Adenosine in the tuberomammillary nucleus inhibits the histaminergic system via A1 receptors and promotes non-rapid eye movement sleep

Adenosine has been proposed to promote sleep through A₁ receptors (A₁R's) and/or A_2A receptors in the brain. We previously reported that A_2A receptors mediate the sleep-promoting effect of prostaglandin D₂, an endogenous sleep-inducing substance, and that activation of these receptors induces sleep and blockade of them by caffeine results in wakefulness. On the other hand, A₁R has been suggested to increase sleep by inhibition of the cholinergic region of the basal forebrain. However, the role and target sites of A₁R in sleep–wake regulation remained controversial. In this study, immunohistochemistry revealed that A₁R was expressed in histaminergic neurons of the rat tuberomammillary nucleus (TMN). In vivo microdialysis showed that the histamine release in the frontal cortex was decreased by microinjection into the TMN of N⁶-cyclopentyladenosine (CPA), an A₁R agonist, adenosine or coformycin, an inhibitor of adenosine deaminase, which catabolizes adenosine to inosine. Bilateral injection of CPA into the rat TMN significantly increased the amount and the delta power density of non-rapid eye movement (non-REM; NREM) sleep but did not affect REM sleep. CPA-promoted sleep was observed in WT mice but not in KO mice for A₁R or histamine H₁ receptor, indicating that the NREM sleep promoted by A₁R-specific agonist depended on the histaminergic system. Furthermore, the bilateral injection of adenosine or coformycin into the rat TMN increased NREM sleep, which was completely abolished by coadministration of 1,3-dimethyl-8-cyclopenthylxanthine, a selective A₁R antagonist. These results indicate that endogenous adenosine in the TMN suppresses the histaminergic system via A₁R to promote NREM sleep.