Induction of transient cerebral ischemia in rats via common carotid occlusion and hypotension.
- Ultrastructural analysis of hippocampal CA1 pyramidal neurons at 6, 24, 48, and 72 hours post-ischemia.
- Administration of protein and RNA synthesis inhibitors to assess their effect on neuronal survival.
Main Results:
- Delayed neuronal death (DND) was observed, with significant degeneration apparent at 72 hours post-ischemia.
- Early ultrastructural changes included polysome disaggregation and the formation of electron-dense material in neurons.
- This dark material accumulated in the soma and dendrites over time and was not mitigated by protein or RNA synthesis inhibitors.
Conclusions:
- DND following transient cerebral ischemia is likely not a form of apoptosis or programmed cell death.
- The accumulating dark material may represent protein complexes or internalized membrane fragments disrupting cellular function.
- These findings suggest a novel mechanism of cell death in the post-ischemic brain.
Abstract:
The ultrastructural changes in the pyramidal neurons of the CA1 region of the hippocampus were studied 6 h, 24 h, 48 h, and 72 h following a transient 10 min period of cerebral ischemia induced by common carotid occlusion combined with hypotension. The pyramidal neurons showed delayed neuronal death (DND), i.e. at 24 h and 48 h postischemia few structural alterations were noted in the light microscope, while at 72 h extensive neuronal degeneration was apparent. The most prominent early ultrastructural changes were polysome disaggregation, and the appearance of electron-dense fluffy dark material associated with tubular saccules. Mitochondria and nuclear elements appeared intact until frank neuronal degeneration. The dark material accumulated with extended periods of recirculation in soma and in the main trunks of proximal dendrites, often beneath the plasma membrane, less frequently in the distal dendrites and seldom in spines. Protein synthesis inhibitors (anisomycin, cycloheximide) and an RNA synthesis inhibitor (actinomycin D), administered by intrahippocampal injections or subcutaneously, did not mitigate neuronal damage. Therefore, DND is probably not apoptosis or a form of programmed cell death. We propose that the dark material accumulating in the postischemic period represents protein complexes, possibly aggregates of proteins or internalized plasma membrane fragments, which may disrupt vital cellular structure and functions, leading to cell death.