Glucose and Lactate Monitoring Across the Rat Sleep–Wake Cycle

Glucose biosensors were prepared by immobilizing glucose oxidase on carbon fiber microelectrodes (CFMEs) either by cross-linking glutaraldehyde vapors or via enzyme entrapment in films of m-phenylenediamine or resorcinol. The enzymatic layer was then covered with a membrane made of Nafion or cellulose acetate. The biosensors were tested using differential normal pulse voltammetry (DNPV) to detect signals. The calibration curves for glucose were linear from 0.3 to 6.5 mM. The DNPV response was essentially insensitive to potentially interfering molecules. Glucose concentrations in plasma and cerebrospinal fluid (CSF) corresponded with those measured by standard procedures. In freely moving rats, cortical extracellular glucose concentration averaged 0.59 ± 0.3 mM. Using combined polysomnographic and DNPV assessment, glucose levels in the cortex were determined during various phases of the sleep–wake cycle. Compared to the waking state (W, 100 %), spontaneous variations were observed during active waking triggered by a water puff (AW, −32 %), slow-wave sleep (SWS, +13 %), and paradoxical sleep (PS, −11 %). To prepare lactate biosensors, CFMEs were coated with lactate oxidase. These sensors, combined with DNPV measurements, showed a linear response to lactate at concentrations ranging from 0.1 to 2.0 mM. Lactate measurement was insensitive to common potentially interfering molecules. In anesthetized rats, cortical lactate concentration averaged 0.41 ± 0.02 mM. Lactate levels in samples of brain tissue, plasma, and CSF corresponded to those measured by standard procedures. In freely moving rats, lactate changes were also related to sleep–wake state. Compared to W (100 %), lactate level increased during AW (+53 %) and decreased during SWS (−16.2 %). However, during PS, lactate level increased (+8.5 %) compared to that noted during SWS. Finally, long-term lactate monitoring revealed the existence of a circadian influence on lactate production rate.