ArticlesOntogeny of REM sleep in rats: possible implications for endogenous depression
Introduction
Pioneering polysomnographic studies 1, 2, 3 found that in altricial mammals, neonates had much more REM sleep than adults of the same species. For example, in rats neonates had about nine times more REM sleep than adults [1]; in cats and humans, neonates had about three to four times more REM sleep than adults 1, 2, 3. The large amount of REM sleep in neonates suggested that neonatal REM sleep had developmental functions, hypotheses that are under empirical investigation 2, 4, 5, 6. This pioneering work suggests several reasons for further investigation of the ontogeny of REM sleep in altricial mammals. (1) Because of technical limitations, early polysomnographic studies of the ontogeny of REM sleep were not able to obtain continuous (24 h/day) polysomnograms for a week or more. Thus, conclusions about developmental changes were based on daily polysomnographic samples that, over time, were collected sequentially from different rats [1]. We have recently developed a new method—the soft head-plug method—for continuous (24 h/day) long-term (weeks) polysomnographic recording of neonatal rats [7]. The new method permits polysomnographic recordings without the limitations of sampling time and sampling different rats. (2) Although the ontogeny of REM sleep amount has been studied, ontogeny of other aspects of REM sleep has not been described. For example, we found no systematic reports of the ontogeny of REM sleep phasic events, REM latency, REM period duration, or REM sleep homeostatic processes. The ontogenies of these REM sleep properties can serve as markers of developmental changes in REM sleep substrates and their possible relationships. (3) The conventional view of the ontogeny of REM sleep is that neonatal REM sleep represents the same state as mature REM sleep. Recent work has suggested a different view, viz., that neonatal REM sleep is an undifferentiated state from which mature REM sleep and non-REM sleep develop or emerge [8]. Thus, further study of the details of the ontogeny of REM sleep may shed light on this controversy. (4) Preliminary data suggested that compared to normal adults, altricial neonates and endogenous depressives had the same distinctive REM sleep characteristics [9]. The finding suggests that further work on the ontogeny of REM sleep may shed light on the hypothesis that REM sleep in endogenous depression represents an immature, incompletely developed, REM sleep system.
The present study investigated the ontogeny of REM sleep in neonatal rats by continuous (24 h/day) polysomnographic recordings for the 2 weeks from age 14 days through age 27 days.
Section snippets
Materials and methods
Nine male Long–Evans neonatal rats had continuous (24 h/day) polysomnography for 2 weeks, from age 14 days to age 28 days. Polysomnographic electrodes were implanted by the soft head-plug method (see below). Two electroencephalographic (EEG) electrodes and two nuchal muscle electrodes were implanted under metofane anesthesia in each rat at age 13 days. The two EEG electrodes were located 1–2 mm and 4–6 mm posterior to bregma about 3–4 mm lateral to the midline. The electrode arrangement
Results
Typical polysomnographic records of the sleep/wake states are displayed in Fig. 1. As shown in the figure, the EEG/EMG patterns of each state were clearly different and easily distinguishable. Like adult REM sleep, neonatal REM sleep was characterized by lower EEG amplitude than slow-wave sleep, and by an absence of muscle tone except for intermittent, momentary phasic muscle contractions or twitches. Like adult slow-wave sleep, neonatal slow wave sleep was characterized by higher EEG amplitude
Discussion
The present findings about the ontogeny of tonic REM sleep in rats were similar to earlier pioneering findings by Jouvet–Mounier [1]. Both studies found a large amount of tonic REM sleep at age 14 days, which decreased substantially over the next 2 weeks and was replaced mainly by wake. There were some minor differences in outcome. The reasons for these differences are not clear. They could include: (1) rat strain—we studied Long–Evans rats, while Jouvet–Mounier did not specify strain; (2)
Acknowledgements
This work was supported by NIMH Grants MH40880 and MH57904.
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