Region-dependent difference in the sleep-promoting potency of an adenosine A2A receptor agonist

Gene expression in the rat brain during prostaglandin D2 and adenosinergically-induced sleep

Previous studies have supported the hypothesis that macromolecular synthesis occurs in the brain during sleep as a response to prior waking activities and that prostaglandin D2 (PGD2) is an endogenous sleep substance whose effects are dependent on adenosine A2a receptor-mediated signaling. We compared gene expression in the cerebral cortex, basal forebrain, and hypothalamus during PGD2-induced and adenosinergically-induced sleep to results from our previously published study of recovery sleep (RS) after sleep deprivation (SD). Immediate early gene expression in the cortex during sleep induced by PGD2- or by the selective adenosine A2a agonist CGS21680 showed limited similarity to that observed during RS while, in the basal forebrain and hypothalamus, widespread activation of immediate early genes not seen during RS occurred. In all three brain regions, PGD2 and CGS21680 reduced the expression of arc, a transcript whose expression is elevated during SD. Using GeneChips, the majority of genes induced by either PGD2 or CGS21680 were induced by both, suggesting activation of the same pathways. However, gene expression induced in the brain after PGD2 or CGS21680 treatment was distinct from that described during RS after SD and apparently involves glial cell gene activation and signaling pathways in neural-immune interactions.

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.

Theacrine, a special purine alkaloid with sedative and hypnotic properties from Cammelia assamica var. kucha in mice

The central nervous system activities of theacrine (1,3,7,9-tetramethyluric acid), a purine alkaloid which is abundantly present in Camellia assamica var. kucha, were investigated in ambulatory activity, pentobarbital-induced sleep and forced swimming test in mice, compared with two other purine alkaloids, caffeine and theobromine. Caffeine treatment led to a marked increase in the ambulatory activity accompanied with decreasing of the immobility time in forced swimming test at both 10 and 30 mg/kg. Under the same conditions, neither theacrine nor theobromine showed obvious excited efficacy. Both doses of theacrine could significantly prolong the sleeping time induced by pentobarbital, while caffeine and theobromine exhibited an inverted effect. These results indicated that theacrine possessed potent sedative and hypnotic properties and its central nervous system effects were different from those of caffeine and theobromine.

Neurobiology of REM and NREM sleep

This paper presents an overview of the current knowledge of the neurophysiology and cellular pharmacology of sleep mechanisms. It is written from the perspective that recent years have seen a remarkable development of knowledge about sleep mechanisms, due to the capability of current cellular neurophysiological, pharmacological and molecular techniques to provide focused, detailed, and replicable studies that have enriched and informed the knowledge of sleep phenomenology and pathology derived from electroencephalographic (EEG) analysis. This chapter has a cellular and neurophysiological/neuropharmacological focus, with an emphasis on rapid eye movement (REM) sleep mechanisms and non-REM (NREM) sleep phenomena attributable to adenosine. The survey of neuronal and neurotransmitter-related brainstem mechanisms of REM includes monoamines, acetylcholine, the reticular formation, a new emphasis on GABAergic mechanisms and a discussion of the role of orexin/hypcretin in diurnal consolidation of REM sleep. The focus of the NREM sleep discussion is on the basal forebrain and adenosine as a mediator of homeostatic control. Control is through basal forebrain extracellular adenosine accumulation during wakefulness and inhibition of wakefulness-active neurons. Over longer periods of sleep loss, there is a second mechanism of homeostatic control through transcriptional modification. Adenosine acting at the A1 receptor produces an up-regulation of A1 receptors, which increases inhibition for a given level of adenosine, effectively increasing the gain of the sleep homeostat. This second mechanism likely occurs in widespread cortical areas as well as in the basal forebrain. Finally, the results of a new series of experimental paradigms in rodents to measure the neurocognitive effects of sleep loss and sleep interruption (modeling sleep apnea) provide animal model data congruent with those in humans.

Effects of caffeine are more marked on daytime recovery sleep than on nocturnal sleep

Caffeine is often used to counteract sleepiness generated by sleep deprivation, jet lag, and shift-work, and is consumed at different times of day. Caffeine also has effects on sleep. However, little is known about the interaction between sleep deprivation, circadian timing, and caffeine consumption on sleep. In this study, we compared the effects of caffeine on nocturnal sleep initiated at habitual circadian time and on daytime recovery sleep. Thirty-four moderate caffeine consumers participated in both caffeine (200 mg) and placebo (lactose) conditions in a double-blind crossover design. Seventeen subjects followed their habitual sleep-wake cycle and slept in the laboratory during the night (Night), while 17 subjects were sleep deprived for one night and recovery sleep started in the morning (DayRec). All subjects received a capsule of 100 mg of caffeine (or placebo) 3 h before bedtime, and the remaining dose 1 h before bedtime. Compared to placebo, caffeine lengthened sleep latency, increased stage 1, and reduced stage 2 and slow-wave sleep (SWS) in both groups. However, caffeine reduced sleep efficiency more strongly in the DayRec group, and decreased sleep duration and REM sleep only in that group. The stronger effects of caffeine on daytime recovery sleep compared to nocturnal sleep are probably the consequence of the combined influence of increasing circadian wake propensity drive and the dissipation of homeostatic sleep pressure. We propose that the reduction of SWS by caffeine during daytime sleep increases the impact of the circadian wake signal on sleep. These results have implications for individuals using caffeine during night time.

Sleep deprivation increases A1 adenosine receptor binding in the human brain: a positron emission tomography study

It is currently hypothesized that adenosine is involved in the induction of sleep after prolonged wakefulness. This effect is partially reversed by the application of caffeine, which is a nonselective blocker of adenosine receptors. Here, we report that the most abundant and highly concentrated A1 subtype of cerebral adenosine receptors is upregulated after 24 h of sleep deprivation. We used the highly selective A1 adenosine receptor (A1AR) radioligand [18F]CPFPX ([18F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine) and quantitative positron emission tomography to assess cerebral A1ARs before and after sleep deprivation in 12 healthy volunteers and a control group (n = 10) with regular sleep. In sleep deprived subjects, we found an increase of the apparent equilibrium total distribution volume in a region-specific pattern in all examined brain regions with a maximum increase in the orbitofrontal cortex (15.3%; p = 0.014). There were no changes in the control group with regular sleep. This is the first molecular imaging study that provides in vivo evidence for an A1AR upregulation in cortical and subcortical brain regions after prolonged wakefulness, indicating that A1AR expression is contributing to the homeostatic sleep regulation.

Challenging sleep in aging: the effects of 200 mg of caffeine during the evening in young and middle-aged moderate caffeine consumers

The aim of this study was to evaluate the effects of a 200-mg administration of caffeine on polysomnographic sleep variables and quantitative sleep electroencephalography (EEG) in 12 young (20-30 years) and 12 middle-aged (40-60 years) moderate caffeine consumers (one to three cups of coffee per day). All subjects were submitted to both a caffeine (200 mg) and placebo (lactose) condition in a double-blind cross-over design. The conditions were separated by 1 week. Compared with the placebo condition, the evening ingestion of caffeine lengthened sleep latency, reduced sleep efficiency, and decreased sleep duration and amount of stage 2 sleep in both age groups. Caffeine also reduced spectral power in delta frequencies in frontal, central and parietal brain areas, but not in prefrontal (PF) and occipital regions. Moreover, caffeine increased spectral power in beta frequencies in frontal and central brain areas in both age groups. A suppression of spectral power in the PF area in low delta frequencies (0.5-1.00 Hz) and a rise in spectral power in the parietal region in high alpha (10.00-12.00 Hz) and beta frequencies (17.00-21.00, 23.00-25.00, 27.00-29.00 Hz) occurred solely in middle-aged subjects. No such changes were noticeable in young subjects. Generally, caffeine produced similar effects in young and middle-aged subjects. Only a few frequency bins showed more effects of caffeine in middle-aged subjects compared with young subjects. Furthermore, sleep EEG results do not entirely support the hypothesis that caffeine fully mimics the effects of a reduction of homeostatic sleep propensity when following a normal sleep-wake cycle.

Adenosine and sleep homeostasis in the Basal forebrain

It is currently hypothesized that the drive to sleep is determined by the activity of the basal forebrain (BF) cholinergic neurons, which release adenosine (AD), perhaps because of increased metabolic activity associated with the neuronal discharge during waking, and the accumulating AD begins to inhibit these neurons so that sleep-active neurons can become active. This hypothesis grew from the observation that AD induces sleep and AD levels increase with wake in the basal forebrain, but surprisingly it still remains untested. Here we directly test whether the basal forebrain cholinergic neurons are central to the AD regulation of sleep drive by administering 192-IgG-saporin to lesion the BF cholinergic neurons and then measuring AD levels in the BF. In rats with 95% lesion of the BF cholinergic neurons, AD levels in the BF did not increase with 6 h of prolonged waking. However, the lesioned rats had intact sleep drive after 6 and 12 h of prolonged waking, indicating that the AD accumulation in the BF is not necessary for sleep drive. Next we determined that, in the absence of the BF cholinergic neurons, the selective adenosine A1 receptor agonist N6-cyclohexyladenosine, administered to the BF, continued to be effective in inducing sleep, indicating that the BF cholinergic neurons are not essential to sleep induction. Thus, neither the activity of the BF cholinergic neurons nor the accumulation of AD in the BF during wake is necessary for sleep drive.

Expression pattern of FOS in orexin neurons during sleep induced by an adenosine A2A receptor agonist

The present study examined the expression pattern of FOS in the hypothalamic peptide neurons during the sleep-dominant state induced by an adenosine A2A receptor agonist. The control rats, those that received the microdialysis-perfusion of their ventral striatum with artificial cerebrospinal fluid in the dark-active phase, spent 24% of the 90-min period prior to sacrifice in non-rapid eye movement (non-REM) sleep and 2.3% of that in REM sleep. These rats exhibited FOS, a transcription factor, in 21% of their orexin neurons and in 1.0% of their melanin-concentrating hormone (MCH) neurons in the perifornical/lateral hypothalamic areas. However, the rats perfused with 50 microM CGS21680, an adenosine A2A receptor agonist, spent 60% of the 90-min period prior to sacrifice in non-REM sleep and 11% of that in REM sleep. These rats exhibited FOS in 1.7% of their orexin neurons and FOS in 0.5% of their MCH neurons. When the sleep-dominant state was disturbed by mild stimulation and the rats were kept in the sleepy state by treatment with a sleep-inducing dose of CGS21680, the rats exhibited FOS in 13.3% of their orexin neurons, which percentage was about half of that for the control rats. These results suggest that the sleep-promoting process induced by this adenosine A2A receptor agonist was associated with a decline in the activity of orexin neurons. MCH neurons are not likely to change their activities during this sleep-promoting process.

Dialysis delivery of an adenosine A2Aagonist into the pontine reticular formation of C57BL/6J mouse increases pontine acetylcholine release and sleep

In vivo microdialysis in C57BL/6J (B6) mouse was used to test the hypothesis that activating adenosine A(2A) receptors in the pontine reticular formation (PRF) increases acetylcholine (ACh) release and rapid eye movement (REM) sleep. Eight concentrations of the adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680; CGS) were delivered to the PRF and ACh in the PRF was quantified. ACh release was significantly increased by dialysis with 3 mum CGS and significantly decreased by dialysis with 10 and 100 microm CGS. Co-administration of the adenosine A(2A) receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385; 30 nM) blocked the CGS-induced increase in ACh release. In a second series of experiments, CGS (3 microm) was delivered by dialysis to the PRF for 2 h while recording sleep and wakefulness. CGS significantly decreased time in wakefulness (-51% in h 1; -54% in h 2), increased time in non-rapid eye movement (NREM) sleep (90% in h 1; 151% in h 2), and increased both time in REM sleep (331% in h 2) and the number of REM sleep episodes (488% in h 2). The enhancement of REM sleep is consistent with the interpretation that adenosine A(2A) receptors in the PRF of the B6 mouse contribute to REM sleep regulation, in part, by increasing ACh release in the PRF. A(2A) receptor activation may promote NREM sleep via GABAergic inhibition of arousal promoting neurons in the PRF.

Differential c-Fos immunoreactivity in arousal-promoting cell groups following systemic administration of caffeine in rats

Despite the widespread use of caffeine, the neuronal mechanisms underlying its stimulatory effects are not completely understood. By using c-Fos immunohistochemistry as a marker of neuronal activation, we recently showed that stimulant doses of caffeine activate arousal-promoting hypothalamic orexin (hypocretin) neurons. In the present study, we investigated whether other key neurons of the arousal system are also activated by caffeine, via dual immunostaining for c-Fos and transmitter markers. Rats were administered three doses of caffeine or saline vehicle during the light phase. Caffeine at 10 and 30 mg/kg, i.p., increased motor activities, including locomotion, compared with after saline or a higher dose, 75 mg/kg. The three doses of caffeine induced distinct dose-related patterns of c-Fos immunoreactivity in several arousal-promoting areas, including orexin neurons and adjacent neurons containing neither orexin nor melanin-concentrating hormone; tuberomammillary histaminergic neurons; locus coeruleus noradrenergic neurons; noncholinergic basal forebrain neurons that do not contain parvalbumin; and nondopaminergic neurons in the ventral tegmental area. At any dose used, caffeine induced little or no c-Fos expression in cholinergic neurons of the basal forebrain and mesopontine tegmentum; dopaminergic neurons of the ventral tegmental area, central gray, and substantia nigra pars compacta; and serotonergic neurons in the dorsal raphe nucleus. Saline controls exhibited only few c-Fos-positive cells in most of the cell groups examined. These results indicate that motor-stimulatory doses of caffeine induce a remarkably restricted pattern of c-Fos expression in the arousal-promoting system and suggest that this specific neuronal activation may be involved in the behavioral arousal by caffeine.

The endogenous somnogen adenosine excites a subset of sleep-promoting neurons via A2A receptors in the ventrolateral preoptic nucleus

Recent research has shown that neurons in the ventrolateral preoptic nucleus are crucial for sleep by inhibiting wake-promoting systems, but the process that triggers their activation at sleep onset remains to be established. Since evidence indicates that sleep induced by adenosine, an endogenous sleep-promoting substance, requires activation of brain A(2A) receptors, we examined the hypothesis that adenosine could activate ventrolateral preoptic nucleus sleep neurons via A(2A) adenosine receptors in rat brain slices. Following on from our initial in vitro identification of these neurons as uniformly inhibited by noradrenaline and acetylcholine arousal transmitters, we established that the ventrolateral preoptic nucleus comprises two intermingled subtypes of sleep neurons, differing in their firing responses to serotonin, inducing either an inhibition (Type-1 cells) or an excitation (Type-2 cells). Since both cell types contained galanin and expressed glutamic acid decarboxylase-65/67 mRNAs, they potentially correspond to the sleep promoting neurons inhibiting arousal systems. Our pharmacological investigations using A(1) and A(2A) adenosine receptors agonists and antagonists further revealed that only Type-2 neurons were excited by adenosine via a postsynaptic activation of A(2A) adenosine receptors. Hence, the present study is the first demonstration of a direct activation of the sleep neurons by adenosine. Our results further support the cellular and functional heterogeneity of the sleep neurons, which could enable their differential contribution to the regulation of sleep. Adenosine and serotonin progressively accumulate during arousal. We propose that Type-2 neurons, which respond to these homeostatic signals by increasing their firing are involved in sleep induction. In contrast, Type-1 neurons would likely play a role in the consolidation of sleep.

Effects on sleep of microdialysis of adenosine A1 and A2a receptor analogs into the lateral preoptic area of rats

Evidence suggests that adenosine (AD) is an endogenous sleep factor. The hypnogenic action of AD is mediated through its inhibitory A1 and excitatory A2A receptors. Although AD is thought to be predominantly active in the wake-active region of the basal forebrain (BF), a hypnogenic action of AD has been demonstrated in several other brain areas, including the preoptic area. We hypothesized that in lateral preoptic area (LPOA), a region with an abundance of sleep-active neurons, AD acting via A1 receptors would induce waking by inhibition of sleep-active neurons and that AD acting via A2A receptors would promote sleep by stimulating the sleep-active neurons. To this end, we studied the effects on sleep of an AD transport inhibitor, nitrobenzyl-thio-inosine (NBTI) and A1 and A2A receptor agonists/antagonists by microdialyzing them into the LPOA. The results showed that, in the sleep-promoting area of LPOA: 1) A1 receptor stimulation or inhibition of AD transport by NBTI induced waking and 2) A2A receptor stimulation induced sleep. We also confirmed that NBTI administration in the wake promoting area of the BF increased sleep. The effects of AD could be mediated either directly or indirectly via interaction with other neurotransmitter systems. These observations support a hypothesis that AD mediated effects on sleep-wake cycles are site and receptor dependent.

An adenosine A2A receptor agonist induces sleep by increasing GABA release in the tuberomammillary nucleus to inhibit histaminergic systems in rats

The adenosine A(2A) receptor (A(2A)R) has been demonstrated to play a crucial role in the regulation of the sleep process. However, the molecular mechanism of the A(2A)R-mediated sleep remains to be elucidated. Here we used electroencephalogram and electromyogram recordings coupled with in vivo microdialysis to investigate the effects of an A(2A)R agonist, CGS21680, on sleep and on the release of histamine and GABA in the brain. In freely moving rats, CGS21680 applied to the subarachnoid space underlying the rostral basal forebrain significantly promoted sleep and inhibited histamine release in the frontal cortex. The histamine release was negatively correlated with the amount of non-rapid eye movement sleep (r = - 0.652). In urethane-anesthetized rats, CGS21680 inhibited histamine release in both the frontal cortex and medial pre-optic area in a dose-dependent manner, and increased GABA release specifically in the histaminergic tuberomammillary nucleus but not in the frontal cortex. Moreover, the CGS21680-induced inhibition of histamine release was antagonized by perfusion of the tuberomammillary nucleus with a GABA(A) antagonist, picrotoxin. These results suggest that the A(2A)R agonist induced sleep by inhibiting the histaminergic system through increasing GABA release in the tuberomammillary nucleus.

Adenosine and sleep–wake regulation

This review addresses three principal questions about adenosine and sleep-wake regulation: (1) Is adenosine an endogenous sleep factor? (2) Are there specific brain regions/neuroanatomical targets and receptor subtypes through which adenosine mediates sleepiness? (3) What are the molecular mechanisms by which adenosine may mediate the long-term effects of sleep loss? Data suggest that adenosine is indeed an important endogenous, homeostatic sleep factor, likely mediating the sleepiness that follows prolonged wakefulness. The cholinergic basal forebrain is reviewed in detail as an essential area for mediating the sleep-inducing effects of adenosine by inhibition of wake-promoting neurons via the A1 receptor. The A2A receptor in the subarachnoid space below the rostral forebrain may play a role in the prostaglandin D2-mediated somnogenic effects of adenosine. Recent evidence indicates that a cascade of signal transduction induced by basal forebrain adenosine A1 receptor activation in cholinergic neurons leads to increased transcription of the A1 receptor; this may play a role in mediating the longer-term effects of sleep deprivation, often called sleep debt.

Dominant localization of adenosine deaminase in leptomeninges and involvement of the enzyme in sleep

Adenosine is an endogenous hypnotic molecule. However, the mechanism by which the level of extracellular adenosine is regulated remains to be elucidated. We found by Northern hybridization and enzyme assay that ecto-5(')-nucleotidase and adenosine deaminase (ADA), major enzymes responsible for the production and degradation of adenosine, respectively, were localized most abundantly in the leptomeninges within the rat brain. Immunohistochemical study showed that ADA was dominantly localized in arachnoid barrier and trabecular cells of the leptomeninges. In vivo microdialysis demonstrated that externally applied adenosine was rapidly metabolized by ADA to inosine in the subarachnoid space. Perfusion of an ADA inhibitor, coformycin, increased the extracellular adenosine level in the subarachnoid space under the rostral basal forebrain. When coformycin was continuously infused into the subarachnoid space, non-rapid eye movement sleep was increased with prolonged duration of the sleep episode. These results demonstrate that the leptomeninges control the extracellular level of adenosine in the subarachnoid space by their high 5(')-nucleotidase and ADA activities and regulate non-rapid eye movement sleep.

Enhanced adenosine A2A receptor facilitation of synaptic transmission in the hippocampus of aged rats

Adenosine either inhibits or facilitates synaptic transmission through A1 or A2A receptors, respectively. Since A2A receptor density increases in the limbic cortex of aged (24 mo) compared with young adult rats (2 mo), we tested if A2A receptor modulation of synaptic transmission was also increased in aged rats. The A2A receptor agonist, CGS21680 (10 nM), caused a larger facilitation of the field excitatory postsynaptic potential (fEPSP) slope in hippocampal slices of aged (38%) than in young rats (19%), an effect prevented by the A2A receptor antagonist, ZM241385 (20 nM). In contrast to young rats, where CGS21680 facilitation of fEPSPs is prevented by the protein kinase C inhibitor, chelerythrine (6 microM), but not by the protein kinase A inhibitor, H-89 (1 microM), the CGS21680-induced facilitation of fEPSP slope in aged rats was prevented by H-89 (1 microM) but not by chelerythrine (6 microM). Also, in contrast to the beta-receptor agonist, isoproterenol (30 microM), CGS21680 (100-1,000 nM) enhanced cAMP levels in hippocampal nerve terminals of aged but not young rats. Finally, we observed a significant increase of both the binding density of [3H]CGS 21680 and the [3H]ZM241385 as well as of the anti-A2A receptor immunoreactivity in hippocampal nerve terminal membranes from aged compared with young rats. This shows that A2A receptor-mediated facilitation of hippocampal synaptic transmission is larger in aged than young rats due to increased A2A receptor density in nerve terminals and to the modified transducing system operated by A2A receptors, from a protein kinase C mediated control of A1 receptors into a direct protein kinase A dependent facilitation of synaptic transmission.

Inhibition of rostral basal forebrain neurons promotes wakefulness and induces FOS in orexin neurons

The present study examined whether the activities of the rostral basal forebrain neurons alter the activities of the orexin (also known as hypocretin) neurons in the tuberal part of the hypothalamus in rats. We performed microdialysis perfusion of the ventromedial portion of the rostral basal forebrain with the GABAA receptor agonist muscimol to inhibit focally the neuronal activities in the rostral basal forebrain. Then, we monitored sleep/wake behaviour and investigated the pattern of activities of orexin neurons by examining the expression of FOS as an indicator of cellular activation. Bilateral perfusion with muscimol (5, 15, and 50 micro m) produced a dose-dependent decrease in the amount of sleep. This perfusion with muscimol at 50 micro m produced FOS-like immunoreactivity in 37% of the orexin neurons located in the tuberal part of the hypothalamus, whereas the FOS-like immunoreactivity was sparse in orexin neurons of the sleeping control rats (P = 0.001 by Mann-Whitney U-test). Unilateral perfusion with muscimol (50 micro m) also suppressed sleep. In this case, FOS-like immunoreactivity was seen in 40% of the orexin neurons on the side ipsilateral to the perfusion site but only in 10% of orexin neurons on the contralateral side (P = 0.018 by Wilcoxon signed rank test). These functional data suggested that a sleep-generating element in the ventromedial part of the rostral basal forebrain provides an inhibitory influence on the activities of the orexin neurons in the tuberal part of the hypothalamus.

Purinergic modulation of neuronal activity in the mesolimbic dopaminergic system in vivo

ATP and its metabolite adenosine activate membrane receptors, termed P2 and P1, respectively. In the present study, the modulation of the mesolimbic neuronal circuit by ATPergic and adenosinergic mechanisms was investigated by microdialysis in the nucleus accumbens (NAc) and by telemetrically recorded EEG from both the NAc and the ventral tegmental area (VTA) of freely moving rats. The basal extracellular dopamine concentration was enhanced after accumbal perfusion with the ATP analog 2-methylthio ATP (2-MeSATP; 100 microM); by contrast, adenosine (100 microM) caused a reduction of extracellular dopamine. When given alone, the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 20 microM) decreased the concentration of dopamine, whereas the P1 receptor antagonist 8-(p-sulfophenyl)theophylline (8-SPT; 100 microM) increased it. In the same animals, P2 receptor stimulation by 2-MeSATP caused neuronal activation, indicated by an elevation of the absolute power in the EEG of the NAc mainly by enhancement of the relative power in the alpha band (8-13 Hz) of the EEG spectrum. By contrast, adenosine led to a depression of the absolute power in the VTA accompanied by an elevation of the delta-band power (0.4-6 Hz) in the NAc corresponding to a slowing of neuronal activity. When given alone, PPADS reduced the absolute EEG power in the NAc accompanied by a decrease in the high-frequency power, but had no effects on the VTA. 8-SPT on its own enhanced the total power in both the NAc and the VTA, reflected by an enhancement in the slow and the high-frequency bands. Whereas the 8-SPT-evoked changes of EEG pattern as well as of dopamine concentration in the NAc were abolished by the co-application of PPADS, the 8-SPT-induced EEG changes in the VTA persisted under these conditions. In conclusion, the accumbal neuronal output, reflected by both dopamine release and neuronal electrical activity, is modulated in a functionally antagonistic manner by P2 and P1 receptor stimulation. It is suggested that an inhibitory GABAergic feedback projection to the VTA is stimulated by adenosine, either directly or indirectly via glutamate release.

Adenosinergic inhibition of basal forebrain wakefulness-active neurons: a simultaneous unit recording and microdialysis study in freely behaving cats

The majority of neurons in the magnocellular basal forebrain are wakefulness-active with highest discharge activity during wakefulness and a marked reduction in activity just before and during the entry to non-rapid eye movement (REM) sleep. We have hypothesized that the reduction of discharge activity of wakefulness-active neurons and a consequent facilitation of the transition from wakefulness to sleep is due to an increase in the extracellular concentration of adenosine during wakefulness. To test the hypothesis, the present study employed microdialysis perfusion of adenosinergic pharmacological agents combined with single unit recording in freely moving cats, so as to determine: 1). if there were dose-dependent effects on behaviorally identified wakefulness-active neurons; 2). whether effects were mediated by the A1 receptor, as contrasted to the A2a receptor; and 3). if effects were specific to wakefulness-active neurons, and not present in sleep-active neurons, those preferentially discharging in nonREM sleep. Both adenosine and the A1 receptor-specific agonist N6-cyclo-hexyl-adenosine reduced the discharge activity of wakefulness-active neurons (n=16) in a dose-dependent manner but had no effect on sleep-active neurons (n=4). The A1 receptor antagonist 8-cyclopentyl-1-3-dimethylxanthine increased the discharge of wakefulness-active neurons (n=5), but the A2a receptor agonist, CGS-16284, had no effect (n=3). Recording sites were histologically localized to the cholinergic basal forebrain. These data support our hypothesis that adenosine acts via the A1 receptor to reduce the activity of wakefulness-promoting neurons, thus providing a cellular mechanism explaining why the increased adenosine concentrations observed in the basal forebrain following prolonged wakefulness act to induce sleep.

An adenosine A2a agonist increases sleep and induces Fos in ventrolateral preoptic neurons

Considerable evidence indicates that adenosine may be an endogenous somnogen, yet the mechanism through which it promotes sleep is unknown. Adenosine may act via A1 receptors to promote sleep, but an A2a receptor antagonist can block the sleep induced by prostaglandin D(2). We previously reported that prostaglandin D(2) activates sleep-promoting neurons of the ventrolateral preoptic area, and we hypothesized that an A2a receptor agonist also should activate these neurons. Rats were instrumented for sleep recordings, and an injection cannula was placed in the subarachnoid space just anterior to the ventrolateral preoptic area. After an 8-10-day recovery period, the A2a receptor agonist CGS21680 (20 pmol/min) or saline was infused through the injection cannula, and the animals were killed 2 h later. The brains were stained using Fos immunohistochemistry, and the pattern of Fos expression was studied in the entire brain. CGS21680 increased non-rapid eye movement sleep and markedly increased the expression of Fos in the ventrolateral preoptic area and basal leptomeninges, but it reduced Fos expression in wake-active brain regions such as the tuberomammillary nucleus. CGS21680 also induced Fos in the shell and core of the nucleus accumbens and in the lateral subdivision of the central nucleus of the amygdala. To determine whether these effects may have been mediated through A1 receptors, an additional group of rats received subarachnoid infusion of the A1 receptor agonist N(6)-cyclopentyladenosine (2 pmol/min). In contrast to CGS21680, infusion of N(6)-cyclopentyladenosine into the subarachnoid space produced only a small decrease in rapid eye movement sleep, and the pattern of Fos expression induced by N(6)-cyclopentyladenosine was notable only for decreased Fos in regions near the infusion site. These findings suggest that an adenosine A2a receptor agonist may activate cells of the leptomeninges or nucleus accumbens that increase the activity of ventrolateral preoptic area neurons. These ventrolateral preoptic area neurons may then coordinate the inhibition of multiple wake-promoting regions, resulting in sleep.

Prostaglandin D2 in sleep-wake regulation: recent progress and perspectives

Prostaglandin (PG) D2 is one of the most active endogenous sleep-promoting substances, which induces physiological sleep in rodents, primates, and most probably in humans as well. In this update article, we review recent experimental results concerning the molecular mechanisms underlying sleep-wake regulation by PGD2, the link between the humoral regulation by the PGD2 system, and the neural network involved in the promotion of non-rapid eye movement (NREM) sleep and the abnormality of NREM sleep regulation found in gene-manipulated mice for PGD synthase.

The role and regulation of adenosine in the central nervous system

Adenosine is a modulator that has a pervasive and generally inhibitory effect on neuronal activity. Tonic activation of adenosine receptors by adenosine that is normally present in the extracellular space in brain tissue leads to inhibitory effects that appear to be mediated by both adenosine A1 and A2A receptors. Relief from this tonic inhibition by receptor antagonists such as caffeine accounts for the excitatory actions of these agents. Characterization of the effects of adenosine receptor agonists and antagonists has led to numerous hypotheses concerning the role of this nucleoside. Previous work has established a role for adenosine in a diverse array of neural phenomena, which include regulation of sleep and the level of arousal, neuroprotection, regulation of seizure susceptibility, locomotor effects, analgesia, mediation of the effects of ethanol, and chronic drug use.

Adenosine A(1) and A(2A) receptors modulate sleep state and breathing in fetal sheep

This study was designed to determine the adenosine (Ado) receptor subtype that mediates the depressant effects of Ado on fetal breathing and rapid eye movements (REM). In chronically catheterized fetal sheep (>0.8 term), intra-arterial infusion of N(6)-cyclopentyladenosine (CPA), an Ado A(1)-receptor agonist, increased the incidence of high-voltage electrocortical (ECoG) activity while virtually abolishing low-voltage activity, REM, and breathing. These effects were blocked by 9-cyclopentyl-1,3-dipropylxanthine (DPCPX), an Ado A(1)-receptor antagonist. Infusion of DPCPX alone increased breath amplitude but had no significant effect on inspiratory duration, breath interval, incidence of REM, or incidence of low-voltage activity. Ado A(2A)-receptor blockade with ZM-241385 increased the incidence of low-voltage ECoG activity, REM, and breathing but had no effect on breath amplitude or respiratory cycle. Both DPCPX and ZM-241385 eliminated the inhibitory effects of Ado on REM and breathing. We conclude that 1) Ado A(1) receptors tonically inhibit fetal respiratory drive, 2) Ado A(2A) receptors tonically inhibit REM-like behavioral state, and 3) both Ado A(1) and A(2A) receptors mediate the depressant effects of Ado on REM and breathing.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.