If everyone slept a full eight hours every night, it is estimated that we would each spend about one third of our lives sleeping. Given that incredible time investment alone, it is clear that sleep must be extraordinarily important to our survival. As such, neuroscientists have put a lot of effort into deciphering the function of sleep as it pertains to the brain. Behavioral sleep research in neuroscience tends to focus on the question of the role of sleep in memory processing, and there are many ways to set out answering that vital question in rodent models.
An important aspect of most sleep studies in rodents is sleep deprivation. These studies will often compare the performance of animals that have been sleep deprived to those that have not. Alternatively, a study may compare the performance of the same group of animals on a task before and after sleep deprivation. There are a few different methods that are commonly used to keep the rodents awake.
The first method is the scientific equivalent of simply prodding an animal so that it cannot get any sleep. This is generally referred to as the gentle stimulation method of sleep deprivation, and usually entails tapping or moving the cage just enough to wake the animals but not enough to spook them (sleep deprivation is stressful enough – no need to compound it with vigorous cage-shaking). Researchers can also introduce novel objects to the home cage, as novel objects and experiences are stimulating to rodents and should keep them awake. Along these lines, the rodents can also be removed from the cage and handled periodically or exposed to novel environments.
A second method of sleep deprivation is known as forced locomotion. Just as it sounds, this method entails keeping animals moving, often with the assistance of a small treadmill or other device that will keep the animal walking.
One last method of sleep deprivation commonly cited in the literature is the flowerpot method. In this setup, rodents are placed on a platform in a tank of water (picture a flowerpot turned upside down surrounded by water, although in the laboratory it is often a small disc). When rodents fall asleep, the resulting loss of muscle tone causes them to fall into the water and wake up. This is a particularly interesting method because it relies on this loss of muscle tone to keep the rodents awake, which varies depending on the stage of sleep that the animal is currently in. For example, an awake animal has full control of their musculature and will stay on the platform. When animals first fall asleep, they maintain some muscle tone, and depending on the size of the platform, they may be able to sleep a little bit without hitting water. However, once animals reach rapid eye movement sleep, or REM sleep, there is a complete loss of muscle tone that will result in falling off the platform. As we will discuss later in this post, the stage of sleep that is deprived may provide more specific insight into the role of sleep in specific memory functions. If deprivation of REM sleep is of particular interest for a study, the flowerpot method is a good choice for selective stage-dependent sleep deprivation. Alternatively, researchers can record from the brains and muscles of sleeping rodents (using electroencephalograms and electromyograms, or EEGs and EMGs, respectively) to determine sleep stage. In this setup, one of the aforementioned sleep deprivation methods can be employed to interrupt the desired stage.
It should be noted that each of these sleep deprivation methods is usually used in studies of acute sleep deprivation. There are relatively fewer studies in rodents on chronic sleep deprivation, not for lack of interest in such studies but often because of the feasibility of employing any of these methods long-term.
Many standard assays for memory performance (e.g. novel object recognition, Morris water maze, fear condition, etc.) can be used in conjunction with these sleep deprivation methods to study the functions of sleep. One important consideration for designing a study on sleep is the neural circuitry that is employed for the memory assay being performed. The hippocampus has been shown to be particularly sensitive to the effects of sleep deprivation. Given this region’s prominent role in the formation of memories, one might be led to assume that the performance of a sleep deprived rodent in a memory task is the result of the effects of sleep in the hippocampus. However, certain memory assays rely more heavily on other brain regions, and if sleep deprivation does not particularly impact these regions, a change in behavior may not be observed.
For example, sleep deprivation has been shown to produce deficits in contextual fear conditioning. This task is known to be dependent upon the hippocampus. However, if the fear conditioning assay was linked to a single stimulus (e.g. a tone) rather than a change in context, the hippocampus may take a back seat to other regions such as the amygdala during that association. In that assay, sleep deprivation may not change freezing behavior as it would in contextual fear conditioning. Similar issues could arise in assays for spatial memory, such as the radial arm maze. If the rodent navigates the maze by using environmental cues around the room, sleep deprivation will probably produce a change in performance, as this process depends on the hippocampus. However, if the rodent’s navigation strategy is action-based (like the rodent reaches an intersection and thinks “turn left”), performance may not change in this instance, as such a strategy relies more on the striatum than the hippocampus. In this particular scenario, one relatively easy way to check the rodent’s strategy is to change where the reward is in the maze (often referred to as reversal-learning). An animal employing a striatum-dependent, action-based strategy will take much longer to adjust to this change, and sleep deprivation should exacerbate that delayed learning.
Another important consideration in sleep studies is the timing of the sleep deprivation. Generally speaking, there are three stages of memory: acquisition, consolidation, and retrieval. The timing of the sleep deprivation can be targeted to influence any of these stages. For example, sleep deprivation before a memory task is generally targeting the acquisition phase of memory formation. Sleep deprivation for several hours after the acquisition of a memory targets the consolidation stage, which is essentially the process that converts recently acquired short-term memories into stable long-term memories. These distinctions are important because different molecular processes underlie each stage of memory. Furthermore, unique molecular processes occur over the course of several hours during consolidation, so careful timing of sleep deprivation relative to memory acquisition can assist in identifying which specific molecular processes are critical to different kinds of memory.
To illustrate the importance of the timing of sleep deprivation, let’s say you’ve designed an experiment in which a rodent is sleep deprived for ten hours following memory acquisition. That sleep deprivation is likely to impact the rodent’s performance on whatever task it was learning. However, that experiment doesn’t say much about what specific molecular processes are important for the memory, given how many things happen during those ten hours. A more informative experiment may involve sleep depriving some rodents for five hours after memory acquisition, and having another group that is permitted to sleep for five hours but then sleep deprived for hours 6 through 10. If one group outperforms the other on the task, even though they’ve both had five hours of sleep deprivation, it may provide some insight into what potential molecular mechanisms are necessary for the formation of that memory and therefore merit further scrutiny.
In summary, conventional rodent memory assays are excellent tools for the study of sleep when combined with established sleep deprivation protocols. Many of these assays are simple to perform and require relatively little training. When designing a sleep study using rodent models, be sure to pay particular attention to the timing of sleep deprivation relative to memory acquisition as well as the circuitry that is known to be at work for a given type of memory task.