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

Phosphodiesterase Diversity and Signal Processing Within cAMP Signaling Networks

A large number of neuromodulators activate G-protein coupled receptors (GPCRs) and mediate their cellular actions via the regulation of intracellular cAMP, the small highly diffusible second messenger. In fact, in the same neuron several different GPCRs can regulate cAMP with seemingly identical timecourses that give rise to distinct signaling outcomes, suggesting that cAMP does not have equivalent access to all its downstream effectors and may exist within defined intracellular pools or domains. cAMP compartmentalization is the process that allows the neuron to differentially interpret these various intracellular cAMP signals into cellular response. The molecular mechanisms that give rise to cAMP compartmentalization are not fully understood, but it is thought that phosphodiesterases (PDEs), the enzymes that degrade cAMP, significantly contribute to this process. PDEs, as the sole mechanism of signal termination for cAMP, hold great promise as therapeutic targets for pathologies that are due to the dysregulation of intracellular cAMP signaling. Due to their diverse catalytic activity, regulation and localization each PDE subtype expressed in a given neuron may have a distinct role on downstream signaling.

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.

Upregulation of glutamate transporter GLT-1 by mTOR-Akt-NF-кB cascade in astrocytic oxygen-glucose deprivation

Excessive extracellular glutamate leads to neuronal death in central nervous system. Excitatory glutamate transporter subtype 2 (GLT-1) carries bulk of glutamate reuptake in cerebral ischemia. Although GLT-1 expression fluctuates during the period of ischemia, little is known about its regulatory mechanism. Here we show an up-regulation of GLT-1 via mammalian target of rapamycin (mTOR)-Akt-nuclear factor-кB (NF-кB) signaling cascade in oxygen glucose deprivation (OGD). We found that brief rapamycin treatment significantly increased GLT-1 expression in cultured astrocytes. Rapamycin increased phosphorylation of raptor at Ser792 and decreased phosphorylation of rictor at Thr1135, suggesting that both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) are involved in GLT-1 expression. This conclusion was further confirmed by raptor and rictor disruption experiments. Akt was activated by mTORC1 inhibition and required for GLT-1 expression because triciribine, a specific inhibitor of Akt, blocked the increase of GLT-1 expression. mTOR-Akt cascade then activated NF-кB and increased кB-motif-binding phosphoprotein (KBBP) expression and GLT-1 transcription. We next demonstrated that mTOR-Akt-NF-кB cascade was activated in OGD and subsequently caused the upregulation of GLT-1. Supporting evidence included: (1) inhibition of Akt or NF-кB occluded OGD-induced GLT-1 upregulation; (2) Raptor knock-down plus OGD did not add to the increase of GLT-1 expression; (3) Intact mTORC2 was required for GLT-1 enhancement. In summary, our data first showed that mTOR-Akt-NF-кB cascade played critical roles to up-regulate GLT-1 in OGD. This signaling cascade may work to promote glutamate uptake in brain ischemia and neurodegenerative diseases.

Chronic zinc exposure decreases the surface expression of NR2A-containing NMDA receptors in cultured hippocampal neurons

BACKGROUND

Zinc distributes widely in the central nervous system, especially in the hippocampus, amygdala and cortex. The dynamic balance of zinc is critical for neuronal functions. Zinc modulates the activity of N-methyl-D-aspartate receptors (NMDARs) through the direct inhibition and various intracellular signaling pathways. Abnormal NMDAR activities have been implicated in the aetiology of many brain diseases. Sustained zinc accumulation in the extracellular fluid is known to link to pathological conditions. However, the mechanism linking this chronic zinc exposure and NMDAR dysfunction is poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We reported that chronic zinc exposure reduced the numbers of NR1 and NR2A clusters in cultured hippocampal pyramidal neurons. Whole-cell and synaptic NR2A-mediated currents also decreased. By contrast, zinc did not affect NR2B, suggesting that chronic zinc exposure specifically influences NR2A-containg NMDARs. Surface biotinylation indicated that zinc exposure attenuated the membrane expression of NR1 and NR2A, which might arise from to the dissociation of the NR2A-PSD-95-Src complex.

CONCLUSIONS

Chronic zinc exposure perturbs the interaction of NR2A to PSD-95 and causes the disorder of NMDARs in hippocampal neurons, suggesting a novel action of zinc distinct from its acute effects on NMDAR activity.

Insulin increases glutamate transporter GLT1 in cultured astrocytes

The astroglial cell-specific glutamate transporter subtype 2 (excitatory amino acid transporter 2, GLT1) plays an important role in excitotoxicity that develops after damage to the central nervous system (CNS) is incurred. Both the protein kinase C signaling pathway and the epidermal growth factor (EGF) pathway have been suggested to participate in the modulation of GLT1, but the modulatory mechanisms of GLT1 expression are not fully understood. In the present study, we aimed to evaluate the effects of insulin on GLT1 expression. We found that short-term stimulation of insulin led to the upregulation of both total and surface expressions of GLT1. Akt phosphorylation increased after insulin treatment, and triciribine, the inhibitor of Akt phosphorylation, significantly inhibited the effects of insulin. We also found that the upregulation of GLT1 expression correlated with increased kappa B motif-binding phosphoprotein (KBBP) and GLT1 mRNA levels. Our results suggest that insulin may modulate the expression of astrocytic GLT1, which might play a role in reactive astrocytes after CNS injuries.

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.

Ionotropic glutamate receptors of amacrine cells of the mouse retina

The mammalian retina contains approximately 30 different morphological types of amacrine cells, receiving glutamatergic input from bipolar cells. In this study, we combined electrophysiological and pharmacological techniques in order to study the glutamate receptors expressed by different types of amacrine cells. Whole-cell currents were recorded from amacrine cells in vertical slices of the mouse retina. During the recordings the cells were filled with Lucifer Yellow/Neurobiotin allowing classification as wide-field or narrow-field amacrine cells. Amacrine cell recordings were also carried out in a transgenic mouse line whose glycinergic amacrine cells express enhanced green fluorescent protein (EGFP). Agonist-induced currents were elicited by exogenous application of NMDA, AMPA, and kainate (KA) while holding cells at −75 mV. Using a variety of specific agonists and antagonists (NBQX, AP5, cyclothiazide, GYKI 52466, GYKI 53655, SYM 2081) responses mediated by AMPA, KA, and NMDA receptors could be dissected. All cells (n= 300) showed prominent responses to non-NMDA agonists. Some cells expressed AMPA receptors exclusively and some cells expressed KA receptors exclusively. In the majority of cells both receptor types could be identified. NMDA receptors were observed in about 75% of the wide-field amacrine cells and in less than half of the narrow-field amacrine cells. Our results confirm that different amacrine cell types express distinct sets of ionotropic glutamate receptors, which may be critical in conferring their unique temporal responses to this diverse neuronal class.

C-type natriuretic peptide modulates glutamate receptors on cultured rat retinal amacrine cells

C-type natriuretic peptide, widely distributed in the CNS, may work as a neuromodulator. In this work, we investigated modulation by C-type natriuretic peptide of functional properties of glutamate receptors in rat retinal GABAergic amacrine cells in culture. Immunocytochemical data revealed that natriuretic peptide receptor-B was strongly expressed on the membrane of cultured GABAergic amacrine cells. By whole cell recording techniques we further identified the glutamate receptor expressed on the GABAergic amacrine cells as an AMPA-preferring subtype. Incubation with C-type natriuretic peptide suppressed the AMPA receptor-mediated current of these cells in a dose-dependent manner by decreasing the efficacy and apparent affinity for glutamate. The effect of C-type natriuretic peptide was reversed by HS-142-1, a guanylyl cyclase-coupled natriuretic peptide receptor-A/B antagonist. Meanwhile, the selective natriuretic peptide receptor-C agonist cANF did not change the glutamate current. In conjunction with the immunocytochemical data, these results suggest that the C-type natriuretic peptide effect may be mediated by natriuretic peptide receptor-B. Furthermore, incubation of retinal cultures in the C-type natriuretic peptide-containing medium elevated cGMP immunoreactivity in the GABAergic amacrine cells, and the C-type natriuretic peptide effect on the glutamate current was mimicked by application of 8-Br-cGMP. It is therefore concluded that C-type natriuretic peptide may modulate the glutamate current by increasing the intracellular concentration of cGMP in these cells via activation of natriuretic peptide receptor-B.

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.

AMPA receptor is involved in transmission of cone signal to ON bipolar cells in carp retina

The present work focuses on characterization of glutamate receptor subtypes mediating cone signal transmission to ON bipolar cells (BCs) in the carp retina, using intracellular recording techniques. Glutamate (5 mM) hyperpolarized cone-dominant ON BCs, which was associated with a suppression of light responses, whereas Co(2+) (1 mM) depolarized these cells and suppressed their light responses. On the other hand, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) of 20 microM caused a membrane depolarization and blocked depolarizing light responses, L-2-amino-4-phosphonobutryic acid (l-AP4) was without effect. The effects of AMPA were reversed by coapplication of GYKI 52466, an AMPA receptor selective non-competitive antagonist, but persisted in the presence of picrotoxin and strychnine. For rod-dominant ON BCs, both l-AP4 and AMPA reversibly blocked depolarizing light responses, but with membrane potential changes of opposite polarities (hyperpolarization for l-AP4 and depolarization for AMPA). In the inner retina, AMPA depolarized transient ON-OFF amacrine cells and blocked both ON and OFF cone-driven depolarizing responses, but l-AP4 did not. These results suggest that AMPA receptors, but not l-AP4 receptors, are involved in synaptic transmission of cone signal to ON bipolar cells in carp retina.

AMPA receptors mediate acetylcholine release from starburst amacrine cells in the rabbit retina

The light response of starburst amacrine cells is initiated by glutamate released from bipolar cells. To identify the receptors that mediate this response, we used a combination of anatomical and physiological techniques. An in vivo, rabbit eyecup was preloaded with [(3)H]-choline, and the [(3)H]-acetylcholine (ACh) released into the superfusate was monitored. A photopic, 3 Hz flashing light increased ACh release, and the selective AMPA receptor antagonist, GYKI 53655, blocked this light-evoked response. Nonselective AMPA/kainate agonists increased the release of ACh, but the specific kainate receptor agonist, SYM 2081, did not increase ACh release. Selective AMPA receptor antagonists, GYKI 53655 or GYKI 52466, also blocked the responses to agonists. We conclude that the predominant excitatory input to starburst amacrine cells is mediated by AMPA receptors. We also labeled lightly fixed rabbit retinas with antisera to choline acetyltransferase (ChAT), AMPA receptor subunits GluR1, GluR2/3, or GluR4, and kainate receptor subunits GluR6/7 and KA2. Labeled puncta were observed in the inner plexiform layer with each of these antisera to glutamate receptors, but only GluR2/3-IR puncta and GluR4-IR puncta were found on the ChAT-IR processes. The same was true of starburst cells injected intracellularly with Neurobiotin, and these AMPA receptor subunits were localized to two populations of puncta. The AMPA receptors are expected to desensitize rapidly, enhancing the sensitivity of starburst amacrine cells to moving or other rapidly changing stimuli.

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.

Phosphorylation of GluR4 AMPA-type glutamate receptor subunit by protein kinase C in cultured retina amacrine neurons

We have previously reported that the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors is potentiated by protein kinase C (PKC) in cultured chick retina amacrine neurons, and that constitutive PKC activity is necessary for basal AMPA receptor activity (Carvalho et al., 1998). In this study, we evaluated the phosphorylation of the GluR4 subunit, which is very abundant in cultured amacrine neurons, to correlate it with the effects of PKC on AMPA receptor activity in these cells. 32P-labelling of GluR4 increased upon AMPA receptor stimulation or cell treatment with phorbol 12-myristate 13-acetate (PMA) before stimulating with kainate. By contrast, phosphorylation of GluR4 was not changed when PKC was inhibited by treating the cells with the selective PKC inhibitor GF 109203X before stimulation with kainate. We conclude that GluR4 is phosphorylated upon PKC activation and/or stimulation of AMPA receptors in cultured amacrine cells. Additionally, AMPA receptor activation with kainate in cultured chick amacrine cells leads to translocation of conventional and novel PKC isoforms to the cell membrane, suggesting that PKC could be activated upon AMPA receptor stimulation in these cells.

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.