Proton modulation of ion channels in isolated horizontal cells of the goldfish retina

Hypoxia increases intracellular calcium in glutamate-activated horizontal cells of goldfish retina via mitochondrial KATP channels and intracellular stores

Central neurons of the common goldfish (Carassius auratus) are exceptional in their capacity to survive Ca2+-induced excitotoxicity and cell death during hypoxia. Horizontal cells (HCs) are inhibitory interneurons of the retina that are tonically depolarized by the neurotransmitter, glutamate, yet preserve intracellular Ca2+ homeostasis. In HCs isolated from goldfish, and in the absence of glutamatergic input, intracellular Ca2+ concentration ([Ca2+]i) is protected from prolonged exposure to hypoxia by mitochondrial ATP-dependent K+ (mKATP) channel activity. In the present study, we investigated the effects of hypoxia upon [Ca2+]i in isolated HCs during tonic activation by glutamate to better predict the effects of hypoxia in the active retina. Dynamic changes in [Ca2+]i were measured using the ratiometric Ca2+ indicator, Fura-2. Application of 100 μM glutamate during hypoxia (PO2 = 25 mmHg) produced a 1.3-fold greater rise in [Ca2+]i compared to the same glutamate stimulus during normoxia. The hypoxia-dependent increase in [Ca2+]i was abolished by application of 5-hydroxydecanoic acid, which renders mKATP channels inactive. Extracellular Ca2+ did not contribute to the elevated [Ca2+]i observed during hypoxia, as the effect persisted in Ca2+-free solution and during application of verapamil, an L-type Ca2+ channel blocker. By contrast, inhibition of the mitochondrial Ca2+ uniporter or ryanodine receptors (with ruthenium red or ryanodine, respectively) abolished the hypoxia-dependent rise in [Ca2+]i. This study reports an mKATP-dependent rise in [Ca2+]i during hypoxia in HCs activated by glutamate, and suggests roles for the mitochondria and intracellular Ca2+ stores in regulating this mechanism.

Seasonal changes in membrane structure and excitability in retinal neurons of goldfish (Carassius auratus) under constant environmental conditions

Seasonal modifications in the structure of cellular membranes occur as an adaptive measure to withstand exposure to prolonged environmental change. Little is known about whether such changes may occur independently of external cues, such as photoperiod or temperature, or how they may impact the central nervous system. We compared membrane properties of neurons isolated from the retina of goldfish (Carassius auratus), an organism well-adapted to extreme environmental change, during the summer and winter months. Goldfish were maintained in a facility under constant environmental conditions throughout the year. Analysis of whole-retina phospholipid composition using mass spectrometry-based lipidomics revealed a two-fold increase in phosphatidylethanolamine species during the winter, suggesting an increase in cell membrane fluidity. Atomic force microscopy was used to produce localized, nanoscale-force deformation of neuronal membranes. Measurement of Young's modulus indicated increased membrane-cortical stiffness (or decreased elasticity) in neurons isolated during the winter. Voltage-clamp electrophysiology was used to assess physiological changes in neurons between seasons. Winter neurons displayed a hyperpolarized reversal potential (Vrev) and a significantly lower input resistance (Rin) compared to summer neurons. This was indicative of a decrease in membrane excitability during the winter. Subsequent measurement of intracellular Ca2+ activity using Fura-2 microspectrofluorometry confirmed a reduction in action potential activity, including duration and action potential profile, in neurons isolated during the winter. These studies demonstrate chemical and biophysical changes that occur in retinal neurons of goldfish throughout the year without exposure to seasonal cues, and suggest a novel mechanism of seasonal regulation of retinal activity.

Mitochondrial KATP channels stabilize intracellular Ca2+ during hypoxia in retinal horizontal cells of goldfish (Carassius auratus)

Neurons of the retina require oxygen to survive. In hypoxia, neuronal ATP production is impaired, ATP-dependent ion pumping is reduced, transmembrane ion gradients are dysregulated, and intracellular Ca2+ concentration ([Ca2+]i) increases enough to trigger excitotoxic cell death. Central neurons of the common goldfish (Carassius auratus) are hypoxia tolerant, but little is known about how goldfish retinas withstand hypoxia. To study the cellular mechanisms of hypoxia tolerance, we isolated retinal interneurons (horizontal cells; HCs), and measured [Ca2+]i with Fura-2. Goldfish HCs maintained [Ca2+]i throughout 1 h of hypoxia, whereas [Ca2+]i increased irreversibly in HCs of the hypoxia-sensitive rainbow trout (Oncorhynchus mykiss) with just 20 min of hypoxia. Our results suggest mitochondrial ATP-dependent K+ channels (mKATP) are necessary to stabilize [Ca2+]i throughout hypoxia. In goldfish HCs, [Ca2+]i increased when mKATP channels were blocked with glibenclamide or 5-hydroxydecanoic acid, whereas the mKATP channel agonist diazoxide prevented [Ca2+]i from increasing in hypoxia in trout HCs. We found that hypoxia protects against increases in [Ca2+]i in goldfish HCs via mKATP channels. Glycolytic inhibition with 2-deoxyglucose increased [Ca2+]i, which was rescued by hypoxia in a mKATP channel-dependent manner. We found no evidence of plasmalemmal KATP channels in patch-clamp experiments. Instead, we confirmed the involvement of KATP in mitochondria with TMRE imaging, as hypoxia rapidly (<5 min) depolarized mitochondria in a mKATP channel-sensitive manner. We conclude that mKATP channels initiate a neuroprotective pathway in goldfish HCs to maintain [Ca2+]i and avoid excitotoxicity in hypoxia. This model provides novel insight into the cellular mechanisms of hypoxia tolerance in the retina.

Retinal horizontal cells of goldfish (Carassius auratus) display subtype-specific differences in spontaneous action potentials in situ

Horizontal cells (HCs) are neurons of the outer retina, which provide inhibitory feedback onto photoreceptors and contribute to image processing. HCs in teleosts are classified into four subtypes (H1-H4), each having different roles: H1-H3 feed back onto different sets of cones, H4 feed back onto rods, and only H1 store and release the inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Dissociated HCs exhibit spontaneous Ca²⁺ -based action potentials (APs), yet it is unclear if APs occur in situ, or if all subtypes exhibit APs. We measured intracellular Ca²⁺ and report APs in slice preparations of the goldfish retina. In HCs furthest from photoreceptors (i.e., H3/H4), APs were less frequent, with greater duration and area under the curve (a measure of Ca²⁺ flux). Next, we classified acutely dissociated HCs into subtypes by integrating the ratio of dendritic field size vs. soma size (r_d/s ). H1 and H2 subtypes had low r_d/s values (<8); H3/H4 had high r_d/s (>12). To verify this model, H1s were identified by immunoreactivity for GABA and 95% of these cells had an r_d/s < 4. In Ca²⁺ imaging experiments, as r_d/s increased, AP duration and area under the curve increased, while frequency decreased. Our results demonstrate the presence of Ca²⁺ -based APs in the goldfish retina in situ and show that HC subtypes H1 through H4 exhibit progressively longer and less frequent spontaneous APs. These results suggest that APs may play an important role in inhibitory feedback, and may have implications for understanding the relative contributions of HC subtypes in the outer retina.

Spontaneous action potentials in retinal horizontal cells of goldfish (Carassius auratus) are dependent upon L-type Ca2+ channels and ryanodine receptors

Horizontal cells (HCs) are interneurons of the outer retina that undergo graded changes in membrane potential during the light response and provide feedback to photoreceptors. We characterized spontaneous Ca²⁺-based action potentials (APs) in isolated goldfish (Carassius auratus) HCs with electrophysiological and intracellular imaging techniques. Transient changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) were observed with fura-2 and were abolished by removal of extracellular Ca²⁺ or by inhibition of Ca²⁺ channels by 50 µM Cd²⁺ or 100 µM nifedipine. Inhibition of Ca²⁺ release from stores with 20 µM ryanodine or 50 µM dantrolene abolished Ca²⁺ transients and increased baseline [Ca²⁺]_i. This increased baseline was prevented by blocking L-type Ca²⁺ channels with nifedipine, suggesting that Ca²⁺-induced Ca²⁺ release from stores may be needed to inactivate membrane Ca²⁺ channels. Caffeine (3 mM) increased the frequency of Ca²⁺ transients, and the store-operated channel antagonist 2-aminoethyldiphenylborinate (100 μM) counteracted this effect. APs were detected with voltage-sensitive dye imaging (FluoVolt) and current-clamp electrophysiology. In current-clamp recordings, regenerative APs were abolished by removal of extracellular Ca²⁺ or in the presence of 5 mM Co²⁺ or 100 µM nifedipine, and APs were amplified with 15 mM Ba²⁺. Collectively, our data suggest that during APs Ca²⁺ enters through L-type Ca²⁺ channels and that Ca²⁺ stores (gated by ryanodine receptors) contribute to the rise in [Ca²⁺]_i. This work may lead to further understanding of the possible role APs have in vision, such as transitioning from light to darkness or modulating feedback from HCs to photoreceptors.NEW & NOTEWORTHY Horizontal cells (HCs) are interneurons of the outer retina that provide inhibitory feedback onto photoreceptors. HCs respond to light via graded changes in membrane potential. We characterized spontaneous action potentials in HCs from goldfish and linked action potential generation to a rise in intracellular Ca²⁺ via plasma membrane channels and ryanodine receptors. Action potentials may play a role in vision, such as transitioning from light to darkness, or in modulating feedback from HCs to photoreceptors.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

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

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

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

The effect of aminosulfonate buffers on the light responses and intracellular pH of goldfish retinal horizontal cells

Retinal horizontal cell feedback acts as a gain control at the first synapse in the visual system and generates center-surround receptive fields in the outer retina. One model of feedback proposes that elevation of protons in the photoreceptor synaptic cleft produces feedback. Most evidence supporting the proton model has depended on the effect of proton buffers, in particular aminosulfonates, but these agents could potentially have effects other than external pH regulation. We therefore determined if the effects of aminosulfonates on horizontal cell rollback, an indicator of feedback, were consistent with external proton buffering. Intracellular recording from horizontal cells in isolated goldfish retina revealed that rollback was blocked only by aminosulfonates with an acid dissociation constant suited for buffering at the pH (7.5) of the Ringer's solution. In isolated goldfish horizontal cells, aminosulfonates, even those that did not block rollback, altered intracellular pH. This suggests that the effect of aminosulfonates on rollback is not because of changing intracellular pH. Measures of both intracellular and extracellular pH revealed that treatment with either glutamate or kainate resulted in acidification. As glutamate produced both internal and external acidification, intracellular and extracellular horizontal cell pH would be expected to increase in response to light, a change consistent with the proton model of feedback.

Guinea pig horizontal cells express GABA, the GABA-synthesizing enzyme GAD 65, and the GABA vesicular transporter

Gamma-aminobutyric acid (GABA) is likely expressed in horizontal cells of all species, although conflicting physiological findings have led to considerable controversy regarding its role as a transmitter in the outer retina. This study has evaluated key components of the GABA system in the outer retina of guinea pig, an emerging retinal model system. The presence of GABA, its rate-limiting synthetic enzyme glutamic acid decarboxylase (GAD(65) and GAD(67) isoforms), the plasma membrane GABA transporters (GAT-1 and GAT-3), and the vesicular GABA transporter (VGAT) was evaluated by using immunohistochemistry with well-characterized antibodies. The presence of GAD(65) mRNA was also evaluated by using laser capture microdissection and reverse transcriptase-polymerase chain reaction. Specific GABA, GAD(65), and VGAT immunostaining was localized to horizontal cell bodies, as well as to their processes and tips in the outer plexiform layer. Furthermore, immunostaining of retinal whole mounts and acutely dissociated retinas showed GAD(65) and VGAT immunoreactivity in both A-type and B-type horizontal cells. However, these cells did not contain GAD(67), GAT-1, or GAT-3 immunoreactivity. GAD(65) mRNA was detected in horizontal cells, and sequencing of the amplified GAD(65) fragment showed approximately 85% identity with other mammalian GAD(65) mRNAs. These studies demonstrate the presence of GABA, GAD(65), and VGAT in horizontal cells of the guinea pig retina, and support the idea that GABA is synthesized from GAD(65), taken up into synaptic vesicles by VGAT, and likely released by a vesicular mechanism from horizontal cells.

A genetically encoded ratiometric sensor to measure extracellular pH in microdomains bounded by basolateral membranes of epithelial cells

Extracellular pH, especially in relatively inaccessible microdomains between cells, affects transport membrane protein activity and might have an intercellular signaling role. We have developed a genetically encoded extracellular pH sensor capable of detecting pH changes in basolateral spaces of epithelial cells. It consists of a chimerical membrane protein displaying concatenated enhanced variants of cyan fluorescence protein (ECFP) and yellow fluorescence protein (EYFP) at the external aspect of the cell surface. The construct, termed pHCECSensor01, was targeted to basolateral membranes of Madin-Darby canine kidney (MDCK) cells by means of a sequence derived from the aquaporin AQP4. The fusion of pH-sensitive EYFP with pH-insensitive ECFP allows ratiometric pH measurements. The titration curve of pHCECSensor01 in vivo had a pK (a) value of 6.5 +/- 0.04. Only minor effects of extracellular chloride on pHCECSensor01 were observed around the physiological concentrations of this anion. In MDCK cells, the sensor was able to detect changes in pH secondary to H(+) efflux into the basolateral spaces elicited by an ammonium prepulse or lactate load. This genetically encoded sensor has the potential to serve as a noninvasive tool for monitoring changes in extracellular pH microdomains in epithelial and other tissues in vivo.