Characterization of Na+-dependent binding sites of [3H]glutamate in synaptic membranes from rat brain

Hydrogen sulfide protects astrocytes against H(2)O(2)-induced neural injury via enhancing glutamate uptake

Excess extracellular glutamate, the main excitatory neurotransmitter, may result in excitotoxicity and neural injury. The present study was designed to study the effect of hydrogen sulfide (H(2)S), a novel neuromodulator, on hydrogen peroxide (H(2)O(2)) -induced glutamate uptake impairment and cellular injuries in primary cultured rat cortical astrocytes. We found that NaHS (an H(2)S donor, 0.1-1000 microM) reversed H(2)O(2)-induced cellular injury in a concentration-dependent manner. This effect was attenuated by L-trans-pyrrolidine-2,4-dicarboxylic (PDC), a specific glutamate uptake inhibitor. Moreover, NaHS significantly increased [(3)H]glutamate transport in astrocytes treated with H(2)O(2), suggesting that H(2)S may protect astrocytes via enhancing glutamate uptake function. NaHS also reversed H(2)O(2)-impaired glutathione (GSH) production. Blockade of glutamate uptake with PDC attenuated this effect, indicating that the effect of H(2)S on GSH production is secondary to the stimulation of glutamate uptake. In addition, it was also found that H(2)S may promote glutamate uptake activity via decreasing ROS generation, enhancing ATP production and suppressing ERK1/2 activation. In conclusion, our findings provide direct evidence that H(2)S has potential therapeutic value for oxidative stress-induced brain damage via a mechanism involving enhancing glutamate uptake function.

Involvement of amino-acid side chains of membrane proteins in the binding of glutathione to pig cerebral cortical membranes

Glutathione (GSH), a general antioxidant and detoxifying compound, is the most abundant thiol-containing peptide in the central nervous system. It has been earlier shown to regulate the functions of glutamate receptors and to possess specific binding sites in both neurons and glial cells. The possible involvement of disulfide bonds, cysteinyl, arginyl, lysyl, glutamyl, and aspartyl residues in the binding of tritiated GSH to specific sites in pig cerebral cortical synaptic membranes was now studied after covalent modification of membrane proteins. Treatment of synaptic membranes with the thiol-modifying reagents 5,5'-dithio-bis(2-nitrobenzoate) (DTNB) and 4,4'-dithiodipyridine (DDP) dramatically enhanced the binding of [3H]GSH in a dose-dependent manner. Dithiothreitol (DTT) alone reduced the binding, but pretreatment of the membranes with DTT potentiated the enhancing effect of DTNB. On the other hand, when the modification with DTNB was followed by treatment with DTT, the enhancement by DTNB was completely reversed. N-ethylmaleimide, a thiol alkylating agent, and phenylisothiocyanate, a thiol- and amino-group modifying compound, reduced the binding, and their effects were additive. The guanidino-modifying agent phenylglyoxal reduced the binding but the carboxyl-modifying reagent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide had no significant effect. The results indicate that cysteinyl side chains and disulfide bonds are essential in the binding of GSH to membrane proteins and that arginyl and lysyl side chains may also be directly involved in this process.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Glutathione and signal transduction in the mammalian CNS

The tripeptide glutathione (GSH) has been thoroughly investigated in relation to its role as antioxidant and free radical scavenger. In recent years, novel actions of GSH in the nervous system have also been described, suggesting that GSH may serve additionally both as a neuromodulator and as a neurotransmitter. In the present article, we describe our studies to explore further a potential role of GSH as neuromodulator/neurotransmitter. These studies have used a combination of methods, including radioligand binding, synaptic release and uptake assays, and electrophysiological recording. We report here the characteristics of GSH binding sites, the interrelationship of GSH with the NMDA receptor, and the effects of GSH on neural activity. Our results demonstrate that GSH binds via its gamma-glutamyl moiety to ionotropic glutamate receptors. At micromolar concentrations GSH displaces excitatory agonists, acting to halt their physiological actions on target neurons. At millimolar concentrations, GSH, acting through its free cysteinyl thiol group, modulates the redox site of NMDA receptors. As such modulation has been shown to increase NMDA receptor channel currents, this action may play a significant role in normal and abnormal synaptic activity. In addition, GSH in the nanomolar to micromolar range binds to at least two populations of binding sites that appear to be distinct from all known excitatory amino acid receptor subtypes. GSH bound to these sites is not displaceable by glutamatergic agonists or antagonists. These binding sites, which we believe to be distinct receptor populations, appear to recognize the cysteinyl moiety of the GSH molecule. Like NMDA receptors, the GSH binding sites possess a coagonist site(s) for allosteric modulation. Furthermore, they appear to be linked to sodium ionophores, an interpretation supported by field potential recordings in rat cerebral cortex that reveal a dose-dependent depolarization to applied GSH that is blocked by the absence of sodium but not by lowering calcium or by NMDA or (S)-2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate antagonists. The present data support a reevaluation of the role of GSH in the nervous system in which GSH may be involved both directly and indirectly in synaptic transmission. A full accounting of the actions of GSH may lead to more comprehensive understanding of synaptic function in normal and disease states.

Specific [3H]glutamate binding in the cerebral cortex and hippocampus of rats during development: effect of homocysteine-induced seizures

Specific [3H]glutamate binding to synaptic membranes from the cerebral cortex and hippocampus of 7-, 12- and 18-day-old rats was examined, both in control animals and during seizures induced by homocysteine. In the cerebral cortex a transient peak of glutamate binding was observed in 7-day-old group, whereas in the hippocampus it occurred in 12-day-old animals. Total specific [3H]glutamate binding was not influenced by preceding seizure activity in either of the age groups and both the studied regions. NMDA- and QA-sensitive glutamate bindings represent the highest portion of the total binding. Moreover, NMDA-sensitive binding in the cerebral cortex of 7-day-old rats is significantly higher as compared to the two more mature groups. The proportion of individual receptor subtypes on total binding in each age group was not influenced by preceding seizure activity. However, NMDA-sensitive binding in the hippocampus of 12-day-old rats, sacrificed during homocysteine-induced seizures, was significantly increased as compared to corresponding controls. In contrast to the effect of NMDA, AMPA, kainate and quisqualate which displaced to a different extent [3H]glutamate binding, homocysteine had no effect when added to membrane preparations. Similarly, [3H]CPP and [3H]AMPA bindings were not affected in the presence of homocysteine. It thus seems unlikely that homocysteine is an effective agonist for conventional ionotropic glutamate receptors. Its potential activity at some of the modulatory sites at the NMDA receptor channel complex or at metabotropic receptors has to be clarified in further experiments.

Differential profiles of binding of a radiolabeled agonist and antagonist at a glycine recognition domain on the N-methyl-D-aspartate receptor ionophore complex in rat brain

Addition of several polyamines, including spermidine and spermine, was effective in inhibiting binding of the antagonist ligand [3H]5,7-dichlorokynurenic acid ([3H]-DCKA) a Gly recognition domain on the N-methyl-D-aspartic acid (NMDA) receptor ionophore complex in rat brain synaptic membranes. In contrast, [3H]DCKA binding was significantly potentiated by addition of proposed polyamine antagonists, such as ifenprodil and (+/-)-alpha-(4-chlorophenyl)-4-[(4-fluorophenyl) methyl]-1-piperidine ethanol, with [3H]Gly binding being unchanged. The inhibition by spermidine was significantly prevented by inclusion of ifenprodil. In addition, spermidine significantly attenuated the abilities of four different antagonists at the Gly domain to displace [3H]DCKA binding virtually without affecting those of four different agonists. Phospholipases A2 and C and p-chloromercuribenzosulfonic acid were invariably effective in significantly inhibiting [3H]DCKA binding with [3H]Gly binding being unaltered. Moreover, the densities of [3H]DCKA binding were not significantly different from those of [3H]-Gly binding in the hippocampus and cerebral cortex, whereas the cerebellum had more than a fourfold higher density of [3H]Gly binding than of [3H]DCKA binding. These results suggest that the Gly domain may have at least two different forms based on the preference to agonists and antagonists in the rodent brain.

Further evidence for multiple forms of an N-methyl-D-aspartate recognition domain in rat brain using membrane binding techniques

Pretreatment with sulfhydryl-reactive agents, such as N-ethylmaleimide and p-chloromercuriphenylsulfonic acid, invariably resulted in marked inhibition of the binding of DL-(E)-2-amino-4-[3H]propyl-5-phosphono-3-pentenoic acid ([3H]CGP 39653), a competitive antagonist at an N-methyl-D-aspartate (NMDA)-sensitive subclass of central excitatory amino acid receptors, in brain synaptic membranes extensively washed and treated with Triton X-100, but did not significantly affect the binding of L-[3H]-glutamic acid ([3H]Glu), an endogenous agonist. The pretreatment was effective in reducing the binding of [3H]-CGP 39653 at equilibrium, without altering the initial association rate, and decreased the affinity for the ligand. Pretreatment with sulfhydryl-reactive agents also enhanced the potencies of NMDA agonists to displace [3H]-CGP 39653 binding and attenuated those of NMDA antagonists, but had little effect on the potencies of the agonists and antagonists to displace [3H]Glu binding. The binding of both [3H]CGP 39653 and [3H]Glu was similarly sensitive to pretreatment with four different proteases in Triton-treated membranes, whereas pretreatment with phospholipase A2 or C markedly inhibited [3H]CGP 39653 binding without altering [3H]Glu binding. Moreover, both phospholipases not only induced enhancement of the abilities of NMDA agonists to displace the binding of [3H]CGP 39653 and [3H]Glu, but also caused diminution of those of NMDA antagonists. These results suggest that both sulfhydryl-reactive agents and phospholipases may predominantly interfere with radiolabeling of the NMDA recognition domain in a state favorable to an antagonist by [3H]CGP 39653, with concomitant facilitation of that in an antagonist-preferring form by [3H]Glu. The possible presence of multiple forms of the NMDA recognition domain is further supported by these data.

Effect of repeated hyperammonemia on Na(+)-dependent binding of glutamate in rat cortical and hippocampal synaptic membranes

Na(+)-dependent binding of L-glutamate in cortical and hippocampal synaptic membranes from hyperammonemic rats was compared to corresponding data in the controls. In hippocampal membranes, repeated hyperammonemia resulted in a 13% and 18% decrease in binding in 20-day-old and 50-day-old rats, respectively. The decrease was statistically significant (P < 0.05) in the older animals and Scatchard analysis revealed a 19% reduction in the number of binding sites without any changes in the affinity. Within the hippocampal formation, the binding in the dentate gyrus was the most sensitive to hyperammonemia where a 21% decrease was found (P < 0.01), whilst the decline of binding in CA1 and CA3 areas of the hippocampus proper was not significant. The results support the idea that excessive accumulation of extracellular glutamate during hyperammonemia is a consequence not only of its increased release, but also of the blocking of Na(+)-dependent binding of glutamate to specific uptake sites.

Anion transport blockers inhibit DL-2-amino-4-phosphonobutyrate responses induced by quisqualate in the rat cerebral cortex

1. Depolarizing responses to DL-2-amino-4-phosphonobutyrate (AP4) and related amino acids have been studied in the rat cerebral cortex slice following the application of quisqualate (Quis). 2. Before exposure to Quis, 500 microM DL-AP4 had little or no effect. However, following a single application of 40 microM Quis for 2 min, DL-AP4 produced depolarizing responses. With repeated applications of DL-AP4, there was a decline in response amplitude. A second application of Quis restored the depolarizing potency of DL-AP4 to a level above that for the first DL-AP4 response after the first Quis application. With a sequence of alternate applications of Quis and DL-AP4, the amplitude of DL-AP4 responses became maximal after the second Quis application. Responses to DL-AP4 could also be induced by the application of 1 microM Quis for 60 min, but were smaller in amplitude. 3. Responses to the normally inactive amino acids L-cysteine (Cys), L-cystathionine (CTN) and L-alpha-aminoadipate (AA) were also induced once Quis was applied. These responses were also maximized after a second application of Quis, except those to L-Cys, which failed to reach a plateau after three Quis applications. 4. The co-application of DL-AP4 with the first Quis application depressed the subsequent mean DL-AP4 response by 47%. Re-application of Quis restored the amplitude of DL-AP4 responses to levels comparable to control. L-alpha-AA also suppressed the induction of DL-AP4 responses, when co-applied with the first Quis exposure, reducing mean response amplitude by 98%. Unlike DL-AP4, however, the effect with L-alpha-AA persisted so that DL-AP4 responses were significantly suppressed compared to control, even after further applications of Quis. 5. The effects of the anion transport blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetoamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) on the induction process and the DL-AP4 responses themselves were examined. DIDS (100 microM) significantly inhibited the DL-AP4 responses, and to a lesser extent the induction of the responses by 40 microM Quis (2 min), while SITS (300 microM) only inhibited the DL-AP4 responses. However, the induction of responses by 1 microM Quis (60 min) was significantly affected by this concentration of SITS. 6. DIDS (100 microM) had no effect on responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA), but selectively potentiated those to Quis. Examination of the full concentration-response curve for Quis revealed that, while the Rmax remained constant, the Hill slope was increased and the EC50 was decreased in the presence of DIDS. SITS (300 microM), however, antagonized responses to AMPA, and had little effect on responses to Quis except at the highest concentration of Quis tested (20 microM), where a potentiation was observed, suggesting that it is a non-NMDA receptor antagonist.7. These observations indicate that the production of depolarizing responses to a number of amino acids, including DL-AP4, in the cerebral cortex is mediated via an anion transport mechanism sensitive to DIDS and SITS, and that the exchange of DL-AP4 for a sequestered excitatory amino acid receptor agonist, probably Quis, could underlie the production of these responses. Indeed, Quis is apparently sequestered via a similar process. However, the involvement of such a process in the induction of these responses remains inconclusive.

D-aspartate binding to the glutamate uptake site in human brain tissue--effects of leucotomy

Binding of [3H]D-aspartate, as an indicator of glutamate uptake sites, was investigated in post-mortem human brain tissue by use of a centrifugation assay to separate free and bound ligand. Binding was displaceable, apparently saturable and to a single site, with mean KD and Bmax values of 2.3 microM and 40.3 nmol/g tissue in the frontal cortex. The method was applied to the study of tissue from frontal and temporal cortices and the caudate nucleus of five psychiatric patients who had undergone a frontal leucotomy. The effects of this neurosurgical procedure were to diminish by almost 50% the density of D-aspartate binding sites in the frontal cortex and caudate nucleus, while the temporal cortex was less affected. It is concluded that the method provides a potentially useful correlate of glutamatergic innervation in human brain tissue.

Comparative studies on binding of 3 different ligands to the N-methyl-D-aspartate recognition domain in brain synaptic membranes treated with Triton X-100

Treatment with a low concentration of Triton X-100 almost tripled the binding of [3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid (CGP 39653), a novel competitive antagonist at an N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors, in synaptic membranes of the rat brain. The binding linearly increased with increasing protein concentrations of up to 0.4 mg/ml and also increased in proportion to incubation time with a plateau within 60 min after the initiation of incubation at 2 degrees C in Triton-treated membranes. Elevation of incubation temperature from 2 degrees C to 30 degrees C resulted in a marked decrease in the binding at equilibrium by 80%, and a maximal level was obtained within 1 min after the initiation of incubation at 30 degrees C with a gradual decline of up to 10 min. Bound [3H]CGP 39653 was rapidly dissociated by the addition of excess unlabeled L-glutamic acid (Glu), and the time required to attain complete dissociation was 60 min at 2 degrees C and 1 min at 30 degrees C, respectively. Among several agonists and antagonists tested, Glu was the most potent displacer of [3H]CGP 39653 binding with progressively less potent displacement by D-2-amino-5-phosphonovaleric, (+-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic (CPP), D-2-amino-7-phosphonoheptanoic, N-methyl-D-aspartic and N-methyl-L-aspartic acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential potentiation by spermidine of abilities of a variety of displacers for [3H]MK-801 binding in hippocampal synaptic membranes

Binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) to an ion channel associated with the N-methyl-D-aspartate (NMDA)-sensitive subtype of brain excitatory amino acid receptors was studied in Triton-treated preparations of synaptic membranes of rat brain. The initial association rate of the binding measured at 30 min after onset of incubation was markedly potentiated by the addition of either L-glutamic acid (Glu) alone or both Glu and glycine (Gly) in a concentration-dependent manner at 10 nM to 0.1 mM. Potentiation occurred to a significantly greater extent in the hippocampus and cerebral cortex than in the cerebellum. In the presence of both Glu and Gly, the endogenous polyamine spermidine (SPD) further potentiated binding in hippocampal and cortical membranes at concentrations above 10 microM without significantly affecting that in cerebellar membranes. The binding of [3H]MK-801 was slowly equilibrated in 16 h. When examined in hippocampal synaptic membranes, the binding at equilibrium was markedly displaced by numerous noncompetitive antagonists for the NMDA receptor. The addition of SPD markedly enhanced potencies of those displacers having a high affinity to [3H]MK-801 binding sites, without affecting other displacers having a low affinity. These results suggest that SPD promotes transition of sites responsible for mediating NMDA responses within the channel to a state with higher affinity for noncompetitive blockers.

Binding of [3H]MK-801, NMDA-displaceable [3H]glutamate, [3H]glycine, [3H]spermidine, [3H]kainate and [3H]AMPA to regionally discrete brain membranes of the gerbil: a biochemical study

Bindings of glutamate receptor agonists and related modulators were investigated in 10 discrete tissues from gerbil brain using a biochemical technique. There appeared considerable discrepancies, in respect of intrahippocampal profiles, from reported data by autoradiography on rat brain. In the gerbil, an almost equivalent level of N-methyl-D-aspartate (NMDA)-displaceable [3H]glutamate binding was found in field CA1 and the dentate gyrus, while approx 30% less in field CA3, a profile which was strikingly similar to that of (+)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]-cycloheptene-5,10-imin e maleate (MK-801) or of [3H]glycine. [3H]Kainate binding was highest in the dentate gyrus followed by field CA3 and then field CA1, the ratio of the highest to the lowest being 3 to 2. Binding of [3H]DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) differed, to a certain extent, from that of [3H]kainate and showed the dentate gyrus followed by field CA1 and then field CA3 in the rank order of decreasing binding. Taking together, intrahippocampal localization of glutamate receptor subtypes in the gerbil, when analyzed with a biochemical binding assay, looks to be less region selective than the distribution obtained on autoradiography in the rat. Thus, it is likely that these different distribution profiles show different status of receptor function respectively, or are due merely to species difference.

Inhibition by calmodulin antagonists of [3H]MK-801 binding in brain synaptic membranes

In brain synaptic membranes not extensively washed, (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5, 10-imine ([3H]MK-801) binding was markedly inhibited in a concentration-dependent manner (at concentrations above 1 microM) by several compounds having antagonistic activity at the Ca(2+)-binding protein calmodulin. Scatchard analysis revealed that N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) inhibited the binding through a significant decrease in the density of binding sites without affecting the affinity at 10 microM. In membranes extensively washed and treated with a low concentration of Triton X-100, L-glutamic acid (Glu) drastically accelerated the initial association rate of [3H]MK-801 binding with glycine (Gly), almost doubling the initial association rate found in the presence of Glu alone. The addition of W-7 invariably reduced the initial association rate observed in the presence of either Glu alone or both Glu and Gly, without significantly altering the dissociation rate of bound [3H]-MK-801, irrespective of the presence of the two stimulatory amino acids. The maximal potencies of Glu, Gly, and spermidine in potentiating the binding were all attenuated by W-7. These results suggest that calmodulin antagonists may interfere with opening processes of an ion channel associated with an N-methyl-D-aspartate-sensitive subclass of excitatory amino acid receptors in rat brain.

Effects of ifenprodil on the N-methyl-d-aspartate receptor ionophore complex in rat brain

The effects of a cerebral anti-ischemic drug ifenprodil on the receptor ionophore complex of an N-methyl-D-aspartate (NMDA)-sensitive subclass of central excitatory amino acid receptors were examined using [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10- imine (MK-801) binding in rat brain synaptic membrane preparations as a biochemical measure. The binding in membrane preparations not extensively washed was markedly inhibited not only by competitive NMDA antagonists such as (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic, D-2-amino-5-phosphonovaleric and D-2-amino-7-phosphonoheptanoic acids, but also by competitive antagonists at the strychnine-insensitive glycine (Gly) site including 7-chlorokynurenic acid and 6,7-dichloroquinoxaline-2,3-dione. Among several proposed ligands for alpha-adrenergic receptors tested, ifenprodil most potently inhibited the binding in these membrane preparations due to a decrease in the density of the binding sites without significantly affecting the affinity. Ifenprodil also inhibited the binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine as well as of [3H]MK-801 to open NMDA channels in a concentration-dependent manner at concentrations above 10 nM in membrane preparations extensively washed but not treated by a detergent, with a Hill coefficient of less than unity. Further treatment of extensively washed membrane preparations with a low concentration of Triton X-100 resulted in an almost complete abolition of [3H]MK-801 binding, and the binding was restored to the level found in membrane preparations not extensively washed following the addition of both L-glutamic acid (Glu) and Gly. Ifenprodil was effective in inhibiting [3H]MK-801 binding via reducing both initial association and dissociation rates in Triton-treated membrane preparations, irrespective of the presence of Glu and Gly added. The binding in Triton-treated membrane preparations was additionally potentiated by the polyamine spermidine in a concentration-dependent manner at concentrations above 10 microM in the presence of both Glu and Gly at maximally effective concentrations. Ifenprodil invariably diminished the abilities of these three stimulants to potentiate [3H]MK-801 binding at concentrations over 1 microM in a manner that the maximal responses each were reduced. These results suggest that ifenprodil does not interfere with the NMDA receptor complex as a specific isosteric antagonist at the polyamine domain in contrast to the prevailing view.

Inhibitory modulation by sodium ions of the N-methyl-D-aspartate recognition site in brain synaptic membranes

Specific binding of radiolabeled L-glutamic acid (Glu) was examined using rat brain synaptic membranes treated with a low concentration of Triton X-100. The binding drastically increased in proportion to increasing concentrations of the detergent used up to 0.1%. Addition of 100 mM sodium acetate significantly potentiated the binding in membranes not treated with Triton X-100, whereas it markedly inhibited the binding in Triton-treated membranes. The binding in Triton-treated membranes was inversely dependent on incubation temperature and reached a plateau within 10 min after the initiation of incubation at 2 degrees C, whereas the time required to attain equilibrium at 30 degrees C was less than 1 min. Sodium acetate invariably inhibited the binding detected at both temperatures independently of the incubation time via decreasing the affinity for the ligand. The binding was significantly displaced by agonists and antagonists for an N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors, but not by those for the other subclasses. Inclusion of sodium acetate reduced the potencies of NMDA agonists to displace the binding without virtually affecting those of NMDA antagonists. Moreover, sodium ions inhibited the ability of Glu to potentiate the binding of N-[3H] [1-(2-thienyl)cyclohexyl]piperidine to open NMDA channels in Triton-treated membranes. These results suggest that sodium ions may play an additional modulatory role in the termination process of neurotransmission mediated by excitatory amino acids via facilitating a transformation of the NMDA recognition site from a state with high affinity for agonists to a state with low affinity.

Age-related decreases of the N-methyl-D-aspartate receptor complex in the rat cerebral cortex and hippocampus

Binding activities of central excitatory amino acid receptors were examined in Triton-treated membrane preparations of the cerebral cortex and hippocampus from brains of rats at 2, 7 and 29 months after birth. Aged rats exhibited a significant reduction of [3H]glutamate (Glu) binding displaceable by N-methyl-D-aspartate (NMDA), as well as strychnine-insensitive [3H]glycine binding in both central structures, as compared with those in young rats. Binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate (MK-801), a non-competitive NMDA antagonist used to label the activated state of ion channels linked to NMDA-sensitive receptors, also decreased with aging irrespective of the experimental conditions employed. Scatchard analysis revealed that reduction of both [3H]Glu binding and [3H]MK-801 binding were due to a significant decrease in the densities of binding sites with aging, with their affinities being unaltered. Binding of [3H]D,L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), which is a specific agonist for quisqualate-sensitive receptors, was unchanged with aging when determined in the absence of 100 mM potassium thiocyanate (KSCN). However, AMPA binding determined in the presence of added KSCN was about 25% reduced in both brain regions of aged rats. Binding of [3H]kainate to kainate-sensitive receptors was unchanged with aging. These results suggest that glutaminergic neurotransmission mediated by NMDA-sensitive receptors may be selectively impaired with aging in the hippocampus and cerebral cortex among 3 different subclasses of excitatory amino acid receptors in the brain.

Neurochemical aspects of the N-methyl-D-aspartate receptor complex

The N-methyl-D-aspartic acid (NMDA)-sensitive subclass of brain excitatory amino acid receptors is supposed to be a receptor-ionophore complex consisting of at least 3 different major domains including an NMDA recognition site, glycine (Gly) recognition site and ion channel site. Biochemical labeling of the NMDA domain using [3H]L-glutamic acid (Glu) as a radioactive ligand often meets with several critical methodological pitfalls and artifacts that cause a serious misinterpretation of the results. Treatment of brain synaptic membranes with a low concentration of Triton X-100 induces a marked disclosure of [3H]Glu binding sensitive to displacement by NMDA with a concomitant removal of other several membranous constituents with relatively high affinity for the neuroactive amino acid. The NMDA site is also radiolabeled by the competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid that reveals possible heterogeneity of the site. The Gly domain is sensitive to D-serine and D-alanine but insensitive to strychnine, and this domain seems to be absolutely required for an opening of the NMDA channels by agonists. The ionophore domain is radiolabeled by a non-competitive type of NMDA antagonist that is only able to bind to the open but not closed channels. The binding of these allosteric antagonists is markedly potentiated by NMDA agonists in a manner sensitive to antagonism by isosteric antagonists in brain synaptic membranes and additionally enhanced by further inclusion of Gly agonists through the Gly domain. Furthermore, physiological and biochemical responses mediated by the NMDA receptor complex are invariably potentiated by several endogenous polyamines, suggesting a novel polyamine site within the complex. At any rate, activation of the NMDA receptor complex results in a marked influx of Ca2+ as well as Na+ ions, which subsequently induces numerous intracellular metabolic alterations that could be associated with neuronal plasticity or excitotoxicity. Therefore, any isosteric and allosteric antagonists would be of great benefit for the therapy and treatment of neurodegenerative disorders with a risk of impairing the acquisition and formation process of memories.

[3H]thienylcyclohexylpiperidine binding activity in brain synaptic membranes treated with Triton X-100

Binding activity of [3H]thienylcyclohexylpiperidine was examined using rat brain synaptic membranes treated with Triton X-100. This compound is proposed to be a noncompetitive antagonist for the N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors. The activity decreased in proportion to increasing concentrations of the detergent up to 0.08%. In vitro addition of L-glutamate (Glu) partially restored the decreased activity caused by this Triton treatment, whereas further addition of glycine (Gly) entirely reversed the loss of activity to the level found in membranes extensively washed but not treated with a detergent. These stimulatory effects were found to be due to the acceleration of the association of ligand. The rank order of potentiation of the activity coincided well with that of the affinity for the NMDA-sensitive subclass among numerous Glu analogs. The potentiation by Gly as well as Glu was invariably prevented by competitive NMDA antagonists, such as DL-2-amino-5-phosphonovalerate and (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate, but not by strychnine. No significant difference was observed between pharmacological profiles of the activities in synaptic membranes treated and not treated with Triton X-100, except haloperidol. The potency of this sigma-ligand to inhibit the activity was greatly reduced by the Triton treatment in the presence of both Glu and Gly. These results suggest that the regulatory properties of Triton-treated synaptic membranes remain unchanged in terms of the interaction within the NMDA receptor complex.

Solubilization of spermidine-sensitive (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ([3H]MK-801) binding activity from rat brain

The receptor-ionophore complex of the N-methyl-D-aspartate (NMDA)-sensitive receptor was solubilized by deoxycholic acid from rat brain using (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne ([3H]MK-801) binding as a marker for the receptor. Gel filtration of the solubilized preparations on a Sephadex G-25 column revealed significant [3H]MK-801 binding sensitive to potentiation by glutamate and glutamate/glycine, which was prevented by competitive antagonists for the NMDA and strychnine-insensitive glycine (GlyB) sites. In contrast to NMDA and glycine, spermidine markedly potentiated the amount of [3H]MK-801 binding in solubilized preparations by increasing the apparent affinity of the ligand. In the presence of all three stimulants, the solubilized preparations exhibited pharmacological profiles similar to those in the membrane preparations. These results clearly indicate that the whole macromolecular NMDA receptor-ionophore complex is solubilized under the experimental conditions used.

Competitive inhibition of NMDA-mediated responses by guanine nucleotides in brain synaptic membranes treated with Triton X-100

The effect of guanine nucleotides on physiological responses mediated by the N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors was examined by using NMDA-sensitive [3H]L-glutamic acid (Glu) binding as well as [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) binding in rat brain synaptic membranes treated with a low concentration of Triton X-100. The NMDA-sensitive [3H]Glu binding was significantly inhibited by the addition of some guanine nucleotides such as GTP, GDP, 5'-guanylylimidodiphosphate and guanosine 5'-O-(3-thiotriphosphate), but not by other nucleotides or nucleosides such as guanosine, cyclic GMP, adenosine, AMP, ADP, ATP, CTP, ITP and UTP. Inclusion of GTP not only attenuated the ability of NMDA to displace [3H]Glu binding in a concentration-dependent manner, but also lowered the affinity of the binding sites for [3H]Glu without altering their densities. The inhibitory potency of an antagonist highly selective to the NMDA receptors (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate on [3H]Glu binding also deteriorated with GTP at concentrations above 10 microM. Addition of Glu induced a concentration-dependent potentiation of [3H]MK-801 binding through an activation of the NMDA-sensitive receptors, and the potency of Glu to potentiate the binding was markedly reduced by the afore-mentioned positive guanine nucleotides in a competitive manner. In contrast, GTP at 0.1 mM non-competitively weakened the stimulatory property of glycine to additionally enhance the binding found in the presence of Glu alone. These results suggest that some guanine nucleotides may have a relatively high affinity for NMDA recognition sites within the NMDA receptor complex in the brain.

Profiles of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine binding in brain synaptic membranes treated with Triton X-100

Binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) was examined using rat brain synaptic membranes treated with a low concentration of Triton X-100. This compound is assumed to be a non-competitive antagonist for the N-methyl-D-aspartate(NMDA)-sensitive subclass of central excitatory amino acid receptors. Binding was quite low but detectable in Triton-treated membranes irrespective of the incubation temperature, and the temperature-dependent portion of the binding was greatly reduced in these Triton-treated membranes. However, binding was drastically potentiated by the inclusion of L-glutamate and its analogous amino acids in a concentration-dependent manner at a concentration range of 10 nM to 0.1 mM. Agonists for the NMDA-sensitive subclass also potentiated binding, with agonists for the other subclasses being ineffective. Glycine at a concentration above 10 nM was not only effective as a stimulant of potentiated binding by glutamate, but was also active in enhancing binding in the absence of added glutamate. Glycine increased both the association and dissociation rates without significantly affecting the dissociation constant. Pharmacological profiles of binding in Triton-treated membranes were not significantly different from those in untreated membranes, except for that of haloperidol. Haloperidol is proposed to be highly selective for brain sigma-receptors on the basis of a potent inhibition of sigma-receptor binding. The inhibitory potency of this sigma-ligand was markedly attenuated in the presence of both glutamate and glycine in Triton-treated membranes, as compared with that in untreated membranes. These results suggest that [3H]TCP binding in Triton-treated membranes is a useful biochemical tool to evaluate predominantly the activated state of ion channels associated with the NMDA-sensitive receptors in terms of freedom from the confounding effects of endogenous amino acids.

Temperature-independent binding of [3H](+-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid in brain synaptic membranes treated by Triton X-100

Specific binding of [3H](+-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), a highly selective antagonist for N-methyl-D-aspartic acid (NMDA) receptors, was examined in brain synaptic membranes treated with Triton X-100 by using a filtration assay method. Elevation of incubation temperature from 2 to 30 degrees C markedly diminished the binding. The binding reached a plateau within 5 min after the initiation of incubation at 2 degrees C, while the time required to attain an equilibrium was 1 min at 30 degrees C. The binding at 2 degrees C was rapidly dissociated by the addition of an excess of unlabeled CPP, NMDA and L-glutamic acid (L-Glu). The binding was also saturable with increasing concentrations of the ligand and displaced by various amino acids structurally related to L-Glu in a stereospecific manner. Competitive but not noncompetitive antagonists for the NMDA receptors invariably inhibited the binding. However, the binding was not prominently affected by agonists for the other subclasses of the brain excitatory amino acid receptors. Both reduced and oxidized forms of glutathione significantly displaced the binding. Scatchard analysis revealed that Triton treatment increased the affinity and density of binding sites which consisted of a single component. Among some endogenous tryptophan metabolites, kynurenic, anthranilic and quinolinic acids inhibited the binding. These results suggest that a filtration assay method is also useful to detect the binding of NMDA receptors in the brain.

6,7-Dichloroquinoxaline-2,3-dione is a competitive antagonist specific to strychnine-insensitive [3H]glycine binding sites on the N-methyl-D-aspartate receptor complex

Multiple binding sites on the N-methyl-D-aspartate (NMDA) receptor complex were examined using rat brain synaptic membranes treated with Triton X-100. Binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne ([3H]MK-801), a noncompetitive NMDA antagonist, in the presence of 10 microM L-glutamate not only was inhibited by different types of antagonists, such as 6,7-dichloro-3-hydroxy-2-quinoxaline-carboxylate, 7-chlorokynurenate, and 6,7-dichloroquinoxaline-2,3-dione (DCQX), but also was abolished by non-NMDA antagonists, including 6-cyano-7-nitroquinoxaline-2,3-dione and 6,7-dinitroquinoxaline-2,3-dione. The inhibition of [3H]MK-801 binding by these compounds was invariably reversed or attenuated by addition of 10 microM glycine. Among these novel antagonists with an inhibitory potency on [3H]MK-801 binding, only DCQX abolished [3H]glycine binding without inhibiting [3H]glutamate and [3H](+-)-3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate bindings. Other antagonists examined were all effective as displacers of the latter two bindings. These results suggest that DCQX is an antagonist highly selective to the strychnine-insensitive glycine binding sites with a relatively high affinity.

Strychnine-insensitive binding of [3H]glycine to synaptic membranes in rat brain, treated with Triton X-100

Binding of radiolabelled glycine, a putative inhibitory neurotransmitter in mammalian lower central structures, was examined by using the synaptic membranes of the brain of rat, treated with Triton X-100. This treatment with Triton markedly potentiated the binding of [3H]glycine detected at 2 degrees C and 30 degrees C. However, this binding was not affected by three different convulsants, strychnine, picrotoxin and bicuculline. The binding was saturable at 2 degrees C, with increasing concentrations of [3H]glycine up to 1 microM. Scatchard analysis revealed that the binding sites consisted of a single component with a Kd of 202 nM and a Bmax of 1.74 pmol/mg protein. The binding was inhibited, not only by various amino acids structurally related to glycine, including D- and L-serine and D-, L- and beta-alanine, but was also eliminated by some peptides containing glycine, such as gamma-D- and gamma-L-glutamylglycine, glycine methylester and N-methyl-glycine. In addition, the strychnine-insensitive binding of [3H]glycine was significantly abolished by numerous quinoxaline antagonists for excitatory amino acid receptors in the brain. These results suggest that synaptic membranes of brain, treated with Triton X-100, are useful to detect the strychnine-insensitive binding of [3H]glycine and superior to untreated membranes in terms of the freedom from the confounding effects of some endogenous amino acids.

Abolition of the NMDA-mediated responses by a specific glycine antagonist, 6,7-dichloroquinoxaline-2,3-dione (DCQX)

Among various quinoxaline derivatives examined, only 6,7-dichloroquinoxaline-2,3-dione (DCQX) competitively displaced the strychnine-insensitive binding of [3H]glycine, without affecting the other binding sites on the N-methyl-D-aspartate (NMDA) receptor complex. This novel specific antagonist abolished the ability of L-glutamate to potentiate [3H]MK-801 binding activity in brain synaptic membranes treated with Triton X-100. Inclusion of glycine reversed this preventive action of DCQX on the potentiation induced by glutamate.

N-methyl-D-aspartate-sensitive [3H]glutamate binding sites in brain synaptic membranes treated with Triton X-100

Specific binding activity of radiolabeled L-glutamic acid, a putative central excitatory neutrotransmitter, was drastically increased with increasing concentrations of Triton X-100 used for pretreatment of rat brain synaptic membranes. The binding in these Triton-treated membranes was a protein dependent, inversely temperature-dependent, stereospecific, structure-selective and saturable process with a high affinity for the amino acid. The binding activity was invariably inhibited by agonists and antagonists for the N-methyl-D-aspartic acid (NMDA)-sensitive subclass, but not by agonists for the other subclasses of excitatory amino acid neurotransmitter receptors in the brain. Scatchard analysis revealed that the binding sites consisted of a single component with a Kd of 24.4 +/- 2.5 nM and a Bmax of 0.94 +/- 0.09 pmol/mg protein. Some endogenous tryptophan metabolites such as kynurenic acid and quinolinic acid also inhibited the binding. These results suggest that synaptic membranes may indeed contain the NMDA-sensitive receptors which are disclosed by Triton X-100 treatment.

Solubilization of stereospecific and quisqualate-sensitive activity of [3H]glutamate binding in the pituitary of the rat

Binding activity of a putative central excitatory neurotransmitter, L-glutamic acid, was solubilized from the pituitary glands of the rat by treatment of the membranous homogenates with a nonionic detergent, Nonidet P-40. The binding activity of [3H]glutamic acid increased linearly with increasing concentrations of the solubilized proteins, up to 15 micrograms. The binding activity reached an equilibrium within 10 min at 2 degrees C, while the time required to attain equilibrium at 30 degrees C was 60 min. Addition of an excess of nonradioactive glutamic acid rapidly decreased the activity detected at 30 degrees C, to the nonspecific binding level. Scatchard analysis of these data revealed that the solubilized binding activity consisted of a single component with a Kd of 0.34 microM and a Bmax of 53.6 pmol/mg protein. L-Glutamic but not D-glutamic acid inhibited the binding activity in a concentration-dependent manner, at the concentration range greater than 10(-8) M. An agonist for a certain subclass of the central glutamate receptors, quisqualic acid, significantly inhibited the solubilized activity, whereas the other two agonists, such as N-methyl-D-aspartic acid and kainic acid, had no significant effect. Reduction of the incubation temperature from 30 degrees C to 2 degrees C resulted in a drastic attenuation of the binding activity due to a decrement in the number of apparent binding sites. These results suggest that the binding activity of [3H]glutamic acid in the pituitary may be derived from a quisqualate-sensitive membranous constituent with a stereospecific high affinity for the central neurotransmitter.

Selective potentiation by L-cysteine of apparent binding activity of [3H]glutathione in synaptic membranes of rat brain

Significant apparent binding activity of [3H]glutathione was detected in synaptic membranous preparations of the rat brain. In vitro addition of sucrose (50-1000 mM) and Triton X-100 (0.02-0.1%) significantly diminished the apparent binding activity, whereas pretreatment of the membranes with Triton X-100 (0.01-0.4%) did not affect the activity. A slight but statistically significant reduction of the apparent binding activity was induced by the in vitro addition (1 mM) of two constituent amino acids, L-glutamic acid and glycine. In contrast, another constituent amino acid, L-cysteine, potently enhanced the binding activity at a concentration higher than 0.1 mM. No prominent alteration of the activity occurred following the inclusion of structurally-related amino acids, dithiothreitol, dithioerythritol and numerous other amino acids. Scatchard analysis revealed that the apparent binding consisted of two independent separate components with Kd values of 0.76 and 11.0 microM, and Bmax values of 4.00 and 27.0 pmol/mg protein respectively. In vitro addition of 1 mM L-cysteine resulted in a single component with a Kd of 8.5 microM and a Bmax of 105 pmol/mg protein. Pretreatment of the membranes with 1 mM L-cysteine potentiated the apparent binding, with a further addition of L-cysteine having no effect. The retina had the highest activity followed by the hypothalamus, striatum, spinal cord, midbrain, hippocampus, medulla-pons, cerebellum and cerebral cortex, which occurred independently of the incubation temperature. In peripheral organs examined, the pituitary possessed higher activity than the retina, with progressively lower activities in the adrenal, liver, spleen, skeletal muscle and heart. No significant activity was detected in the kidney. Addition of 1 mM L-cysteine significantly potentiated the activities at 30 degrees, but not at 2 degrees, in the hippocampus and cerebral cortex without affecting those in other central structures. In contrast, a profound inhibition of the activity was induced by the addition of L-cysteine in the pituitary, adrenal, intestinal mucosa, skeletal muscle and retina independently of the temperature. These results suggest that L-cysteine may selectively potentiate the apparent binding activity of [3H]glutathione in particular regions of the brain, while eliminating that in the peripheral excitable tissues.

Solubilization of quisqualate-sensitive [3H]glutamate binding activity from rat retina

Binding activity of a putative central neurotransmitter, L-glutamic acid, was examined in the supernatant preparations solubilized from rat retinal membranes by Nonidet P-40. [3H]Glutamate binding activity increased linearly with increasing concentrations of the solubilized proteins up to 15 micrograms. The binding activity reached an equilibrium within 10 min at 2 degrees C, while increasing with incubation time up to 60 min at 30 degrees C. Addition of an excess of nonradioactive glutamate rapidly decreased the activity at 30 degrees C. Scatchard analysis revealed that the solubilized retinal binding activity consisted of a single component with a KD of 0.25 microM and a Bmax of 57.4 pmol/mg protein. The solubilized binding activity exhibited a stereospecificity and a structure selectivity to L-glutamate, and was abolished by quisqualate, L-glutamate diethyl ester, and DL-2-amino-3-phosphonopropionate. None of the other agonists and antagonists for the central excitatory amino acid receptors affected the binding activity. Reduction of incubation temperature from 30 degrees C to 2 degrees C resulted in a drastic attenuation of the binding activity due to decrement of the number of the apparent binding sites. Cation-exchange column chromatography revealed that unidentified radioactive material was in fact formed during the incubation of [3H]glutamate with the retinal preparations at 30 degrees C. These results suggest that retinal [3H]glutamate binding activity may be derived at least in part from the quisqualate-sensitive membranous enzyme with a stereospecific and structure-selective high affinity for the central neurotransmitter.

A spider toxin (JSTX) inhibits L-glutamate uptake by rat brain synaptosomes

Joro spider toxin (JSTX), a specific blocker of glutamate receptors, was found to exert a prominent suppressive action on the Na+-dependent binding of L-glutamate to synaptic membranes and on glutamate uptake by synaptosomes in a dose-dependent manner. In contrast, the synthesized 2,4-dihydroxyphenylacetylasparagine (2,4-DHPA-ASN), a common moiety of spider toxins, which has been shown to exhibit almost the same activity as intact JSTX with respect to the inhibition of Na+-independent glutamate binding to its synaptic membrane receptors, shows lower potency in inhibiting Na+-dependent binding and uptake of L-glutamate. From these findings, it is clear that JSTX has the ability to inhibit not only L-glutamate binding to its synaptic membrane receptors but also L-glutamate uptake by synaptosomes, and that polyamines linked to 2,4-DHPA-ASN in the molecule of spider toxins may participate in the inhibition of L-glutamate uptake.

Quantitative autoradiographic characterization of L-[3H] glutamate binding sites in rat vestibular nuclei

Quantitative autoradiography has been used to characterize L-[3H] glutamate binding sites and to describe their distribution in frozen sections of rat vestibular nuclei. Scatchard plots and Hill coefficients of glutamate binding suggest that glutamate interacts with a single population of sites having a KD of about 126 nM and a capacity of 2.5 pmol/mg of protein. Although the level of glutamate binding was not very high compared to the highest levels described for some other brain regions, it was nonetheless substantial. The sites were distributed unevenly in the four vestibular nuclei and their distribution correlated well with the projection areas of the vestibular nerve, which has been described as a glutamate-mediated pathway. The highest numbers of glutamate binding sites were observed in the medial vestibular nuclei. This technique provides a very sensitive assay for characterizing the pharmacological subtypes of glutamate binding in the vestibular nuclei and for analyzing changes in these sites during development or after deafferentation of the vestibular nuclei.

Temperature-dependent and -independent apparent binding activities of [3H]glutathione in brain synaptic membranes

An apparent binding activity of [3H]glutathione was examined by using synaptic membrane preparations of the rat brain. The activity was found to be more than two times as high at 30 degrees C as that found at 2 degrees C. At 2 degrees C, the apparent binding sites consisted of a single component with a Kd of 0.77 microM and a Bmax of 5.60 pmol/mg protein. In contrast, two independent separate sites with Kds of 0.56 and 12.6 microM and Bmaxs of 2.50 and 28.5 pmol/mg protein were observed at 30 degrees C. In vitro addition of Triton X-100 significantly inhibited the apparent binding activities detected at both temperatures, whereas pretreatment of the membranes with the detergent did not significantly affect both binding activities. Among 3 constituent amino acids of glutathione, L-cysteine induced a selective and irreversible potentiation of the apparent activities, which occurred independently of the incubation temperature. Scatchard analysis revealed that L-cysteine drastically increased the number of the low affinity sites without significantly altering their affinity. Apparent binding activities determined at both incubation temperatures were unevenly distributed in the central and peripheral structures. Distribution profile of the temperature-dependent activities was found to be closely related to that of the basal binding activity of [3H]L-glutamic acid, a putative central excitatory neurotransmitter. These results suggest that brain synaptic membranes may indeed contain specific binding sites of [3H]glutathione which have an interaction with the glutamate binding sites. Possible presence of two distinctly different apparent binding sites of [3H]glutathione, such as temperature-independent high affinity sites and temperature-dependent low affinity sites, is also suggested.

Disclosure by triton X-100 of NMDA-sensitive [3H] glutamate binding sites in brain synaptic membranes

Pretreatment of brain synaptic membrane homogenates with Triton X-100 resulted in a drastic disclosure of [3H] glutamate (Glu) binding activity which was sensitive to one of the central Glu receptor agonists, N-methyl-D-aspartic acid (NMDA). The NMDA-sensitive binding was inversely dependent on the incubation temperature, and was a reversible and saturable process. Scatchard analysis revealed that Triton X-100 treatment yielded in a significant enhancement of the affinity with a concomitant increment of the density of binding sites. Electrophysiologically identified agonists and antagonists for the NMDA receptors all significantly inhibited the binding to Triton-treated membranes. These results suggest that Triton-treatment may disclose NMDA-sensitive [3H] Glu binding sites in brain synaptic membranes.

Possible presence of [3H]glutathione (GSH) binding sites in synaptic membranes from rat brain

Reduced as well as oxidized forms of glutathione exhibited a significant displacement of the specific binding of [3H]L-glutamic acid (Glu), a potential candidate for the central excitatory neurotransmitter, to the rat brain synaptic membranes. In order to elucidate these findings, an attempt was made to determine whether or not the synaptic membranes contained the binding sites for this peptide using [3H]glutathione (GSH) as a ligand. The specific binding activity was detected in the synaptic membranous preparations and found to be dependent on the incubation temperature and incubation time. The binding reached a plateau within 60 min of incubation at 2 degrees C and 30 degrees C. [3H]GSH binding increased linearly with increasing concentrations of membranous proteins employed. Scatchard analysis revealed that the binding sites consisted of two separate independent components rather than being comprised of a single constituent. A significant and concentration-dependent displacement of the binding was induced not only by the addition of GSH, but also by the inclusion of some GSH derivatives without SH-moiety, such as the oxidized form of glutathione, S-methyl-glutathione and S-hexyl-glutathione. The binding was also significantly inhibited by various alpha- and gamma-peptides containing L-Glu, but not by those containing D-Glu. Amongst 4 different agonists and antagonists used for the subclassification of the central Glu receptors, an agonist, quisqualic acid, and an antagonist, 2-amino-4-phosphonobutyric acid, exhibited a significant inhibition of the binding at the highest concentration employed. These results suggest that the rat brain synaptic membranes may contain structure-selective, temperature-dependent, high affinity and saturable binding sites for glutathione.

Enhancement of [3H]glutamate binding by N-methyl-D-aspartic acid in rat adrenal

Some neurochemical characteristics of [3H]L-glutamic acid binding sites were studied using membranous homogenate preparations obtained from the rat adrenal. It was found that the binding was inhibited by the addition (10(-7)-10(-3) M) of L-isomers of structure-related compounds in a concentration-dependent manner. A significant inhibition of the binding was induced by L-glutamic acid diethylester, but not by alpha-aminoadipic acid. Scatchard analysis revealed that the binding sites consisted of a single component with a Kd of 0.19 +/- 0.05 microM and a Bmax of 4.11 +/- 0.71 pmol/mg protein, respectively. In vitro addition of sodium acetate (1-100 mM) elicited a stimulatory action on the binding at 2 degrees C, while inducing a significant attenuation of the binding at 30 degrees C. The binding reached a plateau within 30 min of incubation followed by a gradual decline up to 60 min in the presence of 100 mM sodium acetate at 30 degrees C, whereas the binding continued to increase up to 60 min in the absence of sodium acetate. Addition (0.1-10 mM) of N-methyl-D-aspartic acid (NMDA), one of the agonists for central excitatory amino acid neurotransmitter receptors, exerted a significant augmentation of the adrenal binding independently of the incubation temperature in a concentration-dependent manner. The latter facilitation of the binding, however, was not affected by the classical antagonists for central NMDA receptors such as 2-amino-5-phosphonovaleric acid and 2-amino-7-phosphonoheptanoic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization of novel binding sites for [3H]glutamate in rat adrenal

[3H]Glutamate binding sites were solubilized from the rat adrenal glands by treatment of the membranous homogenate preparations with various detergents. The binding in solubilized preparations was dependent on the incubation temperature and incubation time, and reached an equilibrium within 40 min of incubation at 30 degrees C. Scatchard analysis revealed that the binding sites consisted of a single component with a Kd of 0.15 microM and a Bmax of 35.9 pmoles/mg protein, respectively. The binding was significantly displaced by L-isomers of the structure-related compounds, but not by D-isomers. Quisqualic acid exhibited a concentration-dependent inhibition of the binding, whereas neither N-methyl-D-aspartic acid nor kainic acid elicited such a prominent diminution. These results suggest that the rat adrenal indeed contains novel binding sites for the central neurotransmitter candidate.

Differentiation of the Ca2+-stimulated binding from the Cl- -dependent binding of [3H]glutamate in synaptic membranes from rat brain

The effect of Ca2+ as well as Cl- ions on [3H]glutamate (Glu) binding was re-examined using rat brain synaptic membranes frozen at -80 degrees C in 0.32 M sucrose. The inclusion of 20 mM ammonium chloride or 20 mM ammonium chloride plus 2.5 mM calcium acetate disclosed the Cl- -dependent binding or Ca2+-stimulated binding even at 2 min after the initiation of incubation at 30 degrees C and each binding reached a plateau within 30 min. In contrast, the binding reached its maximal value within 10 min followed by a progressive decline up to 60 min in the presence of 100 mM sodium acetate. Scatchard analysis revealed that Cl- as well as Cl-/Ca2+ ions invariably caused a significant increment of the number of binding sites without altering their affinity, whereas Na+ ions induced a prominent increment of the density of binding sites with a concomitant lowering of their affinity. DL-2-Amino-4-phosphonobutyric acid selectively abolished the Cl- -dependent and Ca2+-stimulated bindings without significantly affecting the basal or Na+-dependent binding. Quisqualic acid induced a profound inhibition of both Cl- -dependent and Ca2+-stimulated bindings, to a significantly greater extent than that of the basal and Na+-dependent bindings. D-Aspartic acid exhibited a potent inhibition of the Na+-dependent binding with a significantly less potent displacement of the basal, Cl- -dependent and Ca2+-stimulated bindings. An inhibitor of anion transport, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), not only eliminated the Cl- -dependent binding, but also completely abolished the Ca2+-stimulated binding. Scatchard analysis revealed that DIDS (0.1 mM) prevented the Cl- - and Cl-/Ca2+-induced increment of the density of binding sites with no significant change of their affinity. Pretreatment of the membranes with hydrophilic SH-reactive agents such as N-ethylmaleimide and 5,5'-dithiobis-(2-nitrobenzoic acid) invariably resulted in a more sensitive inhibition of the Ca2+-stimulated binding than that of the Cl- -dependent binding, while hydrophobic reagent p-chloromercuribenzoic acid produced a similarly potent elimination of the Cl- -dependent and Ca2+-stimulated bindings. Calcium-stimulated binding was also found to be sensitively diminished by dithiothreitol and dithioerythritol as compared with the Cl- -dependent binding. In vitro addition of L-ascorbic acid (10(-6)-10(-3) M) attenuated the Ca2+-stimulated binding to a significantly greater extent than the inhibition of the Cl- -dependent binding.(ABSTRACT TRUNCATED AT 400 WORDS)