Differential glutamate-dependent and glutamate-independent adenosine A1 receptor-mediated modulation of dopamine release in different striatal compartments

Adenosine and dopamine are two important modulators of glutamatergic neurotransmission in the striatum. However, conflicting reports exist about the role of adenosine and adenosine receptors in the modulation of striatal dopamine release. It has been previously suggested that adenosine A(1) receptors localized in glutamatergic nerve terminals indirectly modulate dopamine release, by their ability to modulate glutamate release. In the present study, using in vivo microdialysis, we provide evidence for the existence of a significant glutamate-independent tonic modulation of dopamine release in most of the analyzed striatal compartments. In the dorsal, but not in the ventral, part of the shell of the nucleus accumbens (NAc), blockade of A(1) receptors by local perfusion with the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dimethyl-xanthine or by systemic administration of the non-selective adenosine antagonist caffeine induced a glutamate-dependent release of dopamine. On the contrary, A(1) receptor blockade induced a glutamate-independent dopamine release in the core of the NAc and the nucleus caudate-putamen. Furthermore, using immunocytochemical and functional studies in rat striatal synaptosomes, we demonstrate that a fraction of striatal dopaminergic terminals contains adenosine A(1) receptors, which directly inhibit dopamine release independently of glutamatergic transmission.     

Adenosine A 2A receptor antagonists prevent the increase in striatal glutamate levels induced by glutamate uptake inhibitors

Active uptake by neurons and glial cells is the main mechanism for maintaining extracellular glutamate at low, non-toxic concentrations. Activation of adenosine A(2A) receptors increases extracellular glutamate levels, while A(2A) receptor antagonists reduce stimulated glutamate outflow. Whether a modulation of the glutamate uptake system is involved in the effects elicited by A(2A) receptor blockers has never been investigated. This study examined the ability of adenosine A(2A) receptor antagonists to prevent the increase in glutamate levels induced by blockade of the glutamate uptake. In rats implanted with a microdialysis probe in the dorsal striatum, perfusion with 4 mm l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC, a transportable competitive inhibitor of glutamate uptake), or 10 mm dihydrokainic acid (DHK, a non-transportable competitive inhibitor that mainly blocks the glial glutamate transporter GLT-1), significantly increased extracellular glutamate levels. The effects of PDC and DHK were completely prevented by the adenosine A(2A) receptor antagonists SCH 58261 (0.01 mg/kg i.p.) and/or ZM 241385 (5 nm via probe). Since an impairment in glutamate transporter function is thought to play a major role in neurodegenerative disorders, the regulation of glutamate uptake may be one of the mechanisms of the neuroprotective effects of A(2A) receptor antagonists.     

甲硫氨酸腺苷转移酶1A/2A平衡与肝细胞癌

肝细胞癌是一种死亡率极高的癌症,大多数病人发现时已属晚期.甲硫氨酸腺苷转移酶(MAT)是细胞生命活动的关键酶,可以通过催化甲酼氨酸和三磷酸腺苷(ATP)结合,促进生物甲基供体S-腺苷甲酼氨酸(SAMe)的生物合成.正常肝细胞中MAT1A与MAT2A存在动态平衡,共同维持细胞内SAMe稳态;肝细胞癌中MAT1A转变成MAT2A,会使SAMe生物合成减少,为癌细胞生长提供有利条件,故MAT1A表达降低而MAT2A增高.因此,促进MAT2A向MAT1A转化,进而提高MAT1A/MAT2A的比值可能成为治疗肝细胞癌的关键靶点之一.本文就MAT1A/MAT2A平衡在肝细胞癌中的重要作用作一综述,旨在为寻找肝细胞癌防治靶点提供新的思路.     

Genetic and pharmacological inactivation of the adenosine A2A receptor attenuates 3-nitropropionic acid-induced striatal damage

Adenosine A2A receptor (A2AR) antagonism attenuates 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopaminergic neurodegeneration and quinolinic acid-induced excitotoxicity in the neostriatum. As A2ARs are enriched in striatum, we investigated the effect of genetic and pharmacological A2A inactivation on striatal damage produced by the mitochondrial complex II inhibitor 3-nitropriopionic acid (3-NP). 3-NP was administered to A2AR knockout (KO) and wild-type (WT) littermate mice over 5 days. Bilateral striatal lesions were analyzed from serial brain tissue sections. Whereas all of the 3-NP-treated WT mice (C57BL/6 genetic background) had bilateral striatal lesions, only one of eight of the 3-NP-treated A2AR KO mice had detectable striatal lesions. Similar attenuation of 3-NP-induced striatal damage was observed in A2AR KO mice in a 129-Steel background. In addition, the effect of pharmacological antagonism on 3-NP-induced striatal neurotoxicity was tested by pre-treatment of C57Bl/6 mice with the A2AR antagonist 8-(3-chlorostyryl) caffeine (CSC). Although bilateral striatal lesions were observed in all mice treated either with 3-NP alone or 3-NP plus vehicle, there were no demonstrable striatal lesions in mice treated with CSC (5 mg/kg) plus 3-NP and in five of six mice treated with CSC (20 mg/kg) plus 3-NP. We conclude that both genetic and pharmacological inactivation of the A2AR attenuates striatal neurotoxicity produced by 3-NP. Since the clinical and neuropathological features of 3-NP-induced striatal damage resemble those observed in Huntington's disease, the results suggest that A2AR antagonism may be a potential therapeutic strategy in Huntington's disease patients.     

Fetal alcohol spectrum disorders among children in a Brazilian orphanage

Background: The objective was to investigate the frequency of fetal alcohol spectrum disorders (FASD) and ophthalmologic anomalies in orphanage children in Brazil. Methods: A prospective study was performed on 94 children living in an orphanage in Brazil. The children were examined by a multidisciplinary team consisting of specialists in pediatrics, neurology, psychology, neuropsychiatry, and ophthalmology. Results: The main reasons for living in the orphanage, in 61% of the children, were negligence, child abuse, and abandonment. Of all the children studied, 50% had mothers with known alcohol abuse and 47% had one or more diagnoses of neurodevelopmental/behavioral and/or cognitive deficits. General developmental delay was found in 18%, intellectual disability in 3%, cognitive impairment in 27%, attention‐deficit/hyperactivity disorder in 14%, and autism in 3%. Altogether 17% had FASD, comprising three children with fetal alcohol syndrome (FAS), six with partial FAS, and seven with alcohol‐related neurodevelopmental disorder. 16% had ophthalmological findings such as poor vision, strabismus, and dysmorphology of the optic nerves. Twenty‐eight children (30%) were adopted from the orphanage; of these, six had FASD (two FAS, three partial FAS, one alcohol‐related neurodevelopmental disorder), five had attention‐deficit/hyperactivity disorder, and eight had developmental delay. Conclusion: Nearly half of the children living in the orphanage had neurodevelopmental disorders and a considerable number showed signs of damage from prenatal alcohol exposure. A broader look at the problem of FASD in Brazil and other South American countries is desirable to document the burden of disease and provide data for targeting prevention efforts. Birth Defects Research (Part A) 103:178–185, 2015. © 2014 Wiley Periodicals, Inc.     

Caffeine and an adenosine A2A receptor antagonist prevent memory impairment and synaptotoxicity in adult rats triggered by a convulsive episode in early life

Seizures early in life cause long‐term behavioral modifications, namely long‐term memory deficits in experimental animals. Since caffeine and adenosine A_2A receptor (A_2AR) antagonists prevent memory deficits in adult animals, we now investigated if they also prevented the long‐term memory deficits caused by a convulsive period early in life. Administration of kainate (KA, 2 mg/kg) to 7‐days‐old (P7) rats caused a single period of self‐extinguishable convulsions which lead to a poorer memory performance in the Y‐maze only when rats were older than 90 days, without modification of locomotion or anxiety‐like behavior in the elevated‐plus maze. In accordance with the relationship between synaptotoxicity and memory dysfunction, the hippocampus of these adult rats treated with kainate at P7 displayed a lower density of synaptic proteins such as SNAP‐25 and syntaxin (but not synaptophysin), as well as vesicular glutamate transporters type 1 (but not vesicular GABA transporters), with no changes in PSD‐95, NMDA receptor subunits (NR1, NR2A, NR2B) or α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate receptor subunits (GluR1, GluR2) compared with controls. Caffeine (1 g/L) or the A_2AR antagonist, KW6002 (3 mg/kg) applied in the drinking water from P21 onwards, prevented these memory deficits in P90 rats treated with KA at P7, as well as the accompanying synaptotoxicity. These results show that a single convulsive episode in early life causes a delayed memory deficit in adulthood accompanied by a glutamatergic synaptotoxicity that was prevented by caffeine or adenosine A_2AR antagonists.     

Adenosine A2A receptors control neuroinflammation and consequent hippocampal neuronal dysfunction

The blockade of adenosine A(2A) receptors (A2AR) affords a robust neuroprotection in different noxious brain conditions. However, the mechanisms underlying this general neuroprotection are unknown. One possible mechanism could be the control of neuroinflammation that is associated with brain damage, especially because A2AR efficiently control peripheral inflammation. Thus, we tested if the intracerebroventricular injection of a selective A2AR antagonist (SCH58261) would attenuate the changes in the hippocampus triggered by intraperitoneal administration of lipopolysaccharide (LPS) that induces neuroinflammation through microglia activation. LPS administration triggers an increase in inflammatory mediators like interleukin-1β that causes biochemical changes (p38 and c-jun N-terminal kinase phosphorylation and caspase 3 activation) contributing to neuronal dysfunction typified by decreased long-term potentiation, a form of synaptic plasticity. Long-term potentiation, measured 30 min after the tetanus, was significantly lower in LPS-treated rats compared with control-treated rats, while SCH58261 attenuated the LPS-induced change. The LPS-induced increases in phosphorylation of c-jun N-terminal kinase and p38 and activation of caspase 3 were also prevented by SCH58261. Significantly, SCH58261 also prevented the LPS-induced recruitment of activated microglial cells and the increase in interleukin-1β concentration in the hippocampus, indicating that A2AR activation is a pivotal step in mediating the neuroinflammation triggered by LPS. These results indicate that A2AR antagonists prevent neuroinflammation and support the hypothesis that this mechanism might contribute for the ability of A2AR antagonists to control different neurodegenerative diseases known to involve neuroinflammation.     

Interaction of Adenosine and Guanine Derivatives in the Rat Hippocampus: Effects on Cyclic AMP Levels and on the Binding of Adenosine Analogues and GMP

Guanine nucleotides (GN) have been implicated in many intracellular mechanisms. Extracellular actions, probably as glutamate receptor antagonists, have also been recently attributed to these compounds. GN may have a neuroprotective role by inhibiting excitotoxic events evoked by glutamate. Effects of extracellular GN on adenosine-evoked cellular responses have also been reported. However, the exact mechanism of such interaction is not known. In the present study, we showed that GN potentiated adenosine-induced cAMP accumulation in slices of hippocampus from young rats. However, neither GMP nor the metabotropic glutamate receptor agonist, 1S,3R-ACPD, inhibited the binding of the adenosine receptor agonist [³H]NECA (when binding to adenosine A2 receptors), or the binding of the adenosine A2a receptor agonist [³H]CGS 21680 in hippocampal membrane preparations. GppNHp, probably by interacting with G-proteins, decreased [³H]CGS 21680 binding. [³H]GMP binding was assayed in order to evaluate the GN sites which are not G-proteins. [³H]GMP binding was inhibited by GMP and GppNHp, but not by 1S,3R-ACPD. The interaction of endogenous adenosine with the GMP-binding sites was determined by incubating membranes in the presence or absence of adenosine deaminase (ADA). NECA, CADO, CGS 21680 and CPA (only at the highest concentration used) increased GMP binding in the presence of ADA. However, in the absence of ADA, the control levels of GMP binding were as high as in the presence of added ADA plus adenosine agonists, indicating that endogenous adenosine modulates the binding of GMP. If this site has a neuroprotective role, adenosine may be increasing its neuromodulator and proposed protective action.     

Review of published studies of kidney,liver, and gastrointestinal birth defects in fetal alcohol spectrum disorders

OBJECTIVE: Review of published studies of birth defects of the renal, liver, and gastrointestinal organ systems in subjects with fetal alcohol spectrum disorders (FASD). METHOD: We searched PubMed ( http://www.pubmed.gov ) using the following terms: fetal alcohol syndrome and: gastrointestinal tract, kidney, liver, and congenital abnormalities for all years and English only citations. RESULTS: We located 12 studies of FASD and defects of or functional impairments for the liver, 12 of renal abnormalities, and only two with gastrointestinal defects. We did not identify specific patterns of malformations or functional deficits for any of the three organ systems. The existing literature suggests a series of nonspecific outcomes in FASD. CONCLUSIONS: Fetal alcohol spectrum disorder includes a diagnostic category of alcohol‐related birth defects which is clinically difficult to apply. This study adds to the existing literature on birth defects in FASD which is still very limited. The categorical diagnosis of alcohol‐related birth defects requires additional research to determine if a specific pattern of organ specific abnormalities or functional deficits emerges in subjects with FASD. Birth Defects Research (Part A), 2009. © 2009 Wiley‐Liss, Inc.     

Adenosine A2A receptors enhance GABA transport into nerve terminals by restraining PKC inhibition of GAT-1

Neurotransmitter transporters are regulated by phosphorylation but little is known about endogenous substances and receptors that regulate this process. Adenosine is an ubiquitous neuromodulator operating G-protein coupled receptors, which affect the activity of several kinases. We therefore evaluated the influence of adenosine upon the GABA transporter 1 (GAT-1) mediated GABA uptake into hippocampal synaptosomes. Removal of endogenous adenosine (adenosine deaminase, 1 U/mL) decreased GABA uptake, an effect mimicked by blockade of A2A receptors (2-(2-furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine, 50 nM) but not A1 or A2B receptors. A2A receptor activation (4-[2-[[6-amino-9-( N -ethyl-β- d -ribofuranuronamidosyl)-9H-purin-yl]amino]ethyl]benzenepropanoic acid hydrochloride, 3–100 nM) enhanced GABA uptake by increasing the transporter Vmax without change of K_M. This was mimicked by adenylate cyclase activation (forskolin, 10 μM) and prevented by protein kinase A (PKA) inhibition ( N -[2-( p -bromocinnamylamino) ethyl]-5-isoquinolinesulfonamide dihydrochloride, 1 μM), which per se did not influence GABA transport. Blockade of protein kinase C (PKC) (2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide, 1 μM) facilitated GABA transport whereas PKC activation (4-β-phorbol-didecanoate, 250 nM) inhibited it. PKA blockade did not affect the facilitatory action of the PKC inhibitor or the inhibitory action of the PKC activator. However, when adenylate cyclase was activated neither activation nor inhibition of PKC affected GABA uptake. It is concluded that A2A receptors, through activation of the adenylate cyclase/cAMP/PKA transducing pathway facilitate GAT-1 mediated GABA transport into nerve endings by restraining tonic PKC-mediated inhibition.     

Restructuring of basal ganglia circuitry and associated behaviors triggered by low striatal D2 receptor expression: implications for substance use disorders

Dopamine D2 receptors (D2Rs) consistently emerge as a critical substrate for the etiology of some major psychiatric disorders. Indeed, a central theory of substance use disorders (SUDs) postulates that a reduction in D2R levels in the striatum is a determining factor that confers vulnerability to abuse substances. A large number of clinical and preclinical studies strongly support this link between SUDs and D2Rs; however, identifying the mechanism by which low D2Rs facilitate SUDs has been hindered by the complexity of circuit connectivity, the heterogeneity of D2R expression and the multifaceted constellation of phenotypes observed in SUD patient. Animal models are well‐suited for understanding the mechanisms because they allow access to the circuitry and the genetic tools that enable a dissection of the D2R heterogeneity. This review discusses recent findings on the functional role of D2Rs and highlights the distinctive contributions of D2Rs expressed on specific neuronal subpopulations to the behavioral responses to stimulant drugs. A circuit‐wide restructuring of local and long‐range inhibitory connectivity within the basal ganglia is observed in response to manipulation of striatal D2R levels and is accompanied by multiple alterations in dopamine‐dependent behaviors. Collectively, these new findings provide compelling evidence for a critical role of striatal D2Rs in shaping basal ganglia connectivity; even among neurons that do not express D2Rs. These findings from animal models have deep clinical implications for SUD patients with low levels D2R availability where a similar restructuring of basal ganglia circuitry is expected to take place.     

Possible Involvement of Pertussis Toxin-Sensitive G Proteins and D2 Dopamine Receptors in the A1 Adenosine Receptor-Adenylate Cyclase System in Rat Cerebral Cortex

To identify the involvement of dopamine receptors in the transmembrane signaling of the adenosine receptor-G protein-adenylate cyclase system in the CNS, we examined the effects of pertussis toxin (islet-activating protein, IAP) and apomorphine on A1 adenosine agonist (-)N6-R-[3H]phenylisopropyladenosine ([3H]PIA) and antagonist [3H]xanthine amine congener ([3H]XAC) binding activity and adenylate cyclase activity in cerebral cortex membranes of the rat brain. Specific binding to a single class of sites for [3H]XAC with a dissociation constant (KD) of 6.0 +/- 1.3 nM was observed. The number of maximal binding sites (Bmax) was 1.21 +/- 0.13 pmol/mg protein. Studies of the inhibition of [3H]XAC binding by PIA revealed the presence of two classes of PIA binding states, a high-affinity state (KD = 2.30 +/- 1.16 nM) and a low-affinity state (KD = 1.220 +/- 230 nM). Guanosine 5'-(3-O-thio)triphosphate or IAP treatment reduced the number of the high-affinity state binding sites without altering the KD for PIA. Apomorphine (100 microM) increased the KD value 10-fold and decreased Bmax by approximately 20% for [3H]PIA. The effect of apomorphine on the KD value increase was irreversible and due to a conversion from high-affinity to low-affinity states for PIA. The effect was dose dependent and was mediated via D2 dopamine receptors, since the D2 antagonist sulpiride blocked the phenomenon. The inhibitory effect of PIA on adenylate cyclase activity was abolished by apomorphine treatment. There was no effect of apomorphine on displacement of [3H]quinuclidinyl benzilate (muscarinic ligand) binding by carbachol. These data suggest that A1 adenosine receptor binding and function are selectively modified by D2 dopaminergic agents.     

Adenosine A(2A) receptors are required for normal BDNF levels and BDNF-induced potentiation of synaptic transmission in the mouse hippocampus

Brain-derived neurotrophic factor (BDNF), a member of neurotrophin family, enhances synaptic transmission and regulates neuronal proliferation and survival. Both BDNF and its tyrosine kinase receptors (TrkB) are highly expressed in the hippocampus, where an interaction with adenosine A_2A receptors (A_2ARs) has been recently reported. In the present paper, we evaluated the role of A_2ARs in mediating functional effects of BDNF in hippocampus using A_2AR knock-out (KO) mice. In hippocampal slices from WT mice, application of BDNF (10 ng/mL) increased the slope of excitatory post-synaptic field potentials (fEPSPs), an index of synaptic facilitation. This increase of fEPSP slope was abolished by the selective A_2A antagonist ZM 241385. Similarly, genetic deletion of the A_2ARs abolished BDNF-induced increase of the fEPSP slope in slices from A_2AR KO mice The reduced functional ability of BDNF in A_2AR KO mice was correlated with the reduction in hippocampal BDNF levels. In agreement, the pharmacological blockade of A₂Rs by systemic ZM 241385 significantly reduced BDNF levels in the hippocampus of normal mice. These results indicate that the tonic activation of A_2ARs is required for BDNF-induced potentiation of synaptic transmission and for sustaining a normal BDNF tone in the hippocampus.     

Capacitation state-dependent changes in adenosine receptors and their regulation of adenylyl cyclase/cAMP

This study was designed to localize adenosine receptors and to provide evidence that specific receptors are active only in either uncapacitated or capacitated mouse spermatozoa, where they play a role in regulating cAMP production. Using specific antibodies, stimulatory A(2A) receptors were localized primarily on the acrosomal cap region and the flagellar principal piece. Interestingly, the staining was much more pronounced in uncapacitated than in capacitated spermatozoa, suggesting capacitation-dependent changes in epitope accessibility. A(1) receptors showed a very similar distribution, but the staining was markedly greater in capacitated than in uncapacitated cells. After addition of purified decapacitation factor (DF) to capacitated cells, strong staining for A(2A) was regained, suggesting reversibility in epitope accessibility. Chlortetracycline analysis revealed that an agonist specific for A(2A) receptors had no detectable effect on capacitated cells, but after DF-induced decapacitation, the agonist then stimulated capacitation. That agonist also significantly stimulated cAMP production in uncapacitated cells, had no effect on capacitated cells, but regained the ability to stimulate cAMP in the latter following DF treatment. In contrast, an A(1) agonist inhibited cAMP in capacitated cells. These results indicate that specific adenosine receptors function in a reversible manner in one or other capacitation state, resulting in regulation of cAMP.     

Pre-synaptic adenosine A2A receptors control cannabinoid CB1 receptor-mediated inhibition of striatal glutamatergic neurotransmission

An interaction between adenosine A(2A) receptors (A(2A) Rs) and cannabinoid CB(1) receptors (CB(1) Rs) has been consistently reported to occur in the striatum, although the precise mechanisms are not completely understood. As both receptors control striatal glutamatergic transmission, we now probed the putative interaction between pre-synaptic CB(1) R and A(2A) R in the striatum. In extracellular field potentials recordings in corticostriatal slices from Wistar rats, A(2A) R activation by CGS21680 inhibited CB(1) R-mediated effects (depression of synaptic response and increase in paired-pulse facilitation). Moreover, in superfused rat striatal nerve terminals, A(2A) R activation prevented, while A(2A) R inhibition facilitated, the CB(1) R-mediated inhibition of 4-aminopyridine-evoked glutamate release. In summary, the present study provides converging neurochemical and electrophysiological support for the occurrence of a tight control of CB(1) R function by A(2A) Rs in glutamatergic terminals of the striatum. In view of the key role of glutamate to trigger the recruitment of striatal circuits, this pre-synaptic interaction between CB(1) R and A(2A) R may be of relevance for the pathogenesis and the treatment of neuropsychiatric disorders affecting the basal ganglia.