Age-dependent changes of presynaptic neuromodulation via A1-adenosine receptors in rat hippocampal slices

Feedback facilitation by adenosine A2A receptors of ATP release from mouse hippocampal nerve terminals

The adenosine modulation system is mostly composed by inhibitory A₁ receptors (A₁R) and the less abundant facilitatory A_2A receptors (A_2AR), the latter selectively engaged at high frequency stimulation associated with synaptic plasticity processes in the hippocampus. A_2AR are activated by adenosine originated from extracellular ATP through ecto-5'-nucleotidase or CD73-mediated catabolism. Using hippocampal synaptosomes, we now investigated how adenosine receptors modulate the synaptic release of ATP. The A_2AR agonist CGS21680 (10-100 nM) enhanced the K⁺-evoked release of ATP, whereas both SCH58261 and the CD73 inhibitor α,β-methylene ADP (100 μM) decreased ATP release; all these effects were abolished in forebrain A_2AR knockout mice. The A₁R agonist CPA (10-100 nM) inhibited ATP release, whereas the A₁R antagonist DPCPX (100 nM) was devoid of effects. The presence of SCH58261 potentiated CPA-mediated ATP release and uncovered a facilitatory effect of DPCPX. Overall, these findings indicate that ATP release is predominantly controlled by A_2AR, which are involved in an apparent feedback loop of A_2AR-mediated increased ATP release together with dampening of A₁R-mediated inhibition. This study is a tribute to María Teresa Miras-Portugal.

International Union of Basic and Clinical Pharmacology. CXII: Adenosine Receptors: A Further Update

Our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors (2011) contained a number of emerging developments with respect to this G protein-coupled receptor subfamily, including protein structure, protein oligomerization, protein diversity, and allosteric modulation by small molecules. Since then, a wealth of new data and results has been added, allowing us to explore novel concepts such as target binding kinetics and biased signaling of adenosine receptors, to examine a multitude of receptor structures and novel ligands, to gauge new pharmacology, and to evaluate clinical trials with adenosine receptor ligands. This review should therefore be considered a further update of our previous reports from 2001 and 2011. SIGNIFICANCE STATEMENT: Adenosine receptors (ARs) are of continuing interest for future treatment of chronic and acute disease conditions, including inflammatory diseases, neurodegenerative afflictions, and cancer. The design of AR agonists ("biased" or not) and antagonists is largely structure based now, thanks to the tremendous progress in AR structural biology. The A2A- and A2BAR appear to modulate the immune response in tumor biology. Many clinical trials for this indication are ongoing, whereas an A2AAR antagonist (istradefylline) has been approved as an anti-Parkinson agent.

The Neuroprotective Effects of Moderate and Regular Caffeine Consumption in Alzheimer's Disease

The increasing numbers of elderly Alzheimer's disease (AD) patients because of a steady increase in the average lifespan and aging society attract great scientific concerns, while there were fewer effective treatments on AD progression due to unclear exact causes and pathogenesis of AD. Moderate (200-500 mg/d) and regular caffeine consumption from coffee and tea are considered to alleviate the risk of AD and have therapeutic potential. This paper reviewed epidemiological studies about the relationship of caffeine intake from coffee or/and tea with the risk of AD and summarized the caffeine-related AD therapies based on experimental models. And further well-designed and well-conducted studies are suggested to investigate the optimal dosages, frequencies, and durations of caffeine consumption to slow down AD progression and treat AD.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

Functional coupling between adenosine A1 receptors and G-proteins in rat and postmortem human brain membranes determined with conventional guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding or [35S]GTPγS/immunoprecipitation assay

Adenosine signaling plays a complex role in multiple physiological processes in the brain, and its dysfunction has been implicated in pathophysiology of neuropsychiatric diseases such as schizophrenia and affective disorders. In the present study, the coupling between adenosine A₁ receptor and G-protein was assessed by means of two [³⁵S]GTPγS binding assays, i.e., conventional filtration method and [³⁵S]GTPγS binding/immunoprecipitation in rat and human brain membranes. The latter method provides information about adenosine A₁ receptor-mediated Gα_i-3 activation in rat as well as human brain membranes. On the other hand, adenosine-stimulated [³⁵S]GTPγS binding determined with conventional assay derives from functional activation of Gα_i/o proteins (not restricted only to Gα_i-3) coupled to adenosine A₁ receptors. The determination of adenosine concentrations in the samples used in the present study indicates the possibility that the assay mixture under our experimental conditions contains residual endogenous adenosine at nanomolar concentrations, which was also suggested by the results on the effects of adenosine receptor antagonists on basal [³⁵S]GTPγS binding level. The effects of adenosine deaminase (ADA) on basal binding also support the presence of adenosine. Nevertheless, the varied patterns of ADA discouraged us from adding ADA into assay medium routinely. The concentration-dependent increases elicited by adenosine were determined in 40 subjects without any neuropsychiatric disorders. The increases in %E_max values determined by conventional assay according to aging and postmortem delay should be taken into account in future studies focusing on the effects of psychiatric disorders on adenosine A₁ receptor/G-protein interaction in postmortem human brain tissue.

How does adenosine control neuronal dysfunction and neurodegeneration?

The adenosine modulation system mostly operates through inhibitory A₁ (A₁ R) and facilitatory A_2A receptors (A_2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A₁ R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: high-frequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A_2A R; A_2A R switch off A₁ R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A₁ R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A₁ R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A_2A R, probably to bolster adaptive changes, but this heightens brain damage since A_2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A_2A R, whereas astrocytic and microglia A_2A R might control the spread of damage. The A_2A R signaling mechanisms are largely unknown since A_2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A₁ R preconditioning and preventing excessive A_2A R function might afford maximal neuroprotection. The main physiological role of the adenosine modulation system is to sharp the salience of information encoding through a combined action of adenosine A_2A receptors (A_2A R) in the synapse undergoing an alteration of synaptic efficiency with an increased inhibitory action of A₁ R in all surrounding synapses. Brain insults trigger an up-regulation of A_2A R in an attempt to bolster adaptive plasticity together with adenosine release and A₁ R desensitization; this favors synaptotocity (increased A_2A R) and decreases the hurdle to undergo degeneration (decreased A₁ R). Maximal neuroprotection is expected to result from a combined A_2A R blockade and increased A₁ R activation. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Adenosine A1 Receptor-Mediated Endocytosis of AMPA Receptors Contributes to Impairments in Long-Term Potentiation (LTP) in the Middle-Aged Rat Hippocampus

Aging causes multiple changes in the mammalian brain, including changes in synaptic signaling. Previous reports have shown increased extracellular adenosine in the aging brain, and we recently reported that activation of adenosine A1 receptors (A1Rs) induces AMPA receptor (AMPAR) internalization in rat hippocampus. This study investigated whether aging-related changes in the rat hippocampus include altered surface expression of adenosine A1 and A2A receptors, and whether these changes correspond to changes in AMPAR surface expression and altered synaptic plasticity. We found reduced A1R surface expression in middle-aged rat hippocampus, and also reduced GluA1 and GluA2 AMPAR subunit surface expression. Using a chemically-induced LTP (cLTP) experimental protocol, we recorded fEPSPs in young (1 month old) and middle-aged (7-12 month old) rat hippocampal slices. There were significant impairments in cLTP in middle-aged slices, suggesting impaired synaptic plasticity. Since we previously showed that the A1R agonist N(6)-cyclopentyladenosine (CPA) reduced both A1Rs and GluA2/GluA1 AMPARs, we hypothesized that the observed impaired synaptic plasticity in middle-aged brains is regulated by A1R-mediated AMPAR internalization by clathrin-mediated endocytosis. Following cLTP, we found a significant increase in GluA1 and GluA2 surface expression in young rats, which was blunted in middle-aged brains or in young brains pretreated with CPA. Blocking A1Rs with 8-cyclopentyl-1,3-dipropylxanthine or AMPAR endocytosis with either Tat-GluA2-3Y peptide or dynasore (dynamin inhibitor) similarly enhanced AMPAR surface expression following cLTP. These data suggest that age-dependent alteration in adenosine receptor expression contributes to increased AMPAR endocytosis and impaired synaptic plasticity in aged brains.

The role of extracellular adenosine in chemical neurotransmission in the hippocampus and Basal Ganglia: pharmacological and clinical aspects

Now there is general agreement that the purine nucleoside adenosine is an important neuromodulator in the central nervous system, playing a crucial role in neuronal excitability and synaptic/non-synaptic transmission in the hippocampus and basal ganglia. Adenosine is derived from the breakdown of extra- or intracellular ATP and is released upon a variety of physiological and pathological stimuli from neuronal and non-neuronal sources, i.e. from glial cells and exerts effects diffusing far away from release sites. The resultant elevation of adenosine levels in the extracellular space reaches micromolar level, and leads to the activation A₁, A_2A, A_2B and A₃ receptors, localized to pre- and postsynaptic as well as extrasynaptic sites. Activation of presynaptic A₁ receptors inhibits the release of the majority of transmitters including glutamate, acetylcholine, noradrenaline, 5-HT and dopamine, whilst the stimulation of A_2A receptors facilitates the release of glutamate and acetylcholine and inhibits the release of GABA. These actions underlie modulation of neuronal excitability, synaptic plasticity and coordination of neural networks and provide intriguing target sites for pharmacological intervention in ischemia and Parkinson’s disease. However, despite that adenosine is also released during ischemia, A₁ adenosine receptors do not participate in the modulation of excitotoxic glutamate release, which is nonsynaptic and is due to the reverse operation of transporters. Instead, extrasynaptic A1 receptors might be responsible for the neuroprotection afforded by A₁ receptor activation.

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.

Protein expression is altered during spontaneous sleep in aged Sprague Dawley rats

Age-related changes in brain function include those affecting learning, memory, and sleep-wakefulness. Sleep-wakefulness is an essential behavior that results from the interaction of multiple brain regions, peptides, and neurotransmitters. The biological function(s) of sleep, however, remains unknown due to a paucity of information available at the cellular level. Aged rats exhibit alterations in the circadian and homeostatic influences associated with sleep-wake regulation. We recently showed that alterations in cortical profiles occur after timed bouts of spontaneous sleep in young rats. Examination of the cellular response to sleep-wake in old rats may thus provide insight(s) into the biological function(s) of sleep. To test this hypothesis, we monitored cortical profiles in the frontal cortex of young and old Sprague-Dawley rats after timed bouts of spontaneous sleep-wake behavior. Proteins were separated by two-dimensional electrophoresis (2-DE), visualized by fluorescent staining, imaged, and analyzed as a function of behavioral state and age. Old rats showed a 6-fold increase in total protein expression, independent of the behavioral state at sacrifice. When analyzed according to age and behavioral state, there was a decrease (approximately 46%) in the number of phospho-spots present during SWS in aged animals. SWS-associated spots present only in old animals were associated with multiple functions including vesicular transport, cell signaling, oxidation state, cytoskeletal support, and energy metabolism. These data suggest that the intracellular response to the signaling associated with spontaneous sleep is affected by age and is consistent with the idea that the ability of sleep to fulfill its function(s) may become diminished with age.

The substrates of memory: defects, treatments, and enhancement

Recent work has added strong support to the long-standing hypothesis that the stabilization of both long-term potentiation and memory requires rapid reorganization of the spine actin cytoskeleton. This development has led to new insights into the origins of cognitive disorders, and raised the possibility that a diverse array of memory problems, including those associated with diabetes, reflect disturbances to various components of the same mechanism. In accord with this argument, impairments to long-term potentiation in mouse models of Huntington's disease and in middle-aged rats have both been linked to problems with modulatory factors that control actin polymerization in spine heads. Complementary to the common mechanism hypothesis is the idea of a single treatment for addressing seemingly unrelated memory diseases. First tests of the point were positive: Brain-Derived Neurotrophic Factor (BDNF), a potent activator of actin signaling cascades in adult spines, rescued potentiation in Huntington's disease mutant mice, middle-aged rats, and a mouse model of Fragile-X syndrome. A similar reversal of impairments to long-term potentiation was obtained in middle-aged rats by up-regulating BDNF production with brief exposures to ampakines, a class of drugs that positively modulate AMPA-type glutamate receptors. Work now in progress will test if chronic elevation of BDNF enhances memory in normal animals.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

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.

Effect of aging on cerebral A1 adenosine receptors: A [18F]CPFPX PET study in humans

Cerebral A(1) adenosine receptors (A(1)AR) fulfill important neuromodulatory and homeostatic functions. The present study examines possible age-related A(1)AR changes in living humans by positron emission tomography (PET) and the A(1)AR ligand [(18)F]CPFPX. Thirty-six healthy volunteers aged 22-74 years were included. The apparent binding potential (BP'2) of [(18)F]CPFPX in various cerebral regions was calculated non-invasively using the cerebellum as reference region. In addition, the total distribution volume (DV't) was assessed in 10 subjects undergoing arterial blood sampling. There was no significant association between regional DV't and age, gender, caffeine consumption or sleep duration. BP'2 showed a significant age-dependent decrease in all regions except cingulate gyrus (p=0.062). The BP'2 decline ranged from -17% (striatum) to -34% (postcentral gyrus), the average cortical decline being -23%. There was no significant effect of gender, caffeine consumption and sleep duration on BP'2. In line with in vitro animal studies, the present in vivo PET study detected an age-dependent A(1)AR loss in humans that may be of pathophysiological importance in various neurological diseases associated with aging. Because of the discrepant results of the invasive (DV't) and the non-invasive (BP'2) analyses the present study needs further validation.

Synaptic plasticity in early aging

Studies of how aging affects brain plasticity have largely focused on old animals. However, deterioration of memory begins well in advance of old age in animals, including humans; the present review is concerned with the possibility that changes in synaptic plasticity, as found in the long-term potentiation (LTP) effect, are responsible for this. Recent results indicate that impairments to LTP are in fact present by early middle age in rats but only in certain dendritic domains. The search for the origins of these early aging effects necessarily involves ongoing analyses of how LTP is induced, expressed, and stabilized. Such work points to the conclusion that cellular mechanisms responsible for LTP are redundant and modulated both positively and negatively by factors released during induction of potentiation. Tests for causes of the localized failure of LTP during early aging suggest that the problem lies in excessive activity of a negative modulator. The view of LTP as having redundant and modulated substrates also suggests a number of approaches for reversing age-related losses. Particular attention will be given to the idea that induction of brain-derived neurotrophic factor, an extremely potent positive modulator, can be used to provide long periods of normal plasticity with very brief pharmacological interventions. The review concludes with a consideration of how the selective, regional deficits in LTP found in early middle age might be related to the global phenomenon of brain aging.

Chromatographic analysis of dopamine metabolism in a Parkinsonian model

The present study examined the metabolism of released dopamine from rat striatum upon chronic rotenone exposure. The sample separation was carried out by two-dimensional, reversed-phase and ion pair reversed-phase chromatography using on-line solid phase extraction enrichment. Reduced dopamine content and decreased extracellular level of [(3)H] and endogenous dopamine evoked by electrical stimulation indicated the injury of dopaminergic pathway. Sensitivity of dopaminergic neurons were increased to oxidative stress with enhanced release of dopamine and formation of oxidized metabolite dopamine quinone (DAQ). Utilizing multidimensional detection, EC at -100 mV reduction potential, the method has been applied for identification of DAQ and aminochrome (DAC).

Nociceptive response and adenine nucleotide hydrolysis in synaptosomes isolated from spinal cord of hypothyroid rats

Purinergic system exerts a significant influence on the modulation of pain pathways at the spinal site. Adenosine has antinociceptive properties in experimental and clinical situations, while ATP exerts pronociceptive actions in different pain models. In this study we investigated the hydrolysis of ATP to adenosine in synaptosomes from spinal cord in parallel with the nociceptive response of rats at different ages after hypothyroidism induction. Hypothyroidism elicited a significant increase in AMP hydrolysis to adenosine in synaptosomes from spinal cord of rats subjected to neonatal hypothyroidism and in 420-day-old rats submitted to thyroidectomy. Accordingly, these rats presented an analgesic response as a consequence of hypothyroidism. In contrast, the ATP hydrolysis was decreased in the spinal cord of 60-day-old hypothyroid rats in parallel with a significant increase in nociceptive response. These results indicate the involvement of adenine nucleotides in the control of the hypothyroidism-induced nociceptive response during development.

Long-term potentiation is impaired in middle-aged rats: regional specificity and reversal by adenosine receptor antagonists

Memory loss in humans begins early in adult life and progresses thereafter. It is not known whether these losses reflect the failure of cellular processes that encode memory or disturbances in events that retrieve it. Here, we report that impairments in hippocampal long-term potentiation (LTP), a form of synaptic plasticity associated with memory, are present by middle age in rats but only in select portions of pyramidal cell dendritic trees. Specifically, LTP induced with theta-burst stimulation in basal dendrites of hippocampal field CA1 decayed rapidly in slices prepared from 7- to 10-month-old rats but not in slices from young adults. There were no evident age-related differences in LTP in the apical dendrites. Both the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and a positive AMPA receptor modulator (ampakine) offset age-related LTP deficits. Adenosine produced greater depression of synaptic responses in middle-aged versus young adult slices and in basal versus apical dendrites. These results were not associated with variations in A1 receptor densities and may instead reflect regional and age-related differences in adenosine clearance. Pertinent to this, brief applications of A1 receptor antagonists immediately after theta stimulation fully restored LTP in middle-aged rats. We hypothesize that the build-up of extracellular adenosine during theta activity persists into the postinduction period in the basal dendrites of middle-aged slices and thereby activates the A1 receptor-dependent LTP reversal effect. Regardless of the underlying mechanism, the present results provide a candidate explanation for memory losses during normal aging and indicate that, with regard to plasticity, different segments of pyramidal neurons age at different rates.

Caffeine reverses age-related deficits in olfactory discrimination and social recognition memory in rats

Caffeine, a non-selective adenosine receptor antagonist, has been suggested as a potential drug to counteract age-related cognitive decline since critical changes in adenosinergic neurotransmission occur with aging. In the present study, olfactory discrimination and short-term social memory of 3, 6, 12 and 18 month-old rats were assessed with the olfactory discrimination and social recognition tasks, respectively. The actions of caffeine (3.0, 10.0 and 30.0 mg/kg, i.p.), the A1 receptor antagonist DPCPX (1.0 and 3.0 mg/kg, i.p.) and the A2A receptor antagonist ZM241385 (0.5 and 1.0 mg/kg, i.p.) in relation to age-related effects on olfactory functions were also studied. The 12 and 18 month-old rats exhibited significantly impaired performance in both models, demonstrating deficits in their odor discrimination and in their ability to recognize a juvenile rat after a short period of time. Acute treatment with caffeine or ZM241385, but not with DPCPX, reversed these age-related olfactory deficits. The present results suggest the participation of adenosine receptors in the control of olfactory functions and confirm the potential of caffeine for the treatment of aged-related cognitive decline.

Neuroprotection by adenosine in the brain: From A(1) receptor activation to A (2A) receptor blockade

Adenosine is a neuromodulator that operates via the most abundant inhibitory adenosine A(1) receptors (A(1)Rs) and the less abundant, but widespread, facilitatory A(2A)Rs. It is commonly assumed that A(1)Rs play a key role in neuroprotection since they decrease glutamate release and hyperpolarize neurons. In fact, A(1)R activation at the onset of neuronal injury attenuates brain damage, whereas its blockade exacerbates damage in adult animals. However, there is a down-regulation of central A(1)Rs in chronic noxious situations. In contrast, A(2A)Rs are up-regulated in noxious brain conditions and their blockade confers robust brain neuroprotection in adult animals. The brain neuroprotective effect of A(2A)R antagonists is maintained in chronic noxious brain conditions without observable peripheral effects, thus justifying the interest of A(2A)R antagonists as novel protective agents in neurodegenerative diseases such as Parkinson's and Alzheimer's disease, ischemic brain damage and epilepsy. The greater interest of A(2A)R blockade compared to A(1)R activation does not mean that A(1)R activation is irrelevant for a neuroprotective strategy. In fact, it is proposed that coupling A(2A)R antagonists with strategies aimed at bursting the levels of extracellular adenosine (by inhibiting adenosine kinase) to activate A(1)Rs might constitute the more robust brain neuroprotective strategy based on the adenosine neuromodulatory system. This strategy should be useful in adult animals and especially in the elderly (where brain pathologies are prevalent) but is not valid for fetus or newborns where the impact of adenosine receptors on brain damage is different.

Hypothyroidism changes adenine nucleotide hydrolysis in synaptosomes from hippocampus and cerebral cortex of rats in different phases of development

The influence of the thyroid hormones on the normal function of the mammalian central nervous system depends on the brain region and on the developmental stage. Adenine nucleotides and their products also affect the brain function; ATP is an excitatory neurotransmitter, and adenosine has inhibitory effects on neurotransmission. Thus, this study aimed to evaluate the effects of hypothyroidism on the hydrolysis of ATP to adenosine in hippocampal and cortical synaptosomes and blood serum of rats during different phases of development. Rats aged 60 and 420 days old were divided into three groups: control, sham-operated and hypothyroid. Hypothyroidism was induced in these rats by thyroidectomy and methimazole (0.05%) added to their drinking water for 14 days. Neonatal hypothyroidism was induced by adding 0.02% methimazole in the drinking water from day 9 of gestation, and continually until 14 days old. Hypothyroidism increased the AMP hydrolysis in both hippocampus and cerebral cortex synaptosomes of rats in all aged tested. In blood serum, thyroid hormones deficiency increased the AMP hydrolysis in 14-day-old rats and the hydrolysis of ATP, ADP and AMP in 60-day-old rats; however, no alteration was observed in 420-day-old rats. Thus, our results suggest the involvement of the 5'-nucleotidase in synaptic function control in hypothyroidism throughout brain development.

Hyperthyroidism modifies ecto-nucleotidase activities in synaptosomes from hippocampus and cerebral cortex of rats in different phases of development

Here we investigate the possible effects of the hyperthyroidism on the hydrolysis of the ATP to adenosine in the synaptosomes of hippocampus, cerebral cortex and blood serum of rats in different developmental phases. Manifestations of hyperthyroidism include anxiety, nervousness, tachycardia, physical hyperactivity and weight loss amongst others. The thyroid hormones modulate a number of physiological functions in central nervous system, including development, function, expression of adenosine A(1) receptors and transport of neuromodulator adenosine. Thus, hyperthyroidism was induced in male Wistar rats (5-, 60-, 150- and 330-day old) by daily injections of L-thyroxine (T4) for 14 days. Nucleotide hydrolysis was decreased by about 14-52% in both hippocampus and cerebral cortex in 5 to 60-day-old rats. These changes were also observed in rat blood serum. In addition, in 11-month-old rats, inhibition of ADP and AMP hydrolysis persisted in the hippocampus, whereas, in cerebral cortex, an increase in AMP hydrolysis was detected. Thus, hyperthyroidism affects the extracellular nucleotides balance and adenosine production, interfering in neurotransmitter release, development and others physiological processes in different systems.

Ageing-related decline in adenosine A1 receptor binding in the rat brain: An autoradiographic study

The adenosine system has important neuromodulatory and neuroprotective functions in the brain. Several lines of evidence suggest that ageing is associated with major alterations in the adenosine system, which may be partially responsible for changes in sleep, mood, and cognition. In the present study, we examined adenosine A1 receptor density in the rat brain by means of quantitative autoradiography to obtain a detailed anatomical overview of the changes during ageing. A1 receptor binding was assessed in young, old, and senescent animals of 3, 24, and 30 months old, respectively. There was a clear age-dependent reduction in adenosine A1 receptors in most of the brain areas examined, but the magnitude of this reduction varied greatly among regions. Also, whereas some regions displayed a gradual decline in A1 binding sites across the three age classes, other regions showed a particularly strong decrease between the ages of 24 and 30 months. For example, whereas the hippocampus and thalamus showed a gradual decline in A1 binding, some cortical and septal regions showed a more abrupt decline after the age of 24 months. Since particularly in rats many studies have used animals at the age of 24 months or even less, the ageing-related decline in adenosine A1 signaling might have been underestimated.

The diurnal rhythm of adenosine levels in the basal forebrain of young and old rats

There are significant decrements in sleep with age. These include fragmentation of sleep, increased wake time, decrease in the length of sleep bouts, decrease in the amplitude of the diurnal rhythm of sleep, decrease in rapid eye movement sleep and a profound decrease in electroencephalogram Delta power (0.3-4 Hz). Old rats also have less sleep in response to 12 h-prolonged wakefulness (W) indicating a reduction in sleep drive with age. The mechanism contributing to the decline in sleep with aging is not known but cannot be attributed to loss of neurons implicated in sleep since the numbers of neurons in the ventral lateral preoptic area, a region implicated in generating sleep, is similar between young (3.5 months) and old (21.5 months) rats. One possibility for the reduced sleep drive with age is that sleep-wake active neurons may be stimulated less as a result of a decline in endogenous sleep factors. Here, we test this hypothesis by focusing on the purine, adenosine (AD), one such sleep factor that increases after prolonged W. In experiment 1, microdialysis measurements of AD in the basal forebrain at 1 h intervals reveal that old (21.5 months) rats have more extracellular levels of AD compared with young rats across the 24 h diurnal cycle. In experiment 2, old rats kept awake for 6 h (first half of lights-on period) accumulated more AD compared with young rats. If old rats have more AD then why do they sleep less? To investigate whether changes in sensitivity of the AD receptor contribute to the decline in sleep, experiments 3 and 4 determined that for the same concentration of AD or the AD receptor 1 agonist, cyclohexyladenosine, old rats have less sleep compared with young rats. We conclude that even though old rats have more AD, a reduction in the sensitivity of the AD receptor to the ligand does not transduce the AD signal at the same strength as in young rats and may be a contributing factor to the decline in sleep drive in the elderly.

Characterization of monoaminergic systems in brain regions of prematurely ageing mice

We have previously shown that differences in life span among members of Swiss mouse populations appear to be related to their exploration of a T-maze, with a slow exploration ("slow mice") being linked to increased levels of emotionality/anxiety, an impaired immune function and a shorter life span. Thus, we proposed the slow mice as prematurely ageing mice (PAM). We have now compared the monoaminergic systems of the PAM and of the non-prematurely ageing mice (NPAM), in discrete brain regions. PAM had decreased noradrenaline (NA) levels in all the brain regions analysed, whereas the 3-methoxy-4-hydroxyphenyl glycol (MHPG)/NA ratios were not significantly modified. PAM also showed decreased serotonine (5-HT) levels in hypothalamus, striatum and midbrain, as well as increased 5-hydroxyindol-3-acetic acid (5-HIAA)/5-HT ratios in hypothalamus and hippocampus. The dopamine (DA) content was lower in PAM in most regions, whereas the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA and homovanillic acid (HVA)/DA ratios were either increased or unchanged depending on the region analysed. In most cases, the differences between PAM and NPAM involved both sexes. One exception was the hypothalamus where the differences only affected the male mice. The neurochemical alterations found in PAM resemble some changes reported for aged animals and are related with their behavioural features.

Simultaneous measurement of glutamate and dopamine release from isolated guinea pig cochlea

Glutamate is proved to be a neurotransmitter in the mammalian cochlea, transmitting signals between the inner hair cells and the afferent cochlear nerve terminals. The transmission in this synapse is modulated by the lateral olivocochlear efferent fibers by releasing dopamine and other neurotransmitters. This study undertakes to measure simultaneously the release of dopamine and glutamate from isolated guinea pig cochleae. We combined the in vitro microvolume superfusion method, that uses liquid scintillation analysis, to measure [3H]dopamine with high pressure liquid chromatography (HPLC) to determine the glutamate content of the superfusate at rest and during stimulation. The release of both neurotransmitters was significantly increased when electrical field stimulation was applied at a 10 Hz rate. The nonselective sodium-channel inhibitor tetrodotoxin (TTX) at 1 microM completely blocked the effect of stimulation, indicating the neural origin of both dopamine and glutamate. The dopamine receptor antagonist sulpiride at 100 microM and the dopamine receptor agonist bromocriptine at 20 microM did not change the release of glutamate. In contrast, both bromocriptine and sulpiride significantly increased the stimulation-evoked release of dopamine. The effect of sulpiride is most likely due to the blockade of dopamine autoreceptor. Possible explanations why bromocriptine increased the release include: (1) its partional agonist activity; (2) desensitizations of dopamine autoreceptors; or (3) the higher D1 receptor activity of bromocriptine than sulpiride. This study could provide further insights about the role of dopamine and glutamate in cochlear neurotransmission.

Age-dependent decrease in adenosine A1 receptor binding sites in the rat brain. Effect of cis unsaturated free fatty acids

Unsaturated free fatty acids and adenosine operate two neuromodulatory systems with opposite effects on neuronal function. Here, we tested if fatty acids controlled inhibitory adenosine A1 receptors. Arachidonate (AA, 10 microM) decreased the Bmax of an A1 receptor agonist, (R)-[3H]phenylisopropyladenosine (PIA; from 812 to 267 fmol x mg(-1) protein), and antagonist, [3H]1,3-dipropyl-8-cyclopentylxanthine (DPCPX; from 994 to 311 fmol x mg(-1) protein) and decreased the Kd of [3H]PIA (from 1.20 to 0.57 nM) binding to brain membranes of young adult rats (2 months old), these effects being mimicked by other cis but not trans unsaturated or saturated fatty acids. AA (10 microM) increased the potency of the A1 receptor agonist, 2-chloroadenosine to inhibit hippocampal synaptic transmission in young adult rats (EC50 decreased from 337 to 237 nM), which may constitute a safety feedback mechanism to control AA-induced neurotoxicity. Upon aging, there were increased free fatty acid levels and a concomitant decreased density of A1 receptors. This was more marked in hippocampal nerve terminals of aged rats (24 months old) and may be the determinant factor contributing to the lower potency of 2-choloroadenosine in aged rats (EC50 = 955 nM), in spite of the decreased Kd of PIA binding upon aging. The effects of AA on A1 receptor binding were attenuated upon aging, AA being devoid of effects in aged rats. Accordingly, AA (10 microM) failed to modify the potency of 2-choloroadenosine in aged rats (EC50 = 997 nM). However, albumin, which quenches free fatty acids, increased A1 receptor density by 65% and 2-chloroadenosine potency (EC50 = 703 nM) in aged rats, suggesting that the increased fatty acids levels in aged rats may contribute to the decreased potency of A1 receptor agonists in aged rats. Also, the observed saturation of the control by AA of A1 receptors may contribute to the decreased adaptability of neuromodulation to different firing conditions in aged rats.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

K(ATP) channel blockers selectively interact with A(1)-adenosine receptor mediated modulation of acetylcholine release in the rat hippocampus

In this study the role of ATP-sensitive K(+) channels (K(ATP) channels) in the A(1) receptor mediated presynaptic inhibitory modulation of acetylcholine release was investigated in the rat hippocampus. N(6)-Cyclohexyladenosine (CHA), the selective A(1)-adenosine receptor agonist, reduced concentration-dependently the stimulation-evoked (2 Hz, 1 ms, 240 shocks) [3H]acetylcholine ([3H]ACh) release, from in vitro superfused hippocampal slices preloaded with [3H]choline, an effect prevented by the selective A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). By themselves, neither K(ATP) channel openers, i.e. diazoxide, pinacidil and cromakalim, nor glibenclamide and glipizide, the inhibitors of K(ATP) channels, exerted a significant effect on the resting and evoked release of [3H]ACh. Glibenclamide and glipizide (10-100 microM) completely prevented the inhibitory effect of 0.1 microM CHA and shifted the concentration response curve of CHA to the right. 4-Aminopyridine (10-100 microM), the non-selective potassium channel blocker, increased the evoked release of [3H]ACh, but in the presence of 4-aminopyridine, the inhibitory effect of CHA (0.1 microM) still persisted. Oxotremorine, the M(2) muscarinic receptor agonist, decreased the stimulation-evoked release of [3H]ACh, but its effect was not reversed by glibenclamide. 1,3-Diethyl-8-phenylxanthine (DPX), the selective A(1)-antagonist, effectively displaced [3H]DPCPX in binding experiments, while in the case of glibenclamide and glipizide, only slight displacement was observed. In summary, our results suggest that K(ATP) channels are functionally coupled to A(1) receptors present on cholinergic terminals of the hippocampus, and glibenclamide and glipizide, by interacting with K(ATP) channels, relieve this inhibitory neuromodulation.

Parallel modification of adenosine extracellular metabolism and modulatory action in the hippocampus of aged rats

The neuromodulator adenosine can be released as such, mainly activating inhibitory A1 receptors, or formed from released ATP, preferentially activating facilitatory A2A receptors. We tested if changes in extracellular adenosine metabolism paralleled changes in A1/A2A receptor neuromodulation in the aged rat hippocampus. The evoked release and extracellular catabolism of ATP were 49-55% lower in aged rats, but ecto-5'-nucleotidase activity, which forms adenosine, was 5-fold higher whereas adenosine uptake was decreased by 50% in aged rats. The evoked extracellular adenosine accumulation was 30% greater in aged rats and there was a greater contribution of the ecto-nucleotidase pathway and a lower contribution of adenosine transporters for extracellular adenosine formation in nerve terminals. Interestingly, a supramaximal concentration of an A1 receptor agonist, N6-cyclopentyladenosine (250 nM) was less efficient in inhibiting (17% in old versus 34% in young) and A2A receptor activation with 30 nM CGS21680 was more efficient in facilitating (63% in old versus no effect in young) acetylcholine release from hippocampal slices of aged compared with young rats. The parallel changes in the metabolic sources of extracellular adenosine and A1/A2A receptor neuromodulation in aged rats further strengthens the idea that different metabolic sources of extracellular adenosine are designed to preferentially activate different adenosine receptor subtypes.

Modification of adenosine modulation of synaptic transmission in the hippocampus of aged rats

We compared the modulation of synaptic transmission by adenosine A(1) receptors in the hippocampus of aged (24 months) and young adult rats (6 weeks). The adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine, was less potent (P:<0.05) to inhibit synaptic transmission in aged (EC(50)=53 nM) than young adult (EC(50)=14 nM) hippocampal slices, these effects being prevented by the A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). In contrast with the lower effect of the A(1) receptor agonist, it was observed that blockade of A(1) receptors with DPCPX (50 nM), or removal of endogenous extracellular adenosine with adenosine deaminase (2 u ml(-1)), caused a more pronounced disinhibition of synaptic transmission in aged rats. Also consistent with a more intense A(1) receptor-mediated inhibitory tonus by endogenous adenosine in aged rats was the finding that to fully prevent the depression of synaptic transmission induced by 3 min hypoxia, a higher concentration of DPCPX was required in slices from aged (100 nM) than from young (50 nM) rats. It is concluded that in hippocampal slices of aged rats the efficiency of A(1) receptors to modulate synaptic transmission is reduced, but this may be compensated by an enhanced inhibitory tonus by endogenous adenosine.

Tonic adenosine neuromodulation is preserved in motor nerve endings of aged rats

Neuromuscular transmission is decreased in aged subject. Since endogenous adenosine is a potent neuromodulator at motor nerve endings, either inhibiting via A(1) receptors or facilitating via A(2A) receptors acetylcholine release, we now investigated if the tonic effect of endogenous adenosine was modified at phrenic nerve endings of aged rats. The A(2A) receptor antagonist (ZM241385, 50 nM) inhibited (77 +/- 9%) and the A(1) receptor antagonist (DPCPX, 50 nM) facilitated (74 +/- 13%) acetylcholine release from young adult (6 weeks old) rat preparations, indicating a simultaneous tonic activation of A(2A) and A(1) receptors. Tonic modulation by adenosine was unaltered in aged (24 months old) rats, since ZM241385 (50 nM) inhibited (73 +/- 8%) and DPCPX (50 nM) facilitated (91 +/- 20%) acetylcholine release in aged animals similarly to young rats. This indicates that, in contrast to the central nervous system where adenosine neuromodulation is modified in aged animals, the control by adenosine of phrenic nerve function is preserved in aged animals.

Adenosine modulates synaptic plasticity in hippocampal slices from aged rats

Adenosine is known to modulate synaptic plasticity in the hippocampus of young animals through activation of adenosine A1 receptors. The objective of the present study is to investigate whether the modulatory role of adenosine on phenomena of synaptic plasticity is maintained or modified in the hippocampus of aged animals. We compared the effects of the selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 50 nM), on paired-pulse facilitation (PPF), long-term depression (LTD), long-term potentiation (LTP) and depotentiation elicited in hippocampal slices taken from young adult (5-6 weeks) and old (2 years old) male Wistar rats. DPCPX attenuated PPF both in young (1.64 +/- 0.05 vs. 1.76 +/- 0.05%, n = 6) and in old rats (1.33 +/- 0.05 vs. 1.55 +/- 0.1%, n = 6). LTD was only observed in the presence of DPCPX in both young (21.3 +/- 0.6%, n = 4) and old rats (14.4 +/- 0.9%, n = 6). LTP induced by high-frequency stimulation (HFS) was not significantly different in young and old animals, in the presence or in the absence of DPCPX. A larger depotentiation was observed in the presence of DPCPX in young rats (27.6 +/- 4.4% vs. 16.8 +/- 4.7%, n = 7) as well as in old rats (41.3 +/- 5.1% vs. 16.1 +/- 2.7%, n = 6). LTP induced by theta-burst stimulation was observed only in the presence of DPCPX (53.9 +/- 4.9%, n = 5) in young rats, but could be obtained either in the control solution (81.8 +/- 17.9%, n = 7) or in the presence of DPCPX (98.5 +/- 24.2%, n = 7) in old rats. The modulatory role of endogenous adenosine on synaptic plasticity is generally maintained in aged animals. Drugs interfering with adenosine A1 receptor effects could then be used in old animals to modify synaptic plasticity with relevant behavioural consequences.

Determination of adenosine and deoxyadenosine in urine by high-performance liquid chromatography with column switching

The means of measurement of adenosine and deoxyadenosine in urine was developed by separating adenosine and deoxyadenosine from other compounds using high-performance liquid chromatography with column switchings. This method is simple and convenient since no pretreatment of the urine is needed. Using this method, it could be demonstrated that urinary adenosine was higher in an adenosine deaminase (ADA) deficient patient who had a bone marrow transplant treatment (1.97 micromol/mmol creatinine) and in a heterozygote who had a markedly low erythrocyte ADA activity (1% of control ADA activity) (1.33 micromol/mmol creatinine) as compared to normal subjects (0.22+/-0.09 micromol/mmol creatinine, n=11). It was also noted that urinary deoxyadenosine was below the detection limits in the ADA-deficient bone marrow transplant patient, but it was detected in the heterozygote (3.7 micromol/mmol creatinine). Furthermore, it was also demonstrated that a fructose infusion increased the urinary concentration of adenosine from 0.21+/-0.03 to 2.66+/-1.21 micromol/mmol creatinine in five normal subjects.