Desensitization and internalization of adenosine A1 receptors in rat brain by in vivo treatment with R-PIA: involvement of coated vesicles

How does adenosine control neuronal dysfunction and neurodegeneration?

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

Adenosine A2b receptors control A1 receptor-mediated inhibition of synaptic transmission in the mouse hippocampus

Adenosine is a neuromodulator mostly acting through A1 (inhibitory) and A2A (excitatory) receptors in the brain. A2B receptors (A(2B)R) are G(s/q)--protein-coupled receptors with low expression in the brain. As A(2B)R function is largely unknown, we have now explored their role in the mouse hippocampus. We performed electrophysiological extracellular recordings in mouse hippocampal slices, and immunological analysis of nerve terminals and glutamate release in hippocampal slices and synaptosomes. Additionally, A(2B)R-knockout (A(2B)R-KO) and C57/BL6 mice were submitted to a behavioural test battery (open field, elevated plus-maze, Y-maze). The A(2B)R agonist BAY60-6583 (300 nM) decreased the paired-pulse stimulation ratio, an effect prevented by the A(2B)R antagonist MRS 1754 (200 nM) and abrogated in A(2B)R-KO mice. Accordingly, A(2B)R immunoreactivity was present in 73 ± 5% of glutamatergic nerve terminals, i.e. those immunopositive for vesicular glutamate transporters. Furthermore, BAY 60-6583 attenuated the A(1)R control of synaptic transmission, both the A(1)R inhibition caused by 2-chloroadenosine (0.1-1 μM) and the disinhibition caused by the A(1)R antagonist DPCPX (100 nM), both effects prevented by MRS 1754 and abrogated in A(2B)R-KO mice. BAY 60-6583 decreased glutamate release in slices and also attenuated the A(1)R inhibition (CPA 100 nM). A(2B)R-KO mice displayed a modified exploratory behaviour with an increased time in the central areas of the open field, elevated plus-maze and the Y-maze and no alteration of locomotion, anxiety or working memory. We conclude that A(2B)R are present in hippocampal glutamatergic terminals where they counteract the predominant A(1)R-mediated inhibition of synaptic transmission, impacting on exploratory behaviour.

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.

Differential trafficking of adenosine receptors in hippocampal neurons monitored using GFP- and super-ecliptic pHluorin-tagged receptors

Adenosine receptors (ARs) modulate many cellular and systems-level processes in the mammalian CNS. However, little is known about the trafficking of ARs in neurons, despite their importance in controlling seizure activity and in neuroprotection in cerebral ischaemia. To address this we examined the agonist-dependent internalisation of C-terminal GFP-tagged A(1)Rs, A(2A)Rs and A(3)Rs in primary hippocampal neurons. Furthermore, we developed a novel super-ecliptic pHluorin (SEP)-tagged A(1)R which, via the N-terminal SEP tag, reports the cell-surface expression and trafficking of A(1)Rs in real-time. We demonstrate the differential trafficking of ARs in neurons: A(3)Rs internalise more rapidly than A1Rs, with little evidence of appreciable A(2A)R trafficking over the time-course of the experiments. Furthermore, the novel SEP-A(1)R construct revealed the time-course of internalisation and recovery of cell-surface expression to occur within minutes of agonist exposure and removal, respectively. These observations highlight the labile nature of A(1)R and A(3)Rs when expressed at the neuronal plasma membrane. Given the high levels of adenosine in the brain during ischaemia and seizures, internalisation of the inhibitory A(1)R may result in hyperexcitability, increased brain damage and the development of chronic epileptic states.

Adenosine receptors and brain diseases: neuroprotection and neurodegeneration

Adenosine acts in parallel as a neuromodulator and as a homeostatic modulator in the central nervous system. Its neuromodulatory role relies on a balanced activation of inhibitory A(1) receptors (A1R) and facilitatory A(2A) receptors (A2AR), mostly controlling excitatory glutamatergic synapses: A1R impose a tonic brake on excitatory transmission, whereas A2AR are selectively engaged to promote synaptic plasticity phenomena. This neuromodulatory role of adenosine is strikingly similar to the role of adenosine in the control of brain disorders; thus, A1R mostly act as a hurdle that needs to be overcame to begin neurodegeneration and, accordingly, A1R only effectively control neurodegeneration if activated in the temporal vicinity of brain insults; in contrast, the blockade of A2AR alleviates the long-term burden of brain disorders in different neurodegenerative conditions such as ischemia, epilepsy, Parkinson's or Alzheimer's disease and also seem to afford benefits in some psychiatric conditions. In spite of this qualitative agreement between neuromodulation and neuroprotection by A1R and A2AR, it is still unclear if the role of A1R and A2AR in the control of neuroprotection is mostly due to the control of glutamatergic transmission, or if it is instead due to the different homeostatic roles of these receptors related with the control of metabolism, of neuron-glia communication, of neuroinflammation, of neurogenesis or of the control of action of growth factors. In spite of this current mechanistic uncertainty, it seems evident that targeting adenosine receptors might indeed constitute a novel strategy to control the demise of different neurological and psychiatric disorders.

Modulation of adenosine A1 and A2A receptors in C6 glioma cells during hypoxia: involvement of endogenous adenosine

During hypoxia, extracellular adenosine levels are increased to prevent cell damage, playing a neuroprotective role mainly through adenosine A(1) receptors. The aim of the present study was to analyze the effect of hypoxia in both adenosine A(1) and A(2A) receptors endogenously expressed in C6 glioma cells. Two hours of hypoxia (5% O(2)) caused a significant decrease in adenosine A(1) receptors. The same effect was observed at 6 h and 24 h of hypoxia. However, adenosine A(2A) receptors were significantly increased at the same times. These effects were not due to hypoxia-induced alterations in cells number or viability. Changes in receptor density were not associated with variations in the rate of gene expression. Furthermore, hypoxia did not alter HIF-1alpha expression in C6 cells. However, HIF-3alpha, CREB and CREM were decreased. Adenosine A(1) and A(2A) receptor density in normoxic C6 cells treated with adenosine for 2, 6 and 24 h was similar to that observed in cells after oxygen deprivation. When C6 cells were subjected to hypoxia in the presence of adenosine deaminase, the density of receptors was not significantly modulated. Moreover, DPCPX, an A(1) receptor antagonist, blocked the effects of hypoxia on these receptors, while ZM241385, an A(2A) receptor antagonist, was unable to prevent these changes. These results suggest that moderate hypoxia modulates adenosine receptors and cAMP response elements in glial cells, through a mechanism in which endogenous adenosine and tonic A(1) receptor activation is involved.

Adenosine receptor desensitization and trafficking

As with the majority of G-protein-coupled receptors, all four of the adenosine receptor subtypes are known to undergo agonist-induced regulation in the form of desensitization and trafficking. These processes can limit the ability of adenosine receptors to couple to intracellular signalling pathways and thus reduce the ability of adenosine receptor agonists as well as endogenous adenosine to produce cellular responses. In addition, since adenosine receptors couple to multiple signalling pathways, these pathways may desensitize differentially, while the desensitization of one pathway could even trigger signalling via another. Thus, the overall picture of adenosine receptor regulation can be complex. For all adenosine receptor subtypes, there is evidence to implicate arrestins in agonist-induced desensitization and trafficking, but there is also evidence for other possible forms of regulation, including second messenger-dependent kinase regulation, heterologous effects involving G proteins, and the involvement of non-clathrin trafficking pathways such as caveolae. In this review, the evidence implicating these mechanisms is summarized for each adenosine receptor subtype, and we also discuss those issues of adenosine receptor regulation that remain to be resolved as well as likely directions for future research in this field.

Adenosine mediated desensitization of cAMP signaling enhances T-cell responses

Adenosine has long been regarded as a crucial anti-inflammatory agent that protects the host from excessive damage. It has been reported to play an important role in suppressing immune activation, particularly that of T cells. However, it is a general observation that induction of T-cell activation is an efficient event despite the high adenosine levels that are often present in the affected host due to injury or stress. We report here that prior to antigenic stimulation via TCR/CD3, exposure of T cells to adenosine desensitizes adenosine receptors, so as to create a window of time where the T cells are insensitive to this ubiquitous suppressor. T cells from mice that were pre-exposed to this manipulation showed stronger responses to antigenic stimulation; therefore, the P1 adenosine receptor desensitization demonstrated an adjuvant-like effect. Our results suggest that adenosine receptor desensitization may be a mechanism for T cells to escape the general suppression during early points of T-cell activation and may emerge as a potential alternative for vaccine adjuvants.

Role of beta-arrestin1/ERK MAP kinase pathway in regulating adenosine A1 receptor desensitization and recovery

Exposure of cells to adenosine receptor (AR) agonists leads to receptor uncoupling from G proteins and downregulation of the A(1)AR. The receptor levels on the cell surface generally recover on withdrawal of the agonist, because of either translocation of the sequestered A(1)AR back to plasma membrane or de novo synthesis of A(1)AR. To examine the mechanism(s) underlying A(1)AR downregulation and recovery, we treated ductus deferens tumor (DDT(1) MF-2) cells with the agonist R-phenylisopropyladenosine (R-PIA) and showed a decrease in membrane A(1)AR levels by 24 h, which was associated with an unexpected 11-fold increase in A(1)AR mRNA. Acute exposure of these cells to R-PIA resulted in a rapid translocation of beta-arrestin1 to the plasma membrane. Knockdown of beta-arrestin1 by short interfering RNA (siRNA) blocked R-PIA-mediated downregulation of the A(1)AR, suppressed R-PIA-dependent ERK1/2 and activator protein-1 (AP-1) activity, and reduced the induction of A(1)AR mRNA. Withdrawal of the agonist after a 24-h exposure resulted in rapid recovery of plasma membrane A(1)AR. This was dependent on the de novo protein synthesis and on the activity of ERK1/2 but independent of beta-arrestin1 and nuclear factor-kappaB. Together, these data suggest that exposure to A(1)AR agonist stimulates ERK1/2 activity via beta-arrestin1, which subserves receptor uncoupling and downregulation, in addition to the induction of A(1)AR expression. We propose that such a pathway ensures both the termination of the agonist signal and recovery by priming the cell for rapid de novo synthesis of A(1)AR once the drug is terminated.

Effect of chronic gestational treatment with the adenosine A1 receptor agonist R-phenylisopropyladenosine on metabotropic glutamate receptors/phospholipase C pathway in maternal and fetal brain

Pregnant Wistar rats were orally treated with the adenosine receptor agonist R-phenylisopropyladenosine (R-PIA) throughout the gestational period, and the status of the metabotropic glutamate (mGlu) receptor/phospholipase C transduction pathway from maternal and fetal brain was analyzed. In mothers' brains, radioligand binding assays revealed a significant decrease in the Bmax value, without any significant alteration in Kd value. Similar results were observed in the steady-state level of mGlu(1) and mGlu(5) receptors assayed by Western blot, suggesting that both receptor subtypes were modulated by chronic R-PIA treatment. mRNA coding mGlu(1) or mGlu(5) receptors was not altered, suggesting a posttranscriptional modulation as a possible mechanism of the loss of mGlu(1) and mGlu(5) receptors at the membrane surface. Moreover, phospholipase C stimulated by (R,S)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist of group I mGlu receptors, was also significantly decreased after R-PIA treatment, suggesting a heterologous desensitization of group I mGlu/PLC pathway in maternal brain. Western blot and RT-PCR assays showed that neither alphaG(q/11) nor PLCbeta(1) was affected by R-PIA treatment. In fetal brain, no significant differences in mGlu receptors/PLC transduction pathway were observed after R-PIA treatment. These results suggest that chronic R-PIA intake during pregnancy modulates group I mGlu receptor signalling pathway in maternal brain, promoting a down-regulation of mGlu(1) and mGlu(5) receptors and a heterologous desensitization of the mGlu/PLC system.

Interleukin-6 upregulates neuronal adenosine A1 receptors: implications for neuromodulation and neuroprotection

The immunological response in the brain is crucial to overcome neuropathological events. Some inflammatory mediators, such as the immunoregulatory cytokine interleukin-6 (IL-6) affect neuromodulation and may also play protective roles against various noxious conditions. However, the fundamental mechanisms underlying the long-term effects of IL-6 in the brain remain unclear. We now report that IL-6 increases the expression and function of the neuronal adenosine A1 receptor, with relevant consequences to synaptic transmission and neuroprotection. IL-6-induced amplification of A1 receptor function enhances the responses to readily released adenosine during hypoxia, enables neuronal rescue from glutamate-induced death, and protects animals from chemically induced convulsing seizures. Taken together, these results suggest that IL-6 minimizes the consequences of excitotoxic episodes on brain function through the enhancement of endogenous adenosinergic signaling.

Internalization and desensitization of adenosine receptors

Until now, more than 800 distinct G protein-coupled receptors (GPCRs) have been identified in the human genome. The four subtypes of the adenosine receptor (A₁, A_2A, A_2B and A₃ receptor) belong to this large family of GPCRs that represent the most widely targeted pharmacological protein class. Since adenosine receptors are widespread throughout the body and involved in a variety of physiological processes and diseases, there is great interest in understanding how the different subtypes are regulated, as a basis for designing therapeutic drugs that either avoid or make use of this regulation. The major GPCR regulatory pathway involves phosphorylation of activated receptors by G protein-coupled receptor kinases (GRKs), a process that is followed by binding of arrestin proteins. This prevents receptors from activating downstream heterotrimeric G protein pathways, but at the same time allows activation of arrestin-dependent signalling pathways. Upon agonist treatment, adenosine receptor subtypes are differently regulated. For instance, the A₁Rs are not (readily) phosphorylated and internalize slowly, showing a typical half-life of several hours, whereas the A_2AR and A_2BR undergo much faster downregulation, usually shorter than 1 h. The A₃R is subject to even faster downregulation, often a matter of minutes. The fast desensitization of the A₃R after agonist exposure may be therapeutically equivalent to antagonist occupancy of the receptor. This review describes the process of desensitization and internalization of the different adenosine subtypes in cell systems, tissues and in vivo studies. In addition, molecular mechanisms involved in adenosine receptor desensitization are discussed.

Effect of glutamate intake during gestation on adenosine A(1) receptor/adenylyl cyclase pathway in both maternal and fetal rat brain

Pregnant Wistar rats were orally treated with 1 g/L l-glutamate during the entire gestational period and the status of adenosine A(1) receptor (A(1)R)/adenylyl cyclase transduction pathway from maternal and fetal brain was analyzed. Glutamate consumption, estimated from the loss of water from the drinking bottles, was 110 +/- 4.6 mg/kg/day. In mother brains glutamate intake did not significantly alter the B(max) value, although the K(d) value was significantly decreased. However in fetus brain, a significant decrease in B(max) was observed, without an alteration of K(d) value. Similar results were observed by western blot assays using specific A(1)R antibody, suggesting a down-regulation of A(1)R in fetal brain. Concerning alpha subunits of inhibitory G proteins (Gi), alphaGi(3) protein was slightly but significantly decreased in maternal brain without alterations of either Gi(1) or Gi(2). In contrast, alphaGi(1) and alphaGi(2) isoforms were increased in fetal brain. On the other hand, basal, forskolin, and forskolin plus GTPgammaS-stimulated adenylyl cyclase activity was significantly decreased in both maternal and fetal brain, and this was more prominent in fetal than in maternal brain. Finally, A(1)R functionality was significantly decreased in mother brain whereas no significant differences were detected in fetus brain. These results suggest that glutamate administered to pregnant rats modulates A(1)R signaling pathways in both tissues, showing an A(1)R down-regulation in fetal brain, and desensitization in maternal brain.

Adenosine A2A receptors are up-regulated in Pick's disease frontal cortex

Adenosine A2A receptors (A2AR) are highly expressed in striatum. However, they are also present in extrastriatal structures. A2AR were studied in post-mortem human frontal cortex from Pick's disease (PiD) and age-matched non-demented controls by radioligand binding assays, Western-blotting, real-time PCR and adenylyl cyclase activity determination. Saturation binding assay using [3H]ZM 241385, a selective A2A antagonist, as radioligand revealed a significant increase in total adenosine A2AR numbers (Bmax) in frontal cortex from PiD samples (191% of control Bmax), suggesting up-regulation of this receptor. A significant increase in the level of A2AR was also detected by Western-blotting. Furthermore, expression of mRNA coding A2AR determined by quantitative real-time PCR was enhanced. In agreement, stimulation of adenylyl cyclase by CGS 21680, a selective A2A receptor agonist, was significantly strengthened. Up-regulation of A2B receptors and their corresponding mRNA was also observed. These results show that A2A adenosine receptor/adenylyl cyclase transduction pathway is up-regulated and sensitized in frontal cortex brain from PiD.

Adenosine A1 Receptor Protein Levels and Activity Is Increased in the Cerebral Cortex in Creutzfeldt-Jakob Disease and in Bovine Spongiform Encephalopathy-Infected Bovine-PrP Mice

Prion diseases are characterized by neuronal loss, astrocytic gliosis, spongiform change, and abnormal protease-resistant prion protein (PrP) deposition. Creutzfeldt-Jakob disease (CJD) is the most prevalent human prion disease, whereas scrapie and bovine spongiform encephalopathy (BSE) are the most common animal prion diseases. Several candidates have been proposed as mediators of degeneration in prion diseases, one of them glutamate. Recent studies have shown reduced metabotropic glutamate receptor/phospholipase C signaling in the cerebral cortex in CJD, suggesting that this important neuromodulator and neuroprotector pathway is attenuated in CJD. Adenosine is involved in the regulation of different metabolic processes under physiological and pathologic conditions. Adenosine function is mediated by adenosine receptors, which are categorized into 4 types: A1, A2A, A2B, and A3. A1Rs are G-protein-coupled receptors that induce the inhibition of adenylyl cyclase activity. The most dramatic inhibitory actions of adenosine receptors are on the glutamatergic system. For these reasons, we examined the levels of A1Rs in the frontal cortex of 12 patients with CJD and 6 age-matched controls and in BSE-infected bovine-PrP transgenic mice (BoPrP-Tg110 mice) at different postincubation times to address modifications in A1Rs with disease progression. A significant increase in the protein levels of A1Rs was found in the cerebral cortex in CJD and in the murine BSE model at advanced stages of the disease and coincidental with the appearance of PrP expression. In addition, the activity of A1Rs was analyzed by in vitro assays with isolated membranes of the frontal cortex in CJD. Increased activity of the receptor, as revealed by the decreased forskolin-stimulated cAMP production in response to the A1R agonists cyclohexyl adenosine and cyclopentyl adenosine, was observed in CJD cases when compared with controls. Finally, mRNA A1R levels were similar in CJD and control cases, thus suggesting abnormal A1R turnover or dysregulation of raft-associated signaling pathways in CJD. These results show, for the first time, sensitization of A1Rs in prion diseases.

Hypoxia-induced desensitization and internalization of adenosine A1 receptors in the rat hippocampus

Activation of A1 adenosine receptors is important for both the neuromodulatory and neuroprotective effects of adenosine. However, short periods of global ischemia decrease A1 adenosine receptor density in the brain and it is not known if a parallel loss of functional efficiency of A1 adenosine receptors occurs. We now tested if hypoxia leads to changes in the density and efficiency of A1 adenosine receptors to inhibit excitatory synaptic transmission in rat hippocampal slices. In control conditions, the adenosine analog 2-chloroadenosine, inhibited field excitatory post-synaptic potentials with an EC50 of 0.23 microM. After hypoxia (95% N2 and 5% CO2, for 60 min) and reoxygenation (30 min), the EC50 increased to 0.73 microM. This EC50 shift was prevented by the presence of the A1 adenosine receptor antagonist 8-phenyltheophyline, but not by the A(2A)R antagonist 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine, during the hypoxic period. This decreased efficiency of A1 adenosine receptors was not paralleled by a global change of A1 adenosine receptor density or affinity (as evaluated by the binding parameters obtained in nerve terminal membranes). However, the density of biotinylated A1 adenosine receptors at the plasma membrane of nerve terminals was reduced by 30% upon hypoxia/reoxygenation, in a manner prevented by the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine and mimicked by prolonged (60 min) supra-maximal activation of A1 adenosine receptors with 2-chloroadenosine (10 microM). These results indicate that hypoxia leads to a rapid (<90 min) homologous desensitization of A1 adenosine receptor-mediated inhibition of synaptic transmission that is likely due to an internalization of A1 adenosine receptors in nerve terminals.

Induction of adenosine A1 receptor expression by pertussis toxin via an adenosine 5'-diphosphate ribosylation-independent pathway

Pertussis toxin ADP ribosylates G(i) and G(o) transducing proteins and functionally uncouples adenosine A(1) receptor (A(1)AR) from its effectors. We hypothesized that this loss in receptor coupling could lead to de novo A(1)AR synthesis by the cell in a futile attempt to re-establish normal receptor function. To test this hypothesis, we used hamster ductus deferens tumor (DDT(1) MF-2) cells, a cell culture model for studying A(1)AR, and showed that pertussis toxin (100 ng/ml) produced a time-dependent loss in A(1)AR-G(i) interaction and abolished A(1)AR activation of extracellular signal-regulated kinase 1/2. Interestingly, pertussis toxin increased the expression of A(1)AR, as measured by real-time polymerase chain reaction, immunocytochemistry, and [(3)H]cyclopentyl-1,3-dipropylxanthine (DPCPX) binding, suggesting a compensatory response to G(i) protein inactivation. DDT(1) MF-2 cells exposed to pertussis toxin demonstrated nuclear factor kappaB (NF-kappaB) activation within 30 min of exposure, a time point that preceded the loss of function of the A(1)AR. Inhibition of NF-kappaB attenuated the increase in A(1)AR induced by pertussis toxin. Cells exposed to B-oligomer subunit of pertussis toxin, devoid of significant ADP ribosyltransferase activity, showed increased A(1)AR protein expression, preceded by activation of NF-kappaB. B-Oligomer increased intracellular Ca(2+) in DDT(1) MF-2 cells. Chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or inhibition of protein kinase C (PKC) with bisindolylmaleimide hydrochloride reduced the activation of NF-kappaB and [(3)H]DPCPX binding. We conclude that pertussis toxin promotes de novo A(1)AR synthesis by activating NF-kappaB through an ADP ribosylation-independent mechanism involving intracellular Ca(2+) release and PKC activation.

Partners for adenosine A1 receptors

G protein-coupled receptors (GPCRs) are targets for therapy in a variety of neurological diseases. Using adenosine A1 receptors (A1Rs) as paradigm of GPCRs, this review focuses on how protein-protein interactions, from monomers to heteromers, can contribute to hormone/neurotransmitter/neuromodulator regulation. The interaction of A1Rs with other membrane receptors, enzymes, and adaptor and scaffolding proteins is relevant for receptor traffic, internalization, and desensitization, and A1Rs are extremely important in driving signaling through different intracellular pathways. There is even the possibility of linking together GPCR heteromeric complexes with ion channel receptors in a receptor mosaic that might have special integrative value and might constitute the molecular basis for learning and memory.

Allosteric modulators affect the internalization of human adenosine A1 receptors

To study the effect of allosteric modulators on the internalization of human adenosine A(1) receptors, the receptor was equipped with a C-terminal yellow fluorescent protein tag. The introduction of this tag did not affect the radioligand binding properties of the receptor. CHO cells stably expressing this receptor were subjected during 16 h to varying concentrations of the agonist N(6)-cyclopentyladenosine (CPA) in the absence or presence of 10 microM of the allosteric enhancer PD 81,723 ((2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone) or the allosteric inhibitor SCH-202676 (N-(2,3-diphenyl-1,2,4-thiadiazol-5(2H)-ylidene)methanamine). CPA itself was able to internalize 25% and 40% of the receptors at a concentration of 400 nM or 4 muM, respectively. Addition of either PD 81,723 or SCH-202676 alone had no effect on internalization. However, with PD 81,723 a slight amount of internalization was obtained already at 40 nM of CPA and at 400 nM CPA 59% of the receptors internalized. SCH-202676 on the other hand effectively prevented CPA-induced internalization of the receptor.

Effect of chronic gestational treatment with caffeine or theophylline on Group I metabotropic glutamate receptors in maternal and fetal brain

Pregnant rats were treated throughout the gestational period with either caffeine or theophylline, and its effect on the metabotropic glutamate receptor (mGluRs) signal transduction pathway was studied in both maternal and fetal brain. In maternal brain, radioligand binding assays showed that chronic treatment with methylxanthines caused a significant decrease in the total number of mGluRs. This decrease was accompanied by an increase in receptor affinity. Immunodetection showed that mGluR1a and phospholipase C beta1 (PLCbeta1) were significantly decreased in response to chronic methylxanthine treatment, whereas alphaG(q/11) was not affected. A loss was also detected of PLC stimulation mediated by (S)-3,5-dihydroxyphenylglycine (DHPG), a selective Group I mGluR agonist, suggesting desensitization of the mGluR/PLC pathway. In fetal brain, a loss in total mGluRs was observed in fetuses from mothers treated with caffeine or theophylline, without variation in receptor affinity. A decrease in mGluR1a, alphaG(q/11) and PLCbeta1 levels was also observed in response to treatment. However, changes detected in this immature tissue were not associated with variations in PLC activity. These results suggest that chronic caffeine or theophylline treatment down-regulates several mGluR/PLC transduction pathway components in both maternal and fetal brain, causing a loss of receptor responsiveness only in maternal brain.

Different modulation of inhibitory and stimulatory pathways mediated by adenosine after chronic in vivo agonist exposure

After 6 days of in vivo treatment with two selective adenosine receptor agonists, 5'-N-ethylcarboxamido adenosine (NECA) and R-N6-phenylisopropiladenosine (R-PIA), we investigated their effects on adenosine receptors/adenylyl cyclase system in synaptic plasma membranes isolated from rat brain. NECA treatment caused a significant loss of NECA-stimulated adenylyl cyclase activity, suggesting a desensitization of the adenosine A2 receptors-mediated pathway. No significant differences in total adenosine A2 receptors were observed, but Gs protein levels were decreased, suggesting Gs down-regulation as a mechanism for desensitization. On the other hand, NECA treatment caused a significant decrease in high-affinity adenosine A1 receptors population; however, no changes in CHA-inhibited adenylyl cyclase activity or Gi protein level were observed. Finally, when we studied the effects of R-PIA, a selective adenosine A1 receptor agonist, on stimulatory pathway of adenosine, low-affinity adenosine A2 binding sites were decreased without affecting the functionality of the pathway. These results show that adenosine A1 and A2 receptors are modulated in a different way after chronic agonist exposure and suggest the existence of cross-talk mechanisms between both stimulatory an inhibitory pathways mediated by adenosine.

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.

Experimental hypothyroidism modifies specific binding of A1 and A2A analogues to adenosine receptors in the rat kidney

1 Binding kinetic studies with the adenosine analogues [3H]CPA (0.250-50 nm) and [3H]CGS21680 (0.1-100 nm) were performed in renal tissue from control (NL) and thyroidectomised (HTX) rats. We propose that the low renal adenosine content reported in hypothyroid rats may induce changes in the density and/or affinity of adenosine receptor, distributed in the cortex (C), outer medulla (OM), and inner medulla (IM) of the kidney. 2 [3H]CPA and [3H]CGS21680 binding saturation isotherms were fitted by nonlinear regression analysis and evaluated by Furchgott's method. These results revealed high (KH) and low (KL) affinity (KD) sites for both compounds. As expected, a heterogeneous pattern was observed for Bmax and KD values. 3 Bound [3H]CPA and [3H]CGS21680 were displaced by increasing concentrations of nonlabelled DPCPX and NECA, respectively, indicating the presence of A1 and A2A adenosine receptors distributed in the renal segments studied. 4 The relative intrinsic efficacy (epsilon) for [3H]CPA and [3H]CGS21680 showed extreme values (far from 1.0), 0.5 in IM NL and 2.70 in IM HTX for [3H]CGS21680. 5 Our results indicate that A2A adenosine receptor is predominant in IM from HTX, but A1 receptors are expressed preferentially in C in NL. 6 We conclude that the changes observed in number, affinity, and epsilon for the A2A receptor in IM from HTX might be responsible from alterations in medullary function, that is, incapacity for urine concentration as observed in the hypothyroid kidney.

Down-regulation of rat brain adenosine A1 receptors at the end of pregnancy

The status of the adenosine A1 receptor/adenylyl cyclase (A1R/AC) transduction pathway in rat brain was analysed at the end of pregnancy using different approaches. Pregnancy at term caused a significant decrease in the Bmax value obtained by saturation binding assays using [3H]DPCPX as radioligand, suggesting a down-regulation of adenosine A1 receptor. Moreover, A1 receptor immunodetection in pregnant rat membranes and the level of mRNA coding A1 receptor were significantly decreased. This loss of A1 receptor was associated with a significant increase in receptor affinity, since the KD value from the [3H]DPCPX saturation curve and Ki for N6-cyclohexyladenosine (CHA) were decreased in pregnant rats. Surprisingly, CHA-mediated inhibition of adenylyl cyclase was increased, reflecting enhanced receptor responsiveness. On the other hand, immunoblotting of different alphaGi-protein isoforms revealed a significant increase in alphaGi3 level in membranes from pregnant rats. Pre-incubation of membranes with anti-alphaGi3 antibody blocked the guanosine triphosphate (GTP) or CHA inhibitory effect on adenylyl cyclase in both pregnant and non-pregnant rats, pointing to alphaGi3 as the main isoform involved in the A1 receptor response. These results suggest that, at the end of pregnancy, there is a down-regulation of adenosine A1 receptors counterbalanced with a strengthened functionality, probably due to an increase in both alphaGi3 protein and receptor affinity.

Subcellular localization of adenosine A(1) receptors in nerve terminals and synapses of the rat hippocampus

Adenosine is a neuromodulator in the CNS that mainly acts through pre- and postsynaptic A(1) receptors to inhibit the release of excitatory neurotransmitters and NMDA receptor function. This might result from a highly localized distribution of A(1) receptors in the active zone and postsynaptic density of CNS synapses that we now investigated in the rat hippocampus. The binding density of the selective A(1) receptor antagonist, [3H]1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX), was enriched in membranes from Percoll-purified nerve terminals (B(max)=1839+/-52 fM/mg protein) compared to total membranes from the hippocampus (B(max)=984+/-31 fM/mg protein), the same occurring with A(1) receptor immunoreactivity. [3H]DPCPX binding occurred mainly to the plasma membrane rather than to intracellular sites, since the binding of the membrane permeable A(1) receptor ligand [3H]DPCPX to intact hippocampal nerve terminals (B(max)=1901+/-192 fM/mg protein) was markedly reduced (B(max)=321+/-30 fM/mg protein) by the membrane impermeable adenosine receptor antagonist, 8-sulfophenyltheophilline (25 microM). Further subcellular fractionation of hippocampal nerve terminals revealed that A(1) receptor immunoreactivity was strategically located in the active zone of presynaptic nerve terminals, as expected to understand the efficiency of A(1) receptors to depress neurotransmitter release. A(1) Receptors were also present in nerve terminals outside the active zone in accordance with the existence of a presynaptic A(1) receptor reserve. Finally, A(1) receptor immunoreactivity was evident in the postsynaptic density together with NMDA receptor subunits 1, 2A and 2B and with N-and P/Q-type calcium channel immunoreactivity, emphasizing the importance of A(1) receptors in the control of dendritic integration.

Ligand-induced caveolae-mediated internalization of A1 adenosine receptors: morphological evidence of endosomal sorting and receptor recycling

The involvement of caveolae in the internalization of A(1) adenosine receptors (A(1)R) and the receptor sorting and recycling was studied in the smooth muscle cell line DDT(1)MF-2, by binding assays, by confocal microscopy, and at the structural level. The use of cholera toxin-binding subunit adsorbed to gold as a specific probe for labeling the ganglioside GM(1) and immunoelectron microscopy techniques showed that agonist stimulation produced a clustering and sequestration of adenosine receptors in caveolae. Furthermore, pull-down experiments showed there to be a direct interaction between the C-terminal domain of A(1)R and caveolin-1. Addition of exogenous adenosine deaminase (ADA), a protein that binds to A(1)R and acts as a receptor activity modifying protein (RAMP) stimulated R-PIA-induced A(1) receptor internalization. Finally, the sorting and recycling of A(1)R/ADA complexes was analyzed. Detailed electron microscopy revealed that A(1)R/ADA complexes internalize together through caveolae, are differentially sorted in endosomes, and are recycled back to the cell surface by different groups of recycling endosomes. These results give insight into the spatiotemporal regulation and traffic of A(1)R and RAMPs.

Adenosine A1 receptor agonist treatment up-regulates rat brain metabotropic glutamate receptors

Chronic R-N(6)-phenylisopropiladenosine (R-PIA) subcutaneous injection for 6 days significantly increased total glutamate receptor number (180% of control) in rat brain synaptic plasma membranes (SPM), without affecting receptor affinity. A higher increase in metabotropic glutamate (mGlu) receptor number (258% of control) was also detected, indicating that mGlu is the main type of glutamate receptor affected by this treatment. On the other hand, the observed increase in basal and calcium- and Gpp(NH)p-stimulated phospholipase C (PLC) activity after treatment was associated with a significant increase in PLC beta(1) isoform, detected in SPM by immunoblotting assays. Moreover, an increase in PLC activity stimulation with trans-ACPD, in the absence and in the presence of Gpp(NH)p, was detected after R-PIA treatment. These results show that mGlu receptors and its effector system, PLC activity, are up-regulated by chronic exposure to an adenosine A(1) receptor agonist and suggest the existence of a cross-talk mechanism between both signal transduction pathways in rat brain.

Purinergic dysfunction in mania: an integrative model

The purinergic system, which includes the anticonvulsant and antikindling neuromodulator adenosine and the neurotransmitter ATP, modulates second messenger systems, neurotransmitters, energy metabolism and different behaviors, such as sleep, motor activity, cognition, memory, aggressive behavior and social interaction. At the same time, mania is characterized by similar behavioral changes and a molecular basis to explain the pathological activation observed during manic episodes has been also associated with second messenger systems dysfunction and kindling. This hypothesis put forward an integrative model of neuronal communication, associating a reduced adenosinergic activity, mostly at A1 receptors, with the complex network of changes on neurotransmitters pathways related to manic behavior.

Internalization of metabotropic glutamate receptor in C6 cells through clathrin-coated vesicles

C6 glioma cells were treated for hours with 100 microM L-glutamate, quisqualate or trans-ACPD. In all cases, phospholipase C-coupled metabotropic glutamate receptors (mGluRs) present in these cells are down-regulated after this agonist treatment. Cell surface metabotropic glutamate receptor density was minimum at 6 h of agonist treatment and reached near control values after 30 h of treatment. This recovery was associated with a progressive increase in mGluR1 and mGluR1a mRNA level between 6 and 24 h and was not due to agonist removal. Specific L-[3H]glutamate or [3H](+/-)trans-ACPD binding decrease detected in C6 cells after 6 h of 100 microM L-glutamate treatment was associated with a remarkable increase of specific L-[3H]glutamate binding detected in clathrin-coated vesicles isolated from these treated cells. Moreover, this decrease was blocked in the presence of 0.5 M sucrose or 1 microM phenylarsine oxide, suggesting that desensitization and down-regulation of mGluR can be due to an endocytosis process through clathrin-coated pits and vesicles.

Involvement of the cerebellar adenosine A(1) receptor in cannabinoid-induced motor incoordination in the acute and tolerant state in mice

Cannabinoids are known to impair motor function in humans and laboratory animals. We have demonstrated an accentuation of cannabinoid (CP55,940)-induced motor incoordination in mice by the adenosine A(1) receptor-selective agonist N(6)-cyclohexyladenosine (CHA) (4 ng) using an intracerebellar (ICB) microinjection method. This effect was mediated by the A(1) receptor because pre-treatment with ICB 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (100 ng), an adenosine A(1) receptor selective antagonist, completely abolished the accentuation. Furthermore, ICB pre-treatment with DPCPX (100 ng) before ICB CP55,940 (15 microg) attenuated the motor incoordination suggesting a modulation by an endogenous adenosine A(1) system. ICB microinjection of CHA or DPCPX prior to ICB vehicle had no effect on normal motor coordination. ICB microinjection of dipyridamole (25 microg), an adenosine transport inhibitor, significantly accentuated the motor incoordination by ICB CP55,940 (15 microg), providing further support for the involvement of endogenous adenosine in the action of CP55,940. Tolerance to the motor incoordinating effect of ICB CP55,940 was demonstrated following 3 days of i.p. CP55,940 (0.1, 1 or 2 mg/kg every 12 or 24 h; total of six or three injections, respectively). Interestingly, animals which exhibited tolerance to ICB CP55,940 also demonstrated tolerance to the accentuating effect of ICB CHA suggesting cross-tolerance between adenosine agonists and cannabinoids. Cross-tolerance was also demonstrated following 3 days of i.p. CHA (0.25 or 1 mg/kg every 24 h; total of three injections) as further evidence of the modulatory role of the cerebellar adenosine system in the acute manifestation of CP55,940-induced motor incoordination. The involvement of cerebellar adenosine and the A(1) receptor in cannabinoid actions is circumstantially supported by previous evidence that CB(1) receptors and A(1) receptors are both localized on cerebellar granule cell parallel fiber terminals and basket cell neurons where they serve to inhibit the release of neurotransmitters.

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

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

Adenosine A(1) receptor in cultured neurons from rat cerebral cortex: colocalization with adenosine deaminase

Adenosine A(1) receptors (A(1)Rs) have been characterized in primary cultures of neurons from cerebral cortex. The specific adenosine A(1) antagonist 8-cyclopentyl-1,3-[(3)H]dipropylxanthine bound to both membranes and intact cells. When saturation experiments were performed in membranes, a K(D) value of 0.76 nM and a B(max) of 57 fmol/mg of protein were obtained. Competition assays revealed a pharmacological profile characteristic of A(1)Rs. The presence of this receptor was further confirmed by RT-PCR analysis. The expression of the receptor showed no significant changes during the period of culture studied, up to 12 days in vitro. A(1)R agonist inhibited forskolin-stimulated adenylyl cyclase, showing the functional coupling of these receptors with the effector. alphaG(i1, 2) protein level, detected by immunoblot, presented an increase during the period of culture. This increase correlated with an increase in the mRNA level of alphaG(i1) but not alphaG(i2). By immunochemical assays, it is shown that these receptors are expressed in both the neuronal cell body and the proximal dendrites. Colocalization of A(1)Rs with microtubule-associated protein 2 and cell surface adenosine deaminase was shown by confocal microscopy. The high degree of colocalization observed between A(1)Rs and ectoadenosine deaminase in neurons could suggest an important role of the enzyme in adenosine-mediated neuromodulation.

Activation of neuronal adenosine A1 receptors suppresses secretory reflexes in the guinea pig colon

The role of adenosine A1 receptors (A1R) in reflex-evoked short-circuit current (Isc) indicative of chloride secretion was studied in the guinea pig colon. The A1R antagonist 8-cyclopentyltheophylline (CPT) enhanced reflex-evoked Isc. Adenosine deaminase and the nucleoside transport inhibitor S-(4-nitrobenzyl)-6-thioinosine enhanced and reduced reflex-induced Isc, respectively. The A1R agonist 2-chloro-N6-cyclopentyladenosine (CCPA) inhibited reflex-evoked Isc at nanomolar concentrations, and its action was antagonized by CPT. In the presence of either N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide to block the 5-hydroxytryptamine (5-HT)-mediated pathway or piroxicam to block the prostaglandin-mediated pathway, CCPA reduced the residual reflex-evoked Isc. CCPA reduced the response to a 5-HT pulse without affecting the tetrodotoxin-insensitive Isc responses to carbachol or forskolin. Immunoreactivity for A1R was detected in the membrane (10% of neurons) and cytoplasm (90% of neurons) of neural protein gene product 9.5-immunoreactive (or S-100-negative) submucosal neurons, in glia, and in the muscularis mucosa. A1R immunoreactivity in a majority of neurons remained elevated in the cytoplasm despite preincubation with adenosine deaminase or CPT. A1R immunoreactivity colocalized in synaptophysin-immunoreactive presynaptic varicose nerve terminals. The results indicate that endogenous adenosine binding to high-affinity A1R on submucosal neurons acts as a physiological brake to suppress reflex-evoked Isc indicative of chloride secretion.

Epidermal growth factor (EGF)-induced up-regulation and agonist- and antagonist-induced desensitization and internalization of A1 adenosine receptors in a pituitary-derived cell line

This report concerns the study of homologous and heterologous regulation of cell surface A1 adenosine receptors (A1R) in a pituitary-derived cell line. This has been possible by the use of the recently developed anti-A1R antibodies in immunocytochemical assays. Functional desensitization and internalization of A1R in GH4 cells occurred after treatment with agonist but also with antagonist. Epidermal growth factor (EGF) treatment led to the up-regulation of cell surface A1R in GH4 cells. Confocal analysis evidenced an EGF-induced increase of A1R present in intracellular clathrin-coated vesicles. The up-regulation was blocked by actinomycin D thus suggesting the involvement of protein synthesis in the effect induced by the growth factor. These results constitute the first example of adenosine receptor regulation by EGF and one of the few examples of antagonist-induced desensitization and internalization among G-protein-coupled receptors.

Adenosine deaminase and A1 adenosine receptors internalize together following agonist-induced receptor desensitization

A1 adenosine receptors (A1Rs) and adenosine deaminase (ADA; EC 3.5.4. 4) interact on the cell surface of DDT1MF-2 smooth muscle cells. The interaction facilitates ligand binding and signaling via A1R, but it is not known whether it has a role in homologous desensitization of A1Rs. Here we show that chronic exposure of DDT1MF-2 cells to the A1R agonist, N6-(R)-(phenylisopropyl)adenosine (R-PIA), caused a rapid aggregation or clustering of A1 receptor molecules on the cell membrane, which was enhanced by pretreatment with ADA. Colocalization between A1R and ADA occurred in the R-PIA-induced clusters. Interestingly, colocalization between A1R and ADA also occurred in intracellular vesicles after internalization of both protein molecules in response to R-PIA. Agonist-induced aggregation of A1Rs was mediated by phosphorylation of A1Rs, which was enhanced and accelerated in the presence of ADA. Ligand-induced second-messenger desensitization of A1Rs was also accelerated in the presence of exogenous ADA, and it correlated well with receptor phosphorylation. However, although phosphorylation of A1R returned to its basal state within minutes, desensitization continued for hours. The loss of cell-surface binding sites (sequestration) induced by the agonist was time-dependent (t1/2= 10 +/- 1 h) and was accelerated by ADA. All of these results strongly suggest that ADA plays a key role in the regulation of A1Rs by accelerating ligand-induced desensitization and internalization and provide evidence that the two cell surface proteins internalize via the same endocytic pathway.

Desensitization of adenosine A1 receptor-mediated inhibition of adenylyl cyclase in cerebellar granule cells

Agonist-induced desensitization of adenosine A1 receptor-mediated inhibition of adenylyl cyclase has been studied in cerebellar granule cells. Exposure of cells to the adenosine A1 receptor agonist R-phenylisopropyl adenosine (R-PIA) from 2 to 48 h brings about desensitization of this signal transduction pathway. Associated with the desensitization process, a decrease in radioligand binding performed in intact cells with the adenosine A1 receptor agonist [3H]cyclohexyladenosine (CHA) has been detected. Simultaneously, an increase of adenosine A1 radioligand binding has also been detected in microsomes. A decrease in the steady-state level of alpha-Gi in both, plasma membrane and microsomes also has been detected during the desensitization process. These data may account for the desensitization of the inhibitory pathway of the adenylyl cyclase in cerebellar granule cells described herein. After a transient increase in adenosine A1 receptor mRNA, no changes were observed in this parameter after 12 hr of treatment with the adenosine A1 agonist R-PIA, suggesting a post-transcriptional regulation of this receptor during long-term desensitization.

Ligand-induced phosphorylation, clustering, and desensitization of A1 adenosine receptors

Through immunocytochemistry with the use of antibodies against A1 adenosine receptors (A1Rs) and confocal microscopy, we show that stimulation of A1Rs by the agonist (R)-phenylisopropyladenosine [(R)-PIA] caused a rapid (5-15 min) aggregation (clustering) of receptor molecules on the surface of DDT1MF-2 cells. Internalization of the chronically stimulated receptor was slower and occurred concomitantly, with a time-dependent decrease (50%) in the number of cell surface [3H](R)-PIA binding sites. The reduction of binding sites was due partly (30%) to internalization and partly (20%) to the presence of desensitized cell surface receptor molecules that were unable to bind the ligand. Chronic exposure of DDT1MF-2 cells to 50 nM (R)-PIA produced functional desensitization, as deduced from second messenger production assays. Quantification of the content of A1Rs by immunoblotting and flow cytometry in cells pretreated with 50 nM (R)-PIA indicates a time-dependent slow down-regulation of the receptor. Receptor clustering and agonist-induced receptor phosphorylation, which occurred in serine and tyrosine, were simultaneous. The finding that activators of protein kinase A or C were able to induce functional desensitization of A1Rs, phosphorylate A1Rs in serine and threonine, and trigger clustering of the receptor suggests that phosphorylation of A1Rs in serine/threonine is involved in desensitization-related events.