Sleep Promotes Branch-Specific Formation of Dendritic Spines after Learning
The function of sleep is one of the greatest mysteries in neuroscience. Nearly every species of animal requires sleep, yet it is still not known why. One prominent idea, known as the sleep homeostasis hypothesis, suggests that waking results in a global increase in the strengths of synaptic connections in the brain, a phenomenon which is unsustainable because stronger connections consume more energy and take up more space.
The function of sleep, according to this hypothesis, is to downscale or renormalize connection strengths. In support of this theory, previous studies have found that overall synaptic strengths and numerous synaptic proteins are up-regulated during wakefulness and down-regulated during slow-wave sleep. A net loss of synapses was also found during sleep in the developing mouse cortex and in the invertebrate nervous system. These observations suggest that sleep may be involved in the downscaling of synaptic connectivity that has been potentiated during wakefulness.
On the other hand, it is well known that sleep plays an important role in learning and memory. It is known that neurons involved in daytime experiences are reactivated during sleep and exhibit patterns of rhythmic activity. Animal studies have found that ocular dominance plasticity and cortical-evoked local field potential increase rather than decrease after a slow-wave sleep episode. Furthermore, the expression of several proteins required for synaptic plasticity increases during the early hours of sleep. Human studies suggest that the quantity and quality of sleep have a profound impact on learning and memory. Together, these studies support the opposing view that sleep promotes, rather than down-regulates, synaptic plasticity related to learning and memory.
In a recent paper published in the journal Science, Dr. Guang Yang and colleagues employed mice genetically engineered to express a fluorescent protein in neurons, along with motor-learning tests to investigate the role of sleep in learning and memory formation. By using transcranial two-photon microscopy, Yang et al. were able to track and image the growth of postsynaptic dendritic spines along individual dendritic branches before and after mice learned a motor task. Surprisingly, they found that sleep after motor learning promotes the formation of dendritic spines on a subset of branches of individual layer V pyramidal neurons in primary motor cortex. These new spines were formed on different sets of dendritic branches in response to different learning tasks and were protected from being eliminated when a new motor task was learned. Furthermore, they showed that neurons activated during learning of a motor task were reactivated during subsequent non-rapid eye movement sleep.
By disrupting specific phases of sleep, Yang et al. showed that deep or slow-wave sleep was necessary for the formation of new connections and memory retention. These findings demonstrate that sleep has a key role in promoting learning-dependent synapse formation on selected dendritic branches, thereby contributing to memory storage.
This study shows the importance of sleep for children. From birth, children spend much of their waking hours learning new information and motor skills and then sleep for long periods of time. During this period, neurons activated during learning are reactivating during subsequent sleep. This brain activity is critical for creating new synaptic connections between neurons and new memories. This study helps to better understand the underlying mechanisms of how sleep facilitates learning.
—Joseph Cichon, PhD
Read the paper “Sleep promotes branch-specific formation of dendritic spines after learning” in Science, published June 6, 2014.