Opposite modulatory roles for adenosine A1 and A2A receptors on glutamate and dopamine release in the shell of the nucleus accumbens. Effects of chronic caffeine exposure

Previous studies have demonstrated opposing roles for adenosine A1 and A2A receptors in the modulation of extracellular levels of glutamate and dopamine in the striatum. In the present study, acute systemic administration of motor-activating doses of the A2A receptor antagonist MSX-3 significantly decreased extracellular levels of dopamine and glutamate in the shell of the rat nucleus accumbens (NAc) and counteracted both dopamine and glutamate release induced by systemic administration of motor-activating doses of either the A1 receptor antagonist CPT or caffeine. Furthermore, exposure to caffeine in the drinking water (1 mg/mL, 14 days) resulted in tolerance to the effects of systemic injection of CPT or caffeine, but not MSX-3, on extracellular levels of dopamine and glutamate in the NAc shell. The present results show: first, the existence of opposite tonic effects of adenosine on extracellular levels of dopamine and glutamate in the shell of the NAc mediated by A1 and A2A receptors; second, that complete tolerance to caffeine's dopamine- and glutamate-releasing effects which develops after chronic caffeine exposure is attributable to an A1 receptor-mediated mechanism. Development of tolerance to the dopamine-releasing effects of caffeine in the shell of the NAc may explain its weak addictive properties and atypical psychostimulant profile.     

Co-localization and functional interaction between adenosine A(2A) and metabotropic group 5 receptors in glutamatergic nerve terminals of the rat striatum

The anti-Parkinsonian effect of glutamate metabotropic group 5 (mGluR5) and adenosine A(2A) receptor antagonists is believed to result from their ability to postsynaptically control the responsiveness of the indirect pathway that is hyperfunctioning in Parkinson's disease. mGluR5 and A(2A) antagonists are also neuroprotective in brain injury models involving glutamate excitotoxicity. Thus, we hypothesized that the anti-Parkinsonian and neuroprotective effects of A(2A) and mGluR5 receptors might be related to their control of striatal glutamate release that actually triggers the indirect pathway. The A(2A) agonist, CGS21680 (1-30 nM) facilitated glutamate release from striatal nerve terminals up to 57%, an effect prevented by the A(2A) antagonist, SCH58261 (50 nM). The mGluR5 agonist, CHPG (300-600 mum) also facilitated glutamate release up to 29%, an effect prevented by the mGluR5 antagonist, MPEP (10 microm). Both mGluR5 and A(2A) receptors were located in the active zone and 57 +/- 6% of striatal glutamatergic nerve terminals possessed both A(2A) and mGluR5 receptors, suggesting a presynaptic functional interaction. Indeed, submaximal concentrations of CGS21680 (1 nM) and CHPG (100 microm) synergistically facilitated glutamate release and the facilitation of glutamate release by 10 nM CGS21680 was prevented by 10 microm MPEP, whereas facilitation by 300 microm CHPG was prevented by 10 nM SCH58261. These results provide the first direct evidence that A(2A) and mGluR5 receptors are co-located in more than half of the striatal glutamatergic terminals where they facilitate glutamate release in a synergistic manner. This emphasizes the role of the modulation of glutamate release as a likely mechanism of action of these receptors both in striatal neuroprotection and in Parkinson's disease.     

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 A2A receptors inhibit the conductance of NMDA receptor channels in rat neostriatal neurons

Summary Whole-cell patch clamp experiments were carried out in rat striatal brain slices. In a subset of striatal neurons (70–80%), NMDA-induced inward currents were inhibited by the adenosine AZA receptor selective agonist CGS 21680. The non-selective adenosine receptor antagonist 8-(p-sulphophenyl)-theophylline and the AZA receptor selective antagonist 8-(3chlorostyryl) caffeine abolished the inhibitory action of CGS 21680. Intracellular GDP--S, which is known to prevent G protein-mediated reactions, also eliminated the effect of CGS 21680. Extracellular dibutyryl cAMP, a membrane permeable analogue of cAMP, and intracellular Sp-cAMPS, an activator of cAMP-dependent protein kinases (PKA), both abolished the CGS 21680-induced inhibition. By contrast, Rp-cAMPS and PKI 14–24 amide, two inhibitors of PKA had no effect. Intracellular U-73122 (a phospholipase C inhibitor) and heparin (an inositoltriphosphate antagonist) prevented the effect of CGS 21680. Finally, a more efficient buffering of intracellular Ca²⁺ by a substitution of EGTA (11 mM) by BAPTA (5.5 mM) acted like U-73122 or heparin. Hence, AZA receptors appear to negatively modulate NMDA receptor channel conductance via the phospholipase C/inositoltriphosphate/Ca²⁺ pathway rather than the adenylate cyclase/PKA pathway.     

Caffeine inhibition of rat carotid body chemoreceptors is mediated by A2A and A2B adenosine receptors

Caffeine, an unspecific antagonist of adenosine receptors, is commonly used to treat the apnea of prematurity. We have defined the effects of caffeine on the carotid body (CB) chemoreceptors, the main peripheral controllers of breathing, and identified the adenosine receptors involved. Caffeine inhibited basal (IC50, 210 microm) and low intensity (PO2 approximately 66 mm Hg/30 mm K+) stimulation-induced release of catecholamines from chemoreceptor cells in intact preparations of rat CB in vitro. Opposite to caffeine, 5'-(N-ethylcarboxamido)adenosine (NECA; an A2 agonist) augmented basal and low-intensity hypoxia-induced release. 2-p-(2-Carboxyethyl)phenethyl-amino-5'-N-ethylcaboxamido-adenosine hydrochloride (CGS21680), 2-hexynyl-NECA (HE-NECA) and SCH58621 (A2A receptors agents) neither affected catecholamine release nor altered the caffeine effects. The 8-cycle-1,3-dipropylxanthine (DPCPX; an A1/A2B antagonist) and 8-(4-{[(4-cyanophenyl)carbamoylmethyl]-oxy}phenyl)-1,3-di(n-propyl)xanthine (MRS1754; an A2B antagonist) mimicking of caffeine indicated that caffeine effects are mediated by A2B receptors. Immunocytochemical A2B receptors were located in tyrosine hydroxylase positive chemoreceptor cells. Caffeine reduced by 52% the chemosensory discharges elicited by hypoxia in the carotid sinus nerve. Inhibition had two components with pharmacological analysis indicating that A2A and A2B receptors mediate, respectively, the low (17 x 10(-9) m) and high (160 x 10(-6) m) IC50 effects. It is concluded that endogenous adenosine, via presynaptic A2B and postsynaptic A2A receptors, can exert excitatory effects on the overall output of the rat CB chemoreceptors.     

Adenosine A2A receptor stimulation potentiates nitric oxide release by activated microglia

The absence of adenosine A2A receptors, or its pharmacological inhibition, has neuroprotective effects. Experimental data suggest that glial A2A receptors participate in neurodegeneration induced by A2A receptor stimulation. In this study we have investigated the effects of A2A receptor stimulation on control and activated glial cells. Mouse cortical mixed glial cultures (75% astrocytes, 25% microglia) were treated with the A2A receptor agonist CGS21680 alone or in combination with lipopolysaccharide (LPS). CGS21680 potentiated lipopolysaccharide-induced NO release and NO synthase-II expression in a time- and concentration-dependent manner. CGS21680 potentiation of lipopolysaccharide-induced NO release was suppressed by the A2A receptor antagonist ZM-241385 and did not occur on mixed glial cultures from A2A receptor-deficient mice. In mixed glial cultures treated with LPS + CGS21680, the NO synthase-II inhibitor 1400W abolished NO production, and NO synthase-II immunoreactivity was observed only in microglia. Binding experiments demonstrated the presence of A2A receptors on microglial but not on astroglial cultures. However, the presence of astrocytes was necessary for CGS21680 potentiating effect. In light of the reported neurotoxicity of microglial NO synthase-II and the neuroprotection of A2A receptor inhibition, these data suggest that attenuation of microglial NO production could contribute to the neuroprotection afforded by A2A receptor antagonists.     

Dialysis delivery of an adenosine A2A agonist into the pontine reticular formation of C57BL/6J mouse increases pontine acetylcholine release and sleep

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

Glutamatergic Control of Dopamine Release During Stress in the Rat Prefrontal Cortex

Abstract: In vivo microdialysis was used to assess the hypothesis that the stress-induced increase in dopamine release in the prefrontal cortex is mediated by stress-activated glutamate neurotransmission in this region. Local perfusion of an α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, blocked the stress-induced increase in dopamine levels, whereas an NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid, at the dose tested, was not able to alter this response significantly. These data indicate that the effect of stress on dopamine release in the prefrontal cortex is mediated locally by activation of AMPA/kainate receptors, which modulate the release of dopamine in this region.     

Adenosine enhances glial glutamate efflux via A2a adenosine receptors

The present study investigated the effect of adenosine on glial glutamate efflux. Adenosine (from 1 nM to 100 microM) enhanced the release from cultured rat glial cells in a bell-shaped dose-responsive manner for the hippocampus and in a dose-dependent manner for the superior colliculus, and a similar increase was obtained with the A2a adenosine receptor agonist, 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS21680), but not with the A1 adenosine receptor agonist, N6-cyclohexyladenosine (CHA). Adenosine and CGS21680 also enhanced glutamate efflux from Xenopus oocytes injected with the poly (A)+ mRNAs derived from cultured glial cells for the hippocampus and the superior colliculus together with and without the A2a adenosine receptor mRNA, but instead such increase was not found in oocytes expressing A2a adenosine receptors alone. The results of the present study thus suggest that adenosine enhances glutamate efflux from glial cells via A2a adenosine receptors, and this may represent a mechanism underlying the facilitatory action of adenosine on hippocampal and superior colliculus neurotransmissions.     

Adenosine A2a Receptor-Mediated Modulation of Striatal Acetylcholine Release In Vivo

Abstract: To determine the functions of striatal adenosine A_2a receptors in vivo, the effects of a selective agonist, 2-[4-(2-carboxyethyl)phenethylamino]-5'- N -ethylcarboxamidoadenosine hydrochloride (CGS 21680), and an antagonist, ( E )-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine (KF17837), on acetylcholine release were investigated in the striatum of awake freely moving rats using microdialysis. Intracerebroventricular injection of CGS 21680 (10 µg) increased acetylcholine release in striatum and KF17837 (30 mg/kg p.o.) antagonized the CGS 21680-induced acetylcholine elevation. To investigate the contribution of dopaminergic and GABAergic neurons on A_2a receptor-mediated acetylcholine release, the effects of CGS 21680 were studied by using dopamine-depleted rats in the presence or absence of GABA antagonists. In the dopamine-depleted striatum, the intrastriatal application of CGS 21680 (0.3–30 µ M ) increased extracellular acetylcholine, which was significantly greater than that in normal striatum. The CGS 21680-induced elevation of acetylcholine release was still observed in the presence of GABA antagonists bicuculline (30 µ M ) and 2-hydroxysaclofen (100 µ M ) and was similar in both normal and dopamine-depleted striatum. These results suggest that A_2a agonist stimulates acetylcholine release in vivo, and this effect of A_2a agonist is modulated by dopaminergic and GABAergic neurotransmission.     

Excitatory Amino Acid Receptors in the Ventral Tegmental Area Regulate Dopamine Release in the Ventral Striatum

Abstract: The role of excitatory amino acid (EAA) receptors located in the ventral tegmental area (VTA) in tonic and phasic regulation of dopamine release in the ventral striatum was investigated. Microdialysis in conscious rats was used to assess dopamine release primarily from the nucleus accumbens shell region of the ventral striatum while applying EAA antagonists or agonists to the VTA. Infusion of the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (25 and 100 µ M ) into the VTA did not affect dopamine release in the ventral striatum. In contrast, intra-VTA infusion of the NMDA receptor antagonist 2-amino-5-phosphopentanoic acid (100 and 500 µ M ) dose-dependently decreased the striatal release of dopamine. Intra-VTA application of the ionotropic EAA receptor agonists NMDA and AMPA dose-dependently (10 and 100 µ M ) increased dopamine efflux in the ventral striatum. However, infusion of 50 or 500 µ M trans -(±)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD), a metabotropic EAA receptor agonist, did not significantly affect these levels. These data suggest that NMDA receptors in the VTA exert a tonic excitatory influence on dopamine release in the ventral striatum. Furthermore, dopamine neurotransmission in this region may be enhanced by activation of NMDA and AMPA receptors, but not ACPD-sensitive metabotropic receptors, located in the VTA. These data further suggest that EAA regulation of dopamine release primarily occurs in the VTA as opposed to presynaptically at the terminal level.     

GDNF control of the glutamatergic cortico-striatal pathway requires tonic activation of adenosine A2A receptors

Glial cell line-derived neurotrophic factor (GDNF) affords neuroprotection in Parkinson's disease in accordance with its ability to bolster nigrostriatal innervation. We previously found that GDNF facilitates dopamine release in a manner dependent on adenosine A_2A receptor activation. As motor dysfunction also involves modifications of striatal glutamatergic innervation, we now tested if GDNF and its receptor system, Ret ( rearranged during transfection ) and GDNF family receptor α1 controlled the cortico-striatal glutamatergic pathway in an A_2A receptor-dependent manner. GDNF (10 ng/mL) enhanced (by ≈13%) glutamate release from rat striatal nerve endings, an effect potentiated (up to ≈30%) by the A_2A receptor agonist CGS 21680 (10 nM) and prevented by the A_2A receptor antagonist, SCH 58261 (50 nM). Triple immunocytochemical studies revealed that Ret and GDNF family receptor α1 were located in 50% of rat striatal glutamatergic terminals (immunopositive for vesicular glutamate transporters-1/2), where they were found to be co-located with A_2A receptors. Activation of the glutamatergic system upon in vivo electrical stimulation of the rat cortico-striatal input induced striatal Ret phosphorylation that was prevented by pre-treatment with the A_2A receptor antagonist, MSX-3 (3 mg/kg). The results provide the first functional and morphological evidence that GDNF controls cortico-striatal glutamatergic pathways in a manner largely dependent on the co-activation of adenosine A_2A receptors.     

Adenosine A2A receptors control the extracellular levels of adenosine through modulation of nucleoside transporters activity in the rat hippocampus

Adenosine, a neuromodulator of the CNS, activates inhibitory-A1 receptors and facilitatory-A2A receptors; its synaptic levels are controlled by the activity of bi-directional equilibrative nucleoside transporters. To study the relationship between the extracellular formation/inactivation of adenosine and the activation of adenosine receptors, we investigated how A1 and A2A receptor activation modifies adenosine transport in hippocampal synaptosomes. The A2A receptor agonist, CGS 21680 (30 nm), facilitated adenosine uptake through a PKC-dependent mechanism, but A1 receptor activation had no effect. CGS 21680 (30 nm) also increased depolarization-induced release of adenosine. Both effects were prevented by A2A receptor blockade. A2A receptor-mediated enhancement of adenosine transport system is important for formatting adenosine neuromodulation according to the stimulation frequency, as: (1) A1 receptor antagonist, DPCPX (250 nm), facilitated the evoked release of [(3)H]acetylcholine under low-frequency stimulation (2 Hz) from CA3 hippocampal slices, but had no effect under high-frequency stimulation (50 Hz); (2) either nucleoside transporter or A2A receptor blockade revealed the facilitatory effect of DPCPX (250 nm) on [3H]acetylcholine evoked-release triggered by high-frequency stimulation. These results indicate that A2A receptor activation facilitates the activity of nucleoside transporters, which have a preponderant role in modulating the extracellular adenosine levels available to activate A1 receptors.     

Adenosine A2A receptors and metabotropic glutamate 5 receptors are co-localized and functionally interact in the hippocampus: a possible key mechanism in the modulation of N-methyl-D-aspartate effects

Hippocampal metabotropic glutamate 5 receptors (mGlu5Rs) regulate both physiological and pathological responses to glutamate. Because mGlu5R activation enhances NMDA-mediated effects, and given the role played by NMDA receptors in synaptic plasticity and excitotoxicity, modulating mGlu5R may influence both the physiological and the pathological effects elicited by NMDA receptor stimulation. We evaluated whether adenosine A2A receptors (A(2A)Rs) modulated mGlu5R-dependent effects in the hippocampus, as they do in the striatum. Co-application of the A(2A)R agonist CGS 21680 with the mGlu5R agonist (RS)-2-chloro-s-hydroxyphenylglycine(CHPG) synergistically reduced field excitatory postsynaptic potentials in the CA1 area of rat hippocampal slices. Endogenous tone at A(2A)Rs seemed to be required to enable mGlu5R-mediated effects, as the ability of CHPG to potentiate NMDA effects was antagonized by the selective A(2A)R antagonist ZM 241385 in rat hippocampal slices and cultured hippocampal neurons, and abolished in the hippocampus of A(2A)R knockout mice. Evidence for the interaction between A(2A)Rs and mGlu5Rs was further strengthened by demonstrating their co-localization in hippocampal synapses. This is the first evidence showing that hippocampal A(2A)Rs and mGlu5Rs are co-located and act synergistically, and that A(2A)Rs play a permissive role in mGlu5R receptor-mediated potentiation of NMDA effects in the hippocampus.     

Dual regulation of NMDA receptor functions by direct protein-protein interactions with the dopamine D1 receptor

Dopamine D1-like receptors, composed of D1 and D5 receptors, have been documented to modulate glutamate-mediated fast excitatory synaptic neurotransmission. Here, we report that dopamine D1 receptors modulate NMDA glutamate receptor-mediated functions through direct protein-protein interactions. Two regions in the D1 receptor carboxyl tail can directly and selectively couple to NMDA glutamate receptor subunits NR1-1a and NR2A. While one interaction is involved in the inhibition of NMDA receptor-gated currents, the other is implicated in the attenuation of NMDA receptor-mediated excitotoxicity through a PI-3 kinase-dependent pathway.     

Adenosine inhibits the release of interleukin-1beta in activated human peripheral mononuclear cells

The effects of adenosine and subtype-specific activators of adenosine receptors (A1, A2A, A2B and A3) were studied on the release of interleukin-1beta (IL-1beta) from peripheral mononuclear cells, monocytes and lymphocytes. In the cells activated by the protein kinase C specific phorbol ester (phorbol 12-myristate 13-acetate) and Ca(2+) ionophore (A23187) both adenosine and the subtype-specific receptor agonists, CPA (A1), CGS 21680 (A2A) and IB-MECA (A3) induced a concentration-dependent inhibition of IL-1beta release. The rank order of potency in the inhibition of IL-1beta release was CPA=CGS 21680>IB-MECA>adenosine>NECA (in the presence of A1, A2A and A3 receptor inhibitors). The inhibitory actions of CPA, CGS 21680 or IB-MECA were significantly reduced in the presence of DPCPX, ZM 243185 or MRS 1191 as subtype-specific antagonists on A1, A2A and A3 adenosine receptors, respectively. It can be concluded that adenosine inhibits the release of IL-1beta from the activated human peripheral mononuclear cells. In this process A1, A2A and A3 receptors are involved.     

腺苷A2A受体在小鼠颅脑创伤与外周组织损伤模型中的作用差异

本文旨在探索腺苷A2A受体在颅脑创伤、皮肤创伤及放射损伤复合创伤中的作用差异.分别观察和检测野生型小鼠、A2A受体基因敲除小鼠以及给予A2A受体激动剂CGS21680治疗的小鼠在皮肤创伤、放射损伤复合创伤后的伤口愈合时间以及颅脑创伤后的神经功能缺损情况、伤侧皮层脑含水量、脑脊液中谷氨酸浓度.结果表明,CGS21680促进外周组织伤口愈合,却加重颅脑创伤模型的神经功能损害,这与其促进谷氨酸释放有关.相反,A2A受体基因敲除显著延迟小鼠皮肤创伤及放射损伤复合创伤模型的伤口愈合,而在颅脑创伤模型中通过抑制谷氨酸释放产生保护效应.本研究初步证实,A2A受体激活促进谷氨酸大量释放可能是其在中枢损伤与外周损伤产生作用差异的机理之一,这为将来临床应用A2A受体激动剂减轻外周损伤,而用A2A受体拈抗剂减轻颅脑损伤提供了一定的实验依据.     

Caffeine and accumbens shell dopamine

It has been reported that caffeine (1.5-30 mg/kg i.p.) as well as specific A1 (DPCPX, 8-cyclopentyl-1,3-dipropylxanthine) receptor antagonists fail to increase extracellular dopamine (DA) in the shell of the nucleus accumbens (NAc). However, it has also been reported that caffeine (10 and 30 mg/kg i.p.) and the A1 antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) increases NAc shell DA. To clarify this issue rats were implanted with microdialysis probes at different sites in the NAc shell, in the medial prefrontal cortex (PFCX, infralimbic cortex), and at the border between those areas. Irrespective of probe placement within the NAc shell and of the use of different surgical anesthetics (chloral hydrate and ketamine), we failed to observe changes in dialysate DA after 10 and 30 mg/kg i.p. of caffeine. Similarly negative results were obtained with DPCPX and CPFPX, two potent and selective A1 receptor antagonists. A significant increase of DA was obtained after caffeine when probes were located at the border between the NAc shell and the PFCX (10 and 30 mg/kg) or in the PFCX (10 mg/kg). In view of this and of our previous report that caffeine increases dialysate DA in the medial PFCX, we conclude that the increase in dialysate DA by caffeine observed by others arises from the medial PFCX rather than from the NAc shell as a result of placement of microdialysis probes at the border between the NAc shell and the PFCX.     

Protein kinase A and Ca(v)1 (L-Type) channels are common targets to facilitatory adenosine A2A and muscarinic M1 receptors on rat motoneurons

At the rat motor endplate, pre-synaptic facilitatory adenosine A2A and muscarinic M1 receptors are mutually exclusive. We investigated whether these receptors share a common intracellular signalling pathway. Suppression of McN-A-343-induced M1 facilitation of [3H]ACh release was partially recovered when CGS21680C (an A2A agonist) was combined with the cyclic AMP antagonist Rp-cAMPS. Forskolin, rolipram and 8-bromo-cyclic AMP mimicked CGS21680C blockade of M1 facilitation. Both Rp-cAMPs and nifedipine reduced augmentation of [3H]ACh release by McN-A-343 and CGS21680C. Activation of M1 and A2A receptors enhanced Ca2+ recruitment through nifedipine-sensitive channels. Nifedipine inhibition revealed by McN-A-343 was prevented by chelerythrine (a PKC inhibitor) and Rp-cAMPS, suggesting that Ca(v)1 (L-type) channels phosphorylation by PKA and PKC is required. Rp-cAMPS inhibited [3H]ACh release in the presence of phorbol 12-myristate 13-acetate, but PKC inhibition by chelerythrine had no effect on release in the presence of 8-bromo-cyclic AMP. This suggests that the involvement of PKA may be secondary to M1-induced PKC activation. In conclusion, competition of M1 and A2A receptors to facilitate ACh release from motoneurons may occur by signal convergence to a common pathway involving PKA activation and Ca2+ influx through Ca(v)1 (L-type) channels.     

Adenosine A2A receptor activation is determinant for BDNF actions upon GABA and glutamate release from rat hippocampal synaptosomes

Adenosine, through A_2A receptor (A_2AR) activation, can act as a metamodulator, controlling the actions of other modulators, as brain-derived neurotrophic factor (BDNF). Most of the metamodulatory actions of adenosine in the hippocampus have been evaluated in excitatory synapses. However, adenosine and BDNF can also influence GABAergic transmission. We thus evaluated the role of A_2AR on the modulatory effect of BDNF upon glutamate and GABA release from isolated hippocampal nerve terminals (synaptosomes). BDNF (30 ng/ml) enhanced K⁺-evoked [³H]glutamate release and inhibited the K⁺-evoked [³H]GABA release from synaptosomes. The effect of BDNF on both glutamate and GABA release requires tonic activation of adenosine A_2AR since for both neurotransmitters, the BDNF action was blocked by the A_2AR antagonist SCH 58261 (50 nM). In the presence of the A_2AR agonist, {"type":"entrez-protein","attrs":{"text":"CGS21680","term_id":"878113053","term_text":"CGS21680"}}CGS21680 (30 nM), the effect of BDNF on either glutamate or GABA release was, however, not potentiated. It is concluded that both the inhibitory actions of BDNF on GABA release as well as the facilitatory action of the neurotrophin on glutamate release are dependent on the activation of adenosine A_2AR by endogenous adenosine. However, these actions could not be further enhanced by exogenous activation of A_2AR.