Adenosine in sleep and wakefulness

Uncovering residual effects of chronic sleep loss on human performance

Sleep loss leads to profound performance decrements. Yet many individuals believe they adapt to chronic sleep loss or that recovery requires only a single extended sleep episode. To evaluate this, we designed a protocol whereby the durations of sleep and wake episodes were increased to 10 and 32.85 hours, respectively, to yield a reduced sleep-to-wake ratio of 1:3.3. These sleep and wake episodes were distributed across all circadian phases, enabling measurement of the effects of acute and chronic sleep loss at different times of the circadian day and night. Despite recurrent acute and substantial chronic sleep loss, 10-hour sleep opportunities consistently restored vigilance task performance during the first several hours of wakefulness. However, chronic sleep loss markedly increased the rate of deterioration in performance across wakefulness, particularly during the circadian "night." Thus, extended wake during the circadian night reveals the cumulative detrimental effects of chronic sleep loss on performance, with potential adverse health and safety consequences.

The role of adenosine in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder of the central nervous system manifested by cognitive and memory deterioration, a variety of neuropsychiatric symptoms, behavioral disturbances, and progressive impairment of daily life activities. Current pharmacotherapies are restricted to symptomatic interventions but do not prevent progressive neuronal degeneration. Therefore, new therapeutic strategies are needed to intervene with these progressive pathological processes. In the past several years adenosine, a ubiquitously released purine ribonucleoside, has become important for its neuromodulating capability and its emerging positive experimental effects in neurodegenerative diseases. Recent research suggests that adenosine receptors play important roles in the modulation of cognitive function. The present paper attempts to review published reports and data from different studies showing the evidence of a relationship between adenosinergic function and AD-related cognitive deficits. Epidemiological studies have found an association between coffee (a nonselective adenosine receptor antagonist) consumption and improved cognitive function in AD patients and in the elderly. Long-term administration of caffeine in transgenic animal models showed a reduced amyloid burden in brain with better cognitive performance. Antagonists of adenosine A2A receptors mimic these beneficial effects of caffeine on cognitive function. Neuronal cell cultures with amyloid beta in the presence of an A2A receptor antagonist completely prevented amyloid beta-induced neurotoxicity. These findings suggest that the adenosinergic system constitutes a new therapeutic target for AD, and caffeine and A2A receptor antagonists may have promise to manage cognitive dysfunction in AD.

Adenosine receptor ligands and PET imaging of the CNS

Advances in radiotracer chemistry have resulted in the development of novel molecular imaging probes for adenosine receptors (ARs). With the availability of these molecules, the function of ARs in human pathophysiology as well as the safety and efficacy of approaches to the different AR targets can now be determined. Molecular imaging is a rapidly growing field of research that allows the identification of molecular targets and functional processes in vivo. It is therefore gaining increasing interest as a tool in drug development because it permits the process of evaluating promising therapeutic targets to be stratified. Further, molecular imaging has the potential to evolve into a useful diagnostic tool, particularly for neurological and psychiatric disorders. This chapter focuses on currently available AR ligands that are suitable for molecular neuroimaging and describes first applications in healthy subjects and patients using positron emission tomography (PET).

Doxapram and aminophylline on bispectral index under sevoflurane anaesthesia

BACKGROUND AND OBJECTIVE

To evaluate and compare the effect of two clinically available central nervous system stimulants, namely doxapram and aminophylline on arousal from sevoflurane anaesthesia and bispectral index.

METHODS

This randomized, double-blind, placebo-controlled, prospective study was conducted in 90 adult females, ASA I-II, scheduled for elective lower abdominal surgeries at Taipei Medical University Hospital. At 5 min before the completion of surgery, under sevoflurane anaesthesia, patients were divided into three groups to receive doxapram 1 mg kg(-1), aminophylline 2 mg kg(-1) or saline placebo intravenous. Standard vital signs, end-tidal CO(2), end-expiratory sevoflurane concentration, bispectral index and neuromuscular blockade were measured plus clinical parameters of recovery from general anaesthesia.

RESULTS

Compared with the control group, patients receiving doxapram or aminophylline showed a similarly faster recovery from sevoflurane anaesthesia correlated with increase in bispectral index.

CONCLUSION

Intravenous administration of doxapram 1 mg kg(-1) or aminophylline 2 mg kg(-1) hastened the early recovery from sevoflurane anaesthesia. The arousal effect of aminophylline and doxapram appears to be similar.

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.

Caffeine: implications for alertness in athletes

Caffeine is one of the most widely consumed drugs in the world, taken socially and for its alertness- and performance-promoting actions. Extensive reports assert that caffeine increases alertness and cognitive performance levels and, when taken before exercise, demonstrates ergogenic properties. Caffeine ingestion has been associated with increased performance during endurance submaximal, and acute, high-intensity exercise. The exact mechanism of action for the performance effects of caffeine is unknown, although several physiologically and psychologically based theories exist as to how caffeine achieves increased performance capabilities. This paper outlines the known sites of caffeine activity in the body,and discusses these with respect to the effects of caffeine observed during performance assessments.

The Role of Glial Adenosine Receptors in Neural Resilience and the Neurobiology of Mood Disorders

Adenosine receptors were classified into A1- and A2-receptors in the laboratory of Bernd Hamprecht more than 25 years ago. Adenosine receptors are instrumental to the neurotrophic effects of glia cells. Both microglia and astrocytes release after stimulation via adenosine receptors factors that are important for neuronal survival and growth. Neuronal resilience is now considered as of pivotal importance in the neurobiology of mood disorders and their treatment. Both sleep deprivation and electroconvulsive therapy, two effective therapeutic measures in mood disorders, are associated with an increase of adenosine and upregulation of adenosine A1-receptors in the brain. Parameters closely related to adenosine receptor activation such as cerebral metabolic rate and delta power in the sleep EEG provide indirect evidence that adenosinergic signaling may be associated with the therapeutic response to these measures. Thus, neurotrophic effects evoked by adenosine receptors might be important in the mechanism of action of ECT and perhaps also sleep deprivation.

Sleep and circadian abnormalities in a transgenic mouse model of Alzheimer's disease: a role for cholinergic transmission

The Tg2576 mouse model of Alzheimer's disease (AD) exhibits age-dependent amyloid beta (Abeta) deposition in the brain. We studied electroencephalographically defined sleep and the circadian regulation of waking activities in Tg2576 mice to determine whether these animals exhibit sleep abnormalities akin to those in AD. In Tg2576 mice at all ages studied, the circadian period of wheel running rhythms in constant darkness was significantly longer than that of wild type mice. In addition, the increase in electroencephalographic delta (1-4 Hz) power that occurs during non-rapid eye movement sleep after sleep deprivation was blunted in Tg2576 mice relative to controls at all ages studied. Electroencephalographic power during non-rapid eye movement sleep was shifted to higher frequencies in plaque-bearing mice relative to controls. The wake-promoting efficacy of the acetylcholinesterase inhibitor donepezil was lower in plaque-bearing Tg2576 mice than in controls. Sleep abnormalities in Tg2576 mice may be due in part to a cholinergic deficit in these mice. At 22 months of age, two additional deficits emerged in female Tg2576 mice: time of day-dependent modulation of sleep was blunted relative to controls and rapid eye movement sleep as a percentage of time was lower in Tg2576 than in wild type controls. The rapid eye movement sleep deficit in 22 month-old female Tg2576 mice was abolished by brief passive immunization with an N-terminal antibody to Abeta. The Tg2576 model provides a uniquely powerful tool for studies on the pathophysiology of and treatments for sleep deficits and associated cholinergic abnormalities in AD.

Syndromic patent ductus arteriosus: evidence for haploinsufficient TFAP2B mutations and identification of a linked sleep disorder

Patent ductus arteriosus (PDA) is a common congenital heart disease that results when the ductus arteriosus, a muscular artery, fails to remodel and close after birth. A syndromic form of this disorder, Char syndrome, is caused by mutation in TFAP2B, the gene encoding a neural crest-derived transcription factor. Established features of the syndrome are PDA, facial dysmorphology, and fifth-finger clinodactyly. Disease-causing mutations are missense and are proposed to be dominant negative. Because only a small number of families have been reported, there is limited information on the spectrum of mutations and resulting phenotypes. We report the characterization of two kindreds (K144 and K145) with Char syndrome containing 22 and 5 affected members, respectively. Genotyping revealed linkage to TFAP2B in both families. Sequencing of TFAP2B demonstrated mutations in both kindreds that were not found among control chromosomes. Both mutations altered highly conserved bases in introns required for normal splicing as demonstrated by biochemical studies in mammalian cells. The abnormal splicing results in mRNAs containing frameshift mutations that are expected to be degraded by nonsense-mediated mRNA decay, resulting in haploinsufficiency; even if produced, the protein in K144 would lack DNA binding and dimerization motifs and would likely result in haploinsufficiency. Examination of these two kindreds for phenotypes that segregate with TFAP2B mutations identified several phenotypes not previously linked to Char syndrome. These include parasomnia and dental and occipital-bone abnormalities. The striking sleep disorder in these kindreds implicates TFAP2B-dependent functions in the normal regulation of sleep.

Pharmacotherapy for excessive daytime sleepiness

Excessive daytime sleepiness (EDS) has recognized detrimental consequences such as road traffic accidents, impaired psychological functioning and reduced work performance. EDS can result from multiple causes such as sleep deprivation, sleep fragmentation, neurological, psychiatric and circadian rhythm disorders. Treating the underlying cause of EDS remains the mainstay of therapy but in those who continue to be excessively sleepy, further treatment may be warranted. Traditionally, the amphetamine derivatives, methylphenidate and pemoline (collectively sympathomimetic) psychostimulants were the commonest form of therapy for EDS, particularly in conditions such as narcolepsy. More recently, the advent of modafinil has broadened the range of therapeutic options. Modafinil has a safer side-effect profile and as a result, interest in this drug for the management of EDS in other disorders, as well as narcolepsy, has increased considerably. There is a growing school of thought that modafinil may have a role to play in other indications such as obstructive sleep apnea/hypopnea syndrome already treated by nasal continuous positive airway pressure but persisting EDS, shift work sleep disorders, neurological causes of sleepiness, and healthy adults performing sustained operations, particularly those in the military. However, until adequately powered randomised-controlled trials confirm long-term efficacy and safety, the recommendation of wakefulness promoters in healthy adults cannot be justified.

Adenosine uptake inhibitors

Adenosine is a purine nucleoside and modulates a variety of physiological functions by interacting with cell-surface adenosine receptors. Under several adverse conditions, including ischemia, trauma, stress, seizures and inflammation, extracellular levels of adenosine are increased due to increased energy demands and ATP metabolism. Increased adenosine could protect against excessive cellular damage and organ dysfunction. Indeed, several protective effects of adenosine have been widely reported (e.g., amelioration of ischemic heart and brain injury, seizures and inflammation). However, the effects of adenosine itself are insufficient because extracellular adenosine is rapidly taken up into adjacent cells and subsequently metabolized. Adenosine uptake inhibitors (nucleoside transport inhibitors) could retard the disappearance of adenosine from the extracellular space by blocking adenosine uptake into cells. Therefore, it is expected that adenosine uptake inhibitors will have protective effects in various diseases, by elevating extracellular adenosine levels. Protective or ameliorating effects of adenosine uptake inhibitors in ischemic cardiac and cerebral injury, organ transplantation, seizures, thrombosis, insomnia, pain, and inflammatory diseases have been reported. Preclinical and clinical results indicate the possibility of therapeutic application of adenosine uptake inhibitors.

Glucocorticoids influence brain glycogen levels during sleep deprivation

We investigated whether glucocorticoids [i.e., corticosterone (Cort) in rats] released during sleep deprivation (SD) affect regional brain glycogen stores in 34-day-old Long-Evans rats. Adrenalectomized (with Cort replacement; Adx+) and intact animals were sleep deprived for 6 h beginning at lights on and then immediately killed by microwave irradiation. Brain and liver glycogen and glucose and plasma glucose levels were measured. After SD in intact animals, glycogen levels decreased in the cerebellum and hippocampus but not in the cortex or brain stem. By contrast, glycogen levels in the cortex of Adx+ rats increased by 43% ( P < 0.001) after SD, while other regions were unaffected. Also in Adx+ animals, glucose levels were decreased by an average of 28% throughout the brain after SD. Intact sleep-deprived rats had elevations of circulating Cort, blood, and liver glucose that were absent in intact control and Adx+ animals. Different responses between brain structures after SD may be due to regional variability in metabolic rate or glycogen metabolism. Our findings suggest that the elevated glucocorticoid secretion during SD causes brain glycogenolysis in response to energy demands.

Plasticity of purine release during cerebral ischemia: clinical implications?

Adenosine is a powerful modulator of neuronal function in the mammalian central nervous system. During a variety of insults to the brain, adenosine is released in large quantities and exerts a neuroprotective influence largely via the A(1) receptor, which inhibits glutamate release and neuronal activity. Using novel enzyme-based adenosine sensors, which allow high spatial and temporal resolution recordings of adenosine release in real time, we have investigated the release of adenosine during hypoxia/ischemia in the in vitro hippocampus. Our data reveal that during the early stages of hypoxia adenosine is likely released per se and not as a precursor such as cAMP or an adenine nucleotide. In addition, repeated hypoxia results in reduced production of extracellular adenosine and this may underlie the increased vulnerability of the mammalian brain to repetitive or secondary hypoxia/ischemia.

Effect of aminophylline on bispectral index

BACKGROUND

The aim of the present study was to investigate the effects of aminophylline on BIS as well as clinical recovery in patients anesthetized with sevoflurane.

METHODS

Sixty patients with status of ASA I-II scheduled for elective surgery were enrolled in this study. Anesthesia was induced by 2 mg kg(-1) of propofol and 0.5 mg kg(-1) of atracurium, maintained with 1:1 ratio of oxygen and nitrous oxide and 2-2.5% sevoflurane, keeping BIS values at 50 +/- 5. During the last 30 min of the operation no muscle relaxant was given and anesthesia was continued without decreasing anesthetic concentration. After sevoflurane discontinuation, saline was given to Group P, and 5 mg kg(-1) of aminophylline was given to Group A. Bispectral index values, heart rate, blood pressure and oxygen saturation were determined in all the patients before and every min after injection of the test drug for 15 min. The following variables were measured in both groups: eye opening, extubation time, response to command, Aldrete scores, and performing three simple arithmetic calculations.

RESULTS

Between groups there was no statistically significant difference in mean arterial blood pressure, SpO2 and anesthesia time. Heart rate was found to be statistically higher (P < 0.001) at 2 to 6 min in Group A when compared with group P. Eye opening, verbal response, extubation and arithmetic calculation times were significantly shorter (P < 0.001) in Group A. Bispectral index scores were significantly higher in Group A at 1 to 12 min after aminophylline injection when compared with placebo (P < 0.001).

CONCLUSION

Recovery from sevoflurane anesthesia and BIS scores are improved in early period when aminophylline is given at emerging from anesthesia.

Modulation of the excitability of cholinergic basal forebrain neurones by KATP channels

The expression of ATP-sensitive K(+) (K(ATP)) channels by magnocellular cholinergic basal forebrain (BF) neurones was investigated in thin brain slice and dissociated cell culture preparations using a combination of whole-cell, perforated-patch and single-channel recording techniques. Greater than 95% of BF neurones expressed functional K(ATP) channels whose activation resulted in membrane hyperpolarization and a profound fall in excitability. The whole-cell K(ATP) conductance was 14.0 +/- 1.5 nS and had a reversal potential of -91.4 +/- 0.9 mV that shifted by 59.6 mV with a tenfold increase in [K(+)](o). I(KATP) was inhibited reversibly by tolbutamide (IC(50) of 34.1 microM) and irreversibly by glibenclamide (0.3-3 nM) and had a low affinity for [ATP](i) (67% reduction with 6 mm[MgATP](i)). Using perforated-patch recording, a small proportion of the conductance was found to be tonically active. This was weakly potentiated by diazoxide (0.1 mm extracellular glucose) but insensitive to pinacidil (< or =500 microM). Single-channel K(ATP) currents recorded in symmetrical 140 mm K(+)-containing solutions exhibited weak inward rectification with a mean conductance of 66.2 +/- 1.9 pS. Channel activity was inhibited by MgATP (>50 microM) and activated by MgADP (200 microM). The K(+) channels opener diazoxide (200-500 microM) increased channel opening probability (NP(o)) by 486 +/- 120% whereas pinacidil (500 microM) had no effect. In conclusion, the characteristics of the K(ATP) channels expressed by BF neurones are very similar to channels composed of SUR1 and Kir6.2 subunits. In the native cell, their affinity for ATP is close to the resting [ATP](i), potentially allowing them to be modulated by physiologically relevant changes in [ATP](i). The effect of these channels on the level of ascending cholinergic excitation of the cortex and hippocampus is discussed.

Sleep and manipulations of the sleep-wake rhythm in depression

OBJECTIVE

Disturbed sleep is typical for most depressed patients and complaints about disordered sleep are the hallmarks of the disorder. Polysomnographic sleep research has demonstrated that besides impaired sleep continuity, sleep in depression is characterized by a reduction of slow wave sleep and a disinhibition of random eye movement (REM) sleep, with a shortening of REM latency, a prolongation of the first REM period and increased REM density.

METHOD

Our own experimental work has focused on the reciprocal interaction hypothesis of non-REM and REM sleep regulation as a model to explain the characteristic features of depressed sleep.

RESULTS

In agreement with the major tenet of this model, administration of cholinomimetics provoked shortened REM latency in healthy subjects and led to an even stronger REM sleep disinhibition in depressed patients. Manipulations of the sleep-wake cycle, such as sleep deprivation or a phase advance of the sleep period, alleviate depressive symptoms.

CONCLUSION

These data indicate a strong bidirectional relationship between sleep, sleep alterations and depression.

Emotional instability but intact spatial cognition in adenosine receptor 1 knock out mice

Several lines of evidence point to the involvement of adenosine in the regulation of important central mechanisms such as cognition, arousal, aggression and anxiety. In order to elucidate the involvement of the adenosine A1 receptor (A1AR) in spatial learning and the control of exploratory behaviour, we assessed A1AR knockout mice (A1AR-/-) and their wild-type littermates (A1AR+/+) in a place navigation task in the water maze and in a battery of forced and free exploration tests. In the water maze, A1AR-/- mice showed normal escape latencies and were indistinguishable from controls with respect to measures of spatial performance during both training and probe trial. But despite normal performance they showed increased wall hugging, most prominently after the relocation of the goal platform for reversal training. Quantitative analysis of strategy choices indicated that wall hugging was increased mainly at the expense of chaining and passive floating, whereas the frequency of trials characterised as direct swims or focal searching was normal in A1AR-/- mice. These results indicate intact spatial cognition, but mildly altered emotional reactions to the water maze environment. In line with this interpretation, A1AR-/- mice showed normal levels and patterns of activity, but a mild increase of some measures of anxiety in our battery of forced and free exploration paradigms. These results are in line with findings published using a genetically similar line, but demonstrate that the magnitude of the changes and the range of affected behavioural measures may vary considerably depending on the environmental conditions during testing.

Caffeine, fatigue, and cognition

Effects of caffeine and fatigue are discussed with special attention to adenosine-dopamine interactions. Effects of caffeine on human cognition are diverse. Behavioural measurements indicate a general improvement in the efficiency of information processing after caffeine, while the EEG data support the general belief that caffeine acts as a stimulant. Studies using ERP measures indicate that caffeine has an effect on attention, which is independent of specific stimulus characteristics. Behavioural effects on response related processes turned out to be mainly related to more peripheral motor processes. Recent insights in adenosine and dopamine physiology and functionality and their relationships with fatigue point to a possible modulation by caffeine of mechanisms involved in the regulation of behavioural energy expenditure.

Changes in brain glycogen after sleep deprivation vary with genotype

Sleep has been functionally implicated in brain energy homeostasis in that it could serve to replenish brain energy stores that become depleted while awake. Sleep deprivation (SD) should therefore lower brain glycogen content. We tested this hypothesis by sleep depriving mice of three inbred strains, i.e., AKR/J (AK), DBA/2J (D2), and C57BL/6J (B6), that differ greatly in their sleep regulation. After a 6-h SD, these mice and their controls were killed by microwave irradiation, and glycogen and glucose were quantified in the cerebral cortex, brain stem, and cerebellum. After SD, both measures significantly increased by approximately 40% in the cortex of B6 mice, while glycogen significantly decreased by 20-38% in brain stem and cerebellum of AK and D2 mice. In contrast, after SD, glucose content increased in all three structures in AK mice and did not change in D2 mice. The increase in glycogen after SD in B6 mice persisted under conditions of food deprivation that, by itself, lowered cortical glycogen. Furthermore, the strains that differ most in their compensatory response to sleep loss, i.e., AK and D2, did not differ in their glycogen response. Thus glycogen content per se is an unlikely end point of sleep's functional role in brain energy homeostasis.

Sleep as restitution: an introduction

This paper summarizes a symposium that has shown that sleep is a state of altered metabolism, and that disturbances and curtailing of sleep have far-reaching effects on endocrinology, immunology and metabolism, changes that may be linked to disease. As yet, the entire causal chain is weak but there are indications that, in particular, the risk of type 2 diabetes and cardiovascular disease may result from disturbed sleep. It is hypothesized that both insulin resistance and chronic low-grade inflammation may be involved.

Central nervous system effects of caffeine and adenosine on fatigue

Caffeine ingestion can delay fatigue during exercise, but the mechanisms remain elusive. This study was designed to test the hypothesis that blockade of central nervous system (CNS) adenosine receptors may explain the beneficial effect of caffeine on fatigue. Initial experiments were done to confirm an effect of CNS caffeine and/or the adenosine A(1)/A(2) receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) on spontaneous locomotor activity. Thirty minutes before measurement of spontaneous activity or treadmill running, male rats received caffeine, NECA, caffeine plus NECA, or vehicle during four sessions separated by approximately 1 wk. CNS caffeine and NECA (intracerebroventricular) were associated with increased and decreased spontaneous activity, respectively, but caffeine plus NECA did not block the reduction induced by NECA. CNS caffeine also increased run time to fatigue by 60% and NECA reduced it by 68% vs. vehicle. However, unlike the effects on spontaneous activity, pretreatment with caffeine was effective in blocking the decrease in run time by NECA. No differences were found after peripheral (intraperitoneal) drug administration. Results suggest that caffeine can delay fatigue through CNS mechanisms, at least in part by blocking adenosine receptors.

A three-enzyme microelectrode sensor for detecting purine release from central nervous system

As the purines, in particular adenosine, are important signaling agents in the nervous system we have devised a new biosensor for directly measuring their production in real time during physiological activity. Our amperometric adenosine biosensor is made by entrapping 3 enzymes (xanthine oxidase, purine nucleoside phosphorylase and adenosine deaminase) in a composite lactobionamide and amphiphillic polypyrrole matrix around a Pt microelectrode. The resulting sensors are small (25-100 microm diameter), fast responding (10-90% rise time, 2+/-0.23 s), sensitive (100-222 mA M(-1) cm(-2)) and stable (100% activity after 5 days). The sensor was used in vivo to demonstrate the spatial localization of release of adenosine from Xenopus embryo spinal cord during fictive swimming.

Adenosine receptors in the nervous system: pathophysiological implications

Adenosine is a ubiquitous homeostatic substance released from most cells, including neurones and glia. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)) that can inhibit (A(1)) or enhance (A(2)) neuronal communication. Interactions between adenosine receptors and other G-protein-coupled receptors, ionotropic receptors and receptors for neurotrophins also occur, and this might contribute to a fine-tuning of neuronal function. Manipulations of adenosine receptors influence sleep and arousal, cognition and memory, neuronal damage and degeneration, as well as neuronal maturation. These actions might have therapeutic implications for neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, as well as for other neurological situations such as epilepsy, idiopathic pain or even drug addition. Peripheral side effects associated with adenosine receptor agonists limit their usefulness in therapeutics; in contrast, adenosine receptor antagonists appear to have less side effects as it is the case of the well-known non-selective antagonists theophylline (present in tea) or caffeine (abundant in coffee and tea), and their emerging beneficial actions in Parkinson's disease and Alzheimer's disease are encouraging. A(1) receptor antagonism may also be useful to enhance cognition and facilitate arousal, as well as in the periphery when deficits of neurotransmitter release occur (e.g. myasthenic syndromes). Enhancement of extracellular adenosine levels through drugs that influence its metabolism might prove useful approaches in situations such as neuropathic pain, where enhanced activation of inhibitory adenosine A(1) receptors is beneficial. One might then consider adenosine as a fine-tuning modulator of neuronal activity, which via subtle effects causes harmonic actions on neuronal activity. Whenever this homeostasis is disrupted, pathology may be installed and selective receptor antagonism or agonism required.

Lignans isolated from valerian: identification and characterization of a new olivil derivative with partial agonistic activity at A(1) adenosine receptors

A methanolic extract of the roots of Valeriana officinalis (valerian) was investigated for its lignan content. In addition to the lignans 8'-hydroxypinoresinol (1) and pinoresinol-4-O-beta-D-glucoside (2), which had already been isolated from valerian in an earlier study, the 7,9'-monoepoxylignans massoniresinol-4'-O-beta-D-glucoside (3), 4'-O-beta-D-glucosyl-9-O-(6' '-deoxysaccharosyl)olivil (4), and berchemol-4'-O-beta-D-glucoside (5) and the 7,9':7',9-diepoxylignans pinoresinol-4,4'-di-beta-O-D-glucoside (6), 8-hydroxypinoresinol-4'-O-beta-D-glucoside (7), and 8'-hydroxypinoresinol-4'-O-beta-D-glucoside (8) were identified. While lignans 3, 6, 7, and 8 had already been isolated from other plants, lignans 4 and 5 are new natural products. The lignans were investigated in radioligand binding assays at various receptors of the central nervous system, including GABA(A), benzodiazepine, 5-HT(1A), and adenosine A(1) and A(2A) receptors, to investigate their potential contribution to the pharmacological activity of valerian. The novel olivil derivative 4 proved to be a partial agonist at rat and human A(1) adenosine receptors exhibiting A(1) affinity and activity in low micromolar to submicromolar concentrations. Lignan 4 is the first nonnucleoside adenosine receptor agonist not structurally related to adenosine.

Exacerbation of REM sleep behavior disorder by chocolate ingestion: a case report

REM sleep behavior disorder (RBD) is manifest by loss of normal rapid eye movement sleep atonia and the acting out of dreams of often violent content. Both idiopathic and secondary forms of RBD exist. We report on chocolate as a possible new precipitating agent for RBD and comment on a possible mechanism of action in this disorder.

Sleep deprivation decreases glycogen in the cerebellum but not in the cortex of young rats

We tested whether brain glycogen reserves were depleted by sleep deprivation (SD) in Long-Evans rats 20-59 days old. Animals were sleep deprived beginning at lights on and then immediately killed by microwave irradiation. Glycogen and glucose levels were measured by a fluorescence enzymatic assay. In all age groups, SD reduced cerebellar glycogen levels by an average of 26% after 6 h of SD. No changes were observed in the cortex after 6 h of SD, but in the oldest animals, 12 h of SD increased cortical glycogen levels. There was a developmental increase in basal glycogen levels in both the cortex and cerebellum that peaked at 34 days and declined thereafter. Robust differences in cortical and cerebellar glycogen levels in response to enforced waking may reflect regional differences in energy utilization and regulation during wakefulness. These results show that brain glycogen reserves are sensitive to SD.

Treating restless legs syndrome: current pathophysiological concepts and clinical trials

Restless legs syndrome is a distinctive clinical syndrome with a prevalence of about 5% in the general population. One of the outstanding characteristics of restless legs syndrome is its extreme responsiveness to dopaminergic agents. Together with the latest pathophysiological and genetic findings, recent epidemiological and clinical data give a new insight into the classification of restless legs syndrome, thus building the theoretical foundation for the development of new pharmacological methods in its treatment. Current efforts within this area focus on establishing dopaminergic substances for therapy. The hypothesis of a disturbed iron metabolism in restless legs syndrome has been revived by recent theoretical considerations. The present review attempts to explain current strategies of treatment for restless legs syndrome in relation to aetiological, genetic and pathophysiological findings.

Adenosine and caffeine modulate circadian rhythms in the Syrian hamster

Extracellular adenosine accumulates in some brain areas during sleep deprivation. In Syrian hamsters, both sleep deprivation and adenosine A1 agonists can inhibit phase shifts of circadian rhythms to light at night. Sleep deprivation in the day (sleep period) can shift circadian phase. We examined whether the A1 agonist N-CHA mimics this effect. N-CHA (i.p. or i.c.) in the mid-sleep period induced dose-dependent shifts similar to those induced by 3 h sleep deprivation. The adenosine antagonist caffeine administered systemically at the mid-sleep period induced arousal without shifts, and dose-dependently attenuated shifts to a 3 h sleep deprivation procedure (running in a novel wheel). Adenosine may participate in resetting of the circadian clock by manipulations of behavioral state.

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 A2A receptor antagonists are potential antidepressants: evidence based on pharmacology and A2A receptor knockout mice

1. Adenosine, an ubiquitous neuromodulator, and its analogues have been shown to produce 'depressant' effects in animal models believed to be relevant to depressive disorders, while adenosine receptor antagonists have been found to reverse adenosine-mediated 'depressant' effect. 2. We have designed studies to assess whether adenosine A2A receptor antagonists, or genetic inactivation of the receptor would be effective in established screening procedures, such as tail suspension and forced swim tests, which are predictive of clinical antidepressant activity. 3. Adenosine A2A receptor knockout mice were found to be less sensitive to 'depressant' challenges than their wildtype littermates. Consistently, the adenosine A2A receptor blockers SCH 58261 (1 - 10 mg kg(-1), i.p.) and KW 6002 (0.1 - 10 mg kg(-1), p.o.) reduced the total immobility time in the tail suspension test. 4. The efficacy of adenosine A2A receptor antagonists in reducing immobility time in the tail suspension test was confirmed and extended in two groups of mice. Specifically, SCH 58261 (1 - 10 mg kg(-1)) and ZM 241385 (15 - 60 mg kg(-1)) were effective in mice previously screened for having high immobility time, while SCH 58261 at 10 mg kg(-1) reduced immobility of mice that were selectively bred for their spontaneous 'helplessness' in this assay. 5. Additional experiments were carried out using the forced swim test. SCH 58261 at 10 mg kg(-1) reduced the immobility time by 61%, while KW 6002 decreased the total immobility time at the doses of 1 and 10 mg kg(-1) by 75 and 79%, respectively. 6. Administration of the dopamine D2 receptor antagonist haloperidol (50 - 200 microg kg(-1) i.p.) prevented the antidepressant-like effects elicited by SCH 58261 (10 mg kg(-1) i.p.) in forced swim test whereas it left unaltered its stimulant motor effects. 7. In conclusion, these data support the hypothesis that A2A receptor antagonists prolong escape-directed behaviour in two screening tests for antidepressants. Altogether the results support the hypothesis that blockade of the adenosine A2A receptor might be an interesting target for the development of effective antidepressant agents.

Increase of waking and reduction of NREM and REM sleep after nitric oxide synthase inhibition: prevention with GABAA or adenosine A1 receptor agonists

The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) route or was infused directly into the dorsal raphe nucleus (DRN). Subcutaneous (46.0--185.0 micromol/kg) administration of L-NAME increased waking (W), slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) latency, whereas SWS, REMS and the number of REM periods were reduced. Direct application of L-NAME into the DRN (0.37--1.1 micromol) induced an increment of W and a reduction of SWS and REMS. Values corresponding to SWS and REMS latency, and the number of REM periods remained within control levels. Subcutaneous administration of the GABA(A) receptor agonist muscimol (1.7--3.5 micromol/kg) or the adenosine A(1) receptor agonist L-PIA [L(-)N(6)-(2-phenylisopropyl)adenosine] (0.1--0.3 micromol/kg) induced slight but inconsistent changes of W, light sleep (LS), SWS and REMS that did not attain significance. Pretreatment with muscimol (1.7--3.5 micromol/kg, s.c.) or L-PIA (0.1--0.3 micromol/kg, s.c.) antagonized the increase of W and reduction of SWS and REMS induced by s.c. (92.0 micromol/kg) or intra-DRN (0.74 micromol) administration of L-NAME. However, neither muscimol nor L-PIA prevented the increase of REMS latency induced by L-NAME 92.0 micromol/kg, s.c. Our findings tend to indicate that the change of behavioral state observed after systemic or intra-DRN administration of L-NAME is partly related to the reduction of GABA and adenosine at critical sites in the CNS.

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.

SCH 58261 and ZM 241385 differentially prevent the motor effects of CGS 21680 in mice: evidence for a functional 'atypical' adenosine A(2A) receptor

The acute motor effects elicited by drugs acting upon adenosine A(2A) receptors, namely the highly selective agonist CGS 21680 or the antagonists SCH 58261 and ZM 241385, were investigated in mice. CGS 21680 dose-dependently (0.1-2.5 mg/kg i.p.) decreased horizontal and vertical motor activities. The depressant effect of CGS 21680 (0. 5 mg/kg i.p.) was maintained in mice pretreated by the adenosine receptor antagonist 8-(p-sulfophenyl)-theophylline (10-30 mg/kg i.p. ), which poorly penetrates the blood-brain barrier, but was completely lost in adenosine A(2A) receptor knockout mice. Thus, the adenosine A(2A) receptor is critically involved in motor activity. SCH 58261 (1-10 mg/kg i.p.) increased locomotion and rearing with a quick onset, but for a shorter period in mice habituated to the environment than in mice unfamiliar to it. ZM 241385 (7.5-60 mg/kg i. p.) stimulated horizontal and vertical activities with a slow onset at the two highest tested doses, similarly in naive and in habituated mice. The increase in locomotion elicited by ZM 241385 (15-30 mg/kg i.p. and 10-20 nM i.c.v.) was retained in mice treated by CGS 21680 (0.5 mg/kg i.p.) but that elicited by SCH 58261 (1-3-10 mg/kg i.p. and 10-20 nM i.c.v.) partially subsided. In conclusion, both 'striatal-like'/'SCH 58261-sensitive' adenosine A(2A) receptors and 'ZM 241385-sensitive'/'atypical' CGS 21680 binding sites may mediate CGS 21680-induced motor effects. Moreover, our results suggest that 'atypical' CGS 21680 binding sites could be adenosine A(2A) receptors with a peculiar pharmacological profile.

Direct measurement of adenosine release during hypoxia in the CA1 region of the rat hippocampal slice

We have used an enzyme-based, twin-barrelled sensor to measure adenosine release during hypoxia in the CA1 region of rat hippocampal slices in conjunction with simultaneous extracellular field recordings of excitatory synaptic transmission. When loaded with a combination of adenosine deaminase, nucleoside phosphorylase and xanthine oxidase, the sensor responded linearly to exogenous adenosine over the concentration range 10 nM to 20 microM. Without enzymes, the sensor when placed on the surface of hippocampal slices recorded a very small net signal during hypoxia of 40 +/- 43 pA (mean +/- s.e.m.; n = 7). Only when one barrel was loaded with the complete sequence of enzymes and the other with the last two in the cascade did the sensor record a large net difference signal during hypoxia (1226 +/- 423 pA; n = 7). This signal increased progressively during the hypoxic episode, scaled with the hypoxic depression of the simultaneously recorded field excitatory postsynaptic potential and was greatly reduced (67 +/- 6.5 %; n = 9) by coformycin (0.5-2 microM), a selective inhibitor of adenosine deaminase, the first enzyme in the enzymic cascade within the sensor. For 5 min hypoxic episodes, the sensor recorded a peak concentration of adenosine of 5.6 +/- 1.2 microM (n = 16) with an IC(50) for the depression of transmission of approximately 3 microM. In slices pre-incubated for 3-6 h in nominally Ca(2+)-free artificial cerebrospinal fluid, 5 min of hypoxia resulted in an approximately 9-fold greater release of adenosine (48.9 +/- 17.7 microM; n = 6). High extracellular Ca(2+) (4 mM) both reduced the adenosine signal recorded by the sensor during hypoxia (3.5 +/- 0.6 microM; n = 4) and delayed the hypoxic depression of excitatory synaptic transmission.