We have demonstrated previously that mild intraischemic hypothermia confers a marked protective effect on the final histopathological outcome. The present study was carried out to evaluate whether this protective effect involves changes in the degree of local cerebral blood flow reductions, tissue accumulation of free fatty acids, or alterations in the extracellular release of glutamate and dopamine. Rats whose intraischemic brain temperature was maintained at 36° C, 33° C, or 30° C were subjected to 20 minutes of ischemia by four-vessel occlusion combined with systemic hypotension. Levels of local cerebral blood flow, as measured autoradiographically, were reduced uniformly in all experimental animals at the end of the ischemic period. Cortical and striatal free fatty acid levels were measured at the end of ischemia by gas chromatography after tissue extraction and separation by thin layer chromatography. A massive ischemia-induced accumulation of individual free fatty acids was observed in animal groups whose intraischemic brain temperature was maintained at either 36° C or 30° C. Extracellular neurotransmitter levels were measured by microdialysis; the perfusate was collected before, during, and after ischemia. In rats whose intraischemic brain temperature was maintained at 36° C, dopamine and glutamate increased significantly during ischemia and the early period of recirculation (by 500-fold and sevenfold, respectively). In animals whose brain temperature was maintained at 33° C and 30° C, the release of glutamate was completely inhibited, and the release of dopamine was significantly attenuated (by 60%). These results suggest that mild intraischemic hypothermia does not affect the ischemia-induced local cerebral blood flow reduction or free fatty acid accumulation. Received December 27, 1988; accepted March 7, 1989. hypothermia has been attributed mainly to a decrease in brain energy demands, energy failure during the ischemic insult, or both. Recently, we have demonstrated that relatively modest decrements (2° C) in brain temperature during the ischemic insult markedly attenuate neuronal damage in vulnerable brain regions such as the hippocampus or striatum. 8 However, the degree of high-energy phosphate depletion and lactate accumulation at the end of the ischemic insult were not affected by these temperature variations. These results suggest that a moderate reduction in intraischemic brain temperature affects a different consequence of brain ischemia that is important for the development of neuronal damage in these selectively vulnerable brain regions.Neurotransmitter release has been implicated in the pathophysiology of brain damage within hippocampal and striatal areas after ischemia.

We have previously described a marked attenuation of postischemic striatal neuronal death by prior substantia nigra (SN) lesioning. The present study was carried out to evaluate whether the protective effect of the lesion involves changes in the degree of local cerebral blood flow (ICBF) reduction, energy metabolite depletion, or alterations in the extracellular release of striatal dopamine (DA), glutamate (Glu), or gamma-aminobutyric acid (GABA). Control and SN-lesioned rats were subjected to 20 min of forebrain ischemia by four-vessel occlusion combined with systemic hypotension. Levels of ICBF, as measured by the autoradiographic method, and energy metabolites were uniformly reduced in both the ipsi- and contralateral striata at the end of the ischemic period, a finding implying that the lesion did not affect the severity of the ischemic insult itself. Extracellular neurotransmitter levels were measured by microdialysis; the perfusate was collected before, during, and after ischemia. An approximately 500-fold increase in DA content, a 7-fold increase in Glu content, and a 5-fold increase in GABA content were observed during ischemia in nonlesioned animals. These levels gradually returned to baseline by 30 min of reperfusion. In SN-lesioned rats, the release of DA was completely prevented, the release of GABA was not affected, and the release of Glu was partially attenuated. However, excessive extracellular Glu concentrations were still attained, which are potentially toxic. This, taken together with the previous neuropathological findings, suggests that excessive release of DA is important for the development of ischemic cell damage in the striatum.

We evaluated whether regional differences in the magnitude of glutamate, gamma-aminobutyric acid (GABA), and glycine release could explain why some regions are vulnerable to ischemia whereas others are spared. By means of the microdialysis technique, the temporal profile of ischemia-induced changes in extracellular levels of glutamate, GABA, and glycine was compared in regions that demonstrate differing susceptibilities to a 10- and 20-min ischemic insult (dorsal hippocampus, anterior thalamus, somatosensory cortex, and dorsolateral striatum). The degree of ischemia (as established by local cerebral blood flow reduction) and the magnitude of histopathological neuronal damage were also evaluated in these regions. The blood flow reduction was severe and uniform in all regions; however, the histopathological outcome illustrated a different pattern. Whereas the CA1 sector of the hippocampus was severely damaged, the thalamus and cortex were relatively spared from both 10 and 20 min of ischemia. Striatal neurons were resistant to a 10-min insult but severely damaged after 20 min of ischemia. Ischemia-induced increase in glutamate and GABA content were of a similar magnitude and temporal profile in all four brain regions. A uniform increase in extracellular glycine levels was also observed in all four brain structures. The postischemic response, however, was different. Glycine levels remained twofold higher than baseline in the hippocampus but fell to baseline in the cortex and thalamus after both 10- and 20-min insults. In the striatum, glycine levels returned to baseline after 10 min of ischemia but remained relatively high after a 20-min insult.(ABSTRACT TRUNCATED AT 250 WORDS)

Summary: We simultaneously measured neurotransmit ter amino acids by the microdialysis technique and corti cal CBP by laser-Doppler flowmetry in the ischemic pen umbral cortex of rats subjected to 2-h normothermic (36.5-37SC) transient middle cerebral artery (MCA) clip occlusion. Brains were perfusion-fixed 3 days later and infarct volume measured. CBP (% of preischemic values) fell to 32 ± 2% (mean ± SD) during ischemia and rose to 157 ± 68% during recirculation. Extracellular glutamate levels increased from a baseline value of 7 ± 3 f.LM to a peak value of 180 ± 247 f.LM 20-30 min following onset of ischemia but subsequently returned to near baseline lev els after 70 min of ischemia despite ongoing MCA occlu sion. The threshold CBP for moderate glutamate release was 48%. Massive glutamate release was seen during the first 60 min of MCA occlusion in the two animals showing Glutamate-mediated excitotoxicity, though not the sole factor, is now accepted as a major mecha nism of ischemic neuronal damage. In in vitro cell culture studies, the vulnerability of both hippocam pal and cortical neurons to anoxia has been related to glutamate release from presynaptic terminals (Rothman, 1984; Choi et aI., 1987). In in vivo stud ies of both global and focal ischemia, extracellular glutamate levels increase massively during the isch emic period (Benveniste et aI., 1984; Hagberg et aI., Abbreviations used: EI, excitotoxic index; GABA, -y-ami nobutyrate; MeA, middle cerebral artery. 575the largest infarcts and occurred at CBP values .,;20% of control levels. Mean CBP during ischemia exhibited an inverse relationship with infarct volume, and the magni tude of glutamate release during ischemia was positively correlated with infarct volume. Extracellular -y-aminobu tyrate and glycine changes were similar to those of glu tamate but showed no significant correlation with infarct volume. These results suggest that (a) accumulation of extracellular glutamate is an important determinant of in jury in the setting of reversible MCA occlusion and (b) reuptake systems for neurotransmitter amino acids may be functional in the penumbra during transient focal isch emia.

Using the microdialysis technique and laser-Doppler flowmetry, we performed simultaneous measurement of salicylate hydroxylation and glutamate release along with local CBF in the ischemic penumbral cortex of rat brain subjected to normothermic transient middle cerebral artery (MCA) occlusion. Cortical CBF fell to 24 +/- 11% (mean +/- SD) during ischemia and recovered to 84 +/- 16% during reperfusion. Extracellular glutamate levels increased by 6.5-fold above baseline 10 min following MCA occlusion but subsequently returned to near baseline levels in spite of the persistent ischemia. Increase in 2,3- and 2,5-dihydroxybenzoic acid (DHBA) concentrations in the microdialysis perfusate was confirmed during both ischemia and reperfusion phase. Although the temporal profile and amount of salicylate hydroxylation were heterogeneous among individual animals, integrated 2,3-DHBA concentrations during reperfusion were correlated positively with integrated glutamate concentrations during ischemia and negatively with mean postischemic CBF. These relationships suggest a possible association of the enhanced production of 2,3-DHBA during reperfusion with larger amounts of intraischemic glutamate release and lower levels of post-ischemic CBF.