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Vulnerability of mossy fiber targets in the rat hippocampus to forebrain ischemia
Insights
Forebrain ischemia damages CA3 interneurons and hilar neurons, even more than CA1 pyramidal cells, suggesting a role in delayed neuronal death. This study reveals extensive damage in previously understudied hippocampal regions.
Area of Science:
- Neuroscience
- Neurobiology
- Ischemic Brain Injury
Background:
- Forebrain ischemia research in the hippocampus primarily focuses on CA1 pyramidal cell death.
- Dentate granule cells and CA3 pyramidal cells are known to be resistant to ischemia.
- Limited knowledge exists regarding CA3 interneurons and dentate hilus ischemic injury.
Purpose of the Study:
- To investigate the extent of ischemic damage in the CA3 and dentate hilus regions of the hippocampus.
- To identify specific neuronal subtypes vulnerable to forebrain ischemia.
- To explore the potential role of CA3 and hilar neuronal damage in CA1 pyramidal cell death.
Main Methods:
- Utilized the four-vessel-occlusion model to induce forebrain ischemia in rats.
- Employed heat-shock protein (HSP72) immunocytochemistry to detect cellular stress responses.
- Applied a novel Gallyas silver stain to visualize cytoskeletal damage in neurons.
Main Results:
- Identified a subset of CA3 interneurons in the stratum lucidum as highly vulnerable to ischemia, exceeding CA1 pyramidal cell sensitivity.
- Demonstrated significant and previously underestimated damage to multiple hilar neuron types, including mossy cells and spiny fusiform cells.
- Observed early microglial activation and neuronal 'dark' neuron formation in the CA3 stratum lucidum, preceding CA1 damage.
Conclusions:
- Vulnerability of CA3 interneurons and hilar neurons may be linked to dendritic spine density and calcium-permeable non-NMDA receptors.
- Extensive damage in CA3 and hilar regions occurs with a broader dorsoventral distribution than CA1 damage.
- Early injury to spiny CA3 and hilar neurons could be a contributing factor to the delayed neuronal death observed in CA1 pyramidal cells.
Abstract:
Much of the work on forebrain ischemia in the hippocampus has focused on the phenomenon of delayed neuronal death in CA1. It is established that dentate granule cells and CA3 pyramidal cells are resistant to ischemia. However, much less is known about interneuronal involvement in CA3 or ischemic injury in the dentate hilus other than the fact that somatostatin neurons in the latter lose their immunoreactivity. We combined two sensitive methods--heat-shock protein (HSP72) immunocytochemistry and a newly developed Gallyas silver stain for demonstrating impaired cytoskeletal elements--to investigate the extent of ischemic damage to CA3 and the dentate hilus using the four-vessel-occlusion model for inducing forebrain ischemia. HSP72-like immunoreactivity was induced in neuronal populations previously shown to be vulnerable to ischemia. In addition, a distinct subset of interneurons in CA3 was also extremely sensitive to ischemia, even more so than the CA1 pyramidal cells. These neurons are located in the stratum lucidum of CA3 and possess a very high density of dendritic spines. In silver preparations, they were among the first to be impregnated as "dark" neurons, before CA1 pyramidal cells; microglial reaction was also initiated first in the stratum lucidum of CA3. Whereas CA1 damage was most prominent in the septal half of the hippocampus, hilar and CA3 interneuronal damage had a more extensive dorsoventral distribution. Our results also show a far greater extent of damage in hilar neurons than previously reported. At least four hilar cell types were consistently compromised: mossy cells, spiny fusiform cells, sparsely spiny fusiform cells, and long-spined multipolar cells. A common denominator of the injured neurons in CA3 and the hilus was the presence of spines on their dendrites, which in large part accounted for the far greater number of mossy fiber terminals they receive than their non-spiny neighbors. We suggest that the differential vulnerability of neuronal subtypes in these two regions may be attributed to their extremely dense innervation by the mossy fibers and/or the presence of non-NMDA receptor subtypes that are highly permeable to calcium. In addition, early impairment of these spiny CA3 cells and hilar neurons after ischemia may be causal to delayed neuronal death in the CA1 pyramidal cells.