Adenosine acts as a powerful signaling molecule via four distinct G protein-coupled receptors, designated A1, A2A, A2B and A3 adenosine receptors (ARs). A2A and A2B ARs are Gs-coupled, while A1 and A3 ARs inhibit cAMP production via Gi proteins. Antagonists for A1 and A3 ARs may be useful for the treatment of (neuro)inflammatory diseases including acute kidney injury and kidney failure, pulmonary diseases, and Alzheimer's disease. In the present study, we optimized the versatile 2-amino-4-phenylthiazole scaffold by introducing substituents at N2 and C5 to obtain A1 and A3 AR antagonists including dual-target compounds. Selective A1 antagonists with (sub)nanomolar potency were produced, e.g. 11 and 13. These compounds showed species differences being significantly more potent at the rat as compared to the human A1 AR, and were characterized as inverse agonists. Several potent and selective A3 AR antagonists, e.g. 7, 8, 17 and 22 (Ki values of 5-9 nM at the human A3 AR) were prepared, which were much less potent at the rat orthologue. Moreover, dual A1/A3 antagonists (10, 18) were developed showing Ki values between 8 and 42 nM. Docking and molecule dynamic simulation studies using the crystal structure of the A1 AR and a homology model of the A3 AR were performed to rationalize the observed structure-activity relationships.

The use of antagonists for each adenosine receptor (AR) subtype as potent clinical candidates is of growing interest due to their involvement in the treatment of various diseases. The recent resolution of several A1 and A2A ARs X-ray structures provides opportunities for structure-based drug design. In this study, we describe the discovery of novel A1AR antagonists by applying a multistage virtual screening approach, which is based on random forest (RF), e-pharmacophore modeling and docking methods. A multistage virtual screening approach was applied to screen the ChemDiv library (1,492,362 compounds). Among the final hits, 22 compounds were selected for further radioligand binding assay analysis against human A1AR, and 18 compounds (81.82% success) exhibited nanomolar or low micromolar binding potency (Ki). Then, we selected six compounds (pKi > 6) to further evaluate their antagonist profile in a cAMP functional assay, and we found that they had low micromolar antagonistic activity (pIC50 = 5.51-6.38) for the A1AR. Particularly, four of six compounds (pKi > 6) showed very good affinity (pKi = 6.11-7.13) and selectively (>100-fold) for A1AR over A2AAR. Moreover, the novelty analysis suggested that four of six compounds (pKi > 6) were dissimilar to existing A1AR antagonists and hence represented novel A1AR antagonists. Further molecular docking and molecular dynamics (MD) studies showed that the three selective compounds 15, 20 and 22 were stabilized (RMSlig value ≤ 2 Å) inside the binding pocket of A1AR with similar orientations to the docking pose in 100-ns MD simulations, whereas they escaped from the binding area of A2AAR with larger values of RMSlig (RMSlig ≥ 2 Å). We hope that these findings provide new insights into the discovery of drugs targeting A1AR and facilitate research on new drugs and treatments for A1AR-related human pathologies.

1. The pharmacological profile of adenosine A1 receptors in human, guinea-pig, rat and mouse brain membranes was characterized in a radioligand binding assay by use of the receptor selective antagonist, [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]-DPCPX). 2. The affinity of [3H]-DPCPX binding sites in rat cortical and hippocampal membranes was similar. Binding site affinity was higher in rat cortical membranes than in membranes prepared from guinea-pig cortex and hippocampus, mouse cortex and human cortex. pKD values (M) were 9.55, 9.44, 8.85, 8.94, 8.67, 9.39 and 8.67, respectively. The binding site density (Bmax) was lower in rat cortical membranes than in guinea-pig or human cortical membranes. 3. The rank order of potency of seven adenosine receptor agonists was identical in each species. With the exception of 5'-N-ethylcarboxamidoadenosine (NECA), agonist affinity was 3.5-26.2 fold higher in rat cortical membranes than in human and guinea-pig brain membranes; affinity in rat and mouse brain membranes was similar. While NECA exhibited 9.3 fold higher affinity in rat compared to human cortical membranes, affinity in other species was comparable. The stable GTP analogue, Gpp(NH)p (100 microM) reduced 2-chloro-N6-cyclopentyladenosine (CCPA) affinity 7-13.9 fold, whereas the affinity of DPCPX was unaffected. 4. The affinity of six xanthine-based adenosine receptor antagonists was 2.2-15.9 fold higher in rat cortical membranes compared with human or guinea-pig membranes. The rank order of potency was species-independent. In contrast, three pyrazolopyridine derivatives, (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-2-piperidine ethanol (FK453), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-piperidin-2-yl acetic acid (FK352) and 6-oxo-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-1(6H)-pyridazinebutyric acid (FK838) exhibited similar affinity in human, guinea-pig, rat and mouse brain membranes. pKi values (M) for [3H]-DPCPX binding sites in human cortical membranes were 9.31, 7.52 and 7.92, respectively. 5. Drug affinity for adenosine A2A receptors was determined in a [3H]-2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido ade nosine ([3H]-CGS 21680) binding assay in rat striatal membranes. The pyrazolopyridine derivatives, FK453, FK838 and FK352 exhibited pKi values (M) of 5.90, 5.92 and 4.31, respectively, compared with pKi values of 9.31, 8.18 and 7.57 determined in the [3H]-DPCPX binding assay in rat cortical membranes. These novel pyrazolopyridine derivatives therefore represent high affinity, adenosine A1 receptor selective drugs that, in contrast to xanthine based antagonists, exhibit similar affinity for [3H]-DPCPX binding sites in human, rat, mouse and guinea-pig brain membranes.

Adenosine receptors (ARs) have been demonstrated to be potential therapeutic targets against Parkinson's disease (PD). In the present study, we describe a multistage virtual screening approach that identifies dual adenosine A1 and A2A receptor antagonists using deep learning, pharmacophore models, and molecular docking methods. Nineteen hits from the ChemDiv library containing 1,178,506 compounds were selected and further tested by in vitro assays (cAMP functional assay and radioligand binding assay); of these hits, two compounds (C8 and C9) with 1,2,4-triazole scaffolds possessing the most potent binding affinity and antagonistic activity for A1/A2A ARs at the nanomolar level (pKi of 7.16-7.49 and pIC50 of 6.31-6.78) were identified. Further molecular dynamics (MD) simulations suggested similarly strong binding interactions of the complexes between the A1/A2A ARs and two compounds (C8 and C9). Notably, the 1,2,4-triazole derivatives (compounds C8 and C9) were identified as the most potent dual A1/A2A AR antagonists in our study and could serve as a basis for further development. The effective multistage screening approach developed in this study can be utilized to identify potent ligands for other drug targets.

Evidence suggests that pre-ischeamic conditioning (PIC) offers protection against a subsequent ischeamic event. Although some brain areas such as the hippocampus have received much attention, the receptor mechanisms of PIC in other brain regions are unknown. We have previously shown that 10 min oxygen and glucose deprivation (OGD) evokes tolerance to a second OGD event in the caudate. Here we further examine the effect of length of conditioning event on the second OGD event. Caudate mouse brain slices were superfused with artificial cerebro-spinal fluid (aCSF) bubbled with 95%O(2)/5%CO(2). OGD was achieved by reducing the aCSF glucose concentration and by bubbling with 95%N(2)/5%CO(2). After approximately 5 min OGD a large dopamine efflux was observed, presumably caused by anoxic depolarisation. On applying a second OGD event, 60 min later, dopamine efflux was delayed and reduced. We first examined the effect of varying the length of the conditioning event from 5 to 40 min and found tolerance to PIC increased with increasing duration of conditioning. We then examined the receptor mechanism(s) underlying PIC. We found that pre-incubation with either MK-801 or 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) reduced tolerance to the second OGD event. These data suggest that either N-methyl-D-aspartate (NMDA) or adenosine A(1) receptor activation evokes PIC in the mouse caudate.

Blockade of the adenosine A2B receptor (A2BAR) represents a potential novel strategy for the immunotherapy of cancer. In the present study, we designed, synthesized, and characterized irreversible A2BAR antagonists based on an 8-p-sulfophenylxanthine scaffold. Irreversible binding was confirmed in radioligand binding and bioluminescence resonance energy transfer(BRET)-based Gα15 protein activation assays by performing ligand wash-out and kinetic experiments. p-(1-Propylxanthin-8-yl)benzene sulfonyl fluoride (6a, PSB-21500) was the most potent and selective irreversible A2BAR antagonist of the present series with an apparent Ki value of 10.6 nM at the human A2BAR and >38-fold selectivity versus the other AR subtypes. The corresponding 3-cyclopropyl-substituted xanthine derivative 6c (PSB-21502) was similarly potent, but was non-selective versus A1- and A2AARs. Attachment of a reactive sulfonyl fluoride group to an elongated xanthine 8-substituent (12, Ki 7.37 nM) resulted in a potent, selective, reversibly binding antagonist. Based on previous docking studies, the lysine residue K2697.32 was proposed to react with the covalent antagonists. However, the mutant K269L behaved similarly to the wildtype A2BAR, indicating that 6a and related irreversible A2BAR antagonists do not interact with K2697.32. The new irreversible A2BAR antagonists will be useful tools and have the potential to be further developed as therapeutic drugs.

Previous studies evaluated the adenosine receptor antagonists alone to determine their effects on oxidative stress, but little is known about adenosine's protective efficacy when oxidative injury occurs in vivo. Adenosine is a crucial signaling molecule recognized by four distinct G-protein-coupled receptors (GPCRs) (i.e., A1R, A2AR, A2BR, and A3R) and protects cells against pathological conditions. The present study was performed to evaluate the role of antagonist modulation in the setting of paraquat toxicity with adenosine pretreatment. First, PC12 cells were exposed to paraquat (850 μM) and adenosine (30 μM) to develop an in vitro model for the antagonist effect assay. Second, we found that the A1R antagonist DPCPX enhanced the viability of paraquat-induced PC12 cells that underwent adenosine pretreatment. Moreover, the A2AR antagonist ZM241385 decreased the viability of paraquat-induced PC12 cells that underwent adenosine pretreatment. Our findings indicate that adenosine protection requires a dual blockade of A1R and activation of A2AR to work at its full potential, and the A2B and A3 adenosine receptor antagonists increased paraquat-induced oxidative damage. This represents a novel pharmacological strategy based on A1/A2A interactions and can assist in clarifying the role played by AR antagonists in the treatment of neurodegenerative diseases.

The purpose of the study was to investigate whether the co-administration of Mg2+ and Zn2+ with selective A1 and A2A receptor antagonists might be an interesting antidepressant strategy. Forced swim, tail suspension, and spontaneous locomotor motility tests in mice were performed. Further, biochemical and molecular studies were conducted. The obtained results indicate the interaction of DPCPX and istradefylline with Mg2+ and Zn2+ manifested in an antidepressant-like effect. The reduction of the BDNF serum level after co-administration of DPCPX and istradefylline with Mg2+ and Zn2+ was noted. Additionally, Mg2+ or Zn2+, both alone and in combination with DPCPX or istradefylline, causes changes in Adora1 expression, DPCPX or istradefylline co-administered with Zn2+ increases Slc6a15 expression as compared to a single-drug treatment, co-administration of tested agents does not have a more favourable effect on Comt expression. Moreover, the changes obtained in Ogg1, MsrA, Nrf2 expression show that DPCPX-Mg2+, DPCPX-Zn2+, istradefylline-Mg2+ and istradefylline-Zn2+ co-treatment may have greater antioxidant capacity benefits than administration of DPCPX and istradefylline alone. It seems plausible that a combination of selective A1 as well as an A2A receptor antagonist and magnesium or zinc may be a new antidepressant therapeutic strategy.

New compounds with 1H-pyrazolo [3,4-d]pyrimidin-6-amine core scaffolds were synthesized and characterized in vitro to determine their affinity for human A2A and A1 receptors. Among the tested compounds, a few compounds displayed nanomolar binding affinities for both receptors. One particular compound, 11o, showed high binding activities (hA2A Ki = 13.3 nM; hA1 Ki = 55 nM) and full antagonism (hA2A IC50 = 136 nM; hA1 IC50 = 98.8 nM) toward both receptors. Further tests showed that 11o has low hepatic clearance and good pharmacokinetic properties in mice, along with high bioavailability and a high brain plasma ratio. In addition, 11o was associated with very low cardiovascular risk and mutagenic potential, and was well-tolerated in rats and dogs. When tested in an MPTP-induced mouse model of Parkinson's disease, 11o tended to improve behavior. Moreover, 11o dose-dependently reversed haloperidol-induced catalepsy in female rats, with graded ED50 of between 3 and 10 mg/kg. Taken together, these results suggest that this potent dual A2A/A1 receptor antagonist, 11o, is a good candidate for the treatment of Parkinson's disease with an excellent metabolic and safety profile.

Recent research suggests that adenosine receptors (ARs) influence many of the metabolic abnormalities associated with diabetes. A non-xanthine benzylidene indanone derivative 2-(3,4-dihydroxybenzylidene)-4-methoxy-2,3-dihydro-1 H-inden-1-one (2-BI), has shown to exhibit higher affinity at A1/A2A ARs compared to caffeine. Due to its structural similarity to caffeine, and the established antidiabetic effects of caffeine, the current study was initiated to explore the possible antidiabetic effect of 2-BI.

Immunolabeling for adenosine A1 receptors (A1Rs) is high in hippocampal area CA2 in adult rats, and the potentiating effects of caffeine or other A1R-selective antagonists on synaptic responses are particularly robust at Schaffer collateral synapses in CA2. Interestingly, the pronounced staining for A1Rs in CA2 is not apparent until rats are 4 weeks old, suggesting that developmental changes other than receptor distribution underlie the sensitivity of CA2 synapses to A1R antagonists in young animals. To evaluate the role of A1R-mediated postsynaptic signals at these synapses, we tested whether A1R agonists regulate synaptic transmission at Schaffer collateral inputs to CA2 and CA1. We found that the selective A1R agonist CCPA caused a lasting depression of synaptic responses in both CA2 and CA1 neurons in slices obtained from juvenile rats (P14), but that the effect was observed only in CA2 in slices prepared from adult animals (~P70). Interestingly, blocking phosphodiesterase activity with rolipram inhibited the CCPA-induced depression in CA1, but not in CA2, indicative of robust phosphodiesterase activity in CA1 neurons. Likewise, synaptic responses in CA2 and CA1 differed in their sensitivity to the adenylyl cyclase activator, forskolin, in that it increased synaptic transmission in CA2, but had little effect in CA1. These findings suggest that the A1R-mediated synaptic depression tracks the postnatal development of immunolabeling for A1Rs and that the enhanced sensitivity to antagonists in CA2 at young ages is likely due to robust adenylyl cyclase activity and weak phosphodiesterase activity rather than to enrichment of A1Rs.

Caffeine has been used as a first-line drug for treatment of apnea neonatorum for decades due to its high safety and effectiveness. Studies report that caffeine mainly acts as a blocker of Adenosine Receptors (ARs). However, the mechanism of caffeine in reducing apnea neonatorum in the central nervous system has not been fully explored. Medial parabrachial nucleus (MPB) is part of the respiratory center of the pons that may be related to the activity of caffeine. Previous studies have not explored the effect and mechanism of caffeine on MPB neurons. To elucidate this, the current study used antagonists of A1 and A2a receptors to mimic the effect of caffeine in MPB of mice in vitro using the patch-clamp technique. The firing rates and spontaneous post-synaptic currents were recorded. The findings of the study showed that caffeine excited MPB neurons. Notably, the adenosine A1R antagonist 8-cyclopentyl-1,3-dimethyl-xanthine (CPT) but not the adenosine A2aR antagonist Istradefylline (KW6002) mimicked the exciting effect of caffeine, implying that caffeine excited MPB neurons in mice by blocking A1Rs. Further, the results indicated that caffeine could increase efficiency of synaptic transmission to excite MPB neurons. These findings suggest that A1Rs in MPB may be potential targets for caffeine in reducing apnea neonatorum.

The present study describes a modified radioreceptor binding assay using brain homogenate or serum from drug treated animals as the 'competing drug' in a conventional in vitro radioligand binding assay. Method validation involved measurement of the brain and serum concentration of three adenosine receptor antagonists following systemic administration, using a [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) binding assay. The intrinsic [3H]DPCPX binding capacity of test samples was abolished by protein denaturation (80 degrees C, 15 min) and, endogenous ligand was depleted enzymatically, prior to determination of drug concentration. Brain and serum concentrations of the adenosine A1 receptor antagonist, DPCPX increased in a dose related manner when measured 20 min after intraperitoneal injection. Estimated brain concentrations were 13.8, 87.7 and 288 nM following injection of 0.01, 0.1 and 1.0 mg/kg DPCPX, and serum concentrations were 26.5, 195 and 1370 nM respectively. A time dependent decrease in both brain and serum concentration was noted 20-180 min following injection of 1.0 mg/kg DPCPX. The peripheral adenosine receptor antagonists, 1,3-dipropyl-8-p-sulphophenylxanthine (DPSPX; 5.6 mg/kg) and 8-(p-sulphophenyl)theophylline (8-PST; 20 mg/kg), were not detected in brain tissue 20 min after intraperitoneal injection, despite serum concentrations of 56 and 52 microM respectively. This assay provides a useful and versatile method for determining the central penetration of neuroactive drugs.

Extracellular adenosine accumulates in the environment of numerous tumors. For years, this fact has fueled preclinical research to determine whether adenosine receptors (ARs) could be the target to fight cancer. The four ARs discovered so far, A1, A2A, A2B and A3, belong to the class A family of G protein-coupled receptors (GPCRs) and all four have been involved in one way or another in regulating tumor progression. Prompted by the successful anti-cancer immunotherapy, the focus was placed on the ARs more involved in regulation of immune cell differentiation and activation and that are able to establish molecular and functional interactions. This review focuses on the potential of A2A and A2B receptor antagonists in cancer control and in boosting anti-cancer chemotherapy and immunotherapy. The article also overviews the ongoing clinical trials in which A2AR and A2BR ligands are being tested in anti-cancer therapy.

1. Recent studies have shown beneficial effects of an adenosine A(2A) receptor agonist in dt(sz) mutant hamsters, an animal model of paroxysmal dystonia, in which stress and consumption of coffee can precipitate dystonic attacks. This prompted us to examine the effects of adenosine receptor agonists and antagonists on severity of dystonia in dt(sz) hamsters in more detail. 2. The non-selective adenosine A(1)/A(2A) receptor antagonists, caffeine (10 - 20 mg kg(-1) i.p.) and theophylline (10 - 30 mg kg(-1) s.c.), worsened the dystonia in dt(sz) hamsters. 3. Aggravation of dystonia was also caused by the selective adenosine A(1)/A(2A) antagonist CGS 15943 (9-chloro2-2-furyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine) at a dose of 30 mg kg(-1) i.p. and by the adenosine A(1) antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine; 20 - 30 mg kg(-1) i.p.), while the A(2) antagonist DMPX (3,7-dimethyl-1-propargylxanthine; 2 - 4 mg kg(-1) i.p.) and the highly selective A(2A) antagonist ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol; 2 - 5 mg kg(-1) i.p.) failed to exert any effects on dystonia. 4. In contrast to the antagonists, both the adenosine A(1) receptor agonist CPA (N(6)-cyclopentyladenosine; 0.1 - 1.0 mg kg(-1) i.p.) and the A(2A) agonist CGS 21680 (2p-(2carboxyethylphen-ethylamino-5'-N-ethylcarboxamindoadenosine; 0.1 - 2.0 mg kg(-1) i.p.) exerted a striking improvement of dystonia. 5. These data suggest that the precipitating effects of methylxanthines are, at least in part, related to their adenosine receptor antagonistic action. 6. Although adenosine receptor agonists can be regarded as interesting candidates for the therapy of paroxysmal dystonia, adverse effects may limit the therapeutic potential of adenosine A(1) agonists, while beneficial effects of the adenosine A(2A) agonist CGS 21680 were already found at well tolerated doses.

Chronic inflammation plays an important role in the development of neurodegenerative diseases, such as Parkinson's disease (PD). In the present study, we synthesized 25 novel xanthine derivatives with variable substituents at the N1-, N3- and C8-position as adenosine receptor antagonists with potential anti-inflammatory activity. The compounds were investigated in radioligand binding studies at all four human adenosine receptor subtypes, A1, A2A, A2B and A3. Compounds showing nanomolar A2A and dual A1/A2A affinities were obtained. Three compounds, 19, 22 and 24, were selected for further studies. Docking and molecular dynamics simulation studies indicated binding poses and interactions within the orthosteric site of adenosine A1 and A2A receptors. In vitro studies confirmed the high metabolic stability of the compounds, and the absence of toxicity at concentrations of up to 12.5 µM in various cell lines (SH-SY5Y, HepG2 and BV2). Compounds 19 and 22 showed anti-inflammatory activity in vitro. In vivo studies in mice investigating carrageenan- and formalin-induced inflammation identified compound 24 as the most potent anti-inflammatory derivative. Future studies are warranted to further optimize the compounds and to explore their therapeutic potential in neurodegenerative diseases.

The study shows effects of the nonselective adenosine A1/A2A receptor antagonist caffeine and the selective A2A receptor antagonist KW6002 on LPS-induced changes in the extracellular levels of dopamine (DA), glutamate, adenosine, hydroxyl radical, and A2A receptor density in the rat striatum. Intrastriatal LPS (10 μg) injection decreased extracellular level of DA and increased the level of adenosine, glutamate, and hydroxyl radical on the ipsilateral side 24 h after LPS administration. Caffeine (10 and 20 mg/kg i.p.) and KW6002 (1.5 and 3 mg/kg i.p.) given once daily for 6 days and on the 7th day 2 h before and 4 h after LPS injection reversed the LPS-induced changes in extracellular levels of DA, adenosine, glutamate, and hydroxyl radical production. Moreover, LPS-induced decrease in the striatal A2A receptor density was increased by caffeine and KW6002. In order to show the late LPS effect on oxidative damage of DA neurons, the contents of DA, DOPAC, HVA, and hydroxyl radical were determined 72 h after LPS (10 μg) administration into both striata. LPS decreased striatal and substantia nigra content of DA, DOPAC, and HVA while increased striatal but not nigral content of hydroxyl radical. Caffeine (20 mg/kg) and KW60002 (3 mg/kg) given once daily for 6 days and on the 7th day 2 h before and 4 h after intrastriatal injection of LPS normalized the content of DA and its metabolites in both brain regions as well as decreased LPS-induced increase in the striatal level of hydroxyl radical. In conclusion, our data demonstrated antioxidant effects of caffeine and KW6002 in the inflammatory model of PD.

Liver cirrhosis is often complicated by an impaired renal excretion of water and sodium. Diuretics tend to further deteriorate renal function. It is unknown whether chronic selective adenosine A(1) receptor blockade, via inhibition of the hepatorenal reflex and the tubuloglomerular feedback, might exert diuretic and natriuretic effects without a reduction of the glomerular filtration rate. In healthy animals intravenous treatment with the novel A(1) receptor antagonist SLV329 resulted in a strong dose-dependent diuretic (up to 3.4-fold) and natriuretic (up to 13.5-fold) effect without affecting creatinine clearance. Male Wistar rats with thioacetamide-induced liver cirrhosis received SLV329, vehicle or furosemide for 12 weeks. The creatinine clearance of cirrhotic animals decreased significantly (-36.5%, p<0.05), especially in those receiving furosemide (-41.9%, p<0.01). SLV329 was able to prevent this decline of creatinine clearance. Mortality was significantly lower in cirrhotic animals treated with SLV329 in comparison to animals treated with furosemide (17% vs. 54%, p<0.05). SLV329 did not relevantly influence the degree of liver fibrosis, kidney histology or expression of hepatic or renal adenosine receptors. In conclusion, chronic treatment with SLV329 prevented the decrease of creatinine clearance in a rat model of liver cirrhosis. Further studies will have to establish whether adenosine A(1) receptor antagonists are clinically beneficial at different stages of liver cirrhosis.

Recent studies have underscored the importance of the CA2 area in social memory formation. This area, a narrow transition zone between hippocampal CA3 and CA1 areas, is endowed with special connectivity and a distinctive molecular composition. In particular, adenosine A1 receptors (A1R) are enriched in CA2, and based on the prominent synaptic potentiation induced by A1R antagonists (e.g., caffeine) in this area, it has been proposed that CA2 is under the strong tonic control of A1R activation. It is unclear whether this special sensitivity of CA2 to A1R antagonists is due to an elevated extracellular concentration of adenosine or to a different A1R function. Here, using the recording of field potentials evoked simultaneously in CA2 and CA1 by Schaffer collateral stimulation, we confirm that the application of A1R antagonists, caffeine and DPCPX has a stronger effect on synaptic responses in CA2 than in those evoked in CA1. This difference was, at least partially, explained by the action of A1R antagonists on presynaptic A1Rs. We found that caffeine-induced potentiation in CA2 was restricted to Schaffer collateral synapses, but not to those formed by temporoammonic inputs. We also observed that the apparent affinity of an A1R agonist is similar for A1R in both CA2 and CA1 areas, which indicates that the tonic activation of A1R in both areas is comparable. Furthermore, we show that the direct activation of adenylyl cyclase with forskolin in the presence of rolipram, a phosphodiesterase inhibitor, greatly enhances the synaptic potentials in CA2 compared to CA1. The forskolin-induced potentiation was exacerbated in the presence of caffeine or DPCPX, accentuating the differences between the two areas. These results indicate that the tonic activation of A1Rs in area CA2 is not different to that of other hippocampal areas, but it is more efficiently coupled to the downstream effectors.

Acute or chronic administration of guanosine (GUO) induces anxiolytic-like effects, for which the adenosine (ADO) system involvement has been postulated yet without a direct experimental evidence. Thus, we aimed to investigate whether adenosine receptors (ARs) are involved in the GUO-mediated anxiolytic-like effect, evaluated by three anxiety-related paradigms in rats. First, we confirmed that acute treatment with GUO exerts an anxiolytic-like effect. Subsequently, we investigated the effects of pretreatment with ADO or A1R (CPA, CCPA) or A2AR (CGS21680) agonists 10 min prior to GUO on a GUO-induced anxiolytic-like effect. All the combined treatments blocked the GUO anxiolytic-like effect, whereas when administered alone, each compound was ineffective as compared to the control group. Interestingly, the pretreatment with nonselective antagonist caffeine or selective A1R (DPCPX) or A2AR (ZM241385) antagonists did not modify the GUO-induced anxiolytic-like effect. Finally, binding assay performed in hippocampal membranes showed that [3H]GUO binding became saturable at 100-300 nM, suggesting the existence of a putative GUO binding site. In competition experiments, ADO showed a potency order similar to GUO in displacing [3H]GUO binding, whereas AR selective agonists, CPA and CGS21680, partially displaced [3H]GUO binding, but the sum of the two effects was able to displace [3H]GUO binding to the same extent of ADO alone. Overall, our results strengthen previous data supporting GUO-mediated anxiolytic-like effects, add new evidence that these effects are blocked by A1R and A2AR agonists and pave, although they do not elucidate the mechanism of GUO and ADO receptor interaction, for a better characterization of GUO binding sites in ARs.