Therapeutic effect of post-ischemic hypothermia duration on cerebral ischemic injury

To study the efficacy of post-ischemic mild brain hypothermia lasting for different time intervals on cerebral ischemic reperfusion injury.

Male Sprague-Dawley rats were divided into a sham-operated group, normothermia (37-38 degrees C) ischemia group and mild hypothermia (31-32 degrees C) group. The last group was subdivided into four groups: 30 minute hypothermia plus 210 minute normothermia, 60 minute hypothermia plus 180 minute nomothermia,120 minute hypothermia plus 120 minute normothermia, and 240 minute hypothermia (n=8). Global cerebral ischemia was established using the Pulsinelli four-vessel occlusion model. Brain tissue was collected following a 20 minute cerebral ischemia and 240 minute reperfusion, and was used to measure the levels of glutamate (Glu), aspartate (Asp), glycine (Gly), gamma-aminobutyric acid (GABA), dopamine (DA), norepinephrine (NE), serotonin(5-HT) and hydroxyindoleacetic acid (5-HIAA), nitrite (NO(2)), endothelin-1 (ET(1)), tumor necrosis factor alpha(TNFalpha) and interleukin-1beta (IL-1beta). Serum was collected to measure the levels of lactate dehydrogenase (LDH), aspartate aminotransferase (AST), creatine kinase (CK) and its brain band isoenzyme (CK-BB).

Hypothermia lasting for 60-240 minutes delayed the decrease in these amino acids, postponed the decrease in DA, NE and 5-HT and increase in hydroxyindoleacetic acid (5-HIAA), and decreased the levels of IL-1beta, TNFalpha, ET(1) and NO(2) in brain tissue. Hypothermia also decreased the levels of LDH, AST, CK and CK-BB in serum as compared to normothermia group (p<0.05 or p<0.01). Hypothermia lasting for 30 minutes delayed the decreases in these amino acids and 5-HT and increase in 5-HIAA in brain tissue (p<0.05), but failed to influence the levels of IL-1beta, TNFalpha, ET(1) and NO(2) in brain tissue and the amounts of LDH, AST, CK and CK-BB in serum as compared to normothermia ischemia group (p>0.05).

Post-ischemic mild brain hypothermia can significantly suppress the excessive release of amino acids, monoamine neurotransmitters and inflammation response in ischemic tissue. It can also stabilize the function of the cell membrane, which is associated with the mechanism of cerebral protection by mild hypothermia. These results suggest that mild hypothermia should be applied immediately after ischemia and last for more than 60 minutes in order to obtain neuroprotective effects.