Modulation of desensitization at glutamate receptors in isolated crucian carp horizontal cells by concanavalin A, cyclothiazide, aniracetam and PEPA

Lectins modulate the functional properties of GluN1/GluN3-containing NMDA receptors

N-methyl-d-aspartate receptors (NMDARs) play an essential role in excitatory neurotransmission within the mammalian central nervous system (CNS). NMDARs are heteromultimers containing GluN1, GluN2, and/or GluN3 subunits, thus giving rise to a wide variety of subunit combinations, each with unique functional and pharmacological properties. Importantly, GluN1/GluN3A and GluN1/GluN3B receptors form glycine-gated receptors. Here, we combined electrophysiology with rapid solution exchange in order to determine whether the presence of specific N-glycans and/or interactions with specific lectins regulates the functional properties of GluN1/GluN3A and GluN1/GluN3B receptors expressed in human embryonic kidney 293 (HEK293) cells. We found that removing putative N-glycosylation sites alters the functional properties of GluN1/GluN3B receptors, but has no effect on GluN1/GluN3A receptors. Moreover, we found that the functional properties of both GluN1/GluN3A and GluN1/GluN3B receptors are modulated by a variety of lectins, including Concanavalin A (ConA), Wheat Germ Agglutinin (WGA), and Aleuria Aurantia Lectin (AAL), and this effect is likely mediated by a reduction in GluN1 subunit-mediated desensitization. We also found that AAL has the most profound effect on GluN1/GluN3 receptors, and this effect is mediated partly by a single N-glycosylation site on the GluN3 subunit (specifically, N565 on GluN3A and N465 on GluN3B). Finally, we found that lectins mediate their effect only when applied to non-activated receptors and have no effect when applied in the continuous presence of glycine. These findings provide further evidence to distinguish GluN1/GluN3 receptors from the canonical GluN1/GluN2 receptors and offer insight into how GluN1/GluN3 receptors may be regulated in the mammalian CNS.

Calcium dynamics and regulation in horizontal cells of the vertebrate retina: lessons from teleosts

Horizontal cells (HCs) are inhibitory interneurons of the vertebrate retina. Unlike typical neurons, HCs are chronically depolarized in the dark, leading to a constant influx of Ca2+ Therefore, mechanisms of Ca2+ homeostasis in HCs must differ from neurons elsewhere in the central nervous system, which undergo excitotoxicity when they are chronically depolarized or stressed with Ca2+ HCs are especially well characterized in teleost fish and have been used to unlock mysteries of the vertebrate retina for over one century. More recently, mammalian models of the retina have been increasingly informative for HC physiology. We draw from both teleost and mammalian models in this review, using a comparative approach to examine what is known about Ca2+ pathways in vertebrate HCs. We begin with a survey of Ca2+-permeable ion channels, exchangers, and pumps and summarize Ca2+ influx and efflux pathways, buffering, and intracellular stores. This includes evidence for Ca2+-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors and for voltage-gated Ca2+ channels. Special attention is given to interactions between ion channels, to differences among species, and in which subtypes of HCs these channels have been found. We then discuss a number of unresolved issues pertaining to Ca2+ dynamics in HCs, including a potential role for Ca2+ in feedback to photoreceptors, the role for Ca2+-induced Ca2+ release, and the properties and functions of Ca2+-based action potentials. This review aims to highlight the unique Ca2+ dynamics in HCs, as these are inextricably tied to retinal function.

Biochemical and electrophysiological characterization of N-glycans on NMDA receptor subunits

In mammals, excitatory synapses contain two major types of ionotropic glutamate receptors: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors (NMDARs). Both receptor types are comprised of several subunits that are post-translationally modified by N-glycosylation. However, the precise N-glycans that are attached to these receptor types are largely unknown. Here, we used biochemistry to confirm that native NMDARs are extensively N-glycosylated; moreover, we found that the NMDAR GluN2B subunit differs from GluN1 subunits with respect to endoglycosidase H sensitivity. Next, we used a complete panel of lectins to determine the glycan composition of NMDARs in both cerebellar tissue and cultured cerebellar granule cells. Our experiments identified 23 lectins that pulled down both the GluN1 and GluN2B NMDAR subunits. We then performed an electrophysiological analysis using representative lectins and found that pre-incubating cerebellar granule cells with the AAL, WGA, or ConA alters the receptor's biophysical properties; this lectin-mediated effect was eliminated when the cells were deglycosylated with peptide-N-glycosidase F. Similar lectin-mediated effects were observed using HEK293 cells that express recombinant GluN1/GluN2B receptors. Finally, using mutant recombinant GluN subunits expressed in HEK293 cells, we found that 11 out of 12 predicted N-glycosylation sites in GluN1 and 7 out of 7 N-glycosylation sites in GluN2B are occupied by N-glycans. These data provide new insight into the role that N-glycosylation plays in regulating the function of NMDA receptors in the central nervous system. All animal experiments were performed in accordance with relevant institutional ethics guidelines and regulations with respect to protecting animal welfare. We examined the N-glycan composition of NMDA receptors (NMDARs) using deglycosylating enzymes, lectin-based biochemistry, and electrophysiology. Our results revealed that cerebellar NMDARs associate with 23 different lectins that have unique specificities for glycan structures. Furthermore, we found that 11 out of 12 predicted N-glycosylation sites in GluN1 and 7 out of 7 N-glycosylation sites in GluN2B are occupied by N-glycans. These data shed light on the glycan composition of NMDARs, revealing potential targets for the development of novel therapeutic approaches.

Differential modulation by AMPA of signals from red- and green-sensitive cones in carp retinal luminosity-type horizontal cells

Intracellular recordings were made from luminosity-type horizontal cells (LHCs) in the isolated superfused carp retina and the effect of AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid), a glutamate receptor agonist, on these cells was studied. AMPA suppressed the responses of LHCs driven by red-sensitive (R-) cones whereas it potentiated the responses driven by green-sensitive (G-) cones. The AMPA effect could be completely blocked by GYKI 53655, a specific AMPA receptor antagonist, indicating the exclusive involvement of AMPA-preferring receptors. The AMPA effect persisted in the presence of picrotoxin (PTX) or dihydrokainic acid (DHK), suggesting that the feedback from LHCs onto cones and glutamate transporters on cones may not be involved. It is suggested that there may exist different AMPA receptor subtypes with distinct characteristics on LHCs, which mediate signal transfer from R-and G-cones to LHCs, respectively.

Simulation analysis of receptive-field size of retinal horizontal cells by ionic current model

The size of the receptive field of retinal horizontal cells changes with the state of dark/light adaptation. We have used a mathematical model to determine how changes in the membrane conductance affect the receptive-field properties of horizontal cells. We first modeled the nonlinear membrane properties of horizontal cells based on ionic current mechanisms. The dissociated horizontal cell model reproduced the voltage–current (V–I) relationships for various extracellular glutamate concentrations measured in electrophysiological studies. Second, a network horizontal cell model was also described, and it reproduced theV–Irelationship observedin vivo. The network model showed a bell-shaped relationship between the receptive-field size and constant glutamate concentration. The simulated results suggest that the calcium current is a candidate for the bell-shaped length constant relationship.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Heterogeneity and potentiation of AMPA type of glutamate receptors in rat cultured microglia

alpha-amino-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor in rat cultured microglia were analyzed precisely using flop- and flip-preferring allosteric modulators of AMPA receptors, 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetamide (PEPA) and cyclothiazide (CTZ), respectively. Glutamate (Glu)- or kainite (KA)-induced currents were completely inhibited by a specific blocker of AMPA receptor, LY300164, indicating that functional Glu-receptors in cultured microglia are mostly AMPA receptor but not KA receptor in many cells. Glu- and KA-induced currents were potentiated by PEPA and CTZ in a concentration-dependent manner. The ratio of the potentiation by PEPA to the potentiation by cyclothiazide varied with cells between 0.1 and 0.9, suggesting cell-to-cell heterogeneity of AMPA receptor subunits expressed in microglia. Quantitative RT-PCR revealed that GluR1-3 mainly occurred in the flip forms, which agreed with the stronger potentiation of receptor currents by CTZ vs. PEPA. Finally, the potentiation of microglial AMPA receptors by PEPA and CTZ inhibited the Glu-induced release of tumor necrosis factor-alpha (TNF-alpha) unpredictably. The increase in TNF-alpha release by Glu or KA required extracellular Na+ and Ca2+ ions but not mitogen-activated protein kinase (MAPK), suggesting the effects of PEPA and CTZ were not due to the inhibition of MAPK. These results suggest that potentiation of microglial AMPA receptors serves as a negative feedback mechanism for the regulation of TNF-alpha release and may contribute to the ameliorating effects of allosteric modulators of AMPA receptors.

The mechanism of action of aniracetam at synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors: indirect and direct effects on desensitization

The mechanism of action of aniracetam on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors was examined in outside-out patches and at glutamatergic synapses in neurons of the chick cochlear nucleus. A combination of rapid-flow analysis, using glutamate as an agonist, and kinetic modeling indicated that aniracetam slows both the rate of channel closing, and the microscopic rates of desensitization, even for partially liganded receptors. Little effect was observed on the rate of recovery from desensitization or on the response to the weakly desensitizing agonist kainate. Aniracetam's effects on receptor deactivation saturated at lower concentrations than its effects on desensitization, suggesting that cooperativity between homologous binding sites was required to regulate desensitization. Analysis of responses to paired pulses of agonist also indicated that AMPA receptors must desensitize partially even after agonist exposures too brief to permit rebinding. In the presence of aniracetam, evoked excitatory synaptic currents (EPSCs) and miniature EPSCs in low quantal-content conditions had decay times similar to the time course of receptor deactivation. Under these conditions, the time course of both transmitter release and clearance must be <1 to 2 ms. However, in high quantal-content conditions, the evoked EPSC in aniracetam decayed with a time course intermediate between deactivation and desensitization, suggesting that the time course of transmitter clearance is prolonged because of pooling of transmitter in the synaptic cleft. Moreover, by comparing the amounts of paired-pulse synaptic depression and patch desensitization prevented by aniracetam, we conclude that significant desensitization occurs in response to rebinding of transmitter to the AMPA receptors.

Stimulation of sodium pump restores membrane potential to neurons excited by glutamate in zebrafish distal retina

Glutamate either depolarizes or hyperpolarizes retinal neurons. Those are the initial and primary effects. Using a voltage probe (oxonol, DiBaC4 (5)) to study dissociated zebrafish retinal neurons, we find a secondary, longer-term effect: a post-excitatory restoration of membrane potential, termed after-hyperpolarization (AHP). AHP occurs only in neurons that are depolarized by glutamate and typically peaks about 5 min after glutamate application. AHP is seen in dissociated horizontal cells (HCs) and hyperpolarizing, or OFF type, bipolar cells (HBCs). These cells commonly respond with only an AHP component. AHP never occurs in depolarizing, or ON type, bipolar cells (DBCs), which are cell types hyperpolarized by glutamate. AHP is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). It is evoked by kainate, AMPA and the AMPA-selective agonist (S)-5-fluorowillardiine, but not by NMDA, D-aspartate, the kainate-selective agonist SYM 2081 or by DL-2-amino-4-phosphonobutyric acid (DL-AP4). Cells with exclusively AHP responses are tonically depolarized. Resting potentials can be restored by nifedipine, suggesting a tonic, depolarizing action of L-type Ca2+ channels. However AHP is not blocked by nifedipine and is insensitive to [Cl-]o. AHP is blocked by Li+o substitution for Na+o and by ouabain. A mechanism is proposed in which Na+ entering through ionotropic AMPA channels stimulates Na+,K+-ATPase, which, by electrogenic action, restores membrane potential, generating the AHP response. Patterns of ATPase immunoreactivity support localization in the outer plexiform layer (OPL) as cone pedicles, HCs and BCs were positively labelled. Labelling was weaker in the inner plexiform layer (IPL) than in nuclear layers, though two IPL bands of immunoreactive BC terminals could be discerned, one in sublamina a and the other in sublamina b. Persistent stimulation of distal retina by photoreceptor glutamate may induce increased expression and activity of Na+,K+-ATPase, with a consequent impact on distal glutamate responses.

Aniracetam improves contextual fear conditioning and increases hippocampal gamma-PKC activation in DBA/2J mice

DBA/2J (D2) mice display poor contextual learning and have less membrane-bound hippocampal protein kinase C (PKC) compared with C57BL/6 (B6) mice. Aniracetam and oxiracetam were previously shown to improve contextual learning in D2 mice and increase PKC activity. This study investigated a possible mechanism for learning enhancement by examining the effects of aniracetam on contextual fear conditioning and activation of the y isoform of PKC (gamma-PKC) in male D2 mice. In comparison to animals treated with vehicle only (10% 2-hydroxypropyl-beta-cyclodextrin), mice treated with aniracetam (100 mg/kg) 30 min prior to fear conditioning training demonstrated significantly improved contextual learning when tested 30 min and 24 h after training. This corresponded with a significant increase in activated, membrane-bound hippocampal gamma-PKC 30 min after training. No increase in learning or gamma-PKC was found 5 min after training. These results suggest an altered time course of activation of gamma-PKC in response to treatment with aniracetam, which improves learning in D2 mice.

A desensitization-selective potentiator of AMPA-type glutamate receptors

1: We examined the effects of PEPA, an allosteric potentiator of AMPA receptors, on AMPA receptor kinetics. 2: PEPA did not affect the deactivation of glutamate responses but potently attenuated the extent of receptor desensitization without slowing the onset of desensitization in most of the recombinant AMPA receptors (GluR1-flip, GluR1-flop, GluR3-flip, GluR3-flip+GluR2-flip, and GluR3-flop+GluR2-flop) expressed in Xenopus oocytes. For the GluR3-flop subunit, PEPA attenuated the extent of desensitization and only weakly prolonged deactivation (1.3 fold). 3: PEPA did not significantly affect recovery from desensitization in oocytes expressing GluR3-flip, GluR1-flop, and GluR1-flop, but weakly accelerated (2.6 fold) recovery from desensitization in oocytes expressing GluR3-flop. 4: PEPA's effect on desensitization of GluR3-flop-containing receptors is unique in that onset is very slow. 5: Simulation studies using simplified kinetic models for AMPA receptors are utilized to explore the differential effects of PEPA on GluR3-flip and -flop. It is possible to simulate the action on GluR3-flip by modulating two rate constants in a 12-state kinetic model. For simulation of the action on GluR3-flop, the 12-state kinetic model is not enough, and it is necessary to invoke a 13th state, a PEPA-bound receptor to which glutamate cannot bind. 6: These results suggest that attenuation of extent of desensitization represents the principal mechanism underlying the potentiation of AMPA receptors by PEPA, and that PEPA exhibits different mechanisms with respect to GluR3-flip and GluR3-flop.

Selective synaptic distribution of AMPA and kainate receptor subunits in the outer plexiform layer of the carp retina

The subunit composition of ionotropic glutamate receptors (GluRs) is extremely diverse and responsible for the diversity of postsynaptic responses to the release of glutamate, which is the major excitatory neurotransmitter in the retina. To understand the functional consequences of this diversity, it is necessary to reveal the synaptic localization and subunit composition of GluRs. We have used immuno light and electron microscopy to localize AMPA and kainate (GluR1, GluR2/3, GluR4, GluR5-7) subunits in identified carp retinal neurons contributing to the outer plexiform layer. GluR1 could not be detected within the outer plexiform layer. Rod and cone horizontal cells all express only GluR2/3 at the tips of their invaginating dendrites. These receptors are also inserted into the membrane of spinules, light-dependent protrusions of the horizontal cell dendrites, flanking the synaptic ribbon of the cone synapse. Bipolar cells express GluR2/3, GluR4, and GluR5-7 at their terminal dendrites invaginating cone pedicles and rod spherules. Colocalization data suggest that each subunit is expressed by a distinct bipolar cell type. The majority of bipolar cells expressing these receptors seem to be of the functional OFF-type; however, in a few instances, GluR2/3 could also be detected on dendrites of bipolar cells that, based on their localization within the cone synaptic complex, appeared to be of the functional ON-type. The spatial arrangement of the different subunits within the cavity of the cone pedicle appeared not to be random: GluR2/3 was found predominantly at the apex of the cavity, GluR4 at its base and GluR5-7 dispersed between the two.

Blocking AMPA receptor desensitization prolongs spontaneous EPSC decay times and depolarizes H1 horizontal cells in carp retinal slices

Desensitization of H1 horizontal cell (H1 HC) glutamate receptors was investigated in carp retinal slices using cyclothiazide (CTZ), an inhibitor of AMPA receptor desensitization. 100 microM CTZ depolarized H1 HCs and increased the amplitude of light responses, without any prominent changes in their kinetics. Spontaneous EPSCs (sEPSCs) in H1 HCs were observed in the presence of 2.5 mM heptanol, an uncoupling agent of gap junctions. 20 microM GYKI52466 (an AMPA receptor antagonist) blocked the sEPSCs, consistent with the sEPSCs being mediated by AMPA receptors. 100 microM cobalt suppressed the frequency of sEPSCs without changing their mean peak amplitude, suggesting that calcium-dependent transmitter release from cones was not affected by heptanol. CTZ increased the total inward charge transferred per sEPSC by increasing the sEPSC decay time constant twofold, without any significant change in their frequency and mean peak amplitude. This suggests that the depolarizing effect of CTZ on H1 HCs was due to blocking desensitization of AMPA receptors, increasing the inward current induced by glutamate released from cone synaptic terminals. The desensitization of glutamate receptors may function to extend the dynamic range of H1 HC light responses.

Signals from cone photoreceptors to L-type horizontal cells are differentially modulated by low calcium in carp retina

Ca2+ plays crucial roles in both phototransduction and calcium-dependent glutamate release from the photoreceptor terminal. Modulation, by lowering extracellular Ca2+, of red-sensitive (R-) and short wavelength-sensitive (S-) cone-driven light responses of L-type horizontal cells (LHCs) was studied in the isolated superfused carp retina using intracellular recording techniques. Low Ca2+ (nominally Ca2+-free) Ringer's reduced responses of LHCs to both green (500 nm) and red (680 nm) flashes in darkness, with the former being suppressed more substantially than the latter. This differential suppression became more significant when contribution of R-cones to the green-light-induced responses was diminished by a moderate red (680 nm) background light. Application of IBMX, an inhibitor of phosphodiesterase (PDE), increased LHC responses to both red and green flashes equally, resembling the effect of low Ca2+ on phototransduction. In addition, photopic electroretinographic P III responses, reflecting the activity of cones, to red flashes were more potentiated by low Ca2+, compared to those to green flashes, whilst they were both equally potentiated by IBMX. Furthermore, low Ca2+ caused a more pronounced suppression of LHC responses to red flashes than those to green flashes in the presence of IBMX. It is postulated that reduction of LHC responses in low Ca2+ may be due to the 'saturation suppression' caused by the increased glutamate release from the photoreceptor terminal and the differential modulation may reflect a consequence of the dual action of low Ca2+ on the PDE activity in the photoreceptor outer segment and the synaptic strength between cones and LHCs.

Caffeine-sensitive Ca2+ stores in carp retinal bipolar cells

High K+- or caffeine-induced Ca2+ signal was studied in freshly dissociated carp retinal ON-type bipolar cells using a confocal laser-scanning microscope. In response to 35 mM K+ exposure, a rise in [Ca2+]i appeared in both the terminal and soma, but was absent after removal of external Ca2+ or in the presence of 100 microM nifedipine. It is indicated that, for high K+-induced increase of [Ca2+]i, Ca2+ influx through voltage-gated L-type Ca2+ channels is essential and Ca2+ entry through reversed Na+/Ca2+ exchange may be negligible. Interestingly, caffeine-induced elevation of [Ca2+]i was restricted to the soma, and could be abolished by 50 microM ryanodine, suggesting that caffeine-sensitive Ca2+ stores gated by ryanodine receptors were present in the soma but not in the terminal of bipolar cells. After treatment with 50 microM ryanodine for 20 min, the peak of the Ca2+ transients evoked by 35 mM K+ in the soma decreased to 48.2+/-5.7% of the control. The results suggest that depolarization-evoked Ca2+ influx can cause Ca2+ release from caffeine-sensitive Ca2+ stores, and in turn amplify Ca2+ signal in the soma of retinal bipolar cells.

Characterization of AMPA receptors on isolated amacrine-like cells in carp retina

In amacrine-like cells freshly dissociated from crucian carp (Carassius auratus) retina, we recorded whole-cell responses to rapid application of glutamate and kainate. Currents induced by glutamate, but not kainate, usually showed extremely rapid desensitization, and the mean time constant for the decay of the responses to 10 mM glutamate was 2.77 ms. N-methyl-D-aspartate (NMDA) failed to induce any current even with coapplication of glycine and removal of extracellular Mg2 +. 1-(4-aminophenyl)-3-Methylcarbamyl-4-methyl-7,8-methylenedioxy-3, 4-dihydro-5H-2,3-benzodiazepine (GYKI 53655), a selective alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist, was found to completely block glutamate-induced currents, suggesting that the glutamate receptors on these cells are AMPA preferring. The value of EC50 for glutamate and kainate was determined to be 2.73 mM and 97.5 microM, respectively. Noise analysis of fluctuation of whole-cell currents induced by kainate of different concentrations indicated that the mean conductance of the AMPA receptor channels was 5.70 pS. Splice variant analysis of the AMPA receptors was also conducted by comparing the effects of cyclothiazide, a flip receptor-preferring modulator and 4-[2-(phenylsulphonylamino)ethylthio]-2,6-difluoro-phenoxyaceta mide (PEPA), a flop receptor-preferring modulator, on glutamate-induced responses. PEPA was much more potent than cyclothiazide at these receptors with a EC50 of 17.3 microM. The mean ratio of the potentiation by PEPA versus cyclothiazide (P/C ratio) was 4.39. These modulatory effects of cyclothiazide and PEPA were rather similar to those obtained at AMPA receptors assembled from flop variants expressed in Xenopus oocytes, suggesting that the AMPA receptor of the carp amacrine cells may predominantly consist of the flop splice variants.

Zn2+ differentially modulates kinetics of GABA(C) vs GABA(A) receptors in carp retinal bipolar cells

GABA(C) and GABA(A) receptors co-exist in retinal bipolar cells. In the present study the effects of zinc on the kinetics of currents mediated by GABA(C) and GABA(A) receptors were investigated in isolated carp bipolar cells, using whole-cell patch-clamp technique. We observed for the first time that zinc exerted opposite effects on kinetics of the GABA(C) and GABA(A) responses: zinc significantly slowed down activation and desensitization of the GABA(C) response, but accelerated those of the GABA(A) response; zinc dramatically accelerated deactivation of the GABA(C) response, whereas it had no apparent effect on deactivation of the GABA(A) response. These results suggest that zinc may be functionally important in regulating retinal signal transmission.

Zinc modulation of AMPA receptors may be relevant to splice variants in carp retina

With the use of the whole-cell patch clamp technique, we examined effects of zinc on AMPA receptors of isolated carp retinal horizontal cells, predominantly consisting of flop splice variants. We found that zinc ranging from 30 microM to 1 mM failed to modulate glutamate-induced currents of these cells, which is clearly distinct from the results previously obtained in superior colliculus neurons and Xenopus ooctyes. Furthermore, glutamate responses remained unchanged when zinc was co-applied with PEPA, a flop variant-preferential AMPA receptor potentiator. With the co-application of cyclothiazide, a flip variant-preferential AMPA receptor potentiator, however, a dual effect could be observed: zinc potentiated glutamate responses at low concentrations, but inhibited them at higher concentrations. These results suggest that the action of zinc on AMPA receptors may be splice variant-relevant.