Zebrafish as a Tool in the Study of Sleep and Memory-related Disorders.

Sleep is an evolutionarily conserved phenomenon, being an important biological necessity for the learning process and memory consolidation. The brain displays two types of electrical activity during sleep: slow-wave activity or Non-Rapid Eye Movement (NREM) sleep, and desynchronized brain wave activity or Rapid Eye Movement (REM) sleep. There are many theories regarding "Why we need to sleep?"; one of them is the synaptic homeostasis. This theory suggests the role of sleep in the restoration of synaptic homeostasis, which is destabilized by synaptic strengthening triggered by learning during waking and by synaptogenesis during development. Sleep diminishes the plasticity load on neurons and other cells to normalize synaptic strength whereas it reestablishes neuronal selectivity and the ability to learn, leading to the consolidation and integration of memories. The use of zebrafish as a tool to assess sleep and its disorders is growing, although sleep in this animal is not yet divided, for example, into REM and NREM states. However, zebrafish are known to have a regulated daytime circadian rhythm, and their sleep state is characterized by periods of inactivity accompanied by an increase in arousal threshold, preference for resting place, and the "rebound sleep effect" phenomenon, which causes an increased slow-wave activity after a forced waking period. In addition, drugs known to modulate sleep, such as melatonin, nootropics, and nicotine have been tested in zebrafish. In this review, we discuss the use of zebrafish as a model to investigate sleep mechanisms and their regulation, demonstrating this species as a promising model for sleep research.