Conditions of protection by hypothermia and effects on apoptotic pathways in a model of permanent middle cerebral artery occlusion

Purpose and Background

Hypothermia is protective in stroke models, but findings from permanent occlusion models are conflicting. To determine whether hypothermia is effective in such cases, we induced focal ischemia in rats by permanent distal middle cerebral artery (MCA) occlusion, which models a scenario in which the MCA remains occluded but partial reperfusion occurs through collaterals.

Methods

The left MCA was occluded permanently and the CCAs were reopened after 2 h, leading to partial reperfusion, in 6 groups of rats (Table 1) maintained at 37°C, 33°C (mild hypothermia), or 30°C (moderate hypothermia) during and after occlusion. Two days later rats were sacrificed for infarct size measurement. Immunofluorescence staining was used to detect cytochrome c and AIF translocation.

Table 1

Experimental groups

Group	Temp during first 2h	Temp during second 2h	Infarct Size (Mean±SEM) (%)	Reduction in infarct size %	P value (vs group 1)	n
1	37	37	63±3	0		7
2	33	37	64±4	−2	0.848	5
3	37	33	61±4	3	0.710	7
4	33	33	49±3	22	0.004	5
5	30	37	34±5	46	<0.001	7
6	30	30	36±4	43	<0.001	4

Results

Infarct size did not differ from normothermic control (group 1) when hypothermia was mild and relatively brief (groups 2–3; see Table 1). Infarct size in rats exposed to mild hypothermia both during and post-ischemia (group 4) was reduced about 22% relative to group 1. When temperature was decreased to 30°C (group 5) robust protection was observed; an additional 2 h hypothermia during reperfusion in group 6 did not further reduce infarct size. Subcellular translocation of cytochrome c and AIF in the ischemic margin was not blocked by mild hypothermia in groups 2 or 3, but was attenuated in group 4 and blocked in groups 5 and 6.

Conclusion

Mild hypothermia is protective in a model of permanent MCA occlusion if it is extended an additional 2 h. Short durations of hypothermia are protective if temperature is lowered to 30°C. The protection might be achieved by blocking AIF and cytochrome c mediated apoptosis.