Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

Visual deprivation alters development of synaptic function in inner retina after eye opening

Visual deprivation impedes refinement of neuronal function in higher visual centers of mammals. It is often assumed that visual deprivation has minimal effect, if any, on neuronal function in retina. Here we report that dark rearing reduces the light-evoked responsiveness of inner retinal neurons in young mice. We also find that 1 to 2 weeks after eye opening, there is a surge (>4-fold) in the frequency of spontaneous excitatory and inhibitory synaptic events in ganglion cells. Dark rearing reversibly suppresses this surge, but recovery takes >6 days. Frequency changes are not accompanied by amplitude changes, indicating that synaptic reorganization is likely to be presynaptic. These findings indicate there is a degree of activity-dependent plasticity in the mammalian retina that has not been previously described.

BDNF-Induced potentiation of spontaneous twitching in innervated myocytes requires calcium release from intracellular stores

Brain-derived neurotrophic factor (BDNF) can potentiate synaptic release at newly developed frog neuromuscular junctions. Although this potentiation depends on extracellular Ca(2+) and reflects changes in acetylcholine release, little is known about the intracellular transduction or calcium signaling pathways. We have developed a video assay for neurotrophin-induced potentiation of myocyte twitching as a measure of potentiation of synaptic activity. We use this assay to show that BDNF-induced synaptic potentiation is not blocked by cadmium, indicating that Ca(2+) influx through voltage-gated Ca(2+) channels is not required. TrkB autophosphorylation is not blocked in Ca(2+)-free conditions, indicating that TrkB activity is not Ca(2+) dependent. Additionally, an inhibitor of phospholipase C interferes with BDNF-induced potentiation. These results suggest that activation of the TrkB receptor activates phospholipase C to initiate intracellular Ca(2+) release from stores which subsequently potentiates transmitter release.

Inhibition is not required for the production of transient spiking responses from retinal ganglion cells

Ganglion cells responding only transiently to changes in illumination are found in many different vertebrate retinas. The interactions underlying formation of these transient responses are still poorly understood. Two recently proposed hypotheses are (1) functional inhibitory pathways are necessary for transient response production, and (2) direct inhibition of the ganglion cell has little effect on its output. Here, we examine these conclusions by using cell-attached patch-clamp recordings of spiking, whole-cell recordings of synaptic currents, and computer modeling. We found that picrotoxin (a GABA(A) and GABA(C) receptor antagonist), bicuculline (a GABA(A) receptor antagonist), and strychnine (a glycine receptor antagonist), applied either singly or in combination, always failed to convert transient responses to sustained responses. Application of the GABA(B) antagonist CGP35348 in the presence of picrotoxin and strychnine also failed to convert transient responses into sustained responses. Whole-cell recordings of synaptic currents at various holding potentials indicated that direct inhibitory inputs to ganglion cells limit the duration of net excitation, implying that direct inhibition does act to truncate the ganglion cell spiking response. Computer simulations using spiking and synaptic data from combined cell-attached and whole-cell recordings supported this interpretation. We conclude that inhibitory pathways are not required for generation of transient responses, but these pathways do serve to modulate transient ganglion cell spiking responses. We find that this modulation occurs, in part, via inhibitory inputs directly to the ganglion cell.

Localization and developmental expression patterns of the neuronal K-Cl cotransporter (KCC2) in the rat retina

The processing of signals by integrative neurons in the retina and CNS relies strongly on inhibitory synaptic inputs, principally from GABAergic and glycinergic neurons that serve primarily to hyperpolarize postsynaptic neurons. Recent evidence indicates that the neuron-specific K-Cl cotransporter 2 (KCC2) is the major chloride extrusion system permitting hyperpolarizing inhibitory responses. It has been hypothesized that depolarizing GABA responses observed in immature neurons are converted to hyperpolarizing responses in large part by the expression of KCC2 during the second week of postnatal development. The cell-specific localization and developmental expression of KCC2 protein have been examined in relatively few neural tissues and have never been studied in retina, of which much is known physiologically and morphologically about inhibitory synaptic circuits. We examined the localization of KCC2 in adult rat retina with immunohistochemical techniques and determined the time course of its postnatal expression. KCC2 expression was localized in horizontal cells, bipolar cells, amacrine cells, and, most likely, ganglion cells, all of which are known to express GABA receptor subtypes. Developmentally, KCC2 expression in the retina increased gradually from postnatal day 1 (P1) until P14 in the inner retina, whereas expression was delayed in the outer plexiform layer until P7 but reached its adult level by P14. These data support the hypothesis that the function of KCC2 is intimately involved in GABAergic synaptic processing. Furthermore, the delayed temporal expression of KCC2 in the outer plexiform layer indicates that GABAergic function may be differentially regulated in retina during postnatal development and that GABA may produce depolarizing responses in the outer plexiform layer at times when it generates hyperpolarizing responses in the inner plexiform layer.

Molecular analysis of system N suggests novel physiological roles in nitrogen metabolism and synaptic transmission

The amino acid glutamine has a central role in nitrogen metabolism. Although the molecular mechanisms responsible for its transport across cell membranes remain poorly understood, classical amino acid transport system N appears particularly important. Using intracellular pH measurements, we have now identified an orphan protein related to a vesicular neurotransmitter transporter as system N. Functional analysis shows that this protein (SN1) involves H+ exchange as well as Na+ cotransport and, under physiological conditions, mediates glutamine efflux as well as uptake. Together with the pattern of SN1 expression, these unusual properties suggest novel physiological roles for system N in nitrogen metabolism and synaptic transmission.

Automatic detection, characterization, and discrimination of kinetically distinct spontaneous synaptic events

Rapid and reliable detection of randomly occurring small amplitude synaptic events resulting from activation of different classes of ligand-gated receptors is a difficult task. Here, we describe and characterize an amplitude threshold algorithm, written as an IGOR Pro procedure, which detects events as well as characterizes their amplitudes and kinetics. The program was developed to analyze recording traces that each contained both excitatory (glutamate-mediated) and inhibitory (GABA and glycine-mediated) events. By using differences in kinetics, the program could discriminate between the two different classes of events. In summary, the program has the following strengths: (1) it is generally applicable to circumstances in which different populations of elementary events occur concurrently, a drawback of methods that employ matched filtering techniques, (2) it is relatively insensitive to drifts in baseline, and (3) it generates user-accessible arrays of the timing, amplitude and kinetic parameters of the detected events, making customized statistical analysis of event characteristics easily executable.

Caffeine-sensitive calcium stores regulate synaptic transmission from retinal rod photoreceptors

We investigated the role of caffeine-sensitive intracellular stores in regulating intracellular calcium ([Ca(2+)](i)) and glutamatergic synaptic transmission from rod photoreceptors. Caffeine transiently elevated and then markedly depressed [Ca(2+)](i) to below prestimulus levels in rod inner segments and synaptic terminals. Concomitant with the depression was a reduction of glutamate release and a hyperpolarization of horizontal cells, neurons postsynaptic to rods. Caffeine did not affect the rods' membrane potentials indicating that caffeine likely acted via some mechanism(s) other than a voltage-dependent deactivation of the calcium channels. Most of caffeine's depressive action on [Ca(2+)](i), on glutamate release, and on I(Ca) in rods can be attributed to calcium release from stores: (1) caffeine's actions on [Ca(2+)](i) and I(Ca) were reduced by intracellular BAPTA and barium substitution for calcium, (2) other nonxanthine store-releasing compounds, such as thymol and chlorocresol, also depressed [Ca(2+)](i), and (3) the magnitude of [Ca(2+)](i) depression depended on basal [Ca(2+)](i) before caffeine. We propose that caffeine-released calcium reduces I(Ca) in rods by an as yet unidentified intracellular signaling mechanism. To account for the depression of [Ca(2+)](i) below rest levels and the increased fall rate of [Ca(2+)](i) with higher basal calcium, we also propose that caffeine-evoked calcium release from stores activates a calcium transporter that, via sequestration into stores or extrusion, lowers [Ca(2+)](i) and suppresses glutamate release. The effects of store-released calcium reported here operate at physiological calcium concentrations, supporting a role in regulating synaptic signaling in vivo.

Sodium action potentials are not required for light-evoked release of GABA or glycine from retinal amacrine cells

Although most CNS neurons require sodium action potentials (Na-APs) for normal stimulus-evoked release of classical neurotransmitters, many types of retinal and other sensory neurons instead use only graded potentials for neurotransmitter release. The physiological properties and information processing capacity of Na-AP-producing neurons appear significantly different from those of graded potential neurons. To classify amacrine cells in this dichotomy, we investigated whether Na-APs, which are often observed in these cells, are required for functional light-evoked release of inhibitory neurotransmitters from these cells. We recorded light-evoked inhibitory postsynaptic currents (IPSCs) from retinal ganglion cells, neurons directly postsynaptic to amacrine cells, and applied TTX to block Na-APs. In control solution, TTX application always led to partial suppression of the light-evoked IPSC. To isolate release from glycinergic amacrine cells, we used either bicuculline, a GABAA receptor antagonist, or picrotoxin, a GABAA and GABAC receptor antagonist. TTX application only partially suppressed the glycinergic IPSC. To isolate release from GABAergic amacrine cells, we used the glycine receptor blocker strychnine. TTX application only partially suppressed the light-evoked GABAergic IPSC. Glycinergic and GABAergic amacrine cells did not obviously differ in the usage of Na-APs for release. These observations, in conjunction with previous studies of other retinal neurons, indicate that amacrine cells, taken as a class, are the only type of retinal neuron that uses both Na-AP-dependent and -independent modes for light-evoked release of neurotransmitters. These results also provide evidence for another parallel between the properties of retinal amacrine cells and olfactory bulb granule cells.

Analysis of excitatory and inhibitory spontaneous synaptic activity in mouse retinal ganglion cells

Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) were identified and characterized with whole cell and perforated patch voltage-clamp recordings in adult mouse retinal ganglion cells. Pharmacological dissection revealed that all cells were driven by spontaneous synaptic inputs mediated by glutamate and gamma-aminobutyric acid-A (GABAA) receptors. One-half (7/14) of the cells also received glycinergic spontaneous synaptic inputs. Both GABAA and glycine receptor-mediated sIPSCs had rise times (10-90%) of < 1 ms. The decay times of the GABAA receptor-mediated sIPSCs were comparable with those of the glycine receptor-mediated sIPSCs. The average decay time constant for monoexponentially fitted sIPSCs was 63.2 +/- 74.1 ms (mean +/- SD, n = 3278). Glutamate receptor-mediated sEPSCs had an average rise time of 0.50 +/- 0.20 ms (n = 109) and an average monoexponential decay time constant of 5.9 +/- 8.6 ms (n = 2705). Slightly more than two-thirds of the spontaneous synaptic events were monoexponential (68% for sIPSCs and 76% for sEPSCs). The remainder of the events was biexponential. The amplitudes of the spontaneous synaptic events were not correlated with rise times, suggesting that the electrotonic filtering properties of the neurons and/or differences in the spatial location of synaptic inputs could not account for the difference between the decay time constants of the glutamate and GABAA/glycine receptor-mediated spontaneous synaptic events. The amplitudes of sEPSCs were similar to those recorded in tetrodotoxin (TTX), consistent with the events measured in control saline being the response to the release of a single quantum of transmitter. The range of the sIPSC amplitudes in control saline was wider than that recorded in TTX, consistent with some sIPSCs being evoked by presynaptic spikes having an average quantal size greater than one. The rates of sIPSCs and sEPSCs were determined under equivalent conditions by recording with perforated patch electrodes at potentials at which both types of event could be identified. Two groups of ganglion cell were observed; one group had an average sEPSCs/sIPSCs frequency ratio of 0.96 +/- 0.77 (n = 28) and another group had an average ratio of 6.63 +/- 0.82 (n = 7). These findings suggest that a subset of cells is driven much more strongly by excitatory synaptic inputs. We propose that this subset of cells could be OFF ganglion cells, consistent with the higher frequency of spontaneous action potentials found in OFF ganglion cells in other studies.

Compartmentalization of calcium extrusion mechanisms in the outer and inner segments of photoreceptors

Differential localization of calcium channel subtypes in divergent regions of individual neurons strongly suggests that calcium signaling and regulation could be compartmentalized. Region-specific expression of calcium extrusion transporters would serve also to partition calcium regulation within single cells. Little is known about selective localization of the calcium extrusion transporters, nor has compartmentalized calcium regulation within single neurons been studied in detail. Sensory neurons provide an experimentally tractable preparation to investigate this functional compartmentalization. We studied calcium regulation in the outer segment (OS) and inner segment/synaptic terminal (IS/ST) regions of rods and cones. We report these areas can function as separate compartments. Moreover, ionic, pharmacological, and immunolocalization results show that a Ca-ATPase, but not the Na+/K+, Ca2+ exchanger found in the OSs, extrudes calcium from the IS/ST region. The compartmentalization of calcium regulation in the photoreceptor outer and inner segments implies that transduction and synaptic signaling can be independently controlled. Similar separation of calcium-dependent functions is likely to apply in many types of neuron.

Metabotropic glutamate receptor-mediated suppression of an inward rectifier current is linked via a cGMP cascade

Glutamate, the neurotransmitter released by photoreceptors, excites horizontal cells and OFF-type bipolar cells by activating ionotropic receptors. This study investigated an additional action of glutamate in which it modulates a voltage-gated ion channel in horizontal cells. We find that glutamate and APB (2-amino-4-phosphonobutyrate) produce a delayed and moderately prolonged suppression of an inward rectifier current (IRK+). This effect is proposed to occur via an APB-sensitive metabotropic glutamate receptor (mGluR) because common agonists for the ionotropic or APB-insensitive mGluRs are ineffective and the APB-insensitive receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) does not block the actions of glutamate or APB. 8-Br-cGMP, 1-methyl-3-isobutylxanthine (IBMX), and atrial natriuretic peptide (ANP) but not 8-Br-cAMP mimic the suppression of IRK+. The effects of glutamate and APB are blocked by protein kinase inhibitors including Rp-8-pCPT-cGMPS, H-8, and H-7 as well as by ATPgammaS. We hypothesize that the APB receptor suppresses IRK+ via upregulation of cGMP and subsequent activation of a cGMP-dependent protein kinase. This pathway is likely regulated by an ATP-dependent phosphorylation. This is a novel signaling pathway for mGluRs and indicates that at least two distinct APB-activated pathways exist in the retina. Functionally, this APB receptor-mediated action found in horizontal cells would provide a means by which spatially restricted changes of glutamate, produced by local illumination of photoreceptors, could regulate IRK+ and consequently the response properties of these neurons. This would serve to adapt selectively retinal regions stimulated by small regions of the visual world.

Quinine, intracellular pH and modulation of hemi-gap junctions in catfish horizontal cells

Quinine increases the conductance of hemi-gap junctions in horizontal cells. We investigated the mechanisms of alkalinization and the hypothesis that quinine-induced alkalinization produced these conductance increases. We found that quinine-induced alkalinizations were not blocked by cobalt, amiloride, or DIDS. Therefore, this suggests that the alkalinization is not likely due to net proton flux through opened hemi-gap channels nor is it likely due to an action on Cl-/HCO3- exchanger or Na+/H+ exchanger, both of which are known to regulate pHi in the horizontal cells. Quinine increased hemi-gap conductance even when cells were recorded with patch pipets containing up to 80 mM HEPES. We conclude that quinine-induced alkalinization cannot account solely for the hemi-gap junctional conductance increases.

Retinal development: on the crest of an exciting wave

Propagated waves of excitation in developing neural tissues may be a critical feature of maturation. Recent findings shed new light on the mechanisms underlying these waves.

Passive electrical cable properties and synaptic excitation of tiger salamander retinal ganglion cells

The passive electrical properties of 17 ON-OFF retinal ganglion cells were derived from electrophysiological recordings. The parameters for each cells' equivalent model were obtained from the transient current responses to small step changes in clamp potential. Thirteen of the cells could be adequately approximated by a spherical soma connected to an equivalent dendritic cable. Estimates for the cell input conductance (GN), membrane time constant (tau m), the dendritic-to-soma conductance ratio (rho), and the normalized electrotonic length (L) were obtained (mean +/- standard deviation, n = 13): GN = 580 +/- 530 pS, tau m = 97 +/- 72 ms, rho = 2.8 +/- 2.8, and L = 0.34 +/- 0.13. Series resistance averaged 32 +/- 11 M omega. The mean of the derived soma diameters was 18 +/- 6 microns and the mean diameter and length of the equivalent cables were 1.4 +/- 0.6 and 470 +/- 90 microns, respectively. The average of the specific membrane conductances, 1.67 +/- 1.08 S/cm2, corresponded to a membrane resistivity of 60 k omega. cm2. Computer simulations of synaptic inputs were performed on a representative model, with an electrode at the soma and using the worst-case configuration, in which all synaptic inputs were confined to the tips of the dendrites. We draw three conclusions from the modeling: (1) Under voltage clamp, fast spontaneous EPSCs would be significantly attenuated and slowed while the time course of the slower, light-evoked non-NMDA and NMDA EPSCs would be minimally distorted by dendritic filtering. (2) Excitatory synaptic reversal potentials can be accurately determined under voltage clamp. (3) In the absence of GABAergic and glycinergic inhibition, the efficacy at the soma of excitatory conductance changes is essentially independent of their dendritic location. The specific membrane resistivity appears to represent a good compromise between having a small membrane time constant and minimal EPSP attenuation.

Modulation of neuronal function by intracellular pH

Recent studies have revealed that excitation of specific nerve pathways can produce localized changes of pH in nervous tissue. It is important to determine both how these pH changes are generated and, even more importantly, how the excitability of neurons in the localized areas are affected. Evidence indicates that activation of both gamma-aminobutyric acid (GABA) and L-glutamate receptor channels in inhibitory and excitatory pathways, respectively, can raise extracellular pH (pHo) and lower intracellular pH (pHi). At the target location, it has been shown that several types of voltage-gated ion channels in neurons were modified by a change in pHi. These studies, taken together, enable us to hypothesize that intracellular hydrogen ions (H+) might function as neuromodulatory factors, like other types of intracellular second messengers. This hypothesis was tested by using horizontal cells enzymatically dissociated from catfish retina. We found that the high-voltage-activated (HVA) Ca2+ current, inward rectifier K+ current and hemi-gap junctional current are modulated by a change in intracellular H+ concentration, and that L-glutamate suppresses the HVA Ca2+ current by raising the intracellular H+ concentration. These observations support the hypothesis that intracellular H+, acting as a second messenger, governs neuronal excitability via modulation of ionic channel activity. This article reviews recent studies of ours and others on the effect of pHi upon neuronal function.

The relationship between light-evoked synaptic excitation and spiking behaviour of salamander retinal ganglion cells

1. Light-evoked input-output characteristics of ganglion cells in dark-adapted tiger salamander retina were studied in the slice preparation using patch-clamp techniques. Excitatory postsynaptic currents (EPSCs), isolated by blocking inhibitory inputs and evoked by a range of light stimulus intensities, were recorded under whole-cell voltage clamp. Spike responses, evoked by the same light intensities, were recorded extracellularly from the same cells using the cell-attached patch-clamp technique. 2. When N-methyl-D-aspartate (NMDA) receptor-mediated input was blocked by the competitive NMDA antagonist DL-2-amino-5-phosphonoheptanoate (AP7), light-evoked EPSC amplitude and peak firing rate were reduced at all light intensities. In both cases, the data obtained in the presence of AP7 scaled linearly to control data, indicating that NMDA and non-NMDA receptors are activated in the same proportions across the entire 2 log unit stimulus response range of these ganglion cells. 3. The relationship between light-evoked spike frequency and light-evoked EPSC amplitude was linear. The slope of the light-evoked synaptic current-spike frequency relationship was close to the slope of the injected current-spike frequency relationship, indicating that synaptic current and injected current drive spiking in a similar manner. The linearity of the synaptic current-spike frequency relationship was not compromised when NMDA input was blocked by AP7. 4. Light-evoked voltage responses, recorded under whole-cell current clamp, revealed that the average membrane potential during a spike response was depolarized only slightly with increased firing rate. Once the membrane potential surpassed spike threshold, it was maintained by the voltage-gated, spike-generating conductances at a depolarized plateau upon which action potentials were fired. The potential of this plateau varied only slightly with spike frequency. We conclude that the voltage control exerted by the spike-generating currents in ganglion cells prevents a substantial response-dependent decrease in the electrical driving force of the excitatory currents, obviating the need for the voltage-independent synaptic efficacy provided by the combination of NMDA and non-NMDA inputs.

Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells

1. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under voltage-clamp conditions. Consistent with activation of non-NMDA-type glutamate receptors, the sEPSCs reversed at potentials above 0 mV, were blocked by 1 microM CNQX and prolonged by 2 mM aniracetam. 2. The peak conductance of the averaged sEPSCs (n = 70-400) was 130 +/- 60 pS (mean +/- S.D.; 17 cells, ranging from 70 to 290 pS). Amplitude distributions were skewed towards larger amplitudes. 3. The decay of individual and mean sEPSCs was exponential with a mean time constant (tau d) of 3.75 +/- 0.84 ms (n = 13), which was voltage independent. The 10-90% rise time of the sEPSCs was 1.30 +/- 0.44 ms (n = 13). There was no correlation between sEPSC rise time and tau d suggesting that dendritic filtering alone did not shape the time course of sEPSCs. 4. Light-evoked EPSCs in these retinal ganglion cells are mediated by concomitant activation of NMDA and non-NMDA receptors; however, no NMDA component was discerned in the sEPSCs, even when recording at -96 mV in Mg(2+)-free solutions. The decay time course was not altered by 20 microM AP7, an NMDA antagonist, nor was an NMDA component unmasked by adding glycine or D-serine. These results suggest that NMDA and non-NMDA receptors are not coactivated by a single vesicle of transmitter during spontaneous release, and thus are probably not colocalized in the postsynaptic membrane at the sites of spontaneous release. 5. The sEPSCs were an order of magnitude faster than the non-NMDA receptor-mediated EPSCs evoked by light stimuli, and it is proposed that the EPSC time course is determined largely by the extended time course of release of synaptic vesicles from bipolar cells. The quantal content of a light-evoked non-NMDA receptor-mediated EPSC in an on-off cell is about 200 quanta.

Intracellular alkalinization enhances inward rectifier K+ current in retinal horizontal cells of catfish

Isolated cone-driven horizontal cells dissociated from catfish retina were voltage-clamped using the whole-cell patch-clamp technique. The effects of acidification and alkalinization on an anomalous type, inwardly-rectifying K+ current (IRK+) were investigated. The magnitude of IRK+ was enhanced by raising the intracellular pH above 7.4, however, in contrast, intracellular acidification had little effect on this current. The range over which intracellular pH ([pH]i) modulates IRK+ is different from that for modulation of a sustained high-voltage activated calcium current in these same cells and also for proton-sensitive, inward rectifier currents in starfish oocytes, skeletal muscle and heart myocytes.

Actions of nipecotic acid and SKF89976A on GABA transporter in cone-driven horizontal cells dissociated from the catfish retina

Whole-cell voltage-clamp recordings were made from dissociated horizontal cells of the catfish retina. In the presence of picrotoxin (PTX, 100 microM), GABA evoked a sustained inward current at negative holding potentials. Dose-response measurements were well fitted by a logistic curve with a Hill coefficient of 1.11 and EC50 of 9.76 microM. When external Na+ was replaced with Li+, this GABA-induced current was eliminated. The substitution of methanesulfonate for Cl- also suppressed the current. This current was blocked by either nipecotic acid or SKF89976A. However, the mechanisms by which these drugs suppress the GABA-induced current differ. Intracellularly applied SKF89976A blocked the GABA-induced current, while nipecotic acid intracellularly had no effect. beta-Alanine at concentrations greater than 1 mM exerted a slight inhibitory effect. Extracellularly applied SKF89976A produced no current by itself but suppressed GABA-induced currents. Dose-response curves showed that SKF89976A has an IC50 of 0.93 microM and a Hill coefficient of 2.68. Nipecotic acid evoked a current response, like GABA. A Hill coefficient was 1.64 and an EC50 was 7.69 microM. This nipecotic acid-induced current was blocked by substituting Li+ for Na+ or by the addition of SKF89976A. This result is consistent with other studies indicating that nipecotic acid is transported in place of GABA. Extracellular Na+ was required for the prolonged suppression by extracellularly applied SKF89976A, while the extracellular Cl- depletion has no influence on the suppression. The pharmacological profile of this GABA transporter fits the neuronal rather than the glial type of cloned transporters.

Nitric oxide synthase in Müller cells and neurons of salamander and fish retina

Nitric oxide synthase (NOS) is the biosynthetic enzyme of the signaling molecule nitric oxide (NO). NO donors have been reported to modulate conductances in cell types throughout the retina, from photoreceptors to ganglion cells. Previously, NOS immunoreactivity has been reported in amacrine cells and cells within the ganglion cell layer. Here, we have examined the cellular localization of NOS in the retinas of salamander, goldfish, and catfish using both an affinity-purified antiserum to brain NOS and NADPH diaphorase (NADPHd) histochemistry. These markers indicate that an NOS-like enzyme is localized not only to presumptive amacrine cells but also, depending on the species, to photoreceptor ellipsoids, to somata within the ganglion cell layer, and to horizontal cells. In addition to these neurons, our results indicate that Müller cells, the radial glia of the retina, also contain an NOS-like enzyme. In support of this latter conclusion, cells morphologically similar to Müller cells were positive for NADPHd staining in all three species. In salamander, NOS-like immunoreactivity, NADPHd staining, and binding of anti-GFAP (a marker for glia) were localized to cells that were morphologically indistinguishable from Müller cells. In goldfish, reactivity to both anti-NOS and anti-vimentin (a marker for glia) colocalized to radial processes extending through the inner retina to the inner limiting membrane. These observations are the first to indicate the presence of an NOS-like enzyme in Müller cells and suggest that these glia could be a ready source of NO for target neurons throughout the retina.

The contribution of NMDA and non-NMDA receptors to the light-evoked input-output characteristics of retinal ganglion cells

To examine how light-evoked excitatory synaptic inputs to retinal ganglion cells are transformed into output patterns of activity, action potentials were recorded with cell-attached patch-clamp techniques, and then EPSCs and EPSPs were recorded from the same cell in the whole-cell configuration. AP7, an NMDA antagonist, reduced the light-evoked peak spike frequency 36% +/- 21% (mean +/- SD) and reduced the EPSC amplitude, indicating a major role for NMDA receptors in the light response. CNQX, a non-NMDA receptor antagonist, reduced the light-evoked peak spike frequency 28% +/- 22%. CNQX also caused a voltage- and magnesium-dependent delay in spike onset. AP7 and CNQX, however, did not differ significantly in their effect on the EPSC time course, indicating that postsynaptic cellular properties are responsible for the delay observed in the presence of CNQX. These results show that the NMDA receptor contribution to the excitatory response is increased as the cell is depolarized from rest by non-NMDA input.

L-glutamate suppresses HVA calcium current in catfish horizontal cells by raising intracellular proton concentration

Catfish horizontal cells exhibit a high voltage-activated, slowly inactivating calcium (HVA) current. In isolated cells, this current underlies a sustained action potential that can be evoked by depolarization. L-Glutamate at concentrations as low as 1 microM suppressed HVA current and shortened the duration of action potentials. pH indicator dye measurements revealed that L-glutamate acidified cells by up to 0.3 pH units. pH/HVA current dose-response curves indicated that the EC50 for pH inhibition of HVA current was 7.1 and the Hill coefficient was 1.98. L-Glutamate's suppression of HVA current was eliminated when the patch pipette solution contained a high concentration of proton buffer, but was not affected when pipettes contained GTP gamma S. These results support the hypothesis that L-glutamate can modulate HVA current by changing intracellular pH.

Modulation of a sustained calcium current by intracellular pH in horizontal cells of fish retina

A sustained high voltage-activated (HVA), nifedipine- and cadmium-sensitive calcium current and a sustained calcium action potential (AP) were recorded from horizontal cells isolated from catfish retina. pH indicator dyes showed that superfusion with NH4Cl alkalinized these cells and that washout of NH4Cl or superfusion with Na-acetate acidified them. HVA current was slightly enhanced during superfusion of NH4Cl but was suppressed upon NH4Cl washout or application of Na-acetate. When 25 mM HEPES was added to the patch pipette to increase intracellular pH buffering, the effects of NH4Cl and Na-acetate on HVA current were reduced. These results indicated that intracellular acidification reduces HVA calcium current and alkalinization increases it. Sustained APs, recorded with high resistance, small diameter microelectrodes, were blocked by cobalt and cadmium and their magnitude varied with extracellular calcium concentration. These results provide confirmatory evidence that the HVA current is a major component of the AP and indicate that the AP can be used as a measure of how the HVA current can be modified in intact, undialyzed cells. The duration of APs was increased by superfusion with NH4Cl and reduced by washout of NH4Cl or superfusion with Na-acetate. The Na-acetate and NH4Cl washout-dependent shortening of the APs was observed in the presence of intracellular BAPTA, a calcium chelator, IBMX, a phosphodiesterase inhibitor, and in Na-free or TEA-enriched saline. These findings provide supportive evidence that intracellular acidification may directly suppress the HVA calcium current in intact cells. Intracellular pH changes would thereby be expected to modulate not only the resting membrane potential of these cells in darkness, but calcium-dependent release of neurotransmitter from these cells as well. Furthermore, this acidification-dependent suppression of calcium current could serve a protective role by reducing calcium entry during retinal ischemia, which is usually thought to be accompanied by intracellular acidosis.

Suppression of a calcium current by CNQX and kynurenate

During our investigation of a sustained high voltage-activated (HVA) calcium current in retinal horizontal cells, we found that the glutamate receptor antagonists CNQX and kynurenate but not AP7 reversibly reduced the peak amplitude of the HVA current. Changes in the HVA current kinetics or activation voltage were not apparent; there was only a reduction in the peak current. The novel effects of these antagonists on HVA calcium currents reported here could have an impact on many studies involving glutamatergic synaptic transmission.