Hypothermic neuroprotection of peripheral nerve of rats from ischaemia–reperfusion injury

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Abstract
Although there is much information on experimental ischaemic neuropathy, there are only scant data on neuroprotection. We evaluated the effectiveness of hypothermia in protecting peripheral nerve from ischaemia–reperfusion injury using the model of experimental nerve ischaemia. Forty-eight male Sprague–Dawley rats were divided into six groups. We used a ligation–reperfusion model of nerve ischaemia where each of the supplying arteries to the sciatic–tibial nerves of the right hind limb was ligated and the ligatures were released after a predetermined period of ischaemia. The right hind limbs of one group (24 rats) were made ischaemic for 5 h and those of the other group (24 rats) for 3 h. Each group was further divided into three and the limbs were maintained at 37°C (36°C for 5 h of ischaemia) in one, 32°C in the second and 28°C in the third of these groups for the final 2 h of the ischaemic period and an additional 2 h of the reperfusion period. A behavioural score was recorded and nerve electrophysiology of motor and sensory nerves was undertaken 1 week after surgical procedures. At that time, entire sciatic–tibial nerves were harvested and fixed in situ. Four portions of each nerve were examined: proximal sciatic nerve, distal sciatic nerve, mid-tibial nerve and distal tibial nerve. To determine the degree of fibre degeneration, each section was studied by light microscopy, and we estimated an oedema index and a fibre degeneration index. The groups treated at 36–37°C underwent marked fibre degeneration, associated with a reduction in action potential and impairment in behavioural score. The groups treated at 28°C (for both 3 and 5 h) showed significantly less (P < 0.01; ANOVA, Bonferoni post hoc test) reperfusion injury for all indices (behavioural score, electrophysiology and neuropathology), and the groups treated at 32°C had scores intermediate between the groups treated at 36–37°C and 28°C. Our results showed that cooling the limbs dramatically protects the peripheral nerve from ischaemia–reperfusion injury.