In-vivo microdialysis study of extracellular glutamate response to temperature variance in subarachnoid hemorrhage

Neurochemical changes may precede the development of clinical signs in neurological disease. Early identification of such changes may offer an opportunity to avoid or treat complications. Under experimental conditions, extracellular levels of glutamate and other amino acids can be monitored by in-vivo microdialysis in cerebral ischemia, head trauma and epilepsy. Data on the release of glutamate under ischemic conditions in humans are limited. There is no published data on the effects of temperature variation or other manipulations on the extracellular glutamate levels in humans. We report for the first time, the effects of changes in temperature on the extracellular cerebral glutamate levels as measured by in-vivo microdialysis, the dialysate being collected before, during and after cooling in four patients with subarachnoid hemorrhage. Three of the patients had in-vivo microdialysis carried out postoperatively. One patient underwent microdialysis three days prior to the surgical clipping of the aneurysm. In all patients, mild head cooling resulted in a significant decrease in extracellular glutamate levels. The effect of cooling was most apparent when the extracellular glutamate concentrations were high. In two patients, the extracellular glutamate levels increased sharply with fever but returned to normal once the temperature normalized. In vivo microdialysis can be used to measure extracellular glutamate and other neurotransmitters with minimal discomfort in awake humans. This technique offers a unique opportunity to monitor the neurochemistry in critically ill patients and it may aid in developing therapeutic intervention strategies to minimize undesired chemical responses.