What single-unit recording studies tell us about the basic mechanisms of sleep and wakefulness

This paper reviews the presence, localization and characteristics of state-specific neurons in the mouse forebrain, midbrain and hindbrain that are involved in the control of ultradian sleep-wake cycles and shows that all these regions contain basic neural elements capable of generating the sleep-wake cycle. The chronic single-unit recording method in unanaesthetized animals is useful for unravelling the dynamics of sleep-wake switching, in particular because it can analyse events at the level of single neurons, thereby decoding information used by the brain in determining its functional state. A prerequisite is to record a large number of all types of neurons, identify critical wake- and sleep-promoting neurons and determine their activity profiles during the sleep-wake cycle and their trends in spike activity during the state transitions from wakefulness to sleep and from sleep to wakefulness in the same species. Here, I argue that single-unit recordings in unanaesthetized mice help us to (a) determine key neural elements controlling sleep-wake dynamics, (b) elucidate the roles of forebrain and brainstem neurons in ultradian sleep-wake cyclicity and (c) gain a new insight into the functional significance of wakefulness, slow-wave sleep and paradoxical (or rapid eye movement) sleep. I also discuss the merits and limitations of single-unit recording compared with more recent genetic approaches, and I suggest that findings from studies using the classic electrophysiological technique will provide the foundation for future studies using new genetic techniques to dissect the neural networks responsible for the initiation, maintenance and cessation of each wake and sleep state.