Functional activity of the adenosine binding enhancer, PD 81,723, in the in vitro hippocampal slice

Allosteric modulation of purine and pyrimidine receptors

Among the purine and pyrimidine receptors, the discovery of small molecular allosteric modulators has been most highly advanced for the A(1) and A(3) adenosine receptors (ARs). These AR modulators have allosteric effects that are structurally separated from the orthosteric effects in SAR studies. The benzoylthiophene derivatives tend to act as allosteric agonists as well as selective positive allosteric modulators (PAMs) of the A(1) AR. A 2-amino-3-aroylthiophene derivative T-62 has been under development as a PAM of the A(1) AR for the treatment of chronic pain. Several structurally distinct classes of allosteric modulators of the human A(3) AR have been reported: 3-(2-pyridinyl)isoquinolines, 2,4-disubstituted quinolines, 1H-imidazo-[4,5-c]quinolin-4-amines, endocannabinoid 2-arachidonylglycerol, and the food dye Brilliant Black BN. Site-directed mutagenesis of A(1) and A(3) ARs has identified residues associated with the allosteric effect, distinct from those that affect orthosteric binding. A few small molecular allosteric modulators have been reported for several of the P2X ligand-gated ion channels and the G protein-coupled P2Y receptor nucleotides. Metal ion modulation of the P2X receptors has been extensively explored. The allosteric approach to modulation of purine and pyrimidine receptors looks promising for development of drugs that are event and site specific in action.

Synthesis of 2-amino-3-heteroaroylthiophenes and evaluation of their activity as potential allosteric enhancers at the human A1 receptor

2-Amino-3-benzoylthiophenes are allosteric enhancers of agonist binding to the adenosine A(1) receptor. New compounds bearing an heteroaroyl instead of the benzoyl moiety at the 3-position of the thiophene were synthesized. The phenyl ring was replaced with heterocycles that possess heteroatoms able to form hydrogen bonds (2-furanyl, 2-benzofuranyl, 2-pyridinyl in compounds 2-13) or with a thienyl moiety as isoster of the phenyl ring (2-thienyl, 3-thienyl and 5-halo-2-thienyl in compounds 14-29). The effect of several alkyl substituents at positions 4 and 5 of the thiophene ring to increase enhancer activity was determined. The ability of the new molecules to reduce the cAMP content in CHO cells expressing the human adenosine A(1) receptor was evaluated. Compounds 2-13 with hydrogen bond-forming heteroatoms did not show significant activity as allosteric enhancers. On the other hand, compounds 15-16 and 19-20 with an unsubstituted thienyl moiety as replacement for the phenyl ring were nearly as efficacious as PD 81,723, the prototypical A(1) allosteric enhancer. Alkyl substituents at positions 4 and 5 of the thiophene ring were tolerated while a substituted piperidine ring was not tolerated. We conclude that hydrogen bonds could not be formed in the domain of the receptor that accommodates the phenyl ring of 2-amino-3-benzoylthiophene derivatives, indicating that this domain is hydrophobic.

Allosteric enhancers of A1 adenosine receptors increase receptor-G protein coupling and counteract Guanine nucleotide effects on agonist binding

Endogenous ligands of G protein-coupled receptors bind to orthosteric sites that are topologically distinct from allosteric sites. Certain aminothiophenes such as (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluromethyl)-phenyl]-methanone (PD81,723) and 2-amino-4,5,6,7-tetrahydro-benzo[b]thiophen-3-yl)-biphenyl-4-yl-methanone (ATL525) are positive allosteric regulators, or enhancers, of the human A1 adenosine receptor (A1AR). In equilibrium binding assays, 125I-N6-aminobenzyladenosine (125I-ABA) binds to two affinity states of A1AR with KD-high (0.33 microM) and KD-low ( approximately 10 nM). Enhancers have little effect on KD-high but convert all A1AR binding sites to the high-affinity state. Enhancers decrease the potency of guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) as an inhibitor of agonist binding by 100-fold and increase agonist-stimulated guanine nucleotide exchange. The association of 125I-ABA to high-affinity receptors on Chinese hamster ovary (CHO)-hA1 membranes does not follow theoretical single-site association kinetics but is approximated by a bi-exponential equation with t1/2 values of 1.85 and 12.8 min. Allosteric enhancers selectively increase the number of slow binding sites, possibly by stabilizing newly formed receptor-G protein complexes. A new rapid assay method scores enhancer activity on a scale from 0 to 100 based on their ability to prevent the rapid dissociation of 125I-ABA from A1AR in response to GTPgammaS. Compared with PD81,723, ATL525 (100 microM) scores higher (27 versus 79) and has less antagonist activity. ATL525 functionally enhances A1 signaling to inhibit cAMP accumulation in CHO-hA1 cells. These data suggest that simultaneously binding orthosteric and allosteric enhancer ligands convert the A1AR from partly to fully coupled to G proteins and prevents rapid uncoupling upon binding of GTPgammaS.

Hippocampal injury and neurobehavioral deficits are improved by PD 81,723 following hyperglycemic cerebral ischemia

We investigated the effects of PD 81,723, an allosteric enhancer for the adenosine A(1) receptor subtype, on hippocampal injury and Morris water maze (MWM) performance following hyperglycemic cerebral ischemia and reperfusion (4-VO, 10 min) in the rat. PD 81,723 (3 or 10 mg/kg) or the equivalent volume of saline was administered intraperitoneally 30 min prior to ischemia. Moderate hyperglycemia was achieved by administration of D-glucose (3g/kg, i.p.) 15 min prior to induction of ischemia. Morris water maze trials were performed on the 6th, 7th, and 8th days after the ischemic insult. The rat brains were sectioned (8 microm), stained with cresyl violet/acid fuchsin, and evaluated for hippocampal ischemic injury by an experimenter blinded to the treatment conditions. At the higher dose, PD 81,723 (10 mg/kg) had no effect on hippocampal injury or MWM performance following hyperglycemic ischemia compared to corresponding saline-treated animals. In contrast, a lower dose of PD 81,723 (3 mg/kg) resulted in significant (P < 0.05, n = 8) reduction of hippocampal injury following hyperglycemic ischemia. Furthermore, corresponding Morris water maze performance (latency, learning index, and cumulative distance swum) was significantly improved by PD 81,723 (P < 0.05, n = 8). The results of the present study suggest that, in the presence of PD 81,723, an A(1) allosteric enhancer, endogenously produced adenosine is sufficient to exert significant neuroprotection during hyperglycemic ischemia. Moreover, the present study provides further evidence for a neuromodulatory role of adenosine during hyperglycemic ischemia.

Synthesis and biological effects of novel 2-amino-3-naphthoylthiophenes as allosteric enhancers of the A1 adenosine receptor

The current study describes the synthesis and biological evaluation of a novel series of 2-amino-3-naphthoylthiophenes, with variable modifications at the 4- and 5-position of the thiophene as well as the naphthoyl ring. Allosteric enhancer activity was measured in several ways: (1) evaluating the effect on forskolin-stimulated cAMP accumulation in the presence of an A(1)-adenosine agonist (CPA) in Chinese hamster ovary (CHO) cells expressing the cloned human A(1)-adenosine receptor (hA(1)AR); (2) ability of these compounds to displace the binding of [(3)H]DPCPX, [(3)H]ZM 241385, and [(3)H]MRE 3008F20 to the ligand binding site of CHO cells expressing the hA(1), hA(2A), and hA(3) adenosine receptors, respectively; (3) effect on the binding of [(3)H]CCPA to membranes from CHO cells expressing hA(1)AR, to rat brain and human cortex membrane preparations containing native adenosine A(1) receptors; (4) kinetics of the dissociation of [(3)H]CCPA from CHO-hA1 membranes. The pharmacological assays compared the various activities to that of the reference compound PD 81,723 (compound 1). Several compounds appeared to be better than PD 81,723 to enhance the effect of CPA (and thus reduce cAMP content) in the CHO:hA(1) assay. The effect of these compounds at a concentration of 10 microM was slightly greater than that of the same concentration of the PD 81,723 and substantially greater than that of PD 81,723 when responses to 1 microM of each compound were compared. These include compounds 23, 25-29, 31-34, 38, 39, 43, and 58. Cycloalkylthiophenes tended to be more potent then their 4,5-dimethyl analogues, and in the series of cycloalkylthiophenes, tetrahydrobenzo[b]thiophene derivatives appeared to be more potent than the dihydrocyclopentadien[b]thiophene counterparts. Some of the most potent compounds were tested at a concentration of 10 microM for their affinity as competitors to the antagonist binding site of CHO cells expressing hA(1), hA(2A), and hA(3) adenosine receptors. None inhibited binding at the hA(2A)AR, but most of them inhibited binding to the hA(1)AR to varying extents (0-19%) as well as to the hA(3)AR to a substantial degree (0-57%). At a concentration of 10 microM, the compounds 31, 34, 37, 38, and 39 were more active than PD 81,723 to increase the binding of [(3)H]CCPA to CHO:hA(1), human brain and rat cortex membranes. Compound 37 was the most active compound increasing the binding to CHO:hA(1), human brain, and rat cortex membranes by 149, 43, and 27%, respectively (51, 15, and 22%, respectively, for PD 81,723). A good correlation was found between the increments [(3)H]CCPA binding to A(1) receptors expressed in different systems. Unlike the effect on agonist binding, the tested compounds did not increase the binding of the antagonist [(3)H]DPCPX on hCHO-A(1) membranes. Ligand dissociation studies revealed that two compounds (22 and 39) were more potent than 1 to slow the dissociation of [(3)H]CCPA from CHO:hA(1)AR membranes. No clear-cut structure-activity relationship can be observed based on data from the functional assay, but we have identified several compounds, in particular 37 and 39, which appeared to be more potent than 1 and that may be selected for further development.

Effects of the adenosine A1 receptor allosteric modulators PD 81,723 and LUF 5484 on the striatal acetylcholine release

The objective of the present study was to characterize the adenosine A(1) receptor allosteric enhancing and antagonistic actions of (2-amino-4,5,6,7-tetrahydrobenzo[b]thiophen-3-yl)(3,4-dichlorophenyl)methanone (LUF 5484) and (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone (PD 81,723) on striatal acetylcholine release. Upon local administration in conscious rats, LUF 5484 or PD 81,723 caused a concentration-dependent increase of extracellular acetylcholine levels of approximately 40%, which was similar to that obtained by the selective adenosine A(1) receptor antagonists 8-cyclopentyl-1,3-dimethylxanthine (8CPT) and N(6)-cyclopentyl-9-methyladenine (N0840). In interaction experiments, LUF 5484 or PD 81,723 did not change the inhibition of acetylcholine release by the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA), whereas 8CPT caused an eightfold rightward shift. Acetylcholine concentrations were diminished with 62+/-3%, 48+/-11% and 56+/-9% by CPA, CPA+LUF 5484 and CPA+PD 81,723, respectively. In conclusion, the antagonistic action of LUF 5484 and PD 81,723 seems to counteract the putative allosteric actions with respect to the reduction of striatal acetylcholine release.

Modeling the functional effects of allosteric modulators at pharmacological receptors: an extension of the two-state model of receptor activation

Allosteric modulation is a mechanism for modifying pharmacological receptor activity that has largely been ignored in terms of therapeutic drug design, although benzodiazepine receptor ligands are an example of the serendipitous discovery of this class of compound. The current mathematical models of allosteric interactions at (particularly G-protein-coupled) receptors concentrate on the effects of the allosteric ligand on orthosteric ligand binding and ignore potential effects of these compounds on the ability of orthosteric ligands to cause receptor activation. In this report a mathematical model of allosteric interactions at pharmacological receptors has been investigated that explicitly includes effects of the allosteric ligand on receptor activation. This model uses the two-state model of receptor activation as its basis and is qualitatively consistent with currently reported behavior of allosteric modulators. The predictions of this model suggest a series of criteria that should be tested before the effects of an allosteric modulator can be quantified in a nonsystem-dependent manner. It has also been used to provide a potential mechanistic explanation for the functional effects of the A(1) adenosine receptor allosteric enhancer PD 81,723 and a recently reported allosteric modulator of type 1 metabotropic glutamate receptors.

Differential effects of the adenosine A(1) receptor allosteric enhancer PD 81,723 on agonist binding to brain and adipocyte membranes

The benzoylthiophene analog, PD 81,723, has been shown to allosterically enhance agonist binding and functional activation of the mammalian adenosine (ADO) A(1) receptor subtype by putatively maintaining the receptor in a high affinity state. The present studies were conducted to evaluate the ability of PD 81,723 to enhance the binding of [3H]cyclohexyladenosine ([3H]CHA) to A(1) receptors of neural (cerebral cortex) and non-neural (adipocyte) origin in three different species; rat, guinea pig and dog. PD 81, 723 (0.3-100 microM) produced a concentration-dependent enhancement of [3H]CHA binding to rat brain A(1) receptors. These effects were also species-dependent with larger enhancements (150-200% of control) observed in guinea pig and dog brain membranes as compared to the rat (120% of control). In contrast, PD 81,723 did not produce any enhancement of [3H]CHA binding to A(1) receptors in adipocyte membranes from any of the species examined. Additional binding studies were conducted using pharmacological manipulations that have previously been shown to enhance the allosteric effects of PD 81,723. In the presence of 1 mM GTP, the allosteric effects of PD 81,723 (15 microM) were increased in rat, guinea pig and dog brain membranes, however, in adipocyte membranes from each species, no significant alteration in agonist binding was observed. Similarly, the A(1) receptor selective antagonist 8-cyclopentyl-1, 3-dipropylxanthine (added to effectively reduce the intrinsic antagonist properties of PD 81,723) was found to enhance the allosteric effects of PD 81,723 (15 microM) in brain, but produce no alteration of agonist binding in adipocyte membranes from each species. Examination of the dissociation kinetics of [3H]CHA binding from rat brain and adipocyte membranes revealed that PD 81,723 (15 microM) differentially slowed agonist dissociation from brain, but not adipocyte, membranes. Taken together, the present data support the hypothesis that in tissues that are sensitive to PD 81,723, this benzyolthiophene functions to maintain the A(1) receptor in a high-affinity state and that the relative proportions of high-affinity A(1) receptors present in specific tissues may contribute, at least in part, to the apparent differential effects of PD 81,723 on agonist binding. The tissue specific modulation of A(1) receptor function by PD 81,723 also illustrates the possibility that the locus of allosteric modulation by PD 81,723 may be manifest via a specific, but indirect and tissue-dependent, interaction with the A(1) receptor.

Enhancing adenosine A1 receptor binding reduces hypoxic-ischemic brain injury in newborn rats

Hypoxia increases brain adenosine concentrations, which provides neuroprotection through activation of central adenosine A1 receptors. This study was carried out to determine whether PD 81,273, which increases adenosine's binding to A1 receptors, would reduce hypoxia-induced brain injury. PD 81,273 (3 mg/kg, i.p.) decreased by about 50% the weight loss of the left cerebral hemisphere caused by hypoxia-ischemia in neonatal rats. Thus, enhancing adenosine's binding to the A1 receptor decreases hypoxic brain damage.

Adenosine A1 receptor enhancer, PD 81,723, and cerebral ischemia/reperfusion injury in the gerbil

1. The adenosine A1 receptor enhancer, PD 81,723, was tested for its neuroprotective activity in a Mongolian gerbil model of forebrain ischemia/reperfusion cerebral ischemia. 2. Gerbils were injected with PD 81,723 (1, 10 and 125 mg/kg i.p.) 20 min before a 5-min episode of forebrain ischemia. The extent of ischemic injury was assessed by monitoring the increases in locomotor activity and from the degree of damage to the CA1 hippocampal pyramidal cell layer after 5 days of recovery. 3. By both criteria, PD 81,723, at all three dose levels, failed to protect against ischemia/reperfusion evoked cerebral injury.

Enhancement of adenosine A1 receptor functions by benzoylthiophenes in guinea pig tissues in vitro

Previous reports on a series of benzoylthiophenes, including PD 81,723 [2-amino-4,5-dimethyl-3-(3-trifluoromethyl-benzoyl) thiophene], have shown specific enhancement of agonist binding at the adenosine A1 receptor. We have studied the effects of two substituted benzoylthiophenes, PD 78,416 (thieno[2,3-c]pyridine-6(5H)-carboxylic acid, 2-amino-3-benzoyl-4,7-dihydro-ethyl ester) and RS-74513-000 [2-amino-4-ethyl-5-methyl-3-(3-trifluoro-methyl-benzoyl) thiophene] on response elicited by adenosine A1 receptors in isolated guinea pig left atrium and ileum. In the electrically paced left atrium, PD 78,416 antagonized negative inotropic effect elicited by the agonist CPA [N6-cyclopentyladenosine] with a pKB value of 6.2 +/- 0.2 (n = 4). At a low concentration which had no antagonistic effect (0.1 microM), PD 78,416 enhanced the effect of CPA. The concentration-response curve to CPA was shifted leftward by 5.1 fold (95% confidence limits 2.4-11.2). In field stimulated isolated ileum, PD 78,416 (0.1, 0.3, 1 microM) did not enhance or antagonize effects of CPA. At concentrations above 1 microM, PD 78,416 decreased electrically induced contraction. This effect was not sensitive to adenosine deaminase and was not antagonized by the A1 antagonist CPX [8-cyclopentyl-1,3-dipropyl-xanthine] (1 microM). Unlike PD 78,416, RS-74513-000 (0.01, 0.1, 1, 3, 10 microM) did not antagonize or enhance effects of CPA in the left atrium. However, effects of CPA in ileum were enhanced by RS-74513-000 (1 and 3 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of glutamate, aspartate, and GABA release from ischemic rat cerebral cortex

The purpose of this study was to evaluate potential mechanisms of ischemia-evoked amino acid transmitter release. Changes in extracellular levels of transmitter amino acids and lactic acid dehydrogenase (LDH) in rat cerebral cortex during and following four-vessel occlusion elicited global cerebral ischemia were examined using a cortical cup technique. Ischemia-evoked release of glutamate, aspartate and gamma-amino-butyric acid (GABA) was compared in control vs. drug-treated animals. Tetrodotoxin and antagonists of glutamate receptors (DNQX, MK-801, and AP-3) depressed the initial rate of increase in extracellular glutamate and aspartate without altering the total amount of these amino acids collected in the cortical superfusates. Cobalt, a calcium channel antagonist, failed to alter efflux. Acidic amino acid transport inhibitors (dihydrokainate, L-trans-PDC) depressed the rate of onset of glutamate and aspartate release and dihydrokainate depressed total release by 44%. PD 81723, an allosteric enhancer at the A1 adenosine receptor, depressed glutamate efflux, as did L-NAME, an inhibitor of nitric oxide synthase. Extracellular increases in GABA levels were depressed by tetrodotoxin and L-trans-PDC. The GABA transport inhibitor, nipecotic acid, increased the initial rate of onset of GABA release. Increases in LDH levels in the extracellular fluid became apparent during the period of ischemia and continued to increase during the subsequent 90 min of reperfusion. These results suggest that ischemia evokes a release of neurotransmitter amino acids that is only partially dependent upon Ca2+ influx activation or the reversal of amino acid transporters. Nonselective mechanisms, resulting from the disruption of plasma membrane integrity, may contribute significantly to the total ischemia-evoked release of excitatory amino acids.

The adenosine binding enhancer, PD 81,723, inhibits epileptiform bursting in the hippocampal brain slice

The effects of adenosine and the adenosine binding enhancer, PD 81,723, on low magnesium-induced bursting in the in vitro hippocampal slice were examined. Extracellular recordings were obtained from the CA3 pyramidal cell layer while electrically stimulating in the stratum radiatum under low magnesium perfusion. Adenosine (6-100 microM) reduced the duration of epileptiform bursting in a dose-related manner, which was reversible upon washout of adenosine. Application of PD 81,723 (50-100 microM) also resulted in a dose-dependent reduction in the duration of the triggered burst, which was irreversible. These results demonstrate anticonvulsant activity of adenosine and the adenosine binding enhancer, PD 81,723, in the low magnesium model of epilepsy.

Hippocampal localization of 5'-nucleotidase as revealed by immunocytochemistry

The distribution of binding sites for an antibody against ecto-5'-nucleotidase was investigated in the mouse hippocampus by light microscopical immunocytochemistry. The antibody selectively labels a band corresponding to the innervation area of mossy fibre terminals within area CA3. Area CA1 as well as the dendate gyrus are negative. In area CA3 only the proximal but not the distal parts of the apical dendrites of pyramidal cells are labelled. Labelling is in the form of large dots around dendrites of pyramidal cells suggesting that mossy fibre terminals are immunopositive. In contrast, an antibody against the ubiquitous synaptic vesicle protein SV2 labels the large mossy fibre terminals as well as fine and punctate structures in the dendritic and somatic regions throughout the hippocampus. Labelled astrocytes can be found in the entire hippocampus and are frequent in the stratum radiatum and stratum oriens of the CA1 region. Immunopositive astrocytic processes can be found in association with capillary walls. Our results suggest that ecto-5'-nucleotidase may play a crucial role in the hydrolysis of AMP to adenosine at the mossy fibre synapses. Thus, at these synapses, 5'-nucleotidases could function both in completing the extracellular hydrolysis of synaptically released ATP as well as in the extracellular formation of adenosine.