Isoflurane titration improves detection of hippocampal lactate by ¹ H-MRS

Lactate has increasingly been recognized as both an important fuel source and a signaling molecule within the brain. Alterations in brain lactate levels are associated with various neurological diseases. Thus, there is great interest in thein vivodetection and measurement of cerebral lactate levels in animals used for investigation of normal brain function and models of disease. Proton magnetic resonance spectroscopy (¹H-MRS) is a non-invasive technique used to measure lactate and other metabolites within the brain. However, lactate can be difficult to detect with conventional¹H-MRS due to its low abundance and spectral overlap with lipids. In addition, volatile anesthetics used during image acquisition increase lactate production, potentially masking any subtle physiological changes in lactate levels. Here, we made use of a transgenic mouse model in which expression of lactate dehydrogenase A (Ldha), the rate-limiting enzyme of lactate production, was induced within cortical and hippocampal neurons. Unexpectedly,¹H-MRS analysis, under typical isoflurane-induced anesthesia of 4% induction followed by 1.6-2% maintenance, revealed no significant elevation of hippocampal lactate levels in neuronalLdhainduction mice compared to control mice. In contrast,¹H-MRS analysis, using an isoflurane titration protocol in which mice were sequentially exposed to 1.6%, 2%, and then finally 3% isoflurane, revealed significantly higher hippocampal lactate levels inLdhatransgenic mice compared to controls. In addition, significantly fewer mice were required to detect differences in lactate levels using the isoflurane titration protocol compared to conventional isoflurane-induced anesthesia. Our findings highlight the importance of controlling for the effects of anesthesia when detecting changes in hippocampal lactate levelsin vivoand offer a novel protocol for enhanced cerebral lactate detection.