Cold exposure and sleep in the rat: REM sleep homeostasis and body size

Abstract

Study objectives: Exposure to low ambient temperature (Ta) depresses REM sleep (REMS) occurrence. In this study, both short and long-term homeostatic aspects of REMS regulation were analyzed during cold exposure and during subsequent recovery at Ta 24 degrees C.

Design: EEG activity, hypothalamic temperature, and motor activity were studied during a 24-h exposure to Tas ranging from 10 degrees C to -10 degrees C and for 4 days during recovery.

Setting: Laboratory of Physiological Regulation during the Wake-Sleep Cycle, Department of Human and General Physiology, Alma Mater Studiorum-University of Bologna.

Subjects: 24 male albino rats.

Interventions: Animals were implanted with electrodes for EEG recording and a thermistor to measure hypothalamic temperature.

Measurements and results: REMS occurrence decreased proportionally with cold exposure, but a fast compensatory REMS rebound occurred during the first day of recovery when the previous loss went beyond a "fast rebound" threshold corresponding to 22% of the daily REMS need. A slow REMS rebound apparently allowed the animals to fully restore the previous REMS loss during the following 3 days of recovery.

Conclusion: Comparing the present data on rats with data from earlier studies on cats and humans, it appears that small mammals have less tolerance for REMS loss than large ones. In small mammals, this low tolerance may be responsible on a short-term basis for the shorter wake-sleep cycle, and on long-term basis, for the higher percentage of REMS that is quickly recovered following REMS deprivation.

Figure 1

Relationship between the duration of the interval between two consecutive REMS episodes (REMS interval, classes of 2 min) and that of the episode that either precedes (Preceding REMS episode, upper diagram) or follows (Subsequent REMS episode, lower diagram) the interval, during the L period (09:00–21:00) of either baseline (BL; white), 24-h exposure to different low Tas (from −10°C to 10°C, E1; grey), or the first day of the recovery period at Ta 24°C (R1; black). The average duration of the REMS episode was calculated for each animal and for each class on the pool of single REMS episodes and REM sleep clusters and, subsequently, the values for different animals were averaged. Data relative to intervals lasting more than 23 min are shown separately. The best-fit linear regression line is shown for each condition.

Figure 2

Relationship between REMS loss during the 24-h exposure to different low Tas (10°C, circle; 5°C, rhombus; 0°C, square; −10°C, triangle) and subsequent REMS gain during each of the following four days of recovery (R1-R4) at Ta 24°C. Each point represents an individual rat (n=24). Both REMS loss and REMS gain are expressed as the percentage of the daily REMS amount in the baseline (BL) condition. The solid line is the best-fit linear regression line. In the plot relative to R1, the dotted line indicates the hypothetical best-fit regression line if REMS gain fully compensates for the previous REMS loss. In the plots relative to R2–R4, the dotted line indicates the hypothetical best-fit regression line in the case of absence of any REMS gain.

Figure 3

Time course of the loss of REMS related parameters during exposure to different low Tas (from 10°C to −10°C, E1) and the following recovery period at Ta 24°C (R1–R4). Relative cumulative loss (% of daily baseline levels ± S.E.M.) of the total amount of REMS (filled circles) and theta energy in REMS (empty squares) is shown. Each point represents the average 2-h value for 24 animals; either the upper or the lower limit of S.E.M. is indicated by a thin line (solid for the amount, dashed for the energy).

Figure 4

Relationship between REMS “ultradian” (upper diagram) and “fast rebound” (lower diagram) thresholds and either body (filled symbol) or brain (empty symbol) mass, in the rat (circle), cat (triangle), and human (square). Values are shown on a double log plot. The best power function curve estimation (broken line) is shown for each relationship.