Adenosine, energy metabolism and sleep homeostasis

Adenosine is directly linked to the energy metabolism of cells. In the central nervous system (CNS) an increase in neuronal activity enhances energy consumption as well as extracellular adenosine concentrations. In most brain areas high extracellular adenosine concentrations, through A1 adenosine receptors, decrease neuronal activity and thus the need for energy. Adenosine may be a final common pathway for various sleep factors. We have identified a relatively specific area, the basal forebrain (BF), which appears to be central in the regulation/execution of recovery sleep after sleep deprivation (SD), or prolonged wakefulness. Adenosine concentration increases in this area during SD, and this increase induces sleep while prevention of the increase during SD abolishes recovery sleep. The increase in adenosine is associated with local changes in energy metabolism as indicated by increases in levels of pyruvate and lactate and increased phosphorylation of AMP-activated protein kinase. The increases in adenosine and sleep are associated with intact cholinergic system since specific lesion of the BF cholinergic cells abolishes both. Whether adenosine during SD is produced by the cholinergic neurons or astrocytes associated with them remains to be explored. An interesting, but so far unexplored question regards the relationship between the local, cortical regulation of sleep homeostasis and the global regulation of the state of sleep as executed by lower brain mechanisms, including the BF. The increase in adenosine concentration during SD also in cortical areas suggests that adenosine may have a role in the local regulation of sleep homeostasis. The core of sleep need is probably related to primitive functions of life, like energy metabolism. It can be noted that this assumption in no way excludes the possibility that later in evolution additional functions may have developed, e.g., related to complex neuronal network functions like memory and learning.