Sleep, like waking, is a complex phenomenon comprising fluctuations in many neural and physiological systems. When we fall asleep, our skeletal muscle loses tone, the electrical activity in our cerebral cortex (i.e., the EEG) changes, and, during active sleep, our eyes dart around and our limbs twitch. The challenge of studying infant sleep is that these various components, which have been studied extensively in adults, do not always present themselves clearly in infants. For example, the EEG of infant rats before eleven days of age does not exhibit the clearly differentiable activity upon which researchers rely so heaviliy when judging adult sleep. These and other factors mean that we must assess infant sleep on its own terms rather than judge it against an adult standard. This is a central tenet of infant research, and one that we forget at our peril.

We begin with behavior. The movie below provides a bird's eye view of sleep and wake activity in 2-, 5-, and 8-day-old rats. Each pup has been lightly secured on its back inside a temperature-controlled chamber that keeps it warm. At each age, the fine twitching movements of the limbs, head, and tail are indicative of active sleep. These movements are called myoclonic twitches. Occasionally, the pups exhibit coordinated movements of the limbs, such as stretching and kicking, that are indicative of the waking state. (If you listen to the audio channel of the movie, you can hear the nuchal muscle activity and easily relate it to the pups' behavior.)

Several years ago, we found that we could measure EMG activity in the nuchal muscle (i.e., the muscle that elevates the head) and reliably relate this measure of muscle activity to behavior. At that point, we had a foundation upon which we could begin examining the neural bases of sleep and wakefulness early in the infant rat's postnatal life. We have now explored medullary, mesopontine, and forebrain mechanisms involved in the modulation of sleep and wakefulness. Here are some relevant publications:

Karlsson, K. Æ., Gall, A. J., Mohns, E. J., Seelke, A. M. H., & Blumberg, M. S. The neural substrates of infant sleep in rats. PLoS Biology, 3, 891-901, 2005. pdf  

Seelke, A. H., Karlsson, K. Æ., Gall, A. J., & Blumberg, M. S. Extraocular muscle activity, rapid eye movements, and the development of active and quiet sleep. European Journal of Neuroscience, 22, 911-920, 2005. pdf

Seelke, A. M. H., & Blumberg, M. S. The microstructure of active and quiet sleep as cortical delta activity emerges in infant rats. Sleep, 31, 691-699, 2008. pdf

You can find a complete list of publications here.

One of the goals of our research is to identify the role that sleep plays in the development of the nervous system. We view sleep as essential to the process by which sensory and motor systems establish the topographic relations (or somatotopic maps) that make normal function possible. This process is particularly critical during early development but also continues throughout life. We believe that sleep, especially active sleep, is critical to this process because it provides a period of relative quiescence when discrete signals can be sent and received by the nervous system.

The figure below illustrates the likely flow of twitch-related activity from its production of the twitch within the brainstem to the movement of the limb to the generation of sensory feedback that then produces spindle burst activity in somatosensory cortex as well as hippocampal activity. As we now know, the corpus callosum modulates this activity; by cutting the corpus callosum early in development, we are now able to explore mechanisms of neural plasticity.

Below are two publications related to twitching and its effects on cortical and hippocampal activity:

Mohns, E. J., & Blumberg, M. S. Synchronous bursts of neuronal activity in the developing hippocampus: Modulation by active sleep, and association with emerging gamma and theta rhythms. Journal of Neuroscience, 28, 10134-10144, 2008. pdf

Marcano-Reik, A. J., & Blumberg, M. S. The corpus callosum modulates spindle-burst activity within homotopic regions of somatosensory cortex in newborn rats. European Journal of Neuroscience, 28, 1457-1468, 2008. pdf

You can find a complete list of publications here

Perhaps the most interesting developmental changes in sleep and wakefulness relate to the temporal organization of these states. For example, we have documented seminal developmental changes in the temporal organization of sleep-wake bouts and are seeking to identify the neural mechanisms that underlie these developmental changes in Norway rats:

Blumberg, M. S., Seelke, A. M. H., Lowen, S. B., & Karlsson, K. Æ. Dynamics of sleep-wake cyclicity in developing rats. Proceedings of the National Academy of Sciences, 102, 14860-14864, 2005. pdf

Gall, A. J., Todd, W. D., Ray, B., Coleman, C. M., & Blumberg, M. S. The development of day-night differences in sleep and wakefulness in Norway rats and the effect of bilateral enucleation. Journal of Biological Rhythms, 23, 232-241, 2008. pdf

Gall, A. J., Joshi, B., Best, J., Florang, V. R., Doorn, J. A., & Blumberg, M. S. Developmental emergence of power-law wake behavior depends upon the functional integrity of the locus coeruleus. Sleep, 32, 920-926, 2009. pdf

Developmental analyses can also be helpful for exploring evolutionary issues pertaining to sleep-wake organization. We are currently adopting a developmental comparative approach to understand circadian rhythmicity using nocturnal (i.e., night-active) Norway rats and diurnal (i.e., day-active) Nile grass rats. By tracking the development of sleep and wakefulness across early development and exploring their neural control, we are identifying the key components that have been evolutionarily altered to produce the phenotypes associated with these different species. Over the coming years we hope to expand this approach to different questions and species.

Our research questions drive our methods. Sometimes, especially when working with infant subjects, new methods must be devised. For example, we were among the first to devise methods for recording brain activity throughout the neuraxis in unanesthetized infant rats.

Our current methods include (or can include):

* Behavioral and electrographic (i.e., EMG) recordings;

* Voltage-sensitive dye imaging of cortical activity;

* Neurophysiological recordings using surface EEG, silicon depth electrodes, and octrodes;

* Chemical and electrical lesions;

* Pharmacological manipulations, including cortical infusions and local inactivation (e.g., with fluorescent muscimol);

* In vivo electrochemistry (i.e., amperometry) for real-time measurements of extracellular concentrations of glutamate, choline, and GABA;

* c-fos immunohistochemistry;

* Running-wheel activity for measures of circadian rhythmicity;

* PCR genotyping.

 

 

Our research is funded by grants from the National Institutes of Health.