Figure 7. Acute inhibition of orexin neurons induces SWS and decreases the activity of serotonergic DR neurons. A, Schematic drawing of in vivo extracellular recordings from the DR and photic illumination of the hypothalamus using fiber optics. EEG and EMG electrodes were implanted to determine sleep/wakefulness state. Plastic fiber optics (0.5 mm diameter) were bilaterally inserted into the hypothalamus. Orange light from LED (590 ± 5 nm) was applied through these fibers. Firing frequency of serotonergic DR neurons was recorded through a glass electrode inserted into the DR. B, Averaged shape of spikes classified as presumably serotonergic neurons (top traces) and nonserotonergic neurons (bottom traces) in the DR. Serotonergic DR neurons have longer action potential duration (∼1 ms) as well as a deflection in the negative component (arrow, top left trace) or a shoulder on the falling phase (arrow, top right trace). C, Bar graph summarizing the average EEG power density in the delta, theta, alpha, and beta bands during wakefulness, SWS, and REM sleep of wild-type littermate mice, orexin/Halo mice, and orexin/ataxin-3 mice during the light on period (n = 13). D, Bar graph summarizes the firing frequency of serotonergic neurons in the DR during wakefulness, SWS, and REM sleep recorded from wild-type littermate mice (n = 9–22), orexin/Halo mice (n = 6–26), and orexin/ataxin-3 mice (n = 12–17). E, Confirmation of an extracellular recording from neurons in the DR. The arrow shows PSB labeling (blue), indicating the location of the tip of the recording electrode (left). Scale bar, 500 μm. The right is a higher magnification of the rectangle in the left (right). Scale bar, 300 μm. F, G, Representative EEG/EMG recordings and firing frequency of DR neurons during orange light illumination in wild-type mice (F) and orexin/Halo mice (G). H, Power spectral analysis of EEG recorded from orexin/Halo mice (n = 12). The top graph shows power spectra of spontaneously occurring wakefulness, SWS, and REM sleep observed in orexin/Halo mice during the lights-on period (n = 6–26). The bottom graph shows power spectra: Pre, 30 s before illumination; Post, 30 s after illumination; 0–30 s, the first half 30 s illumination; 30–60 s, the second half 30 s illumination in orexin/Halo mice during the lights on period (n = 6–26). I, The average EEG power densities in the delta, theta, alpha, and beta wave bands (n = 12). Average power significantly increased in the second half of light illumination in all four frequency bands. J, Bar graph summarizing the firing frequency of serotonergic DR neurons during orange light illumination of orexin/Halo mice (n = 12). K, Bar graph summarizing integrated EMG levels. Integrated EMG levels were normalized to Pre, 30 s before illumination. Values are represented as means ± SEM. *p < 0.05.