Essential role of dopamine D2 receptor in the maintenance of wakefulness, but not in homeostatic regulation of sleep, in mice

Abstract

Dopamine (DA) and its D(2) receptor (R) are involved in cognition, reward processing, and drug addiction. However, their roles in sleep-wake regulation remain unclear. Herein we investigated the role of D(2)R in sleep-wake regulation by using D(2)R knock-out (KO) mice and pharmacological manipulation. Compared with WT mice, D(2)R KO mice exhibited a significant decrease in wakefulness, with a concomitant increase in non-rapid eye movement (non-REM, NREM) and REM sleep and a drastic decrease in the low-frequency (0.75-2 Hz) electroencephalogram delta power of NREM sleep, especially during the first 4 h after lights off. The KO mice had decreased mean episode duration and increased episode numbers of wake and NREM sleep, many stage transitions between wakefulness and NREM sleep during the dark period, suggesting the instability of the wake stage in these D(2)R KO mice. When the KO mice were subjected to a cage change or an intraperitoneal saline injection, the latency to sleep in the KO mice decreased to half of the level for WT mice. The D(2)R antagonist raclopride mimicked these effects in WT mice. When GBR12909, a dopamine transport inhibitor, was administered intraperitoneally, it induced wakefulness in WT mice in a dose-dependent manner, but its arousal effect was attenuated to one-third in the D(2)R KO mice. However, these 2 genotypes showed an identical response in terms of sleep rebound after 2, 4, and 6 h of sleep deprivation. These results indicate that D(2)R plays an essential role in the maintenance of wakefulness, but not in homeostatic regulation of NREM sleep.

Figure 1.

Sleep–wake profiles of WT and D₂R KO mice under baseline conditions. A, Time course changes in wakefulness, NREM, and REM sleep. Each circle represents the hourly mean amount of each stage. Open and closed circles stand for the WT and D₂R KO profiles, respectively. The horizontal open and filled bars on the χ-axes indicate the 12 h light and 12 h dark periods, respectively. B, Total time spent in wakefulness, NREM, and REM sleep during the 12 h dark and 12 h light phases. Open and filled bars show the profiles for the WT and D₂R KO, respectively. Values are the means ± SEM (n = 10). *p < 0.05, **p < 0.01, compared with corresponding WT littermate value, assessed by two-way ANOVA followed by the PLSD test.

Figure 2.

A–F, Episode numbers, and mean durations (A–C) and stage transition (D–F) during the first 4 h of darkness, 12 h dark and 12 h light phases. Open and filled bars show the profiles for the WT and D₂R KO, respectively. G–I, EEG power density during the first 4 h of darkness, 12 h dark period, and 12 h light period. The curves represent logarithmic mean values of absolute power densities [power density (μV²/Hz) = power value (μV²)/(frequency resolution 0.25 Hz × adjustment parameter of hanning window 1.5)]. The horizontal bars indicate where there is a statistical difference (p < 0.05). Values are the means ± SEM (n = 10). *p < 0.05, **p < 0.01 compared with the corresponding WT littermate value, assessed by two-way ANOVA followed by the PLSD test.

Figure 3.

Latency to NREM and REM sleep after host cage change or saline injection intraperitoneally at 10:00 A.M. A, B, Top and middle, Typical examples of polygraphic recordings and corresponding hypnograms illustrating the effects of cage change (A) or injection of saline (B) on WT and D₂R KO mice. Bottom, Average latency to NREM and REM sleep after cage changes (A) or injection of saline (B). Open and filled bars show the profiles for the WT and KO mice, respectively. C, D, Average latency to NREM and REM sleep after administration of raclopride, a D₂R antagonist, to WT mice combined with a cage change (C) or not (D). Open and filled bars show the profiles for the vehicle and raclopride, respectively. Values are the means ± SEM (n = 6∼8). **p < 0.01 compared with the corresponding value for WT littermates, assessed by one-way ANOVA followed by the PLSD test.

Figure 4.

Effect of inhibition of dopamine transport on wakefulness. A, B, Time course changes in wakefulness in WT (A) and D₂R KO (B) mice treated with the DAT inhibitor GBR12909. Each circle represents the hourly mean amount of wakefulness. Open and closed circles stand for the profiles of vehicle and GBR12909 treatments, respectively. The horizontal open and filled bars on the χ-axes indicate the 12 h light and 12 h dark periods, respectively. C, D, Dose–response effect on total time spent in wakefulness for 6 h after administration of vehicle or GBR12909 to WT (C) and the KO (D) mice. Open and filled bars show the profiles of vehicle and GBR12909 treatments, respectively. Values are the means ± SEM (n = 6–8). *p < 0.05, **p < 0.01 compared with corresponding vehicle control, assessed by two-way ANOVA followed by the PLSD test.

Figure 5.

Sleep–wake profiles of WT and D₂R KO mice after SD. A, B, Time course changes in NREM and REM sleep in WT (A) and D₂R KO (B) mice for 6 h SD (2:00–8:00 P.M.). Each circle represents the hourly mean amount of sleep. Open and closed circles stand for the baseline and SD profiles, respectively. The horizontal open and filled bars on the χ-axes indicate the 12 h light and 12 h dark periods, respectively. C, Total amount of NREM and REM sleep for 6 h after 4 and 6 h SD. D–F, Slow-wave activity of NREM for 6 h after SD of WT (D) and D₂R KO (E) mice, and 24 h baseline (F). Each delta power of NREM sleep in the range of 0.75–4 Hz was first summated and then normalized as a percentage of the corresponding mean delta power of NREM sleep during 12 h (D, E) or 24 h baseline (F), respectively. Open and shaded columns or circles show the baseline and SD, respectively. Values are the means ± SEM (n = 10). *p < 0.05, **p < 0.01 compared with the corresponding baseline value or between the two genotypes, assessed by two-way ANOVA followed by the PLSD test.