Adaptive changes in adenosine receptors following long-term treatment with the adenosine receptor agonist R-phenylisopropyl adenosine

Expression of A1 adenosine receptors in the developing avian retina: in vivo modulation by A(2A) receptors and endogenous adenosine

Little is known about the mechanisms that regulate the expression of adenosine receptors during CNS development. We demonstrate here that retinas from chick embryos injected in ovo with selective adenosine receptor ligands show changes in A1 receptor expression after 48 h. Exposure to A1 agonist N⁶-cyclohexyladenosine (CHA) or antagonist 8-Cyclopentyl-1, 3-dipropylxanthine (DPCPX) reduced or increased, respectively, A1 receptor protein and [³H]DPCPX binding, but together, CHA+DPCPX had no effect. Interestingly, treatment with A(2A) agonist 3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino] ethyl]phenyl] propanoic acid (CGS21680) increased A1 receptor protein and [³H]DPCPX binding, and reduced A(2A) receptors. The A(2A) antagonists 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-trizolo[1,5-c] pyrimidine (SCH58261) and 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazo-5-yl-amino]ethyl)phenol (ZM241385) had opposite effects on A1 receptor expression. Exposure to CGS21680 + CHA did not change A1 receptor levels, whereas CHA + ZM241385 or CGS21680 + DPCPX had no synergic effect. The blockade of adenosine transporter with S-(4-nitrobenzyl)-6-thioinosine (NBMPR) also reduced [³H]DPCPX binding, an effect blocked by DPCPX, but not enhanced by ZM241385. [³H]DPCPX binding kinetics showed that treatment with CHA reduced and CGS21680 increased the Bmax, but did not affect Kd values. CHA, DPCPX, CGS21680, and ZM241385 had no effect on A1 receptor mRNA. These data demonstrated an in vivo regulation of A1 receptor expression by endogenous adenosine or long-term treatment with A1 and A(2A) receptors modulators.

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.

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.

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.

Modification of adenosine A1 and A2A receptor density in the hippocampus of streptozotocin-induced diabetic rats

Adenosine A(1) and A(2A) receptors are neuromodulatory systems that can control mnemonic behavior, which is modified by diabetes. Since the density of these adenosine receptors can change upon chronic noxious brain conditions, we now tested if the density of A(1) and A(2A) receptors was modified in the hippocampus of streptozotocin-induced diabetic rats. The binding density of the selective A(1) receptor antagonist, (3)H-DPCPX, was decreased by 36% in total hippocampal membranes 7 days after induction of diabetes and this down-regulation was maintained after 30 and 90 days, which was also confirmed by Western blot analysis of A(1) receptor immunoreactivity. In contrast, the binding density of the selective A(2A) receptor antagonist, (3)H-SCH 58261, was enhanced by 83% in total hippocampal membranes 7 days after induction of diabetes and this up-regulation persisted after 30 and 90 days. These results show that the balance between inhibitory A(1) and facilitatory A(2A) adenosine receptors is modified in the hippocampus of streptozotocin-induced diabetic rats. Thus, the most abundant A(1) receptors are down-regulated and there is an up-regulation of A(2A) receptors, suggesting a gain of function of hippocampal A(2A) receptors compared to A(1) receptors in diabetes, as has been observed in other chronic noxious brain conditions where A(2A) receptor blockade affords robust neuroprotection.

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.

Long-term Effect of Convulsive Behavior on the Density of Adenosine A1 and A2A Receptors in the Rat Cerebral Cortex

PURPOSE

Adenosine is a neuromodulator that has been proposed to act as an anticonvulsant mainly via inhibitory A1 receptors, but recent data show that genetic deletion of facilitatory A 2A receptors might also attenuate convulsions. Since both A1 and A 2A receptors are prone to down- and upregulation in different stressful situations, we investigated if convulsive behavior leads to a long-term change in A1 and A 2A receptor density in the rat cerebral cortex.

METHODS

Stage 4-5 convulsions (Racine's scale) were induced in adult Wistar rats either through amygdala stimulation (kindling) or by intraperitoneal injection of kainate (10 mg/ml). Rats were killed after 4 weeks to evaluate adenosine A1 and A 2A receptor density in the cerebral cortex using both Western blot and membrane binding assays.

RESULTS

The binding density of the A1 antagonist, 3H-DPCPX, decreased by 40. +/- 4.4% and by 20.7 +/- 0.5% after kindling or kainate injection. Likewise, A1 receptor immunoreactivity in cortical membranes from kindled or kainate-injected rats decreased by 19.1 +/- 3.3% and 12.7 +/- 5.7%, respectively. In contrast, the binding density of the A 2A receptor antagonist 3H-SCH 58261 increased by 293 +/- 34% and by 159 +/- 32% in cortical membranes from kindled or kainate-injected rats, and A 2A receptor immunoreactivity also increased by 151 +/- 12% and 79.6 +/- 7.0%.

CONCLUSIONS

This indicates that after convulsive behavior there is a long-term decrease of A1 receptors accompanied by an increased density of A 2A receptors, suggesting that A 2A antagonists rather than A1 agonists may be more promising anticonvulsive drugs.

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.

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.

Differential desensitization of responses mediated by presynaptic and postsynaptic A1 adenosine receptors

G-protein-coupled receptors (GPCRs) often desensitize during continuous activation, but it is not known whether desensitization is influenced by subcellular location. In hippocampal neurons, activation of adenosine A1 receptors (A1Rs) or GABA(B) receptors on synaptic terminals inhibits neurotransmitter release, whereas activation of the same receptors on cell bodies and dendrites decreases excitability by activating inwardly rectifying potassium (GIRK) channels. Here we report that responses mediated by presynaptic A1Rs desensitize more slowly than responses mediated by postsynaptic (somatodendritic) A1Rs in cultured neurons. Agonist treatment for 2 hr has no effect on adenosine-induced presynaptic inhibition, whereas such treatment nearly abolishes adenosine-induced activation of postsynaptic GIRK channels. Agonist treatment for longer periods (>12 hr) eventually desensitizes A1R-mediated presynaptic inhibition. Presynaptic and postsynaptic responses both recover from desensitization after agonist removal, but recovery of presynaptic inhibition requires more time. Desensitization of postsynaptic responses apparently occurs at the level of the receptor, because postsynaptic G-proteins and GIRK channels appear to be fully functional. Inhibition of voltage-gated calcium channels by postsynaptic A1Rs also desensitizes rapidly, although this desensitization is less complete than is observed for activation of postsynaptic GIRK channels. Comparison of concentration-response curves for presynaptic and postsynaptic responses suggests that a receptor reserve exists for presynaptic inhibition, but that the magnitude of this reserve is insufficient to account for the absence of presynaptic desensitization after brief agonist exposure. These results suggest that agonist-induced desensitization of responses mediated by neuronal GPCRs may depend on the subcellular location of the receptors.

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.

Neonatal cerebral hypoxia-ischemia: the effect of adenosine receptor antagonists

The effects of nonselective (theophylline), A1-(DPCPX) or A2A-selective (SCH 58261) adenosine receptor antagonists administered before or after neonatal hypoxia-ischemia (HI) were studied on the extent of brain injury in 7-day-old rats evaluated after 14 days. A possible effect of theophylline (20 mg/kg) on expression of immediate early genes was studied with in situ hybridization. Theophylline (20, 30 or 60 mg/kg) given prior to HI reduced brain damage by 48% (P < 0.001), 36% (P < 0.01) and 34% (P < 0.05), respectively, compared to control rats. This effect was not explained by changes in temperature, cerebral blood flow, blood gas/acid base status or blood glucose during the insult. Theophylline enhanced the upregulation of c-fos and NFGI-A during reperfusion but did not prevent the decrease in adenosine A1 receptor mRNA. Posttreatment with SCH 58261 (0.2 or 2 mg/kg) reduced brain damage by 19% (P < 0.05) and 14% (NS), respectively, compared to control rats which was unrelated to the core temperature. DPCPX (2 or 10 mg/kg) had no effect on the development of brain injury. In conclusion, nonselective and A2A adenosine receptor antagonists reduced brain injury in a model of HI in immature animals.