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The neural substrates of infant sleep in rats
Karl A E Karlsson1, Andrew J Gall, Ethan J Mohns
1Program in Behavioral and Cognitive Neuroscience, Department of Psychology, University of Iowa, Iowa City, USA.
Insights
Infant rat sleep involves muscle atonia and twitches, controlled by brainstem areas similar to adults. This challenges theories about early sleep development lacking higher brain activity.
Area of Science:
- Neuroscience
- Developmental Biology
- Sleep Research
Background:
- Sleep is crucial but poorly understood in infants, often studied only in adults.
- Infant sleep is characterized by muscle atonia and myoclonic twitches, but its neural basis is unclear.
- The absence of state-dependent neocortical activity is perceived as an impediment to studying early sleep.
Purpose of the Study:
- To investigate the neural circuitry underlying muscle atonia and myoclonic twitches in infant rats.
- To identify brainstem areas projecting to the medullary inhibitory area (MIA) and their role in sleep.
- To compare the neural substrates of infant sleep with those of adult sleep.
Main Methods:
- Described a medullary inhibitory area (MIA) in week-old rats.
- Investigated projections from the subcoeruleus (SubLC), pontis oralis (PO), and dorsolateral pontine tegmentum (DLPT) to the MIA.
- Performed lesions in the SubLC, PO, and DLPT to observe effects on muscle atonia and twitches.
Main Results:
- Neurons in SubLC, PO, and DLPT exhibit state-dependent activity, suggesting roles in muscle tone modulation and myoclonic twitches.
- Lesions in SubLC and PO resulted in REM sleep without atonia (decreased atonia, unchanged twitching).
- Lesions in DLPT increased atonia but decreased twitching.
Conclusions:
- The neural substrates of infant rat sleep are remarkably similar to those of adult sleep.
- Supraspinal neural elements contribute to infant sleep even before state-dependent neocortical activity.
- Findings challenge existing theories on the development of sleep regulation.
Abstract:
Sleep is a poorly understood behavior that predominates during infancy but is studied almost exclusively in adults. One perceived impediment to investigations of sleep early in ontogeny is the absence of state-dependent neocortical activity. Nonetheless, in infant rats, sleep is reliably characterized by the presence of tonic (i.e., muscle atonia) and phasic (i.e., myoclonic twitching) components; the neural circuitry underlying these components, however, is unknown. Recently, we described a medullary inhibitory area (MIA) in week-old rats that is necessary but not sufficient for the normal expression of atonia. Here we report that the infant MIA receives projections from areas containing neurons that exhibit state-dependent activity. Specifically, neurons within these areas, including the subcoeruleus (SubLC), pontis oralis (PO), and dorsolateral pontine tegmentum (DLPT), exhibit discharge profiles that suggest causal roles in the modulation of muscle tone and the production of myoclonic twitches. Indeed, lesions in the SubLC and PO decreased the expression of muscle atonia without affecting twitching (resulting in "REM sleep without atonia"), whereas lesions of the DLPT increased the expression of atonia while decreasing the amount of twitching. Thus, the neural substrates of infant sleep are strikingly similar to those of adults, a surprising finding in light of theories that discount the contribution of supraspinal neural elements to sleep before the onset of state-dependent neocortical activity.
