Adenosine A(1) receptor agonism in the immature rat brain and heart

Effect of chronic maternal L-Glu intake during gestation and/or lactation on oxidative stress markers, AMPA Glu1 receptor and adenosine A1 signalling pathway from foetal and neonatal cerebellum

L-Glutamate (L-Glu) is an amino acid present in the diet that plays a fundamental role in the central nervous system, as the main excitatory neurotransmitter participating in learning and memory processes. In addition, the nucleoside adenosine has a crucial role in L-Glu metabolism, by regulating the liberation of this neurotransmitter through four different receptors: A1, A2A, A2B and A3, which activate (A2A and A2B) or inhibit (A1 and A3) adenylate cyclase pathway. L-Glu at high concentrations can act as a neurotoxin and induce oxidative stress. The study of the oxidative stress correlated with an excess of L-Glu consumption during maternity is key to understand its effects on foetuses and neonates. Previous studies have shown that there is a change in the receptor levels in the brain of pregnant rats and their foetuses when mothers are administered L-Glu during gestation; however, its effect on the cerebellum is unknown. Cerebellum is known to be responsible for motor, cognitive and emotional functions, so its possible involvement after L-Glu consumption is an important issue to study. Therefore, the aim of the present work was to study the effect of L-Glu exposure during gestation and lactation on oxidative stress biomarkers and neurotransmitter receptors from the cerebellum of foetuses and neonates. After maternal L-Glu intake during gestation, oxidative stress was increased, as the ionotropic L-Glu receptors, and GluR1 AMPA subunit levels were altered in foetuses. A1 adenosine receptor suffered changes after L-Glu treatment during gestation, lactation or both, in lactating neonate cerebellum, while adenylate cyclase activity remain unaltered. Further studies will be necessary to elucidate the importance of L-Glu intake and its possible excitotoxicity in the cerebellum of Wistar rats during the pregnancy period and their involvement in long-term neurodegeneration.

Hypoxia Depresses Synaptic Transmission in the Primary Motor Cortex of the Infant Rat—Role of Adenosine A1 Receptors and Nitric Oxide

The acute and long-term consequences of perinatal asphyxia have been extensively investigated, but only a few studies have focused on postnatal asphyxia. In particular, electrophysiological changes induced in the motor cortex by postnatal asphyxia have not been examined so far, despite the critical involvement of this cortical area in epilepsy. In this study, we exposed primary motor cortex slices obtained from infant rats in an age window (16–18 day-old) characterized by high incidence of hypoxia-induced seizures associated with epileptiform motor behavior to 10 min of hypoxia. Extracellular field potentials evoked by horizontal pathway stimulation were recorded in layers II/III of the primary motor cortex before, during, and after the hypoxic event. The results show that hypoxia reversibly depressed glutamatergic synaptic transmission and neuronal excitability. Data obtained in the presence of specific blockers suggest that synaptic depression was mediated by adenosine acting on pre-synaptic A1 receptors to decrease glutamate release, and by a nitric oxide (NO)/cGMP postsynaptic pathway. These effects are neuroprotective because they limit energy failure. The present findings may be helpful in the preclinical search for therapeutic strategies aimed at preventing acute and long-term neurological consequences of postnatal asphyxia.

Behavioral, genetic and biochemical changes in the brain of the offspring of female mice treated with caffeine during pregnancy and lactation

The intrauterine environment is a critical location for exposure to exogenous and endogenous factors that trigger metabolic changes through fetal programming. Among the external factors, chemical compounds stand out, which include caffeine, since its consumption is common among women, including during pregnancy. Thereby, the aim of the present study was to evaluate the behavioral, genetic, and biochemical parameters in the offspring of female mice treated with caffeine during pregnancy and lactation. Swiss female mice (60 days old) received tap water or caffeine at 0.3 or 1.0 mg/mL during copulation (7 days), pregnancy (21 days) and lactation (21 days). After the end of the lactation period, the offspring were divided into groups (water, caffeine 0.3 or 1.0 mg/mL) with 20 animals (10 animals aged 30 days and 10 animals aged 60 days per group per sex). Initially, the offspring were submitted to behavioral tasks and then euthanized for genetic and biochemical analysis in the brain (cortex, striatum, and hippocampus). Behavioral changes in memory, depression, and anxiety were observed in the offspring: 30-day-old female offspring at 1.0 mg /mL dose presented anxiogenic behavior and male offspring the 0.3 mg/mL dose at 30 days of age did not alter long-term memory. Furthermore, an increase in DNA damage and oxidative stress in the brain were observed in the offspring of both sexes. Furthermore, there were changes in Ape-1, BAX, and Bcl-2 in the female offspring hippocampus at 30 days of life. Thus, with this study, we can suggest genotoxicity, oxidative stress, and behavioral changes caused by caffeine during pregnancy and lactation in the offspring that were not treated directly, but received through their mothers; thus, it is important to raise awareness regarding caffeine consumption among pregnant and lactating females.

Caffeine and Its Neuroprotective Role in Ischemic Events: A Mechanism Dependent on Adenosine Receptors

Ischemia is characterized by a transient, insufficient, or permanent interruption of blood flow to a tissue, which leads to an inadequate glucose and oxygen supply. The nervous tissue is highly active, and it closely depends on glucose and oxygen to satisfy its metabolic demand. Therefore, ischemic conditions promote cell death and lead to a secondary wave of cell damage that progressively spreads to the neighborhood areas, called penumbra. Brain ischemia is one of the main causes of deaths and summed with retinal ischemia comprises one of the principal reasons of disability. Although several studies have been performed to investigate the mechanisms of damage to find protective/preventive interventions, an effective treatment does not exist yet. Adenosine is a well-described neuromodulator in the central nervous system (CNS), and acts through four subtypes of G-protein-coupled receptors. Adenosine receptors, especially A₁ and A_2A receptors, are the main targets of caffeine in daily consumption doses. Accordingly, caffeine has been greatly studied in the context of CNS pathologies. In fact, adenosine system, as well as caffeine, is involved in neuroprotection effects in different pathological situations. Therefore, the present review focuses on the role of adenosine/caffeine in CNS, brain and retina, ischemic events.

Caffeine inhibits hypoxia-induced nuclear accumulation in HIF-1α and promotes neonatal neuronal survival

Apnea of prematurity (AOP) defined as cessation of breathing for 15-20 s, is commonly seen in preterm infants. Caffeine is widely used to treat AOP due to its safety and effectiveness. Caffeine releases respiratory arrest by competing with adenosine for binding to adenosine A1 and A2A receptors (A1R and A2AR). Long before its use in treating AOP, caffeine has been used as a psychostimulant in adult brains. However, the effect of caffeine on developing brains remains unclear. We found that A1R proteins for caffeine binding were expressed in the brains of neonatal rodents and preterm infants (26-27 weeks). Neonatal A1R proteins colocalized with PSD-95, suggesting its synaptic localization. In contrast, our finding on A2R expression in neonatal neurons was restricted to the mRNA level as detected by single cell RT/PCR due to the lack of specific A2AR antibody. Furthermore, caffeine (200 μM) at a dose twice higher than the clinically relevant dose (36-130 μM) had minor or no effects on several basic neuronal functions, such as neurite outgrowth, synapse formation, expression of A1R and transcription of CREB-1 and c-Fos, further supporting the safety of caffeine for clinical use. We found that treatment with CoCl2 (125 μM), a hypoxia mimetic agent, for 24 h triggered neuronal death and nuclear accumulation of HIF-1α in primary neuronal cultures. Subsequent treatment with caffeine at a concentration of 100 μM alleviated CoCl2-induced cell death and prevented nuclear accumulation of HIF-1α. Consistently, caffeine treatment in early postnatal life of neonatal mice (P4-P7) also prevented subsequent hypoxia-induced nuclear increase of HIF-1α. Together, our data support the utility of caffeine in alleviating hypoxia-induced damages in developing neurons.

Transient Disruption of Adenosine Signaling During Embryogenesis Triggers a Pro-epileptic Phenotype in Adult Zebrafish

Adenosinergic signaling has important effects on brain function, anatomy, and physiology in both late and early stages of development. Exposure to caffeine, a non-specific blocker of adenosine receptor, has been indicated as a developmental risk factor. Disruption of adenosinergic signaling during early stages of development can change the normal neural network formation and possibly lead to an increase in susceptibility to seizures. In this work, morpholinos (MO) temporarily blocked the translation of adenosine receptor transcripts, adora1, adora2aa, and adora2ab, during the embryonic phase of zebrafish. It was observed that the block of adora2aa and adora2aa + adora2ab transcripts increased the mortality rate and caused high rate of malformations. To test the susceptibility of MO adora1, MO adora2aa, MO adora2ab, and MO adora2aa + adora2ab animals to seizure, pentylenetetrazole (10 mM) was used as a convulsant agent in larval and adult stages of zebrafish development. Although no MO promoted significant differences in latency time to reach the seizures stages in 7-day-old larvae, during the adult stage, all MO animals showed a decrease in the latency time to reach stages III, IV, and V of seizure. These results indicated that transient interventions in the adenosinergic signaling through high affinity adenosine receptors during embryonic development promote strong outcomes on survival and morphology. Additionally, long-term effects on neural development can lead to permanent impairment on neural signaling resulting in increased susceptibility to seizure.

Desensitization of adenosine A(1) receptors in rat immature cortical neurons

Adenosine plays an important neuroprotective role in brain, usually mediated by the activation of adenosine A₁ receptors. Prolonged activation of a G-protein-coupled receptor generally leads to the partial loss of the responsiveness of receptor-mediated transduction pathways (desensitization). Rat immature cortical neurons were treated with 100 nM⁻N⁶-phenylisopropyladenosine (R-PIA), a selective A₁ receptor agonist, and the effect on adenosine A₁ receptor/adenylyl cyclase pathway was studied. Incubation with R-PIA for 6, 12, 24 and 48 h elicited a time-dependent decrease in adenosine A₁ receptors in plasma membranes (92, 58, 43 and 26% of control, respectively), which was associated with variations in microsomal fraction (21, 56, 124 and 233% of control, respectively), suggesting the internalization and down-regulation of adenosine A₁ receptors. Moreover, real-time PCR assays showed a significant increase in mRNA levels coding adenosine A₁ receptor after the longest treatment period (48 h). In addition, αGi₁₋₂ protein levels detected in microsomes and mRNA levels coding αGi₁ protein were increased after 48 h of treatment with R-PIA, suggesting the synthesis of new αGi₁ proteins. Finally, adenylyl cyclase inhibition elicited by 2-Chloro-N6-cyclopentyladenosine (CPA), a selective adenosine A₁ receptor agonist, was significantly reduced after 12, 24 and 48h of treatment (37, 24 and 23%, respectively) as compared to controls (54%), suggesting the desensitization of adenosine A₁ receptor/adenylyl cyclase pathway. These results suggest that adenosine A₁ receptors desensitize slowly after prolonged receptor activation in immature cortical neurons, showing mechanisms of desensitization similar to those described not only in fetal but also in adult rat brain.

Adverse and protective influences of adenosine on the newborn and embryo: implications for preterm white matter injury and embryo protection

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (ARs). In the postnatal period, A1AR activation may contribute to white matter injury in the preterm infant by altering oligodendrocyte (OL) development. In models of perinatal brain injury, caffeine is neuroprotective against periventricular white matter injury (PWMI) and hypoxic-ischemic encephalopathy (HIE). Supporting the notion that blockade of adenosine action is of benefit in the premature infant, caffeine reduces the incidence of bronchopulmonary dysplasia and CP in clinical studies. In comparison with the adverse effects on the postnatal brain, adenosine acts via A1ARs to play an essential role in protecting the embryo from hypoxia. Embryo protective effects are blocked by caffeine, and caffeine intake during early pregnancy increases the risk of miscarriage and fetal growth retardation. Adenosine and adenosine antagonists play important modulatory roles during mammalian development. The protective and deleterious effects of adenosine depend on the time of exposure and target sites of action.

Methylxanthines during pregnancy and early postnatal life

World-wide, many fetuses and infants are exposed to methylxanthines via maternal consumption of coffee and other beverages containing these substances. Methylxanthines (caffeine, theophylline and aminophylline) are also commonly used as a medication for apnea of prematurity.The metabolism of methylxanthines is impaired in pregnant women, fetuses and neonates, leading to accumulating levels thereof. Methylxanthines readily passes the placenta barrier and enters all tissues and thus may affect the fetus/newborn at any time during pregnancy or postnatal life, given that the effector systems are mature.At clinically relevant doses, the major effector system for methylxanthines is adenosine receptors. Animal studies suggest that adenosine receptors in the cardiovascular, respiratory and immune system are developed at birth, but that cerebral adenosine receptors are not fully functional. Furthermore animal studies have shown protective positive effects of methylxanthines in situations of hypoxia/ischemia in neonates. Similarly, a positive long-term effect on lung function and CNS development was found in human preterm infants treated with high doses of caffeine for apneas. There is now evidence that the overall benefits from methylxanthine therapy for apnea of prematurity outweigh potential short-term risks.On the other hand it is important to note that experimental studies have indicated that long-term effects of caffeine during pregnancy and postnatally may include altered behavior and altered respiratory control in the offspring, although there is currently no human data to support this.Some epidemiology studies have reported negative effects on pregnancy and perinatal outcomes related to maternal ingestion of high doses of caffeine, but the results are inconclusive. The evidence base for adverse effects of caffeine in first third of pregnancy are stronger than for later parts of pregnancy and there is currently insufficient evidence to advise women to restrict caffeine intake after the first trimester.

Effects of 2-chloroadenosine on cortical epileptic afterdischarges in immature rats

Adenosine may represent an endogenous anticonvulsant in the brain. This study focused on the possible anticonvulsant action of an adenosine agonist, 2-chloroadenosine, against cortical epileptic afterdischarges (ADs) in immature rats. Three age groups of rat pups with implanted electrodes were studied: 12-, 18- and 25-days-old. The compound, 2-chloroadenosine, was injected after the first successful stimulation at doses of 1, 4 or 10 mg/kg intraperitoneally, and stimulation at the same intensity was repeated three more times. Movements directly elicited by stimulation, as well as clonic seizures accompanying electroencephalography (EEG) ADs, were markedly suppressed in only the 18-day-old animals. The effects in the 12- and especially the 25-day-old rats were moderate. The duration of the ADs decreased in all three age groups with 2-chloroadenosine treatment, and the shortest AD duration was seen in the treated, 12-day-old rats. The AD suppression also lasted longer in this age group than it did in the older animals. After a brief suppression of the second AD, the treated, 25-day-old group exhibited a significant AD rebound during the third and fourth stimulations. Taken together, our data show that 2-chloroadenosine exhibits an anticonvulsant effect that is dose- and age-dependent.

The neuroprotective effect of cannabidiol in an in vitro model of newborn hypoxic-ischemic brain damage in mice is mediated by CB(2) and adenosine receptors

To investigate the mechanisms involved in cannabidiol (CBD)-induced neuroprotection in hypoxic-ischemic (HI) immature brain, forebrain slices from newborn mice underwent oxygen and glucose deprivation in the presence of vehicle, or CBD alone or with selective antagonists of cannabinoid CB(1) and CB(2), and adenosine A(1) and A(2) receptors. CBD reduced acute (LDH efflux to the incubation medium) and apoptotic (caspase-9 concentration in tissue) HI brain damage by reducing glutamate and IL-6 concentration, and TNFalpha, COX-2, and iNOS expression. CBD effects were reversed by the CB(2) antagonist AM630 and by the A(2A) antagonist SCH58261. The A(1A) antagonist DPCPX only counteracted the CBD reduction of glutamate release, while the CB(1) antagonist SR141716 did not modify any effect of CBD. In conclusion, CBD induces robust neuroprotection in immature brain, by acting on some of the major mechanisms underlying HI cell death; these effects are mediated by CB(2) and adenosine, mainly A(2A), receptors.

Differential effects of the adenosine A1 receptor agonist adenosine amine congener on renal, femoral and carotid vascular conductance in preterm fetal sheep

1. Adenosine A(1) receptor activation is critical for endogenous neuroprotection from hypoxia-ischaemia, raising the possibility that treatment with A(1) receptor agonists may be an effective physiological protection strategy for vulnerable preterm infants. However, the A(1) receptor can mediate unwanted systemic effects, including vasoconstriction of the afferent glomerular arteriole. There is limited information on whether this occurs at doses that improve cerebral perfusion in the immature brain. 2. Therefore, in the present study, we examined whether infusion of the selective A(1) receptor agonist adenosine amine congener (ADAC) is associated with reduced renal perfusion in chronically instrumented preterm (0.7 gestation) fetal sheep. In the present study, ADAC was given in successive doses of 2.5, 5.0 and 15.0 microg, 45 min apart. 3. Treatment with ADAC was associated with a marked reduction in renal vascular conductance (and blood flow), whereas carotid conductance was increased and there was no significant effect on femoral conductance. In contrast with the stable effects of increasing ADAC dose on vascular conductance, there was a significant dose-related fall in fetal heart rate and blood pressure. 4. In conclusion, these short-term data support the concern that A(1) receptor agonist infusion can selectively impair renal perfusion, even at low doses.

Transient inhibition of astrocytogenesis in developing mouse brain following postnatal caffeine exposure

Caffeine is frequently administered in human preterm newborns. Although some data suggest a potential risk for the developing brain, its impact has not been fully evaluated. We used a murine model of postnatal caffeine treatment in which mouse pups received intraperitoneal injections of caffeine from postnatal days 3 to 10. Caffeine exposure resulted in a transient reduction of glial fibrillary acidic protein and S100beta protein expression in various brain areas during the first 2 postnatal weeks (19.8% and 23.2% reduction in the hippocampus at P15, respectively). This effect was dose-dependent and at least partly involved a reduction of glial proliferation, as a caffeine-induced decrease of 5-bromodeoxyuridine incorporation was observed in the dentate gyrus and subventricular zone (25.8% and 26.6%, respectively) and no increase of programmed cell death (cleaved caspase-3 immunostaining) was observed at postnatal day 7. This effect could be reproduced with an antagonist of A(2a) adenosine receptor (A(2a)R) and was blocked by co-injection of an agonist. These results suggest that postnatal caffeine treatment might induce an alteration of astrocytogenesis via A(2a)R blockade during brain development. Although no obvious neuritic abnormalities (microtubule-associated protein 2 and synaptophysin immunostaining) were observed, postnatal caffeine treatment could have long-term consequences on brain function.

Developmental downregulation of excitatory GABAergic transmission in neocortical layer I via presynaptic adenosine A(1) receptors

Layer I of the developing cortex contains a dense GABAergic fiber plexus. These fibers provide excitatory inputs to Cajal-Retzius (CR) cells, the early born neurons in layer I. CR cells possess an extensive axonal projection and form synaptic contacts with excitatory, presumably pyramidal, neurons before birth. Interestingly, activity of CR cells declines during the first postnatal week, but mechanism(s) underlying this phenomenon is not yet known. Here we recorded inhibitory postsynaptic currents (IPSCs) in CR cells at postnatal day (P) 1-2 and P5-7. Blockade of adenosine A(1) receptors (A(1)Rs) increased the amplitude of evoked IPSCs (eIPSCs) and decreased paired-pulse ratio at P5-7 but not at P1-2. A(1)R activation decreased the mean eIPSC amplitude at P5-7, but failed to affect eIPSCs at P1-2. Ecto-adenosine triphosphatase (ATPase) inhibition completely abolished the A(1)R-mediated effects suggesting that extracellular ATP is the main source of adenosine. Because A(1)R blockade did not affect the median miniature IPSC amplitude, our results demonstrate that adenosine reduces gamma-aminiobutyric acid (GABA) release probability via presynaptic A(1)Rs at P5-7. As neuronal activity in layer I can depolarize pyramidal neurons influencing thereby glutamatergic synaptogenesis in the lower cortical layers, postnatal weakening of GABAergic transmission by adenosinergic system might reflect a developmental downregulation of this excitatory drive when glutamatergic synapses are formed.

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.

Shift of adenosine kinase expression from neurons to astrocytes during postnatal development suggests dual functionality of the enzyme

Adenosine is a potent modulator of excitatory neurotransmission, especially in seizure-prone regions such as the hippocampal formation. In adult brain ambient levels of adenosine are controlled by adenosine kinase (ADK), the major adenosine-metabolizing enzyme, expressed most strongly in astrocytes. Since ontogeny of the adenosine system is largely unknown, we investigated ADK expression and cellular localization during postnatal development of the mouse brain, using immunofluorescence staining with cell-type specific markers. At early postnatal stages ADK immunoreactivity was prominent in neurons, notably in cerebral cortex and hippocampus. Thereafter, as seen best in hippocampus, ADK gradually disappeared from neurons and appeared in newly developed nestin- and glial fibrillary acidic protein (GFAP)-positive astrocytes. Furthermore, the region-specific downregulation of neuronal ADK coincided with the onset of myelination, as visualized by myelin basic protein staining. After postnatal day 14 (P14), the transition from neuronal to astrocytic ADK expression was complete, except in a subset of neurons that retained ADK until adulthood in specific regions, such as striatum. Moreover, neuronal progenitors in the adult dentate gyrus lacked ADK. Finally, recordings of excitatory field potentials in acute slice preparations revealed a reduced adenosinergic inhibition in P14 hippocampus compared with adult. These findings suggest distinct roles for adenosine in the developing and adult brain. First, ADK expression in young neurons may provide a salvage pathway to utilize adenosine in nucleic acid synthesis, thus supporting differentiation and plasticity and influencing myelination; and second, adult ADK expression in astrocytes may offer a mechanism to regulate adenosine levels as a function of metabolic needs and synaptic activity, thus contributing to the differential resistance of young and adult animals to seizures.

Perinatal white matter injury: The changing spectrum of pathology and emerging insights into pathogenetic mechanisms

Perinatal brain injury in survivors of premature birth has a unique and unexplained predilection for periventricular cerebral white matter. Periventricular white-matter injury (PWMI) is now the most common cause of brain injury in preterm infants and the leading cause of chronic neurological morbidity. The spectrum of chronic PWMI includes focal cystic necrotic lesions (periventricular leukomalacia; PVL) and diffuses myelination disturbances. Recent neuroimaging studies support that the incidence of PVL is declining, whereas focal or diffuse noncystic injury is emerging as the predominant lesion. Factors that predispose to PVL during prematurity include hypoxia, ischemia, and maternal-fetal infection. In a significant number of infants, PWMI appears to be initiated by perturbations in cerebral blood flow that reflect anatomic and physiological immaturity of the vasculature. Ischemic cerebral white matter is susceptible to pronounced free radical-mediated injury that particularly targets immature stages of the oligodendrocyte lineage. Emerging experimental data supports that pronounced ischemia in the periventricular white matter is necessary, but not sufficient to generate PWMI. The developmental predilection for PWMI to occur during prematurity appears to be related to both the timing of appearance and regional distribution of susceptible oligodendrocyte progenitors. Injury to oligodendrocyte progenitors may contribute to the pathogenesis of PWMI by disrupting the maturation of myelin-forming oligodendrocytes. Chemical mediators that may contribute to white-matter injury include reactive oxygen species glutamate, cytokines, and adenosine. As our understanding of the pathogenesis of PWMI improves, it is anticipated that new strategies for directly preventing brain injury in premature infants will develop.

Different roles of adenosine A1, A2A and A3 receptors in controlling kainate-induced toxicity in cortical cultured neurons

Adenosine is a neuromodulator that can control brain damage through activation of A(1), A(2A) and A(3) receptors, which are located in both neurons and other brain cells. We took advantage of cultured neurons to investigate the role of neuronal adenosine receptors in the control of neurotoxicity caused by kainate and cyclothiazide. Both A(1), A(2A) and A(3) receptors were immunocytochemically identified in cortical neurons. Activation of A(1) receptors with 100 nM CPA did not modify the extent of neuronal death whereas the A(1) receptor antagonist, DPCPX (50 nM), attenuated neurotoxicity by 28 +/- 5%, and effect similar to that resulting from the removal of endogenous adenosine with 2U/ml of adenosine deaminase (27 +/- 3% attenuation of neurotoxicity). In the presence of adenosine deaminase, DPCPX had no further effect and CPA now exacerbated neurotoxicity by 42 +/- 4%. Activation of A(2A) receptor with 30 nM CGS21680 attenuated neurotoxicity by 40 +/- 8%, an effect prevented by the A(2A) receptor antagonists, SCH58261 (50 nM) or ZM241385 (50 nM), which by themselves were devoid of effect. Finally, neither A(3) receptor activation with Cl-IB-MECA (100-500 nM) nor blockade with MRS1191 (5 microM) modified neurotoxicity. These results show that A(1) receptor activation enhances and A(2A) receptor activation attenuates neurotoxicity in cultured cortical neurons, indicating that these two neuronal adenosine receptors directly control neurodegeneration. Interestingly, the control by adenosine of neurotoxicity in cultured neurons is similar to that observed in vivo in newborn animals and is the opposite of what is observed in adult brain preparations where A(1) receptor activation and A(2A) receptor blockade are neuroprotective.

Maternal caffeine intake impairs MK-801-induced hyperlocomotion in young rats

Here we have investigated the effects of maternal caffeine intake (1 g/l) on MK-801-induced hyperlocomotion in rat pups. Animals submitted to caffeine treatment during the gestational and lactational period were separated in two groups: caffeine-treated group (up to 21 days old) and washout group (caffeine treatment up to 7 days old). MK-801 (0.25 mg/kg, i.p.) promoted hyperlocomotion in control rats, but this stimulatory effect was significantly decreased in caffeine-treated and washout groups. The permanent effect after caffeine withdrawal suggests durable or adaptive changes during neurodevelopment, mainly on adenosine receptors or neurotransmitter systems modulated by adenosine, such as the glutamatergic system.

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.

Chronic caffeine or theophylline intake during pregnancy inhibits A1 receptor function in the rat brain

The aim of this work was to study whether caffeine or theophylline chronically consumed during pregnancy affect inhibitory adenylyl cyclase pathway mediated by adenosine, in rat brain of both mothers and full-term fetuses. Immunoblotting analysis revealed a significant decrease in alphaGi(1,2) subunit level (27-29% in mothers, 15-18% in fetuses), associated with a significant increase in the mRNA level coding alphaGi(1) in both maternal and fetal rat brain (12-22%) after methylxanthine intake. No significant differences in alphaGi(3) level were detected in any case. On the other hand, forskolin- and forskolin plus guanosine-5'-O(3-thiotriphosphate) tetralithium salt-stimulated adenylyl cyclase activity was significantly decreased (30-36%) in maternal brain. Moreover, adenylyl cyclase inhibition elicited by N(6)-cyclohexyladenosine, specific adenosine A(1) receptor agonist, was also significantly decreased in caffeine- (40.5%) and theophylline- (55.0%) treated mothers, suggesting a desensitization of adenosine A(1) receptor/adenylyl cyclase pathway in maternal brain. However, no significant differences were detected in fetal brain between control and treated animals. Therefore, caffeine or theophylline chronic intake during pregnancy differently modulates inhibitory adenylyl cyclase pathway mediated by adenosine in maternal and fetal brain causing a loss of the system responsiveness only in maternal brain but down-regulating Gi(1) protein in both mother and fetus brain.

Preconditioning and the developing brain

Preconditioning occurs when a subinjurious exposure renders the brain less vulnerable to a subsequent damaging exposure. In this essay, various models of preconditioning in the immature brain are discussed. Adenosine, excitatory amino acids, nitric oxide, hypoxia-inducible factor, ATP-sensitive K+ channels, caspases, heat shock proteins, inflammatory mediators and gene expression all seem to be involved in sensing, transducing and executing preconditioning resistance. Also reviewed in this essay is evidence that some subinjurious exposures render the brain more vulnerable to a subsequent damaging exposure. We believe that unraveling the mechanisms of how the developing brain becomes inherently resilient or vulnerable will offer important insights into the pathogenesis of injury. Preconditioning of the brain or induction of tolerance of the immune system might be utilized in the future to decrease CNS vulnerability and the occurrence of perinatal brain injury.

Changes in purine levels and adenosine receptors in kindled seizures in the rat

Adenosine is an inhibitory modulator of neuronal activity and its possible involvement in seizures is of interest. We have examined changes in adenosine, its metabolites and receptors in brains of hippocampus-kindled rats, a model of partial epilepsy. Purine levels were measured by in vivo microdialysis and showed a small increase in adenosine and a dramatic increase in its metabolites after kindled seizures. Adenosine A1 receptor binding using [H]DPCPX was unaltered after seizures, whereas A1 agonist stimulated binding of GTP[gamma-S] and A1 mRNA expression increased in the CA3 and other regions. Striatal adenosine A2A mRNA and receptor binding with [H]SCH-58261 decreased. These findings indicate that kindled seizures increase adenosine release and metabolism and induces adaptive changes in adenosine receptors.

Controversies surrounding xanthine therapy

The methylxanthines aminophylline, theophylline and caffeine have been used for more than 30 years to treat apnoea of prematurity. Today, they are among the most commonly prescribed drugs in neonatal medicine. Methylxanthines reduce the frequency of idiopathic apnoea and the need for mechanical ventilation by acting as non-specific inhibitors of adenosine A(1) and A(2a) receptors. However, recent and rapidly growing research into the actions of adenosine and its receptors raises concerns about the safety of methylxanthine therapy in very preterm infants. Possible adverse effects include impaired growth, lack of neuroprotection during acute hypoxic-ischaemic episodes and abnormal behaviour. An international controlled clinical trial is underway to examine the long-term efficacy and safety of methylxanthine therapy in very low birth weight babies.

Key neuroprotective role for endogenous adenosine A1 receptor activation during asphyxia in the fetal sheep

BACKGROUND AND PURPOSE

The fetus is well known to be able to survive prolonged exposure to asphyxia with minimal injury compared with older animals. We and others have observed a rapid suppression of EEG intensity with the onset of asphyxia, suggesting active inhibition that may be a major neuroprotective adaptation to asphyxia. Adenosine is a key regulator of cerebral metabolism in the fetus.

METHODS

We therefore tested the hypothesis that infusion of the specific adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), given before 10 minutes of profound asphyxia in near-term fetal sheep, would prevent neural inhibition and lead to increased brain damage.

RESULTS

DPCPX treatment was associated with a transient rise and delayed fall in EEG activity in response to cord occlusion (n=8) in contrast with a rapid and sustained suppression of EEG activity in controls (n=8). DPCPX was also associated with an earlier and greater increase in cortical impedance, reflecting earlier onset of primary cytotoxic edema, and a significantly smaller reduction in calculated cortical heat production after the start of cord occlusion. After reperfusion, DPCPX-treated fetuses but not controls developed delayed onset of seizures, which continued for 24 hours, and sustained greater selective hippocampal, striatal, and parasagittal neuronal loss after 72-hour recovery.

CONCLUSIONS

These data support the hypothesis that endogenous activation of the adenosine A1 receptor during severe asphyxia mediates the initial suppression of neural activity and is an important mechanism that protects the fetal brain.

Adenosine A1 receptor activation reduces myocardial reperfusion effects on intrinsic cardiac nervous system

The intrinsic cardiac nervous system is the final common integrator of regional cardiac function. The ischemic myocardium modifies this nervous system. We sought to determine the role that intrinsic cardiac neuronal P(1) purinergic receptors play in transducing myocardial ischemia and the subsequent reperfusion. The activity generated by ventricular neurons was recorded concomitant with cardiac hemodynamic variables in 44 anesthetized pigs. Regional ventricular ischemia was induced by briefly occluding (30 s) the ventral interventricular coronary artery distal to the arterial blood supply of identified ventricular neurons. Adenosine (100 microM) was administered to these neurons via their local arterial blood supply during or immediately after transient coronary artery occlusion. Occlusion was also performed following local administration of adenosine A(1) [8-cyclopentyl-1,3-dipropylxanthine (DPCPX)] or A(2) [3,7-dimethyl-1-propargylxanthine (DMPX)] receptor blocking agents. The activity generated by ventricular neurons was modified by transient coronary artery occlusion and the subsequent reperfusion (||Delta|| 112 +/- 14 and 168 +/- 34 impulses/min, respectively; P < 0.01 vs. preischemic states). Locally administered adenosine attenuated neuronal responses to reperfusion (-75%; P < 0.01 compared with normal reperfusion) but not ischemia. The neuronal stabilizing effects that adenosine elicited during reperfusion persisted in the presence of DMPX but not DPCPX. It is concluded that activation of neuronal adenosine A(1) receptors stabilizes the intrinsic cardiac nervous system during reperfusion.

Aggravated brain damage after hypoxic ischemia in immature adenosine A2A knockout mice

BACKGROUND AND PURPOSE

Cerebral hypoxic ischemia (HI) is an important cause of brain injury in the newborn infant. Adenosine is believed to protect against HI brain damage. However, the roles of the different adenosine receptors are unclear, particularly in young animals. We examined the role of adenosine A2A receptors (A2AR) using 7-day-old A2A knockout (A2AR(-/-)) mice in a model of HI.

METHODS

HI was induced in 7-day-old CD1 mice by exposure to 8% oxygen for 30 minutes after occlusion of the left common carotid artery. The resulting unilateral focal lesion was evaluated with the use of histopathological scoring and measurements of residual brain areas at 5 days, 3 weeks, and 3 months after HI. Behavioral evaluation of brain injury by locomotor activity, rotarod, and beam-walking test was made 3 weeks and 3 months after HI. Cortical cerebral blood flow, assessed by laser-Doppler flowmetry, and rectal temperature were measured during HI.

RESULTS

Reduction in cortical cerebral blood flow during HI and rectal temperature did not differ between wild-type (A2AR(+/+)) and knockout mice. In the A2AR(-/-) animals, brain injury was aggravated compared with wild-type mice. The A2AR(-/-) mice subjected to HI displayed increased forward locomotion and impaired rotarod performance in adulthood compared with A2AR(+/+) mice subjected to HI, whereas beam-walking performance was similarly defective in both groups.

CONCLUSIONS

These results suggest that, in contrast to the situation in adult animals, A2AR play an important protective role in neonatal HI brain injury.

The adenosine A1 receptor agonist adenosine amine congener exerts a neuroprotective effect against the development of striatal lesions and motor impairments in the 3-nitropropionic acid model of neurotoxicity

Huntington's disease is a genetic neurodegenerative disorder characterized clinically by both motor and cognitive impairments and striatal lesions. At present, there are no pharmacological treatments able to prevent or slow its development. In the present study, we report the neuroprotective effect of adenosine amine congener (ADAC), a specific A1 receptor agonist known to be devoid of any of the side effects that usually impair the clinical use of such compounds. Remarkably, in a rat model of Huntington's disease generated by subcutaneous infusion of the mitochondrial inhibitor 3-nitropropionic acid (3NP), we have observed that an acute treatment with ADAC (100 microg x kg(-1) x d(-1)) not only strongly reduces the size of the striatal lesion (-40%) and the remaining ongoing striatal degeneration (-30%), but also prevents the development of severe dystonia of hindlimbs. Electrophysiological recording on corticostriatal brain slices demonstrated that ADAC strongly decreases the field EPSP amplitude by 70%, whereas it has no protective effect up to 1 microm against the 3NP-induced neuronal death in primary striatal cultures. This suggests that ADAC protective effects may be mediated presynaptically by the modulation of the energetic impairment-induced striatal excitotoxicity. Altogether, our results indicate that A1 receptor agonists deserve further experimental evaluation in animal models of Huntington's disease.

Adenosine A1 receptor down-regulation in mothers and fetal brain after caffeine and theophylline treatments to pregnant rats

Pregnant rats were treated daily with 1 g/L of caffeine or theophylline in their drinking water during pregnancy and the effect of these methylxanthines on adenosine A1 receptor was assayed using binding and reverse transcription polymerase chain reaction (RT-PCR) assays in brains from both mothers and full-term fetuses. In plasma membranes from pregnant rat brain, caffeine and theophylline caused a significant decrease in total receptor numbers, of the same order in both cases (30%), with no significant changes on receptor affinity. The effect of these adenosine receptor antagonists on plasma membranes from fetal brains was more marked, being detected at approximately 50% of the total receptors detected in control conditions. However, in this tissue, a significant increase in the receptor affinity, of the same order in both cases, was also detected after antagonist administration. No significant variation on the potency of caffeine and theophylline as antagonists was detected after treatments in mothers; however, higher affinities were detected in fetuses. A decrease in the total receptor numbers in fetal brain was associated with an increase in the mRNA coding A1 receptor, as determined by RT-PCR assays, not having detected any mRNA difference in maternal brain. No variation in the levels of mRNA coding A2A receptor was detected in any case. These results suggest that maternal caffeine or theophylline intake modulates adenosine A1 receptor, causing a down-regulation of adenosine A1 receptor in brain in both mothers and fetuses.

A1 adenosine receptor activation induces ventriculomegaly and white matter loss

A1 adenosine receptors (A1ARs) are widely expressed in the brain during development. To examine whether A1AR activation can alter postnatal brain formation, neonatal rats from postnatal days 3 to 14 were treated with the A1AR agonist N6-cyclopentyladenosine (CPA) in the presence or absence of the peripheral A1AR antagonist 8-(p-sulfophenyl)-theophylline (8SPT). CPA or CPA + 8SPT treatment resulted in reductions in white matter volume, ventriculomegaly, and neuronal loss. Quantitative electron microscopy revealed reductions in total axon volume following A1AR agonist treatment. We also observed reduced expression of myelin basic protein in treated animals. Showing that functional A1ARs were present over the ranges of ages studies, high levels of specific [3H]CCPA binding were observed at PD 4, 7 and 14, and receptor-G protein coupling was present at each age. These observations show that activation of A1ARs with doses of CPA that mimic the effects of high adenosine levels results in damage to the developing brain.