Anterior Insula Regulates Multiscale Temporal Organization of Sleep and Wake Activity

Abstract

The role of specific cortical regions in sleep-regulating circuits is unclear. The anterior insula (AI) has strong reciprocal connectivity with wake and sleep-promoting hypothalamic and brainstem regions, and we hypothesized that the AI regulates patterns of sleep and wakefulness. To test this hypothesis, we lesioned the AI in rats (n = 8) and compared sleep, wake, and activity regulation in these animals with nonlesioned controls (n = 8) with 24-h sleep recordings and chronic infrared activity monitoring. Compared to controls, animals with AI lesions had decreased wakefulness and increased rapid eye movement (REM) sleep and non-REM (NREM) sleep. AI-lesioned animals had shorter wake bouts, especially during the active dark phase. AI-lesioned animals also had more transitions from NREM to REM sleep, especially during the inactive light phase. Chronic infrared monitoring revealed that AI-lesioned animals also had a disturbed temporal organization of locomotor activity at multiple time scales with more random activity fluctuations from 4 to 12 h despite intact circadian rhythms. These results suggest that the AI regulates sleep and activity and contributes to the regulation of sleep and motor behavior rhythmicity across multiple time scales. Dysfunction of the AI may underlie changes in sleep-wake patterns in neurological diseases.

Keywords: NREM sleep; REM sleep; activity; insula; multiscale; sleep; wake.

Figure 1

Photomicrographs (A) and overlays (B) of anterior insula (AI) lesions. Stereotaxic injections of ibotenic acid result in cell body lesions within the AI, as seen in Nissl body staining of coronal sections (A). Overlays of AI lesion area in individual animals are shown in color online. Overlays show extensive lesion of the agranular anterior insular cortex (B). The overlay shown in (A) is also shown in (B), Ra107 in orange.

Figure 2

Overall percentages of wake (A), NREM (B), and REM (C) sleep in control and AI-lesioned rats, over 24 h, 12 h of dark, and 12 h of light, show increased NREM sleep and decreased wakefulness in AI-lesioned rats, as well as increased REM sleep. Total number of episodes of wake (D), NREM (E), and REM (F) sleep in control and AI-lesioned rats, over 24 h, 12 h of dark, and 12 h of light, showing an overall increase in REM bouts, especially during the light phase. Average percentages and average EEG fast Fourier transform (FFT) power of wake (G, H), NREM sleep (I, J), and REM sleep (K, L) are shown. The decrease in wake across the dark phase (G) in AI-lesioned animals is also marked by an increase in theta (5-10 Hz) band power, while the increase in NREM sleep during the dark phase (I) is also marked by decreased delta (0.5-4.5 Hz) and increased theta band power (J). While the amount of REM sleep is increased, especially during the light phase (K), the FFT power is similar, with a small increase in theta power (L). *p < 0.05. CTL = control animals; AI = anterior insula–lesioned animals; W = wake; NR = NREM sleep; R = REM sleep.

Figure 3

The average duration and frequency distributions of wake (A, B), NREM sleep (C, D), and REM sleep (E, F) episodes is shown across 24 h. Average bout durations show a decrease in wake episode duration in AI animals overall and an increase in NREM sleep duration. The cumulative percentage of bouts, by the bout duration, shows an increase in long-duration NREM sleep bouts in AI-lesioned animals (D). Consistent with the average bout duration of REM sleep (E), AI-lesioned animals have more REM sleep bouts across bout duration bins, but the cumulative percentage of the distribution of bout durations is similar in both groups (F). *p < 0.05. CTL = control animals; AI = anterior insula–lesioned animals; W = wake; NR = NREM sleep; R = REM sleep.

Figure 4

The total number of transitions between wake to NREM sleep (A), NREM sleep to wake (B), and NREM sleep to REM sleep (C) is shown across 24 h, the dark phase, and the light phase. AI lesions have increased transitions from NREM to REM sleep, especially during the light phase. Example hypnograms for sleep-wake states are shown for 1 control animal (D, F) and 1 AI-lesioned animal (E, G), showing wake to NREM transitions during the dark phase (D, E) and NREM to REM sleep transitions during the light phase (F, G). Transitions are marked as vertical bars, and bouts are marked as gray dots. AI animals have more frequent REM sleep episodes compared to control animals during the light phase and more NREM during the dark phase. *p < 0.05, mean ± SEM. CTL = control animals; AI = anterior insula–lesioned animals; W = wake; NR = NREM sleep; R = REM sleep.

Figure 5

Example activity plots for 1 control animal (A) show self-similar activity patterns across multiple time scales (i.e., the temporal structure is not readily distinguishable at different time scales). Measurements of the FFT power of activity across multiple time scales from 1 to 24 h (B) show multiple peaks in the Fourier amplitude at 6, 8, 12, and 24 h (mean). Comparisons of control and anterior insula (AI) lesion groups show a degradation of signal power at the 12-h time scale in AI-lesioned rats compared to controls. Detrended fluctuation analysis of activity shows normal activity signals in an example control animal but a degradation in the function from 4 to 12 h in an AI-lesioned animal (C). The average scaling exponent fit for 2 time ranges, 0.1 to 1 h (α₁) and 4 to 12 h (α₂), reveals a disruption at the α₂ range, but not the α₁ range, in AI-lesioned rats compared to controls (D). *p < 0.05, mean ± SEM.