Neonatal maternal separation disrupts regulation of sleep and breathing in adult male rats

Abstract

Study objectives: Neonatal maternal separation (NMS) disrupts development of cardiorespiratory regulation. Adult male rats previously subjected to NMS are hypertensive and show a hypoxic ventilatory response greater than that of controls. These results have been obtained in awake or anesthetised animals, and the consequences of NMS on respiratory control during normal sleep are unknown. This study tested the following.

Hypotheses: NMS augments respiratory variability across sleep-wake states, and NMS-related enhancement of the hypoxic ventilatory response occurs during sleep.

Methods: Two groups of adult rats were used: controls (no treatment) and rats subjected to NMS. Ventilatory activity, coefficient of variation, and hypoxic ventilatory response were compared between groups and across sleep-wake states.

Subjects: Male Sprague Dawley rats-NMS: n=11; controls: n=10. Pups subjected to NMS were isolated from their mother for 3 hours per day from postnatal days 3 to 12. Controls were undisturbed.

Measurements and results: At adulthood, sleep-wake states were monitored by telemetry, and ventilatory activity was measured using whole-body plethysmography. Sleep and breathing were measured for 2.5 hours (in the morning) while the rats were breathing room air. Data were analysed in 20-second epochs. Rats were then exposed to a brief (90-sec) hypoxic episode (nadir = 12% O2) to measure the hypoxic ventilatory response. The coefficient of variability for tidal volume and breathing frequency decreased during sleep but remained more elevated in NMS rats than in controls. During non-rapid eye movement sleep, the breathing-frequency response to hypoxia of NMS rats was significantly greater than that of controls.

Conclusion: Neonatal maternal separation results in persistent disruption of respiratory control during sleep.

Figure 1

Effects of neonatal maternal separation (NMS) on sleep architecture during normoxia. (A) Total recording time and (B) proportion of time spent in each sleep-wake state during the 2.5 hours of baseline recording. (C) Mean duration of each bout of sleep-wake state. Measurements were performed on adult rats previously subjected to NMS (black bars, n = 11) and control rats (undisturbed; white bars, n = 10). † indicates a mean value statistically different from the corresponding control value at P < 0.05. Non-REM refers to non-rapid eye movement sleep.

Figure 2

Comparison of breathing frequency and its variability across sleep-wake states between controls (open bars, n = 10) and rats previously subjected to neonatal maternal separation (NMS; black bars, n = 11). Values originate from recordings performed during 2.5 hours under normoxic conditions. (A) Absolute values, (B) data expressed as a percentage change from wakefulness, and (C) coefficient of variation. † indicates a mean value statistically different from corresponding control value at P < 0.05. * indicates a value significantly different from the corresponding wakefulness value at P < 0.05. REM refers to rapid eye movement sleep.

Figure 3

Comparison of tidal volume and its variability across sleep-wake states between controls (open bars, n = 10) and rats previously subjected to neonatal maternal separation (NMS; black bars, n = 11). Values originate from recordings performed during 2.5 hours under normoxic conditions. (A) Absolute values, (B) data expressed as a percentage change from wakefulness, and (C) coefficient of variation. † indicates a mean value statistically different from the corresponding control value at P < 0.05. * indicates a value significantly different from the corresponding wakefulness value at P < 0.05. REM refers to rapid eye movement sleep.

Figure 4

Comparison of minute ventilation and its variability across sleep-wake states between controls (open bars, n = 10) and rats previously subjected to neonatal maternal separation (NMS; black bars, n = 11). Values originate from recordings performed during 2.5 hours under normoxic conditions. (A) Absolute values, (B) data expressed as a percentage change from wakefulness, and (C) coefficient of variation. † indicates a mean value statistically different from the corresponding control value at P < 0.05. * indicates a value significantly different from the corresponding wakefulness value at P < 0.05. REM refers to rapid eye movement sleep.

Figure 5

(A) Comparison of the breathing frequency response to hypoxia (90 sec) between controls (white circles) and rats previously subjected to neonatal maternal separation (NMS; black circles) during non-rapid eye movement (non-REM) sleep. The figure also shows the change in frequency during the return to normoxia (recovery: 120 sec). The small circles show the fraction of inspired O₂ measured in the chamber during the experiment (right-hand axis). In this panel, data were obtained every 10 seconds. (B) Comparison of the mean breathing-frequency responses measured at the end of hypoxia (last 2 data bins; 20 sec) obtained in control (white bars) and NMS rats (black bars) during wakefulness (control: n = 4; NMS: n = 3) and REM sleep (control: n = 3; NMS: n = 4).

Figure 6

Comparison of the time course of the minute-ventilation response to hypoxia between controls (open circles) and rats previously subjected to neonatal maternal separation (NMS; black circles). Minute-ventilation responses were measured during (A) non-rapid eye movement (non-REM) sleep, (B) REM sleep, and (C) wakefulness. In these panels, data were obtained every 10 seconds. The smaller inserts (panels B, D, and F) report the tidal volume responses measured at the end of the hypoxic stimulus (last 2 data bins; 20 sec) in each sleep-wake state. Data are expressed as percentage change from baseline. Each point or bar represents the mean value ± SEM. † indicates a value significantly different from the corresponding control value at P < 0.05.

Figure 7

Correlation between the minute ventilation response to hypoxia (last 20 sec of hypoxia expressed as a percentage change from baseline; Figure 6 and the coefficient of variation for this variable during the 2.5 hours of recording under normoxia (Figure 4). These measurements were obtained during non-rapid eye movement (non-REM) sleep. Open circles: control rats (n = 5); black circles: rats previously subjected to neonatal maternal separation (NMS; n = 6).