Coexpression of N-methyl-D-aspartate and phencyclidine receptors in Xenopus oocytes injected with rat brain mRNA

SNAP-25 is a target of protein kinase C phosphorylation critical to NMDA receptor trafficking

Protein kinase C (PKC) enhances NMDA receptor (NMDAR)-mediated currents and promotes NMDAR delivery to the cell surface via SNARE-dependent exocytosis. Although the mechanisms of PKC potentiation are established, the molecular target of PKC is unclear. Here we show that synaptosomal-associated protein of 25 kDa (SNAP-25), a SNARE protein, is functionally relevant to PKC-dependent NMDAR insertion, and identify serine residue-187 as the molecular target of PKC phosphorylation. Constitutively active PKC delivered via the patch pipette potentiated NMDA (but not AMPA) whole-cell currents in hippocampal neurons. Expression of RNAi targeting SNAP-25 or mutant SNAP-25(S187A) and/or acute disruption of the SNARE complex by treatment with BoNT A, BoNT B or SNAP-25 C-terminal blocking peptide abolished NMDAR potentiation. A SNAP-25 peptide and function-blocking antibody suppressed PKC potentiation of NMDA EPSCs at mossy fiber-CA3 synapses. These findings identify SNAP-25 as the target of PKC phosphorylation critical to PKC-dependent incorporation of synaptic NMDARs and document a postsynaptic action of this major SNARE protein relevant to synaptic plasticity.

Receptor systems: will mining the receptorome yield novel targets for pharmacotherapy?

We have recently defined the receptorome as 'that part of the proteome encoding receptors'. In this article, I provide a general overview of the members of the receptorome as well as methods used to screen the receptorome-both in silico and physically. Case histories of receptorome-based discovery efforts are then highlighted and the relevance of this approach to the discovery and validation of molecular targets for drug abuse treatment is emphasized.

Characterization of two novel N-methyl-D-aspartate antagonists: EAA-090 (2-[8,9-dioxo-2,6-diazabicyclo [5.2.0]non-1(7)-en2-yl]ethylphosphonic acid) and EAB-318 (R-alpha-amino-5-chloro-1-(phosphonomethyl)-1H-benzimidazole-2-propanoic acid hydrochloride)

Two novel N-methyl-d-aspartate (NMDA) antagonists with unique chemical structures, EAA-090 (2-[8,9-dioxo-2, 6-diazabicyclo[5.2.0]non-1(7)-en2-yl]ethylphosphonic acid) and EAB-318 (R-alpha-amino-5-chloro-1-(phosphonomethyl)-1H-benzimidazole-2-propanoic acid hydrochloride), were compared with CGS-19755 (Selfotel) in ligand binding, electrophysiology, and neuroprotection assays. CGS-19755, EAA-090 and EAB-318 inhibited [(3)H]3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid binding to NMDA receptors with IC(50) values of 55, 28, and 7.9 nM, respectively. All three compounds decreased the duration of spontaneous synaptic currents and inhibited NMDA-activated currents in rat hippocampal neurons. IC(50) values for inhibition of current induced by 10 microM NMDA were 795, 477, and 69 nM for CGS-19755, EAA-090, and EAB-318, respectively. The NMDA antagonists protected chick embryo retina slices and cultured rat hippocampal and cortical neurons from glutamate- and NMDA-induced neurotoxicity. In experiments in which different NMDA receptor splice variants and subtypes were expressed in Xenopus oocytes, all three antagonists preferentially blocked NMDA-elicited currents mediated by N-methyl-d-aspartate receptor (NR)1 splice variants containing the N-terminal insertion. They also favored NR2A-versus NR2B- or NR2C-containing NMDA receptors, with EAA-090 showing the greatest selectivity. EAA-090 was 10 times more potent at blocking NR2A-versus NR2B- or NR2C-containing NMDA receptors. In addition to being the most potent NMDA antagonist, EAB-318 inhibited alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors. The combination of NMDA and AMPA/kainate block enabled EAB-318 to protect neurons against ischemia induced cell death.

A role for extracellular Na+ in the channel gating of native and recombinant kainate receptors

Ionotropic glutamate receptors of the kainate and AMPA subtypes share a number of structural features, both topographical and in terms of stoichiometry. In addition, AMPA and kainate receptors share similar pharmacological and biophysical properties in that they are activated by common agonists and display rapid activation and desensitization characteristics. However, we show here that in contrast to AMPA receptor-mediated responses (native or recombinant GluR3 receptor), the response of native and recombinant (GluR6) kainate receptors to glutamate was drastically reduced in the absence of extracellular Na+ (i.e., when replaced by Cs+). Removal of Na+ increases the rate of desensitization, indicating that external Na+ modulates channel gating. Whereas the size of the substituting cation is important in mimicking the action of Na+ (Li+>K+>Cs+), modulation was voltage independent. These results indicate the existence of different gating mechanisms for AMPA and kainate receptors. By using chimeric AMPA-kainate receptors derived from GluR3 and GluR6, we have identified a key residue in the S2 segment of GluR6 (M770) that is largely responsible for the sensitivity of the receptor to external Na+. Thus, these results show the existence of a specific kainate receptor gating mechanism that requires external Na+ to be operative.

A role for extracellular Na+ in the channel gating of native and recombinant kainate receptors

Ionotropic glutamate receptors of the kainate and AMPA subtypes share a number of structural features, both topographical and in terms of stoichiometry. In addition, AMPA and kainate receptors share similar pharmacological and biophysical properties in that they are activated by common agonists and display rapid activation and desensitization characteristics. However, we show here that in contrast to AMPA receptor-mediated responses (native or recombinant GluR3 receptor), the response of native and recombinant (GluR6) kainate receptors to glutamate was drastically reduced in the absence of extracellular Na+ (i.e., when replaced by Cs+). Removal of Na+ increases the rate of desensitization, indicating that external Na+ modulates channel gating. Whereas the size of the substituting cation is important in mimicking the action of Na+ (Li+>K+>Cs+), modulation was voltage independent. These results indicate the existence of different gating mechanisms for AMPA and kainate receptors. By using chimeric AMPA-kainate receptors derived from GluR3 and GluR6, we have identified a key residue in the S2 segment of GluR6 (M770) that is largely responsible for the sensitivity of the receptor to external Na+. Thus, these results show the existence of a specific kainate receptor gating mechanism that requires external Na+ to be operative.

Differential contributions of NMDA and non-NMDA receptors to spinal Fos expression evoked by superficial tissue and muscle inflammation in the rat

The role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the spinal cord in the transmission of nociceptive afferents from superficial tissue and muscle was studied by examining the effects of NMDA or non-NMDA receptor antagonists on Fos expression in the spinal dorsal horn. Muscle inflammation was induced by injection of turpentine oil into the gastrocnemius muscle, whereas superficial tissue inflammation was induced by an intraplantar injection of turpentine oil into the hindpaw. The NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5), the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) or normal saline were intrathecally administered 15 min before an intramuscular or intraplantar injection of turpentine oil. Muscle inflammation evoked expression of Fos-like immunoreactive neurons staining in neurons that were predominantly distributed in the middle portions of laminae I-II(outer) and the lateral portions of laminae V-VI of the ipsilateral dorsal horn at the spinal L(4)-L(5). DNQX, but not AP-5, significantly reduced the total number of Fos-like immunoreactive neurons evoked by muscle inflammation. In contrast, superficial tissue inflammation evoked expression of Fos-like immunoreactive neurons in the medial portions of laminae I-II(outer) and V-VI of the ipsilateral dorsal horn at the spinal L(4)-L(5) that was blocked by AP-5, but not by DNQX. Injection of normal saline did not influence the numbers of Fos-LI neurons. These results indicate that different glutamate receptors in the dorsal horn of the spinal cord may mediate nociceptive input from superficial tissue (particularly skin) and muscle. DNQX receptors may mediate transmission of nociceptive information originating in muscle, while NMDA receptors may preferentially mediate transmission of nociceptive information originating in skin.

Protein kinase C potentiation of N-methyl-D-aspartate receptor activity is not mediated by phosphorylation of N-methyl-D-aspartate receptor subunits

N-methyl-D-aspartate receptors (NMDARs) are Ca(2+)-permeable glutamate-gated ion channels whose physiological properties in neurons are modulated by protein kinase C (PKC). The present study was undertaken to determine the role in PKC-induced potentiation of the NR1 and NR2A C-terminal tails, which serve as targets of PKC phosphorylation [Tingley, W. G., Ehlers, M. D., Kameyama, K., Doherty, C., Ptak, J. B., Riley, C. T. & Huganir, R. L. (1997) J. Biol. Chem. 272, 5157-5166]. Serine residue 890 in the C1 cassette is a primary target of PKC phosphorylation and a critical residue in receptor clustering at the membrane. We report herein that the presence of the C1 cassette reduces PKC potentiation and that mutation of Ser-890 significantly restores PKC potentiation. Splicing out or deletion of other C-terminal cassettes singly or in combination had little or no effect on PKC potentiation. Moreover, experiments involving truncation mutants reveal the unexpected finding that NMDARs assembled from subunits lacking all known sites of PKC phosphorylation can show PKC potentiation. These results indicate that PKC-induced potentiation of NMDAR activity does not occur by direct phosphorylation of the receptor protein but rather of associated targeting, anchoring, or signaling protein(s). PKC potentiation of NMDAR function is likely to be an important mode of NMDAR regulation in vivo and may play a role in NMDA-dependent long-term potentiation.

Mutation of structural determinants lining the N-methyl-D-aspartate receptor channel differentially affects phencyclidine block and spermine potentiation and block

Spermine and other endogenous polyamines potentiate, block and permeate the N-methyl-D-aspartate receptor channel. To identify structural determinants of the N-methyl-D-aspartate channel that mediate spermine's actions, we generated mutant receptors with asparagine (N) to glutamine (Q) or arginine (R) substitutions in the selectivity filter of the channel. We demonstrate that mutation of the three critical asparagines in this domain differentially affects block by phencyclidine and both potentiation and block by spermine. N-to-Q and N-to-R mutations in the N site of the NR1 subunit (N598 in NR1(011), N619 in NR1(100)) and N-to-Q mutations in the N and N + 1 sites (N595 and N596 in NR2A, respectively) of the NR2 subunit (Q/NN, R/NN, N/QN, N/NQ, Q/QN and Q/NQ receptors) reduced affinity for phencyclidine. The Q/NN receptor showed markedly reduced potentiation by spermine, with little or no change in spermine block. The R/NN receptor showed markedly reduced spermine potentiation and affinity for spermine at its block site. The N/QN, N/NQ and Q/QN mutant receptors showed somewhat enhanced spermine block, while the Q/ NQ double mutant exhibited significantly more enhanced spermine block. Thus, the asparagine residues critical to Ca2+ permeability and Mg2+ block of N-methyl-D-aspartate channels are also critical to block by spermine and phencyclidine. To examine the interaction of spermine and phencyclidine within the channel, we performed competition studies. Spermine appeared to compete with phencyclidine for binding to the receptor; however, blocks by phencyclidine and by spermine were not additive. The findings suggest that spermine can bind to a site in the external vestibule of the channel to impede phencyclidine binding, but allow Na+ influx.

NS-257, a novel competitive AMPA receptor antagonist, interacts with kainate and NMDA receptors

In this study, we examined the effects of a novel, water-soluble, putative competitive AMPA receptor antagonist, 1,2,3,6,7, 8-hexahydro-3-(hydroxyimino)-N,N,7-trimethyl-2-oxobenzo[2,1- b:3, 4-c']dipyrrole-5-sulfonamide (NS-257) on AMPA, kainate and NMDA receptors using the two-electrode voltage-clamp technique in Xenopus oocytes. All glutamate receptor subtypes were inhibited by NS-257 in a voltage-independent way. When kainate was applied to oocytes injected with total mouse brain mRNA, mainly AMPA receptors were activated. The antagonistic effects of NS-257 on these kainate-induced currents were concentration-dependent and competitive. In the same way, NS-257 blocked kainate-induced currents recorded from oocytes expressing homomeric GluR-1 receptors. In our experiments higher concentrations (>1 microM) of NS-257 also produced inhibitory effects on kainate and to a lesser extent on NMDA receptor function as indicated by recordings from GluR-6 or NR-1b/2A cRNA injected oocytes. While NMDA receptor function was inhibited in a competitive fashion, kainate responses recorded from homomeric GluR-6 receptors were blocked in a mixed competitive-noncompetitive manner. This mixed antagonistic action of NS-257 might have been caused by preincubating oocytes with concanavalin A, which blocks desensitization of kainate receptors. Although NS-257 appeared to be a less potent AMPA receptor antagonist then other known antagonists like NBQX, its main advantage over all other reported compounds so far is its higher aqueous solubility which still represents the major weakness of the other AMPA receptor antagonists, especially for clinical use.

The phosphoprotein DARPP-32 mediates cAMP-dependent potentiation of striatal N-methyl-D-aspartate responses

The signal transduction pathway underlying the cAMP-dependent modulation of rat striatal N-methyl-D-aspartate (NMDA) responses was investigated by using the two-electrode voltage-clamp technique. In oocytes injected with rat striatal poly(A)+ mRNA, activation of cAMP-dependent protein kinase (PKA) by forskolin potentiated NMDA responses. Inhibition of protein phosphatase 1 (PP1) and/or protein phosphatase 2A (PP2A) by the specific inhibitor calyculin A occluded the PKA-mediated potentiation of striatal NMDA responses, suggesting that the PKA effect was mediated by inhibition of a protein phosphatase. Coinjection of oocytes with striatal mRNA and antisense oligodeoxynucleotides directed against the protein phosphatase inhibitor DARPP-32 dramatically reduced the PKA enhancement of NMDA responses. NMDA responses recorded from oocytes injected with rat hippocampal poly(A)+ mRNA were not affected by stimulation of PKA. When oocytes were coinjected with rat hippocampal poly(A)+ mRNA plus complementary RNA coding for DARPP-32, NMDA responses were potentiated after stimulation of PKA. The results provide evidence that DARPP-32, which is enriched in the striatum, may participate in the signaling between the two major afferent striatal pathways, the glutamatergic and the dopaminergic projections, by the cAMP-dependent regulation of striatal NMDA currents.

Ca2+ influx amplifies protein kinase C potentiation of recombinant NMDA receptors

Protein kinase C (PKC) potentiates NMDA receptors in hippocampal, trigeminal, and spinal neurons. Although PKC phosphorylates the NMDA receptor subunit NR1 at four residues within the C terminal splice cassette C1, the molecular mechanisms underlying PKC potentiation of NMDA responses are not yet known. The present study examined the role of Ca2+ in PKC potentiation of recombinant NMDA receptors expressed in Xenopus oocytes. We found that Ca2+ influx through PKC-potentiated NMDA receptors can further increase the NMDA response ("Ca2+ amplification"). Ca2+ amplification required a rise in intracellular Ca2+ concentration at or near the intracellular end of the channel and was independent of Ca2+-activated Cl- current. Ca2+ amplification depended on extracellular Ca2+ concentration during NMDA application and not during PKC activation. Ca2+ amplification was reduced by the membrane-permeant Ca2+-chelating agent BAPTA-AM. Mutant receptors with greatly reduced Ca2+ permeability did not exhibit Ca2+ amplification. Receptors containing the NR1 N-terminal splice cassette showed more Ca2+ amplification, possibly because of their larger basal current and therefore greater Ca2+ influx. Contrary to expectation, splicing out the two C-terminal splice cassettes of NR1 enhanced PKC potentiation in a manner independent of extracellular Ca2+. This observation indicates that PKC potentiation does not require phosphorylation of the C1 cassette of the NR1 subunit. PKC potentiation of NMDA receptors in vivo is likely to be affected by Ca2+ amplification of the potentiated signal; the degree of amplification will depend in part on alternative splicing of the NR1 subunit, which is regulated developmentally and in a cell-specific manner.

Ca2+ influx amplifies protein kinase C potentiation of recombinant NMDA receptors

Protein kinase C (PKC) potentiates NMDA receptors in hippocampal, trigeminal, and spinal neurons. Although PKC phosphorylates the NMDA receptor subunit NR1 at four residues within the C terminal splice cassette C1, the molecular mechanisms underlying PKC potentiation of NMDA responses are not yet known. The present study examined the role of Ca2+ in PKC potentiation of recombinant NMDA receptors expressed in Xenopus oocytes. We found that Ca2+ influx through PKC-potentiated NMDA receptors can further increase the NMDA response ("Ca2+ amplification"). Ca2+ amplification required a rise in intracellular Ca2+ concentration at or near the intracellular end of the channel and was independent of Ca2+-activated Cl- current. Ca2+ amplification depended on extracellular Ca2+ concentration during NMDA application and not during PKC activation. Ca2+ amplification was reduced by the membrane-permeant Ca2+-chelating agent BAPTA-AM. Mutant receptors with greatly reduced Ca2+ permeability did not exhibit Ca2+ amplification. Receptors containing the NR1 N-terminal splice cassette showed more Ca2+ amplification, possibly because of their larger basal current and therefore greater Ca2+ influx. Contrary to expectation, splicing out the two C-terminal splice cassettes of NR1 enhanced PKC potentiation in a manner independent of extracellular Ca2+. This observation indicates that PKC potentiation does not require phosphorylation of the C1 cassette of the NR1 subunit. PKC potentiation of NMDA receptors in vivo is likely to be affected by Ca2+ amplification of the potentiated signal; the degree of amplification will depend in part on alternative splicing of the NR1 subunit, which is regulated developmentally and in a cell-specific manner.

Serotonin 5-HT2 receptor activation potentiates N-methyl-D-aspartate receptor-mediated ion currents by a protein kinase C-dependent mechanism

Modulation of N-methyl-D-aspartate (NMDA) receptor-mediated ion currents by serotonin was investigated with a two-electrode voltage clamp technique in Xenopus oocytes injected with rat brain RNA. After a 1-min application of 200 nM serotonin a transient potentiation of the NMDA receptor-mediated ion currents was observed. The serotonin-induced enhancement was mimicked by the protein kinase C activators 1-oleoyl-2-acetyl-sn-glycerol (100 microM) and phorbol 12-myristate 13-acetate (10 nM), whereas the inactive phorbol ester 4-alpha-phorbol 12-myristate 13-acetate (10 nM) had no effect. From these observations it was concluded that protein kinase C was involved in the enhancement of NMDA-induced currents. In agreement with this conclusion, it was found that the serotonin effect was inhibited by the protein kinase C inhibitors sphingosine (1 microM) or staurosporine (1 microM) added 20 min before NMDA application and by oocyte injection of protein kinase C (PKC)-inhibitor peptide (500 ng/oocyte) 1 hr prior to recordings. The serotonin receptor involved was identified as a 5-HT2 receptor subtype by the finding that 200 nM of the selective 5-HT2 receptor agonist alpha-methyl-5-hydroxytryptamine mimicked the potentiation of NMDA-induced ion currents by serotonin. Furthermore, the observed potentiation was significantly reduced by co-application of serotonin with 100 microM of the selective 5-HT2 receptor antagonist ketanserin. These results indicate that 5-HT2 receptors enhance NMDA receptor function via phosphoinositol hydrolysis and subsequent stimulation of PKC.

The endogenous agonist quinolinic acid and the non endogenous homoquinolinic acid discriminate between NMDAR2 receptor subunits

Quinolinic acid is an endogenous neurotoxin with NMDA receptor agonist properties. As such it may be the etiologic agent in many diseases. In this paper the NMDA receptor agonist properties of quinolinic acid, as well as those of homoquinolinic acid, a non endogenous analogue, were investigated in Xenopus oocytes injected with 12-day-old rat cortical mRNA or with recombinant NMDA receptors. In oocytes injected with cortical mRNA, quinolinic acid was a weak NMDA receptor agonist: millimolar concentrations were necessary to induce responses that were smaller than maximal responses induced by NMDA; homoquinolinic acid and NMDA had similar affinities but different efficacies: maximal responses induced by homoquinolinic acid were larger than maximal responses induced by NMDA. Cortical mRNA, as verified by RT-PCR and restriction analysis, contains various NMDA subunits. In order to investigate if the low affinity or efficacy of quinolinic acid could be explained by receptor composition, the pharmacological properties of the putative agonists were investigated in oocytes expressing binary combinations of recombinant NMDA receptors. Quinolinic acid did not activate receptors containing NR1 + NR2C but did activate receptors containing NR1 + NR2A and NR1 + NR2B even if only at millimolar concentrations; homoquinolinic acid activated all subunit combinations but was less efficient than NMDA only in the NR1 + NR2C subunit combination. The relative efficacies of quinolinic acid and homoquinolinic acid were evaluated by comparing the maximal responses induced by these agonists with those induced by NMDA and glutamate in the same oocytes. The rank order of potency was quinolinic acid < NMDA < homoquinolinic acid < or = glutamate for the NR1 + NR2A and NR1 + NR2B combinations whereas for NR1 + NR2C it was quinolinic acid << << homoquinolinic acid < NMDA < or = glutamate. The use of quinolinic acid and homoquinolinic acid may thus help to identify endogenous receptors containing the NR2C subunit.

Spermine potentiation of recombinant N-methyl-D-aspartate receptors is affected by subunit composition

The present study shows that both the NR1 and NR2 subunits critically affect spermine potentiation of heteromeric recombinant N-methyl-D-aspartate receptors. NR1(011), the most prominent NR1 splice variant in rat forebrain, and NR1(100), prominent in midbrain, were expressed in Xenopus oocytes singly and in combination with NR2A, NR2B, and NR2C subunits. As for NR1(011) homomers, NR1(011)/NR2B receptors exhibited spermine potentiation by two mechanisms: by increasing glycine affinity and by increasing current through receptors with bound N-methyl-D-aspartate and glycine. NR1(011)/NR2A receptors exhibited only the increase in glycine affinity, and NR1(011)/NR2C receptors exhibited neither. As for NR1(100) homomers, NR1(100)/NR2B and NR1(100)/NR2A receptors exhibited spermine potentiation only by increasing the glycine affinity. Spermine produced no potentiation of NR1(100)/NR2C receptors. Thus, the NR2B subunit "permits" both forms of spermine potentiation, the NR2A subunit permits spermine potentiation only by increasing the glycine affinity, and th NR2C subunit permits neither form of potentiation. Spermine actions on NR1/NR2 showed little voltage dependence. These observations are of interest because the NR1 and NR2 subunits are differentially distributed and developmentally regulated. At early postnatal ages, NR2B subunit mRNA was more highly expressed than NR2A and NR2C mRNAs in hippocampus, neocortex, and caudate-putamen. These findings account for many of the observed differences among neurons in polyamine actions and suggest that these actions will vary in a cell-specific and age-related manner.

Splice variants of the N-methyl-D-aspartate receptor NR1 identify domains involved in regulation by polyamines and protein kinase C

The N-methyl-D-aspartate (NMDA) receptor NR1 gene encodes RNA that is alternatively spliced to generate at least seven variants. The variants arise from splicing in or out of three exons; one encodes a 21-amino acid insert in the N-terminal domain, and two encode adjacent sequences of 37 and 38 amino acids in the C-terminal domain. Splicing out of the second C-terminal exon deletes a stop codon and results in an additional open reading frame encoding an unrelated sequence of 22 amino acids before arriving at a second stop codon. We denote the NR1 variants by the presence or absence of the three alternatively spliced exons (from 5' to 3'); thus, NR1(111) has all three exons, NR1(000) has none, and NR1(100) has only the N-terminal exon. We report here electrophysiological characterization of six splice variants of the NR1 receptor expressed in Xenopus oocytes. NR1 receptors that lacked the N-terminal exon (NR1(000), NR1(010), and NR1(011)) exhibited a relatively high affinity for NMDA (EC50 approximately 13 microM) and marked potentiation by spermine. In contrast, those receptor variants with the N-terminal insert (NR1(100), NR1(101), and NR1(111)) showed a lower agonist affinity and little or no spermine potentiation at saturating glycine. All six variants showed spermine potentiation at low glycine and inhibition by spermine at more negative potentials. Variants differing only in the C-terminal domain differed little in agonist affinity and spermine potentiation. These findings indicate that the N-terminal insert either participates in agonist and polyamine binding domains or indirectly modifies their conformations. The splice variants differed in the extent to which they could be potentiated by activators of protein kinase C (PKC) from 3- to 20-fold. Presence of the N-terminal insert and absence of the C-terminal sequences increased potentiation by PKC. These findings identify the contributions of the separate polypeptide domains to modulation by polyamines and PKC and provide further support for the concept that subunit composition determines functional properties of NMDA receptors.

Whole cell and single channel analysis of the kinetics of glycine-sensitive N-methyl-D-aspartate receptor desensitization

1. The kinetics of glycine-sensitive, N-methyl-D-aspartate (NMDA) receptor desensitization were investigated in cultured neurones with the patch clamp technique. 2. The degree of fast NMDA-receptor desensitization was inversely related to glycine concentration. Thus, increasing concentrations of glycine from 30 nM to 2.5 microM potentiated desensitized NMDA responses (873% +/- 101%) to a greater degree than peak responses (260% +/- 27%). 3. The desensitization was due to a decrease in the affinity of glycine for the strychnine-insensitive, glycine modulatory site (glycineB site) following activation of the NMDA-receptor complex. Thus, the A50 for glycine in potentiating peak responses (77 nM, 95% confidence limited 58-104 nM) was five fold lower than that for plateau responses (399 nM, 340-468 nM). 4. The rate of desensitization was related to glycine concentration such that a reciprocal plot of desensitization rate (1/tau S-1) against glycine concentration had a slope of 9.5* 10(6) M-1 S-1. 5. Recovery from desensitization following step increases in glycine or L-alanine concentration in the continuous presence of NMDA (200 microM) reflected the association kinetics of the glycineB agonist used. 6. The rate and degree of NMDA receptor desensitization was independent of holding potential. 7. NMDA receptor desensitization was also evident at the single channel level. 8. The glycineB antagonist 7-chlorokynurenic acid (7-Chl-Kyn 3 and 10 microM) concentration-dependently induced an identical form of desensitization in the presence of 1 microM glycine. 9. In contrast, the competitive NMDA antagonist (+/-)-amino-phosphonovaleric acid (APV 30 to 300 microM) concentration-dependently antagonized and slowed the onset kinetics of NMDA responses.

Cloning of an apparent splice variant of the rat N-methyl-D-aspartate receptor NMDAR1 with altered sensitivity to polyamines and activators of protein kinase C

Molecular cloning identified complementary DNA species, from a rat ventral midbrain library, encoding apparent splice variants of the N-methyl-D-aspartate (NMDA) receptor NMDAR1 (which we now term NR1a). Sequencing revealed that one variant, NR1b, differs from NR1a by the presence of a 21-amino acid insert near the amino end of the N-terminal domain and by an alternate C-terminal domain in which the last 75 amino acids are replaced by an unrelated sequence of 22 amino acids. NR1b is virtually identical to NR1a in the remainder of the N- and C-terminal domains, at the 5' and 3' noncoding ends, and within the predicted transmembrane domains and extracellular and cytoplasmic loops. These findings suggest that the two forms of the receptor arise by differential splicing of a transcript from the same gene. Sequencing of other clones indicates the existence of a third variant, NR1c, identical to NR1b in its C terminus but lacking the N-terminal insert. NR1b RNA injected into Xenopus oocytes generated functional homomeric NMDA channels with electrophysiological properties distinct from those of NR1a homomeric channels. NR1b channels exhibited a lower apparent affinity for NMDA and for glutamate. NR1b channels exhibited a lower affinity for D-2-amino-5-phosphonovaleric acid and a higher affinity for Zn2+. The two receptor variants showed nearly identical affinities for glycine, Mg2+, and phencyclidine. Spermine potentiation of NMDA responses, prominent in oocytes injected with rat forebrain message, was also prominent for NR1a receptors, but was greatly reduced or absent for NR1b receptors. Treatment with the protein kinase C activator phorbol 12-myristate 13-acetate potentiated NMDA responses in NR1b-injected oocytes by about 20-fold; potentiation of NMDA responses in NR1a-injected oocytes was much less, about 4-fold. These findings support a role for alternate splicing in generating NMDA channels with different functional properties.

Alternative splicing generates functionally distinct N-methyl-D-aspartate receptors

We have used expression cloning in Xenopus oocytes to isolate two different cDNAs encoding functional N-methyl-D-aspartate (NMDA) receptor subunits. The two receptors (NMDA-R1A and -R1B) display different pharmacologic properties as a consequence of alternative exon addition within the putative ligand-binding domain. The splicing choice is regulated such that R1B is the predominant form of receptor in the cerebellum, whereas R1A predominates in other brain regions. Expression of either of the subunits alone in oocytes results in an NMDA-evoked inward current with electrophysiologic properties closely resembling those of the NMDA receptors observed in neurons. Thus, the complex properties exhibited by the NMDA receptor in neurons can be generated by the expression of a single receptor subunit.

Equilibrium and kinetic study of glycine action on the N-methyl-D-aspartate receptor in cultured mouse brain neurons

1. The characteristics of the activation of the N-methyl-D-aspartate (NMDA) response by glycine were studied using whole-cell and outside-out patch clamp recording techniques. 2. Glycine concentration-response (C-R) curves were measured in the presence of 10 microM-NMDA and fitted with the Hill equation modified to account for the response to NMDA observed in the absence of added glycine. The mean value of the apparent dissociation constant (KD) was 150 nM, and the mean value of the Hill coefficient (nH) was 1.1. When the KD was corrected for the concentration of contaminating glycine in nominally glycine-free solutions, estimated assuming that there is no response in the absence of glycine, the value was 130 nM. 3. The question of how many glycine binding sites there are on each NMDA receptor-channel complex was addressed by examining the curvature at the foot of the glycine C-R curve. An equation that allowed estimation of both the concentration of contaminating glycine and of the value of nH was fitted to glycine C-R data up to 50 nM. The mean value of nH was found to be 1.0, consistent with the idea that there is one glycine binding site. 4. The kinetics of the interaction of glycine with the NMDA receptor were measured by fitting single exponential curves to the current relaxation following a jump in glycine concentration in the presence of 10 microM-NMDA. The plot of the inverse of the relaxation time constant as a function of glycine concentration after the concentration jump was linear. The association rate constant was estimated from these data as 1.2 x 10(7) M-1 s-1 and the dissociation rate as 1.0 s-1. 5. Experiments were devised to allow the evaluation of the KD and dissociation rates of glycine in the absence of NMDA. They led to a value for KD of 80 nM, slightly but significantly lower than the value of 150 nM estimated in the presence of 10 microM-NMDA. The glycine dissociation rate in the absence of NMDA was found to be 0.7 s-1, not significantly different from that measured in the presence of 10 microM-NMDA. 6. The results are consistent with a model of the NMDA receptor with a single glycine binding site. The characteristics of glycine binding are similar in the absence and the presence of 10 microM-NMDA, although NMDA binding may cause a small increase in the glycine KD.(ABSTRACT TRUNCATED AT 250 WORDS)

Purified unitary kainate/alpha-amino-3-hydroxy-5-methylisooxazole-propionate (AMPA) and kainate/AMPA/N-methyl-D-aspartate receptors with interchangeable subunits

We have purified and characterized two vertebrate excitatory amino acid ionotropic receptors from the Xenopus central nervous system. Each is a unitary receptor (i.e., having more than one class of excitatory amino acid agonist specificity within one protein oligomer). The first is a unitary non-N-methyl-D-aspartate (non-NMDA) receptor and the second is a unitary NMDA/non-NMDA receptor. The specific agonist-activated channel activity and pharmacology of each type were recognized by patch-clamping lipid bilayers in which the isolated protein was reconstituted. In the second case, the NMDA and the non-NMDA sites could not be physically separated and exhibited functional interaction. Parallel evidence for this was obtained when poly(A) RNA from Xenopus brain was translated in oocytes: a noncompetitive inhibition of the response to L-kainate is produced by NMDA to a maximum depression of 30% at 1 mM NMDA. Each isolated oligomer contains 42-kDa subunits of the non-NMDA ligand binding type, but the second type has an additional NMDA-receptor-specific 100-kDa subunit. Thus, a subunit-exchange hypothesis can account for the known multiplicity of excitatory amino acid receptor types.

Protein kinase C-mediated enhancement of NMDA currents by metabotropic glutamate receptors in Xenopus oocytes

1. N-Methyl-D-aspartate (NMDA) receptors were expressed in Xenopus oocytes injected with rat brain RNA. The modulation of NMDA-induced currents was examined by activating protein kinase C (PKC) either directly (using phorbol esters) or indirectly (via metabotropic glutamate agonists). 2. Bath application of the PKC activator, 4-beta-phorbol-12,13-dibutyrate (PDBu) resulted in a two-fold increase in the NMDA-evoked current at all holding potentials examined (-80 to 0 mV). The inactive (alpha) stereoisomer of phorbol ester was ineffective. 3. The increase was observed under conditions that eliminate the oocyte's endogenous calcium-dependent chloride current, which often contributes to the NMDA response in oocytes. 4. The PDBu effect was specific to the NMDA subclass of glutamate receptors in that no increase was observed in the responses to two other glutamate agonists, kainate and AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid). 5. Stimulation of PKC by activation of metabotropic receptors via either quisqualate or trans-ACPD (trans-1-aminocyclopentane-1,3-dicarboxylic acid) also led to an increase in NMDA currents. 6. Both methods of enhancement induced transient effects. PDBu effects lasted 10-45 min, depending upon both dose and length of application. Quisqualate and trans-ACPD effects were shorter, lasting less than 10 min under these conditions of application. 7. Both methods of enhancement were blocked by the PKC inhibitor, staurosporine. In addition, the phorbol ester-induced enhancement of NMDA responses occluded further enhancement by quisqualate. 8. The results suggest a role for metabotropic glutamate receptors in modulation of NMDA-mediated processes.

Electric organ polyamines and their effects on the acetylcholine receptor

1. The electric organ of Torpedo nobiliana contained putrescine (PUT), spermidine (SPD), spermine (SPM), and cadaverine (CAD). Traces of acetylated SPD and SPM were occasionaly seen. 2. Upon fractionation of the tissue by differential centrifugation, the polyamines (PA) were found predominantly in the soluble fraction. The postsynaptic membrane fraction, containing a high concentration of acetylcholine receptor (AChR), was proportionally enriched in SPM. The molar ratio of SPM to AChR was approximately two in these membranes. 3. The effect of exogeneous PA on AChR function was studied by two methods: carbamoylcholine (CCh)-dependent 86Rb+ influx into receptor-rich membrane vesicles and [alpha-125I]bungarotoxin (Bgt) binding to the AChR. 4. SPM inhibited both ion influx and the rate of Bgt binding at concentrations above 1 mM, and therefore it appears to act as a competitive antagonist of the AChR. 5. At submicromolar concentrations, and only after preincubation with the receptor-rich membrane, SPM and PUT increased the ion influx by about 20% over control values. 6. Preincubation with 100 nM SPM did not affect the equilibrium binding of iodinated toxin or the rate of toxin binding, and therefore SPM was not uncovering new receptors. 7. By measuring the initial rate of toxin binding after different periods of preincubation with 1 microM CCh, the rate of the slow phase of receptor desensitization was determined. This rate was not changed by 100 nM SPM. 8. Although these results suggest that at low concentrations SPM is a positive modulator of the AChR, the precise mechanism of action is not determined yet.

Actions of ketamine, phencyclidine and MK-801 on NMDA receptor currents in cultured mouse hippocampal neurones

1. Stable N-methyl-D-aspartic acid (NMDA) receptor-mediated currents in cultured mouse hippocampal neurons were evoked by 20 ms pressure pulse applications of L-aspartate, repeatedly applied at 30 or 40 s intervals, to the cell body region of the neurone. We have characterized the voltage- and use-dependent blockade of the currents by three dissociative anaesthetics: ketamine, phencyclidine (PCP) and MK-801 in mouse hippocampal neurones grown in dissociated tissue culture. 2. We have used a simple model of the blockade, based on the 'guarded receptor hypothesis' to interpret our data. The model assumes that receptors are maximally activated at the peak of the response with an open probability (Po) approaching 1, that there is no desensitization and that the blocking drug only associates with, or dissociates from, receptor channels which have been activated by agonist (e.g. open channels). 3. The model allows us to estimate forward and reverse rate constants for binding of the blockers to open channels from measurements of the steady-state level of blockade and the rate of change of the current amplitude per pulse during onset and offset of blockade. As predicted by the model, the estimated reverse rate was independent of blocker concentration while the forward rate increased with concentration. Changing the level of positively charged ketamine (pKa 7.5) tenfold by changing pH from 6.5 to 8.5 caused a corresponding change in the forward rate while having no effect on the reverse rate. Most of the voltage dependence of the blockade could be accounted for by reduction of the reverse rate by depolarization. 4. Estimated forward rate constants for ketamine, PCP and MK-801 were similar to one another when measured under similar conditions and were 3 x 10(4) - 3 x 10(5) M-1 S-1. Most of the differences in potency of the three blockers could be accounted for by differences in the reverse rate constants which were approximately 0.2, 0.03 and 0.003 s-1 for ketamine, PCP and MK-801, respectively. The estimated rate constants actually are the product of the rate constants and 1/Po. Suggestions that maximum Po is much less than 1 for NMDA channels imply that both forward and reverse rate constants of blockade may in fact be larger than we have calculated. However, their magnitudes, relative to one another, are unaffected by this consideration. 5. The reverse rate constant of blockade increased at positive potentials. This increase was prevented when the neurone was loaded with N-methyl-D-glucamine, an impermeant cation which prevented outward currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Polyamines potentiate responses of N-methyl-D-aspartate receptors expressed in xenopus oocytes

Glutamate, the major excitatory neurotransmitter in the central nervous system, activates at least three types of channel-forming receptors defined by the selective agonists N-methyl-D-aspartate (NMDA), kainate, and quisqualate [or more selectively by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)]. Activation of the NMDA receptor requires glycine as well as NMDA or glutamate. Recent studies have provided evidence that certain polyamines potentiate the binding by NMDA receptors of glycine and the open channel blocker MK-801. To determine whether polyamines alter channel opening, we examined their effects on rat brain glutamate receptors expressed in Xenopus oocytes. Our results demonstrate that spermine potentiates the response of the NMDA receptor but has no effect on responses to kainate and quisqualate. Furthermore, spermine increases the maximum response to NMDA and glycine and acts, at least in part, by increasing the apparent affinity of the NMDA receptor/channel complex for glycine. The present findings and the fact that polyamines are a natural constituent of brain suggest that polyamines may play a role in the regulation of glutamatergic transmission.

A kinetic analysis of the modulation of N-methyl-D-aspartic acid receptors by glycine in mouse cultured hippocampal neurones

1. Responses to N-methyl-D-aspartic acid (NMDA) were recorded from mouse embryonic hippocampal neurones in dissociated culture, using whole-cell patch-clamp recording. A fast perfusion system, with an exchange time constant of less than 10 ms, was used to study modulation of NMDA receptor desensitization by glycine. 2. The onset of NMDA receptor desensitization was well fitted by a single-exponential function; with 30 nM-glycine the time constant was 250 ms, corresponding to a rate of 4 s-1. The rate of onset of desensitization became faster with increasing glycine concentration, with a slope of 0.87 x 10(7) M-1 s-1. Recovery from desensitization, studied with a twin-pulse technique, was also well fitted by a single-exponential function; with 30 nM-glycine the time constant of recovery was 1.95 s-1. The rate of recovery from desensitization became faster with increasing glycine concentration, with a slope of 0.76 x 10(7) M-1 s-1. These results are consistent with a model in which the effect of glycine occurs via an increase in the rate constant for recovery from desensitization, with little effect on the rate constant for onset of desensitization. Over the range 30-300 nM-glycine, the ratio of the rate constants calculated for recovery and onset of desensitization was a good predictor of the degree of desensitization recorded at equilibrium. 3. Concentration jump experiments with glycine were performed with 100 microM-NMDA present continuously, and for a single binding site model gave estimates of the association (1.1 x 10(7) M-1 s-1) and dissociation (3.1 s-1) rate constants for interaction of glycine with the NMDA receptor. In the presence of NMDA, concentration jumps from 3 microM-glycine to lower concentrations gave relaxations which became slower with decreasing glycine concentration over the range 1 microM-30 nM. A similar slowing of desensitization occurred when the glycine concentration was altered over the same range. 4. Glycine analogues of lower affinity produced desensitization with faster kinetics. D-Alanine, 150 nM, produced desensitization with a time constant of 175 ms, faster than recorded with an equipotent concentration of glycine (50 nM, time constant 259 ms). Responses of similar peak amplitude, recorded with 60 microM-L-alanine, and 500 microM-D,L-homoserine, did not produce strong desensitization, consistent with desensitization too rapid to resolve in our experiments.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of N-methyl-D-aspartic acid receptor desensitization by glycine in mouse cultured hippocampal neurones

1. Responses to N-methyl-D-aspartic acid (NMDA) were recorded from mouse embryonic hippocampal neurones in dissociated culture, using the tight-seal, whole-cell, patch-clamp technique for voltage clamp. A rapid perfusion system, with an exchange time constant of less than 10 ms, was used to apply NMDA under conditions which minimized slow, calcium-sensitive desensitization. With no added glycine, responses to 100 microM-NMDA applied for 1.5 s declined by greater than 90%, due to an additional component of desensitization of time constant 250 ms. 2. Adding glycine to the extracellular solution, over the range 30 nM to 3 microM, both potentiated responses to NMDA and to L-glutamate, and reduced fast desensitization. In the presence of 3 microM-glycine responses to NMDA declined by only 10%. Similar potentiation and reduction of desensitization was obtained with 3 microM concentrations of the glycine analogues D-alanine and D-serine. 3. Analysis of dose-response curves for the action of glycine on responses to 100 microM-NMDA revealed a 3-fold higher potency of glycine for potentiation of peak versus steady-state responses, with concentrations for half-activation of 134 and 382 nM, respectively. The competitive glycine antagonist 7-chlorokynurenic acid produced a similar shift of both the peak and steady-state dose-response curves for glycine, consistent with an equilibrium dissociation constant of 280 nM for interaction of 7-chlorokynurenic acid with the glycine binding site on NMDA receptors. 4. In the presence of 100 nM-glycine, 10 microM-7-chlorokynurenic acid produced nearly complete block of the response to 3 nM-NMDA, suggesting that glycine is absolutely required for activation of the NMDA receptor channel complex. 5. In some neurones responses to NMDA showed essentially no desensitization in the presence of 3 microM-glycine. Under these conditions, 7-chlorokynurenic acid produced a concentration-dependent block of both the initial and equilibrium response to NMDA, with a 4-fold greater sensitivity for block of the steady-state current (IC50 = 2.25 microM) than for block of the peak current (IC50 = 8.96 microM). As a result, in the presence of 7-chlorokynurenic acid, responses to NMDA showed strong desensitization, even in the presence of 3 microM-glycine. 6. Our results show that glycine-evoked potentiation of NMDA receptor activity is accompanied by reduced desensitization.(ABSTRACT TRUNCATED AT 400 WORDS)

The glycine site of the N-methyl-D-aspartate receptor channel: differences between the binding of HA-966 and of 7-chlorokynurenic acid

The mechanisms of action of three different glycine-site antagonists of the N-methyl-D-aspartate (NMDA)-receptor channel were analyzed employing [3H]glycine direct binding assays, as well as functional glycine- and glutamate-induced uncompetitive blocker binding assays. The latter assays measure apparent channel opening. All three antagonists tested, viz., 7-chlorokynurenic acid (7-Cl-KYNA), kynurenic acid (KYNA), and 1-hydroxy-3-aminopyrrolidone-2 (HA-966), inhibited the binding of [3H]glycine to the NMDA receptor in a dose-dependent manner. These antagonists also inhibited the glycine-induced increase in accessibility of the uncompetitive blocker [3H]N-[1-(2-thienyl)cyclohexyl]-piperidine ([3H]TCP) to the channel. 7-Cl-KYNA and KYNA, but not HA-966, completely blocked the glutamate-induced binding of [3H]TCP, in a manner similar to the non-competitive manner in which the selective NMDA antagonist D-(-)-2-amino-5-phosphonovaleric acid (AP-5) inhibited glycine-induced [3H]TCP binding. The inhibitory effects of HA-966 and of AP-5 on glutamate-induced [3H]TCP binding were overcome when glutamate concentrations were increased. Of the three antagonists, 7-Cl-KYNA appears to be the most potent (Ki = 0.4-1.0 microM) and the most selective glycine antagonist. KYNA was found to act at both the glycine (Ki = 40-50 microM) and the glutamate sites. In contrast, HA-966 (Ki = 6-17 microM) appears to act either on a domain distinct from the glutamate and the glycine sites, but tightly associated with the latter, or at the glycine site, but according to a mechanism distinct from that of 7-Cl-KYNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycine decreases desensitization of N-methyl-D-aspartate (NMDA) receptors expressed in Xenopus oocytes and is required for NMDA responses

In Xenopus oocytes injected with rat brain mRNA, as in neurons, glycine greatly potentiated responses of the N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. Injected oocytes generated a partially desensitizing inward current in response to NMDA with 30 nM added glycine. As the added glycine concentration was increased from 30 nM to 1 microM, the NMDA response was increased and exhibited less desensitization. The relationship between the NMDA peak response and added glycine concentration indicated a single component response with apparent affinity of 0.29 microM and a Hill coefficient of 0.77. The desensitized response was also fit by the Hill relation with a lower affinity but similar coefficient. The time course of desensitization at 500 microM NMDA was exponential with a time constant (350 msec) that was independent of glycine concentration between 0.03 and 0.3 microM. At higher glycine concentration a slower component of decay (tau = 1.4 sec) was observed. This component was enhanced by increasing the extracellular Ca2+. NMDA without added glycine evoked a small transient response. However this response was suppressed completely by prewashing with the glycine antagonist 7-chlorokynurenic acid, suggesting that it may have been due to glycine contamination. The dose-response relation for low concentrations of glycine indicated that the measured level of glycine contamination accounted for these responses. These results indicate that glycine has at least two actions at the NMDA receptor: it enables channel opening by the agonist and decreases desensitization.

The PCP site of the NMDA receptor complex

Evidence from electropharmacological experimentation favors the hypothesis that the PCP site is intimately associated with the channel domain of the NMDA receptor. But it is too early to state that this site lies deep within the NMDA channel pore. Determining the molecular details of the PCP site will require a complete and detailed kinetic analysis of NMDA single channel behavior. Furthermore, it is likely that hydrophobic receptor site(s) are responsible for some aspects of the blockade by at least some members of the dissociative anaesthetic family.

Allosteric modulation of N-methyl-D-aspartate receptors

In this review we have attempted to describe the basis for current models of the NMDA receptor, and justify the need for the various binding sites that have been proposed. The NMDA receptor is clearly a complex molecule with a number of modulatory sites, any of which may have great functional significance. From the data presented above it is apparent that the NMDA recognition site is closely coupled with the glycine site, and can also be regulated by Zn2+. The glycine site is reciprocally coupled to the NMDA site, and may also be coupled to a divalent-cation site outside the channel. However, the glycine site is insensitive to Zn2+. The Zn2+ site is probably not inside the channel to any degree, but can profoundly affect the ability of NMDA site ligands to operate the channel. However, the determination of reciprocal effects at the Zn2+ site await the development of a suitably potent and selective ligand for this site. Several lines of evidence suggest that the phencyclidine and channel-blocking Mg2+ site are located within the NMDA-operated ion channel. Glutamate, glycine, and Zn2+ alter the binding of ligands to these sites. However, this is most likely to be due to alteration of access of the ligands to their sites rather than a direct allosteric coupling. It does appear that phencyclidine site drugs and Mg2+ bind to separate sites within the channel, and that these separate sites are allosterically coupled. This complex series of interactions, many of which are mediated by endogenous agents, may allow very fine control over the expression of NMDA receptor-mediated synaptic transmission. In addition to these ligand-produced modulatory effects, there may also be covalent modification of the channel by receptor phosphorylation. Furthermore, the voltage sensitivity of some of the effects allows control of NMDA receptor-mediated signaling by alteration of the membrane potential in the postsynaptic cell, which can be achieved in a wide variety of ways. The level of sophistication possible in adjusting the responsiveness of this receptor seems entirely appropriate given its central involvement in a wide variety of fundamental neurobiological events, and underscores the deleterious pathological sequelae of the system tilting out of balance. At the same time, the wide array of possible therapeutic targets raises hopes that it may soon be possible to treat effectively some severely debilitating and currently untreatable diseases.

Nonpsychotropic cannabinoid acts as a functional N-methyl-D-aspartate receptor blocker

Binding studies using the enantiomers of the synthetic cannabinoid 7-hydroxy-delta 6-tetrahydrocannabinol 1,1-dimethylheptyl homolog in preparations of rat brain cortical membranes reveal that the (+)-(3S,4S) enantiomer HU-211 blocks N-methyl-D-aspartate (NMDA) receptors in a stereospecific manner and that the interaction occurs at binding sites distinct from those of other noncompetitive NMDA antagonists or of glutamate and glycine. Moreover, HU-211 induces stereotype and locomotor hyperactivity in mice and tachycardia in rat, effects typically caused by NMDA receptor antagonists. HU-211 is also a potent blocker of NMDA-induced tremor, seizures, and lethality in mice. This compound may therefore prove useful as a nonpsychoactive drug that protects against NMDA-receptor-mediated neurotoxicity.

Excitatory amino acids: the involvement of second messengers in the signal transduction process

1. Excitatory amino acids (EAA) can activate second messenger systems in addition to a direct gating of ion channels. A discrete coupling between novel EAA receptor subtypes and second messenger systems has been previously proposed. 2. EAAs have been suggested to activate both adenylate and guanylate cyclases and also to induce phosphoinositide (PI) turnover. The increased PI turnover was observed in both central neurons and glia, and a "quisqualate-type" receptor has been most frequently involved, which may differ from the quisqualate receptor previously defined by electrophysiological studies. 3. The roles of EAA-induced calcium influx into neurons and raised intracellular calcium levels are discussed regarding the activation of phosphoinositide turnover. 4. This review examines the data supporting a link between EAA receptors and second messengers and considers whether there is any need for adopting new EAA receptor subtypes. Also, the use of the Xenopus laevis oocyte for expressing EAA receptors and studying any putative links to second messenger systems is discussed.

Regulation of NMDA receptor desensitization in mouse hippocampal neurons by glycine

Responses to the excitatory amino acid N-methyl-D-aspartate (NMDA) are markedly potentiated by nanomolar concentrations of glycine. This is due to the action of glycine at a novel strychnine-resistant binding site with an anatomical distribution identical to that for NMDA receptors, suggesting that the NMDA receptor channel complex contains at least two classes of amino-acid recognition site. Antagonists at the glycine-binding site associated with NMDA receptors act as potent non-competitive antagonists, but do not alter the mean open time or conductance, as estimated by fluctuation analysis. The mechanisms by which glycine acts on NMDA receptors are unknown, but single-channel recording experiments show an increase in opening frequency with no change in mean open time or conductance, suggesting that glycine could regulate transitions to states that are intermediate between binding of NMDA receptor agonists and ion-channel gating. It has been suggested that glycine acts as a co-agonist at the NMDA receptor, and that responses to NMDA cannot be obtained in the complete absence of glycine, but in these experiments the response to NMDA was measured at equilibrium, and it is unlikely that sufficient temporal resolution was achieved to detect rapid alterations in receptor gating. Using a fast perfusion system we find that glycine regulates desensitization at NMDA receptors; this has a major effect on the response to NMDA measured at equilibrium, as would occur with slower applications of agonist. Reduction of NMDA receptor desensitization by glycine provides an example of a novel mechanism for regulation of ion-channel activity.

Indole-2-carboxylic acid: a competitive antagonist of potentiation by glycine at the NMDA receptor

The N-methyl-D-aspartate (NMDA) class of excitatory amino acid receptors regulates the strength and stability of excitatory synapses and appears to play a major role in excitotoxic neuronal death associated with stroke and epilepsy. The conductance increase gated by NMDA is potentiated by the amino acid glycine, which acts at an allosteric site tightly coupled to the NMDA receptor. Indole-2-carboxylic acid (I2CA) specifically and competitively inhibits the potentiation by glycine of NMDA-gated current. In solutions containing low levels of glycine, I2CA completely blocks the response to NMDA, suggesting that NMDA alone is not sufficient for channel activation. I2CA will be useful for defining the interaction of glycine with NMDA receptors and for determining the in vivo role of glycine in excitotoxicity and synapse stabilization.

N-methyl-D-aspartate activates different channels than do kainate and quisqualate

In the mammalian central nervous system, the excitatory amino acid transmitter L-glutamate activates three pharmacologically distinguishable receptors, the N-methyl-D-aspartate (NMDA), kainate, and quisqualate receptors. The present paper addresses the issue of whether these three receptors operate independent channels or whether they share channels that may have several conductance substates. The Xenopus oocyte provides a system for expression of exogenous mRNAs that permits detailed study of receptor structure and function. In oocytes injected with rat brain mRNA, NMDA has a stoichiometry of channel activation different from that for kainate and quisqualate. NMDA activates its own channels as indicated by simple summation or near-summation of currents evoked by NMDA with those evoked by quisqualate or kainate. Deviations from summation are ascribable to lack of selectivity in which an agonist at one receptor acts as a weak antagonist at another receptor. A further indication of separate channels is that block of NMDA channels by Mg2+ or phencyclidine has no effect on kainate or quisqualate responses evoked during the block. Interactions of kainate and quisqualate are more complex, but they can be explained by lack of complete specificity of these agonists for their own receptors.

mRNA from NCB-20 cells encodes the N-methyl-D-aspartate/phencyclidine receptor: a Xenopus oocyte expression study

The mouse neuroblastoma--Chinese hamster brain hybrid cell line NCB-20 is the only clonal cell line in which binding studies indicate the presence of phencyclidine (PCP) receptors. We report here that Xenopus oocytes injected with NCB-20 cell poly(A)+ RNA express N-methyl-D-aspartate (NMDA)-activated channels and that these channels include the PCP receptor site. In injected oocytes, NMDA application evoked a partially desensitizing inward current that was potentiated by glycine, blocked by the competitive antagonist D-2-amino-5-phosphonovaleric acid, blocked by Mg2+ and by Zn2+, and blocked in a use-dependent manner by the PCP receptor ligands PCP and MK-801. There was little or no response to kainate or quisqualate (agonists of the other excitatory amino acid receptors), to gamma-aminobutyric acid (an inhibitory transmitter), or to glycine (an inhibitory transmitter as well as an allosteric potentiator of NMDA channels). Thus, NMDA/PCP receptors expressed from NCB-20 cell mRNA exhibit properties similar to those of the neuronal receptors. The absence of expression of other excitatory amino acid receptors in this system makes it particularly useful for study of NMDA-evoked responses without interference from responses mediated by other receptors. Moreover, NCB-20 mRNA may be an appropriate starting material for cloning the cDNA(s) encoding the NMDA/PCP-receptor complex.

Modulation of Neuropeptide-lnduced Membrane Currents by Protein Kinase C in Xenopus Oocytes Injected with GH Pituitary Cell Poly(A) RNA

Abstract Protein kinase C was activated in Xenopus laevis oocytes by phorbol ester treatment and its effects on the inositol trisphosphate/Ca(2+) transmembrane signalling pathway analysed. Induction of the pathway was achieved by ligand stimulation of TRH receptors translated from GH(3) pituitary cell mRNA. In voltage-clamped oocytes bath application of peptide, injection of guanosine 5'-(3-O-thio) triphosphate (GTPgammaS), inositol trisphosphate or Ca(2+) all elicited inward membrane currents. Treatment of oocytes with tumour-promoting phorbol esters for 35 min almost completely abolished the ligand and GTPgammaS-induced responses. In contrast, phorbol ester treatment enhanced inositol trisphosphate-generated membrane currents. Ca(2+)-mediated responses remained unaffected by tumour promoters. The data indicate a dual role for protein kinase C in the modulation of transmembrane signalling: a feedback mechanism prevents phosphoinositide turnover whereas a feedforward reaction triggers the effect of intracellular inositol trisphosphate on the Ca(2+) release.

The mechanism of action and pharmacological specificity of the anticonvulsant NMDA antagonist MK-801: a voltage clamp study on neuronal cells in culture

1. Some possible molecular mechanisms of action of the anxiolytic, anticonvulsant and neuroprotective agent MK-801 have been examined in 'whole-cell' voltage clamp recordings performed on rat hippocampal and cortical neurones, bovine adrenomedullary chromaffin cells and N1E-115 neuroblastoma cells maintained in cell culture. 2. Transmembrane currents recorded from rat hippocampal and cortical neurones in response to locally applied N-methyl-D-aspartate (NMDA) were antagonized by MK-801 (0.1-3.0 microM). Blockade was use-dependent, and little influenced by transmembrane potential. MK-801 (3 microM) had no effect on currents evoked by kainate (100 microM). 3. The antagonism of NMDA-induced currents by MK-801 was only slowly and incompletely reversed when the cell membrane potential was clamped at -60 mV during washout. Prolonged applications of NMDA at +40, but not -60 mV during washout, markedly accelerated recovery from block. 4. In contrast to MK-801, ketamine (10 microM) blocked NMDA-induced currents in a voltage-dependent manner. Blockade increased with membrane hyperpolarization and was completely reversible upon washout. 5. MK-801 (1-10 microM) produced a voltage- and concentration-dependent block of membrane currents elicited by ionophoretically applied acetylcholine (ACh) recorded from bovine chromaffin cells. The block was readily reversible upon washout. 6. gamma-Aminobutyric acidA (GABAA) receptor-mediated chloride currents of chromaffin cells were unaffected by MK-801 (1-100 microM). In contrast, such currents were potentiated by diazepam (1 microM). MK-801 (100 microM) had no effect on currents evoked by GABA on hippocampal neurones. 7. MK-801 (10 microM) had little effect on membrane currents recorded from N1E-115 neuroblastoma cells in response to ionophoretically applied 5-hydroxytryptamine (5-HT). Such currents were antagonized by the 5-HT3 receptor antagonist GR 38032F (1 nM) and also by MK-801 at high concentration (100 microM). 8. Voltage-activated, tetrodotoxin-sensitive, sodium currents of chromaffin cells were unaffected by 10 microM MK-801. However, at a relatively high concentration (100 microM), MK-801 reduced the amplitude of such currents to approximately 77% of control. 9. The relevance of the present results to the central actions of MK-801 is discussed.

Further evidence demonstrating that N-methyl-D-aspartate and kainate activate distinct ion channels

Several excitatory amino acid receptors encoded by rat brain mRNA were expressed in Xenopus oocytes. Experimental protocols using an open channel blocker (MK-801) were designed to test the common receptor-channel hypothesis in which N-methyl-D-aspartate (NMDA) and kainate activate the same ion channel but induce different open channel conformations with different ionic permeabilities. The present data demonstrate that NMDA exposes previously trapped MK-801 molecules to the transmembrane field and accelerates their dissociation from the channel at positive potentials, while kainate lacks this effect. Therefore, kainate does not activate the same ion channel as NMDA does. Furthermore, differential inhibition of the NMDA response or the kainate response by the competitive antagonists D-2-amino-5-phosphonopentanoic acid (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) indicates that NMDA and kainate do not share the same binding site. Thus, these several lines of evidence demonstrate that two distinct receptor-channels are activated by NMDA and kainate, respectively.

Interaction of [3H]MK-801 with multiple states of the N-methyl-D-aspartate receptor complex of rat brain

N-Methyl-D-aspartate (N-Me-D-Asp) and phencyclidine receptors interactively mediate central nervous system processes including psychotomimetic effects of drugs as well as neurodegenerative, cognitive, and developmental events. To elucidate the mechanism of this interaction, effects of N-Me-D-Asp agonists and antagonists and of glycine-like agents upon binding of the radiolabeled phencyclidine receptor ligand [3H]MK-801 were determined in rat brain. Scatchard analysis revealed two discrete components of [3H]MK-801 binding after 4 hr of incubation. Incubation in the presence of L-glutamate led to an increase in apparent densities but not in affinities of both components of [3H]MK-801 binding as well as conversion of sites from apparent low to high affinity. Incubation in the presence of combined D-serine and L-glutamate led to an increase in the apparent density of high-affinity [3H]MK-801 binding compared with incubation in the presence of either L-glutamate or D-serine alone. These data support a model in which phencyclidine receptor ligands bind differentially to closed as well as open conformations of the N-Me-D-Asp receptor complex and in which glycine-like agents permit or factilitate agonist-induced conversion of N-Me-D-Asp receptors from closed to open conformations.

Requirement for glycine in activation of NMDA-receptors expressed in Xenopus oocytes

Receptors for N-methyl-D-aspartate (NMDA) are involved in many plastic and pathological processes in the brain. Glycine has been reported to potentiate NMDA responses in neurons and in Xenopus oocytes injected with rat brain messenger RNA. Glycine is now shown to be absolutely required for activation of NMDA receptors in oocytes. In voltage-clamped oocytes, neither perfusion nor rapid pressure application of NMDA onto messenger RNA-injected oocytes caused a distinct ionic current without added glycine. When glycine was added, however, NMDA evoked large inward currents. The concentration of glycine required to produce a half-maximal response was 670 nanomolar, and the glycine dose-response curve extrapolated to zero in the absence of glycine. Several analogs of glycine could substitute for glycine, among which D-serine and D-alanine were the most effective. The observation that D-amino acids are effective will be important in developing drugs targeted at the glycine site.