Dopaminergic D1 and D2 receptors are essential for the arousal effect of modafinil

Abstract

Modafinil is a wake-promoting compound with low abuse potential used in the treatment of narcolepsy. Although the compound is reported to affect multiple neurotransmitter systems such as catecholamines, serotonin, glutamate, GABA, orexin, and histamine, however, the molecular mechanism by which modafinil increases wakefulness is debated. Herein we used dopamine (DA) D(2) receptor (D(2)R)-deficient mice combined with D(1)R- and D(2)R-specific antagonists to clarify the role of DA receptors in the arousal effects of modafinil. In wild-type mice, intraperitoneal modafinil induced wakefulness in a dose-dependent manner. Pretreatment with either D(1)R antagonist SCH23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine] at 30 microg/kg or D(2)R antagonist raclopride at 2 mg/kg blocked the arousal effects of low-dose modafinil at 22.5 and 45 mg/kg. When modafinil was given at 90 and 180 mg/kg, pretreatment of D(1)R antagonist did not affect the wakefulness at all, whereas D(2)R antagonist significantly attenuated the wakefulness to the half level compared with vehicle control. Similarly, D(2)R knock-out (KO) mice exhibited attenuated modafinil-induced wakefulness. However, pretreatment of D(2)R KO mice with D(1)R antagonist completely abolished arousal effects of modafinil. These findings strongly indicate that dopaminergic D(1)R and D(2)R are essential for the wakefulness induced by modafinil.

Figure 1.

A–L, Time course changes in wakefulness of WT mice after intraperitoneal administration of modafinil at four doses (A–D) and pretreatment with D₁R antagonist SCH23390 (E–H) or D₂R antagonist raclopride (I–L). Each circle represents the hourly mean amounts of wakefulness. Open and closed circles stand for the baseline- and experimental-day profiles, respectively. On the experimental day, modafinil was given at 10:00 A.M., as indicated by the closed arrow, and D₁R or D₂R antagonist was given at 9:30 A.M., as indicated by the open arrow. The vehicle was used for the baseline day. The horizontal open and filled bars on the x-axes indicate the 12 h light and dark periods, respectively. Values are the means ± SEM (n = 6–8; 6 mice used in B, D, K, and L; 7 in A, E, F, H, and I; and 8 in C, G, and J). *p < 0.05, **p < 0.01 by the paired t test.

Figure 2.

A–C, Total time of WT mice spent in wakefulness and in NREM and REM sleep for 9 h after modafinil administration (A) or pretreatment with D₁R antagonist SCH23390 (B) or D₂R antagonist raclopride (C). Open and filled bars show the profiles for the baseline and experimental days, when the mice were treated with vehicle (for baseline day) or with modafinil or modafinil plus the indicated DA receptor antagonist (for experimental day). Modafinil was given at 10:00 A.M., and pretreatment with D₁R or D₂R antagonist was given at 9:30 A.M. on the experimental day. Values are the means ± SEM (n = 6–8). *p < 0.05, **p < 0.01 compared with its own control, by two-way ANOVA, followed by the PLSD test.

Figure 3.

A–H, Time course changes in wakefulness of D₂R KO mice after intraperitoneal administration of modafinil at four doses (A–D) or after pretreatment with D₁R antagonist SCH23390 before modafinil (E–H). Each circle represents the hourly mean amount of wakefulness. Open and closed circles stand for the baseline- and experimental-day profiles, respectively. On the experimental day, modafinil was given at 10:00 A.M., as indicated by the closed arrow, and pretreatment with the D₁R antagonist was done at 9:30 A.M., as indicated by the open arrow. The vehicle was used on the baseline day. The horizontal open and filled bars on the x-axes indicate the 12 h light and dark periods, respectively. Values are the means ± SEM (n = 6–8; 6 mice used in E and B; 7 in A, D, F, and H; and 8 in C and G). *p < 0.05, **p < 0.01 by the paired t test.

Figure 4.

A, B, Total time spent in wakefulness and in NREM and REM sleep for 9 h after modafinil administration (A) and pretreatment with D₁R antagonist before modafinil (B) to D₂R KO mice. On the experimental day, modafinil was given at 10:00 A.M., and pretreatment with the D₁R antagonist was given at 9:30 A.M. The vehicle was used on the baseline day. Each value is presented as the mean ± SEM (n = 6–8). *p < 0.05, **p < 0.01, significantly different from the control, as assessed by one-way ANOVA followed by the PLSD test.

Figure 5.

Typical examples of EEG and EMG recordings and corresponding hypnograms in mice treated with vehicle or modafinil at 180 mg/kg. A–G, WT mice were treated with vehicle (A), modafinil only (B), SCH23390 + modafinil (C), or raclopride + modafinil (D), and D₂R KO mice were treated with vehicle (E), modafinil only (F), and SCH23390 + modafinil (G). The open arrows indicate pretreatment with D₁R or D₂R antagonists or its vehicle, and closed arrows represent the time of modafinil injection or its vehicle. W, Wakefulness; R, REM sleep; N, NREM sleep.

Figure 6.

Effects of pretreatment with SCH23390 on wake duration elicited by modafinil, which is defined as the time from the injection of modafinil or vehicle to the appearance of the first NREM sleep episode. A–D, WT mice were treated with modafinil only (A) or SCH23390 + modafinil (B), and D₂R KO mice were treated with modafinil only (C) or SCH23390 + modafinil (D). Open and filled bars show the profiles for the vehicle and drug treatments, respectively. Each value represents the mean ± SEM (n = 6–8). *p < 0.05, **p < 0.01 compared with the vehicle injection and as assessed by ANOVA followed by the PLSD test.