Similarities of adenosine uptake systems in astrocytes and neurons in primary cultures

Purinergic mechanisms in neuroinflammation: An update from molecules to behavior

The principle functions of neuroinflammation are to limit tissue damage and promote tissue repair in response to pathogens or injury. While neuroinflammation has utility, pathophysiological inflammatory responses, to some extent, underlie almost all neuropathology. Understanding the mechanisms that control the three stages of inflammation (initiation, propagation and resolution) is therefore of critical importance for developing treatments for diseases of the central nervous system. The purinergic signaling system, involving adenosine, ATP and other purines, plus a host of P1 and P2 receptor subtypes, controls inflammatory responses in complex ways. Activation of the inflammasome, leading to release of pro-inflammatory cytokines, activation and migration of microglia and altered astroglial function are key regulators of the neuroinflammatory response. Here, we review the role of P1 and P2 receptors in mediating these processes and examine their contribution to disorders of the nervous system. Firstly, we give an overview of the concept of neuroinflammation. We then discuss the contribution of P2X, P2Y and P1 receptors to the underlying processes, including a discussion of cross-talk between these different pathways. Finally, we give an overview of the current understanding of purinergic contributions to neuroinflammation in the context of specific disorders of the central nervous system, with special emphasis on neuropsychiatric disorders, characterized by chronic low grade inflammation or maternal inflammation. An understanding of the important purinergic contribution to neuroinflammation underlying neuropathology is likely to be a necessary step towards the development of effective interventions. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Glycogenolysis and purinergic signaling

Both ATP and glutamate are on one hand essential metabolites in brain and on the other serve a signaling function as transmitters. However, there is the major difference that the flux in the pathway producing transmitter glutamate is comparable to the rate of glucose metabolism in brain, whereas that producing transmitter ATP is orders of magnitude smaller than the metabolic turnover between ATP and ADP. Moreover, de novo glutamate production occurs exclusively in astrocytes, whereas transmitter ATP is produced both in neurons and astrocytes. This chapter deals only with ATP and exclusively with its formation and release in astrocytes, and it focuses on potential associations with glycogenolysis, which is known to be indispensable for the synthesis of glutamate. Glycogenolysis is dependent upon an increase in free intracellular Ca(2+) concentration (Ca(2+)]i). It can be further stimulated by cAMP, but in contrast to widespread beliefs, cAMP can on its own not induce glycogenolysis. Astrocytes generate ATP from accumulated adenosine, and this process does not seem to require glycogenolysis. A minor amount of the generated ATP is utilized as a transmitter, and its synthesis requires the presence of the mainly intracellular nucleoside transporter ENT3. Many transmitters as well as extracellular K(+) concentrations high enough to open the voltage-sensitive L-channels for Ca(2+) cause a release of transmitter ATP from astrocytes. Adenosine and ATP induce release of ATP by action at several different purinergic receptors. The release evoked by transmitters or elevated K(+) concentrations is abolished by DAB, an inhibitor of glycogenolysis.

Adenosine and adenosine uptake inhibitors potentiate the neuromuscular blocking action of rocuronium mediated by adenosine A(1) receptors in isolated rat diaphragms

BACKGROUND

Adenosine, which pre-junctionally modulates neuromuscular transmission, and adenosine uptake inhibitors, which increase extracellular adenosine, have been used clinically. We investigated the effects of adenosine, dipyridamole and midazolam on the neuromuscular blocking action of rocuronium.

METHODS

Isometric twitch tensions of rat nerve-hemidiaphragm preparations elicited by indirect (phrenic nerve) supra-maximal stimulation at 0.1 Hz were evaluated (n=6 in all data).

RESULTS

Pre-treatments with adenosine (0.1 and 1 microM) and CCPA (1 microM, adenosine A(1) receptor agonist), but not that with CGS21680 (0.5 microM, A(2) receptor agonist), shifted the rocuronium concentration-twitch tension curves to the left and decreased the rocuronium concentration for 50% twitch depression (IC(50)) compared with the control (P<0.01). The leftward shift induced by 1 microM adenosine was inhibited by pre-treatments with theophylline (50 microM, non-selective adenosine receptor antagonist) and DPCPX (0.2 microM, A(1) receptor antagonist) but not by that with DPMA (5 microM, A(2) receptor antagonist). Pre-treatments with dipyridamole and midazolam, adenosine uptake inhibitors, shifted the curve to the left and decreased IC(50) at supra-therapeutic concentrations (10 and 2.5 microM, respectively) but not at clinical concentrations (2 and 0.5 microM, respectively), and the leftward shifts were inhibited by pre-treatment with DPCPX (0.2 microM).

CONCLUSION

The results indicate that adenosine potentiates the neuromuscular blocking action of rocuronium mediated by adenosine A(1) receptors and that supra-therapeutic concentrations of dipyridamole and midazolam also potentiate the action of rocuronium by increasing endogenous adenosine concentration.

Adenosine released by astrocytes contributes to hypoxia-induced modulation of synaptic transmission

Astrocytes play a critical role in brain homeostasis controlling the local environment in normal as well as in pathological conditions, such as during hypoxic/ischemic insult. Since astrocytes have recently been identified as a source for a wide variety of gliotransmitters that modulate synaptic activity, we investigated whether the hypoxia-induced excitatory synaptic depression might be mediated by adenosine release from astrocytes. We used electrophysiological and Ca2+ imaging techniques in hippocampal slices and transgenic mice, in which ATP released from astrocytes is specifically impaired, as well as chemiluminescent and fluorescence photometric Ca2+ techniques in purified cultured astrocytes. In hippocampal slices, hypoxia induced a transient depression of excitatory synaptic transmission mediated by activation of presynaptic A1 adenosine receptors. The glia-specific metabolic inhibitor fluorocitrate (FC) was as effective as the A1 adenosine receptor antagonist CPT in preventing the hypoxia-induced excitatory synaptic transmission reduction. Furthermore, FC abolished the extracellular adenosine concentration increase during hypoxia in astrocyte cultures. Several lines of evidence suggest that the increase of extracellular adenosine levels during hypoxia does not result from extracellular ATP or cAMP catabolism, and that astrocytes directly release adenosine in response to hypoxia. Adenosine release is negatively modulated by external or internal Ca2+ concentrations. Moreover, adenosine transport inhibitors did not modify the hypoxia-induced effects, suggesting that adenosine was not released by facilitated transport. We conclude that during hypoxia, astrocytes contribute to regulate the excitatory synaptic transmission through the release of adenosine, which acting on A1 adenosine receptors reduces presynaptic transmitter release. Therefore, adenosine release from astrocytes serves as a protective mechanism by down regulating the synaptic activity level during demanding conditions such as transient hypoxia.

Nucleoside transporter expression and function in cultured mouse astrocytes

Uptake of purine and pyrimidine nucleosides in astrocytes is important for several reasons: (1) uptake of nucleosides contributes to nucleic acid synthesis; (2) astrocytes synthesize AMP, ADP, and ATP from adenosine and GTP from guanosine; and (3) adenosine and guanosine function as neuromodulators, whose effects are partly terminated by cellular uptake. It has previously been shown that adenosine is rapidly accumulated by active uptake in astrocytes (Hertz and Matz, Neurochem Res 14:755-760, 1989), but the ratio between active uptake and metabolism-driven uptake of adenosine is unknown, as are uptake characteristics for guanosine. The present study therefore aims at providing detailed information of nucleoside transport and transporters in primary cultures of mouse astrocytes. Reverse transcription-polymerase chain reaction identified the two equilibrative nucleoside transporters, ENT1 and ENT2, together with the concentrative nucleoside transporter CNT2, whereas CNT3 was absent, and CNT1 expression could not be investigated. Uptake studies of tritiated thymidine, formycin B, guanosine, and adenosine (3-s uptakes at 1-4 degrees C to study diffusional uptake and 1-60-min uptakes at 37 degrees C to study concentrative uptake) demonstrated a fast diffusional uptake of all four nucleosides, a small, Na(+)-independent and probably metabolism-driven uptake of thymidine (consistent with DNA synthesis), larger metabolism-driven uptakes of guanosine (consistent with synthesis of DNA, RNA, and GTP) and especially of adenosine (consistent with rapid nucleotide synthesis), and Na(+)-dependent uptakes of adenosine (consistent with its concentrative uptake) and guanosine, rendering neuromodulator uptake independent of nucleoside metabolism. Astrocytes are accordingly well suited for both intense nucleoside metabolism and metabolism-independent uptake to terminate neuromodulator effects of adenosine and guanosine.

Involvement of astrocytes in purine‐mediated reparative processes in the brain

Astrocytes are involved in multiple brain functions in physiological conditions, participating in neuronal development, synaptic activity and homeostatic control of the extracellular environment. They also actively participate in the processes triggered by brain injuries, aimed at limiting and repairing brain damages. Purines may play a significant role in the pathophysiology of numerous acute and chronic disorders of the central nervous system (CNS). Astrocytes are the main source of cerebral purines. They release either adenine-based purines, e.g. adenosine and adenosine triphosphate, or guanine-based purines, e.g. guanosine and guanosine triphosphate, in physiological conditions and release even more of these purines in pathological conditions. Astrocytes express several receptor subtypes of P1 and P2 types for adenine-based purines. Receptors for guanine-based purines are being characterised. Specific ecto-enzymes such as nucleotidases, adenosine deaminase and, likely, purine nucleoside phosphorylase, metabolise both adenine- and guanine-based purines after release from astrocytes. This regulates the effects of nucleotides and nucleosides by reducing their interaction with specific membrane binding sites. Adenine-based nucleotides stimulate astrocyte proliferation by a P2-mediated increase in intracellular [Ca2+] and isoprenylated proteins. Adenosine also, via A2 receptors, may stimulate astrocyte proliferation, but mostly, via A1 and/or A3 receptors, inhibits astrocyte proliferation, thus controlling the excessive reactive astrogliosis triggered by P2 receptors. The activation of A1 receptors also stimulates astrocytes to produce trophic factors, such as nerve growth factor, S100beta protein and transforming growth factor beta, which contribute to protect neurons against injuries. Guanosine stimulates the output of adenine-based purines from astrocytes and in addition it directly triggers these cells to proliferate and to produce large amount of neuroprotective factors. These data indicate that adenine- and guanine-based purines released in large amounts from injured or dying cells of CNS may act as signals to initiate brain repair mechanisms widely involving astrocytes.

The involvement of adenosine neuromodulation in pentobarbital-induced field excitatory postsynaptic potentials depression in rat hippocampal slices

We investigated the contribution of adenosine neuromodulation to mechanisms of pentobarbital-induced depression of excitatory synaptic transmission in vitro. Transverse hippocampal slices were prepared from brains removed from isoflurane-anesthetized male Wistar rats. Field excitatory postsynaptic potentials (fEPSPs), elicited by orthodromic electrical stimulation of Schaffer collateral at 0.05 Hz, were recorded from the CA1 region in oxygenated artificial cerebrospinal fluid. Amplitude of fEPSP was analyzed for assessing drug effects. Pentobarbital (100 microM) transiently depressed fEPSPs (P<0.01); i.e., fEPSP was initially depressed to approximately 60% of control and then recovered to approximately 80% of control. The fEPSP depression was partially suppressed by pretreatment with 50 microM aminophylline, a nonselective adenosine receptor antagonist, and 0.2 microM 3, 7-Dimethyl-1-propagylxanthine, an adenosine A(1) receptor antagonist (P<0.01 each). However, the fEPSP depression was not affected by pretreatment with 10 microM 8-cyclopentyl-1, 3-dipropylxanthine, an A(2) receptor antagonist, or 10 microM bicuculline, a gamma-aminobutyric acid (GABA) A receptor antagonist. The results indicate that adenosine neuromodulation through A(1) receptors and other undefined mechanisms, which are independent from GABAergic mechanisms, are involved in pentobarbital-induced depression of excitatory synaptic transmission.

Purine uptake and release in rat C6 glioma cells: nucleoside transport and purine metabolism under ATP-depleting conditions

Adenosine, through activation of membrane-bound receptors, has been reported to have neuroprotective properties during strokes or seizures. The role of astrocytes in regulating brain interstitial adenosine levels has not been clearly defined. We have determined the nucleoside transporters present in rat C6 glioma cells. RT-PCR analysis, (3)H-nucleoside uptake experiments, and [(3)H]nitrobenzylthioinosine ([(3)H]NBMPR) binding assays indicated that the primary functional nucleoside transporter in C6 cells was rENT2, an equilibrative nucleoside transporter (ENT) that is relatively insensitive to inhibition by NBMPR. [(3)H]Formycin B, a poorly metabolized nucleoside analogue, was used to investigate nucleoside release processes, and rENT2 transporters mediated [(3)H]formycin B release from these cells. Adenosine release was investigated by first loading cells with [(3)H]adenine to label adenine nucleotide pools. Tritium release was initiated by inhibiting glycolytic and oxidative ATP generation and thus depleting ATP levels. Our results indicate that during ATP-depleting conditions, AMP catabolism progressed via the reactions AMP --> IMP --> inosine --> hypoxanthine, which accounted for >90% of the evoked tritium release. It was surprising that adenosine was not released during ATP-depleting conditions unless AMP deaminase and adenosine deaminase were inhibited. Inosine release was enhanced by inhibition of purine nucleoside phosphorylase; ENT2 transporters mediated the release of adenosine or inosine. However, inhibition of AMP deaminase/adenosine deaminase or purine nucleoside phosphorylase during ATP depletion produced release of adenosine or inosine, respectively, via the rENT2 transporter. This indicates that C6 glioma cells possess primarily rENT2 nucleoside transporters that function in adenosine uptake but that intracellular metabolism prevents the release of adenosine from these cells even during ATP-depleting conditions.

N(6)-2-(4-aminophenyl)ethyl-adenosine enhances the anticonvulsive action of conventional antiepileptic drugs in the kindling model of epilepsy in rats

APNEA [(N(6)-2-(4-aminophenyl)ethyl-adenosine; a non-selective adenosine A(3) receptor agonist; 2-4 mgkg(-1)] had no significant effect on seizure parameters (seizure severity, seizure duration and afterdischarge duration) in amygdala-kindled rats. Subsequently, APNEA was combined with antiepileptic drugs administered at doses ineffective in fully kindled rats. Co-administration of APNEA (0.5-2 mg kg(-1)) with carbamazepine (2.5-20 mg kg(-1)) resulted in the significant reduction of all studied seizure parameters. Moreover, 8-cyclopentyl-1,3-dimethylxanthine 8-CPX (a selective adenosine A(1) receptor antagonist; 5 mg kg(-1)) partially reduced the anticonvulsive activity of a combination of APNEA (2 mg kg(-1)) with carbamazepine (20 mg kg(-1)), but not that of carbamazepine (20 mgkg(-1))+APNEA (0.5 mg kg(-1)). When APNEA (2 mg kg(-1)) was combined with phenobarbital (20 mg kg(-1)), valproate (75 mg kg(-1)) or clonazepam (0.003 mg kg(-1)), seizure and afterdischarge durations were significantly shortened. 8-CPX (5 mg kg(-1)) totally reversed the APNEA (2 mg kg(-1))-induced enhancement of the anticonvulsive action of valproate. However, when the non-selective adenosine A(3) receptor agonist was administered together with diphenylhydantoin, no protection was observed in the kindling model of epilepsy. The interaction at the pharmacokinetic level can be excluded because APNEA did not interfere with the free plasma level of antiepileptics used in this study. It may be concluded that the interaction of APNEA with carbamazepine involves A(3) adenosine receptor-dependent events.

Transient depression of excitatory synaptic transmission induced by adenosine uptake inhibition in rat hippocampal slices

The transient property of the dipyridamole-induced depression of excitatory synaptic transmission was analyzed using field EPSPs (fEPSPs) recorded from the CA1 region in rat hippocampal slices. The fEPSPs were depressed by 1 microM dipyridamole and then gradually recovered to the control level. The depression was antagonized by aminophylline or DPCPX, although it was not significantly affected by DMPX. The results suggest that the fEPSP depression is induced by a mechanism through the A(1) receptor.

Chronic hypoxia enhances adenosine release in rat PC12 cells by altering adenosine metabolism and membrane transport

Acute exposure to hypoxia causes a release of adenosine (ADO) that is inversely related to the O2 levels in oxygen-sensitive pheochromocytoma (PC12) cells. In the current study, chronic exposure (48 h) of PC12 cells to moderate hypoxia (5% O2) significantly enhanced the release of ADO during severe, acute hypoxia (1% O2). Investigation into the intra- and extracellular mechanisms underpinning the secretion of ADO in PC12 cells chronically exposed to hypoxia revealed changes in gene expression and activities of several key enzymes associated with ADO production and metabolism, as well as the down-regulation of a nucleoside transporter. Decreases in the enzymatic activities of ADO kinase and ADO deaminase accompanied by an increase in those of cytoplasmic and ecto-5'-nucleotidases bring about an increased capacity to produce intra- and extracellular ADO. This increased potential to generate ADO and decreased capacity to metabolize ADO indicate that PC12 cells shift toward an ADO producer phenotype during hypoxia. The reduced function of the rat equilibrative nucleoside transporter rENT1 also plays a role in controlling extracellular ADO levels. The hypoxia-induced alterations in the ADO metabolic enzymes and the rENT1 transporter seem to increase the extracellular concentration of ADO. The biological significance of this regulation is unclear but is likely to be associated with modulating cellular activity during hypoxia.

Involvement of the adenosine neuromodulatory system in the benzodiazepine-induced depression of excitatory synaptic transmissions in rat hippocampal neurons in vitro

We investigated whether adenosine neuromodulation is involved in a benzodiazepine (midazolam)-induced depression of excitatory synaptic transmissions in the CA1 and dentate gyrus (DG) regions in rat hippocampal slices. Field excitatory postsynaptic potentials (fEPSPs), evoked by electrical stimulation of the CA1-Schaffer collateral or the DG-perforant path, were recorded with extracellular microelectrodes from CA1-stratum radiatum or DG-stratum moleculare in oxygenated ACSF. The initial slope of the fEPSPs was analyzed for assessing the drug effects. Midazolam (1 microM) transiently depressed CA1- and DG-fEPSPs. The fEPSPs were depressed to approximately 75% of the control values, and then gradually recovered. The depression was not affected by bicuculline, a GABAA receptor antagonist, although it was completely antagonized by aminophylline, an adenosine receptor antagonist. Dipyridamole (5 microM), an adenosine uptake inhibitor, depressed the fEPSPs in a similar manner to midazolam. An adenosine deaminase inhibitor, EHNA, also transiently depressed the fEPSPs, but in a different manner. Exogenous adenosine persistently depressed the fEPSPs. The effects of the drugs were not significantly different in the CA1 and DG regions. The results suggest that midazolam (1 microM) depresses excitatory synaptic transmissions through the adenosine neuromodulatory system by inhibiting adenosine uptake in the CA1 and DG regions of the hippocampus.

Adenosine and the nervous system: pharmacological data and therapeutic perspectives

1. Adenosine acts on a family of G-protein-coupled receptors called purinoreceptors. 2. Four subtypes have been cloned and pharmacologically characterized. 3. The principal pharmacological data and structure-function relations for agonist interactions with P1 receptors are presented. 4. We conclude that the potent role of adenosine in the nervous system may be interesting for the development of drugs targeted at purines and their receptors.

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

Enhancement by benzodiazepines of the inhibitory effect of adenosine on skeletal neuromuscular transmission

1. Interactions of benzodiazepines with adenosine on the neuromuscular transmission were studied in mouse diaphragm preparations. 2. In tubocurarine (0.6-0.8 microM)-partially paralyzed preparations, diazepam (35 microM) and Ro 5-4864 (3-30 microM), a peripheral type benzodiazepine receptor agonist, potentiated the inhibitory effect of adenosine on indirect twitch responses. 3. The central type receptor agonist, clonazepam did not affect the inhibitory effect of adenosine. 4. The peripheral benzodiazepine receptor antagonist, PK11195 (1-10 microM) attenuated the adenosine inhibition and antagonized the potentiation by Ro 5-4864. 5. Ro 5-4864 failed to enhance further the inhibitory effect of adenosine in the presence of dipyridamole, an adenosine uptake inhibitor that also potentiated adenosine inhibition. 6. Neither Ro 5-4864 nor PK 11195 affected the inhibition produced by a stable adenosine analogue, 2-chloroadenosine, which is not a substrate for the adenosine uptake system. 7. Ro 5-4864 did not affect endplate potentials (e.p.ps) in the absence of adenosine, but reduced the amplitude of e.p.ps in the presence of adenosine without affecting miniature e.p.ps. 8. It is suggested that benzodiazepines potentiate the adenosine-effected presynaptic inhibition of neuromuscular transmission by an inhibition of adenosine uptake through activation of peripheral type benzodiazepine receptors.

Ionic dependence of adenosine uptake into cultured astrocytes

Adenosine uptake in cultured astrocytes is dependent on various ions and energy metabolism. The Na(+)-gradient plays an important role, since nigericin, ouabain, amiloride and substitution of Na+ with choline inhibited adenosine uptake. The proton-gradient was of importance, since carbonylcyanide m-chlorophenylhydrozone (CCCP) and omeprazole also inhibited adenosine uptake. Furthermore, adenosine uptake was dependent on Cl- anion. Substitution of Cl- with isethionate, as well as DIDS or furosemide inhibited adenosine uptake. Adenosine uptake was also sensitive to Ca2+ gradient, removal of extracellular Ca2+ and calcimycin inhibited adenosine uptake. Adenosine uptake was not dependent on extracellular K+ and was not affected by valinomycin. Although, K(+)-channel openers (BRL 34195 and nicorandil) as well as the K(+)-channel antagonist, glyburide, inhibited adenosine uptake, the inhibitory effect of BRL 34915 was not antagonized by glyburide. Rotenone and 2,4-dinitrophenol also inhibited adenosine uptake. Ionic dependence and metabolic energy dependence of adenosine uptake suggest that uptake is primarily an active process.

Stimulation of reactive astrogliosis in vivo by extracellular adenosine diphosphate or an adenosine A2 receptor agonist

Adenosine and its nucleotides adenosine triphosphate (ATP) and adenosine diphosphate (ADP) stimulate the proliferation of brain astrocytes in vitro and augment the effects of other growth factors. Following brain injury, hypoxia, or around solid tumors with necrotic centers, such as glioblastoma multiformes, high concentrations of adenine nucleotides and adenosine are released into the extracellular space; extracellular adenosine concentrations can rise 30-100-fold to a concentration in excess of 100 microM. Increased concentrations of extracellular adenosine and adenine nucleotides may contribute to reactive astrocytic proliferation following brain injury. To test this hypothesis, adenosine, an adenosine analog 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA), or ADP was micro-injected into rat cortex. The number of glial fibrillary acidic protein-immunopositive cells was compared between the treated and contralateral saline-injected hemispheres. Within 48 hr, astrocyte density around the CPCA (100 microM) infusion site was almost double that around the control saline infusion site. In hemispheres into which CPCA was infused, there was an increase in astrocytes in the subpial region along fiber tracts and around blood vessels, characteristic of Scherer's secondary structures found in association with malignant astrocytic brain tumors. The increased astrogliosis elicited by CPCA was abolished by coinfusion of the adenosine A2 receptor antagonist 1,3-dipropyl-7-methylxanthine (DPMX). While microinjection of adenosine (1 mM) failed to stimulate astrogliosis, microinjection of ADP (500 microM) also resulted in a significant reactive astrogliosis and accumulation of astrocytes similar to Scherer's secondary structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of adenosine receptors in a model of cultured neurons from rat forebrain

The neuromodulator adenosine is acting through specific receptors coupled to adenylate cyclase via G-proteins. The expression of both adenosine receptors A1 and A2 as well as forskolin binding sites was investigated by radioligand binding techniques in 8-day-old neurons isolated from fetal rat forebrain and cultured in chemically-defined medium. Adenosine A1 receptors were specifically labeled with [3H]chloro-N6-cyclopentyladenosine (CCPA), whereas [3H]CGS 21680 was used for the analysis of A2 receptors. Cultured neurons exhibited high affinity binding sites for CCPA (Bmax = 160 fmol/mg protein; Kd = 2.9 nM), and for CGS 21680 (Bmax = 14 fmol/mg protein; Kd = 1.7 nM). These data correlate well with those obtained in crude membranes isolated from the newborn rat forebrain. The incubation of culture membranes in the additional presence of guanylyl-5'-imidodiphosphate (Gpp(NH)p, a GTP analogue) led to significantly increased Kd-values, suggesting the association of adenosine receptors with G-proteins. Finally, cultured neurons also bound specifically [3H]forskolin with characteristics close to those found in the newborn brain, indicating that cultured neurons appear as an appropriate model for studying the neuromodulatory properties of adenosine.

Nucleoside transport sites in a cultured human retinal cell line established by SV-40 T antigen gene

Adenosine, an important neuromodulatory compound in the brain and retina, is a potent vasodilator in most vascular beds throughout the body. Its actions are potentiated by inhibitors of nucleoside transport into cells. Knowledge of the existence of specific adenosine uptake systems in mammalian retina and the inhibition of the uptake by nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport, raises the possibility that the associated nucleoside transport system may be of pharmacological importance in retinal function. We have characterized the binding of the nucleoside transporter probe, [3H]NBMPR, to a cultured human retinal cell line established by transfection of SV-40 T antigen plasmid-DNA. The binding was specific, saturable and reversible. Scatchard analysis of the saturation data revealed that NBMPR binds to a homogeneous population of high affinity binding sites (KD = 0.65 +/- 0.22 nM; Bmax = 466 +/- 157 fmol/mg protein) characteristically similar to the binding sites in human retinal tissue (KD = 0.32 +/- 0.01 nM; Bmax = 292 +/- 41 fmol/mg protein). Selected compounds inhibited the binding in the cell line and retinal tissue with the same rank order of potency, suggesting that the transporters in the cell line and retinal tissue are similar. The data showed that the cell line is a useful model for the study of nucleoside transporter function in human retina.

Morphine-evoked release of adenosine from the spinal cord occurs via a nucleoside carrier with differential sensitivity to dipyridamole and nitrobenzylthioinosine

We have investigated the potential role of a bi-directional nucleoside carrier in the release of endogenous adenosine from spinal cord synaptosomes by examining the effects of dipyridamole and nitrobenzylthioinosine (NBI) on evoked release of adenosine. When 40 pmol adenosine were added to synaptosomes, only 70 +/- 2% was recovered, suggesting 30% uptake of adenosine. Dipyridamole (0.1-10 microM) reduced this uptake and also increased basal adenosine release, probably due to inhibition of the re-uptake of adenosine derived from released nucleotide. In contrast, NBI (0.1-10 microM) had no effect on either uptake of added adenosine or on basal release of adenosine. Addition of K+ (24 mM) and morphine (10 microM) produced a 50-60% increase in the release of adenosine, and this was reduced 35-98% by both dipyridamole and NBI (0.01-10 microM). Dipyridamole (0.01-1 microM) had no effect on the release of nucleotides (detected as adenosine) induced by noradrenaline, 5-hydroxytryptamine (5-HT) and capsaicin (50 microM each), although 10 microM dipyridamole significantly reduced release evoked by noradrenaline and 5-HT. This latter effect of dipyridamole was determined not to be due to inhibition of ATP release when measured directly. Within the spinal cord, there is a removal system for adenosine which is dipyridamole-sensitive but NBI-insensitive. Release of adenosine, but not nucleotides, appears to occur via this carrier system. The inhibition of release by NBI, but its lack of effect on uptake, suggests the involvement of heterogeneous carrier molecules in adenosine uptake and release from the spinal cord.

Effects of benzodiazepine administration on A1 adenosine receptor binding in-vivo and ex-vivo

The adenosine receptor has been implicated in the central mechanism of action of benzodiazepines. The specific binding of an A1-selective adenosine antagonist radioligand, [3H]8-cyclopentyl-1,3-dipropylxanthine, was measured in-vivo in mice treated with alprazolam (2 mg kg-1, i.p.), lorazepam (2 mg kg-1, i.p.) and vehicle. Binding studies were performed in-vivo and ex-vivo in mice receiving continuous infusion of alprazolam (2 mg kg-1 day-1), lorazepam (2 mg kg-1 day-1) and vehicle by mini-osmotic pumps for 6 days. Continuous infusion of alprazolam and lorazepam significantly decreased specific binding by 34 and 53%, respectively, compared with vehicle treatment (P less than 0.01). Single doses of alprazolam and lorazepam induced a similar trend in specific binding in-vivo (P = 0.07). There were no alterations in A1-receptor density (Bmax) or affinity (Kd) in cortex, hippocampus or brainstem in ex-vivo studies. Benzodiazepine treatment may diminish A1- receptor binding in-vivo by inhibiting adenosine uptake or by direct occupancy of the A1 adenosine receptor recognition site.

Metabolism of extracellular adenine nucleotides by cultured rat brain astrocytes

Intact astrocytes cultured from newborn rat cerebral cortex rapidly converted extracellular ATP to ADP. The ATPase responsible was apparently not saturated, even at 750 microM ATP. In contrast, the conversion of ADP to AMP was slow, and the reaction was limiting for the subsequent dephosphorylation process. Adenosine formation was the only fate for AMP. The reaction was catalyzed by 5'-nucleotidase with an apparent Km of 55 microM for AMP and appeared to be inhibited by high concentrations of ATP and ADP. Astrocytes were able to take up adenosine with an apparent Km value of 45 microM. Uptake was inhibited by dipyridamole but not by anti-5'-nucleotidase IgG. The results support the proposal that astrocytes play a role in modulating synaptic events involving ATP and adenosine.

Adenosine in vertebrate retina: localization, receptor characterization, and function

1. The uptake of [3H] adenosine into specific populations of cells in the inner retina has been demonstrated. In mammalian retina, the exogenous adenosine that is transported into cells is phosphorylated, thereby maintaining a gradient for transport of the purine into the cell. 2. Endogenous stores of adenosine have been demonstrated by localization of cells that are labeled for adenosine-like immunoreactivity. In the rabbit retina, certain of these cells, the displaced cholinergic, GABAergic amacrine cells, are also labeled for adenosine. 3. Purines are tonically released from dark-adapted rabbit retinas and cultured embryonic chick retinal neurons. Release is significantly increased with K+ and neurotransmitters. The evoked release consists of adenosine, ATP, and purine metabolites, and while a portion of this release is Ca2+ dependent, one other component may occur via the bidirectional purine nucleoside transporter. 4. Differential distributions of certain enzymes involved in purine metabolism have also been localized to the inner retina. 5. Heterogeneous distributions of the two subtypes of adenosine receptors, A1 and A2, have been demonstrated in the mammalian retina. Coupling of receptors to adenylate cyclase has also been demonstrated. 6. Adenosine A1 receptor agonists significantly inhibit the K(+)-stimulated release of [3H]-acetylcholine from the rabbit retina, suggesting that endogenous adenosine may modulate the light-evoked or tonic release of ACh.

Do adenosinergic substrates mediate methylxanthine effects upon reinforcement thresholds for electrical brain stimulation in the rat?

Caffeine and other methylxanthines elevate reinforcement threshold for electrical brain stimulation with an order of potency suggesting that the effect is mediated by antagonism of adenosine A2 receptors. The purpose of this study was to evaluate further the possible mechanism by which caffeine and other methylxanthines elevate reinforcement thresholds for ICSS. Drugs known to affect adenosinergic transmission in predictable ways, adenosine receptor agonists and antagonists and benzodiazepine agonists and inverse agonists, were tested to determine their effect upon reinforcement threshold. Both the selective A1 adenosine agonist, R(-)-PIA, and the non-selective A1/A2 adenosine agonist NECA failed to alter reinforcement thresholds, as did CGS 15943, a potent non-xanthine non-selective adenosine receptor antagonist. Chlordiazepoxide, a benzodiazepine agonist, lowered reinforcement thresholds and FG 7142, a benzodiazepine inverse agonist, elevated reinforcement thresholds, perhaps corresponding to their anxiolytic and anxiogenic subjective effects in humans. However, another benzodiazepine agonist, midazolam and another inverse agonist, beta-CCE, did not alter reinforcement thresholds. These results fail to support a general role for adenosinergic systems in the threshold-elevating effect of methylxanthines.

Catecholaminergic traits of chick sympathetic neurons may be differentially regulated by a cGMP-dependent pathway

The purine metabolites inosine and adenosine selectively increase the catecholamine, but not the acetylcholine production in cultured chick superior cervical ganglion neurons via an as yet unknown intracellular pathway. In order to elucidate some of the molecular events involved in this differential regulation of neurotransmitter production by purines, the SCG neurons were cultured in the presence of cyclic nucleotide analogs and activators of adenylate and guanylate cyclase. Neither 8-bromo-cyclic AMP (8-Br-cAMP), 8-bromo-cyclic GMP (8-Br-cGMP), or forskolin, an activator of adenylate cyclase, could mimic the effect of inosine, i.e. differentially increase catecholamine production. Sodium nitroprusside, an activator of guanylate cyclase, however, has a strong potentiating action on the effect of inosine. The noradrenergic properties of chick sympathetic neurons may thus be differentially modulated by a cGMP-dependent pathway.

The accumulation of [3H]phenylisopropyl adenosine ([3H]PIA) and [3H]adenosine into rabbit retinal neurons is inhibited by nitrobenzylthioinosine (NBI)

Uptake of [3H]adenosine and [3H]R-phenylisopropyladenosine (R-PIA) into retinal cells was assessed autoradiographically, in the presence and absence of the purine nucleoside transport inhibitor, nitrobenzylthioinosine (NBI). Under control conditions, both purine nucleosides were accumulated in cell bodies localized to the ganglion cell layer, and the inner nuclear layer. In the presence of NBI, significantly less accumulation of nucleosides within cell bodies was observed, particularly within the inner nuclear layer, suggesting that most of the uptake occurred via the transport of both substrates. The stereoisomer of adenosine, L-[3H]adenosine, was not accumulated into retinal cells consistent with the view that the accumulation of both adenosine and R-PIA occurs via the purine nucleoside transporter.

Radioligand binding to adenosine receptors and adenosine uptake sites in different brain regions of normal and narcoleptic dogs

The present study compares the characteristics of radioligand binding to adenosine receptors and adenosine uptake sites in 100- and 50-day-old normal and narcoleptic dogs. Binding to A1 receptors was quantified using a selective A1 agonist ([3H]N6-[(R)-1-methyl-2-phenylethyl] adenosine, [3H]R-PIA) and an antagonist ([3H]dipropyl-8-cyclopentyl-xanthine, [3H]CPX). Differences in the binding of [3H]R-PIA and that of [3H]5'-ethylcarboxamide adenosine ([3H]NECA), which binds to both A1 and A2 receptors with similar affinities, were used to quantify A2 receptors. Nucleoside transport sites were labeled with [3H]nitrobenzylthioinosine ([3H]NBTI), a potent inhibitor of nucleoside transport systems. The present study offered no evidence that either adenosine A1 receptors and adenosine uptake sites in the frontal cortex or adenosine A2 receptors in the putamen were altered in narcoleptic dogs. However, we found that adenosine A1 receptors in the dog exist in different affinity states and that the affinity state in which the receptor is found depends on the brain region examined. A characterization of these low- and high-affinity sites was performed and results indicated that these sites cannot be explained by a single interaction of the A1 receptor with a single G-protein population.

Physiological and pharmacological properties of adenosine: therapeutic implications

Adenosine is a nucleoside which has been shown to participate in the regulation of physiological activity in a variety of mammalian tissues, and has been recognized as a homeostatic neuromodulator. It exerts its actions via membrane-bound receptors which have been characterized using biochemical, electrophysiological and radioligand binding techniques. Adenosine has been implicated in the pharmacological actions of several classes of drugs. A number of studies strongly suggest that the nucleoside may regulate cellular activity in many pathological disorders and, in that respect, adenosine derivatives appear as promising candidates for the development of new therapeutic compounds, such as anticonvulsant, anti-ischemic, analgesic and neuroprotective agents.

The effect of soluflazine on adenosine receptors in the rat brain

Soluflazine, a potent adenosine transport inhibitor, was intracerebroventricularly administered to rats via ALZET mini osmotic pumps (4nmole, 0.5 L/hr) for 14 days and the effect on adenosine receptors was determined in specific brain areas. Soluflazine decreased adenosine A1 radioligand binding in the hippocampus as measured by [3H]R-PIA, and lowered adenosine A2 binding sites in the striatum, as estimated by the "NECA minus R-PIA" assay. Previous work from our lab has shown the ability of diazepam and triazolam to decrease adenosine binding in the same brain areas. The data show that a specific adenosine transport inhibitor produces the same effect on adenosine receptors as benzodiazepines, and suggest a role for adenosine in the CNS effects of benzodiazepines.

Adenosine analogs attenuate tolerance-dependence on alprazolam

1. Tolerance to and physical dependence on alprazolam were induced in mice by administering two doses of a slow release preparation. 2. Physical dependence was evaluated by the abstinence syndrome induced by flumazenil. Tolerance was studied by measuring the motor incoordination induced by a test dose of alprazolam. 3. The intensity of tolerance was decreased by the administration of L-phenylisopropyl adenosine (L-PIA), cyclopentyl adenosine (CPA), cyclohexyl adenosine (CHA), N-ethylcarboxamide adenosine (NECA), 8-phenyltheophylline (8-PTP) and theophylline (TP). 4. The intensity of the abstinence syndrome induced by flumazenil was attenuated by L-PIA, CPA NECA, TP and 8-PTP. 5. The results suggest that benzodiazepines may exert, at least in part, their effects by involving adenosine in the central nervous system.

Adenosine transport by rat and guinea pig synaptosomes: basis for differential sensitivity to transport inhibitors

Adenosine transport by rat and guinea pig synaptosomes was studied to establish the basis for the marked differences in the potency of some transport inhibitors in these species. An analysis of transport kinetics in the presence and absence of nitrobenzylthioinosine (NBTI) using synaptosomes derived from several areas of rat and guinea pig brain indicated that at least three systems contributed to adenosine uptake, the Km values of which were approximately 0.4, 3, and 15 microM in both species. In both species, the system with the Km of 3 microM was potently (IC50 of approximately 0.3 nM) and selectively inhibited by NBTI. This NBTI-sensitive system accounted for a greater proportion of the total uptake in the guinea pig than in the rat and was inhibited by dipyridamole, mioflazine, and related compounds more potently in the guinea pig. Preliminary experiments with other species indicate that adenosine transport in the mouse is similar to that in the rat, whereas in the dog and rabbit, it is more like that in the guinea pig. In the rat, none of the systems appeared to require Na+, but the two systems possessing the higher affinities for adenosine were inhibited by veratridine- and K(+)-induced depolarization. The transport systems were active over a broad pH range, with maximal activity between pH 6.5 and 7.0. Our results are consistent with the possibility that adenosine transport systems may be differentiated into uptake and release systems.

Effects of adenosine deaminase inhibition on active uptake and metabolism of adenosine in astrocytes in primary cultures

The use of a relatively specific adenosine deaminase inhibitor, 2'-deoxycoformycin (1.0 microM), has revealed an active transport of adenosine into astrocytes in primary cultures. The abolishment of part of the metabolic degradation and of a concentration gradient, which may favour influx, did not lead to a decreased total uptake. The concentration of labelled, i.e. exchangeable adenosine rose to become several fold higher than in the medium. Thus, as previously shown in neurons, the uptake of adenosine into astrocytes occurs by an active and concentrative process. As a result of the increase in the adenosine concentration when the inhibitor was present, evidence for an increased phosphorylation to the nucleotides (i.e. ATP, ADP, AMP) was obtained. This is in contrast to previous findings in neurons where the incorporation of labelled adenosine into these compounds was decreased in the presence of 2'-deoxycoformycin. This difference may suggest that the salvage pathway from inosine to adenine nucleotides is crucial for nucleotide synthesis in neurons, but not in astrocytes.

All nucleoside transporters in bovine chromaffin cells are nitrobenzylthioinosine sensitive

Nitrobenzylthioinosine is an effective inhibitor of adenosine transport in chromaffin cells. When adenosine transport was measured at a 0.15 microM adenosine concentration, in the presence of variable concentrations of nitrobenzylthioinosine, ranging from 10(-14) to 10(-6) M, a half-maximal inhibitory concentration value (IC50) of 1 +/- 0.3 nM was deduced. This compound has the capacity to inhibit the total adenosine transport at 10(-7) M concentration. Nitrobenzylthioinosine acts in a non-competitive manner in blocking adenosine transport, as deduced from a Dixon plot, with a constant inhibition value (K1) of 0.01 +/- 0.003 nM. The results suggest that all nucleoside transporters present in bovine chromaffin cells are sensitive to the nitrobenzylthioinosine inhibition.

Effects of prolonged administration of triazolam on adenosine A1 and A2 receptors in the brain of rats

Continuous subcutaneous administration of triazolam, a benzodiazepine with short plasma half-life, for 10 days either decreased (31%, 2 mg/day) or increased (15%, 0.5 mg/day) radioligand binding to adenosine A2 receptors in the rat striatum. In a similar manner, we have shown previously that diazepam (5-10 mg/day), a benzodiazepine with a long plasma half-life attenuated radioligand binding to adenosine A2 receptors in the rat striatum by 45-25%.

Adenosine metabolism in neurons and astrocytes in primary cultures

Metabolic fate of [8-14C]adenosine was studied in primary cultures of either astrocytes or neurons from the mouse brain. In astrocytes the main metabolic route was the formation of nucleotides. Thus, synthesis of adenosine triphosphate (ATP) amounted to about 0.2 nmol X min-1 X mg-1 protein. The deamination occurred less rapidly. The total rate of formation of inosine was difficult to establish because a considerable amount of labeled inosine accumulated in the medium. The initial incorporation of radioactivity into inosine in the medium was extremely rapid, probably because of the action of an ectoenzyme. However, the labeling of inosine in the medium also continued to increase slowly throughout the incubation, maybe as a result of release of intracellularly formed inosine. The total inosine formation rate during the incubation amounted to at most 0.1 nmol X min-1 X mg-1. Hypoxanthine was formed at a corresponding rate but was released to a lesser extent. In neurons much less label was incorporated into ATP. The major metabolite was inosine, formed intracellularly at a rate of 0.2 nmol X min-1 X mg-1. In addition, there was an immediate rapid labeling of inosine (and to a lesser extent hypoxanthine) in the medium, again suggesting the action of an ectoenzyme. Neither neurons nor astrocytes released a measurable amount of nucleotides to the medium. The cellular differences in adenosine metabolism are probably of relevance for the interpretation of adenosine metabolism in brain in situ. The ectoenzyme may be of importance for rapid termination of the neuromodulator activity of adenosine, and the rapid nucleotide formation in astrocytes is in agreement with a high metabolic activity of these cells.

Inhibition of adenosine deaminase activity reveals an intense active transport of adenosine into neurons in primary cultures

It is often assumed that adenosine transport into brain cells occurs by facilitated diffusion and that the continued net uptake of adenosine depends on its subsequent metabolism, which keeps the intracellular concentration of unmetabolized adenosine low and thus maintains a concentration gradient. If that is the case, inhibition of adenosine metabolism should decrease uptake. We have previously reported a considerable deamination of accumulated adenosine to inosine in primary cultures of cerebral cortical neurons. A relatively specific adenosine deaminase inhibitor, 2'-deoxycoformycin, was used in the present study. In the presence of this drug, the adenosine content (pool size) increased many fold without any decrease in total influx of adenosine. Influx of accumulated adenosine took place against a concentration gradient, demonstrating that a metabolic degradation of accumulated adenosine is not required to drive adenosine uptake. This does not preclude that under normal conditions some adenosine may get into the cells by diffusion.

Influence of PLA2-PG system on purine release and cAMP content in dissociated primary glial cultures from rat striatum

Purine release and prostaglandin (PG) outflow were simultaneously evaluated from untreated glial primary cultures of rat striatum, at rest and under field electrical stimulation. Purine release was also assayed from sister cultured cells in which a suitable pharmacological treatment with 1 x 10(-6) M dexamethasone or 1 x 10(-4) M indomethacin had produced a complete inhibition of the phospholipase A2-prostaglandin (PLA2-PG) system. Purine release from untreated cells seems to be regulated by specific receptor sites for released adenosine (Ado); A1 receptors exert an inhibitory control on purine release while A2 receptors facilitate it. PG release appears to be related to A1-mediated Ado activity, since culture treatment with 1 x 10(-10) M 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or 1 x 10(-4) M N-ethylmaleimide (NEM), A1 receptor inhibitory agents able to increase purine release, induced a significant reduction of the evoked PG outflow. Purine amount, released from glial cells with inhibited PLA2-PG system, was remarkably greater than that one assayed from control cultured cells. In so treated cultures, no additive effect, NEM-induced, was detected, while the addition of a mixture of PGs partially reduced the increased purine outflow. An electrically evoked cAMP accumulation, significantly greater than that found in controls, was even detected in cultured cells with inhibited PLA2-PG system. Since 10 micrograms/ml adenosine deaminase (ADA) reduced while DPCPX enhanced the evoked cAMP accumulation, it seems partially due to released Ado and accounts for a prevalent A2-stimulating rather than an A1-inhibitory control on adenylate cyclase activity. Thus, in cultured glial cells, the PLA2-PG system, likely linked to A1 receptor sites, concurs to control purine release and seems to affect less directly cAMP accumulation.

Effects of two nucleoside transport inhibitors, dipyridamole and soluflazine, on purine release from the rat cerebral cortex

The effects of two nucleoside transport inhibitors, dipyridamole and soluflazine, on adenosine, inosine and oxypurine release from the normoxic and hypoxic/ischemic rat cerebral cortex have been studied. Dipyridamole (500 micrograms/kg) enhanced adenosine release during hypoxic/ischemic challenges in comparison with saline-injected controls. It decreased the hypoxia/ischemia-elicited releases of inosine, hypoxanthine and xanthine. Both basal and hypoxia/ischemia-elicited releases of uric acid were elevated. Soluflazine, administered topically or systemically, failed to enhance adenosine release and did not consistently alter the hypoxia/ischemia-evoked releases of inosine, hypoxanthine and xanthine. Basal release of uric acid was elevated. The failure of either drug to elevate the basal or hypoxia/ischemia-evoked releases of adenosine above predrug levels illustrates one of the problems which may be inherent in the use of bidirectional nucleoside transport inhibitors for the manipulation of adenosine levels in the cerebral interstitial fluid.

The cellular localization of adenosine receptors in rat neostriatum

Using quantitative autoradiography of ligand binding sites combined with lesions of specific neuronal pathways, the cellular locations of A1 and A2 adenosine receptors, as well as a third binding site for the adenosine receptor ligand, [3H]N-ethylcarboxamidoadenosine, and a nucleoside transporter were investigated in rat neostriatum. Intrastriatal kainic acid administration resulted in the loss of 50% of A1 adenosine receptors and virtually abolished ligand binding to A2 receptors. A small reduction in [3H]cyclohexyladenosine binding to striatal A1 receptors was found after lesioning the corticostriatal input. A2 receptor sites were unaffected by this treatment. Destruction of dopaminergic neurons using 6-hydroxydopamine or the raphestriatal serotoninergic input using 5,7-dihydroxytryptamine affected neither A1 nor A2 binding sites. These results indicate the localization of both A1 and A2 adenosine receptors on neurons intrinsic to the neostriatum and probably postsynaptic to the dopaminergic input. In addition, a binding site for [3H]N-ethylcarboxamidoadenosine which is not affected by the adenosine receptor agonist, R-phenylisopropyladenosine, was also partly abolished after kainic acid injection. In contrast, no significant change in the binding of the nucleoside transporter ligand, [3H]nitrobenzylthioinosine, was observed after any lesions, indicating the widespread association of this site with various cell types.

Desensitization of adenosine A2 receptors in the striatum of the rat following chronic treatment with diazepam

Following prolonged treatment (7 days) with diazepam (10 mg/kg/day, using ALZET mini-osmotic pumps) in rats, the function of adenosine receptors was assessed in specific structures of the brain, using both agonist ligand binding and adenylate cyclase assays. Binding to A1 receptors was quantified using [3H]N6-[(R)-1-methyl-2-phenylethyl] adenosine, a selective ligand at A1 receptors. Differences in the binding of this ligand and that of [3H]5'-N-ethylcarboxamide adenosine, which binds to both A1 and A2 subtypes of receptors with similar affinities, were used to quantify A2 receptors. Treatment with diazepam failed to alter the binding of [3H]N6-[(R)-1-methyl-2-phenylethyl] adenosine in all areas of the brain studied. However, the binding of A2 receptors and A2 receptor-mediated stimulation of adenylate-cyclase were significantly attenuated in striatal membranes from diazepam-treated rats. Thus, the present study indicated that functional adenosine A2 receptors were desensitized after prolonged treatment with diazepam, since decreased agonist binding to A2 receptors paralleled an attenuation in the stimulation by adenosine of the activity of adenylate cyclase, an effect mediated by the A2 receptor. These results further indicate that the changes in adenosine A2 receptors correlated with significant short-lasting alterations in the sleep-wake cycle during the withdrawal of diazepam. The alterations in sleep-wakefulness did not correlate with the effect of diazepam on benzodiazepine receptors since no changes were observed in the binding of benzodiazepine receptors.

Cultures of glial cells release purines under field electrical stimulation: the possible ionic mechanisms

Dissociated primary cultures of glial cells released a remarkable amount of purines, at rest and during field electrical stimulation. The HPLC identification of labelled compounds derived from 3H-Adenosine (3H-Ado) (employed to preload the cultures) indicated that nucleotides and nucleosides were represented in the superfusate in equivalent proportions (43.86% and 56.14% respectively). Very much higher amounts of unlabelled purines prevalently constituted by nucleotides compounds (91.10%) were also released and detectable in the superfusate. In all the experimental conditions their evoked release did not result frequency-dependent. Since: a linear increase related to the stimulation frequencies was found for the released labelled compounds; no labelled purines were assayed in 5 x 10-5M Dipyridamole-treated cultures; any significant presence of labelled nucleotides, inosine and hypoxantine was not found in cultures simultaneously treated with 1 x 10-5M 2'-deoxycoformycin and 1 x 10-4M 1-(-5-isoquinolinsulfonyl)-2-methylpiperizine (H7) (3H-Ado amounts resulted more than doubled in these experimental conditions); labelled compounds have been assumed as tracers of a glial purine rate whose release can be connected to electrically-evoked action potentials. Purine outflow from glial cells is not sodium dependent, in fact TTX (5 x 10-7M) did not affect their basal or electrically-evoked release. A remarkable calcium-dependence was also evidentiated by the 1 x 10-4M Verapamil-induced inhibition of basal and evoked release. TEA (1 x 10-2M), a specific inhibitor of potassium efflux throughout calcium-mediated specific channels, strongly reduced the evoked purine outflow and any additive effect of its was not detectable when administered simultaneously to the calcium antagonist. These findings indicate that the frequency-dependent purine release from cultured glial cells is linked to ionic mechanisms, which calcium and potassium are mainly involved in.

Pharmacological characterization of rapidly accumulated adenosine by dissociated brain cells from adult rat

Mechanically dissociated brain cells from adult rats were used to study biochemically and pharmacologically their capacity to accumulate rapidly [3H]adenosine. The assay, which used an inhibitor-stop method to prevent further uptake into cells, was characterized with respect to protein and optimal substrate concentrations, and incubation times that ranged from 5 to 180 s. The accumulation of [3H]adenosine using 15-s incubation periods, conditions under which less than 10% of accumulated [3H]adenosine was metabolized, was best described kinetically by a two-component system with Km and Vmax values for the high-affinity component of 0.8 microM and 6.2 pmol/mg protein/15 s and for the low-affinity component 259 microM and 2,217 pmol/mg protein/15 s, respectively. The potencies with which nucleosides, adenosine deaminase resistant adenosine receptor agonists, and nucleoside uptake inhibitors competed for these uptake components were determined. Of the nucleosides examined, adenosine was the "preferred" substrate for the uptake site. The Ki value of adenosine for the high-affinity component was 10.7 microM. Inosine and uridine competed for a single lower affinity uptake system: Ki values were 142 and 696 microM, respectively. Nucleoside uptake inhibitors--nitrobenzylthioinosine, dipyridamole, and dilazep--were the most potent inhibitors of [3H]adenosine accumulation tested: the Ki values for the high-affinity system were 0.11, 1.3, and 570 nM, respectively. The adenosine analogs S-phenylisopropyladenosine, R-phenylisopropyladenosine, and cyclohexyladenosine inhibited the high-affinity component with Ki values of 2.3, 9.3, and 14.5 microM, respectively. N-Ethylcarboxamidoadenosine competed for a single lower affinity uptake system: Ki, 292 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium and calcium dependence of purine release from rat cerebral cortical slices

Electrically evoked purine release from rat cerebral cortical slices was evaluated, using a HPLC analysis combined with radioactivity measurement of the identified fractions. Two different pools of released purines have been identified: one probably related to cell metabolism and the other strictly linked to the nervous transmission. Since a linear increase, due to the stimulation frequencies, was found for the purines released from this second pool, a possible dependence on sodium and calcium transmembrane fluxes was evaluated. Pretreatment of the slices with TTX (5 x 10(-7) M) caused only a partial inhibitory effect on purine release (50%). This effect was probably related to the drug activity on the neuronal component of slices, since TTX induces an almost complete inhibition of purine release from isolated neurons "in cultures" and does not affect it from glial cells. Verapamil (1 x 10(-4) M), a calcium-channel blocker at glial and neuronal level, and TEA (3 x 10(-2) M), a specific inhibitor of calcium-mediated potassium efflux from glial cells, administered to the slices alone or in combination, showed a partial calcium-dependence of purine release. These results suggest a glial role in modulation of electrically-evoked purine release. These cells could exert a "buffering action" that regulates the calcium-mediated potassium availability, by which neuronal activity might be influenced.

Chronic administration of diazepam downregulates adenosine receptors in the rat brain

Following chronic administration (10 or 20 days) of diazepam (5 mg/kg/day, subcutaneous pellets) or RO 15-1788 (5 mg/kg/day, intraperitoneally), adenosine and benzodiazepine receptors in different rat brain areas were assessed by radioligand binding studies using [3H]R-PIA for A1 receptors, [3H]NECA and [3H]R-PIA for A2 receptors and [3H]FNZ for benzodiazepine receptors. Chronic administration of diazepam for 10, but not for 20 days, decreased A2 receptors in the striatum by 46% (p less than 0.05) and A1 receptors in the hippocampus by 13% (p less than 0.05). Administration of diazepam for 10 days and 20 days failed to alter [3H]FNZ binding in all brain areas studied. However, 20 days of diazepam administration decreased the magnitude of GABA enhancement of [3H]FNZ binding in the cortex by 25% (p less than 0.05). In contrast, chronic administration of RO 15-1788 failed to alter [3H]R-PIA, [3H]NECA and [3H]FNZ binding in all brain areas. These results suggest that adenosine receptors may play a role in the CNS actions of benzodiazepines.

Adenosine transporters in chromaffin cells: subcellular distribution and characterization

Adenosine transporters in freshly isolated and cultured chromaffin cells were quantified by the [3H]dipyridamole binding technique, showing a maximal bound capacity of 0.4 +/- 0.05 pmol/10(6) cells (240,000 +/- 20,000 transporters by cell). Scatchard analysis showed a similar affinity for [3H]dipyridamole in isolated cells and subcellular fractions (Kd = 5 +/- 0.6 nM). For enriched plasma membrane preparations and chromaffin granule membranes, the maximal binding capacities were also very similar, 2.3 +/- 0.3 and 1.8 +/- 0.4 pmol/mg protein, respectively. When [3H]nitrobenzylthioinosine was employed as a radioligand, the maximal bound capacity in cultured chromaffin cells was 0.053 +/- 0.004 pmol/10(6) cells (32,000 +/- 3000 transporters per cell) with a high affinity constant (Kd = 0.25 +/- 0.03 nM); similar values were obtained in all subcellular fractions (Kd = 0.1 +/- 0.01). Also, plasma and chromaffin granule membranes showed similar maximal binding values (0.4 +/- 0.06 pmol/mg protein). Photoincorporation studies with [3H]nitrobenzylthioinosine into plasma membrane polypeptides showed the presence of three molecular species of 115 +/- 10; 58 +/- 6 and 42 +/- 5 kDa. Chromaffin granule membranes showed only the 105 +/- 9 and 51 +/- 4 molecular species.

Autoradiographic studies on the uptake of adenosine and on binding of adenosine analogues in neurons and astrocytes of cultured rat cerebellum and spinal cord

The cellular localization of the uptake of [3H]adenosine and of binding of labelled adenosine analogues was studied in explant cultures of rat cerebellum and spinal cord by means of autoradiography. [3H]Adenosine was taken up by many neurons and astrocytes in both cerebellar and spinal cord cultures. The uptake of adenosine was inhibited in the absence of sodium or at 0 degrees C, suggesting an active transport mechanism. In both types of cultures, a great number of neurons showed binding sites for the A1-receptor agonist [3H]R-N6-phenylisopropyladenosine and for the mixed A1/A2-agonist [3H]N(ethyl)carboxamidoadenosine. Binding sites for both radioligands were also found on astrocytes, suggesting that these cells have receptors for the purinergic neurotransmitter adenosine. This suggestion is further supported by recent electrophysiological studies from our laboratory demonstrating that adenosine and its analogues produce hyperpolarizations of astrocytes which are blocked by the adenosine antagonist theophylline.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

The role of adenosine in the central actions of the benzodiazepines

1. Evidence is presented which indicates that the central actions of the benzodiazepines cannot be fully accounted for by assuming an action only at the GABAA-Cl- channel supramolecular complex. 2. The hypothesis is presented, together with supporting evidence, that inhibition of adenosine uptake can account for many of the actions of the benzodiazepines. 3. New findings showing that Ro 15-1788 and Ro 5-4864 have both potentiative and antagonistic interactions with adenosine are discussed. 4. The proconvulsant beta-carbolines are shown to be adenosine antagonists. 5. The concept that benzodiazepine action may involve several mechanisms is presented.