The Electrophysiology of Electric Organs of Marine Electric Fishes : III. The electroplaques of the stargazer, Astroscopus y-graecum

The electroplaques of Astroscopus y-graecum were studied in situ with microelectrode recordings. Despite the distant taxonomic relations and the different origins of the organs, their properties in the teleost and torpedine marine electric fishes are remarkably similar. Only the innervated membrane (the dorsal) is electrogenically reactive in Astroscopus, and it, too, does not respond to electrical stimuli. As in the torpedine fishes, the uninnervated membrane of the electroplaques offers a very low resistance to the discharge of the innervated membrane. Additional direct evidence for electrical inexcitability of the reactive surface was obtained by denervating one of the bilateral organs. The denervated one did not respond to strong electrical stimuli which evoked responses in the opposite, innervated organ. The denervated electroplaques had a normal resting potential and were depolarized by acetylcholine and carbamylcholine similarly to normal cells. Other properties related to electrical inexcitability were also demonstrated. A pharmacological finding of considerable theoretical significance is that desensitization occurred on depolarizing cells with acetylcholine but was absent on depolarizing them with carbamylcholine.

The Electrophysiology of Electric Organs of Marine Electric Fishes : II. The electroplaques of main and acccessory organs of Narcine brasiliensis

Studies on the electric organs of Narcine brasiliensis and particularly of the responses of the electroplaques of the accessory organ confirm and amplify data obtained on the electroplaques of Torpedo nobiliana. Only the innervated surface is electrogenically reactive and the uninnervated surface has a low resistance, as in Torpedo electroplaques. However, in the accessory organ of Narcine the innervated surface is the dorsal, rather than the ventral, and it has a different pattern of innervation. The responses of single cells of the accessory organ exhibit marked facilitation on repetitive stimulation. The facilitated responses, like the individual responses of Torpedo and of the main organ of Narcine, are electrochemically graded on changing the membrane potential with applied currents, and are inverted in sign when outward currents through the innervated face are very strong.

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.

Expanded Prussian blue analogues incorporating [Re6Se8(CN)6](3-/4-) clusters: adjusting porosity via charge balance

Face-capped octahedral [Re(6)Se(8)(CN)(6)](3-/4-) clusters are used in place of octahedral [M(CN)(6)](3-/4-) complexes for the synthesis of microporous Prussian blue type solids with adjustable porosity. The reaction between [Fe(H(2)O)(6)](3+) and [Re(6)Se(8)(CN)(6)](4-) in aqueous solution yields, upon heating, Fe(4)[Re(6)Se(8)(CN)(6)](3).36H(2)O (4). A single-crystal X-ray analysis confirms the structure of 4 to be a direct expansion of Prussian blue (Fe(4)[Fe(CN)(6)](3).14H(2)O), with [Re(6)Se(8)(CN)(6)](4-) clusters connected through octahedral Fe(3+) ions in a cubic three-dimensional framework. As in Prussian blue, one out of every four hexacyanide units is missing from the structure, creating sizable, water-filled cavities within the neutral framework. Oxidation of (Bu(4)N)(4)[Re(6)Se(8)(CN)(6)] (1) with iodine in methanol produces (Bu(4)N)(3)[Re(6)Se(8)(CN)(6)] (2), which is then metathesized to give the water-soluble salt Na(3)[Re(6)Se(8)(CN)(6)] (3). Reaction of [Co(H(2)O)(6)](2+) or [Ni(H(2)O)(6)](2+) with 3 in aqueous solution affords Co(3)[Re(6)Se(8)(CN)(6)](2).25H(2)O (5) or Ni(3)[Re(6)Se(8)(CN)(6)](2).33H(2)O (6). Powder X-ray diffraction data show these compounds to adopt structures based on the same cubic framework present in 4, but with one out of every three cluster hexacyanide units missing as a consequence of charge balance. In contrast, reaction of [Ga(H(2)O)(6)](3+) with 3 gives Ga[Re(6)Se(8)(CN)(6)].6H(2)O (7), wherein charge balance dictates a fully occupied cubic framework enclosing much smaller cavities. The expanded Prussian blue analogues 4-7 can be fully dehydrated, and retain their crystallinity with extended heating at 250 degrees C. Consistent with the trend in the frequency of framework vacancies, dinitrogen sorption isotherms show porosity to increase along the series of representative compounds 7, Ga(4)[Re(6)Se(8)(CN)(6)](3).38H(2)O, and 6. Furthermore, all of these phases display a significantly higher sorption capacity and surface area than observed in dehydrated Prussian blue. Despite incorporating paramagnetic [Re(6)Se(8)(CN)(6)](3-) clusters, no evidence for magnetic ordering in compound 6 is apparent at temperatures down to 5 K. Reactions related to those employed in preparing compounds 4-6, but carried out at lower pH, produce the isostructural phases H[cis-M(H(2)O)(2)][Re(6)Se(8)(CN)(6)].2H(2)O (M = Fe (8), Co (9), Ni (10)). The crystal structure of 8 reveals a densely packed three-dimensional framework in which [Re(6)Se(8)(CN)(6)](4-) clusters are interlinked through a combination of protons and Fe(3+) ions.

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.

Gating properties of gap junction channels assembled from connexin43 and connexin43 fused with green fluorescent protein

We used cell lines expressing wild-type connexin43 (Cx43) and Cx43 fused with enhanced green fluorescent protein (Cx43-EGFP) to examine mechanisms of gap junction channel gating. Previously it was suggested that each hemichannel in a cell-cell channel possesses two gates, a fast gate that closes channels to a nonzero conductance or residual state via fast (< approximately 2 ms) transitions and a slow gate that fully closes channels via slow transitions (> approximately 10 ms). Here we demonstrate that transjunctional voltage (V(j)) regulates both gates and that they are operating in series and in a contingent manner in which the state of one gate affects gating of the other. Cx43-EGFP channels lack fast V(j) gating to a residual state but show slow V(j) gating. Both Cx43 and Cx43-EGFP channels exhibit slow gating by chemical uncouplers such as CO(2) and alkanols. Chemical uncouplers do not induce obvious changes in Cx43-EGFP junctional plaques, indicating that uncoupling is not caused by dispersion or internalization of junctional plaques. Similarity of gating transitions during chemical gating and slow V(j) gating suggests that both gating mechanisms share common structural elements. Cx43/Cx43-EGFP heterotypic channels showed asymmetrical V(j) gating with fast transitions between open and residual states only when the Cx43 side was relatively negative. This result indicates that the fast V(j) gate of Cx43 hemichannels closes for relative negativity at its cytoplasmic end.

Functional alterations in gap junction channels formed by mutant forms of connexin 32: evidence for loss of function as a pathogenic mechanism in the X-linked form of Charcot-Marie-Tooth disease

CMTX, the X-linked form of Charcot-Marie-Tooth disease, is an inherited peripheral neuropathy arising in patients with mutations in the gene encoding the gap junction protein connexin 32 (Cx32). In this communication, we describe the expression levels and biophysical parameters of seven mutant forms of Cx32 associated with CMTX, when expressed in paired Xenopus oocytes. Paired oocytes expressing the R15Q and H94Q mutants show junctional conductances not statistically different from that determined for Cx32WT, though both show a trend toward reduced levels. The S85C and G12S mutants induce reduced levels of junctional conductance. Three other mutants (R15W, H94Y and V139M) induce no conductance above baseline when expressed in paired oocytes. Analysis of the conductance voltage relations for these mutants shows that the reduced levels of conductance are entirely (H94Y and V139M) or partly (S85C and R15W) explicable by a reduced open probability of the mutant hemichannels. The R15Q and H94Q mutations also show alterations in the conductance voltage relations that would be expected to minimally (H94Q) or moderately (R15Q) reduce the available gap junction communication pathway. The reduction in G12S induced conductance cannot be explained by alterations in hemichannel open probability and are more likely due to reduced junction formation. These results demonstrate that many CMTX mutations lead to loss of function of Cx32. For these mutations, the loss of function model is likely to explain the pathogenesis of CMTX.

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.

Microglia at brain stab wounds express connexin 43 and in vitro form functional gap junctions after treatment with interferon-gamma and tumor necrosis factor-alpha

Gap junctional communication between microglia was investigated at rat brain stab wounds and in primary cultures of rat and mouse cells. Under resting conditions, rat microglia (FITC-isolectin-B4-reactive cells) were sparsely distributed in the neocortex, and most (95%) were not immunoreactive for Cx43, a gap junction protein subunit. At brain stab wounds, microglia progressively accumulated over several days and formed aggregates that frequently showed Cx43 immunoreactivity at interfaces between cells. In primary culture, microglia showed low levels of Cx43 determined by Western blotting, diffuse intracellular Cx43 immunoreactivity, and a low incidence of dye coupling. Treatment with the immunostimulant bacterial lipopolysaccharide (LPS) or the cytokines interferon-gamma (INF-gamma) or tumor necrosis factor-alpha (TNF-alpha) one at a time did not increase the incidence of dye coupling. However, microglia treated with INF-gamma plus LPS showed a dramatic increase in dye coupling that was prevented by coapplication of an anti-TNF-alpha antibody, suggesting the release and autocrine action of TNF-alpha. Treatment with INF-gamma plus TNF-alpha also greatly increased the incidence of dye coupling and the Cx43 levels with translocation of Cx43 to cell-cell contacts. The cytokine-induced dye coupling was reversibly inhibited by 18 alpha-glycyrrhetinic acid, a gap junction blocker. Cultured mouse microglia also expressed Cx43 and developed dye coupling upon treatment with cytokines, but microglia from homozygous Cx43-deficient mice did not develop significant dye coupling after treatment with either INF-gamma plus LPS or INF-gamma plus TNF-alpha. This report demonstrates that microglia can communicate with each other through gap junctions that are induced by inflammatory cytokines, a process that may be important in the elaboration of the inflammatory response.

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.

Molecular determinants of membrane potential dependence in vertebrate gap junction channels

The conductance, g(j), of many gap junctions depends on voltage between the coupled cells (transjunctional voltage, V(j)) with little effect of the absolute membrane potential (V(m)) in the two cells; others show combined V(j) and V(m) dependence. We examined the molecular determinants of V(m) dependence by using rat connexin 43 expressed in paired Xenopus oocytes. These junctions have, in addition to V(j) dependence, V(m) dependence such that equal depolarization of both cells decreases g(j). The dependence of g(j) on V(m) was abolished by truncation of the C-terminal domain (CT) at residue 242 but not at 257. There are two charged residues between 242 and 257. In full-length Cx43, mutations neutralizing either one of these charges, Arg243Gln and Asp245Gln, decreased and increased V(m) dependence, respectively, suggesting that these residues are part of the V(m) sensor. Mutating both residues together abolished V(m) dependence, although there is no net change in charge. The neutralizing mutations, together or separately, had no effect on V(j) dependence. Thus, the voltage sensors must differ. However, V(j) gating was somewhat modulated by V(m), and V(m) gating was reduced when the V(j) gate was closed. These data suggest that the two forms of voltage dependence are mediated by separate but interacting domains.

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.

Connexin43 null mice reveal that astrocytes express multiple connexins

The gap junction protein connexin43 (Cx43) is the primary component of intercellular channels in cardiac tissue and in astrocytes, the most abundant type of glial cells in the brain. Mice in which the gene for Cx43 is deleted by homologous recombination die at birth, due to profound hypertrophy of the ventricular outflow tract and stenosis of the pulmonary artery. Despite this significant cardiovascular abnormality, brains of connexin43 null [Cx43 (-/-)] animals are shown to be macroscopically normal and to display a pattern of cortical lamination that is not detectably different from wildtype siblings. Presence of Cx40 and Cx45 in brains and astrocytes cultured from both Cx43 (-/-) mice and wildtype littermates was confirmed by RT-PCR, Northern blot analyses and by immunostaining; Cx46 was detected by RT-PCR and Northern blot analyses. Presence of Cx26 in astrocyte cultures was indicated by RT-PCR and by Western blot analysis, although we were unable to resolve whether it was contributed by contaminating cells; Cx30 mRNA was detected by Northern blot in long term (2 weeks) but not fresh cultures of astrocytes. These studies thus reveal that astrocyte gap junctions may be formed of multiple connexins. Presumably, the metabolic and ionic coupling provided by these diverse gap junction types may functionally compensate for the absence of the major astrocyte gap junction protein in Cx43 (-/-) mice, providing whatever intercellular signaling is necessary for brain development and cortical lamination.

Electrical synapses, a personal perspective (or history)

Gap junctions are the morphological substrate of one class of electrical synapse. This memoir records the author's involvement in the development of our knowledge of the physiology and ultrastructure of electrical synapses. The answer to whether neurotransmission is electrical or chemical is either. One lesson is that Occam's razor sometimes cut too deep; the nervous system does its operations in a number of different ways and a unitarian approach can lead one astray [M.V.L. Bennett, Nicked by Occam's razor: unitarianism in the investigation of synaptic transmission, Biol. Bull. 168 (1985) 159-167]. Electrical synapses can do many things that chemical synapses can do, and do them just as slowly. The new molecular, cellular and physiological techniques will clarify where gap junctions and electrical coupling do and do not occur and permit experimental manipulation with high specificity.

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.

Clustering of connexin 43-enhanced green fluorescent protein gap junction channels and functional coupling in living cells

Communication-incompetent cell lines were transfected with connexin (Cx) 43 fused with enhanced green fluorescent protein (EGFP) to examine the relation between Cx distribution determined by fluorescence microscopy and electrical coupling measured at single-channel resolution in living cell pairs. Cx43-EGFP channel properties were like those of wild-type Cx43 except for reduced sensitivity to transjunctional voltage. Cx43-EGFP clustered into plaques at locations of cell-cell contact. Coupling was always absent in the absence of plaques and even in the presence of small plaques. Plaques exceeding several hundred channels always conferred coupling, but only a small fraction of channels were functional. These data indicate that clustering may be a requirement for opening of gap junction channels.

Seeing is relieving: electrical synapses between visualized neurons

Two reports of electrical synapses between identified inhibitory neurons in the neocortex suggest that many neuron types known to communicate chemically may also be electrically coupled.

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.

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.