Zhi-Li Huang

Arousal effect of orexin A depends on activation of the histaminergic system

Orexin neurons are exclusively localized in the lateral hypothalamic area and project their fibers to the entire central nervous system, including the histaminergic tuberomammillary nucleus (TMN). Dysfunction of the orexin system results in the sleep disorder narcolepsy, but the role of orexin in physiological sleep–wake regulation and the mechanisms involved remain to be elucidated. Here we provide several lines of evidence that orexin A induces wakefulness by means of the TMN and histamine H1 receptor (H1R). Perfusion of orexin A (5 and 25 pmol/min) for 1 hr into the TMN of rats through a microdialysis probe promptly increased wakefulness for 2 hr after starting the perfusion by 2.5- and 4-fold, respectively, concomitant with a reduction in rapid eye movement (REM) and non-REM sleep. Microdialysis studies showed that application of orexin A to the TMN increased histamine release from both the medial preoptic area and the frontal cortex by ≈2-fold over the baseline for 80 to 160 min in a dose-dependent manner. Furthermore, infusion of orexin A (1.5 pmol/min) for 6 hr into the lateral ventricle of mice produced a significant increase in wakefulness during the 8 hr after starting infusion to the same level as the wakefulness observed during the active period in wild-type mice, but not at all in H1R gene knockout mice. These findings strongly indicate that the arousal effect of orexin A depends on the activation of histaminergic neurotransmission mediated by H1R.

Adenosine A2A, but not A1, receptors mediate the arousal effect of caffeine.

Caffeine, a component of tea, coffee and cola, induces wakefulness. It binds to adenosine A1 and A2A receptors as an antagonist, but the receptor subtype mediating caffeine-induced wakefulness remains unclear. Here we report that caffeine at 5, 10 and 15 mg kg−1 increased wakefulness in both wild-type mice and A1 receptor knockout mice, but not in A2A receptor knockout mice. Thus, caffeine-induced wakefulness depends on adenosine A2A receptors.

Dominant localization of prostaglandin D receptors on arachnoid trabecular cells in mouse basal forebrain and their involvement in the regulation of non-rapid eye movement sleep

Infusion of prostaglandin (PG) D2 into the lateral ventricle of the brain induced an increase in the amount of non-rapid eye movement sleep in wild-type (WT) mice but not in mice deficient in the PGD receptor (DP). Immunofluorescence staining of WT mouse brain revealed that DP immunoreactivity was dominantly localized in the leptomeninges (LM) of the basal forebrain but that PGD synthase immunoreactivity was widely distributed in the LM of the entire brain. Electron microscopic observation indicated that DP-immunoreactive particles were predominantly located on the plasma membranes of arachnoid trabecular cells of the LM. The region with the highest DP immunoreactivity was clearly defined as bilateral wings in the LM of the basal forebrain located lateral to the optic chiasm in the proximity of the ventrolateral preoptic area, one of the putative sleep centers, and the tuberomammillary nucleus, one of the putative wake centers. The LM of this region contained DP mRNA 70-fold higher than that in the cortex as judged from the results of quantitative reverse transcription–PCR. PGD2 infusion into the subarachnoid space of this region increased the extracellular adenosine level more than 2-fold in WT mice but not in the DP-deficient mice. These results indicate that DPs in the arachnoid trabecular cells of the basal forebrain mediate an increase in the extracellular adenosine level and sleep induction by PGD2.

Dopaminergic D1 and D2 Receptors Are Essential for the Arousal Effect of Modafinil

Modafinil is a wake-promoting compound with low abuse potential used in the treatment of narcolepsy. Although the compound is reported to affect multiple neurotransmitter systems such as catecholamines, serotonin, glutamate, GABA, orexin, and histamine, however, the molecular mechanism by which modafinil increases wakefulness is debated. Herein we used dopamine (DA) D2 receptor (D2R)-deficient mice combined with D1R- and D2R-specific antagonists to clarify the role of DA receptors in the arousal effects of modafinil. In wild-type mice, intraperitoneal modafinil induced wakefulness in a dose-dependent manner. Pretreatment with either D1R antagonist SCH23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine] at 30 μg/kg or D2R antagonist raclopride at 2 mg/kg blocked the arousal effects of low-dose modafinil at 22.5 and 45 mg/kg. When modafinil was given at 90 and 180 mg/kg, pretreatment of D1R antagonist did not affect the wakefulness at all, whereas D2R antagonist significantly attenuated the wakefulness to the half level compared with vehicle control. Similarly, D2R knock-out (KO) mice exhibited attenuated modafinil-induced wakefulness. However, pretreatment of D2R KO mice with D1R antagonist completely abolished arousal effects of modafinil. These findings strongly indicate that dopaminergic D1R and D2R are essential for the wakefulness induced by modafinil.

Arousal Effect of Caffeine Depends on Adenosine A2A Receptors in the Shell of the Nucleus Accumbens

Caffeine, the most widely used psychoactive compound, is an adenosine receptor antagonist. It promotes wakefulness by blocking adenosine A2A receptors (A2ARs) in the brain, but the specific neurons on which caffeine acts to produce arousal have not been identified. Using selective gene deletion strategies based on the Cre/loxP technology in mice and focal RNA interference to silence the expression of A2ARs in rats by local infection with adeno-associated virus carrying short-hairpin RNA, we report that the A2ARs in the shell region of the nucleus accumbens (NAc) are responsible for the effect of caffeine on wakefulness. Caffeine-induced arousal was not affected in rats when A2ARs were focally removed from the NAc core or other A2AR-positive areas of the basal ganglia. Our observations suggest that caffeine promotes arousal by activating pathways that traditionally have been associated with motivational and motor responses in the brain.

Lipocalin-type prostaglandin D synthase produces prostaglandin D2 involved in regulation of physiological sleep.

Prostaglandin (PG) D2 has been proposed to be essential for the initiation and maintenance of the physiological sleep of rats because intracerebroventricular administration of selenium tetrachloride (SeCl4), a selective inhibitor of PGD synthase (PGDS), was shown to reduce promptly and effectively the amounts of sleep during the period of infusion. However, gene knockout (KO) mice of PGDS and prostaglandin D receptor (DP1R) showed essentially the same circadian profiles and daily amounts of sleep as wild-type (WT) mice, raising questions about the involvement of PGD2 in regulating physiological sleep. Here we examined the effect of SeCl4 on the sleep of WT and KO mice for PGDS and DP1R and that of a DP1R antagonist, ONO-4127Na, on the sleep of rats. The i.p. injection of SeCl4 into WT mice decreased the PGD2 content in the brain without affecting the amounts of PGE2 and PGF2α. It inhibited sleep dose-dependently and immediately after the administration during the light period when mice normally sleep, increasing the wake time; and the treatment with this compound resulted in a distinct sleep rebound during the following dark period. The SeCl4-induced insomnia was observed in hematopoietic PGDS KO mice but not at all in lipocalin-type PGDS KO, hematopoietic and lipocalin-type PGDS double KO or DP1R KO mice. Furthermore, the DP1R antagonist ONO-4127Na reduced sleep of rats by 30% during infusion into the subarachnoid space under the rostral basal forebrain at 200 pmol/min. These results clearly show that the lipocalin-type PGDS/PGD2/DP1R system plays pivotal roles in the regulation of physiological sleep.

Essential Role of Dopamine D2 Receptor in the Maintenance of Wakefulness, But Not in Homeostatic Regulation of Sleep, in Mice

Dopamine (DA) and its D2 receptor (R) are involved in cognition, reward processing, and drug addiction. However, their roles in sleep–wake regulation remain unclear. Herein we investigated the role of D2R in sleep–wake regulation by using D2R knock-out (KO) mice and pharmacological manipulation. Compared with WT mice, D2R KO mice exhibited a significant decrease in wakefulness, with a concomitant increase in non-rapid eye movement (non-REM, NREM) and REM sleep and a drastic decrease in the low-frequency (0.75–2 Hz) electroencephalogram delta power of NREM sleep, especially during the first 4 h after lights off. The KO mice had decreased mean episode duration and increased episode numbers of wake and NREM sleep, many stage transitions between wakefulness and NREM sleep during the dark period, suggesting the instability of the wake stage in these D2R KO mice. When the KO mice were subjected to a cage change or an intraperitoneal saline injection, the latency to sleep in the KO mice decreased to half of the level for WT mice. The D2R antagonist raclopride mimicked these effects in WT mice. When GBR12909, a dopamine transport inhibitor, was administered intraperitoneally, it induced wakefulness in WT mice in a dose-dependent manner, but its arousal effect was attenuated to one-third in the D2R KO mice. However, these 2 genotypes showed an identical response in terms of sleep rebound after 2, 4, and 6 h of sleep deprivation. These results indicate that D2R plays an essential role in the maintenance of wakefulness, but not in homeostatic regulation of NREM sleep.

The Role of Adenosine in the Regulation of Sleep

This paper presents an overview of the current knowledge about the role of adenosine in the sleep-wake regulation with a focus on adenosine in the central nervous system, regulation of adenosine levels, adenosine receptors, and manipulations of the adenosine system by the use of pharmacological and molecular biological tools. The endogenous somnogen prostaglandin (PG) D(2) increases the extracellular level of adenosine under the subarachnoid space of the basal forebrain and promotes physiological sleep. Adenosine is neither stored nor released as a classical neurotransmitter and is thought to be formed inside cells or on their surface, mostly by breakdown of adenine nucleotides. The extracellular concentration of adenosine increases in the cortex and basal forebrain during prolonged wakefulness and decreases during the sleep recovery period. Therefore, adenosine is proposed to act as a homeostatic regulator of sleep and to be a link between the humoral and neural mechanisms of sleep-wake regulation. Both the adenosine A(1) receptor (A(1)R) and A(2A)R are involved in sleep induction. The A(2A)R plays a predominant role in the somnogenic effects of PGD(2). By use of gene-manipulated mice, the arousal effect of caffeine was shown to be dependent on the A(2A)R. On the other hand, inhibition of wake-promoting neurons via the A(1)R also mediates the sleep-inducing effects of adenosine, whereas activation of A(1)R in the lateral preoptic area induces wakefulness, suggesting that A(1)R regulates the sleep-wake cycle in a site-dependent manner. The potential therapeutic applications of agonists and antagonists of these receptors in sleep disorders are briefly discussed.

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 (A1Rs) and/or A2A receptors in the brain. We previously reported that A2A 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.

Adenosine A2A receptors in ventral striatum, hypothalamus and nociceptive circuitry. Implications for drug addiction, sleep and pain

Adenosine A2A receptors localized in the dorsal striatum are considered as a new target for the development of antiparkinsonian drugs. Co-administration of A2A receptor antagonists has shown a significant improvement of the effects of l-DOPA. The present review emphasizes the possible application of A2A receptor antagonists in pathological conditions other than parkinsonism, including drug addiction, sleep disorders and pain. In addition to the dorsal striatum, the ventral striatum (nucleus accumbens) contains a high density of A2A receptors, which presynaptically and postsynaptically regulate glutamatergic transmission in the cortical glutamatergic projections to the nucleus accumbens. It is currently believed that molecular adaptations of the cortico-accumbens glutamatergic synapses are involved in compulsive drug seeking and relapse. Here we review recent experimental evidence suggesting that A2A antagonists could become new therapeutic agents for drug addiction. Morphological and functional studies have identified lower levels of A2A receptors in brain areas other than the striatum, such as the ventrolateral preoptic area of the hypothalamus, where adenosine plays an important role in sleep regulation. Although initially believed to be mostly dependent on A1 receptors, here we review recent studies that demonstrate that the somnogenic effects of adenosine are largely mediated by hypothalamic A2A receptors. A2A)receptor antagonists could therefore be considered as a possible treatment for narcolepsy and other sleep-related disorders. Finally, nociception is another adenosine-regulated neural function previously thought to mostly involve A1 receptors. Although there is some conflicting literature on the effects of agonists and antagonists, which may partly be due to the lack of selectivity of available drugs, the studies in A2A receptor knockout mice suggest that A2A receptor antagonists might have some therapeutic potential in pain states, in particular where high intensity stimuli are prevalent.