A good night sleep is essential for refreshing oneself after a hectic day. It’s now known to us that sleep is essential not only for healthy body, but also for healthy mind. The role of sleep in the consolidation of memory has already been established. But the cellular and molecular processes behind this sleep induced memory consolidation are not yet known.
Researches have shown conflicting views regarding the role of sleep in synaptic plasticity. Some research supports the idea of synaptic down-scaling, whereas other supports the idea of synaptic up scaling during sleep.
A study conducted by Yang et al at New York University, School of Medicine, showed that sleep promotes formation of post-synaptic dendritic spines on a subset of branches of individual layer V pyramidal neurons in the mortor cortex of mice.
The study can be divided into two parts. In 1st part, they assessed spine formation following training. For this, they used two groups of mice. The experimental group consisting of mice that were given motor training for 2 days and the control group consisting of untrained mice. Rotarod motor training was given to the experimental group. The results were analyzed based on time frame of experiment. Firstly, within 6 hours of motor training and then within 24 hours and after 24 hours of motor training. In 2nd part, they gave rotarod training to mice and divided the mice into two categories: non-sleep deprived and sleep deprived mice. Then, they assessed the formation of new dendritic spines in sleep deprived vs non sleep deprived mice.
The results showed significantly higher formation of dendritic spines in trained mice in first 6 hours of training as compared to untrained controls. This formation of dendritic spines in trained mice was continuous in the 1st day. The spine formation remained confined to about 30% of dendrities after 24 hours of motor learning. Likewise, there was significant reduction in dendritic spines in sleep deprived mice as compared to non-sleep deprived mice. The reduction in the dendritic spines after sleep deprivation cannot be compensated either by motor training or by sleeping again.
Sleep consist of two phases: Rapid eye movement(REM) phase and NREM(non rapid eye movement) phase. Sleep deprivation in REM phase did not disrupt the formation of dendritic spines. Its in the NREM phase of sleep that dendritic spines are formed. Also, the study showed via calcium imaging that it’s the same neurons that are activated during neuronal activation at wakefulness and NREM phase of sleep. This neuronal reactivation during NREM phase is critical in the formation of new dendritic spines.
Concluding the results, rotarod training is important for the formation of new synapses as shown by increase in dendritic spines in trained mice as compared to untrained controls in the 1st part of the experiment. However, there was significant reduction of dendritic spines in sleep deprived mice undergoing motor training suggesting that sleep is necessary for formation of new synapses after learning. Formation of new synapses after learning occurs during NREM phase of sleep and neuronal reactivation is responsible for it.
Guang Yang et al, Sleep promotes branch-specific formation of dendritic spines after learning, Science 344,1173 (2014); DOI: 10.1126/science.1249098