Desensitization of AMPA receptors on horizontal cells isolated from crucian carp retina

Orexin-A differentially modulates AMPA-preferring responses of ganglion cells and amacrine cells in rat retina

By activating their receptors (OX1R and OX2R) orexin-A/B regulate wake/sleeping states, feeding behaviors, but the function of these peptides in the retina remains unknown. Using patch-clamp recordings and calcium imaging in rat isolated retinal cells, we demonstrated that orexin-A suppressed α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-preferring receptor-mediated currents (AMPA-preferring currents) in ganglion cells (GCs) through OX1R, but potentiated those in amacrine cells (ACs) through OX2R. Consistently, in rat retinal slices orexin-A suppressed light-evoked AMPA-preferring receptor-mediated excitatory postsynaptic currents in GCs, but potentiated those in ACs. Intracellular dialysis of GDP-β-S or preincubation with the Gi/o inhibitor pertussis toxin (PTX) abolished both the effects. Either cAMP/the protein kinase A (PKA) inhibitor Rp-cAMP or cGMP/the PKG blocker KT5823 failed to alter the orexin-A effects. Whilst both of them involved activation of protein kinase C (PKC), the effects on GCs and ACs were respectively eliminated by the phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor and phosphatidylcholine (PC)-PLC inhibitor. Moreover, in GCs orexin-A increased [Ca(2+)]i and the orexin-A effect was blocked by intracellular Ca(2+)-free solution and by inositol 1,4,5-trisphosphate (IP3) receptor antagonists. In contrast, orexin-A did not change [Ca(2+)]i in ACs and the orexin-A effect remained in intracellular or extracellular Ca(2+)-free solution. We conclude that a distinct Gi/o/PI-PLC/IP3/Ca(2+)-dependent PKC signaling pathway, following the activation of OX1R, is likely responsible for the orexin-A effect on GCs, whereas a Gi/o/PC-PLC/Ca(2+)-independent PKC signaling pathway, following the activation of OX2R, mediates the orexin-A effect on ACs. These two actions of orexin-A, while working in concert, provide a characteristic way for modulating information processing in the inner retina.

Cell-specific cre recombinase expression allows selective ablation of glutamate receptors from mouse horizontal cells

In the mouse retina, horizontal cells form an electrically coupled network and provide feedback signals to photoreceptors and feedforward signals to bipolar cells. Thereby, horizontal cells contribute to gain control at the first visual synapse and to the antagonistic organization of bipolar and ganglion cell receptive fields. However, the nature of horizontal cell output remains a matter of debate, just as the exact contribution of horizontal cells to center-surround antagonism. To facilitate studying horizontal cell function, we developed a knockin mouse line which allows ablating genes exclusively in horizontal cells. This knockin line expresses a Cre recombinase under the promoter of connexin57 (Cx57), a gap junction protein only expressed in horizontal cells. Consistently, in Cx57+/Cre mice, Cre recombinase is expressed in almost all horizontal cells (>99%) and no other retinal neurons. To test Cre activity, we crossbred Cx57+/Cre mice with a mouse line in which exon 11 of the coding sequence for the ionotropic glutamate receptor subunit GluA4 was flanked by two loxP sites (GluA4fl/fl). In GluA4fl/fl:Cx57+/Cre mice, GluA4 immunoreactivity was significantly reduced (∼50%) in the outer retina where horizontal cells receive photoreceptor inputs, confirming the functionality of the Cre/loxP system. Whole-cell patch-clamp recordings from isolated horizontal cell somata showed a reduction of glutamate-induced inward currents by ∼75%, suggesting that the GluA4 subunit plays a major role in mediating photoreceptor inputs. The persistent current in GluA4-deficient cells is mostly driven by AMPA and to a very small extent by kainate receptors as revealed by application of the AMPA receptor antagonist GYKI52466 and concanavalin A, a potentiator of kainate receptor-mediated currents. In summary, the Cx57+/Cre mouse line provides a versatile tool for studying horizontal cell function. GluA4fl/fl:Cx57+/Cre mice, in which horizontal cells receive less excitatory input, can thus be used to analyze the contribution of horizontal cells to retinal processing.

Synaptic contribution of Ca2+-permeable and Ca2+-impermeable AMPA receptors on isolated carp retinal horizontal cells and their modulation by Zn2+

Ca(2+)-permeable and Ca(2+)-impermeable AMPA receptors are co-expressed on carp retinal horizontal cells. In the present study, we examined the synaptic contribution and Zn(2+) modulatory effect of these two AMPA receptor subtypes using whole-cell patch clamp technique. Specific Ca(2+)-permeable AMPA receptor antagonist (1-naphthyl acetyl spermine, NAS) and selective Ca(2+)-impermeable AMPA receptor blocker (pentobarbital, PB) were used to separate the glutamate-response in isolated H1 horizontal cell mediated by these two subtypes of AMPA receptors respectively. Application of 100 microM NAS substantially suppressed the current elicited by 3 mM glutamate and the remaining NAS-insensitive component was completely blocked by application of 100 microM PB. In addition, Zn(2+) had dual effects on Ca(2+)-permeable AMPA receptor-mediated current: at low concentration (10 microM), Zn(2+) potentiated the current, but at higher concentrations (100 and 1000 microM), Zn(2+) reduced the current in a dose-dependent manner. However, Zn(2+) (10, 100 and 1000 microM) failed to modulate the NAS-insensitive current mediated by Ca(2+)-impermeable AMPA receptors. Overall, our results suggest that Ca(2+)-permeable AMPA receptors contribute more to the cell's glutamate-response than Ca(2+)-impermeable AMPA receptors. Furthermore, Zn(2+) has dual effects on the Ca(2+)-permeable AMPA receptor activity without affecting Ca(2+)-impermeable AMPA receptors.

Pharmacological characterization, localization, and regulation of ionotropic glutamate receptors in skate horizontal cells

Glutamate is believed to be the primary excitatory neurotransmitter in the vertebrate retina, and its fast postsynaptic effects are elicited by activating NMDA-, kainate-, or AMPA-type glutamate receptors. We have characterized the ionotropic glutamate receptors present on retinal horizontal cells of the skate, which possess a unique all-rod retina simplifying synaptic circuitry within the outer plexiform layer (OPL). Isolated external horizontal cells were examined using whole-cell voltage-clamp techniques. Glutamate and its analogues kainate and AMPA, but not NMDA, elicited dose-dependent currents. The AMPA receptor antagonist GYKI 52466 at 100 microm abolished glutamate-elicited currents. Desensitization of glutamate currents was removed upon coapplication of cyclothiazide, known to potentiate AMPA receptor responses, but not by concanavalin A, which potentiates kainate receptor responses. The dose-response curve to glutamate was significantly broader in the presence of the desensitization inhibitor cyclothiazide. Polyclonal antibodies directed against AMPA receptor subunits revealed prominent labeling of isolated external horizontal cells with the GluR2/3 and GluR4 antibodies. 1-Naphthylacetyl spermine, known to block calcium-permeable AMPA receptors, significantly reduced glutamate-gated currents of horizontal cells. Downregulation of glutamate responses was induced by increasing extracellular ion concentrations of Zn2+ and H+. The present study suggests that Ca2+-permeable AMPA receptors likely play an important role in shaping the synaptic responses of skate horizontal cells and that alterations in extracellular concentrations of calcium, zinc, and hydrogen ions have the potential to regulate the strength of postsynaptic signals mediated by AMPA receptors within the OPL.

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.

Depolarization of isolated horizontal cells of fish acidifies their immediate surrounding by activating V-ATPase

In order to interpret the formation of receptive field surrounds in retinal neurons, a proton-mediated mechanism was proposed to mediate feedback from horizontal cells (HCs) to cone photoreceptors. To verify the idea that depolarized HCs release protons, we measured, by a fluorescence ratiometric technique, the pH of the immediate external surface (pHs) of HCs isolated from the carp or goldfish retina. When HCs stained by 5-hexadecanoylaminofluorescein, a pH-sensitive lipophilicdye, were depolarized by bath-application of kainate or high-K+ medium, pHs was lowered. The amount of pHs change was monotonically dependent on the degree of depolarization, as much as 0.21 +/- 0.05 pH units by 100 mV depolarization (induced by 100 mm K+). Acidification was suppressed by 400 nm bafilomycin A1, a specific inhibitor of the vacuolar type H+ pump (V-ATPase), suggesting that depolarization released protons from HCs via the voltage-sensitive H+ pump. Immunocytochemical analysis, using an anti-V-ATPase antibody, revealed the existence of V-ATPase in dissociated HCs. These results support the hypothesis that the feedback from HCs to cones could be proton mediated.

Glycine receptors are functionally expressed on bullfrog retinal cone photoreceptors

Using immunocytochemical and whole cell recording techniques, we examined expression of glycine receptors on bullfrog retinal cone photoreceptors. Immunofluorescence double labeling experiments conducted on retinal sections and isolated cell preparations showed that terminals and inner segments of cones were immunoreactive to both alpha1 and beta subunits of glycine receptors. Moreover, application of glycine induced a sustained inward current from isolated cones, which increased in amplitude in a dose-dependent manner, with an EC50 (concentration of glycine producing half-maximal response) of 67.3+/-4.9 microM, and the current was blocked by the glycine receptor antagonist strychnine, but not 5,7-dichlorokynurenic acid (DCKA) of 200 microM, a blocker of the glycine recognition site at the N-methyl-D-aspartate (NMDA) receptor. The glycine-induced current reversed in polarity at a potential close to the calculated chloride equilibrium potential, and the reversal potential was changed as a function of the extracellular chloride concentration. These results suggest that strychnine-sensitive glycine receptors are functionally expressed in bullfrog cones, which may mediate signal feedback from glycinergic interplexiform cells to cones in the outer retina.

Functional N-Methyl-D-Aspartate Receptors Are Expressed in Cone-Driven Horizontal Cells in Carp Retina

Glutamate works as a major excitatory neurotransmitter in the vertebrate retina. Whole-cell recordings made from isolated carp cone horizontal cells (H1 cells) showed that N-methyl-D-aspartate (NMDA), co-applied with glycine, induced inward currents that were blocked by the NMDA receptor competitive antagonist D-2-amino-5-phosphonopentanoate (D-AP5) and 5,7-dichlorokynurenic acid (DCKA), a selective NMDA receptor antagonist acting at the glycine site on the NMDA receptor complex. Moreover, calcium imaging showed that NMDA caused a significant elevation of intracellular calcium levels ([Ca(2+)](i)) of H1 cells, which was also blocked by D-AP5. In contrast, neither inward currents nor changes in [Ca(2+)](i) could be induced by NMDA in rod horizontal cells (H4 cells). Intracellular recordings made from H1 cells in the isolated retina, superfused with Ringer's containing 1 mM Mg(2+), in the dark demonstrated that NMDA reduced the light-off overshoot of H1 cells. We therefore conclude that the functional NMDA receptor is expressed in carp H1 cells, from which this receptor has been thought to be absent, and this receptor may play a role in modulating cone-driven signal of horizontal cells in the dark.

An ionotropic GABA receptor with novel pharmacology at bullfrog cone photoreceptor terminals

Characteristics of ionotropic gamma-aminobutyric acid (GABA) receptors at bullfrog cone terminals were studied by patch clamp techniques in isolated cell and retinal slice preparations. GABA-induced inward currents from isolated cones reversed in polarity at a potential, very close to the chloride equilibrium potential, and they were completely suppressed by picrotoxin. Unexpectedly, the GABA current was dose-dependently potentiated by the well-known GABA(A) receptor antagonist bicuculline (BIC), but was suppressed by gabazine, another GABA(A) antagonist, and imidazole-4-acetic acid (I4AA), a GABA(C) receptor antagonist. Similarly, currents induced by both GABA(A) agonist muscimol and GABA(C) agonist cis-4-aminocrotonic acid (CACA) were also potentiated by BIC. Furthermore, currents induced from cones by GABA and kainate-caused depolarization of horizontal cells in retinal slice preparations were both potentiated by BIC. All these results suggest that the ionotropic GABA receptor at the bullfrog cone terminal exhibits novel pharmacology, distinct from both traditional GABA(A) and GABA(C) receptors.

Modulation by melatonin of glutamatergic synaptic transmission in the carp retina

Melatonin is involved in a variety of physiological functions through activating specific receptors coupled to GTP-binding protein. Melatonin and its receptors are abundant in the retina. Here we show for the first time that melatonin modulates glutamatergic synaptic transmission from cones to horizontal cells (HCs) in carp retina. Immunocytochemical data revealed the expression of the MT1 receptor on carp HCs. Whole-cell recordings further showed that melatonin of physiological concentrations potentiated glutamate-induced currents from isolated cone-driven HCs (H1 cells) in a dose-dependent manner, by increasing the efficacy and apparent affinity of the glutamate receptor. The effects of melatonin were reversed by luzindole, but not by K 185, indicating the involvement of the MT1 receptor. Like melatonin, methylene blue (MB), a guanylate cyclase inhibitor, also potentiated the glutamate currents, but internal infusion of cGMP suppressed them. The effects of melatonin were not observed in cGMP-filled and MB-incubated HCs. These results suggest that the melatonin effects may be mediated by decreasing the intracellular concentration of cGMP. Consistent with these observations, melatonin depolarized the membrane potential of H1 cells and reduced their light responses, which could also be blocked by luzindole. These effects of melatonin persisted in the presence of the antagonists of receptors for dopamine, GABA and glycine, indicating a direct action of melatonin on H1 cells. Such modulation by melatonin of glutamatergic transmission from cones to HCs is thought to be in part responsible for circadian changes in light responsiveness of cone HCs in teleost retina.

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.

Ca2+-permeable and Ca2+-impermeable AMPA receptors coexist on horizontal cells

Fura-2 fluorescent calcium imaging was used for analyzing the subtype of AMPA receptors in freshly dissociated horizontal cells of carp retina. Exogenous application of AMPA induced an increase of intracellular concentration of free Ca2+ ([Ca2+]i) in horizontal cells, while the [Ca2+]i increase was partly inhibited by nifedipine. The residual [Ca2+]i increase was completely eliminated by joro spider toxin-3, a blocker of Ca2+-permeable AMPA receptors. On the other hand, the application of pentobarbital, which blocked Ca2+-impermeable AMPA receptors, could also partly inhibit the increase of [Ca2+]i, implying that the application of AMPA induced the activation of both Ca2+-permeable and Ca2+-impermeable AMPA receptors and the consequent activation of voltage-gated Ca2+ channels. Taken together, these results suggested that Ca2+-permeable and Ca2+-impermeable AMPA receptors were coexpressed on horizontal cells.

Role of Ca2+ store in AMPA-triggered Ca2+ dynamics in retinal horizontal cells

Fura-2 fluorescent calcium imaging was applied to measure [Ca(2+)](i) in freshly dissociated horizontal cells of carp retina, and a model containing endoplasmic reticulum (ER) membrane processes and plasma membrane processes was constructed for quantitative analyses of the AMPA-triggered calcium dynamics. A transient increase followed by a sustained steady level of [Ca(2+)](i) was observed when 100 microM AMPA was applied, while the initial transient increase of [Ca(2+)](i) was suppressed by exogenously applied ryanodine. The model analyses results suggest that the AMPA-triggered calcium dynamics involves a number of cytoplasmic and endoplasmic processes that interact with each other. It also suggests that calcium store is an important part contributing to the transient calcium signal.

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.

Glycinergic input to carp retinal ganglion cells may be mediated by glycine receptors with homologous kinetics

Current responses of carp retinal ganglion cells (RGCs) retrogradely labeled and freshly dissociated to rapid application of glycine were recorded by whole-cell patch clamp techniques and effects of glycine antagonists on these responses were analyzed. The current response to maintained application of glycine at a concentration higher than 30 microM exhibited desensitization, which was well fitted to a monoexponential function. Strychnine (1 microM), a glycine receptor antagonist, completely blocked the response to 100 microM glycine. Strychnine at a concentration range between 10 and 200 nM suppressed the response to 100 microM glycine in a dose-dependent manner, and only a slow-activated and sustained current eventually remained in the presence of 200 nM strychnine. Power spectral density (PSD) analysis revealed no changes in the density-frequency dependence caused by strychnine. It was further shown that dissociation of strychnine from glycine receptors was rather slow. Moreover, Zn(2+) exerted similar dual action on this sustained response and the response in Ringer's: potentiating and reducing them at low and high concentrations of Zn(2+), respectively. 5,7-Dichlorokynurenic acid (DCKA, 500 microM), a selective blocker of the glycine recognition site at the NMDA receptor, partially reduced the glycine response, but without changing its kinetics. These results suggest that glycinergic input to carp ganglion cells may be mediated by strychnine-sensitive glycine receptors with homologous kinetics, and slow dissociation of strychnine from glycine receptors may partially account for the changes in glycine response kinetics occurring in the presence of strychnine.

Suppression by zinc of AMPA receptor-mediated synaptic transmission in the retina

Zinc is strikingly co-localized with glutamate-containing vesicles in the synaptic terminals of retinal photoreceptors, and it is thought to be co-released with glutamate onto postsynaptic neurons such as horizontal cells and bipolar cells. Here we examined exogenous zinc modulation of glutamate receptors on cultured retinal horizontal cells using patch-clamp recording and endogenous zinc effect on intact horizontal cells using intracellular recording techniques. Application of 3, 30, and 300 microM zinc reduced the whole cell peak current of response to 200 microM glutamate by 2, 30, and 56%, respectively. Zinc suppression of glutamate response persisted in the presence of 10 microM cyclothiazide (CTZ). Glutamate responses of outside-out patches were completely abolished by 30 microM 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), and the receptor desensitization was blocked by 30 microM CTZ, indicating that receptor target for the zinc action on horizontal cells is alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid (AMPA) receptors. Zinc decreased the amplitude of outside-out patch peak current without an effect on either its 10-90% rise time or the rate of receptor desensitization. Dose-response curves for glutamate show that zinc reduced the maximal current evoked by glutamate and increased EC(50) from 50 +/- 3 to 70 +/- 6 microM without changing the Hill coefficient. Chelation of endogenous zinc with 1 mM Ca-EDTA depolarized horizontal cells in the intact retina by 3 mV, consistent with relief of the partial glutamate receptor inhibition by zinc. Overall, the results describe a unimodal form of zinc modulation of AMPA-type glutamate receptor responses not previously described in native neuronal preparations and a novel role for endogenous zinc in modulating neurotransmission.

Presynaptic depolarization rate controls transmission at an invertebrate synapse

Second-order neurons L1-3 of the locust ocellar pathway make inhibitory synapses with each other. Although the synapses transmit graded potentials, transmission depresses rapidly and completely so that a synapse only transmits when the presynaptic terminal depolarizes rapidly. The rate at which a presynaptic neuron depolarizes determines the rate at which a postsynaptic neuron hyperpolarizes, and neurotransmitter is only released during a fixed 2 ms long period. Consequently, the amplitude of a postsynaptic potential depends on the rate rather than the amplitude of a presynaptic depolarization. Following a postsynaptic potential, a synapse recovers from depression over about a second. The synapse recovers from depression even if the presynaptic terminal is held depolarized.

A metabotropic glutamate receptor regulates transmitter release from cone presynaptic terminals in carp retinal slices

The role of group III metabotropic glutamate receptors (mGluRs) in photoreceptor-H1 horizontal cell (HC) synaptic transmission was investigated by analyzing the rate of occurrence and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in H1 HCs uncoupled by dopamine in carp retinal slices. Red light steps or the application of 100 microM cobalt reduced the sEPSC rate without affecting their peak amplitude, which is consistent with hyperpolarization or the suppression of Ca(2+) entry into cone synaptic terminals reducing vesicular transmitter release. Conversely, postsynaptic blockade of H1 HC AMPA receptors by 500 nM CNQX reduced the amplitude of sEPSCs without affecting their rate. This analysis of sEPSCs represents a novel methodology for distinguishing between presynaptic and postsynaptic sites of action. The selective agonist for group III mGluRs, l-2-amino-4-phosphonobutyrate (L-APB or L-AP4; 20 microM), reduced the sEPSC rate with a slight reduction in amplitude, which is consistent with a presynaptic action on cone synaptic terminals to reduce transmitter release. During L-APB application, recovery of sEPSC rate occurred with 500 microM (s)-2-methyl-2-amino-4-phosphonobutyrate (MAP4), a selective antagonist of group III mGluR, and with 200 microM 4-aminopyridine (4-AP), a blocker of voltage-dependent potassium channels. Whole-cell recordings from cones in the retinal slice showed no effect of L-APB on voltage-activated Ca(2+) conductance. These results suggest that the activation of group III mGluRs suppresses transmitter release from cone presynaptic terminals via a 4-AP-sensitive pathway. Negative feedback, operating via mGluR autoreceptors, may limit excessive glutamate release from cone synaptic terminals.

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.

Analysis of spontaneous EPSCs in retinal horizontal cells of the carp

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under Whole-cell voltage clamp from carp type 1 horizontal cells (H1 cells) uncoupled by dopamine in retinal slices. Red light steps, which hyperpolarise cones and reduce glutamate release, induced outward current responses accompanied by a suppression of sEPSCs. sEPSCs decayed exponentially with a mean time constant of 0.71+/-0.07 ms and had a reversal potential near 0 mV. Power spectral analysis of sEPSCs revealed a similar decay time constant. They were suppressed by a non-NMDA receptor antagonist, CNQX at 10 microM, and a relatively specific AMPA receptor antagonist, GYKI52466 at 20 microM. The presence of sEPSCs suggests that the release of glutamate from cone synaptic terminals is vesicular. The reduction in mean sEPSC frequency with red light was not accompanied by a significant change in the mean sEPSC conductance increase (482+/-59 pS), suggesting that a decrease in the vesicular release rate from cones does not alter the vesicular glutamate concentration (quantal contents). The results suggest that the spontaneous events in H1 cells were contributed by non-NMDA (possibly AMPA) type glutamate receptors modulated by the red cone input.

GABA enhances short wavelength-sensitive cone input and reduces red cone input to carp L-type horizontal cells

Light responses of cone-driven horizontal cells were recorded intracellularly in the isolated superfused carp retina and the effects of gamma-aminobutyric acid (GABA) on signals from red-sensitive (R-) and short-wavelength-sensitive (S-) cones (green cones and/or blue cones) were studied. In the presence of a bright red (694 nm) background light, which substantially suppressed signal from R-cones, the responses of L-type horizontal cells (L-HCs) to 532-nm flashes, predominantly driven by the S-cone input, were potentiated by application of GABA. In contrast, the responses of these cells to 694-nm flashes driven by the R-cone input, were suppressed, when signal from S-cones was suppressed by a bright 532-nm background light. Both the effects could be reversed by co-application of bicuculline, suggesting the involvement of GABA(A) receptors. It was unlikely that the potentiation by GABA of the S-cone driven responses of the L-HCs was mediated by actions of GABA on the cone photoreceptors. The dual action of GABA persisted in the dopamine-depleted retina, indicating no involvement of the dopaminergic interplexiform cells. We speculate that this dual action may be partially due to differential modulation by GABA of different postsynaptic mechanisms respectively mediating signal transfer from R-cones and S-cones to L-HCs.

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.

Zn2+ differentially modulates glycine receptors versus GABA receptors in isolated carp retinal third-order neurons

It was previously reported that strychnine-sensitive glycine receptors and GABAA receptors co-existed on carp retinal third-order neurons (amacrine/ganglion cells) (Li, P. and Yang, X.-L., Strong synergism between GABAA and glycine receptors on isolated carp third-order neurons. NeuroReport, 9 (1998) 2785-2789.). In this study, the effects of the divalent cation Zn2+ on these two receptors were studied in amacrine/ganglion cells acutely isolated from carp, with the use of the whole-cell patch clamp recording technique. The glycine-induced currents were steadily potentiated by Zn2+ of lower concentrations (0.1-10 microM), while being dose-dependently inhibited by Zn2+ of higher concentrations ( > 100 microM). Both the effects involved changes in apparent glycine affinity of the glycine receptor. In contrast, Zn2+ consistently suppressed the GABA(A) mediated currents of these cells, and did not show similar dual effect. The differential modulation by Zn2+ of glycine receptors versus GABA(A) receptors provides a versatility for regulating inhibitory glycinergic and GABAergic inputs converging on most amacrine/ganglion cells.

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

In horizontal cells freshly dissociated from crucian carp (Carassius auratus) retina, we examined the effects of modulators of glutamate receptor desensitization, concanavalin A, cyclothiazide, aniracetam and 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetam ide (PEPA), on responses to rapid application of glutamate and kainate, using whole-cell voltage-clamp techniques. Incubation of concanavalin A suppressed the peak response but weakly potentiated the equilibrium response of horizontal cells to glutamate. Cyclothiazide blocked glutamate-induced desensitization in a dose-dependent manner, which resulted in a steady increase of the equilibrium current. The concentration of cyclothiazide causing a half-maximal potentiation for the equilibrium response was 85 microM. Furthermore, cyclothiazide shifted the dose-response relationship of the equilibrium current to the right, but slightly suppressed the kainate-induced sustained current. These effects of concanavalin A and cyclothiazide are consistent with the supposition that glutamate receptors of carp horizontal cells may be an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-preferring subtype. In order to further characterize the AMPA receptors of horizontal cells, modulation by aniracetam and PEPA of glutamate- and kainate-induced currents was studied. Aniracetam, a preferential modulator of flop variants of AMPA receptors, considerably blocked desensitization of glutamate-induced currents, but only slightly potentiated kainate-induced currents. It was further found that PEPA, a flop-preferring allosteric modulator of AMPA receptor desensitization, slightly suppressed the peak current, while it dramatically potentiated the equilibrium current induced by glutamate in a dose-dependent manner. PEPA was much potent than aniracetam at these receptors and showed the effect on glutamate-induced desensitization even at a concentration as low as 3 microM. PEPA also potentiated non-desensitizing currents induced by kainate, but with much less extent. These modulatory effects of concanavalin A, cyclothiazide, aniracetam and PEPA on AMPA receptors in carp horizontal cells were rather similar to those obtained at AMPA receptors assembled from flop variants expressed in Xenopus oocyte and HEK cell. Consequently, we speculate that the AMPA receptor on carp horizontal cells may predominantly carry the flop splice variants.

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.