Contribution of calpain activation to early stages of hippocampal damage during oxygen-glucose deprivation.

Calpains are Ca(2+)-activated enzymes which cleave cytoskeletal and other proteins, contributing to neuronal damage in conditions of pathological intracellular Ca(2+) elevation, including stroke. However, the consequences of calpain overactivation have typically been observed hours after insult. To identify the earliest events attributable to calpain activation, and thus potentially isolate calpain substrates involved in acute neuronal damage, we dynamically recorded the effects of calpain inhibition in an in vitro model of stroke. Extracellular DC potentials and fEPSPs were monitored together with changes of light transmittance (as a measure of cell and mitochondrial swelling) and Rh 123 fluorescence (to monitor mitochondrial membrane potential; DeltaPsi(m)) in hippocampal slices obtained from P12-P17 rats. No differences were observed in the latencies of fEPSP disruption or onset of extracellular DC shifts associated with hypoxic spreading depression (HSD) evoked by oxygen-glucose deprivation (OGD) under control conditions or in the presence of calpain inhibitor III (MDL 28170). However, a significant difference was observed in transmitted light signals during OGD with calpain inhibition. Given the potential contribution of mitochondrial swelling to changes in light transmittance, these experiments were also conducted in the presence of cyclosporin A to block opening of the mitochondrial permeability transition pore (MPTP). Our results indicate that differences in OGD-induced changes of light transmittance in the presence of MDL 28170 are not likely the result of MPTP blockade or changes in dendritic beading. We propose that calpain inhibition may alter changes in light transmittance by limiting conformational changes of mitochondria.

Lack of phenotype for LTP and fear conditioning learning in calpain 1 knock-out mice.

We previously proposed the hypothesis that calpain activation played an important role in long-term potentiation (LTP) of synaptic transmission in hippocampus. Two forms of calpain are predominant in brain tissues, calpain 1 (mu-calpain), activated by micromolar calcium concentration and calpain 2 (m-calpain), activated by millimolar calcium concentration in vitro. In the present study, we tested the role of calpain 1 in LTP and in learning and memory using calpain 1 knock-out mice. Changes in learning and memory were assessed using both context and tone fear conditioning. No differences in freezing responses were observed between the knock-out and the wild-type animals during the acquisition phase of the training, eliminating the possibility that the knock-out animals could be differentially affected by the foot shock. Likewise, no differences in freezing responses elicited by either the context or the tone were observed during the retention phase. No differences in short-term potentiation (STP) or LTP were observed in hippocampal slices from the knock-out and matched wild-type mice. Several interpretations might explain these negative results. First, it is conceivable that calpain 2 plays a more dominant role in neurons, and that calpain 1 makes a minor contribution as opposed to its suspected predominant role in the hematopoietic system. Alternatively, it is conceivable that some as yet unknown compensatory mechanisms take effect, and that calpain 2 or another calpain isoform substitutes for the missing calpain 1.