Differential vulnerability of CA1 vs CA3 pyramidal neurons after ischemia: possible relationship to sources of Zn2+ accumulation and its entry into and prolonged effects on mitochondria.

Excitotoxic mechanisms contribute to the degeneration of hippocampal pyramidal neurons after recurrent seizures and brain ischemia. However, susceptibility differs, with CA1 neurons preferentially degenerating after global ischemia, and CA3 neurons after limbic seizures. Whereas most studies address contributions of excitotoxic Ca(2+) entry, it is apparent that Zn(2+) also contributes, reflecting accumulation in neurons either after synaptic release and entry through post-synaptic channels or upon mobilization from intracellular Zn(2+) binding proteins like metallothionein-III (MT-III). Using mouse hippocampal slices to study acute oxygen glucose deprivation (OGD) triggered neurodegeneration, we find evidence for early contributions of excitotoxic Ca(2+) and Zn(2+) accumulation in both CA1 and CA3, as indicated by the ability of Zn(2+) chelators or Ca(2+) entry blockers to delay pyramidal neuronal death in both regions. However, using knockout animals (of MT-III and vesicular Zn(2+) transporter, ZnT3) and channel blockers revealed substantial differences in relevant Zn(2+) sources, with critical contributions of pre-synaptic release and its permeation through Ca(2+) (and Zn(2+)) permeable AMPA channels in CA3, and Zn(2+) mobilization from MT-III predominating in CA1. To assess consequences of the intracellular Zn(2+) accumulation, we employed OGD exposures slightly shorter than those causing acute neuronal death; under these conditions, cytosolic Zn(2+) rises persisted for 10-30 min after OGD, followed by recovery over ∼40-60 min. Furthermore, the recovery appeared to be accompanied by mitochondrial Zn(2+) accumulation (via the mitochondrial Ca(2+) uniporter, MCU) in CA1 but not in CA3 neurons, that was markedly diminished in MT-III knockouts, suggesting that it depended upon Zn(2+) mobilization from this protein.

SIGNIFICANCE STATEMENT: The basis for the differential vulnerabilities of CA1 vs CA3 pyramidal neurons is unclear. Present studies of events during and after acute OGD highlight a possible important difference, with rapid synaptic entry of Ca(2+) and Zn(2+) contributing more in CA3, but with delayed and long lasting accumulation of Zn(2+) within mitochondria occurring in CA1 but not CA3 pyramidal neurons. These data may be consistent with observations of prominent mitochondrial dysfunction as a critical early event in the delayed degeneration of CA1 neurons after ischemia, and support a hypothesis that mitochondrial Zn(2+) accumulation in the early reperfusion period may be a critical and targetable upstream event in the injury cascade.