Adenosine A₁ and A_2A Receptors in Sleep Disorders: Mechanisms and Therapeutic Implications

Sleep-wake regulation is controlled by circadian and homeostatic processes, with adenosine acting as a key molecular mediator of homeostatic sleep pressure. Extracellular adenosine accumulates during wakefulness as a result of neuronal energy metabolism, particularly in the basal forebrain, and declines during recovery sleep, thereby reflecting the physiological need for sleep. The sleep-promoting effects of adenosine are mediated primarily by two G protein-coupled receptor subtypes, the adenosine A₁ receptor and the adenosine A_2A receptor. The adenosine A₁ receptor, coupled to inhibitory Gi/o proteins and widely expressed in the cortex, hippocampus, thalamus, and basal forebrain, suppresses wake-promoting neuronal activity and facilitates slow-wave activity during non-rapid eye movement sleep. In contrast, the adenosine A_2A receptor, coupled to stimulatory Golf proteins and enriched in the striatum and nucleus accumbens, promotes sleep by activating neurons in the preoptic hypothalamus and engaging the indirect basal ganglia pathway. Despite these well-established roles, the contributions of dysregulation of the adenosine A₁ receptor and the adenosine A_2A receptor to specific sleep disorders remain incompletely understood. This review examines how signaling of the adenosine A₁ receptor and the adenosine A_2A receptor is altered in insomnia, obstructive sleep apnea, narcolepsy, and restless legs syndrome, and evaluates the therapeutic potential of receptor-selective strategies for adenosine receptor-targeted treatment.