Specific binding of 2-[3H]chloroadenosine to rat brain cortical membranes

The impact of inosine on hippocampal synaptic transmission and plasticity involves the release of adenosine through equilibrative nucleoside transporters rather than the direct activation of adenosine receptors

Inosine has robust neuroprotective effects, but it is unclear if inosine acts as direct ligand of adenosine receptors or if it triggers metabolic effects indirectly modifying the activity of adenosine receptors. We now combined radioligand binding studies with electrophysiological recordings in hippocampal slices to test how inosine controls synaptic transmission and plasticity. Inosine was without effect at 30 μM and decreased field excitatory post-synaptic potentials by 14% and 33% at 100 and 300 μM, respectively. These effects were prevented by the adenosine A₁ receptor antagonist DPCPX. Inosine at 300 (but not 100) μM also decreased the magnitude of long-term potentiation (LTP), an effect prevented by DPCPX and by the adenosine A_2A receptor antagonist SCH58261. Inosine showed low affinity towards human and rat adenosine receptor subtypes with K_i values of > 300 µM; only at the human and rat A₁ receptor slightly higher affinities with K_i values of around 100 µM were observed. Affinity of inosine at the rat A₃ receptor was higher (K_i of 1.37 µM), while it showed no interaction with the human orthologue. Notably, the effects of inosine on synaptic transmission and plasticity were abrogated by adenosine deaminase and by inhibiting equilibrative nucleoside transporters (ENT) with dipyridamole and NBTI. This shows that the impact of inosine on hippocampal synaptic transmission and plasticity is not due to a direct activation of adenosine receptors but is instead due to an indirect modification of the tonic activation of these adenosine receptors through an ENT-mediated modification of the extracellular levels of adenosine.

Regulation of heptaspanning-membrane-receptor function by dimerization and clustering

G-protein-coupled receptors form homomers and heteromers; agonist-induced conformational changes within interacting receptors of the oligomer modify their pharmacology, signalling and/or trafficking. When these receptors are activated, the oligomers rearrange and cluster and a novel mechanism of receptor-operation regulation by oligomer intercommunication is possible. This intercommunication would be assisted by components of the plasma membrane and by scaffolding proteins. Receptor cross-sensitization, cross-desensitization and novel, integrated receptor responses can then develop between oligomeric receptor complexes of the cluster without direct contact between them. This concept gives a new perspective to the understanding of neurotransmission and neuronal plasticity.

Adenosine as a signaling molecule in the retina: biochemical and developmental aspects

The nucleoside adenosine plays an important role as a neurotransmitter or neuromodulator in the central nervous system, including the retina. In the present paper we review compelling evidence showing that adenosine is a signaling molecule in the developing retina. In the chick retina, adenosine transporters are present since early stages of development before the appearance of adenosine A1 receptors modulating dopamine-dependent adenylate cyclase activity or A2 receptors that directly activate the enzyme. Experiments using retinal cell cultures revealed that adenosine is taken up by specific cell populations that when stimulated by depolarization or neurotransmitters such as dopamine or glutamate, release the nucleoside through calcium-dependent transporter-mediated mechanisms. The presence of adenosine in the extracellular medium and the long-term activation of adenosine receptors is able to regulate the survival of retinal neurons and blocks glutamate excitoxicity. Thus, adenosine besides working as a neurotransmitter or neuromodulator in the mature retina, is considered as an important signaling molecule during retinal development having important functions such as regulation of neuronal survival and differentiation.

Enzymatic and extraenzymatic role of ecto-adenosine deaminase in lymphocytes

Adenosine deaminase (ADA, EC 3.5.4.4) is an enzyme of the purine metabolism which has been the object of considerable interest mainly because the congenital defect causes severe combined immunodeficiency (SCID). In the last 10 years, ADA, which was considered to be cytosolic, has been found on the cell surface of many cells and, therefore, it can be considered an ecto-enzyme. There is recent evidence about a specific role of ecto-ADA, which is different from that of intracellular ADA. Apart from degrading extracellular adenosine (Ado) or 2'-deoxyadenosine (dAdo), which are toxic for lymphocytes, ecto-ADA has an extraenzymatic function via its interaction with CD26. ADA/CD26 interaction results in co-stimulatory signals in T cells. This co-stimulation is blocked by HIV-1, thus evidencing a role for ecto-ADA in the pathophysiology of AIDS. The fact that, besides CD26, ADA can interact with different cell-surface proteins opens new perspectives in the research for a role of ecto-ADA in the function of the immune system and in the interactions that take place between different cells in the development of the immune system. The most interesting aspect is the possible participation of the ecto-enzyme in cell-to-cell contacts during ontogenesis and maturation of immunocompetent cells.

Cell surface adenosine deaminase: much more than an ectoenzyme

During the last 10 years, adenosine deaminase (ADA), an enzyme considered to be cytosolic, has been found on the cell surface of many cells, therefore it can be considered an ectoenzyme. EctoADA, which seems to be identical to intracellular ADA and has a globular structure, does not interact with membranes but with membrane proteins. Two of these cell surface receptors for ectoADA have been identified: CD26 and A1 adenosine receptors (A1R). Apart from degradation of extracellular adenosine another functional role of ectoADA has been assigned. EctoADA is able to transmit signals when interacting with either CD26 or A1R. In this way, it acts as a co-stimulatory molecule which facilitates a variety of specific signalling events in different cell types. The heterogeneous distribution of the enzyme in the nervous system indicates that ectoADA may be a neuroregulatory molecule. On the other hand, ectoADA might act as a bridge between two different cells thus raising the possibility that it may be important for the development of the nervous system.

Purinergic modulation of ethanol-induced sleep time in long-sleep and short-sleep mice

The long-sleep (LS) and short-sleep (SS) mice were selectively bred for differences in sensitivity to the depressant effects of ethanol. In addition to their differential sensitivity to ethanol, they are also differentially sensitive to purinergic agonists and antagonists. This suggests that there may be differences in the purinergic systems of these lines of mice which may aid in understanding how they differ in ethanol sensitivity. We have investigated whether these drugs are capable of modifying acute ethanol sensitivity as measured by ethanol-induced loss of the righting response (ethanol sleep time), waking blood and brain ethanol concentrations, and blood ethanol elimination rate. The purinergic agonists cyclohexyladenosine (CHA), L-phenylisopropyladenosine (PIA), 2-chloroadenosine (CAD), and N-ethylcarboxamidoadenosine (NEC) increased sleep time in both LS and SS mice, however, LS mice were generally more affected than SS. The LS and SS mice were also differentially sensitive to the purinergic antagonists, theophylline and caffeine. Blood and brain ethanol concentration on awakening suggested that CNS sensitivity to acute ethanol administration was altered by pretreatment with agonists but not antagonists. Two agonists, CHA and NEC, significantly lowered ethanol elimination in both lines of mice while PIA, CAD, and the antagonists theophylline, and caffeine were without affect on elimination rate. These data support previous observations that adenosine-mediated systems may be involved in the modulation of ethanol sensitivity.

Characteristics of an adenosine A1 binding site in human placental membranes

Binding sites were solubilized from human placental membrane using 1.5% sodium cholate and were assayed using polyethylene glycol precipitation. These soluble binding sites had properties of an adenosine A1 binding site. 2-[3H]Chloroadenosine and N-[3H]-ethylcarboxamidoadenosine (NECA) binding were time dependent and reversible. Scatchard plots indicate two classes of binding sites with Kd values of 6 and 357 nM for 2-chloro[8-3H]adenosine and 0.1 and 26 nM with [3H]NECA. The specificity of [3H]NECA binding was assessed by the ability of adenosine analogs to complete for binding sites. Using this approach the estimated IC50 values were 60 nM for (R-PIA), 160 nM for S-PIA, 80 nM for NECA, and 20 nM for 2-chloroadenosine. Binding of [3H]NECA to the soluble sites is inhibited to 48% of the control value by 100 microM guanylyl-5'-imidodiphosphate (Gpp(NH)p). The IC50 value for NECA binding to the soluble binding site was increased from 80 nM to 1500 by Gpp(NH)p. There was a shift of binding affinity from a mixture of high and low affinity to only low affinity with 100 microM Gpp(NH)p. Despite these alterations a NECA prelabeled molecular species of 150 kDa did not decrease in molecular weight upon the addition of 100 microM Gpp(NH)p during high-performance liquid chromatography on a Superose 12 column. Other evidence to support the concept of preferential solubilization and assay of a small population of A1 binding sites was obtained. Following solubilization adenosine A2-like binding sites could be detected only in reconstituted vesicles. The existence of small amounts of A1 binding sites in intact human placental membranes was directly demonstrated using the A1 agonist ligand N6-[3H]cyclohexyladenosine and the A1 antagonist ligand 8-[3H]cyclopentyl-1,3-dipropylxanthine. JAR choriocarcinoma cells have "A2-like" membrane binding sites. In contrast to placental membranes, only A2-like binding sites could be solubilized from JAR choriocarcinoma cells. These observations indicate that human placental membranes contain adenosine A1 binding sites in addition to A2-like binding sites. These sites are guanine nucleotide sensitive, but do not shift to a lower molecular weight form upon assumption of a low affinity state.

Interactions between caffeine and adenosine agonists in producing embryo resorptions and malformations in mice

Caffeine has previously been demonstrated to be teratogenic in mice, causing primarily limb reduction defects and cleft palate. To investigate the possibility that the mechanism of caffeine teratogenesis is related to its central stimulant activity, which is thought to result from its activity as an adenosine antagonist, the effects of the stable adenosine agonists L-phenylisopropyladenosine (L-PIA) and chloroadenosine on caffeine teratogenicity were examined. It was found that these adenosine analogs have embryolethal effects when administered intraperitoneally on Days 11 and 12 of gestation at extremely low dosages (minimal effect levels of 1 and 10 mumol/kg, respectively). Caffeine at dosages as low as 129 mumol/kg ip protected against the embryolethal effects of 5 mumol/kg L-PIA if administered simultaneously with L-PIA but not if administered 30 to 60 min after L-PIA. The maternotoxicity of either of the adenosine agonists or caffeine alone was ameliorated by coadministration of the other, particularly when the molar excess of caffeine was approximately 20-fold. These results suggest that the embryotoxicity and maternotoxicity of the adenosine agonists and the maternotoxicity of caffeine are mediated by binding to adenosine receptors. When coadministered with teratogenic dosages of caffeine (1160 to 1290 mumol/kg), L-PIA and chloroadenosine at dosage levels of 1 to 100 mumol/kg did not prevent and in some cases potentiated the teratogenic effects of caffeine. This lack of protection by L-PIA and chloroadenosine suggests that adenosine receptors are not the primary site of action for caffeine-induced teratogenicity. The embryolethal effects of L-PIA and chloroadenosine and the teratogenicity of caffeine occurred only at maternotoxic dosages. However, an analysis of the correlation structure indicates that the embryotoxicities of L-PIA and caffeine were not secondary to maternotoxicity as measured by body weight change.

Characteristics of high affinity and low affinity adenosine binding sites in human cerebral cortex

The binding characteristics of human brain cortical membrane fractions were evaluated to test the hypothesis that there are A1 and A2 adenosine binding sites. The ligands used were 2-chloro[8-3H]adenosine and N6-[adenine-2,8-3H]cyclohexyladenosine. Binding of chloroadenosine to human brain cortical membranes was time dependent, reversible and concentration dependent. The Kd calculated for chloroadenosine by Scatchard analysis of equilibrium data was 280 nM, with a Bmax of 1.6 pmoles/mg protein, suggesting a single class of binding sites. The specificity of chloroadenosine binding was assessed by the ability of adenosine analogs to compete for binding sites. Using this approach, the apparent Kd was estimated to be 0.74 microM for 5'-N-ethyl-carboxamideadenosine, 1 microM cyclohexyladenosine, and 13 microM for N6-(L-2-phenylisopropyl)adenosine. Isobutylmethylxanthine and theophylline, receptor antagonists, had apparent Kd values of 84 microM and 105 microM, respectively. Hill slope factors ranged from 0.3 to 0.6. Chloroadenosine binding to human brain cortical membranes approached equilibrium at 90 minutes, with a T1/2 of 10 minutes. The kob was 0.080 min-1 and the k1 was 7.5 X 10(4) min-1 M-1. Reversibility of chloroadenosine binding at equilibrium was completed at approximately 10 minutes with a k2 value of 0.074 min-1. The Kd calculated from the rate constants was 990 nM. Cyclohexyladenosine binding was concentration dependent. The Kd calculated for cyclohexyladenosine via Scatchard analysis of equilibrium data was 5 nM with a Bmax of 0.35 pmoles/mg protein. Cyclohexyladenosine binding was displaced by 3 known receptor agonists: N6-(L-2-phenyliso propyl)adenosine (Kd 4 nM), 2-chloroadenosine (Kd 10 nM) and 5H-N-ethyl-carboxamideadenosine (Kd 6 nM). The apparent Kd values for the agonists were 1 to 3 orders of magnitude lower with this ligand as compared to radioactive chloroadenosine. Binding was also displaced by 2 known antagonists, isobutylmethylxanthine and theophylline, with apparent Kd values of 4 microM and 8 microM, respectively. Hill slope factors ranged from 0.5 to 0.8. Our data support the existence of two adenosine binding sites in human cortex compatible with the low affinity (A2) and high affinity (A1) adenosine receptors.

Characteristics of adenosine binding sites in atrial sarcolemmal membranes

The studies reported here involve an exploration of the sites on atrial myocyte membranes with which adenosine interacts to produce its potent physiological effects in atrial muscle. Specific, high affinity binding of the stable adenosine analogs 2-chloro[3H]adenosine (2-ClAdo) and [3H]adenosine 5'-N-ethylcarboxamide (NECA) to atrial sarcolemmal membranes was measured in kinetic and equilibrium studies at 4 degrees C and 35 degrees C. Analysis of the [3H]2-ClAdo binding isotherm indicated the presence of two classes of binding site with equilibrium Kassoc values estimated to be 5.7 X 10(7) M-1 and 2.7 X 10(6) M-1. Displacement of bound [3H]2-ClAdo by adenosine 5'-N-cyclopropylcarboxamide (NCPCA) and by several N6-substituted adenosine analogs confirmed the presence of two classes of binding site. Analysis of the [3H]NECA binding also revealed the presence of two types of binding site for this ligand. The methylxanthines isobutylmethylxanthine and theophylline displaced bound [3H]2-ClAdo whereas adenosine uptake inhibitors and several other purines showed little activity. These atrial membrane binding sites exhibit many of the characteristics of the physiological adenosine receptors studied in intact atria. Furthermore, the [3H]2-ClAdo binding sites were sensitive to treatment with proteolytic enzymes, suggesting that these sites exist on sarcolemmal membrane proteins.

Adenosine: an endogenous modulator of hippocampal noradrenaline release

In slices of hippocampus from the rabbit, preincubated with [3H]noradrenaline and then continuously superfused, the modulation of the release of noradrenaline by adenosine receptors was studied. Electrical field stimulation of the slices elicited a release of [3H]noradrenaline which was inhibited in a concentration-dependent manner by various adenosine receptor agonists. From the order of potency: cyclohexyladenosine greater than (-)phenylisopropyladenosine [(-)PIA] greater than 5'-N-ethylcarboxamide-adenosine (NECA) greater than 2-chloro-adenosine greater than adenosine (+)phenylisopropyladenosine greater than ATP, the inhibitory adenosine receptor was classified as A1- (Ri-) receptor. The effect of the agonist was strongly reduced by adenosine receptor antagonists, the methylxanthines. A role for endogenous adenosine in the modulation of hippocampal noradrenaline release is supported by these findings: (1) that blockade of adenosine receptors by methylxanthines, especially by 8-phenyltheophylline, increased, whereas (2) inhibition of the uptake of adenosine decreased the evoked release of noradrenaline and (3) that deamination of endogenous extracellular adenosine by addition of adenosine deaminase to the medium enhanced the evoked transmitter release. Inhibitors of endogenous adenosine deaminase and 5'-nucleotidase were without effect. It is concluded that release of noradrenaline in the hippocampus is inhibited at the level of the noradrenergic nerve terminals by endogenous adenosine via A1 (or Ri) receptors.

[125I]N6-(3-iodo-4-hydroxyphenyl)isopropyladenosine: the use of the diastereomers as ligands for adenosine receptors in rat brain

The diastereomers of N6-(4-hydroxyphenyl)isopropyladenosine (HPIA) were synthesized, radioiodinated and used as ligands for adenosine receptors in rat brain membranes. In contrast to the S-isomer, specific binding (measured by displacement with 10 microM N6-R-phenylisopropyladenosine (PIA] of [125I]N6-[R-(3-iodo-4-hydroxyphenyl)isopropyl] adenosine ([125I]R- IHPIA) required about 2 h for equilibrium but bound with a higher affinity. Scatchard analysis of binding data were compatible with the existence of single binding site with Kd values of 0.7 nM (R-isomer) and 10 nM (S-isomer), and maximal binding of 228 and 237 fmol/mg, respectively, at 30 degrees C. Computer-based non-linear curve fitting resulted in estimates of 0.67 and 7.5 nM, and of 208 and 173 fmol/mg, for the R- and S-isomer, respectively. Dissociation of the R-isomer in the presence of 10 microM PIA was biphasic, both phases being increased in the presence of 100 microM Gpp(NH)p. The ratio of the rate of dissociation for the slower phase, k2, and the rate of association, k1, yielded a further estimate for Kd of 0.3 nM. The specific binding of [125I]R-IHPIA was displaced by adenosine receptor agonists with the following order of decreasing potency: N6-[R-(phenyl)-isopropyl]adenosine = N6-[R-(4-hydroxyphenyl)isopropyl]adenosine greater than 5'-N-ethylcarboxamidoadenosine greater than 2-chloroadenosine greater than N6-[S-(phenyl)isopropyl]adenosine = N6-[S-(4-hydroxyphenyl)isopropyl]adenosine which is typical of binding to Ri/A1 type receptors. Receptor antagonists displaced this binding in the following order of decreasing potency: 8-(p-hydroxyphenyl) theophylline greater than 8-phenyltheophylline greater than theophylline.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothermia-induced supersensitivity to adenosine for responses mediated via A1-receptors but not A2-receptors

Four isolated tissues were examined in which the responses to adenosine are mediated via either A1- or A2-receptors. The responses examined were the inhibition of cholinergic transmission of field-stimulated guinea-pig ileum (A1), inhibition of noradrenergic transmission of field-stimulated rat vas deferens (A1), inhibition of developed tension of rat paced left atria (A1) and relaxation of carbachol-contracted guinea-pig trachea (A2). Cumulative concentration-response curves for adenosine and 2-chloroadenosine were constructed at 37, 30 or 27 degrees C. When plotted as a percentage of the maximum response, the concentration-response curves were displaced to the left by cooling in the ileum, vas deferens and atria, indicative of supersensitivity. This increase in sensitivity does not arise from inhibition of uptake or deamination by cooling, since it occurs equally for adenosine and 2-chloroadenosine, the latter being immune to these processes. In contrast, the sensitivity of the trachea was not affected (2-chloroadenosine) or reduced (adenosine) by cooling. Thus responses mediated via adenosine receptors of the A1 subtype exhibit hypothermia-induced supersensitivity, whereas those mediated via A2-receptors do not. This suggests a fundamental temperature-dependent difference between the two adenosine receptor subtypes.

Demonstration of adenosine receptors on mouse cerebral smooth muscle membranes

Adenosine receptors have been identified on brain cortical membranes and microvascular preparations. However, they have not been demonstrated on specific microvascular elements in isolation. 2-3H-chloroadenosine was used as a ligand to investigate the presence of adenosine receptors on isolated mouse cerebral smooth muscle membranes. The binding studies reveal the presence of a high affinity binding site with a Kd value of 33.3 nM and a maximal binding capacity (Bmax) of 283 fmol/mg protein. These findings demonstrate that there is an adenosine receptor on cerebral smooth muscle membranes.

[3H]5'-N-ethylcarboxamide adenosine binds to both Ra and Ri adenosine receptors in rat striatum

Adenosine analogs such as 5'-N- ethylcarboxamide adenosine and N6-cyclohexyladenosine stimulate or inhibit adenosine cyclase activity in preparations of rat striatum depending on the assay conditions. N6-cyclohexyladenosine inhibits but does not stimulate adenosine cyclase activity in preparations of hippocampus. These findings suggest that the striatum contains both Ra (stimulatory) and Ri (inhibitory) adenosine receptors while the hippocampus contains only Ri receptors. We have previously shown that [3H]N6-cyclohexyladenosine binds to Ri receptors in rat hippocampus ( Yeung and Green 1983). Comparisons of the characteristics of [3H]5'-N- ethylcarboxamide adenosine and [3H]N6-cyclohexyladenosine binding to hippocampus show that [3H]5'-N- ethylcarboxamide adenosine also binds to Ri receptors with high affinity. [3H]5'-N- ethylcarboxamide adenosine binds to Ri receptors in the striatum and to a second site that is present in striatum but not hippocampus. High affinity binding of both ligands to Ri receptors can be blocked by treatments with N-ethylmaleimide that do not markedly affect [3H]5'-N- ethylcarboxamide adenosine binding to the second site in the striatum. The pharmacological characteristics of the second site indicate that it is the Ra adenosine receptor.

Evidence for an adenosine receptor in human tissues

Our observations suggest that [3H] chloroadenosine, an adenosine receptor agonist, identifies binding sites in human placenta with characteristics of the high affinity, adenosine receptor. The binding is time dependent, reversible and saturable. The potency series of adenosine receptor methylxanthine antagonists in displacing [3H] chloroadenosine is appropriate.

[3H]adenosine binding to bovine coronary arteries and myocardium

The characteristics of [3H]adenosine binding to bovine myocardial and coronary tissue were investigated using the microsomal fraction obtained by fractionated centrifugation. Binding experiments consisting of equilibrium, kinetic and competition studies were performed employing a filtration assay. The coronary microsomal fraction yielded two classes of binding sites with different affinities and binding capacities. The approximate binding parameters were: KD = 1.7 x 10(-7) M, Bmax = 1.2 pmol/mg protein, and KD = 2.6 x 10(-6) M, Bmax = 6.6 pmol/mg protein, respectively. Only one single class of binding sites was found in the myocardium as indicated by a linear Scatchard plot (KD = 3.7 x 10(-7) M, Bmax = 11.7 pmol/mg protein). In both tissues competition experiments performed with a number of adenosine analogues displayed similar specificity, i.e. high competing potency of adenosine nucleotides, whereas, 2-chloroadenosine, L-PIA, 2'-deoxyadenosine and theophylline showed little or no affinity. The results presented in this paper do not provide evidence for the predominance of typical Ra sites according to the definition of Londos et al. (Proc. Natl. Acad. Sci. U.S.A. 77, 2551, 1980) in bovine coronary arterial tissue or myocardium.

Adenosine receptor binding: structure-activity analysis generates extremely potent xanthine antagonists

Structure-activity analysis of alkylxanthine derivatives at adenosine receptor binding sites has been employed to design more potent adenosine receptor antagonists. Receptor affinities of xanthines were determined by measuring inhibition of the binding of N6-[3H]cyclohexyladenosine to bovine brain membranes. 1,3-Dipropyl substitutions enhance potency compared to the 1,3-dimethyl substitution in theophylline. An 8-phenyl substituent produces a considerable increase in potency, which is augmented by certain para substitutions on the 8-phenyl ring. Combining an ortho amino with a para-chloro substituent on the 8-phenyl ring affords further increases in potency. Combining all of these substituents results in 1,3-dipropyl-8-(2-amino-4-chlorophenyl) xanthine, a compound of extraordinary receptor affinity, with a Ki for adenosine A1 receptors of 22 pM. It is 4,000,000 times more potent than xanthine itself and 70,000 times more potent than theophylline.

Regulation of purine metabolism by plasma membrane and cytoplasmic 5'-nucleotidases

The contribution of plasma membrane 5'-nucleotidase (E.C. 3.1.3.5) to intracellular purine degradation and release was evaluated in cultured human lymphoblasts. B-lymphoblasts and T-lymphoblasts are characterized by high and low levels of plasma membrane 5'-nucleotidase activity, respectively. After radiolabeling of the cellular adenine nucleotide pools with [8-14C]adenine, deoxyglucose-induced purine nucleotide degradation resulted in a 2-2.5 times greater release of cellular radioactivity from the B-lymphoblasts than from the T-lymphoblasts. Specific inhibition of plasma membrane 5'-nucleotidase with 50 microM alpha, beta-methylene adenosine diphosphate (AMPCP) did not decrease purine release during deoxyglucose-induced nucleotide degradation. Similarly, the inhibition of B-lymphoblast membrane 5-nucleotidase did not alter the incorporation of [8-14C]adenine into the nucleotide pool. Therefore, to explain the relatively high release of purine nucleotide degradation products in B-lymphoblasts when compared with T-lymphoblasts, cytoplasmic 5'-nucleotidase activity was investigated in these cell lines. B-lymphoblasts have seven times more cytoplasmic 5'-nucleotidase activity for dAMP and two to three times more activity for other purine nucleoside 5'-monophosphates than do T-lymphoblasts at pH 7.4. Membrane and cytoplasmic nucleotidase activities are produced by different enzymes that can be distinguished by differences in pH optima, Michaelis constants for purine substrates, divalent cation requirements, and susceptibilities to AMPCP inhibition. The data suggest that plasma membrane 5'-nucleotidase hydrolyzes extracellular nucleoside 5'-monophosphates only. Cytoplasmic 5'-nucleotidase most likely regulates the degradation of intracellular nucleoside 5'-monophosphates and may be responsible for the increased purine release observed in B-lymphoblasts.

Binding of 2',5'-dideoxyadenosine to brain membranes. Comparison to P-site inhibition of adenylate cyclase

Membranes from rat cerebral cortex and striatum contain a relatively large number of high-affinity binding sites for [3H]2',5'-dideoxyadenosine, [3H]adenine arabinoside, and [3H]adenosine. The binding of [3H]2',5'-dideoxyadenosine and [3H]adenine arabinoside was virtually unaffected by relatively specific agonists and antagonists for adenosine receptors, such as 2-chloroadenosine, N6-phenylisopropyladenosine or theophylline. Binding of [3H]adenosine was partially blocked by such receptor ligands. The specific binding of all three ligands was antagonized by a variety of adenosine analogs which inhibit adenylate cyclase by interaction with the so-called P-site associated with this enzyme. However, potencies of adenosine analogs as P-site inhibitors of adenylate cyclase and as antagonists of binding do not correlate well. 5'-Methylthioadenosine had high potency and efficacy versus binding of [3H]2',5'-dideoxyadenosine but had virtually no effect on activity of adenylate cyclase. 2-Fluoroadenosine was less potent than adenosine as an antagonist of specific binding of [3H]2',5'-dideoxyadenosine, while 2-fluoroderivatives of adenosine, adenine arabinoside and adenine xylofuranoside were more potent than the parent compounds as P-site inhibitors. The significance of the binding sites for [3H]2',5'-dideoxyadenosine remains unclear, but their presence complicates the use of [3H]adenosine and certain analogs as ligands for adenosine membrane sites associated with adenylate cyclase.

Adenosine mediates sedative action of various centrally active drugs

Adenosine and its analogs depress the firing of neurons in various brain regions. The primary mode of action of adenosine in exerting this action appears to be the depression of calcium entry, thus decreasing presynaptic neurotransmitter release. Adenosine uptake inhibitors and adenosine deaminase inhibitors potentiate the depressant actions of adenosine. Caffeine and theophylline, methylxanthines, antagonize these actions. Adenosine is therefore likely to be released and to exert an ongoing modulation of the neuron excitability in the intact brain. Adenosine uptake by nerve terminals appears to be important in regulating the extracellular concentration of adenosine and thus of adenosine's action. A number of groups of centrally active sedative, anxiolytic and anticonvulsant drugs inhibit adenosine uptake by brain synaptosomal preparations. It is proposed that these agents exert their sedative effects by inhibiting adenosine uptake and thus potentiating depressant actions by locally released adenosine on neuronal activity.

Release of norepinephrine and dopamine from brain vesicular preparations: effects of adenosine analogues

1. Adenosine analogues inhibit calcium-dependent K+-evoked release of [3H]norepinephrine from guinea pig cerebral cortical and hippocampal vesicular preparations. Inhibition requires high concentrations (100 microM) of the adenosine analogues and is abolished in the presence of high concentrations (2 mM) of calcium ions. The inhibitory effect of 2-chloroadenosine is blocked by theophylline. The structure activity profile (N6-D-phenylisopropyladenosine greater than or equal to N6-L-phenylisopropyladenosine greater than or equal to 2-chloroadenosine greater than N6-cyclohexyladenosine, adenosine 5'-cyclopropylcarboxamide) is not that expected of either A1 (high-affinity) or A2 (low-affinity) adenosine receptors. 2. Calcium-dependent K+-evoked release of [3H]dopamine from guinea pig striatal vesicular preparations is inhibited by apomorphine. However, only 2-chloroadenosine causes an inhibition of K+-evoked release of [3H]dopamine. Other adenosine analogues such as D- and L-phenylisopropyladenosine and adenosine 5'-cyclopropylcarboxamide cause a facilitation of K+-evoked release. The facilitation is abolished or reduced in the presence of high concentrations (2 mM) of calcium ions. The sites of action of adenosine analogues do not appear to have structural requirements identical to those expected of A1 (high-affinity) or A2 (low-affinity) adenosine receptors. 3. The results indicate that adenosine analogues can have either inhibitory or facilitory effects on K+-evoked release of catecholamines from central synaptic terminals.

Adenosine receptor agonists inhibit K+-evoked Ca2+ uptake by rat brain cortical synaptosomes

The uptake of Ca2+ by a K+-depolarized rat brain cerebral cortical crude synaptosomal preparation (P2 fraction) was investigated. The characteristics of the Ca2+ uptake system are similar to those observed by other investigators. The preparation is also a suitable model with which to study the effects of adenosine on Ca2+ uptake and neurotransmitter release, as it is generally accepted that K+-evoked Ca2+ uptake is intimately related to depolarization-induced release of neurotransmitters. We have demonstrated that an extracellular receptor is involved in mediating the adenosine-evoked inhibition of K+-evoked Ca2+ uptake. The pharmacological properties of the receptor suggest that it may be similar in some respects to the A2-receptor associated with adenylate cyclase. The adenosine uptake inhibitor, dipyridamole, potentiated the action of adenosine, suggesting that re-uptake is important in controlling the extracellular adenosine concentration and thus in the regulation of the adenosine receptor. The adenosine receptor antagonist theophylline inhibited the effects of adenosine. Calmodulin inhibited K+-evoked uptake of Ca2+ by the synaptosomal fraction.

Adenosine receptors mediating cyclic AMP production in the rat hippocampus

In the transversely cut rat hippocampus, adenosine caused a dose-dependent increase in the accumulation of [3H]cyclic AMP from [3H]ATP. Adenosine breakdown products were inactive. AMP was somewhat less effective than adenosine, and its effect could be partially, but not completely, abolished by alpha, beta-methylene-ADP and GMP, which inhibited its metabolism by 5'-nucleotidase. The effect of adenosine was unaffected by inhibitors of adenosine deaminase, but enhanced by several inhibitors of adenosine uptake. Some analogues of adenosine, including N6-phenylisopropyladenosine (PIA), 2-chloroadenosine and adenosine 5'-ethylcarboxamide (NECA), were more active than adenosine, whereas others such as 2-deoxyadenosine and 9-(tetrahydro-2-furyl)adenine (SQ 22536) actually inhibited the response. The effect of PIA was highly stereospecific. The action of adenosine was inhibited by several alkylxanthines, the most potent of which was 8-phenyltheophylline. [3H]Cyclohexyladenosine (CHA) bound specifically to cell membranes from the rat hippocampus. The extent of binding was similar to that found in other cortical areas. The relative potency of some adenosine analogues and alkylxanthines to displace labelled CHA was essentially similar to their potency as effectors of the cyclic AMP system. Adenosine contributed to the cyclic AMP-elevating effect of alpha-adrenoceptor-stimulating drugs and several amino acids, but not to that seen with isoprenaline. The cyclic AMP increase seen following depolarization was only partially adenosine-dependent. The present results demonstrate that the rat hippocampus contains adenosine receptors mediating cyclic AMP accumulation and that these receptors have similar characteristics to those mediating pyramidal cell depression. Adenosine-induced cyclic AMP accumulation may be used as a biochemical correlate to electrophysiology and as a convenient parameter to assess the influence of drugs on adenosine mechanisms in the rat hippocampus.

An A1-adenosine receptor, characterized by [3H] cyclohexyladenosine binding, mediates the depression of evoked potentials in a rat hippocampal slice preparation

In slices of rat hippocampus, adenosine and several adenosine derivatives depressed evoked neuronal responses to afferent stimulation. The nanomolar potency of adenosine derivatives and their relative effectiveness indicate that the depression of evoked potentials is mediated via an A1-adenosine receptor. A remarkable similarity was found between the relative potencies of nucleoside derivatives with respect to their electrophysiological effects and to their inhibition of high affinity [3H] cyclohexyladenosine ([3H]CHA) binding to rat brain membranes. We conclude that the [3H] CHA binding site in rat brain membranes represents a physiological receptor of the A1-type.

Cell-membrane receptors for purines. Review

Purines are involved in many aspects of cell chemistry - intermediary metabolism, nucleic acid synthesis, and the supply of high-energy phosphates to various active transport systems. In addition, however, there appear to be specific receptor molecules located within the plasma membrane of some cells, which mediate changes of cell function in response to purines present in the extracellular fluid. It is the purpose of this review to summarize the kind of functions subserved by those receptors as well as the basic structural requirements for their activation.

Adenosine receptors in rat brain membranes: characterization of high affinity binding of [3H]-2-chloroadenosine

1. [3H]-2-chloroadenosine has been found to be a suitable ligand for the study of adenosine receptors in rat brain synaptic membranes. 2. Binding sites labelled by [3H]-2-chloroadenosine had a high affinity with a KD value of 23.5 nM. 3. Binding is heat sensitive, pH dependent and probably involves protein molecules. 4. The IC50 values for 2-chloroadenosine, adenosine, L-N6-phenylisopropyladenosine and D-N6-phenylisopropyladenosine, N6-cyclohexyladenosine and adenosine-5'-N-ethyl-carboxamide inhibition of [3H]2-chloroadenosine binding are in good agreement with the values obtained in studies of the ability of these compounds to inhibit adenylate cyclase, suggesting that [3H]-2-chloroadenosine binding sites reported here are comparable to the adenosine A1 receptor site. 5. There are regional differences in [3H]-2-chloroadenosine binding to brain membranes. 6. This difference is probably due to the discrepancies in the number of binding sites, and is probably not caused by changing affinities of receptors to the ligand.

In vivo behavioral assessment of central nervous system purinergic receptors

Administered intracisternally, adenosine (ADO), 2-chloroadenosine (CADO), adenosine-5'-cyclopropylcarboxamide (ACC) and adenosine-5'-ethylcarboxamide (AEC) caused dose-related increases in hot plate reaction times in mice. The rank order of potency was AEC=ACC greater than CADO greater than ADO and ACC exerted demonstrable effects with doses as low as 10 ng/mouse. ADO itself was more potent than AMP, ADP, ATP and several other related compounds of interest. Theophylline, caffeine and 8-phenyltheophylline antagonized the antinocisponsive effect of CADO or ACC. Papaverine (an adenosine uptake blocker) and erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA: an adenosine deaminase inhibitor) potentiated the effect of ADO. EHNA did not potentiate the action of CADO in this procedure. The antinocisponsive effect of CADO was not antagonized by a host of neurally active agents including naloxone, clonidine and RO 20-1724. Time course studies indicated that the antinocisponsive effect of ADO was transient with the peak effect occurring 5 min after injection and disappearing by 60 min, whereas the effect of CADO persisted for at least 90 min. Intracisternally administered CADO also caused a pronounced hypothermia, loss of muscle tone and was active in the mouse writhing test. Taken together, these data demonstrate that purine exert potent in vivo behavioral effects and are consonant with the existence of a central purinergic P1-receptor which is amenable to selective pharmacological manipulation.

Effects of 2-chloroadenosine on electric potentials in brain synaptic membrane vesicles

Isolated synaptic plasma membrane vesicles developed an internal negative membrane potential (delta psi) following loading with potassium succinate and incubation in NaCl, sodium succinate, or Tris succinate media. Membrane delta psi was monitored by measuring triphenyl[3H]methylphosphonium ion ([3H]TPMP+) accumulation by these vesicles. Estimates of delta psi ranged from --6.9 mV for vesicles incubated in sodium succinate to --28 mV for membranes incubated in NaCl. Intravesicular TPMP+ accumulation was strongly dependent on the K+ diffusion potential and was enhanced by the K+ ionophore valinomycin and by the adenosine analog 2-chloroadenosine (2-Cl-Ado). The stimulation of TPMP+ influx by 2-Cl-Ado was dependent on the concentration of this agent, independent of Cl- fluxes, and sensitive to inhibition by the methylxanthine theophylline. The increase of delta psi of the synaptic membrane vesicles caused by 2-Cl-Ado paralleled the hyperpolarization of neurons produced by adenosine and 2-Cl-Ado in physiological systems.

Alkylxanthines elevate hippocampal excitability. Evidence for a role of endogenous adenosine

The effects of four xanthines (theophylline, 8-phenyltheophylline, isobutylmethylxanthine (IBMX), and 7-benzyl-IBMX) were studied in the hippocampal slice in vitro. These agents increased the excitability of this preparation with 8-phenyltheophylline being the most potent, 7-benzyl-IBMX the least potent, and theophylline and IBMX having intermediate potencies. A similar rank order was observed in terms of the potencies of these xanthines in antagonizing a) electrophysiological responses to adenosine, and b) adenosine-stimulated cyclic AMP formation. These results indicate that the excitatory actions of xanthines in the vivo hippocampus can be most easily explained on the basis of their ability to block adenosine's actions; the CNS excitatory actions of these drugs in vivo may depend upon a similar mechanism of action.

Adenosine receptors and behavioral actions of methylxanthines

Central stimulant actions of 10 methylxanthines in mice correlate with affinities for adenosine receptors labeled with N6-[3H]cyclohexyladenosine. Affinities of methylxanthines for adenosine receptors are consonant with central levels attained at behaviorally effective doses. The much higher concentrations of methylxanthines required to influence benzodiazepine receptor binding do not correlate with behavioral potency. N6-(L-Phenylisopropyl)adenosine (L-PIA), a metabolically stable analog of adenosine with high affinity for adenosine receptors, is an extremely potent behavioral depressant, reducing locomotor activity of mice at doses as little as 0.05 mumol/kg. The D isomer, which has much less affinity for adenosine receptors, is much less active as a central depressant. Theophylline stimulates locomotor activity and reverses depressant effects of L-PIA. Caffeine or 1,7-dimethylxanthine, when administered alone, elicits biphasic effects, with locomotor depression at lower doses and stimulation at higher doses. When administered with L-PIA, even low doses of caffeine produce marked stimulation. 3-Isobutyl-1-methylxanthine given alone elicits only behavioral depression. However, like theophylline and caffeine, isobutylmethylxanthine reverses the L-PIA-evoked depression, converting it into pronounced locomotor stimulation. The data strongly suggest that the behavioral stimulant effects of methylxanthines involve a blockade of central adenosine receptors.

Depression of the firing of rat cerebral cortical neurons by 2-azido analogues of adenine nucleotides

Iontophoretically applied 2-azido photoaffinity analogues of adenosine 5'-triphosphate (ATP) and adenosine 5'-monophosphate (AMP) have been tested on rat cerebral cortical neurons. 2-Azido-AMP and 2-azido-ATP had a powerful depressant action on the spontaneous firing of cortical neurons. Both compounds were equipotent with AMP in terms of the maximum depression elicited by equivalent current applications but the rate of recovery was markedly slowed after application of the azido analogues. The adenosine antagonist 8-(p-sulphophenyl)theophylline prevented 2-azido-AMP and 2-azido-ATP from exerting their effects. The 2-azido analogues of adenosine may be useful ligands for further studies on central adenosine receptors.

Biochemical characterization of putative central purinergic receptors by using 2-chloro[3H]adenosine, a stable analog of adenosine

After pretreatment of rat brain synaptic membranes with adenosine deaminase to remove endogenous adenosine, 2-chloro[3H]adenosine, a stable analog of adenosine, binds to two sites with Kd values of 1.3 and 16 nM and corresponding Bmax values of 207 and 380 fmol/mg of protein. Binding is reversible, and the highest density of sites occurs in enriched synaptosomal fractions. In peripheral tissue, negligible binding is observed in heart, kidney, and liver, while testicle has 11 fmol of binding sites/mg of protein. In brain, caudate and hippocampus have the highest density of sites, and spinal cord and hypothalamus have the lowest. This high-affinity binding is stereospecific; the L diasteromer of N6-phenylisopropyladenosine is approximately 30-times more potent as a displacer of 2-chloro[3H]adenosine than the D isomer and is also sensitive to theophylline (IC50 = 8.8 microM) and other purine-related compounds. Several putative neurotransmitters, neurotransmitter antagonists, and other centrally active compounds have no effect on binding. The data are consistent with the hypothesis that 2-chloro[3H]adenosine is binding to central purinergic receptors.

The effect of morphine on purine and acetylcholine release from rat cerebral cortex: evidence for a purinergic component in morphine's action

Morphine enhances the release of adenosine and its metabolites from the rat cerebral cortex and inhibits the release of acetylcholine. Naloxone antagoinizes the effects of morphine on both purine and acetylcholine release. The adenosine antagonists, caffeine and theophylline, reduce morphine's effects on acetylcholine release, and at the same time increase the spontaneous release of acetylcholine. It is suggested that morphine, acting at a naloxone-sensitive site, enhances the level of extracellular adenosine, which in turn inhibits the release of acetylcholine, and that some of morphine's actions are mediated by a purinergic step.