Release of endogenous glutamate from isolated cone photoreceptors of the lizard

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.

Probing neurochemical structure and function of retinal ON bipolar cells with a transgenic mouse

Retinal ON bipolar cells make up about 70% of all bipolar cells. Glutamate hyperpolarizes these cells by binding to the metabotropic glutamate receptor mGluR6, activating the G-protein G(o1), and closing an unidentified cation channel. To facilitate investigation of ON bipolar cells, we here report on the production of a transgenic mouse (Grm6-GFP) in which enhanced green fluorescent protein (EGFP), under control of mGluR6 promoter, was expressed in all and only ON bipolar cells. We used the mouse to determine density of ON bipolar cells, which in central retina was 29,600 cells/mm(2). We further sorted the fluorescent cells and created a pure ON bipolar cDNA library that was negative for photoreceptor unique genes. With this library, we determined expression of 27 genes of interest. We obtained positive transcripts for G(o) interactors: regulators of G-protein signaling (RGS), Ret-RGS1 (a variant of RGS20), RGS16, RGS7, purkinje cell protein 2 (PCP2, also called L7 or GPSM4), synembryn (RIC-8), LGN (GPSM2), RAP1GAP, and Gbeta5; cGMP modulators: guanylyl cyclase (GC) 1alpha1, GC1beta1, phosphodiesterase (PDE) 1C, and PDE9A; and channels: inwardly rectifying potassium channel Kir2.4, transient receptor potential TRPC2, and sperm-specific cation channels CatSper 2-4. The following transcripts were not found in our library: AGS3 (GPSM1), RGS10, RGS19 (GAIP), calbindin, GC1alpha2, GC1beta2, PDE5, PDE2A, amiloride-sensitive sodium channel ACCN4, and CatSper1. We then localized Kir2.4 to several cell types and showed that, in ON bipolar cells, the channel concentrates in their dendritic tips. The channels and modulators found in ON bipolar cells likely shape their light response. Additional uses of the Grm6-GFP mouse are also discussed.

Silencing acid-sensing ion channel 1a alters cone-mediated retinal function

The action of extracellular protons on retinal activity and phototransduction occurs through pH-sensitive elements, mainly membrane conductances present on the different cell types of the outer and inner nuclear layers and of the ganglion cell layer. Acid-sensing ion channels (ASICs) are depolarizing conductances that are directly activated by protons. We investigated the participation of ASIC1a, a particular isoform of ASICs, in retinal physiology in vivo using electroretinogram measurements. In situ hybridization and immunohistochemistry localized ASIC1a in the outer and inner nuclear layers (cone photoreceptors, horizontal cells, some amacrine and bipolar cells) and in the ganglion cell layer. Both the in vivo knockdown of ASIC1a by antisense oligonucleotides and the in vivo blocking of its activity by PcTx1, a specific venom peptide, were able to decrease significantly and reversibly the photopic a- and b-waves and oscillatory potentials. Our study indicates that ASIC1a is an important channel in normal retinal activity. Being present in the inner segments of cones and inner nuclear layer cells, and mainly at synaptic cleft levels, it could participate in gain adaptation to ambient light of the cone pathway, facilitating cone hyperpolarization in brightness and modulating synaptic transmission of the light-induced visual signal.

Retinal bipolar cell input mechanisms in giant danio. I. Electroretinographic analysis

Electroretinograms (ERGs) were recorded from the giant danio (Danio aequipinnatus) to study glutamatergic input mechanisms onto bipolar cells. Glutamate analogs were applied to determine which receptor types mediate synaptic transmission from rods and cones to on and off bipolar cells. Picrotoxin, strychnine, and tetrodotoxin were used to isolate the effects of the glutamate analogs to the photoreceptor-bipolar cell synapse. Under photopic conditions, the group III metabotropic glutamate receptor (mGluR) antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) only slightly reduced the b-wave, whereas the excitatory amino acid transporter (EAAT) blocker dl-threo-beta-benzyl-oxyaspartate (TBOA) removed most of it. Complete elimination of the b-wave required both antagonists. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) blocked the d-wave. Under scotopic conditions, rod and cone inputs onto on bipolar cells were studied by comparing the sensitivities of the b-wave to photopically matched green and red stimuli. The b-wave was >1 log unit more sensitive to the green than to the red stimulus under control conditions. In CPPG or l-AP4 (l-(+)-2-amino-4-phosphonobutyric acid, a group III mGluR agonist), the sensitivity of the b-wave to the green stimulus was dramatically reduced and the b-waves elicited by the 2 stimuli became nearly matched. The d-wave elicited by dim green stimuli, which presumably could be detected only by the rods, was eliminated by NBQX.

IN CONCLUSION

1) cone signals onto on bipolar cells involve mainly EAATs but also mGluRs (presumably mGluR6) to a lesser extent; 2) rods signal onto on bipolars by mainly mGluR6; 3) off bipolar cells receive signals from both photoreceptor types by AMPA/kainate receptors.

Metabotropic glutamate receptors in vertebrate retina

A striking feature in visual information processing is the fact that the primary signaling elements, the rods and the cones, are hyperpolarized and thus inhibited by light, the physiological stimulus. Light effectively shuts down neurotransmitter release by the photoreceptors onto the second-order retinal neurons. It has long been recognized that a sign-inverting synapse utilizing a specialized receptor is required to translate the inhibitory photoreceptor response into an excitatory signal suitable for transmission to the visual cortex. Although the first clues to the underlying mechanism were discovered in the 1970s, the actual receptor initiating the sign inversion in the ON bipolar cells was only identified in 1993. This receptor was found to belong to the family of metabotropic glutamate receptors (mGluRs) and is referred to as mGluR6. Subsequent studies have focused on the intracellular transduction pathway allowing mGluR6 to mediate a hyperpolarizing response to the neurotransmitter glutamate. The mGluR family of receptors comprises seven additional members, all of which are also found in retinal cells. Their function is to modulate rather than to transmit visual signals. In this brief overview, I describe the basic properties of mGluRs and summarize their roles in retinal signaling.

The dynamic characteristics of the feedback signal from horizontal cells to cones in the goldfish retina

1. The dynamic properties of the microcircuitry formed by cones and horizontal cells in the isolated goldfish retina were studied. Cones project to horizontal cells and horizontal cells feed back to cones via a relatively slow negative feedback pathway. 2. The time constant of the feedback signal in cones and of the effect this feedback signal had on the responses of second-order neurons was determined using whole-cell patch clamp and intracellular recording techniques. 3. It was found that the feedback signal in cones had a time constant of around 80 ms, whereas the time constant of the effect this feedback signal had on the second-order neurons ranged from 36 to 116 ms. This range of time constants can be accounted for by the non-linearity of the Ca(2+) current in the cones. In depolarized cones, the feedback-mediated response in second-order neurons had a similar time constant to that of the direct light response of the cone, whereas in hyperpolarized cones, the time constant of the feedback-mediated response in second-order neurons was considerably larger. 4. Further, it was shown that there was no delay in the feedback pathway. This is in contrast to what has been deduced from the response properties of second-order neurons. In one type of horizontal cell, the responses to red light were delayed relative to the responses to green light. This delay in the second-order neurons can be accounted for by the interaction of the direct light response of the medium-wavelength-sensitive cones (M-cones) with the feedback response of the M-cones received from the horizontal cells.

The open- and closed-loop gain-characteristics of the cone/horizontal cell synapse in goldfish retina

Under constant light-adapted conditions, vision seems to be rather linear. However, the processes underlying the synaptic transmission between cones and second-order neurons (bipolar cells and horizontal cells) are highly nonlinear. In this paper, the gain-characteristics of the transmission from cones to horizontal cells and from horizontal cells to cones are determined with and without negative feedback from horizontal cells to cones. It is shown that 1) the gain-characteristic from cones to horizontal cells is strongly nonlinear without feedback from horizontal cells, 2) the gain-characteristic between cones and horizontal cells becomes linear when feedback is active, and 3) horizontal cells feed back to cones via a linear mechanism. In a quantitative analysis, it will be shown that negative feedback linearizes the synaptic transmission between cones and horizontal cells. The physiological consequences are discussed.

Slice blotting: a method for detecting the release of immunoreactive substances from living brain tissue

The intricate circuitry of the CNS forms highly organized structures containing a multitude of transmitters, modulators and other chemical signals expressed in specific patterns and pathways. Anatomical studies with immunohistochemical and molecular biological techniques have mapped in fine detail the distribution of these substances in fixed tissue. However, the release of neuroactive substances is often under precise spatial control and is regulated by numerous physiological factors. Understanding such complex intercellular signaling systems will require the development of new spatially resolved methods for detecting secretion in living systems. A simple but powerful method is described here for visualizing and quantifying the time-integrated spatial pattern of release of chemical signals from living neural tissue. The method combines the in vitro brain slice preparation with immunostaining protocols used for antigen detection on Western blots. It has widespread potential application in biological research because it can map in vitro patterns of release of cytokines, growth factors, chemoattractants, chemorepellents, morphogens, enzymes, and other paracrine signals in spatially organized systems, subject to a variety of stimuli and conditions.

Imaging of glutamate release from the goldfish retinal slice

A fluorometric procedure to image release of the neurotransmitter glutamate from living retinal slices is described. Patterns of endogenous glutamate efflux were imaged with a cooled CCD camera in goldfish retinal slices as NADH fluorescence produced by a cycling glutamate dehydrogenase (GDH). Basal and potassium evoked glutamate effluxes were strongly localized to the outer and inner plexiform layers, supporting the model that photoreceptors and bipolar cells release glutamate as their prime fast neurotransmitter.

Imaging of endogenous neurotransmitter release

A method for imaging endogenous release of two common neurotransmitters from tissue slices is described. This detection employs fluorometric, enzymatic assays for glutamate and for GABA, the progress of which is directly monitored by a cooled CCD camera. The assays are highly specific for their substrate, allowing accurate determination of either glutamate or GABA release by tissues. The images captured allow visualization of the release of these transmitters, permitting a direct correlation of transmitter release with well localized regions of the tissue examined. This procedure is advantageous for studies on endogenous neurotransmission.

Postsynaptic response kinetics are controlled by a glutamate transporter at cone photoreceptors

We evaluated the role of the sodium/glutamate transporter at the synaptic terminals of cone photoreceptors in controlling postsynaptic response kinetics. The strategy was to measure the changes in horizontal cell response rate induced by blocking transporter uptake in cones with dihydrokainate (DHK). DHK was chosen as the uptake blocker because, as we show through autoradiographic uptake measurements, DHK specifically blocked uptake in cones without affecting uptake in Mueller cells. Horizontal cells depolarized from about -70 to -20 mV as the exogenous glutamate concentration was increased from approximately 1 to 40 microM, so horizontal cells can serve as "glutamate electrodes" during the light response. DHK slowed the rate of hyperpolarization of the horizontal cells in a dose-dependent way, but didn't affect the kinetics of the cone responses. At 300 microM DHK, the rate of the horizontal cell hyperpolarization was slowed to only 17 +/- 8.5% (mean +/- SD) of control. Translating this to changes in glutamate concentration using the slice dose response curve as calibration in Fig. 2, DHK reduced the rate of removal of glutamate from approximately 0.12 to 0.031 microM/s. The voltage dependence of uptake rate in the transporter alone was capable of modulating glutamate concentration: we blocked vesicular released glutamate with bathed 20 mM Mg2+ and then added 30 microM glutamate to the bath to reestablish a physiological glutamate concentration level at the synapse and thereby depolarize the horizontal cells. Under these conditions, a light flash elicited a 17-mV hyperpolarization in the horizontal cells. When we substituted kainate, which is not transported, for glutamate, horizontal cells were depolarized but light did not elicit any response, indicating that the transporter alone was responsible for the removal of glutamate under these conditions. This suggests that the transporter was both voltage dependent and robust enough to modulate glutamate concentration. The transporter must be at least as effective as diffusion in removing glutamate from the synapse because there is only a very small light response once the transporter is blocked. The transporter, via its voltage dependence on cone membrane potential, appears to contribute significantly to the control of postsynaptic response kinetics.

Reducing extracellular Cl- suppresses dihydropyridine-sensitive Ca2+ currents and synaptic transmission in amphibian photoreceptors

A reduction in extracellular chloride suppresses light-evoked currents of second-order retinal neurons (bipolar and horizontal cells) by reducing release of glutamate from photoreceptors. The underlying mechanisms responsible for this action of reduced extracellular Cl- were studied with a combination of electrophysiological recordings from single neurons in a retinal slice preparation and image analyses of intracellular Ca2+ (Fura-2) and pH [2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester] in dissociated photoreceptors. The results show that reducing extracellular Cl- suppresses a dihydropyridine (DHP)-sensitive Ca2+ current (I(Ca)) in photoreceptors. It is proposed that suppression of I(Ca) results in suppression of photoreceptor neurotransmission. The suppressive effect of low Cl- on I(Ca) is not due to antagonism by the substituting anion nor is it mediated by changes in extracellular or intracellular pH. We conclude that normal extracellular levels of Cl- are important for maintenance of the voltage-gated Ca2+ channels that support neurotransmission from photoreceptors. Several ideas are presented about the mechanisms by which Cl- supports photoreceptor neurotransmission and the possibility that modulations of Cl- might play a physiological role in the regulation of Ca2+ channels in photoreceptors and, hence, photoreceptor function.

Clearance of neurotransmitter from the cone synaptic cleft in goldfish retina

Dendrites of bipolar and horizontal cells protrude deeply into the synaptic terminals of cones in goldfish retina. This arrangement gives the impression that the cone synaptic terminal surrounds a morphologically shielded compartment, the cone synaptic cleft, from which clearance of neurotransmitter by diffusion is limited. In this study the time constant of this clearance has been approached in two ways: (1) the morphological parameters determining the clearance (extracellular synaptic volume and leak area), were estimated using morphometric methods. These data were introduced into a diffusion model of the cone pedicle, yielding a time constant for the clearance of < 1 msec; (2) the time constant of the light onset response when the glutamate transporter in the cone was blocked with DL-threo-beta-hydroxyaspartate or dihydrokainate, was interpreted as the time constant of the clearance, yielding values of almost 650 msec compared to around 90 msec in control conditions. The decay time-constant of the Ca-dependent tail-currents in cones was used, as an approximation of the dynamics of the intracellular Ca-concentration and thus of the glutamate release by the cones. The decay time constant was about 800 msec. This suggests that the intracellular Ca-concentration in the synaptic terminal and hence the glutamate release by the cones drops with a similar large time constant. These results indicate that the cone pedicle in goldfish does not limit the clearance of neurotransmitters from the synaptic cleft and that the fast light onset response of horizontal cells under control conditions is due to activation of glutamate transporters by hyperpolarization of the cone membrane potential while the glutamate release drops slowly. The slow horizontal cell light onset response in beta-hydroxyaspartate or dihydrokainate may be due to a slow reduction of the glutamate release by the cones at light onset.

Glutamate responses of bipolar cells in a slice preparation of the rat retina

Whole-cell currents from >70 voltage-clamped bipolar cells were recorded in a slice preparation of the rat retina. The recorded cells were identified and classified by intracellular staining with Lucifer yellow. Glutamate, the specific agonists (+/-)-2-amino-4-phosphonobutyric acid (AP-4) and kainate (KA), and the antagonist 6-cyanoquinoxaline-2,3-dione (CNQX) were applied. The cells could be isolated from presynaptic influences by the co-application of bicuculline, strychnine, and cobalt ions. Responses to AP-4 were elicited only from bipolar cells with axons stratifying in the inner part of the inner plexiform layer (IPL). AP-4 caused an outward current in these cells attributable to the closure of nonspecific cation channels. Responses to kainate representing a direct action of the drug on the recorded cells were observed only in bipolar cells with axons stratifying in the outer part of the IPL. KA caused a CNQX-sensitive inward current in these cells, associated with openings of nonspecific cation channels. The results predict that cone bipolar (CB) cells with axons terminating in the outer IPL are OFF-bipolars, whereas those with axons terminating in the inner IPL are ON-bipolars. Most of the cells expressed GABA-gated Cl- conductances. In rod bipolar and in some CB cells, only part of the GABA-induced currents could be blocked by the application of bicuculline, suggesting the presence of GABAc receptors in addition to GABAA receptors.

Cone photoreceptors respond to their own glutamate release in the tiger salamander

Pulse-like currents resembling miniature postsynaptic currents were recorded in patch-clamped isolated cones from the tiger salamander retina. The events were absent in isolated cones without synaptic terminals. The frequency of events was increased by either raising the osmotic pressure or depolarizing the cell. It was decreased by the application of either glutamate or the glutamate-transport blockers dihydrokainate and D,L-threo-3-hydroxyaspartate. The events required external Na+ for which Li+ could not substitute. The reversal potential of these currents followed the equilibrium potential for Cl- when internal Cl- concentration was changed. Thus, these miniature currents appear to represent the presynaptic activation of the glutamate receptor with glutamate transporter-like pharmacology, caused by the photoreceptor's own vesicular glutamate release. Using a noninvasive method to preserve the intracellular Cl- concentration, we showed that glutamate elicits an outward current in isolated cones. Fluorescence of the membrane-permeable form of fura-2 was used to monitor Ca2+ entry at the cone terminal as a measure of membrane depolarization. The increase in intracellular Ca2+ concentration, elicited by puff application of 30 mM KCl, was completely suppressed in the presence of 100 microM glutamate. Puff application of glutamate alone had no measurable depolarizing effect. These results suggest that the equilibrium potential for Cl-, ECl, was more negative than the activation range for Ca2+ channels and that glutamate elicited an outward current, hyperpolarizing the cones.

Pharmacology of selective and non-selective metabotropic glutamate receptor agonists at L-AP4 receptors in retinal ON bipolar cells

Retinal ON bipolar cells possess metabotropic glutamate receptors (mGluRs) which are sensitive to L-2-amino-4-phosphonobutyric acid (L-AP4). Recent studies suggest there are multiple subtypes of L-AP4 receptors. In order to provide a more complete description of the pharmacology of the retinal L-AP4 receptor, we examined the actions of a number of compounds which are active at L-AP4 receptors and other mGluRs. Four groups of compounds were studied: (1) AP4 analogues (e.g. L-AP5, L-SOP, cyclobutylene AP5, and N-Me-AP4), (2) non-selective mGluR agonists (ibotenate and quisqualate), (3) selective mGluR agonists (L-CCG-I), and (4) agonists proposed to be selective for specific mGluR subtypes (DCG-IV and t-ADA). Concentration-response curves were obtained using the b-wave of the electroretinogram (ERG) as an assay for L-AP4 receptor activation. Whole cell voltage clamp recordings from ON bipolar cells in the retinal slice preparation of the mudpuppy were used to determine whether the compounds acted as L-AP4 receptor agonists. All compounds were L-AP4 receptor agonists, except t-ADA which was ineffective. The results reveal pharmacological differences between L-AP4 receptors in mudpuppy ON bipolar cells and those in other systems, consistent with the proposal that there are multiple L-AP4 receptor subtypes. For example, retinal L-AP4 receptors are more potently activated by L-AP5 than L-SOP, whereas L-SOP has been shown to be more potent than L-AP5 in L-AP4 receptors in the lateral perforant path (LPP) of the rat hippocampus. L-SOP is also relatively more potent at the cloned L-AP4 receptors mGluR4, 6, and 7 than in mudpuppy ON bipolar cells in situ. The different potencies of these compounds in retina and LPP is ascribed to both steric and charge factors. The results with DCG-IV and t-ADA are consistent with the proposal that these are subtype-selective agonists, but DCG-IV is likely to be selective only at very low concentrations (< or = 1 microM).

Activation of glutamate receptors in response to membrane depolarization of hair cells isolated from chick cochlea

1. Experiments were performed to identify the neurotransmitter released from hair cells of chick cochlea. An isolated hair cell was closely apposed to a cultured granule cell of the rat cerebellum, and both cells were whole-cell voltage clamped by utilizing a nystatin perforated patch technique. 2. Depolarization of hair cells to potentials more positive than -20 mV induced currents in the granule cell in a 10 mM Ca2+ extracellular medium. Amplitudes of induced currents were dependent on the membrane potential of granule cells and showed an outward-going rectification. The induced current in granule cells was reversibly suppressed by a local application of 2-amino-5-phosphonovalerate (APV), which indicates that the current was generated through the activation of an NMDA subtype of the glutamate receptor expressed on the granule cell. 3. The current amplitude of the granule cell was dependent on the size of hair cell depolarization. The size of current induced in a granule cell held at +55 mV was progressively increased with hair cell depolarization from -20 to +10 mV. At more positive potentials, the current amplitude was decreased. This voltage dependence was similar to but did not exactly match that of Ca2+ currents in the hair cell. The granule cell current appeared at more positive membrane potentials than the Ca2+ current in hair cells. 4. When intracellular Ca2+ concentration was increased by UV irradiation of the hair cell loaded with a caged Ca2+ compound, nitr-5, the closely apposed granule cell generated an outward current when voltage clamped at +55 mV. 5. These observations (paragraphs 2-4) imply that the most likely neurotransmitter released from the hair cell at its synapse with the afferent nerve terminal is glutamate.