Effects of mono- and divalent ions on the binding of the adenosine analogue CGS 21680 to adenosine A2 receptors in rat striatum

Multifunctional scaffolds for bone repair following age-related biological decline: Promising prospects for smart biomaterial-driven technologies

The repair of large bone defects due to trauma, disease, and infection can be exceptionally challenging in the elderly. Despite best clinical practice, bone regeneration within contemporary, surgically implanted synthetic scaffolds is often problematic, inconsistent, and insufficient where additional osteobiological support is required to restore bone. Emergent smart multifunctional biomaterials may drive important and dynamic cellular crosstalk that directly targets, signals, stimulates, and promotes an innate bone repair response following age-related biological decline and when in the presence of disease or infection. However, their role remains largely undetermined. By highlighting their mechanism/s and mode/s of action, this review spotlights smart technologies that favorably align in their conceivable ability to directly target and enhance bone repair and thus are highly promising for future discovery for use in the elderly. The four degrees of interactive scaffold smartness are presented, with a focus on bioactive, bioresponsive, and the yet-to-be-developed autonomous scaffold activity. Further, cell- and biomolecular-assisted approaches were excluded, allowing for contemporary examination of the capabilities, demands, vision, and future requisites of next-generation biomaterial-induced technologies only. Data strongly supports that smart scaffolds hold significant promise in the promotion of bone repair in patients with a reduced osteobiological response. Importantly, many techniques have yet to be tested in preclinical models of aging. Thus, greater clarity on their proficiency to counteract the many unresolved challenges within the scope of aging bone is highly warranted and is arguably the next frontier in the field. This review demonstrates that the use of multifunctional smart synthetic scaffolds with an engineered strategy to circumvent the biological insufficiencies associated with aging bone is a viable route for achieving next-generation therapeutic success in the elderly population.

Mechanism of μ-Opioid Receptor-Magnesium Interaction and Positive Allosteric Modulation

In the era of opioid abuse epidemics, there is an increased demand for understanding how opioid receptors can be allosterically modulated to guide the development of more effective and safer opioid therapies. Among the modulators of the μ-opioid (MOP) receptor, which is the pharmacological target for the majority of clinically used opioid drugs, are monovalent and divalent cations. Specifically, the monovalent sodium cation (Na⁺) has been known for decades to affect MOP receptor signaling by reducing agonist binding, whereas the divalent magnesium cation (Mg²⁺) has been shown to have the opposite effect, notwithstanding the presence of sodium chloride. Although ultra-high-resolution opioid receptor crystal structures have revealed a specific Na⁺ binding site and molecular dynamics (MD) simulation studies have supported the idea that this monovalent ion reduces agonist binding by stabilizing the receptor inactive state, the putative binding site of Mg²⁺ on the MOP receptor, as well as the molecular determinants responsible for its positive allosteric modulation of the receptor, are unknown. In this work, we carried out tens of microseconds of all-atom MD simulations to investigate the simultaneous binding of Mg²⁺ and Na⁺ cations to inactive and active crystal structures of the MOP receptor embedded in an explicit lipid-water environment and confirmed adequate sampling of Mg²⁺ ion binding with a grand canonical Monte Carlo MD method. Analyses of these simulations shed light on 1) the preferred binding sites of Mg²⁺ on the MOP receptor, 2) details of the competition between Mg²⁺ and Na⁺ cations for specific sites, 3) estimates of binding affinities, and 4) testable hypotheses of the molecular mechanism underlying the positive allosteric modulation of the MOP receptor by the Mg²⁺ cation.

Peeking at G-protein-coupled receptors through the molecular dynamics keyhole

Molecular dynamics is a state of the art computational tool for the investigation of biophysics phenomenon at a molecular scale, as it enables the modeling of dynamic processes, such as conformational motions, molecular solvation and ligand binding. The recent advances in structural biology have led to a bloom in published G-protein-coupled receptor structures, representing a solid and valuable resource for molecular dynamics studies. During the last decade, indeed, a plethora of physiological and pharmacological facets of this membrane protein superfamily have been addressed by means of molecular dynamics simulations, including the activation mechanism, allosterism and, very recently, biased signaling. Here, we try to recapitulate some of the main contributions that molecular dynamics has recently produced in the field.

Ca2+ allostery in PTH-receptor signaling

The parathyroid hormone (PTH) and its related peptide (PTHrP) activate PTH receptor (PTHR) signaling, but only the PTH sustains G_S-mediated adenosine 3',5'-cyclic monophosphate (cAMP) production after PTHR internalization into early endosomes. The mechanism of this unexpected behavior for a G-protein-coupled receptor is not fully understood. Here, we show that extracellular Ca²⁺ acts as a positive allosteric modulator of PTHR signaling that regulates sustained cAMP production. Equilibrium and kinetic studies of ligand-binding and receptor activation reveal that Ca²⁺ prolongs the residence time of ligands on the receptor, thus, increasing both the duration of the receptor activation and the cAMP signaling. We further find that Ca²⁺ allostery in the PTHR is strongly affected by the point mutation recently identified in the PTH (PTH^R25C) as a new cause of hypocalcemia in humans. Using high-resolution and mass accuracy mass spectrometry approaches, we identified acidic clusters in the receptor's first extracellular loop as key determinants for Ca²⁺ allosterism and endosomal cAMP signaling. These findings coupled to defective Ca²⁺ allostery and cAMP signaling in the PTHR by hypocalcemia-causing PTH^R25C suggest that Ca²⁺ allostery in PTHR signaling may be involved in primary signaling processes regulating calcium homeostasis.

Tritium-labeled agonists as tools for studying adenosine A2B receptors

A selective agonist radioligand for A_2B adenosine receptors (A_2BARs) is currently not available. Such a tool would be useful for labeling the active conformation of the receptors. Therefore, we prepared BAY 60-6583, a potent and functionally selective A_2BAR (partial) agonist, in a tritium-labeled form. Despite extensive efforts, however, we have not been able to establish a radioligand binding assay using [³H]BAY 60-6583. This is probably due to its high non-specific binding and its moderate affinity, which had previously been overestimated based on functional data. As an alternative, we evaluated the non-selective A_2BAR agonist [³H]NECA for its potential to label A_2BARs. [³H]NECA showed specific, saturable, and reversible binding to membrane preparations of Chinese hamster ovary (CHO) or human embryonic kidney (HEK) cells stably expressing human, rat, or mouse A_2BARs. In competition binding experiments, the AR agonists 2-chloroadenosine (CADO) and NECA displayed significantly higher affinity when tested versus [³H]NECA than versus the A_2B-antagonist radioligand [³H]PSB-603 while structurally diverse AR antagonists showed the opposite effects. Although BAY 60-6583 is an A_2BAR agonist, it displayed higher affinity versus [³H]PSB-603 than versus [³H]NECA. These results indicate that nucleoside and non-nucleoside agonists are binding to very different conformations of the A_2BAR. In conclusion, [³H]NECA is currently the only useful radioligand for determining the affinity of ligands for an active A_2BAR conformation.

Mechanistic insights into allosteric regulation of the A2A adenosine G protein-coupled receptor by physiological cations

Cations play key roles in regulating G-protein-coupled receptors (GPCRs), although their mechanisms are poorly understood. Here, 19F NMR is used to delineate the effects of cations on functional states of the adenosine A2A GPCR. While Na+ reinforces an inactive ensemble and a partial-agonist stabilized state, Ca2+ and Mg2+ shift the equilibrium toward active states. Positive allosteric effects of divalent cations are more pronounced with agonist and a G-protein-derived peptide. In cell membranes, divalent cations enhance both the affinity and fraction of the high affinity agonist-bound state. Molecular dynamics simulations suggest high concentrations of divalent cations bridge specific extracellular acidic residues, bringing TM5 and TM6 together at the extracellular surface and allosterically driving open the G-protein-binding cleft as shown by rigidity-transmission allostery theory. An understanding of cation allostery should enable the design of allosteric agents and enhance our understanding of GPCR regulation in the cellular milieu.

Nanolayered hybrid mediates synergistic co-delivery of ligand and ligation activator for inducing stem cell differentiation and tissue healing

Cellular behaviors, such as differentiation, are regulated by complex ligation processes involving cell surface receptors, which can be activated by various divalent metal cations. The design of nanoparticle for co-delivery of ligand and ligation activator can offer a novel strategy to synergistically stimulate ligation processes in vivo. Here, we present a novel layered double hydroxide (LDH)-based nanohybrid (MgFe-Ado-LDH), composed of layered MgFe hydroxide nanocarriers sandwiching the adenosine cargo molecule, maintained through an electrostatic balance, to co-deliver the adenosine (Ado) ligand from the interlayer spacing and the Mg2+ ion (ligation activator) through the dissolution of the MgFe nanocarrier itself. Our findings demonstrate that the MgFe-Ado-LDH nanohybrid promoted osteogenic differentiation of stem cells through the synergistic activation of adenosine A2b receptor (A2bR) by the dual delivery of adenosine and Mg2+ ions, outperforming direct supplementation of adenosine alone. Furthermore, the injection of the MgFe-Ado-LDH nanohybrid and stem cells embedded within hydrogels promoted the healing of rat tibial bone defects through the rapid formation of fully integrated neo-bone tissue through the activation of A2bR. The newly formed bone tissue displayed the key features of native bone, including calcification, mature tissue morphology, and vascularization. This study demonstrates a novel and effective strategy of bifunctional nanocarrier-mediated delivery of ligand (cargo molecule) and activation of its ligation to receptor by the nanocarrier itself for synergistically inducing stem cell differentiation and tissue healing in vivo, thus offering novel design of biomaterials for regenerative medicine.

Validation of a Na+-shift binding assay for estimation of the intrinsic efficacy of ligands at the A2A adenosine receptor

INTRODUCTION

Determination of the intrinsic efficacy of ligands at the A_2A receptor is important for selecting drug candidates, e.g. in the case of inflammatory diseases where agonists are searched for or in Parkinson disease (antagonists).

METHODS

Three functional binding assays were compared with up to seven ligands with different efficacies: the GTP-shift method based on the decrease of affinity observed with agonists when GTP is added to the competition binding assay; the K_i ratio method based on the different affinity states of the receptor when using an agonist or antagonist radioligand and the Na⁺-shift assay based on the difference of affinity of agonists when tested in a medium containing a divalent cation (50mM MgCl₂) favoring the G protein coupled agonist-receptor complex or sodium (100mM NaCl) as negative allosteric modulator.

RESULTS

The Na⁺-shift assay proposed herein successfully discriminated the full agonists CGS21680, NECA and adenosine (IC₅₀ ratio=13-14) from the weak inverse agonists ZM241385 and IBMX (IC₅₀ ratio=0.85) and the partial agonists LUF5834 and regadenoson (IC₅₀ ratios equal to 3 and 10, respectively).

DISCUSSION

We conclude that the Na⁺-shift assay proposed herein for the A_2A receptors has been validated and represents a rapid, economic and efficient functional binding assay to be used in a drug development program for early estimation of the intrinsic efficacy of hits.

Effects of extracellular sodium on mu-opioid receptors coupled to potassium channels coexpressed in Xenopus oocytes

Wild-type or mutant H297N or H297Q of the mu-opioid receptor were co-expressed with the inwardly rectifying potassium channel GIRK1 in oocytes from Xenopus laevis. Under voltage clamp, pairs of concentration response curves were generated using the agonist normorphine in a bathing medium containing 38.5 mM sodium or an identical medium in which the sodium was replaced by an equimolar concentration of choline. The maximum currents were greater in the presence of sodium by about 30% at wild-type receptors and by about 100% at the mutant receptors. The EC(50) values tended to increase somewhat as well, though these differences reached statistical significance only for the mutant H297Q. Flame photometry detected no change in the intracellular sodium or potassium concentrations of oocytes, suggesting that the effect of sodium was solely extracellular. Thus sodium, long known for its effects on in vitro ligand binding at mu-opioid receptors, also affects overall transduction as revealed in the Xenopus oocyte model of a complete, living cell system.

Adenosine A(2A) receptor facilitation of hippocampal synaptic transmission is dependent on tonic A(1) receptor inhibition

Adenosine tonically inhibits synaptic transmission through actions at A(1) receptors. It also facilitates synaptic transmission, but it is unclear if this facilitation results from pre- and/or postsynaptic A(2A) receptor activation or from indirect control of inhibitory GABAergic transmission. The A(2A) receptor agonist, CGS 21680 (10 nM), facilitated synaptic transmission in the CA1 area of rat hippocampal slices (by 14%), independent of whether or not GABAergic transmission was blocked by the GABA(A) and GABA(B) receptor antagonists, picrotoxin (50 microM) and CGP 55845 (1 microM), respectively. CGS 21680 (10 nM) also inhibited paired-pulse facilitation by 12%, an effect prevented by the A(2A) receptor antagonist, ZM 241385 (20 nM). These effects of CGS 21680 (10 nM) were occluded by adenosine deaminase (2 U/ml) and were made to reappear upon direct activation of A(1) receptors with N(6)-cyclopentyladenosine (CPA, 6 nM). CGS 21680 (10 nM) only facilitated (by 17%) the K(+)-evoked release of glutamate from superfused hippocampal synaptosomes in the presence of 100 nM CPA. This effect of CGS 21680 (10 nM), in contrast to the isoproterenol (30 microM) facilitation of glutamate release, was prevented by the protein kinase C inhibitors, chelerythrine (6 microM) and bisindolylmaleimide (1 microM), but not by the protein kinase A inhibitor, H-89 (1 microM). Isoproterenol (30 microM), but not CGS 21680 (10-300 nM), enhanced synaptosomal cAMP levels, indicating that the CGS 21680-induced facilitation of glutamate release involves a cAMP-independent protein kinase C activation. To discard any direct effect of CGS 21680 on adenosine A(1) receptor, we also show that in autoradiography experiments CGS 21680 only displaced the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentyladenosine ([(3)H]DPCPX, 0.5 nM) with an EC(50) of 1 microM in all brain areas studied and CGS 21680 (30 nM) failed to change the ability of CPA to displace DPCPX (1 nM) binding to CHO cells stably transfected with A(1) receptors. Our results suggest that A(2A) receptor agonists facilitate hippocampal synaptic transmission by attenuating the tonic effect of inhibitory presynaptic A(1) receptors located in glutamatergic nerve terminals. This might be a fine-tuning role for adenosine A(2A) receptors to allow frequency-dependent plasticity phenomena without compromising the A(1) receptor-mediated neuroprotective role of adenosine.

Effect of chronic manganese treatment on adenosine tissue levels and adenosine A2a receptor binding in diverse regions of mouse brain

In the present study the effects of chronic manganese (Mn) treatment on adenosine A2a receptor binding in mouse brain have been assessed. Male albino mice were divided in two groups: In the Mn-treated group, the animals were injected intraperitoneally (i.p.) with MnCl2 (5 mg/kg/day) five days per week during 9 weeks; in the control group, they were injected likewise with a saline solution. A significant decrease of the Kd without alteration of Bmax in the cerebellum and, an increase of the Kd and Bmax in hippocampus of mice treated with Mn were found. Also, an increase of Kd in frontal cortex was observed. The binding parameters in caudate nucleus, olfactory bulb and hypothalamus were not altered by Mn. A significant decrease in the adenosine concentration in caudate nucleus, olfactory bulb and hypothalamus, without significant changes in hippocampus, frontal cortex and cerebellum was also detected. These findings suggest that chronic administration of Mn could affect adenosine receptor function and turnover, depending on the brain region analyzed.

Regulation of agonist binding to rat ET(B) receptors by cations and GTPgammaS

Endothelins exert their physiological effects through interaction with cell surface receptors that are members of the G-protein-coupled receptor family. The endothelin receptor subtype B (ET(B) receptor) is abundantly expressed in rat cerebellum. Since agonist binding to G-protein-coupled receptors may be modulated by cations and guanine nucleotides, we investigated the effects of cations and guanosine 5'-O-(2-thiotriphosphate) (GTPgammaS) on 125I-endothelin-1 (125I-ET-1) binding to rat cerebellar membranes. Both Na+ and Mg2+-stimulated 125I-ET-1 binding causing an increase in receptor affinity for the agonist. While the effect of the divalent cation was evident at relatively low concentrations (5-10 mM), the stimulatory activity of the monovalent cation appeared at relatively high concentrations (50 mM). Additive activities of 25-50 mM NaCl and 1 mM MgCl2 suggested that monovalent and divalent cations increased receptor affinity for ET-1 by different mechanisms. In the presence of 5 mM MgCl2, 50 mM NaCl caused an additional modest reduction of the Kd value. Whereas 5 mM MgCl2 affected the displacement curves of both ET-3 and suc-[Glu9, Ala11,15]-endothelin-1 (8-21) (IRL 1620), the influence of 50 mM NaCl on these curves was less substantial. All together, these results suggest that modulation of receptor affinity by NaCl depends on the nature of the displacing agonist. In the presence of 5 mM MgCl2 or 50 mM NaCl, a partial regulation of 125I-ET-1 binding by GTPgammaS was detectable, while in the absence of cations no GTPgammaS-dependent inhibition was evident.

Functional uncoupling of adenosine A(2A) receptors and reduced responseto caffeine in mice lacking dopamine D2 receptors

Dopamine D(2) receptors (Rs) and adenosine A(2A)Rs are coexpressed on striatopallidal neurons, where they mediate opposing actions. In agreement with the idea that D(2)Rs tonically inhibit GABA release from these neurons, stimulation-evoked GABA release was significantly greater from striatal/pallidal slices from D(2)R null mutant (D(2)R(-/-)) than from wild-type (D(2)R(+/+)) mice. Release from heterozygous (D(2)R(+/-)) slices was intermediate. However, contrary to predictions that A(2A)R effects would be enhanced in D(2)R-deficient mice, the A(2A)R agonist CGS 21680 significantly increased GABA release only from D(2)R(+/+) slices. CGS 21680 modulation was observed when D(2)Rs were antagonized by raclopride, suggesting that an acute absence of D(2)Rs cannot explain the results. The lack of CGS 21680 modulation in the D(2)R-deficient mice was also not caused by a compensatory downregulation of A(2A)Rs in the striatum or globus pallidus. However, CGS 21680 significantly stimulated cAMP production only in D(2)R(+/+) striatal/pallidal slices. This functional uncoupling of A(2A)Rs in the D(2)R-deficient mice was not explained by reduced expression of G(s), G(olf), or type VI adenylyl cyclase. Locomotor activity induced by the adenosine receptor antagonist caffeine was significantly less pronounced in D(2)R(-/-) mice than in D(2)R(+/+) and D(2)R(+/-) mice, further supporting the idea that D(2)Rs are required for caffeine activation. Caffeine increased c-fos only in D(2)R(-/-) globus pallidus. The present results show that a targeted disruption of the D(2)R reduces coupling of A(2A)Rs on striatopallidal neurons and thereby responses to drugs that act on adenosine receptors. They also reinforce the ideas that D(2)Rs and A(2A)Rs are functionally opposed and that D(2)R-mediated effects normally predominate.

Autoradiographic comparison of the potency of several structurally unrelated adenosine receptor antagonists at adenosine A1 and A(2A) receptors

We have examined the potency of several adenosine receptor antagonists at adenosine A1 and A2A receptors using quantitative autoradiography and have compared the results with those of previous studies using the same radioligands in membrane preparations. The agonists [3H]cyclohexyladenosine and [3H]2-[p-(2-carbonylethyl)-phenylethylamino]-5'-N-ethylcarbo xamido adenosine ([3H]CGS 21680) were used as radioligands for the two receptors. The results show that 1,3-dipropyl-8-cyclopentyl xanthine (DPCPX) is almost 1000-fold and 8-chloro-4-cyclohexyl-amino-1-(trifluoromethyl)[1,2,4]triazolo[4,3-a] quinoxaline (CP-68,247) about 300-fold more potent at adenosine A1 receptors in cortex and striatum than at striatal adenosine A2A receptors. Conversely, 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo [1,5-c]pyrimidine (SCH 58261) is approximately 1000-fold and 4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-yl amino]ethyl)phenol (ZM 241,385) about 400-fold more potent at adenosine A2A than at A1 receptors. Caffeine and its metabolites did not show any selectivity. Other studied antagonists were non-selective or showed a modest (20- to 40-fold) adenosine A2A receptor selectivity. Thus, only a few of the antagonists show such high selectivity that it is not offset by differences in drug distribution and levels of receptor subtype expression.

Mechanisms of inhibitory effects of zinc and cadmium ions on agonist binding to adenosine A1 receptors in rat brain

The dose-dependent inhibition of zinc and cadmium ions of agonist binding to A1 adenosine receptors in rat brain is prevented by histidine and cysteine, respectively. In the present study, the possible different mechanisms of Zn2+ and Cd2+ inhibitions were examined. The effects of Zn2+ and Cd2+ on equilibrium binding parameters of the agonists N6-cyclohexyl-[2,8-3H]-adenosine ([3H]CHA) or chloro-N6-cyclopentyl-adenosine ([3H]CCPA) and the antagonist cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) were compared with those effects of reagents or binding conditions which altered histidyl or cysteinyl residues of the A1 receptor. Zn2+ pretreatment did not change A1 agonist or antagonist affinity, but did reduce the Bmax. The inhibitory effects of Zn2+ pretreatments were also maintained after several membrane washings. Diethylpyrocarbonate, a histidine-specific alkylating reagent, behaved like zinc ions: pretreatment with A1 agonist protected the histidyl residues of the [3H]CHA binding site against modification by Zn2+, while the modification of the protonation state of the nitrogen of the imidazole group of histidines by changing pH indicated that the interactions of Zn2+ with the histidyl residues were feasible with their unprotonated form. These findings suggest the formation of coordination bonds between Zn2+ and histidines critical for [3H]CHA or [3H]DPCPX binding, which may prevent the ligand interaction with the specific sites without modifying the binding kinetics of radioligand to the non-chelated recognition sites. Cd2+ pretreatment reduced the [3H]CCPA affinity, but did not modify the affinity of the antagonist [3H]DPCPX, the Bmax remaining unaffected. As with cadmium effects, the oxidation of the thiol group of cysteine by dithionitrobenzoic acid (DTNB) reduced [3H]CCPA affinity without changing the number of binding sites. The reducing reagent dithiothreitol, which alone was unable to modify [3H]CCPA binding, overcame the inhibiting effects of both Cd2+ and DTNB. These findings suggest that cadmium ions may oxidize SH groups of cysteines localized on the A1 receptor molecule or a cysteine localized in the region of G(i)alpha subunit involved in the coupling with receptors. This mechanism can justify potential conformational modifications of the receptor molecule producing the decrease in affinity.

Study of adenosine A2 receptors in membrane preparations from optic tectum of chicks

Binding properties of the subtypes of adenosine A2 receptors in membrane preparations and the effects of adenosine receptor ligands on cAMP accumulation in slices from the optic tectum of neonatal chicks have been investigated. [3H]2-[4-(2-p-carboxyethyl)phenylamino]-5'-N-ethylcarboxaminoadenosin e (CGS 21680), a selective ligand for adenosine A2a receptors, did not bind to optic tectal membranes, as observed with rat striatal membranes. CGS 21680 also did not induce cyclic AMP accumulation in optic tectum slices. However, 5'-N-ethylcarboxamidoadenosine (NECA), 2-chloro-adenosine or adenosine induced a 2.5- to 3-fold increase on cyclic AMP accumulation in this preparation. [3H]NECA binds to fresh non-washed-membranes obtained from optic tectum of chicks, displaying one population of binding sites, which can be displaced by NECA, 8-phenyltheophylline, 2-chloro-adenosine, but is not affected by CGS 21680. The estimated K(D) value was 400.90 +/- 80.50 nM and the Bmax was estimated to be 2.51 +/- 0.54 pmol/mg protein. Guanine nucleotides, which modulate G-proteins activity intracellularly, are also involved in the inhibition of glutamate responses by acting extracellularly. Moreover, we have previously reported that guanine nucleotides potentiate, while glutamate inhibits, adenosine-induced cyclic AMP accumulation in slices from optic tectum of chicks. However, the guanine nucleotides, GMP or GppNHp and the metabotropic glutamate receptors agonist, 1S,3R-ACPD did not alter the [3H]NECA binding observed in fresh non-washed-membranes. Therefore, the adenosine A2 receptor found in the optic tectum must be the adenosine A2b receptor which is available only in fresh membrane preparations, and its not modulated by guanine nucleotides or glutamate analogs.

Adenosine A2A receptor interactions with receptors for other neurotransmitters and neuromodulators

Adenosine, by activating adenosine A2A receptors, seems to have a crucial function in regulating the activation of multiple receptors that affect neurotransmitter release and/or synaptic transmission, in particular receptors for neuropeptides (calcitonin gene related peptide (CGRP) and vasoactive intestinal peptide (VIP)), and NMDA receptors, metabotropic glutamate receptors, nicotinic autofacilitatory receptors, dopamine receptors and adenosine A1 receptors. The manner in which these A2A receptors are involved in interactions with the receptors for other neurotransmitters and or neuromodulators opens novel avenues for the action of this 'omnipresent' nucleoside. Either by direct receptor-receptor modulation or by post-receptor mechanisms, adenosine, in its 'obsession' to protect cells from insults, uses as many receptor systems as possible to synchronize synaptic transmission, in order to exert what seems to be the 'destiny' of this nucleoside--protection of the nervous system.

Vasopressin-induced calcium signaling in cultured cortical neurons

Earlier autoradiographic studies from our laboratory detected vasopressin recognition sites in the mammalian cerebral cortex [R.E. Brinton, K.W. Gee, J.K. Wamsley, T.P. Davis, H.I. Yamamura, Regional distribution of putative vasopressin receptors in rat brain and pituitary by quantitative autoradiography, Proc. Natl. Acad. Sci. U. S.A., 81 (1984) 7248-7252; C. Chen, R.D. Brinton, T.J. Shors, R.F. Thompson, Vasopressin induction of long-lasting potentiation of synaptic transmission in the dentate gyrus, Hippocampus, 3 (1993) 193-204]. More recently, we have detected mRNA for the V1a vasopressin receptors (V1aRs) in cultured cortical neurons [R.S. Yamazaki, Q. Chen, S.S. Schreiber, R.D. Brinton, V1a Vasopressin receptor mRNA expression in cultured neurons, astroglia, and oligodendroglia of rat cerebral cortex, Mol. Brain Res., 45 (1996) 138-140]. To determine whether these recognition sites are functional receptors, we have pursued the signal transduction mechanism associated with the V1a vasopressin receptor in enriched cultures of cortical neurons. Results of these studies demonstrate that exposure of cortical neurons to the selective V1 vasopressin receptor agonist, [Phe2,Orn8]-vasotocin, (V1 agonist) induced a significant accumulation of [3H]inositol-1-phosphate ([3H]IP1). V1 agonist-induced accumulation of [3H]IP1 was concentration dependent and exhibited a linear dose response curve. Time course analysis of V1 agonist-induced accumulation of [3H]IP1 revealed a significant increase by 20 min which then decreased gradually over the remaining 60 min observation period. V1 agonist-induced accumulation of [3H]IP1 was blocked by a selective V1a vasopressin receptor antagonist, (Phenylac1, D-Tyr(Me)2, Arg6,8, Lys-NH29)-vasopressin. Results of calcium fluorometry studies indicated that V1 agonist exposure induced a marked and sustained rise in intracellular calcium which was abolished in the absence of extracellular calcium. The loss of the rise in intracellular calcium was not due to a failure to induce PIP2 hydrolysis since activation of the phosphatidylinositol pathway occurred in the absence of extracellular calcium. V1 agonist activation of calcium influx was then investigated. V1 agonist-induced 45Ca2+ uptake was concentration dependent with a biphasic time course at 250 nM. Preincubation with the L-type calcium channel blocker, nifedipine, blocked V1 agonist-induced calcium influx suggesting V1 agonist-induced L-type calcium channel activation in cortical neurons. Furthermore, V1 agonist-induced calcium influx was blocked by both bisindolyleimide I (PKC inhibitor) and U-73122 (PLC inhibitor) suggesting a modulation of V1 agonist-induced L-type calcium channel activation by downstream components of the phosphatidylinositol signaling pathway such as protein kinase C. These results indicate that in cultured cortical neurons, V1a vasopressin receptor activation leads to induction of the phosphatidylinositol signaling pathway, influx of extracellular calcium via L-type calcium channel activation, and a rise in intracellular calcium which is dependent on V1a receptor activated influx of extracellular calcium. These data are the first to demonstrate an effector mechanism for the V1 vasopressin receptor in the cerebral cortex and provide a potential biochemical mechanism that may underlie vasopressin enhancement of memory function.

5'-substituted adenosine analogs as new high-affinity partial agonists for the adenosine A1 receptor

5'-(Alkylthio)-, 5'-(methylseleno)-, and 5'-(alkylamino)-substituted analogues of N6-cyclopen-tyladenosine (CPA) were synthesized in 30-50% overall yields. The affinities of these compounds for the adenosine A1 and A2A receptors were determined in rat brain membranes. The 5'-substituted CPA analogues proved selective for the adenosine A1 receptors, displaying affinities in the nanomolar range. The compounds were also evaluated for their ability to stimulate [35S]GTP gamma S binding, also in rat brain membranes. The Ki values in receptor binding studies corresponded well to the EC50 values thus obtained. Intrinsic activities of the compounds were tested in vitro by determining the GTP shift in receptor binding studies as well as the maximal binding of [35S]GTP gamma S. It appeared that the 5'-thio and 5'-seleno derivatives in particular behaved as partial agonists.

Distribution and postnatal ontogeny of adenosine A2A receptors in rat brain: comparison with dopamine receptors

In adult rat brain, adenosine A2A receptors and dopamine D2 receptors are known to be located on the same cells where they interact in an antagonistic manner. In the present study we wanted to examine when this situation develops and compared the postnatal ontogeny of the binding of the adenosine A2A receptor agonist [3H]CGS 21680, the binding of the dopamine D1 receptor antagonist [3H]SCH 23390 and the dopamine D2 receptor antagonist [3H]raclopride. All three radioligands bound to the striatum at birth and this binding increased several-fold during the postnatal period. [3H]SCH 23390 binding developed first (mostly during the first week), followed by [3H]raclopride binding (first to third week) and [3H]CGS 21680 binding (only during second and third week). For all three radioligands the binding tended to decrease between 21 days and adulthood. This occurred earlier and was more pronounced in the globus pallidus than in the other examined structures. The increase in [3H]CGS 21680 binding from newborn to adult was mainly due to four-fold increase in the number of binding sites. The pharmacology of [3H]CGS 21680 binding to caudate-putamen was similar in newborn, one-week-old and adult animals, and was indicative of A2A receptors. The binding was inhibited by guanylyl imidodiphosphate at all ages, indicating that A2A receptors are G-protein-coupled already at birth. In contrast to the large increase in [3H]CGS 21680 binding, there was a decrease in the levels of A2A messenger RNA during the postnatal period in the caudate-putamen. In cerebral cortex [3H]CGS 21680 bound to a different site than the A2A receptor. From birth to adulthood cortical binding of [3H]CGS 21680 increased four-fold and that of the adenosine A1 agonist [3H]cyclohexyladenosine 19-fold. During early postnatal development [3H]SCH 23390 binding was higher in deep than in superficial cortical layers, but this difference disappeared in adult animals. There was binding of both [3H]CGS 21680 and [3H]cyclohexyladenosine to the olfactory bulb, suggesting a role of the two adenosine receptors in processing of olfactory information. [3H]CGS 21680 binding was present in the external plexiform layer and glomerular layer, and increased during development, but the density of binding sites was about one tenth of that seen in caudate putamen. [3H]cyclohexyladenosine showed a very different labelling pattern, resembling that observed with [3H]SCH 23390. Postnatal changes in adenosine receptors may explain age-dependent differences in stimulatory caffeine effects and endogenous protection against seizures. Since A2A receptors show a co-distribution with D2 receptors throughout development, caffeine may partly exert such actions by regulating the activity of D2 receptor-containing striatopallidal neurons.

Characterization of human A2A adenosine receptors with the antagonist radioligand [3H]-SCH 58261

1. We have characterized the binding of the new potent and selective antagonist radioligand [3H]-5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazol o[1,5- c]pyrimidine, [3H]-SCH 58261, to human cloned A2A adenosine receptors. 2. In Chinese hamster ovary (CHO) cells transfected with the human cloned A2A receptor, [3H]-SCH 58261 specific binding (about 70%) was rapid, saturable, reversible and proportional to protein concentration. The kinetic KD value was 0.75 nM. Saturation experiments showed that [3H]-SCH 58261 labelled a single class of recognition sites with high affinity (KD = 2.3 nM) and limited capacity (apparent Bmax = 526 fmol mg-1 protein). 3. Competition experiments revealed that binding of 0.5 nM [3H]-SCH 58261 was displaced by adenosine receptor agonists with the following order of potency: 2-hexynyl-5'-N-ethylcarboxamidoadenosine (2HE-NECA) > 5'-N-ethylcarboxamidoadenosine (NECA) = 2-phenylaminoadenosine (CV 1808) > 2-[4-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosi ne (CGS 21680) > R-N6-phenylisopropyladenosine (R-PIA) > or = N6-cyclohexyladenosine (CHA) > S-N6-phenylisopropyladenosine (S-PIA). 4. Adenosine receptor antagonists inhibited [3H]-SCH 58261 binding with the following order: 5-amino-9-chloro-2-(2-furyl)-[1,2,4]-triazolo[1,5-c] quinazoline (CGS 15943) > SCH 58261 > xanthine amine congener (XAC) > (E,18%-Z,82%)7-methyl-8-(3,4-dimethoxystyryl)-1,3- dipropylxanthine (KF 17837S) > 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) > theophylline. 5. Affinity values and rank order of potency of both receptor agonists and antagonists were similar to those previously obtained in human platelet and rat striatal membranes, except for CV 1808 which was more potent than CGS 21680. SCH 58261 was a competitive antagonist at inhibiting NECA-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation in CHO cells transfected with human A2A receptors. Good agreement was found between binding and functional data. 6. Thus, the new antagonist radioligand is preferable to the receptor agonist radioligand [3H]-CGS 21680 hitherto used to examine the pharmacology of human cloned A2A adenosine receptors.

Getting it together: signal transduction in G-protein coupled receptors by association of receptor domains

The mechanism of signal transduction by G-protein coupled receptors is unknown. Here, we propose that these receptors signal in a way that is qualitatively similar to that seen in the chemokine and endocrine hormone receptor families; the signal occurs when two domains of the receptor are brought together, although this is not the only requirement for signaling.

[125I]4-aminobenzyl-5'-N-methylcarboxamidoadenosine (125I)AB-MECA) labels multiple adenosine receptor subtypes in rat brain

Adenosine modulates neuronal activity and neurotransmitter release through interaction with cell surface receptors. Four adenosine receptor subtypes, A1, A2A, A2B, and A3 receptors, have been cloned and characterized. The agonist ligand, [125I]AB-MECA ([125I]4-aminobenzyl-5'N-methylcarboxamidoadenosine) has high affinity for recombinant A1 and A3 receptors [Olah et al., Mol. Pharmacol, 45 (1994) 978-982]. Rodent A3 receptors are relatively insensitive to xanthines; inhibition of A1 receptors with xanthines allows selective detection of A3 receptors despite the lack of selectivity of the ligand. We studied whether [125I]AB-MECA is useful for localization and characterization of A3 receptors in rat brain. The autoradiographic distribution of total [125I]AB-MECA (400 pM) binding closely resembled the pattern of A1 receptor binding, with highest levels in cerebellum, hippocampus, and thalamus, and moderate levels in cortex and striatum. Drug competition studies confirmed that almost all [125I]AB-MECA binding could be attributed to labeling of A1 receptors. Xanthine amine congener (1 microM) reduced specific [125I]AB-MECA binding by > 95%, indicating that xanthine-resistant A3 receptors represent a quantitatively minor subtype. Despite the use of a radioligand with high affinity and high specific activity, the low density of A3 receptors in rat brain appears insufficient to allow localization, or even consistent detection, of this receptor subtype. In the presence of DPCPX (50 nM, to block A1 receptors), residual [125I]AB-MECA binding to A2A receptors was observed in the striatum. Thus [125I]AB-MECA labels primarily A1 and A2A adenosine receptors in rat brain.

A2 adenosine receptors: their presence and neuromodulatory role in the central nervous system

1. Adenosine is an endogenous neuromodulator that exerts its depressant effect on neurons by acting on the A1 adenosine receptor subtype. Excitatory actions of adenosine, mediated by the activation of the A2 adenosine receptor subtype, have also been shown in the central nervous system. 2. Adenosine A2a receptors are highly localized in the striatum, as demonstrated by the binding assay of the A2a selective agonist, CGS2680, and by analysis of the A2 receptor mRNA localization with in situ hybridization histochemistry. However, adenosine A2a, receptors, albeit at lower levels, are also localized in other brain regions, such as the cortex and the hippocampus. 3. In the striatum, adenosine A2a, receptors are implicated in the control of motor activity. Evidences exists of an antagonistic interaction between adenosine A2a and dopamine D2 receptors. 4. Utilizing selective agonists and antagonists for adenosine A2a receptors, their role in the modulation of the release of several neurotransmitters (acetylcholine, dopamine, glutamate, GABA) has been extensively studied in the brain (striatum, cortex, hippocampus). Controversial results have been obtained and, because the overall effect of endogenous adenosine in the brain is that of an inhibitory tonus, the physiological meaning of the excitatory A2 receptor remains to be clarified.

Magnesium ion augmentation of inhibitory effects of adenosine on dopamine release in the rat striatum

The effects of adenosine and magnesium ion (Mg2+) on striatal dopamine release were studied in awake rats by in vivo microdialysis. The mean striatal basal levels of dopamine release at Mg2+ free perfusate were 56.95 +/- 5.30 fmol/sample (for 20 min). By varying the Mg2+ levels in perfusate from 0 mmol/L to 1, 10 or 40 mmol/L, the dopamine release was inhibited by Mg2+ in a level-dependent manner. Perfusion with modified Ringer's solution containing zero Mg2+ and from 5 to 50 mumol/L adenosine, non-selective adenosine agonist, as well as 0.1 mumol/L 2-chloro-N6-cyclopentyladenosine (CCPA), selective adenosine A1 agonist, showed no effect on dopamine release. However, from 5 to 50 mumol/L adenosine and from 0.1 to 1 mumol/L CCPA plus Mg2+ (1 and 40 mumol/L) perfusion decreased the dopamine release. This inhibitory effect of adenosine and CCPA on striatal dopamine release was enhanced by an increase in extracellular Mg2+ levels. Levels of 50 mumol/L of 8-cyclopentyl-1,3-dimethylxanthine (CPT), a selective adenosine A1 receptor antagonist, in perfusate increased the dopamine release under conditions both with and without Mg2+. This stimulatory effect of CPT on striatal dopamine release was reduced by an increase in extracellular Mg2+ levels. As a result, CPT antagonized the inhibitory effects of adenosine and CCPA on dopamine release under conditions of the presence and absence of Mg2+. These results suggest that the inhibition of striatal dopamine release by adenosine was mediated by adenosine A1 receptor. This inhibition was intensified by Mg2+. This study also revealed that the concentrations of Mg2+, which ranged from physiological to supraphysiological, reduced the striatal dopamine release; furthermore it was found that the physiological concentration of Mg2+ potentiated the effects of adenosine agonists, but inhibited adenosine antagonist. Thus, the present study, using in vivo microdialysis preparations, suggests Mg2+ inhibits the calcium ion channels and enhances the adenosinergic function in the central nervous system.

Binding of the radioligand [3H]-SCH 58261, a new non-xanthine A2A adenosine receptor antagonist, to rat striatal membranes

1. The present study describes the binding to rat striatal A2A adenosine receptors of the new potent and selective antagonist radioligand, [3H]-5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazol o [1,5-c] pyrimidine, [3H]-SCH 58261. 2. [3H]-SCH 58261 specific binding to rat striatal membranes ( > 90%) was saturable, reversible and dependent upon protein concentration. Saturation experiments revealed that [3H]-SCH 58261 labelled a single class of recognition sites with high affinity (Kd = 0.70 nM) and limited capacity (apparent Bmax = 971 fmol mg-1 of protein). The presence of 100 microM GTP in the incubation mixture did not modify [3H]-SCH 58261 binding parameters. 3. Competition experiments showed that [3H]-SCH 58261 binding is consistent with the labelling of A2A striatal receptors. Adenosine receptor agonists competed with the binding of 0.2 nM [3H]-SCH 58261 with the following order of potency: 2-hexynyl-5'-N-ethyl carboxamidoadenosine (2HE-NECA) > 5'-N-ethylcarboxamidoadenosine (NECA) > 2-[4-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosi ne (CGS 21680) > 2-phenylaminoadenosine (CV 1808) > R-N6-phenylisopropyladenosine (R-PIA) > N6-cyclohexyladenosine (CHA) = 2-chloro-N6-cyclopentyladenosine (CCPA) > S-N6-phenylisopropyladenosine (S-PIA). 4. Adenosine antagonists inhibited [3H]-SCH 58261 binding with the following order: 5-amino-9-chloro-2-(2-furyl)-[1,2,4]-triazolo[1,5-c] quinazoline (CGS 15943) > 5-amino-8-(4-fluorobenzyl)-2-(2-furyl)-pyrazolo [4,3-e]-1,2,4-triazolo [1,5-c] pyrimidine (8FB-PTP) = SCH 58261 > xanthine amine congener (XAC) = (E,18%-Z,82%)7-methyl-8-(3,4-dimethoxystyryl)-1,3-dipropylxanthine (KF 17837S) > 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) > or = 8-phenyltheophylline (8-PT). 5. The Ki values for adenosine antagonists were similar to those labelled with the A2A agonist [3H]-CGS 21680. Affinities of agonists were generally lower. The A1-selective agonist, R-PIA, was found to be about 9 fold more potent than its stereoisomer, S-PIA, thus showing the stereoselectivity of [3H]-SCH 58261 binding. Except for 8-PT, the adenosine agonists and antagonists examined inhibited [3H]-SCH 58261 binding with Hill coefficients not significantly different from unity. 6. The present results indicate that [3H]-SCH 58261 is the first non-xanthine adenosine antagonist radioligand which directly labels A2A striatal receptors. High receptor affinity, good selectivity and very low non-specific binding make [3H]-SCH 58261 an excellent probe for studying the A2A adenosine receptor subtype in mammalian brain.

Evidence for high-affinity binding sites for the adenosine A2A receptor agonist [3H] CGS 21680 in the rat hippocampus and cerebral cortex that are different from striatal A2A receptors

The binding of the adenosine A2A receptor selective agonist 2-[4-(2-p-carboxyethyl)phenylamino] -5'-N-ethylcarboxamidoadenosine (CGS 21680) to the rat hippocampal and cerebral cortical membranes was studied and compared with that to striatal membranes. [3H] CGS 21680, in the concentration range tested (0.2-200 nM), bound to a single site with a Kd of 58 nM and a Bmax of 353 fmol/mg protein in the hippocampus, and with a Kd of 58 nM and a Bmax of 264 fmol/mg protein in the cortex; in the striatum, the single high-affinity [3H] CGS 21680 binding site had a Kd of 17 nM and a Bmax of 419 fmol/mg protein. Both guanylylimidodiphosphate (100 microM) and Na+ (100 mM) reduced the affinity of [3H] CGS 21680 binding in the striatum by half and virtually abolished [3H] CGS 21680 binding in the hippocampus and cortex. The displacement curves of [3H] CGS 21680 binding with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), N6-cyclohexyladenosine (CHA), 5'-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine (CADO) were biphasic in the hippocampus and cortex as well as in the striatum. The predominant [3H]CGS 21680 binding site in the striatum (80%) had a pharmacological profile compatible with A2A receptors and was also present in the hippocampus and cortex, representing 10-25% of [3H]CGS 21680 binding. The predominant [3H]CGS 21680 binding site in the hippocampus and cortex had a pharmacological profile distinct from A2A receptors: the relative potency order of adenosine antagonists DPCPX, 1,3-dipropyl- 8-¿4-[(2-aminoethyl)amino]carbonylmethyl- oxyphenyl¿ xanthine (XAC), 8-(3-chlorostyryl)caffeine (CSC), and (E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)- methylxanthine (KF 17,837) as displacers of [3H] CGS 21680 (5 nM) binding in the hippocampus and cerebral cortex was DPCPX > XAC >> CSC approximately KF 17,837, and the relative potency order of adenosine agonists CHA, NECA, CADO, 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5'-N- ethylcarboxamidoadenosine (APEC), and 2-phenylaminoadenosine (CV 1808) was CHA approximately NECA > or = CADO > APEC approximately CV1808 > CGS 21680. In the presence of DPCPX (20 nM), [3H] CGS 21680 (0.2-200 nM) bound to a site (A2A-like) with a Kd of 20 nM and a Bmax of 56fmol/mg protein in the hippocampus and with a Kd of 22 nM and a Bmax of 63fmol/mg protein in the cortex. In the presence of CSC (200 nM), [3H]CGS 21680(0.2-200 nM) bound to a second high-affinity site with a Kd of 97 nM and a Bmax of 255 fmol/mg protein in the hippocampus and with a Kd of 112 nM and a Bmax of 221 fmol/mg protein in the cortex. Two pharmacologically distinct [3H]CGS 21680 binding sites were found in synaptosomal membranes of the hippocampus and cortex and in the striatum, one corresponding to A2A receptors and the other to the second high-affinity [3H]CGS 21680 binding site. In contrast, the pharmacology of [3H]CHA binding was similar in synaptosomal membranes of the three brain areas. The present results establish the existence of at least two high-affinity [3H]CGS 21680 binding sites in the CNS and demonstrate that the [3H]CGS 21680 binding site predominant in the hippocampus and cerebral cortex has different binding characteristics from the classic A2A adenosine receptor, which predominates in the striatum.

8-substituted adenosine and theophylline-7-riboside analogues as potential partial agonists for the adenosine A1 receptor

A series of 8-substituted adenosine and theophylline-7-riboside analogues (28 and 9 compounds, respectively) was tested on adenosine A1 and A2A receptors as an extensive exploration of the adenosine C8-region. Alkylamino substituents at the 8-position cause an affinity decrease for adenosine analogues, but an affinity increase for theophylline-7-riboside derivatives. The affinity decrease is probably due to a direct steric hindrance between the C8-substituent and the binding site as well as to electronic effects, not to a steric influence on the ribose moiety to adopt the anti conformation. The 8-substituents increase the affinity of theophylline-7-riboside analogues probably by binding to a lipophilic binding site. The intrinsic activity was tested in vitro for some 8-substituted adenosine analogues, by determining the GTP shift in receptor binding studies and the inhibition of adenylate cyclase in a culture of rat thyroid FRTL-5 cells, and in vivo in the rat cardiovascular system for 8-butylaminoadenosine. Thus, it was shown that 8-ethyl-, 8-butyl-, and 8-pentylamino substituted analogues of adenosine may be partial agonists in vitro, and that 8-butylaminoadenosine is a partial agonist for the rat cardiovascular A1 receptor in vivo.

Further characterization of the binding of the adenosine receptor agonist [3H]CGS 21680 to rat brain using autoradiography

2-[p-(2-carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadeno sine (CGS 21680) is considered a selective ligand for adenosine A2A receptors, which are known to be enriched in striatum and olfactory tubercle. We have investigated the characteristics of [3H]CGS 21680 binding in several brain regions using quantitative autoradiography. In agreement with previous data the radioligand was found to label the caudate-putamen, accumbens nucleus, olfactory tubercle and globus pallidus, but also many other structures, e.g. cerebral and cerebellar cortex, hippocampus, thalamus and some brainstem nuclei, were labelled. Cortical and striatal binding of [3H]CGS 21680 was unaltered by high concentrations of the adenosine transport inhibitor dipyridamole or the phosphodiesterase inhibitor rolipram but was displaced by 1,3-diethyl-8-phenylxanthine, the A2 selective adenosine antagonist CP 66,713, and the A2A selective agonist SHA 118. These three agents were approximately equipotent in striatum, cortex and hippocampus. The A2 selective agonist CV 1808 was a 4-5 times more potent displacer in cortex and hippocampus than in the striatum. [3H]CGS 21680 binding was strongly magnesium-dependent in all the studied brain regions, in contrast to the binding of adenosine A1 agonists. The binding of [3H]CGS 21680 to cerebral cortex and hippocampus, but not the binding to striatum, was displaced by the adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine in nanomolar concentrations. The present study provides evidence that in cerebral cortex and hippocampus, most of the [3H]CGS 21680 binds to a receptor site that is distinct from the striatal A2A receptor and the classical adenosine A1 receptor and may represent a hitherto unrecognized binding site.

A2a/D2 receptor interactions are not observed in COS-7 cells transiently transfected with dopamine D2 and adenosine A2a receptor cDNA

The rat D2 receptor and the dog A2a receptor subcloned into the pXM vector were transiently transfected into COS-7 cells using the DEAE-dextran method. The transfected cells expressed approx. 200 fmol D2 receptors/mg protein and approx. 5 pmol/mg protein of the A2a receptor as judged by binding experiments with [3H]raclopride [or[3H]-N-propyl-apomorphine (NPA)] and [3H]-CGS 21680, respectively. The high affinity KD values were 0.43 and 19 nM for D2 and A2a receptors, respectively, in agreement with results obtained from other cells and tissues. The non-selective adenosine receptor agonist NECA stimulated cAMP accumulation both in non-transfected and transfected COS-7 cells with only a slight difference in potency, suggesting that most of the stimulation is due to activation of A2b receptors known to be present on virtually every cell. The two A2a selective agonists CGS 21680 and CV-1808 were essentially inactive in transfected COS-7 cells, but were very active in PC-12 cells known to possess functional A2a receptors. Dopamine did not decrease cAMP accumulation in the transfected COS-7 cells. CGS 21680 (30 nM) did not affect the binding characteristics of D2 receptors in the co-transfected COS-7 cells in contrast to the increased KH, KL and RH values found previously in rat striatal membranes after CGS 21680 treatment. The present findings indicate that transiently transfected A2a and D2 receptors in COS-7 cells have normal binding properties, but couple poorly to adenylyl cyclase, despite the presence of Gs protein and adenylyl cyclase in these cells. Our results also demonstrate that the previously reported interactions between A2a receptors and D2 receptors do not occur when only the receptor proteins are expressed in COS-7 cells, suggesting that the two receptor molecules do not interact directly to influence binding characteristics.

Mechanism of the hippocampal loss of adenosine sensitivity in calcium-free media

Adenosine depresses the excitability of pyramidal neurones in the hippocampus. This effect is lost in calcium-free media and we have now investigated the mechanism of this. Extracellular recordings were made of antidromically and orthodromically evoked population potentials from CA1 region of rat hippocampal slices. It was observed that the activity of adenosine can be restored in the presence of procaine or carbamazepine, known inhibitors of sodium channels. The GABAB agonist baclofen was able to depress potential size but did not restore sensitivity to adenosine. It is concluded that the loss of postsynaptic sensitivity to adenosine in calcium-free solution results from the increased sodium conductances.

The effect of calcium removal on the suppression by adenosine of epileptiform activity in the hippocampus: demonstration of desensitization

1. Previous work has suggested that presynaptic effects of adenosine may be dependent on divalent cations. The present study was undertaken to determine whether a similar requirement existed at postsynaptic sites. 2. Extracellular recordings were made in the CA1 pyramidal cell layer of rat hippocampal slices following orthodromic stimulation of Schaffer collateral fibres in stratum radiatum or antidromic stimulation of the alveus. In antidromic stimulation experiments, CaCl2 was omitted (calcium-free medium) or reduced to 0.24 mM (low calcium medium) and in some experiments MgSO4 was increased to 2 mM. Kynurenic acid at concentrations of 1 and 5 mM in calcium-free medium and 1 mM in low calcium medium had no effect on secondary spike size. 3. Adenosine and baclofen induced a concentration-dependent reduction in the amplitude of orthodromic potentials with maximum effects at 20 and 5 microM respectively. 4. In nominally calcium-free medium, bursts of multiple population spikes were obtained in response to antidromic stimulation. Adenosine had little effect in reducing the secondary spike amplitude. At high concentration (2 mM) an initial depression was seen which declined within 3-5 min. 5. Sensitivity to adenosine was restored in low calcium medium or by raising magnesium. Although raising the divalent cation concentration increased the inhibitory effect of adenosine, desensitization was still seen. 6. 2-Chloroadenosine (100-500 microM) and R-PIA (50 microM), which are not substrates for either the nucleoside transporters or adenosine deaminase, were inactive in the absence of calcium. S-(2-hydroxy-5 nitrobenzyl)-6-thioinosine, an adenosine uptake blocker, at a concentration 100 MicroM had no effect on secondary potential size and did not restore adenosine sensitivity in calcium-free medium.7. Thapsigargin, which discharges intracellular calcium stores, had no significant effect at 1 MicroM on the bursts of action potentials and did not change the effect of 0.5 mM adenosine in calcium-free medium.8. Unlike adenosine, baclofen concentration-dependently reduced the secondary spike size in calcium free medium and no sign of recovery was observed during maintained superfusion for up to 45 min. No cross-desensitization was seen between baclofen and adenosine.9. Applications of adenosine locally by pressure to neuronal somata or dendrites still resulted in desensitized responses in calcium-free medium.10. It is concluded that the postsynaptic sensitivity to adenosine is dependent on the concentration of divalent cations in the extracellular space implying an effect of cations on adenosine receptor activation or transduction processes.

The binding of the adenosine A2 receptor selective agonist [3H]CGS 21680 to rat cortex differs from its binding to rat striatum

The binding of the reportedly A2A selective agonist CGS 21680 (2-[p-(2-carboxyethyl)phenylethylamino]-5'N-ethylcarboxamidoadenos ine) to cortex and striatum was examined in parallel using quantitative receptor autoradiography. [3H]CGS 21680 bound to a single site in rat striatum with KD 2.3 nM and Bmax 320 fmol/mg grey matter. In addition [3H]CGS 21680 bound to a single site in the cerebral cortex with KD 47 nM and Bmax 100 fmol/mg grey matter. In cat cortex [3H]CGS 21680 (2 nM) binding was strong and particularly evident in the most superficial layers. The potency order for inhibition of 2 nM [3H]CGS 21680 binding to rat striatum was NECA (5'-N-ethylcarboxamidoadenosine; IC50 9.0 nM) > 2-CADO (2-chloroadenosine; 87 nM) > R-PIA (N6-(R)-phenylisopropyladenosine; 110 nM). The potency order for inhibition of 2 nM [3H]CGS 21680 binding to rat cortex was NECA (3.0 nM) > 2-CADO (14 nM) > or = R-PIA (16 nM). Gpp(NH)p (5'-guanylyl imidodiphosphate) inhibited [3H]CGS 21680 binding to both cortex and striatum, but more potently in cortex (IC50 100 nM vs. 470 nM). The present results show that there is a cortical binding site for [3H]CGS 21680 which appears to be different from the the striatal A2A receptor, the A2B receptor and the A1 receptor.