Extracellular levels of lactate, but not oxygen, reflect sleep homeostasis in the rat cerebral cortex

Abstract

Study objective: It is well established that brain metabolism is higher during wake and rapid eye movement (REM) sleep than in nonrapid eye movement (NREM) sleep. Most of the brain's energy is used to maintain neuronal firing and glutamatergic transmission. Recent evidence shows that cortical firing rates, extracellular glutamate levels, and markers of excitatory synaptic strength increase with time spent awake and decline throughout NREM sleep. These data imply that the metabolic cost of each behavioral state is not fixed but may reflect sleep-wake history, a possibility that is investigated in the current report.

Design: Chronic (4d) electroencephalographic (EEG) recordings in the rat cerebral cortex were coupled with fixed-potential amperometry to monitor the extracellular concentration of oxygen ([oxy]) and lactate ([lac]) on a second-by-second basis across the spontaneous sleep-wake cycle and in response to sleep deprivation.

Setting: Basic sleep research laboratory.

Patients or participants: Wistar Kyoto (WKY) adult male rats.

Interventions: N/A.

Measurements and results: Within 30-60 sec [lac] and [oxy] progressively increased during wake and REM sleep and declined during NREM sleep (n = 10 rats/metabolite), but with several differences. [Oxy], but not [lac], increased more during wake with high motor activity and/or elevated EEG high-frequency power. Meanwhile, only the NREM decline of [lac] reflected sleep pressure as measured by slow-wave activity, mirroring previous results for cortical glutamate.

Conclusions: The observed state-dependent changes in cortical [lac] and [oxy] are consistent with higher brain metabolism during waking and REM sleep in comparison with NREM sleep. Moreover, these data suggest that glycolytic activity, most likely through its link with glutamatergic transmission, reflects sleep homeostasis.

Keywords: EEG; Lactate; cerebral cortex; in vivo amperometry; oxygen; rat; sleep; slow-wave activity.

Figure 1

Increases in [lac] and [oxy] are typically observed during waking and REM sleep, behavioral states characterized by elevated neuronal activity, whereas these energy metabolites typically decrease during NREM sleep. (A and B) The average [lac] and [oxy] across the 24-hr period (n = 10 rats each). [Lac] was significantly lower in the light period than in the dark period (P < 0.05) (C and D) [Lac] and [oxy] in the prefrontal cortex of individual rats are depicted for each 4-sec epoch of wake (red), NREM sleep (blue), and REM sleep (green) during undisturbed baseline days. The boxed regions in C and D can be seen at higher resolution in E and F, respectively.

Figure 2

After a short delay (< 1 min), [lac] and [oxy] increase during waking and REM sleep and decrease during NREM sleep. Changes in [lac] (A) or [oxy] (B) during wake, NREM sleep, and REM sleep relative to the behavioral state onset (n = 10 rats each). Because of intrinsic variability in episode duration, data (mean ± standard error of the mean) presented here are shown to include all time points at which at least 15% of all episodes for that behavioral state can contribute. (C) Average cortical concentration of lactate, oxygen, and glutamate in 4-sec epochs for the first 3-min after wake or NREM sleep onset. Glutamate results were derived from data previously collected by Dash et al.

Figure 3

During waking, [oxy] in cortical areas activated by movement increases more when locomotor activity is high, whereas [lac] or [glu] do not change in association with locomotor activity. Each panel depicts the concentration (mean ± standard error of the mean) of lactate (A), oxygen (B), or glutamate (C) in 4-sec epochs across all episodes of mostly active or mostly quiet wake. All episodes were classified as either active or quiet based on total electromyographic activity across the duration of the wake episode. *P < 0.01. Glutamate results were derived from data previously collected by Dash et al.

Figure 4

[Oxy] changes in association with the amount of high- and low-frequency electroencephalographic (EEG) band-limited power (BLP; an index of cortical activity), whereas [lac] and [glu] show no relationship with BLP at any frequency. BLP was calculated for every 4-sec epoch of wake in 3 lower frequency windows (gray lines); Delta (0.5-4Hz), Theta (5-9Hz), Alpha (10-18Hz) and 5 higher frequency windows (black lines): LowBeta (22-30), HighBeta (30.25-40Hz), LowGamma (40.25-59Hz), MedGamma (61-80Hz), HighGamma (80.25-100Hz). Changes in [lac] (A), [oxy] (B), and [glu] (C) during each 4-sec epoch of wake were binned across 4 quartiles of increasing power within each frequency band and are presented as mean ± standard error of the mean. Post hoc testing for lower frequencies (0.5-18 Hz) and higher frequencies (22-100 Hz), Fisher least significant difference: *P < 0.05; **P < 0.01. Glutamate results were derived from data previously collected by Dash et al.

Figure 5

During both spontaneous and recovery nonrapid eye movement (NREM) sleep after sleep deprivation, increased sleep pressure is associated with larger declines in [lac], whereas [oxy] does not change in association with sleep pressure. Average changes in [lac] (A) and [oxy] (B) during episodes of early (hr 1-4 after lights onset) and late (hr 8-12 after lights onset) spontaneous NREM sleep. (C and D) Top panels: average [lac] and [oxy] for each 4-sec epoch of wake (red), NREM sleep (blue), and REM sleep (green) during 3 hr of sleep deprivation and ensuing 3 hr of recovery sleep in individual rats. Bottom panels: electromyographic activity is depicted for each 4-sec epoch (gray) along with a 1-min moving average overlay (black). (E and F) Average [lac], [glu], or [oxy] (n = 5 each) across the 3-hr sleep deprivation. Concentrations were averaged across 30-min bins and plotted at the middle time point for each bin. (G and H) Average [lac] and [oxy] (relative to sleep onset) during the first 9 hr from light onset (baseline days) and during the first 9 hr after the end of sleep deprivation (n = 5 rats each). Concentrations were binned and averaged across 1-hr intervals and plotted at the middle time point for each bin. (I and J) The relationship between the rates of decline in [lac] and [oxy] and slow-wave activity (SWA) during NREM sleep following either undisturbed baseline (empty gray circles; n = 10 rats each) or sleep deprivation (filled black circles; n = 5 rats each). SWA and analyte concentrations were binned and averaged for 2-hr intervals across the light period. *P < 0.05. Glutamate results were derived from data previously collected by Dash et al.