Schizophrenia: Evidence implicating hippocampal GluN2B protein and REST epigenetics in psychosis pathophysiology

The hippocampus is strongly implicated in the psychotic symptoms of schizophrenia. Functionally, basal hippocampal activity (perfusion) is elevated in schizophrenic psychosis, as measured with positron emission tomography (PET) and with magnetic resonance (MR) perfusion techniques, while hippocampal activation to memory tasks is reduced. Subfield-specific hippocampal molecular pathology exists in human psychosis tissue which could underlie this neuronal hyperactivity, including increased GluN2B-containing NMDA receptors in hippocampal CA3, along with increased postsynaptic density protein-95 (PSD-95) along with augmented dendritic spines on the pyramidal neuron apical dendrites. We interpret these observations to implicate a reduction in the influence of a ubiquitous gene repressor, repressor element-1 silencing transcription factor (REST) in psychosis; REST is involved in the age-related maturation of the NMDA receptor from GluN2B- to GluN2A-containing NMDA receptors through epigenetic remodeling. These CA3 changes in psychosis leave the hippocampus liable to pathological increases in neuronal activity, feedforward excitation and false memory formation, sometimes with psychotic content.

Effects of global ischemia and estradiol pretreatment on phosphorylation of Akt, CREB and STAT3 in hippocampal CA1 of young and middle-aged female rats

Transient global ischemia induces selective, delayed neuronal death of pyramidal neurons in the hippocampal CA1. Whereas long term treatment of middle-aged female rats with estradiol at physiological doses ameliorates neuronal death, the signaling pathways that mediate the neuroprotection are, as yet, unknown. Protein kinase B (Akt) and downstream transcription factors, the cAMP response element binding protein (CREB) and signal transducer and activator of transcription (STAT3) are critical players in cellular survival following injury. The present study was undertaken to determine whether long term estradiol alters the phosphorylation status and activity of Akt, STAT3 and CREB in ovariohysterectomized, middle-aged and young female rats subjected to global ischemia. Irrespective of either hormone or ischemic condition, middle-aged females exhibited lower levels of p-CREB and higher levels of Akt and STAT3 in CA1 than young females, as assessed by Western blot. In middle-aged animals, ischemia increased the phosphorylation status/activity of Akt and STAT3, and decreased the phosphorylation status/activity of CREB in the hippocampal CA1. Whereas estradiol did not detectably alter the phosphorylation status/activity of Akt or STAT3, it prevented the ischemia-induced decrease in nuclear p-CREB. Similar results were observed for the young females. Collectively, these data demonstrate that CREB, STAT3, and Akt are involved in the molecular response to global ischemia and that age influences the status of CREB, STAT3 and Akt activity in CA1 under physiological as well as pathological conditions, further emphasizing the importance of including older rodents in neuroprotection studies.

Altered mTOR signaling and enhanced CYFIP2 expression levels in subjects with fragile X syndrome

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and autism. The protein (FMRP) encoded by the fragile X mental retardation gene (FMR1), is an RNA-binding protein linked to translational control. Recently, in the Fmr1 knockout mouse model of FXS, dysregulated translation initiation signaling was observed. To investigate whether an altered signaling was also a feature of subjects with FXS compared to typical developing controls, we isolated total RNA and translational control proteins from lymphocytes of subjects from both groups (38 FXS and 14 TD). Although we did not observe any difference in the expression level of messenger RNAs (mRNAs) for translational initiation control proteins isolated from participant with FXS, we found increased phosphorylation of the mammalian target of rapamycin (mTOR) substrate, p70 ribosomal subunit 6 kinase1 (S6K1) and of the mTOR regulator, the serine/threonine protein kinase (Akt), in their protein lysates. In addition, we observed increased phosphorylation of the cap binding protein eukaryotic initiation factor 4E (eIF4E) suggesting that protein synthesis is upregulated in FXS. Similar to the findings in lymphocytes, we observed increased phosphorylation of S6K1 in brain tissue from patients with FXS (n = 4) compared to normal age-matched controls (n = 4). Finally, we detected increased expression of the cytoplasmic FMR1-interacting protein 2 (CYFIP2), a known FMRP interactor. This data verify and extend previous findings using lymphocytes for studies of neuropsychiatric disorders and provide evidence that misregulation of mTOR signaling observed in the FXS mouse model also occurs in human FXS and may provide useful biomarkers for designing targeted treatments in FXS.

Differences in NMDA receptor expression during human development determine the response of neurons to HIV-tat-mediated neurotoxicity

HIV infection of the CNS can result in neurologic dysfunction in a significant number of infected individuals. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence of HIV infection in neurons. Thus, neuronal damage and dropout are likely due to indirect effects of HIV infection of other CNS cells, through elaboration of inflammatory factors and neurotoxic viral proteins, including the viral transactivating protein tat. We and others demonstrated that tat induces apoptosis in differentiated mature human neurons. We now demonstrate that the high level of tat toxicity observed in human neurons involves specific developmental stages that correlate with N-methyl-D-aspartate receptor (NMDAR) expression, and that tat toxicity is also dependent upon the species being analyzed. Our results indicate that tat treatment of primary cultures of differentiated human neurons with significant amounts of NMDAR expression induces extensive apoptosis. In contrast, tat treatment induces only low levels of apoptosis in primary cultures of immature human neurons with low or minimal expression of NMDAR. In addition, tat treatment has minimal effect on rat hippocampal neurons in culture, despite their high expression of NMDAR. We propose that this difference may be due to low expression of the NR2A subunit. These findings are important for an understanding of the many differences among tissue culture systems and species used to study HIV-tat-mediated toxicity.

Chronic estradiol treatment increases CA1 cell survival but does not improve visual or spatial recognition memory after global ischemia in middle-aged female rats

Transient global ischemia induces selective, delayed neuronal death in the hippocampal CA1 and cognitive deficits. Physiological levels of 17beta-estradiol ameliorate ischemia-induced neuronal death and cognitive impairments in young animals. In view of concerns regarding hormone therapy in postmenopausal women, we investigated whether chronic estradiol treatment initiated 14 days prior to ischemia attenuates ischemia-induced CA1 cell loss and impairments in visual and spatial memory, in ovariohysterectomized (OVX), middle-aged (9-11 months) female rats. To determine whether the duration of hormone withdrawal affects the efficacy of estradiol treatment, hormone treatment was initiated immediately (0 week), 1 week, or 8 weeks after OVX. Age-matched, OVX and gonadally intact females were studied at each OVX interval. Ischemia was induced 1 week after animals were pretested on a variety of behavioral tasks. Global ischemia produced significant neuronal loss in the CA1 and impaired performance on visual and spatial recognition. Chronic estradiol modestly but significantly increased the number of surviving CA1 neurons in animals at all OVX durations. However, in contrast with previous results in young females, estradiol did not preserve visual or spatial memory performance in middle-aged females. All animals displayed normal locomotion, spontaneous alternation and social preference, indicating the absence of global behavioral impairments. Therefore, the neuroprotective effects of estradiol are different in middle-aged than in young rats. These findings highlight the importance of using older animals in studies assessing potential treatments for focal and global ischemia.

Global ischemia-induced increases in the gap junctional proteins connexin 32 (Cx32) and Cx36 in hippocampus and enhanced vulnerability of Cx32 knock-out mice

Gap junctions are conductive channels that connect the interiors of coupled cells. In the hippocampus, GABA-containing hippocampal interneurons are interconnected by gap junctions, which mediate electrical coupling and synchronous firing and thereby promote inhibitory transmission. The present study was undertaken to examine the hypothesis that the gap junctional proteins connexin 32 (Cx32; expressed by oligodendrocytes, interneurons, or both), Cx36 (expressed by interneurons), and Cx43 (expressed by astrocytes) play a role in defining cell-specific patterns of neuronal death in hippocampus after global ischemia in mice. Global ischemia did not significantly alter Cx32 and Cx36 mRNA expression and slightly increased Cx43 mRNA expression in the vulnerable CA1, as assessed by Northern blot analysis and in situ hybridization. Global ischemia induced a selective increase in Cx32 and Cx36 but not Cx43 protein abundance in CA1 before onset of neuronal death, as assessed by Western blot analysis. The increase in Cx32 and Cx36 expression was intense and specific to parvalbumin-positive inhibitory interneurons of CA1, as assessed by double immunofluorescence. Protein abundance was unchanged in CA3 and dentate gyrus. The finding of increase in connexin protein without increase in mRNA suggests regulation of Cx32 and Cx36 expression at the translational or post-translational level. Cx32(Y/-) null mice exhibited enhanced vulnerability to brief ischemic insults, consistent with a role for Cx32 gap junctions in neuronal survival. These findings suggest that Cx32 and Cx36 gap junctions may contribute to the survival and resistance of GABAergic interneurons, thereby defining cell-specific patterns of global ischemia-induced neuronal death.

Activation of metabotropic glutamate receptor 1 accelerates NMDA receptor trafficking

Regulation of neuronal NMDA receptors (NMDARs) by group I metabotropic glutamate receptors (mGluRs) is known to play a critical role in synaptic transmission. The molecular mechanisms underlying mGluR1-mediated potentiation of NMDARs are as yet unclear. The present study shows that in Xenopus oocytes expressing recombinant receptors, activation of mGluR1 potentiates NMDA channel activity by recruitment of new channels to the plasma membrane via regulated exocytosis. Activation of mGluR1alpha induced (1) an increase in channel number times channel open probability, with no change in mean open time, unitary conductance, or reversal potential; (2) an increase in charge transfer in the presence of NMDA and the open channel blocker MK-801, indicating an increased number of functional NMDARs in the cell membrane; and (3) increased NR1 surface expression, as indicated by cell surface Western blots and immunofluorescence. Botulinum neurotoxin A or expression of a dominant negative mutant of synaptosomal associated protein of 25 kDa molelcular mass (SNAP-25) greatly reduced mGluR1alpha-mediated potentiation, indicating that receptor trafficking occurs via a SNAP-25-mediated form of soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor-dependent exocytosis. Because group I mGluRs are localized to the perisynaptic region in juxtaposition to synaptic NMDARs at glutamatergic synapses in the hippocampus, mGluR-mediated insertion of NMDARs may play a role in synaptic transmission and plasticity, including long-term potentiation.

mGluR1-mediated potentiation of NMDA receptors involves a rise in intracellular calcium and activation of protein kinase C

Potentiation of ionotropic glutamate receptor activity by metabotropic glutamate receptors (mGluRs) is thought to modulate activity at glutamatergic synapses in the hippocampus. However, the precise pathway by which this modulation occurs is not well understood. The present study tests the hypothesis that mGluR1-mediated potentiation of N-methyl-D-aspartate receptors (NMDARs) occurs via a phospholipase C (PLC)-initiated cascade. NMDAR functional activity was examined by whole-cell recording from Xenopus oocytes expressing recombinant NMDARs and mGluR1alpha. The mGluR1 agonist (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) significantly potentiated NMDA-elicited currents. mGluR1alpha-mediated potentiation of NMDA responses was eliminated by the PLC inhibitor U-73122. Buffering of intracellular Ca2+ by BAPTA-AM or depletion of intracellular Ca2+ by the Ca2+/ATPase inhibitor thapsigargin greatly reduced ACPD potentiation. ACPD potentiation was reduced by the specific protein kinase C (PKC) inhibitor Ro-32-0432 and eliminated by the broad spectrum kinase inhibitor staurosporine. ACPD produced no further potentiation after potentiation of NMDARs by the PKC-activating phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA). Thus, Group I mGluRs potentiate NMDA responses via activation of PLC; at least part of the potentiation is due to rise in intracellular Ca2+ and stimulation of PKC. Cytochalasin D, which disrupts the actin cytoskeleton, blocked ACPD-elicited chloride currents and ACPD-induced potentiation of NMDAR currents, consistent with a role for cytoskeletal protein(s) in the signaling pathway. As Group I mGluRs are localized to the perisynaptic region in juxtaposition to NMDARs at glutamatergic synapses, mGluR-mediated potentiation of NMDAR activity may play a role in synaptic transmission and plasticity including LTP.

Protein kinase C modulates NMDA receptor trafficking and gating

Regulation of neuronal N-methyl-D-aspartate receptors (NMDARs) by protein kinases is critical in synaptic transmission. However, the molecular mechanisms underlying protein kinase C (PKC) potentiation of NMDARs are uncertain. Here we demonstrate that PKC increases NMDA channel opening rate and delivers new NMDA channels to the plasma membrane through regulated exocytosis. PKC induced a rapid delivery of functional NMDARs to the cell surface and increased surface NR1 immunofluorescence in Xenopus oocytes expressing NMDARs. PKC potentiation was inhibited by botulinum neurotoxin A and a dominant negative mutant of soluble NSF-associated protein (SNAP-25), suggesting that receptor trafficking occurs via SNARE-dependent exocytosis. In neurons, PKC induced a rapid delivery of functional NMDARs, assessed by electrophysiology, and an increase in NMDAR clusters on the surface of dendrites and dendritic spines, as indicated by immunofluorescence. Thus, PKC regulates NMDAR channel gating and trafficking in recombinant systems and in neurons, mechanisms that may be relevant to synaptic plasticity.

Insulin promotes rapid delivery of N-methyl-D- aspartate receptors to the cell surface by exocytosis

Insulin potentiates N-methyl-d-aspartate receptors (NMDARs) in neurons and Xenopus oocytes expressing recombinant NMDARs. The present study shows that insulin induced (i) an increase in channel number times open probability (nP(o)) in outside-out patches excised from Xenopus oocytes, with no change in mean open time, unitary conductance, or reversal potential, indicating an increase in n and/or P(o); (ii) an increase in charge transfer during block of NMDA-elicited currents by the open channel blocker MK-801, indicating increased number of functional NMDARs in the cell membrane with no change in P(o); and (iii) increased NR1 surface expression, as indicated by Western blot analysis of surface proteins. Botulinum neurotoxin A greatly reduced insulin potentiation, indicating that insertion of new receptors occurs via SNARE-dependent exocytosis. Thus, insulin potentiation occurs via delivery of new channels to the plasma membrane. NMDARs assembled from mutant subunits lacking all known sites of tyrosine and serine/threonine phosphorylation in their carboxyl-terminal tails exhibited robust insulin potentiation, suggesting that insulin potentiation does not require direct phosphorylation of NMDAR subunits. Because insulin and insulin receptors are localized to glutamatergic synapses in the hippocampus, insulin-regulated trafficking of NMDARs may play a role in synaptic transmission and plasticity, including long-term potentiation.

The AMPAR subunit GluR2: still front and center-stage

Abnormal influx of Ca(2+) through AMPA-type glutamate receptors (AMPARs) is thought to contribute to the neuronal death associated with a number of brain disorders. AMPARs exist as both Ca(2+)-impermeable and Ca(2+)-permeable channels. AMPARs are encoded by four genes designated GluR1 (GluR-A) through GluR4 (GluR-D). The presence of the GluR2 subunit renders heteromeric AMPA receptor assemblies Ca(2+)-impermeable. Molecular diversity of AMPARs under physiological and pathological conditions is generated by differential spatio-temporal patterns of GluR expression, by alternative RNA splicing and editing and by targeting and trafficking of receptor subunits at dendritic spines. The GluR2 gene is under transcriptional control by the RE1 element specific transcription factor, a gene silencing factor which renders it neuron-specific. GluR2 transcripts are edited by ADAR2 (double-stranded RNA-specific editase 1). AMPAR targeting and trafficking to spines are regulated by synaptic activity and are critical to synaptic plasticity. Recent studies involving animal models of transient forebrain ischemia and epilepsy show that GluR2 mRNA and GluR2 subunit expression are downregulated in vulnerable neurons prior to cell death. Ca(2+) imaging and electrical recording from individual pyramidal neurons in hippocampal slices reveal changes in AMPAR functional properties after ischemia. In slices from post-ischemia animals, CA1 neurons with robust action potentials exhibit greatly enhanced AMPA-elicited rises in intracellular Ca(2+). Excitatory postsynaptic currents in post-ischemic CA1 exhibit an enhanced Ca(2+)-dependent component that appears to be mediated by Ca(2+)-permeable AMPARs. These studies provide evidence for Ca(2+) influx through AMPARs in neurons destined to die. To examine whether acute GluR2 downregulation, even in the absence of a neurological insult, can induce neuronal death, we performed knockdown experiments in rats and gerbils with antisense oligonucleotides targeted to GluR2 mRNA. GluR2 antisense oligonucleotide induced neuronal cell death of pyramidal neurons and enhanced pathogenicity of brief ischemic episodes. These observations provide evidence for Ca(2+) influx through AMPARs in neurons destined to die and implicate Ca(2+)-permeable AMPARs in the pathogenesis of ischemia-induced neuronal death.

Remodeling of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit composition in hippocampal neurons after global ischemia

Transient global ischemia induces selective delayed cell death, primarily of principal neurons in the hippocampal CA1. However, the molecular mechanisms underlying ischemia-induced cell death are as yet unclear. The present study shows that global ischemia triggers a pronounced and cell-specific reduction in GluR2 [the subunit that limits Ca(2+) permeability of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors] in vulnerable CA1 neurons, as evidenced by immunofluorescence of brain sections and Western blot analysis of microdissected hippocampal subfields. At 72 h after ischemia (a time before cell death), virtually all CA1 pyramidal neurons exhibited greatly reduced GluR2 immunolabeling throughout their somata and dendritic processes. GluR2 immunolabeling was unchanged in pyramidal cells of the CA3 and granule cells of the dentate gyrus, regions resistant to ischemia-induced damage. Immunolabeling of the AMPA receptor subunit GluR1 was unchanged in CA1, CA3, and dentate gyrus. Western analysis indicated that GluR2 subunit abundance was markedly reduced in CA1 at 60 and 72 h after the ischemic insult; GluR1 abundance was unchanged in all subfields at all times examined. These findings, together with the previous observation of enhanced AMPA-elicited Ca(2+) influx in postischemic CA1 neurons, show that functional GluR2-lacking, Ca(2+)-permeable AMPA receptors are expressed in vulnerable neurons before cell death. Thus, the present study provides an important link in the postulated causal chain between global ischemia and delayed death of CA1 pyramidal neurons.

Status epilepticus decreases glutamate receptor 2 mRNA and protein expression in hippocampal pyramidal cells before neuronal death

Kainic acid (KA)-induced status epilepticus in adult rats leads to delayed, selective death of pyramidal neurons in the hippocampal CA1 and CA3. Death is preceded by down-regulation of glutamate receptor 2 (GluR2) mRNA and protein [the subunit that limits Ca(2+) permeability of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors] in CA1 and CA3, as indicated by in situ hybridization, immunolabeling, and quantitative Western blotting. GluR1 mRNA and protein are unchanged or slightly increased before cell death. These changes could lead to formation of GluR2-lacking, Ca(2+)-permeable AMPA receptors and increased toxicity of endogenous glutamate. GluR2 immunolabeling is unchanged in granule cells of the dentate gyrus, which are resistant to seizure-induced death. Thus, formation of Ca(2+)-permeable AMPA receptors may be a critical mediator of delayed neurodegeneration after status epilepticus.

Patterns of developmental expression of the RNA editing enzyme rADAR2

To date, two structurally related RNA-editing enzymes with adenosine deaminase activity have been identified in mammalian tissue: ADAR1 and ADAR2 [Bass B. I. et al. (1997) RNA 3, 947-949]. In rodents, ADAR2 undergoes alternative RNA splicing, giving rise to two splice variants that differ by the presence or absence of a 10-amino-acid insert in the carboxy-terminal catalytic domain. However, the physiological significance of the splicing and its regional and developmental regulation are as yet unknown. The present study examined spatial and temporal patterns of ADAR2 gene transcripts within specific neuronal populations of rat brain. The two rodent ADAR2 isoforms were expressed at comparable levels at all ages examined. rADAR2 messenger RNA expression was first detectable in the thalamic nuclei formation at embryonic day E19. The rADAR2b insert and rADAR2a splice probes produced images similar to that of the rADAR2 pan probe. At birth, rADAR2a messenger RNA splice variants were abundantly expressed in the thalamic nuclei. No signal for any probe was detectable in other brain regions, including neocortex, hippocampus, striatum and cerebellum at this stage of development. During the first week of postnatal life, rADAR2 messenger RNA expression (detected with the pan probe) increased gradually in several brain regions, with low expression detected at postnatal day P7 in the olfactory bulb, inferior colliculus, and within the pyramidal and granule cell layers of the hippocampus. Hybridization patterns of the rADAR2a variant probe reached peak expression at about the second week of life, while peak expression of the rADAR2b probe was reached at about the third week of life. At the end of the first week of life (P7), expression of both splice variants was strongest in the thalamic nuclei. By P14, rADAR2 messenger RNA expression was more consolidated in the deeper structures, including the thalamic nuclei and the granule cell layer of the cerebellum. By P21, maximal levels of rADARb expression were observed in the thalamic nuclei, inferior colliculus, cerebellum and pontine nuclei. In the adult, rADAR2 messenger RNA expression was of highest intensity in the thalamic nuclei, with high levels of expression in the olfactory bulb, inferior colliculus, cerebellum and pontine nuclei. At the level of the hippocampus, positive labelling was restricted to the CA3 region of the Ammon's horn and the dentate gyrus, with weak signals in the CA1 subfield. rADAR2 pan expression was at near background levels throughout the neocortex and caudate putamen. In summary, our study shows that ADAR2 messenger RNA expression is regulated in a cell-specific manner throughout development. At early ages, ADAR2 messenger RNA is expressed only within (and restricted to) the thalamic nuclei. By the third postnatal week, expression of the editase enzyme is more widely distributed throughout the olfactory bulb, CA3 and dentate gyrus of the hippocampus, thalamus, inferior colliculus and the molecular cell layer of the cerebellum. ADAR2 is thought to act at specific nucleotide positions in primary transcripts encoding glutamate receptor subunits, thereby altering gating and ionic permeability properties of AMPA- and kainate-activated channels. ADAR2 also acts at pre-messenger RNA encoding the serotonin 5HT-2C receptor to alter G-protein coupling. Thus, RNA editing may be an important mechanism for fine-tuning of the physiological and pharmacological properties of transmitter receptors of the central nervous system.

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.

Knockdown of AMPA receptor GluR2 expression causes delayed neurodegeneration and increases damage by sublethal ischemia in hippocampal CA1 and CA3 neurons

Considerable evidence suggests that Ca(2+)-permeable AMPA receptors are critical mediators of the delayed, selective neuronal death associated with transient global ischemia and sustained seizures. Global ischemia suppresses mRNA and protein expression of the glutamate receptor subunit GluR2 and increases AMPA receptor-mediated Ca(2+) influx into vulnerable neurons of the hippocampal CA1 before the onset of neurodegeneration. Status epilepticus suppresses GluR2 mRNA and protein in CA3 before neurodegeneration in this region. To examine whether acute downregulation of the GluR2 subunit, even in the absence of a neurological insult, can cause neuronal cell death, we performed GluR2 "knockdown" experiments. Intracerebral injection of antisense oligodeoxynucleotides targeted to GluR2 mRNA induced delayed death of pyramidal neurons in CA1 and CA3. Antisense-induced neurodegeneration was preceded by a reduction in GluR2 mRNA, as indicated by in situ hybridization, and in GluR2 protein, as indicated by Western blot analysis. GluR2 antisense suppressed GluR2 mRNA in the dentate gyrus but did not cause cell death. The AMPA receptor antagonist 6-cyano-7-nitroquinoxiline-2,3-dione (CNQX) and the Ca(2+)-permeable AMPA receptor channel blocker 1-naphthyl acetyl spermine protected against antisense-induced cell death. This result indicates that antisense-induced cell death is mediated by Ca(2+)-permeable AMPA receptors. GluR2 antisense and brief sublethal global ischemia acted synergistically to cause degeneration of pyramidal neurons, consistent with action by a common mechanism. These findings demonstrate that downregulation of GluR2 is sufficient to induce delayed death of specific neuronal populations.

AMPA receptor protein expression and function in astrocytes cultured from hippocampus

Glutamate receptors guide the proliferation, migration, and differentiation of glial cells. Here, we characterize AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid) and NMDA receptor protein expression and function and mRNA expression in hippocampal glial cultures. By immunocytochemistry, GluR2 (the subunit that limits the Ca(2+) permeability of AMPA receptors) exhibited prominent labeling in hippocampal glial cultures. Double-labeling of GluR2 with GFAP and with A2B5 revealed GluR2 subunit expression on type-1 and type-2 astrocyte lineage cells. GluR1 subunit expression was more prominent in type-1 than in type-2 astrocytes. To characterize functional properties of glutamate receptors expressed in cultured hippocampal astrocytes, we performed whole-cell patch clamp recording. Application of L-glutamate, AMPA, and kainate, but not NMDA, to small, rounded cells (morphologically identified as type-2 astrocytes) elicited inward currents which were blocked by the AMPA/kainate antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX). Cyclothiazide potentiated AMPA- and kainate-elicited currents, indicative of AMPA-preferring receptors. Current voltage analysis indicated that type-2 astrocyte AMPA receptors were electrically linear, indicative of GluR2-containing, Ca(2+)-impermeable AMPA receptors. By Northern blot analysis, GluR1 mRNA was highest in astrocyte cultures from cerebellum and hippocampus and moderate in astrocyte cultures from neocortex and striatum. GluR3 mRNA was detectable in astrocyte cultures from cerebellum and neocortex. GluR2 and NR1 mRNA expression were not detected in astrocytes cultured from any brain region examined. In situ hybridization studies showed wide expression of GluR1 mRNA in cultured astrocytes; GluR2 and GluR3 mRNAs were near background levels. Thus, cultured type-2 astrocytes express functional AMPA receptors in a cell-specific and region-specific manner, consistent with their role in neuronal-glial communication.

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.

Spermine and arcaine block and permeate N-methyl-D-aspartate receptor channels

Polyamines such as spermine are thought to be endogenous regulators of NMDA (N-methyl-D-aspartate)-type glutamate receptors. Polyamine block of NMDA receptors was studied in excised outside-out patches from rat hippocampal neurons and Xenopus oocytes expressing recombinant receptors. Extracellular spermine and arcaine reduced NMDA single-channel conductance in a voltage-dependent manner, with partial relief of block evident at large inside negative membrane potentials. Reducing extracellular Na+ concentration increased the apparent affinities for spermine and arcaine, indicating strong interaction between spermine and permeant ions. Internal spermine also blocked NMDA channels in a voltage-dependent manner, with relief of block evident at large inside positive potentials. The Woodhull model of channel block by an impermeant ion adequately described the actions of external spermine from -60 to +60 mV, but failed for more negative potentials. Eyring rate theory for a permeable blocker with two barriers and one binding site adequately described the voltage-dependent block and relief from block by both external and internal spermine over the range of -120 to +60 mV. These findings indicate that polyamines block and permeate neuronal NMDA receptor channels from the extracellular and intracellular sides, although sensitivity to internal spermine is probably too low to be physiologically relevant.

The expression of GABA(A) receptor subunits in the substantia nigra is developmentally regulated and region-specific

The substantia nigra pars reticulata (SNR) controls the spread of seizures. GABA(A)ergic drug (agonist or antagonist) infusions into the SNR have age-specific and site-specific effects on flurothyl-induced seizures. Developmental and cell-specific regulation of GABA(A) receptor subunit expression may be responsible for these specific effects. To test this hypothesis, in situ hybridization was used to examine regional expression of alpha1 and gamma2L GABA(A) receptor subunit mRNAs in the SNR during development. Distinct temporal and spatial patterns of expression were observed. In rats at postnatal days (PN) 21-60, fewer neurons were labeled with probes directed to alpha1 and gamma2L subunits in SNRanterior compared with SNRposterior. In addition, neurons in SNRanterior contained higher amounts of hybridization grains than in SNRposterior. In PN 15 rats, the labeling of neurons was relatively diffuse throughout the anterior and posterior SNR regions with moderate amounts of hybridization grains for both subunits. The finding of age-related differential distribution of alpha1 and gamma2L subunit mRNAs in the SNR suggests that GABA(A) receptor heterogeneity may play a role in the age-specific and site-specific effects of GABA(A)ergic agents on seizures in the SNR.

Aurintricarboxylic acid prevents GLUR2 mRNA down-regulation and delayed neurodegeneration in hippocampal CA1 neurons of gerbil after global ischemia

Aurintricarboxylic acid (ATA), an inhibitor of endonuclease activity and other protein-nucleic acid interactions, blocks apoptosis in several cell types and prevents delayed death of hippocampal pyramidal CA1 neurons induced by transient global ischemia. Global ischemia in rats and gerbils induces down-regulation of GluR2 mRNA and increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced Ca2+ influx in CA1 before neurodegeneration. This result and neuroprotection by antagonists of AMPA receptors suggests that formation of AMPA receptors lacking GluR2, and therefore Ca2+ permeable, leads to excessive Ca2+ influx in response to endogenous glutamate; the resulting delayed neuronal death in CA1 exhibits many characteristics of apoptosis. In this study, we examined the effects of ATA on expression of mRNAs encoding glutamate receptor subunits in gerbil hippocampus after global ischemia. Administration of ATA by injection into the right cerebral ventricle 1 h before (but not 6 h after) bilateral carotid occlusion prevented the ischemia-induced decrease in GluR2 mRNA expression and the delayed neurodegeneration. These findings suggest that ATA is neuroprotective in ischemia by blocking the transcriptional changes leading to down-regulation of GluR2, rather than by simply blocking endonucleases, which presumably act later after Ca2+ influx initiates apoptosis. Maintaining formation of Ca2+ impermeable, GluR2 containing AMPA receptors could prevent delayed death of CA1 neurons after transient global ischemia, and block of GluR2 down-regulation may provide a further strategy for neuroprotection.

Functional significance of cysteine residues in the delta opioid receptor studied by site-directed mutagenesis

Previous work suggested that sulfhydryl groups and disulfide bridges have important functions in opioid binding to the delta opioid receptor. The question regarding which cysteines are essential for ligand binding was approached by replacement of cysteine residues in the cloned delta opioid receptor using site-directed mutagenesis. The wild-type and mutant receptors were expressed stably in Chinese hamster ovary cells. The two extracellular cysteine residues and the six located in transmembrane domains were mutated to serine or alanine, one at a time. Replacement of either of the extracellular cysteines produced a receptor devoid of delta agonist and antagonist binding activity. Immunofluorescence cytochemistry, performed with anti delta opioid receptor antibodies in washed cell monolayers in one of these mutants (Cys-Ser121), and immunoblots, performed on cell extracts, indicate that the receptor was expressed and seems to be associated with the cell membrane. The existence of an essential extracellular disulfide bridge, previously postulated by analogy to other G protein coupled receptors, is strongly supported by our results. Replacement of any one of the six transmembrane cysteines was virtually without effect on the ability of the receptor to bind delta agonists and antagonists. Since there is strong evidence that the transmembrane domains are involved in ligand binding, these results suggest that the cysteine residues, even those near or at the binding site, are not essential for receptor binding. Furthermore, these results support the idea that the striking effects of sulfhydryl reagents on ligand binding of opioid receptors are likely to be due to steric hindrance by the large moieties transferred to the sulfhydryl groups of cysteine residues by these reagents.

Non-plaque dystrophic dendrites in Alzheimer hippocampus: a new pathological structure revealed by glutamate receptor immunocytochemistry

Alzheimer's disease is a progressive dementia characterized by a pronounced neurodegeneration in the entorhinal cortex, hippocampal CA1, and subiculum. Excitatory amino acid receptor-mediated excitotoxicity is postulated to play a role in the neurodegeneration in Alzheimer's disease. The present study investigated immunocytochemical localization of excitatory amino acid receptor subunits in the hippocampus of twelve Alzheimer's disease and eleven controls, matched for age, sex and post mortem interval. Immunocytochemistry with antibodies specific for glutamate receptors GluR1, GluR2(4) (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid), GluR5/6/7 (kainate) and NR1 (N-methyl-D-aspartate) receptor subunits demonstrated that virtually all projection neurons in all subfields contained subunits from each receptor class. However, regional differences in immunoreactivity were apparent in Alzheimer's disease vs normal human brain. In the vulnerable regions (i.e. CA1) immunolabelling of GluR1, GluR2(4), GluR5/6/7 and NR1 was reduced, presumably due to cell loss. In contrast, GluR2(4) immunolabelling appeared to be increased in the inner portion of the molecular layer of the dentate gyrus. In addition to cellular labelling, GluR1, GluR2(4) and NR1 immunolabelling revealed a novel pathological structure in 12 of 12 Alzheimer's disease, but none of the control brains. The lesions were juxtacellular clusters of granular immunoreactivity in the neuropil of the pyramidal cell layer. Ultrastructural analysis revealed these to be cellular processes containing dense vesicles and flocculent material with immunolabelling localized to plasma and vesicular membranes. They were not specifically associated with amyloid fibrils and did not contain paired helical filaments and they were also distinct from granulovacuolar degeneration. Several structures contained Hirano body filaments indicating that the dystrophic processes were most likely dendritic in origin. Additional studies are needed to determine the pathogenesis of these lesions, which could provide an additional index of dendritic deterioration in Alzheimer's disease.

Developmental regulation of glutamate and GABA(A) receptor gene expression in rat hippocampus following kainate-induced status epilepticus

In adult rats, kainic acid-induced status epilepticus markedly reduces GluR2 (the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid, AMPA subunit that limits Ca2+ permeability), receptor mRNA in the vulnerable CA3 and may contribute to delayed neurodegeneration. In rat pups resistant to kainate seizure-induced hippocampal neurodegeneration by silver impregnation, glutamate or GABA(A) alpha1-receptor mRNAs were unaltered in CA3 neurons 24 h after status epilepticus. In the dentate gyrus, GluR1 and GluR2 mRNAs were transiently increased in P14 but not P5 pups. Immunocytochemistry revealed no apparent differences in the distribution patterns of GluR1, GluR2, or GluR2/3 receptor proteins in the CA3 or dentate gyrus of P14 pups. Status epilepticus-induced alterations in receptor GluR2 and GABA(A) alphal mRNAs and AMPA protein expression vary with developmental age. Sustained expression at young ages may contribute to the resistance of developing hippocampal neurons to seizure-induced damage.

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.

Status epilepticus-induced alterations in metabotropic glutamate receptor expression in young and adult rats

In adult rats, kainic acid induces status epilepticus and delayed, selective cell loss of pyramidal neurons in the hippocampal CA3. In pup rats, kainate induces status epilepticus but not the accompanying neuronal cell death. The precise mechanisms underlying this age-dependent vulnerability to seizure-induced cell death are not understood. Metabotropic glutamate receptors (mGluRs) are developmentally and spatially regulated throughout the hippocampus and are implicated in seizure-induced damage. In the present study we used in situ hybridization to examine possible changes in mGluR expression at the level of the hippocampus after status epilepticus in postnatal day 10 (P10) pup and adult (P40) rats. Status epilepticus did not alter expression of mGluR1, mGluR3, or mGluR5 mRNAs. In pup and adult rats, status epilepticus induced a reduction in expression of mGluR2 mRNA in granule cells of the dentate gyrus. This change could lead to augmented glutamate release at mossy fiber synapses on CA3 pyramidal cells and thereby promote hyperexcitation. In pup but not adult rats, mGluR4 mRNA expression was enhanced in CA3 pyramidal neurons. Upregulation of presynaptic mGluR4 in pup CA3 neurons could lead to reduced transmitter release from CA3 axons, including recurrent collaterals, thereby reducing vulnerability of neonatal CA3 neurons to seizure-induced damage. These findings indicate that status epilepticus affects mGluR expression in a gene- and cell-specific manner, and that these changes vary with the developmental stage.

The GluR2 (GluR-B) hypothesis: Ca(2+)-permeable AMPA receptors in neurological disorders

The abnormal influx of Ca2+ through glutamate receptor channels is thought to contribute to the loss of neurons associated with a number of brain disorders. Until recently, the NMDA receptor was the only glutamate receptor known to be Ca(2+)-permeable. It is now well established that AMPA receptors exist not only in Ca(2+)-impermeable but also in Ca(2+)-permeable forms. AMPA receptors are encoded by four genes designated gluR1 (gluR-A) through gluR4 (gluR-D). The presence of the gluR2 subunit renders heteromeric AMPA receptor assemblies Ca(2+)-impermeable. Recent studies involving animal models of transient forebrain ischemia and epilepsy show that gluR2 mRNA is downregulated in vulnerable neurons. These observations suggest that downregulation of gluR2 gene expression may serve as a 'molecular switch' leading to the formation of Ca(2+)-permeable AMPA receptors and enhanced toxicity of endogenous glutamate following a neurological insult.

Global ischemia induces downregulation of Glur2 mRNA and increases AMPA receptor-mediated Ca2+ influx in hippocampal CA1 neurons of gerbil

Transient, severe forebrain or global ischemia leads to delayed cell death of pyramidal neurons in the hippocampal CA1. The precise molecular mechanisms underlying neuronal cell death after global ischemia are as yet unknown. Glutamate receptor-mediated Ca2+ influx is thought to play a critical role in this cell death. In situ hybridization revealed that the expression of mRNA encoding GluR2 (the subunit that limits Ca2+ permeability of AMPA-type glutamate receptors) was markedly and specifically reduced in gerbil CA1 pyramidal neurons after global ischemia but before the onset of neurodegeneration. To determine whether the change in GluR2 expression is functionally significant, we examined the AMPA receptor-mediated rise in cytoplasmic free Ca2+ level ([Ca2+]i) in individual CA1 pyramidal neurons by optical imaging with the Ca2+ indicator dye fura-2 and by intracellular recording. Seventy-two hours after ischemia, CA1 neurons that retained the ability to fire action potentials exhibited a greatly enhanced AMPA-elicited rise in [Ca2+]i. Basal [Ca2+]i in these neurons was unchanged. These findings provide evidence for Ca2+ entry directly through AMPA receptors in pyramidal neurons destined to die. Downregulation of GluR2 gene expression and an increase in Ca2+ influx through AMPA receptors in response to endogenous glutamate are likely to contribute to the delayed neuronal death after global ischemia.

Developmental regulation and cell-specific expression of N-methyl-D-aspartate receptor splice variants in rat hippocampus

The present study demonstrates cell-specific and developmental regulation of 5' and 3' splicing of the N-methyl-D-aspartate receptor NR1 subunit within specific neuronal populations of the hippocampus. At birth, NR1 transcripts lacking exon 5 (encoding the amino-terminal splice cassette N1) exhibit mature patterns of labelling within the hippocampus, with high levels of expression in the CA1, CA3, and dentate gyrus. In contrast, exon 5-containing (NR1(1XX)) transcripts are expressed at low levels until P8, at which time expression is prominent and essentially uniform in the CA1, CA3, and dentate gyrus. Exon 5 expression increases at a faster rate in CA3 than in CA1 or dentate gyrus. By the third week postnatal (postnatal day P21), exon 5-containing transcripts exhibit a distinct gradient of labelling, with more intense expression in CA3, than in CA1 or dentate gyrus. By P21 pyramidal neurons of the CA1 and granule cells of the dentate gyrus express mainly NR1(0XX) receptor messenger RNAs (lacking exon 5). Because splicing in of the N1 splice cassette confers greater current amplitude and enhanced potentiation by protein kinase C, these observations predict elevated levels of synaptic activity in the CA1 early in postnatal life, a time at which synaptic plasticity is enhanced. The carboxy-terminal splice cassettes C1 and C2 are regulated independently within the hippocampus. Whereas NR1(X11) (C1-, C2-containing) and NR1(X01) (C2 only) receptors exhibit high levels of expression in CA1, CA3, and dentate gyrus, NR1(X00) receptors are expressed more intensely in pyramidal neurons of CA3. NR1(X10) receptor expression is very low in all cells and at all times examined, even in adults. Because splicing in of the C1 cassette is thought to regulate receptor targeting, clustering, and cytoskeletal interactions, N-methyl-D-aspartate receptors in the two hippocampal subfields may play differing roles in synaptogenesis and the formation of new neuronal contacts. Moreover, cell-specific patterns of NR1(X11) receptor messenger RNAs parallel those of NR1(0XX) receptor messenger RNAs; and cell-specific patterns of NR1(1XX) (N1-containing) receptor messenger RNAs parallels those of NR1(X00) (C1-, C2-lacking) receptor messenger RNAs throughout development. These observations suggest that NR1(100) receptors, which exhibits the greatest potentiation by protein kinase C, are likely to be important in CA1 during the second and third weeks postnatal. Cell-specific expression of NR1 splice variants undoubtedly contributes to functional diversity of N-methyl-D-aspartate receptor properties in neuronal populations within the hippocampus. Developmental regulation of NR1 splicing is likely to influence synaptic plasticity and the formation of new synaptic contacts. Moreover, findings from this study suggest that a change in NR1 splicing following a neurological injury could significantly alter glutamate pathogenicity in a particular population of cells.

The GluR2 hypothesis: Ca(++)-permeable AMPA receptors in delayed neurodegeneration

Increased glutamate-receptor-mediated Ca++ influx is considered an important factor underlying delayed neurodegeneration following ischemia or seizures. Until recently, the NMDA receptor was the only glutamate receptor known to be Ca(++)-permeable. It is now well established that glutamate receptors of the AMPA type, encoded by a gene family designated GluR1-GluR4, exist in both Ca(++)-permeable and Ca(++)-impermeable forms, depending on their subunit composition and degree of RNA editing. Recombinant channels assembled without GluR2 are permeable to Ca++; channels assembled with (edited) GluR2 are Ca(++)-impermeable. AMPA receptors in most adult neurons are hetero-oligomers containing GluR2 subunits, but some neurons have GluR2-less, Ca(++)-permeable receptors. The "GluR2 hypothesis" predicts that a relative reduction in the expression of GluR2 results in enhanced Ca++ influx through newly synthesized AMPA receptors, thereby increasing neurotoxicity of endogenous glutamate. Recent observations indicate reduction in GluR2 expression and predict formation of Ca(++)-permeable AMPA receptors following global ischemia and kainate-induced status epilepticus; these changes are likely to be a major factor contributing to the delayed neurodegeneration that follows these pathological events. The delayed neurodegeneration appears to be primarily apoptotic. Thus, there are at least three strategies for neuroprotection: block of formation of GluR2-less receptors, which may be possible at several levels; block of the GluR2-less receptors themselves; and block of the subsequent apoptosis.

Alternatively spliced isoforms of the NMDARI receptor subunit

Molecularly diverse forms of the NMDA-receptor subunit NRI are formed by alternative RNA splicing. Differential splicing of three exons generates as many as eight NRI splice variants, seven of which have been identified in cDNA libraries. The alternatively spliced exons encode a 21 amino acid sequence in the N-terminus domain (termed NI), and adjacent sequences of 37 and 38 amino acids in the C-terminus domain (termed C1 and C2, respectively). Splicing out the exon segment that encodes the C2 cassette removes the first stop codon, resulting in a new open reading frame that encodes an unrelated sequence of 22 amino acids (C2') before a second stop codon is reached. Differential RNA splicing alters the structural, physiological and pharmacological properties of receptors that comprise NRI subunits. Diversity of NMDA receptors is also caused by differential association with members of the NR2 gene family. The finding of cell-specific expression and developmental regulation of NRI splice variants, and of the NR2 subunits, provides an explanation for the diversity of properties of NMDA receptors in different neuronal populations.

Maturation and segregation of brain networks that modify seizures

The mature brain is less susceptible to seizures than the immature brain. We demonstrate that in the mature substantia nigra (SN) there are two topographically discrete GABAA-sensitive regions which differ in the amount of mRNA expression of the GABAA receptor alpha 1 subunit. These two regions mediate separate anticonvulsant and proconvulsant effects and use divergent projection networks. By contrast, in the immature SN there is no special topography of mRNA expression of the alpha 1 subunit and only the proconvulsant network is present. The decreased seizure susceptibility of the mature brain may be related to postnatal segregation of GABAA-sensitive networks.

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.

AMPA/kainate receptor gene expression in normal and Alzheimer's disease hippocampus

Alzheimer's disease is a progressive dementia characterized by pronounced degeneration of certain populations of neurons in the hippocampus and cerebral cortex of the brain. One theory is that glutamate receptor-mediated toxicity plays a role in cell loss associated with Alzheimer's disease. We used in situ hybridization to examine GluR1, GluR2, and GluR3 messengerRNAs (encoding alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/kainate receptor subunits) in sections of autopsy samples of Alzheimer's disease brains and age-, sex-, and post-mortem delay-matched brains from non-demented (control) subjects. GluR1 and GluR2 exhibited a heterogeneous distribution in control brain. GluR1 was expressed in granule cells of the dentate gyrus, in pyramidal cells of the CA1 and CA3 hippocampal subfields and in neurons of the subiculum and entorhinal cortex. GluR2 mRNA was at high density in the dentate gyrus and in CA3, but was at low density in CA1, subiculum, and entorhinal cortex. GluR3 hybridization was at very low levels but selectively localized to the dentate gyrus and CA3. In cerebellum, GluR1 was found in granule and Purkinje cell layers. In sections from Alzheimer's disease brain, a high degree of intersubject variability was observed: some samples showed markedly reduced GluR1 mRNA levels in dentate gyrus, CA1 and CA3 relative to controls; others showed no changes. Microscopic observation of emulsion-dipped sections revealed that the reduction of GluR1 seen in the dentate gyrus and CA3 of some Alzheimer's disease subjects was not due to cell loss.(ABSTRACT TRUNCATED AT 250 WORDS)

Kainate-induced status epilepticus alters glutamate and GABAA receptor gene expression in adult rat hippocampus: an in situ hybridization study

In adult rats, intraperitoneal administration of kainic acid, a glutamic acid analog and potent neurotoxin, induces persistent seizure activity that results in electrographic alterations and neuropathology that closely resemble human temporal lobe epilepsy. We used in situ hybridization to identify regions of altered glutamate and GABAA receptor gene expression following kainate-induced status epilepticus. In the CA3/CA4 area, the hippocampal region most vulnerable to neurodegeneration after kainate acid treatment, expression of GluR2 (the AMPA/kainate receptor subunit that limits Ca2+ permeability) and GluR3 was decreased markedly at 12 and 24 hr, times preceding neurodegeneration. These findings raise the possibility that increased formation of Ca(2+)-permeable AMPA/kainate receptors in the CA3/CA4 area may enhance glutamate pathogenicity. Expression of the GABAA alpha 1, subunit was also reduced, indicating a possible decrease in inhibitory transmission, which would also enhance excitotoxicity. GluR1 and NR1 expression was not significantly changed. In the dentate gyrus, a region resistant to neurodegeneration, concomitant increases in GluR2 and GluR3 expression were observed; GluR1, NR1, and GABAA alpha 1 mRNAs were not detectably altered. Analysis of emulsion-dipped sections revealed that the changes in GluR2, GluR3, and GABAA alpha 1 expression represented changes in mRNA content per neuron and were specific to pyramidal cells of the CA3/CA4 area and to granule cells of the dentate gyrus. These findings indicate that kainate seizures modify hippocampal glutamate and GABAA receptor expression in a cell-specific manner. Timing of the changes in glutamate and GABAA receptor mRNAs indicates that these changes may play a causal role in hippocampal neuronal cell loss following kainate-induced seizures.

Mutagenesis rescues spermine and Zn2+ potentiation of recombinant NMDA receptors

Alternative splicing generates distinct forms of the NMDA receptor subunit NR1. NR1 subunits with an N-terminal insert (termed N1) form receptors in Xenopus oocytes with greatly reduced potentiation by spermine and Zn2+. Oocytes expressing NR1 receptors with N1 exhibited larger NMDA currents than oocytes expressing corresponding receptors without N1. In the present study, we used mutational analysis to investigate structural features of the N1 insert that control current amplitude and spermine and Zn2+ potentiation. Neutralization of positive charges in N1 rescued spermine and Zn2+ potentiation. Positive charges in N1 did not affect spermine or Zn2+ affinity. Neutralization of positive charges in N1 diminished the responses to the level of NR1 receptors lacking N1. The positively charged N1 may increase NMDA currents by causing a conformational change similar to that produced by spermine and Zn2+ in NR1 receptors lacking N1.

NMDA and non-NMDA receptor gene expression following global brain ischemia in rats: effect of NMDA and non-NMDA receptor antagonists

Transient forebrain or global ischemia in rats induces selective and delayed damage of hippocampal CA1 neurons. In a previous study, we have shown that expression of GluR2, the kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit that governs Ca2+ permeability, is preferentially reduced in CA1 at a time point preceding neuronal degeneration. Postischemic administration of the selective AMPA receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX), protects CA1 neurons against delayed death. In this study we examined the effects of NBQX (at a neuroprotective dose) and of MK-801 (a selective NMDA receptor antagonist, not protective in this model) on kainate/AMPA receptor gene expression changes after global ischemia. We also examined the effects of transient forebrain ischemia on expression of the NMDA receptor subunit NMDAR1. In ischemic rats treated with saline, GluR2 and GluR3 mRNAs were markedly reduced in CA1 but were unchanged in CA3 or dentate gyrus. GluR1 and NMDAR1 mRNAs were not significantly changed in any region examined. Administration of NBQX or MK-801 did not alter the ischemia-induced changes in kainate/AMPA receptor gene expression. These findings suggest that NBQX affords neuroprotection by a direct blockade of kainate/AMPA receptors, rather than by a modification of GluR2 expression changes.

Glutamate receptor gene expression in spinal cord of arthritic rats

Injury to peripheral tissue leads to hyperalgesia that appears to be partly mediated by functional changes at the level of the spinal cord. Glutamate receptors are thought to play a role in acute and short-term (minutes to hours) spinal cord nociceptive responses and may be involved in prolonged or chronic pain (hours to days). We used in situ hybridization to examine AMPA/kainate (GluR1, GluR2, and GluR3) and NMDA (NR1) receptor gene expression in spinal cord following induction of prolonged inflammation by a unilateral intraarticular injection of lipopolysaccharide (LPS; 10 micrograms) into the hindpaw. In control rats, GluR1 expression was prominent throughout the layers of the gray matter of the spinal cord. Microscopic examination revealed labeling of neuronal cell somata in all major nuclei. GluR2 was abundant in substantia gelatinosa and motor nuclei; emulsion-dipped sections exhibited intense labeling over densely packed neurons in the superficial laminae of dorsal horn and individual motoneurons of ventral horn. GluR3 and NR1 were expressed at low levels throughout spinal cord gray matter. One day after LPS injection, when joint swelling was maximal, GluR1 expression was bilaterally decreased by 25% in the substantia gelatinosa at the level of the lumbar cord. In contrast, no significant change was apparent in GluR2, GluR3, or NR1 expression in any nucleus of the cord. At 72 hr after injection, when joint diameter approached control values, all four transcripts were expressed at near control levels. These findings provide evidence for a specific decrease in GluR1 expression in the cord in response to joint inflammation.

Developmental regulation of mRNAs encoding rat brain kainate/AMPA receptors: a northern analysis study

Functionally diverse kainate/alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) receptors are generated by assembly of glutamate receptor (GluR)1, 2, and 3 subunits into homomeric and heteromeric channels. We examined GluR1, 2, and 3 gene expression in embryonic, neonatal, and adult rat brain by northern analysis under conditions of high stringency. In the adult, hybridization to a GluR1 riboprobe revealed the presence of an abundant RNA species, 5.2 kb in size, and minor bands of 3.2 and 3.9 kb. GluR2 hybridized to two species, 3.9 and 5.9 kb, of comparable abundance, presumably attributable to alternate splice products. Hybridization to the GluR3 riboprobe showed a major species of 5.2 kb. This pattern of RNA species was invariant over all the brain regions examined. Examination of GluR expression in development revealed that in the postnatal period, GluR1, 2, and 3 mRNAs are regulated as a function of age. In adult rat brain, GluR1 and 2 mRNA expression was highest in hippocampus; GluR3 was expressed at highest density in hippocampus and frontal cortex. The three transcripts were first detected at embryonic day 16 and then exhibited changes in expression levels in a region-specific manner. In hippocampus, all three transcripts exhibited elevated expression in the late neonatal period; in frontal cortex, elevated expression was observed for GluR2 and 3 only. In striatum, all three transcripts were expressed at relatively low levels throughout development, with a modest peak at postnatal day 14. In cerebellum, the GluR1 mRNA level was high from postnatal day 28 to adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonergic lesions decrease mu- and delta-opiate receptor binding in discrete areas of the hypothalamus and in the midbrain central gray

Serotonergic nerve terminals in the brain were lesioned by intraventricular infusion of the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) and levels of mu- and delta-opiate binding were measured in brain areas implicated in reproductive behavior and gonadotropin secretion. The lesion decreased mu-receptor binding in the preoptic area (mPOA) and the midbrain central gray, while delta-receptor binding was decreased in the mPOA and the dorsomedial nucleus of the hypothalamus. Hypothalamic serotonergic lesions also attenuated morphine inhibition of female sexual behavior. These results indicate the existence of serotonergic-opiate interactions in select regions of the brain and suggest that these interactions may be important in the regulation of lordosis behavior.

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.

Effects of polyamines on NMDA-induced currents in rat hippocampal neurons: a whole-cell and single-channel study

Actions of the polyamines spermine and spermidine on NMDA-induced currents were examined in cultured hippocampal neurons from embryonic rat. In whole-cell patch experiments using voltage-clamp, spermine (300 microM) produced about a two-fold potentiation of responses to NMDA (at -70 mV in the presence of saturating glycine); half-maximal potentiation was elicited at 207 microM. The potentiation produced by spermine was somewhat greater at positive potentials. The onset of potentiation was fast (t1/2 < 1 s), indicative of an extracellular site of action. Spermidine was of comparable potency but less efficacious than spermine in potentiating NMDA responses. In excised outside-out patches, spermine exhibited two actions on NMDA-induced single-channel responses. In some patches, it increased the channel open probability; both frequency of channel opening and burst length were increased with no significant change in the mean open duration, which accounted for much of the potentiation seen in whole-cell experiments. In all patches, spermine decreased channel conductance at negative voltages, an effect ascribable to fast channel block (with a possible contribution by charge screening). These results are consistent with opposing actions of polyamines mediated at distinct sites on the NMDA receptor.

Switch in glutamate receptor subunit gene expression in CA1 subfield of hippocampus following global ischemia in rats

Severe, transient global ischemia of the brain induces delayed damage to specific neuronal populations. Sustained Ca2+ influx through glutamate receptor channels is thought to play a critical role in postischemic cell death. Although most kainate-type glutamate receptors are Ca(2+)-impermeable, Ca(2+)-permeable kainate receptors have been reported in specific kinds of neurons and glia. Recombinant receptors assembled from GluR1 and/or GluR3 subunits in exogenous expression systems are permeable to Ca2+; heteromeric channels containing GluR2 subunits are Ca(2+)-impermeable. Thus, altered expression of GluR2 in development or following a neurological insult or injury to the brain can act as a switch to modify Ca2+ permeability. To investigate the molecular mechanism underlying delayed postischemic cell death, GluR1, GluR2, and GluR3 gene expression was examined by in situ hybridization in postischemic rats. Following severe, transient forebrain ischemia GluR2 gene expression was preferentially reduced in CA1 hippocampal neurons at a time point that preceded their degeneration. The switch in expression of kainate/AMPA receptor subunits coincided with the previously reported increase in Ca2+ influx into CA1 cells. Timing of the switch indicates that it may play a causal role in postischemic cell death.

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.

Are Ca(2+)-permeable kainate/AMPA receptors more abundant in immature brain?

The permeability properties of kainate/AMPA receptors are determined by subunit composition. The GluR1 and GluR3 subunits form Ca(2+)-permeable channels and exhibit inward rectification; heteromeric receptors containing the GluR2 subunit are Ca(2+)-impermeable and electrically linear. These observations raise the possibility of a developmental 'switch' in which turning on or off of GluR2 expression regulates the level of Ca2+ permeable kainate/AMPA receptors. We examined the ratio of GluR1 and GluR3 to GluR2 gene expression in developing and adult rat brain by in situ hybridization. A larger value of this ratio is likely to be associated with greater Ca2+ permeability. Our data suggest that in neocortex, striatum and cerebellum the number of Ca(2+)-permeable kainate/AMPA receptors is high at P4 and declines monotonically with age. In hippocampus, the number increases from P7 to P21, after which it declines. These findings provide evidence for a developmental 'switch' in which Ca2+ permeable glutamate receptors are turned off following early developmental events.

High efficiency reconstitution of a phencyclidine/MK-801 receptor binding site solubilized from rat forebrain membranes

Phencyclidine (PCP) receptors have been solubilized from rat forebrain membranes with the zwitterionic detergent 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate. Specific binding of the potent PCP receptor ligands [3H]thienyl-phencyclidine (TCP) and [3H]MK-801 was restored by incorporating extracted membrane protein into lipid vesicles prepared from a total brain lipid extract. A nearly quantitative recovery of solubilized receptor activity was achieved; this was dependent upon both the concentration of detergent used during membrane solubilization and the concentration of added lipid used during the reconstitution. The single, saturable, binding site measured for both [3H]TCP and [3H]MK-801 in solubilized and reconstituted preparations exhibited properties similar to those of the high affinity PCP binding site labeled by these ligands in brain membranes. The ability of ligands selective for this site (MK-801, TCP, and dexoxadrol) to competitively displace specific [3H]TCP binding was retained after solubilization and reconstitution, although IC50 values measured for these ligands were shifted to higher concentrations. Levoxadrol and haloperidol were ineffective at displacing the radioligand binding in both membrane and vesicle preparations. The additive and dose-dependent ability of glutamate and glycine to enhance [3H]TCP binding to the solubilized/reconstituted receptor further suggests that a direct interaction with the N-methyl-D-aspartate receptor/ion channel complex has been preserved in the vesicle preparations. The photoaffinity labeling of two polypeptides (Mr 98,000 and 59,000) by azido-[3H]PCP was demonstrated in the vesicle preparations; this was largely prevented by competitive displacement of the radioligand with PCP before photolysis. These results establish both an essential lipid dependency and polypeptide composition for the high affinity, haloperidol-insensitive, PCP receptor in brain.

Neuroanatomical localization of kappa 1 and kappa 2 opioid receptors in rat and guinea pig brain

The neuroanatomical localization of kappa opioid receptors in rat and guinea pig brain was determined by quantitative in vitro receptor autoradiography. Our study shows striking differences in kappa 1 and kappa 2 receptor distributions both between species and within each species. In the rat brain, kappa 1 sites (labeled by [3H]U-69,593) are of low density and confined to a small number of structures. These include the claustrum, endopiriform nucleus, caudate putamen, nucleus accumbens, midline nuclear group of the thalamus, superficial grey layer of the superior colliculus, and central grey. kappa 2 sites (labeled by [3H]ethylketocyclazocine or [3H]bremazocine under conditions in which mu, delta, and kappa 1 binding was suppressed) are more widely distributed throughout all levels of rat brain. kappa 2 sites occur at high density in the caudate putamen, nucleus accumbens, amygdala, thalamus, and interpeduncular nuclei. In guinea pig brain, kappa 1 sites predominate and are of high density in layers I and VI of the neocortex, claustrum, endopiriform nucleus, caudate putamen, nucleus accumbens, and molecular layer of the cerebellum. As in rat brain, kappa 2 sites in guinea pig are more uniformly and widely distributed throughout the brain than are kappa 1 sites. The highest density of kappa 2 sites is in the dorsal parabrachial nucleus, interpeduncular nuclei, mammillary nuclei, and posterior thalamic nuclei. Results from this study demonstrate important interspecies differences in the distribution of kappa 1 and kappa 2 opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of three glutamate receptor genes in developing rat brain: an in situ hybridization study

Non-N-methyl-D-aspartate glutamate receptors (GluRs) are encoded by a gene family, known members of which are designated GluR-1, -2, -3, -4, and -5. The present study examined the developmental pattern of GluR-1, -2, and -3 gene expression in rat brain. In situ hybridization revealed different spatial patterns throughout the brain for the cognate mRNAs at all ages examined, as well as different temporal patterns during development. In the adult all three mRNAs were expressed prominently in the pyramidal and granule layers of the hippocampus and in the Purkinje cell layer of the cerebellum, where detailed differences were apparent at the cellular level. In neocortex, GluR-2 mRNA exhibited prominent lamination and regional differences, which were less marked for GluR-1 and -3 mRNAs. In caudate-putamen GluR-2 mRNA was at high levels, but GluR-1 and -3 mRNAs were not. At early ages transcripts were transiently elevated relative to adult levels. GluR-1 mRNA reached peak expression in cortex at postnatal day 14 (P14) (225% of adult), in striatum at P4 (255% of adult), in hippocampus at P14 (195% of adult), and in cerebellum at P21 (150% of adult). GluR-3 exhibited more modest peaks in neocortex and hippocampus. In contrast, GluR-2 mRNA was at near adult levels throughout the first days of postnatal life and exhibited a peak only in cerebellum at P14 (168% of adult). The finding of differential developmental regulation of the GluR-1, -2, and -3 genes indicates that the receptors they encode may have different influences on synaptic plasticity, neuronal survival, and susceptibility to excitatory amino acid toxicity.

Interaction of Mg2+ and phencyclidine in use-dependent block of NMDA channels

The interaction between Mg2+ and phencyclidine (PCP) in blocking open N-methyl-D-aspartate (NMDA) channels was investigated in Xenopus oocytes injected with rat brain mRNA. These receptors exhibit the pharmacological and physiological properties of the neuronal receptors, and the oocyte is readily amenable to electrical recording and application of well-controlled chemical stimuli. We found that Mg2+ at physiological concentrations greatly impeded the ability of PCP to block the NMDA channel. The interaction between Mg2+ and PCP was competitive; 0.5 mM Mg2+ caused a four-fold decrease in the potency of PCP in blocking open NMDA channels. Moreover, Mg2+ speeded the recovery from PCP block in the presence of agonist, suggesting that Mg2+ reduced reblock of NMDA channels by PCP that had escaped from open channels. Our observations suggest that the presence of Mg2+ in the channel tends to prevent PCP entry and block. Since depolarization is likely to reduce channel occupancy by Mg2+ more than that by PCP, neural activity may have an important influence on the actions of PCP and related drugs.

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.