3-[(±)-2-Carboxypiperazin-4-yl]propyl-1-Phosphonic Acid Recognizes Two N-Methyl-D-Aspartate Binding Sites in Rat Cerebral Cortex Membranes

Binding of 3-[(+-)-2-carboxypiperazin-4-yl][3H]-propyl-1-phosphonic acid ([3H]CPP), a competitive inhibitor of N-methyl-D-aspartate (NMDA), has been studied in synaptic plasma membranes from rat cerebral cortex. Computer analysis of saturation and homologous displacement isotherms deriving from these plasma membranes indicated the existence of two binding sites: a specific, saturable, high-affinity binding site with a pKD value of 7.53 +/- 0.03 (29.5 nM) and a maximum binding value (Bmax) of 2.25 +/- 0.36 pmol/mg of protein, and a low-affinity site with a KD of approximately 600 nM and a Bmax of 7.0 pmol/mg of protein. It is argued that, in the light of current literature evidence, the low-affinity binding site may represent an agonist-dependent receptor, linked to physiological processes such as neurotransmitter release and channel regulation, whereas the high-affinity binding site may be linked to an antagonist-preferred receptor, for which no function has yet been reported.

Selective down-regulation of [(125)I]Y0-alpha-conotoxin MII binding in rat mesostriatal dopamine pathway following continuous infusion of nicotine

Prolonged exposure to nicotine, as occurs in smokers, results in up-regulation of all the neuronal nicotinic acetylcholine receptor subtypes studied so far, the only differences residing in the extent and time course of the up-regulation. alpha6beta2*-Nicotinic acetylcholine receptors are selectively enriched in the mesostriatal dopaminergic system and may play a crucial role in nicotine dependence. Here we show that chronic nicotine treatment (3mg/kg/day for two weeks, via s.c. osmotic minipumps) caused a significant decrease (36% on average) in the binding of [(125)I]Y(0)-alpha-conotoxin MII (a selective ligand for alpha6beta2*-nicotinic acetylcholine receptors in this system) to all the five regions of the rat dopaminergic pathway analyzed in this study. After one week of withdrawal, binding was still lower than control in striatal terminal regions (namely the caudate putamen and the accumbens shell and core). In somatodendritic regions (the ventral tegmental area and the substantia nigra) the decrease was significant at the end of the treatment and recovered within one day of withdrawal. This effect was not due to displacement of [(125)I]Y(0)-alpha-conotoxin MII binding by residual nicotine. In fact the binding was not changed by 565 ng/g nicotine (obtained with a single injection of nicotine), a concentration much higher than that found in the brain of rats chronically treated with nicotine (240 ng/g). In addition, consistent with previous studies reporting an up-regulation of other subtypes of nicotinic acetylcholine receptors, we found that nicotine exposure significantly increased (40% on average) the binding of [(125)I]epibatidine (a non-selective agonist at most neuronal heteromeric nicotinic acetylcholine receptors) in three up to five regions containing only alpha-conotoxin MII-insensitive [(125)I]epibatidine binding sites, namely the primary motor, somatosensory and auditory cortices. In conclusion, this work is the first to demonstrate that alpha6beta2*-nicotinic acetylcholine receptors, unique within the nicotinic acetylcholine receptor family, are down-regulated following chronic nicotine treatment in rat dopaminergic mesostriatal pathway, a finding that may shed new light in the complex mechanisms of nicotine dependence.

Allosteric modulation of [3H]-CGP39653 binding through the glycine site of the NMDA receptor: further studies in rat and human brain

Binding of D,L-(E)-2-amino-4-[(3)H]-propyl-5-phosphono-3-pentenoic acid ([(3)H]-CGP39653), a selective antagonist at the glutamate site of the NMDA receptor, is modulated by glycine in rat brain tissue. We have further investigated this phenomenon in rodent and human brain by means of receptor binding and quantitative autoradiography techniques. In rat cerebral cortical membranes the glycine antagonist 3-[2-(Phenylaminocarbonyl)ethenyl]-4,6-dichloro-indole-2-carboxylic acid sodium salt (GV150526A) did not change basal [(3)H]-CGP39653 binding, but competitively reversed the high affinity component of [(3)H]-CGP39653 binding inhibition by glycine, with a pK(B) value of 8.38, in line with its affinity for the glycine site (pK(i)=8.49 vs. [(3)H]-glycine). Glycine (10 microM) significantly decreased [(3)H]-CGP39653 affinity for the NMDA receptor (with no change in the B(max)), whereas enhanced L-glutamate affinity (P<0.05, paired-samples Student's t-test). In rat brain sections the addition of GV150526A (30 microM) to the incubation medium increased [(3)H]-CGP39653 binding to 208% of control (average between areas), indicating the presence of endogenous glycine. The enhancement presented significant regional differences (P<0.05, two-way ANOVA), with striatum higher than cerebral cortex (282 and 187% of control, respectively; P<0.05, Fisher's LSD). On the contrary, there was not any significant variation in affinity values of [(3)H]-CGP39653, L-glutamate, glycine and GV150526A in striatal and cortical membranes. These results confirmed the existence of regionally distinct NMDA receptors subtypes with different glycine/glutamate allosteric modulation. Whole brain autoradiography revealed an uneven distribution of [(3)H]-CGP39653 binding sites in human brain. High levels of binding were determined in hippocampus and in cingulate, frontoparietal and insular cortex. Intermediate to low levels of binding were found in diencephalic nuclei and basal ganglia. [(3)H]-CGP39653 binding was increased to 216% of control (mean between areas) by 30 microM GV150526A. The enhancement, however, did not present significant regional differences. These results introduce GV150526A as a useful tool to identify NMDA receptor subtypes by means of receptor autoradiography; moreover, they demonstrate that the allosteric inhibition of [(3)H]-CGP39653 binding by glycine parallels an increase in receptor affinity to the endogenous ligand L-glutamate. Finally, this study provides the first detailed anatomical description of the regional distribution of [(3)H]-CGP39653 binding sites in human brain.

Receptor binding characteristics of the novel NMDA receptor glycine site antagonist [3H]GV150526A in rat cerebral cortical membranes

Binding of the glycine site antagonist 3-[2-(Phenylamino-carbonyl)ethenyl]-4,6-dichloro-indole-2-carboxylic acid sodium salt ([3H]GV150526A) was characterised in rat cerebral cortical membranes. Saturation experiments indicated the existence of a high affinity binding site, with a pK(d) value of 9.08 (K(d)=0. 8 nM) and a B(max) of 3.4 pmol/mg of protein. A strong linear correlation was observed between the displacement potencies for [3H]GV150526A and [3H]glycine of 13 glycine site ligands (r=0.991). The association kinetics of [3H]GV150526A binding was monophasic, with a k(on) value of 0.047 (nM)(-1) min(-1). Dissociation was induced by the addition of an excess of glycine, GV150526A, or 5,7-dichlorokynurenic acid (DCKA), another glycine antagonist. With GV150526A and DCKA, the dissociation curves presented similar k(off) values (0.068 and 0.069 min(-1), respectively), as expected from ligands binding to the same site. Conversely, a significantly lower k(off) value (0.027 min(-1)) was found with glycine. Although these data may suggest that glycine agonists and antagonists bind to discrete sites with an allosteric linkage (rather than interacting competitively), the reason for this difference remains to be elucidated. It is concluded that [3H]GV150526A can be considered a new valuable tool to further investigate the properties of the glycine site of the NMDA receptor.

Substituted indole-2-carboxylates as potent antagonists of the glycine binding site associated with the NMDA receptor

A novel series of indole-2-carboxylate analogues of GV150526 (1) in which the propenoic double bond was substituted with different "probes" or replaced by a isosteric cyclopropyl moiety were synthesized and evaluated for their affinity profile in order to obtain further information on the pharmacophoric model of the glycine binding site associated to the NMDA receptor.

Potent antihyperalgesic activity without tolerance produced by glycine site antagonist of N-methyl-D-aspartate receptor GV196771A

Central sensitization is a condition of enhanced excitability of spinal cord neurons that contributes to the exaggerated pain sensation associated with chronic tissue or nerve injury. N-methyl-D-aspartate (NMDA) receptors are thought to play a key role in central sensitization. We have tested this hypothesis by characterizing in vitro and in vivo a novel antagonist of the NMDA receptor acting on its glycine site, GV196771A. GV196771A exhibited an elevated affinity for the NMDA glycine binding site in rat cerebral cortex membranes (pKi = 7.56). Moreover, GV196771A competitively and potently antagonized the activation of NMDA receptors produced by glycine in the presence of NMDA in primary cultures of cortical, spinal, and hippocampal neurons (pKB = 7.46, 8. 04, and 7.86, respectively). In isolated baby rat spinal cords, 10 microM GV196771A depressed wind-up, an electrical correlate of central sensitization. The antihyperalgesic properties of GV196771A were studied in a model of chronic constriction injury (CCI) of the rat sciatic nerve and in the mice formalin test. In the CCI model GV196771A (3 mg/kg twice a day p.o.), administered before and then for 10 days after nerve ligature, blocked the development of thermal hyperalgesia. Moreover, GV196771A (1-10 mg/kg p.o.) reversed the hyperalgesia when tested after the establishment of the CCI-induced hyperalgesia. In the formalin test GV196771A (0.1-10 mg/kg p.o.) dose-dependently reduced the duration of the licking time of the late phase. These antihyperalgesic properties were not accompanied by development of tolerance. These observations strengthen the view that NMDA receptors play a key role in the events underlying plastic phenomena, including hyperalgesia. Moreover, antagonists of the NMDA glycine site receptor could represent a new analgesic class, effective in conditions not sensitive to classical opioids.

[3H]5,7-dichlorokynurenic acid recognizes two binding sites in rat cerebral cortex membranes

Binding of [3H]5,7-dichlorokynurenic acid ([3H]DCKA), a competitive antagonist of the strychnine-insensitive glycine site of the N-methyl-D-aspartate (NMDA) receptor channel complex, was characterized in synaptic plasma membranes from rat cerebral cortex. Non linear curve fitting of [3H]DCKA saturation and homologous displacement isotherms indicated the existence of two binding sites: a specific, saturable, high affinity site, with a pKD value of 7.24 (KD = 57.5 nmol/l) and a maximum binding value (Bmax) of 6.9 pmol/mg of protein and a second site, with micromolar affinity. The pharmacological profile of both binding components was determined by studying the effect on [3H]DCKA and [3H]glycine binding of a series of compounds known to interact with different excitatory and inhibitory amino acid receptors. These studies confirmed the identity of the high affinity site of [3H]DCKA binding with the strychnine-insensitive glycine site of the NMDA receptor channel complex. 3-[2-(Phenylaminocarbonyl)ethenyl]-4,6-dichloroindole-2-carb oxylic acid sodium salt (GV 150526A), a new, high affinity, selective glycine site antagonist (1), was the most potent inhibitor of this component of binding (pKi = 8.24, Ki = 5.6 nmol/l). The low affinity component of [3H]DCKA binding was insensitive to the agonists glycine and D-serine and the partial agonist (+/-)-3-amino-1-hydroxy-2-pyrrolidone (HA 966), though recognised by glycine site antagonists. The precise nature of this second, low affinity [3H]DCKA binding site remains to be elucidated.

[3H]MK-801 binding and the mRNA for the NMDAR1 subunit of the NMDA receptor are differentially distributed in human and rat forebrain

The distributions of [3H]MK-801 binding and the NMDA NR1 subunit mRNA were studied using receptor autoradiography and in-situ hybridization in rat and human brain whole-hemisphere coronal sections. Receptor protein detected by radioligand autoradiography and the mRNA for the key subunit of the receptor presented similar distributions in the forebrain, with a few areas showing an imbalance between the levels of mRNA and receptor protein. Human frontal cortex showed a relative abundance of NMDAR1 mRNA as compared to [3H]MK-801 binding. The same area in rat brain did not show any difference in the two distributions. In comparison, the rat claustrum presented a relative excess of NMDAR1 mRNA which was not detected in human sections. Human caudate nucleus exhibited relatively high levels of [3H]MK-801 binding that were unmatched in rat caudate. The hippocampi of either species presented similar levels of [3H]MK-801 binding and NMDAR1 mRNA, but when the two signals were measured in specific subfields of the hippocampal formation, the differential distribution of the two signals reflected the anatomy of hippocampal connections assuming a preferential dendritic distribution for MK-801 binding. Interestingly, rat and human hippocampi also showed some important species-dependent difference in the relative distribution of the receptor protein and mRNA. The data presented show an overall good correlation between the mRNA for the key subunit of the NMDA receptor and the functional receptor detected with radioligand binding and highlight the presence of local differences in their ratio. This may reflect different splicing of the mRNA for the NMDAR1 subunit in specific brain areas of rat and human. The species-dependent differences in the relative distribution of the mRNA for the key subunit of the NMDA receptor and that of a marker of functional receptors also highlights important differences in the NMDA function in rat and human brain.

Substituted indole-2-carboxylates as in vivo potent antagonists acting as the strychnine-insensitive glycine binding site

A series of indole-2-carboxylates bearing suitable chains at the C-3 position of the indole nucleus was synthesized and evaluated in terms of in vitro affinity using [3H]glycine binding assay and in vivo potency by inhibition of convulsions induced by N-methyl-D-aspartate (NMDA) in mice. 3-[2-[(Phenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxyl ic acid (8) was an antagonist at the strychnine-insensitive glycine binding site (noncompetitive inhibition of the binding of [3H]TCP, pA2 = 8.1) displaying nanomolar affinity for the glycine binding site (pKi = 8.5), coupled with high glutamate receptor selectivity (> 1000-fold relative to the affinity at the NMDA, AMPA, and kainate binding sites). This indole derivative inhibited convulsions induced by NMDA in mice, when administered by both iv and po routes (ED50 = 0.06 and 6 mg/kg, respectively). The effect of the substituents on the terminal phenyl ring of the C-3 side chain was investigated. QSAR analysis suggested that the pKi value decreases with lipophilicity and steric bulk of substituents and increases with the electron donor resonance effect of the groups present in the para position of the terminal phenyl ring. According to these results the terminal phenyl ring of the C-3 side chain should lie in a nonhydrophobic pocket of limited size, refining the proposed pharmacophore model of the glycine binding site associated with the NMDA receptor.

Regionally different N-methyl-D-aspartate receptors distinguished by ligand binding and quantitative autoradiography of [3H]-CGP 39653 in rat brain

1. Binding of D,L-(E)-2-amino-4-[3H]-propyl-5-phosphono-3-pentenoic acid ([3H]-CGP 39653), a high affinity, selective antagonist at the glutamate site of the N-methyl-D-aspartate (NMDA) receptor, was investigated in rat brain by means of receptor binding and quantitative autoradiography techniques. 2. [3H]-CGP 39653 interacted with striatal and cerebellar membranes in a saturable manner and to a single binding site, with KD values of 15.5 nM and 10.0 nM and receptor binding densities (Bmax values) of 3.1 and 0.5 pmol mg-1 protein, respectively. These KD values were not significantly different from that previously reported in the cerebral cortex (10.7 nM). 3. Displacement analyses of [3H]-CGP 39653 in striatum and cerebellum, performed with L-glutamic acid, 3-((+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and glycine showed a pharmacological profile similar to that reported in the cerebral cortex. L-Glutamic acid and CPP produced complete displacement of specific binding with Ki values not significantly different from the cerebral cortex. Glycine inhibited [3H]CGP 39653 binding with shallow, biphasic curves, characterized by a high and a low affinity component. Furthermore, glycine discriminated between these regions (P < 0.005, one-way ANOVA), since the apparent Ki of the high affinity component of the glycine inhibition curve (KiH) was significantly lower (Fisher's protected LSD) in the striatum than the cortex (33 nM and 104 nM, respectively). 4. Regional binding of [3H]-CGP 39653 to horizontal sections of rat brain revealed a heterogeneous distribution of binding sites, similar to that reported for other radiolabelled antagonists at the NMDA site (D-2-[3H]-amino-5-phosphonopentanoic acid ([3H]-D-AP5) and [3H]-CPP). High values of binding were detected in the hippocampal formation, cerebral cortex and thalamus, with low levels in striatum and cerebellum. 5. [3H]-CGP 39653 binding was inhibited by increasing concentrations of L-glutamic acid, CPP and glycine. L-Glutamic acid and CPP completely displaced specific binding in all regions tested, with similar IC50 values throughout. Similarly, glycine was able to inhibit the binding in all areas considered: 10 microM and 1 mM glycine reduced the binding to 80% and 65% of control (average between areas) respectively. The percentage of specific [3H]-CGP 39653 binding inhibited by 1 mM glycine varied among regions (P < 0.05, two-ways ANOVA). Multiple comparison, performed by Fisher's protected LSD method, showed that the inhibition was lower in striatum (72% of control), with respect to cortex (66% of control) and hippocampal formation (58% of control). 6. The inhibitory action of 10 microM glycine was reversed by 100 microM 7-chloro-kynurenic acid (7-CKA), a competitive antagonist of the glycine site of the NMDA receptor channel complex, in all areas tested. Moreover, reversal by 7-CKA was not the same in all regions (P < 0.05, two-ways ANOVA). In fact, in the presence of 10 microM glycine and 100 microM 7-KCA, specific [3H]-CGP 39653 binding in the striatum was 131% of control, which was significantly greater (Fisher's protected LSD) than binding in the hippocampus and the thalamus (104% and 112% of control, respectively). 7. These results demonstrate that [3H]-CGP 39653 binding can be inhibited by glycine in rat brain regions containing NMDA receptors; moreover, they suggest the existence of regionally distinct NMDA receptor subtypes with a different allosteric mechanism of [3H]-CGP 39653 binding modulation through the associated glycine site.

Pharmacological analysis of carboxyphenylglycines at metabotropic glutamate receptors

Three carboxyphenylglycine derivatives were examined for their activity on glutamate metabotropic receptors negatively linked to adenylate cyclase. Chinese hamster ovary cells stably expressing mGlu2 and mGlu4 were utilised for this study. A receptor binding analysis was also performed for the main classes of glutamate ionotropic receptors and for the glycine binding site on the NMDA-receptor complex. In mGlu2 expressing cells (S)4-carboxy-3-hydroxyphenylglycine and (S)4-carboxy-phenylglycine antagonized forskolin-stimulated cAMP levels, with EC50 of 21 and 970 microM, respectively, acting as agonists at this receptor subtype, whereas (RS) alpha-methyl-4-carboxyphenylglycine antagonized glutamate response in these cells. None of these compounds showed any agonistic or antagonistic activity on mGlu4 expressing cells. No affinity for the ionotropic receptors (NMDA, AMPA and kainate) and for the glycine site of the NMDA-receptor complex was found using the receptor binding approach, except for (RS)4-carboxy-3-hydroxyphenylglycine which showed a pKi of 5.68 in ((+/-)2-carboxypiperazin-4-yl)propyl-1-phosphonic acid binding for NMDA receptor, although this can be ascribed to the (R) form of the racemic mixture.

1-Aminocyclopropane-1-carboxylic acid derivatives: a new, general synthesis and NMDA receptor complex binding affinity study

A new synthesis of 1-aminocyclopropane-1-carboxylic acid and of its (E)- and (Z)-2-substituted analogues (R = CH3;i-Pr;C6H5) has been performed by means of the "diazo-addition" method, starting from N-(diphenylmethylene)-2,3-dehydro-1-amino-1-carboxylate precursors. The (E)- and (Z)-2-phenyl and the (Z)-2-methylcyclopropaneamino acids have been obtained with high diastereospecificity. All the cyclopropaneamino acids prepared were tested for their affinity for some glutamate receptors and resulted inactive, with the exception of compounds (E)-1b and (Z)-1c which showed a shallow displacement of [3H]-glycine binding.

Allosteric modulation of [3H]CGP 39653 binding by glycine in rat brain

D,L-(E)-2-Amino-4-propyl-5-phosphono-3-pentenoic acid (CGP 39653), a new, high-affinity, selective NMDA receptor antagonist, interacts with rat cortical membranes in a saturable way and apparently to a single binding site, with a KD of 10.7 nM and a receptor density of 2.6 pmol/mg of protein. Displacement analysis of [3H]CGP 39653 binding shows a pharmacological profile similar to that reported for another NMDA antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP). Glycine, however, is able to discriminate between the two ligands; in fact, it does not affect [3H]CPP binding but inhibits [3H]CGP 39653 binding in a biphasic way. D-Serine, another agonist at the strychnine-insensitive glycine binding site of the NMDA receptor complex, inhibits [3H]CGP 39653 binding in the same way as glycine, with a potency that correlates with its binding affinity at the glycine site. In addition, 7-chlorokynurenic acid, an antagonist at the glycine site, is able to reverse the displacement of [3H]CGP 39653 by glycine in a dose-dependent manner. Furthermore, the dissociation rate constant of [3H]CGP 39653 is enhanced in the presence of glycine, whereas the presence of NMDA receptor ligands does not modify the rate of dissociation of [3H]CGP 39653 from the receptor. These results indicate that part of the binding of the NMDA antagonist CGP 39653 can be potently modified by glycine through an allosteric mechanism, and suggest the existence of two antagonist preferring NMDA receptor subtypes that are differentially modulated through the glycine binding site.

Induction/repair of strand breakage in mature and nascent DNA of cultured Chinese hamster ovary cells exposed to hydrogen peroxide

Treatment of cultured mammalian cells with hydrogen peroxide results in the production of extensive DNA damage. Strand breakage was produced at the level of either nascent or mature DNA and the former target appeared slightly more resistant than the latter. Although inhibitors of the enzyme poly(ADP-ribose) polymerase similarly retarded the repair of such lesions, removal of DNA strand breaks was much slower for the newly synthesized DNA as compared to mature DNA.

Autoradiographic studies on lymph node populations from non-Hodgkin B lymphomas

The authors have investigated the cytokinetic behaviour of 23 cases of non-Hodgkin B cell lymphomas. In all cases diagnosis was established on the basis of histological, cytoimmunological and cytochemical data. The study, carried out with the aid of autoradiographic techniques, indicates that the various histological subclasses are characterized by a different metabolic and kinetic behaviour: the aggressiveness of the malignant lymphoma is closely related to the rate of growth of the lymphomatous cell populations, as assessed by the mitotic index and the labelling index with 3H-thymidine. The study of the synthesis and metabolism of RNA provides not only useful information on the energy requirements necessary for the neoplastic growth, but, when employed in association with immunohistochemical techniques for the detection of intracytoplasmic Ig synthesis, it defines more accurately the functional characteristics and the differentiation potentialities of the neoplastic populations in different cases.