Identification and localization of K+ channels in the mouse retina

Calcium/calmodulin-dependent protein kinase II associates with the K+ channel isoform Kv4.3 in adult rat optic nerve

Spikes are said to exhibit "memory" in that they can be altered by spikes that precede them. In retinal ganglion cell axons, for example, rapid spiking can slow the propagation of subsequent spikes. This increases inter-spike interval and, thus, low-pass filters instantaneous spike frequency. Similarly, a K+ ion channel blocker (4-aminopyridine, 4AP) increases the time-to-peak of compound action potentials recorded from optic nerve, and we recently found that reducing autophosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) does too. These results would be expected if CaMKII modulates spike propagation by regulating 4AP-sensitive K+ channels. As steps toward identifying a possible substrate, we test whether (i) 4AP alters optic nerve spike shape in ways consistent with reducing K+ current, (ii) 4AP alters spike propagation consistent with effects of reducing CaMKII activation, (iii) antibodies directed against 4AP-sensitive and CaMKII-regulated K+ channels bind to optic nerve axons, and (iv) optic nerve CaMKII co-immunoprecipitates with 4AP-sensitive K+ channels. We find that, in adult rat optic nerve, (i) 4AP selectively slows spike repolarization, (ii) 4AP slows spike propagation, (iii) immunogen-blockable staining is achieved with anti-Kv4.3 antibodies but not with antibodies directed against Kv1.4 or Kv4.2, and (iv) CaMKII associates with Kv4.3. Kv4.3 may thus be a substrate that underlies activity-dependent spike regulation in adult visual system pathways.

Retinoschisin and novel Na/K-ATPase interaction partners Kv2.1 and Kv8.2 define a growing protein complex at the inner segments of mammalian photoreceptors

The RS1 gene on Xp 22.13 encodes retinoschisin which is known to directly interact with the retinal Na/K-ATPase at the photoreceptor inner segments. Pathologic mutations in RS1 cause X-linked juvenile retinoschisis (XLRS), a hereditary retinal dystrophy in young males. To further delineate the retinoschisin-Na/K-ATPase complex, co-immunoprecipitation was performed with porcine and murine retinal lysates targeting the ATP1A3 subunit. This identified the voltage-gated potassium (Kv) channel subunits Kv2.1 and Kv8.2 as direct interaction partners of the retinal Na/K-ATPase. Colocalization of the individual components of the complex was demonstrated at the membrane of photoreceptor inner segments. We further show that retinoschisin-deficiency, a frequent consequence of molecular pathology in XLRS, causes mislocalization of the macromolecular complex during postnatal retinal development with a simultaneous reduction of Kv2.1 and Kv8.2 protein expression, while the level of retinal Na/K-ATPase expression remains unaffected. Patch-clamp analysis revealed no effect of retinoschisin-deficiency on Kv channel mediated potassium ion currents in vitro. Together, our data suggest that Kv2.1 and Kv8.2 together with retinoschisin and the retinal Na/K-ATPase are integral parts of a macromolecular complex at the photoreceptor inner segments. Defective compartmentalization of this complex due to retinoschisin-deficiency may be a crucial step in initial XLRS pathogenesis.

Expression and Localization of Kcne2 in the Vertebrate Retina

Purpose

To characterize the retinal expression and localization of Kcne2, an ancillary (β) ion-channel subunit with an important role in fine-tuning cellular excitability.

Methods

We analyzed available single-cell transcriptome data from tens of thousands of murine retinal cells for cell-type-specific expression of Kcne2 using state-of-the-art bioinformatics techniques. This evidence at the transcriptome level was complemented with a comprehensive immunohistochemical characterization of mouse retina (C57BL/6, ages 8-12 weeks) employing co-labeling techniques and cell-type-specific antibody markers. We furthermore examined how conserved the Kcne2 localization pattern in the retina was across species by performing immunostaining on zebrafish, cowbird, sheep, mice, and macaque.

Results

Kcne2 is distinctly expressed in cone photoreceptors and rod bipolar cells. At a subcellular level, the bulk of Kcne2 immunoreactivity can be observed in the outer plexiform layer. Here, it localizes into cone pedicles and likely the postsynaptic membrane of the rod bipolar cells. Thus, the vast majority of Kcne2 immunoreactivity is observed in a thin band in the outer plexiform layer. In addition to this, faint Kcne2 immunoreactivity can also be observed in cone inner segments and the somata of a small subset of cone ON bipolar cells. Strikingly, the localization of Kcne2 in the outer plexiform layer was preserved among all of the species studied, spanning at least 300 million years of evolution of the vertebrate kingdom.

Conclusions

The data we present here suggest an important and specific role for Kcne2 in the highly specialized photoreceptor-bipolar cell synapse.

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.

Progressive myoclonic epilepsy-associated gene Kctd7 regulates retinal neurovascular patterning and function

Neuron function relies on and instructs the development and precise organization of neurovascular units that in turn support circuit activity. However, our understanding of the molecular cues that regulate this relationship remains sparse. Using a high-throughput screening pipeline, we recently identified several new regulators of vascular patterning. Among these was the potassium channel tetramerization domain-containing protein 7 (KCTD7). Mutations in KCTD7 are associated with progressive myoclonic epilepsy, but how KCTD7 regulates neural development and function remains poorly understood. To begin to identify such mechanisms, we focus on mouse retina, a tractable part of the central nervous system that contains precisely ordered neuron subtypes supported by a trilaminar vascular network. We find that deletion of Kctd7 induces defective patterning of the adult retina vascular network, resulting in increased branching, vessel length, and lacunarity. These alterations reflect early and specific defects in vessel development, as emergence of the superficial and deep vascular layers were delayed. These defects are likely due to a role for Kctd7 in inner retina neurons. Kctd7 is absent from vessels but present in neurons in the inner retina, and its deletion resulted in a corresponding increase in the number of bipolar cells in development and increased vessel branching in adults. These alterations were accompanied by retinal function deficits. Together, these data suggest that neuronal Kctd7 drives growth and patterning of the vasculature and that neurovascular interactions may participate in the pathogenesis of KCTD7-related human diseases.

Potential independent action of sigma receptor ligands through inhibition of the Kv2.1 channel

The sigma-1 receptor (σ1-R) and sigma-2 receptor (σ2-R) are potential drug targets for treatment of cancer, pain, depression, retinal degeneration and other neuronal diseases. Previous reports show that sigma-1 receptor modulates the activities of multiple channels. We are interested in possible sigma receptor modulation of Kv2.1, a K⁺ channel abundant in retinal photoreceptors. We tested the effect of established sigma receptor ligands on Kv2.1 channels which were stably expressed in HEK293 cells. Surprisingly, σ1-R antagonists inhibited Kv2.1 currents in both wild type and σ1-R knockout HEK293 cells that we engineered using the CRISPR/Cas9 technology. Moreover, PB28, a σ1-R antagonist and also σ2-R agonist, inhibited Kv2.1 in σ1-R knockout cells, but this action was not blocked by the σ2-R antagonists that did not have an effect on Kv2.1. We also observed inhibition of electroretinogram by PB28 in wild type as well as σ1-R knockout mice. Thus, the results in this study indicate that the Kv2.1-inhibiting function of the sigma ligands is not sigma receptor dependent, suggesting a direct effect of these ligands on the Kv2.1 channel.

Differential effect of brief electrical stimulation on voltage-gated potassium channels

Electrical stimulation of neuronal tissue is a promising strategy to treat a variety of neurological disorders. The mechanism of neuronal activation by external electrical stimulation is governed by voltage-gated ion channels. This stimulus, typically brief in nature, leads to membrane potential depolarization, which increases ion flow across the membrane by increasing the open probability of these voltage-gated channels. In spiking neurons, it is activation of voltage-gated sodium channels (Na_V channels) that leads to action potential generation. However, several other types of voltage-gated channels are expressed that also respond to electrical stimulation. In this study, we examine the response of voltage-gated potassium channels (K_V channels) to brief electrical stimulation by whole cell patch-clamp electrophysiology and computational modeling. We show that nonspiking amacrine neurons of the retina exhibit a large variety of responses to stimulation, driven by different K_V-channel subtypes. Computational modeling reveals substantial differences in the response of specific K_V-channel subtypes that is dependent on channel kinetics. This suggests that the expression levels of different K_V-channel subtypes in retinal neurons are a crucial predictor of the response that can be obtained. These data expand our knowledge of the mechanisms of neuronal activation and suggest that K_V-channel expression is an important determinant of the sensitivity of neurons to electrical stimulation.NEW & NOTEWORTHY This paper describes the response of various voltage-gated potassium channels (K_V channels) to brief electrical stimulation, such as is applied during prosthetic electrical stimulation. We show that the pattern of response greatly varies between K_V channel subtypes depending on activation and inactivation kinetics of each channel. Our data suggest that problems encountered when artificially stimulating neurons such as cessation in firing at high frequencies, or "fading," may be attributed to K_V-channel activation.

Frequency-selective transmission of graded signals in large monopolar neurons of blowfly Calliphora vicina compound eye

The functional roles of voltage-gated K(+)(Kv) channels in visual system interneurons remain poorly studied. We have addressed this problem in the large monopolar cells (LMCs) of the blowfly Calliphora vicina, using intracellular recordings and mathematical modeling methods. Intracellular recordings were performed in two cellular compartments: the synaptic zone, which receives input from photoreceptors, and the axon, which provides graded potential output to the third-order visual neurons. Biophysical properties of Kv conductances in the physiological voltage range were examined in the dark with injections of current in the discontinuous current-clamp mode. Putative LMC types 1/2 and 3 (L1/2 and L3, respectively) had dissimilar Kv channelomes: L1/2 displayed a prominent inactivating Kv conductance in the axon, while L3 cells were characterized by a sustained delayed-rectifier Kv conductance. To study the propagation of voltage signals, the data were incorporated into the previously developed mathematical model. We demonstrate that the complex interaction between the passive membrane properties, Kv conductances, and the neuronal geometry leads to a resonance-like filtering of signals with peak frequencies of transmission near 15 and 40 Hz for L3 and L1/2, respectively. These results point to distinct physiological roles of different types of LMCs.

Deletion of the Kv2.1 delayed rectifier potassium channel leads to neuronal and behavioral hyperexcitability

The Kv2.1 delayed rectifier potassium channel exhibits high-level expression in both principal and inhibitory neurons throughout the central nervous system, including prominent expression in hippocampal neurons. Studies of in vitro preparations suggest that Kv2.1 is a key yet conditional regulator of intrinsic neuronal excitability, mediated by changes in Kv2.1 expression, localization and function via activity-dependent regulation of Kv2.1 phosphorylation. Here we identify neurological and behavioral deficits in mutant (Kv2.1(-/-) ) mice lacking this channel. Kv2.1(-/-) mice have grossly normal characteristics. No impairment in vision or motor coordination was apparent, although Kv2.1(-/-) mice exhibit reduced body weight. The anatomic structure and expression of related Kv channels in the brains of Kv2.1(-/-) mice appear unchanged. Delayed rectifier potassium current is diminished in hippocampal neurons cultured from Kv2.1(-/-) animals. Field recordings from hippocampal slices of Kv2.1(-/-) mice reveal hyperexcitability in response to the convulsant bicuculline, and epileptiform activity in response to stimulation. In Kv2.1(-/-) mice, long-term potentiation at the Schaffer collateral - CA1 synapse is decreased. Kv2.1(-/-) mice are strikingly hyperactive, and exhibit defects in spatial learning, failing to improve performance in a Morris Water Maze task. Kv2.1(-/-) mice are hypersensitive to the effects of the convulsants flurothyl and pilocarpine, consistent with a role for Kv2.1 as a conditional suppressor of neuronal activity. Although not prone to spontaneous seizures, Kv2.1(-/-) mice exhibit accelerated seizure progression. Together, these findings suggest homeostatic suppression of elevated neuronal activity by Kv2.1 plays a central role in regulating neuronal network function.

Potassium ion channels in retinal ganglion cells (review)

Retinal ganglion cells (RGCs) consolidate visual processing and constitute the last step prior to the transmission of signals to higher brain centers. RGC death is a major cause of visual impairment in optic neuropathies, including glaucoma, age‑related macular degeneration, diabetic retinopathy, uveoretinitis and vitreoretinopathy. Discharge patterns of RGCs are primarily determined by the presence of ion channels. As the most diverse group of ion channels, potassium (K+) channels play key roles in modulating the electrical properties of RGCs. Biochemical, molecular and pharmacological studies have identified a number of K+ channels in RGCs, including inwardly rectifying K+ (Kir), ATP‑sensitive K+ (KATP), tandem‑pore domain K+ (TASK), voltage‑gated K+ (Kv), ether‑à‑go‑go (Eag) and Ca2+‑activated K+ (KCa) channels. Kir channels are important in the maintenance of the resting membrane potential and controlling RGC excitability. KATP channels are involved in RGC survival and neuroprotection. TASK channels are hypothesized to contribute to the regulation of resting membrane potentials and firing patterns of RGCs. Kv channels are important regulators of cellular excitability, functioning to modulate the amplitude, duration and frequency of action potentials and subthreshold depolarizations, and are also important in RGC development and protection. Eag channels may contribute to dendritic repolarization during excitatory postsynaptic potentials and to the attenuation of the back propagation of action potentials. KCa channels have been observed to contribute to repetitive firing in RGCs. Considering these important roles of K+ channels in RGCs, the study of K+ channels may be beneficial in elucidating the pathophysiology of RGCs and exploring novel RGC protection strategies.

Glucose sensitivity of mouse olfactory bulb neurons is conveyed by a voltage-gated potassium channel

The olfactory bulb has recently been proposed to serve as a metabolic sensor of internal chemistry, particularly that modified by metabolism. Because the voltage-dependent potassium channel Kv1.3 regulates a large proportion of the outward current in olfactory bulb neurons and gene-targeted deletion of the protein produces a phenotype of resistance to diet-induced obesity in mice, we hypothesized that this channel may play a role in translating energy availability into a metabolic signal. Here we explored the ability of extracellular glucose concentration to modify evoked excitability of the mitral neurons that principally regulate olfactory coding and processing of olfactory information. Using voltage-clamp electrophysiology of heterologously expressed Kv1.3 channels in HEK 293 cells, we found that Kv1.3 macroscopic currents responded to metabolically active (d-) rather than inactive (l-) glucose with a response profile that followed a bell-shaped curve. Olfactory bulb slices stimulated with varying glucose concentrations showed glucose-dependent mitral cell excitability as evaluated by current-clamp electrophysiology. While glucose could be either excitatory or inhibitory, the majority of the sampled neurons displayed a decreased firing frequency in response to elevated glucose concentration that was linked to increased latency to first spike and decreased action potential cluster length. Unlike modulation attributed to phosphorylation, glucose modulation of mitral cells was rapid, less than one minute, and was reversible within the time course of a patch recording. Moreover, we report that modulation targets properties of spike firing rather than action potential shape, involves synaptic activity of glutamate or GABA signalling circuits, and is dependent upon Kv1.3 expression. Given the rising incidence of metabolic disorders attributed to weight gain, changes in neuronal excitability in brain regions regulating sensory perception of food are of consequence.

Photoreceptor inner and outer segments

Photoreceptors are exquisitely adapted to transform light stimuli into electrical signals that modulate neurotransmitter release. These cells are organized into several compartments including the unique outer segment (OS). Its whole function is to absorb light and transduce this signal into a change of membrane potential. Another compartment is the inner segment where much of metabolism and regulation of membrane potential takes place and that connects the OS and synapse. The synapse is the compartment where changes in membrane potentials are relayed to other neurons in the retina via release of neurotransmitter. The composition of the plasma membrane surrounding these compartments varies to accommodate their specific functions. In this chapter, we discuss the organization of the plasma membrane emphasizing the protein composition of each region as it relates to visual signaling. We also point out examples where mutations in these proteins cause visual impairment.

Melatonin inhibits tetraethylammonium-sensitive potassium channels of rod ON type bipolar cells via MT2 receptors in rat retina

By challenging specific receptors, melatonin synthesized and released by photoreceptors regulates various physiological functions in the vertebrate retina. Here, we studied modulatory effects of melatonin on K+ currents of rod-dominant ON type bipolar cells (Rod-ON-BCs) in rat retinal slices by patch-clamp techniques. Double immunofluorescence experiments conducted in isolated cell and retinal section preparations showed that the melatonin MT₂ receptor was expressed in somata, dendrites and axon terminals of rat Rod-ON-BCs. Electrophysiologically, application of melatonin selectively inhibited the tetraethylammonium (TEA)-sensitive K+ current component, but did not show any effect on the 4-aminopyridine (4-AP)-sensitive component. Consistent with the immunocytochemical result, the melatonin effect was blocked by co-application of 4-phenyl-2-propionamidotetralin (4-P-PDOT), a specific MT₂ receptor antagonist. Neither protein kinase A (PKA) nor protein kinase G (PKG) seemed to be involved because both the PKA inhibitor Rp-cAMP and the PKG inhibitor KT5823 did not block the melatonin-induced suppression of the K+ currents. In contrast, application of the phospholipase C (PLC) inhibitor U73122 or the protein kinase C (PKC) inhibitor bisindolylmaleimide IV (Bis IV) eliminated the melatonin effect, and when the Ca²+ chelator BAPTA-containing pipette was used, melatonin failed to inhibit the K+ currents. These results suggest that suppression of the TEA-sensitive K+ current component via activation of MT₂ receptors expressed on rat Rod-ON-BCs may be mediated by a Ca²+-dependent PLC/inositol 1,4,5-trisphosphate (IP₃/PKC signaling pathway.

"Cone dystrophy with supernormal rod electroretinogram": a comprehensive genotype/phenotype study including fundus autofluorescence and extensive electrophysiology

PURPOSE

The purpose of this study was to characterize the clinical, electrophysiologic, and genetic features in "cone dystrophy with supernormal rod electroretinogram (ERG)."

METHODS

Twenty-four cases between 5 and 59 years of age were ascertained. Full-field ERGs, incorporating the international standards, were used to derive intensity-ERG response functions. ON-OFF ERGs were performed. Fundus autofluorescence imaging was performed on 15 subjects. Deoxyribonucleic acid was available in 18 cases and was screened for a mutation in KCNV2.

RESULTS

Photophobia and nyctalopia were common. Autofluorescence was variable but often showed a ring-like area of high density that in middle-aged individuals, usually surrounded by an area of macular retinal pigment epithelial atrophy. Scotopic ERG amplitudes overlapped with the normal range but had characteristic a- and b-wave intensity-response functions; all had a broadened a-wave to the brightest flash. Photopic ERGs were abnormal; there was a delay in some ON and most OFF responses. Mutations in KCNV2 were detected in 18 cases, including 4 with novel mutations.

CONCLUSION

Individuals with mutations in KCNV2 manifest a wide range of macular and autofluorescence abnormalities. A ring-like area of parafoveal high density autofluorescence is common. ERG amplitudes are variable, but the intensity-ERG response functions and bright flash ERG waveforms are pathognomonic.

Histamine enhances voltage-gated potassium currents of ON bipolar cells in macaque retina

PURPOSE

The goal was to understand the functions of retinopetal axons containing histamine. In prior work, type 3 histamine receptors (HR3) have been localized to the tips of ON bipolar cell dendrites in macaque retinas. Voltage-gated potassium channels have also been localized to bipolar cell dendrites, and the hypothesis tested in the present study was that these are modulated by histamine.

METHODS

Whole-cell recordings of potassium currents were made from bipolar cells in slice preparations of macaque retina. In voltage-clamp mode, the cells were held at -60 mV and stepped to values from -60 to 80 mV. Recordings of the membrane potential were also made in current-clamp mode. Histamine, the HR3 agonist (R) alpha-methylhistamine (RAMH), tetraethyl ammonium (TEA), and 4-aminopyridine (4-AP) were applied in the superfusate.

RESULTS

Histamine produced a dose-dependent increase in potassium currents in a subset of bipolar cells. At 5 microM, histamine increased the currents by 15% or more in the ON bipolar cells but not in the OFF bipolar cells. RAMH at 5 microM increased the amplitude of the potassium currents in the ON bipolar cells. In 10 mM TEA, potassium currents were reduced in all the bipolar cells, and there was no effect of histamine. Histamine hyperpolarized the resting membrane potential of the ON bipolar cells by 5 mV.

CONCLUSIONS

By enhancing potassium currents in the ON bipolar cells, histamine is expected to reduce the amplitude of the light responses and limit their duration. The hyperpolarization of the resting membrane potential would also reduce neurotransmitter release at their output synapses.

Characterization of the Heteromeric Potassium Channel Formed by Kv2.1 and the Retinal Subunit Kv8.2 in Xenopus Oocytes

Kv8.2 (KCNV2) subunits do not form homotetrameric potassium channels, although they coassemble with Kv2.1 to constitute functional heteromers. High expression of Kv8.2 was reported in the human retina and its mutations were linked to the visual disorder “cone dystrophy with supernormal rod electroretinogram.” We detected abundant Kv8.2 expression in the photoreceptor layer of mouse retina, where Kv2.1 is also known to be present. When the two subunits were coexpressed in Xenopus oocytes in equal amounts, Kv8.2 abolished the current of Kv2.1. If the proportion of Kv8.2 was reduced then the current of heteromeric channels emerged. Kv8.2 shifted the steady-state activation of Kv2.1 to more negative potentials, without affecting the voltage dependence of inactivation. This gave rise to a window current within the −40 to −10 mV membrane potential range. Ba2+ inhibited the heteromeric channel and shifted its activation to more positive potentials. These electrophysiological and pharmacological properties resemble those of the voltage-gated K+ current (named IKx) described in amphibian retinal rods. Furthermore, oocytes expressing Kv2.1/Kv8.2 developed transient hyperpolarizing overshoots in current-clamp experiments, whereas those expressing only Kv2.1 failed to do so. Similar overshoots are characteristic responses of photoreceptors to light flashes. We demonstrated that Kv8.2 G476D, analogous to a disease-causing human mutation, eliminated Kv2.1 current, if the subunits were coexpressed equally. However, Kv8.2 G476D did not form functional heteromers under any conditions. Therefore we suggest that the custom-tailored current of Kv2.1/Kv8.2 functionally contributes to photoreception, and this is the reason that mutations of Kv8.2 lead to a genetic visual disorder.

Differential distribution of voltage-gated potassium channels Kv 1.1-Kv1.6 in the rat retina during development

The discharge behavior of neurons depends on a variable expression and sorting pattern of voltage-dependent potassium (Kv) channels that changes during development. The rodent retina represents a neuronal network whose main functions develop after birth. To obtain information about neuronal maturation we analyzed the expression of subunits of the Kv1 subfamily in the rat retina during postnatal development using immunocytochemistry and immunoelectron microscopy. At postnatal day 5 (P5) all the alpha-subunits of Kv1.1-Kv1.6 channels were found to be expressed in the ganglion cell layer (GCL), most of them already at P1 or P3. Their expression upregulates postnatally and the pattern and distribution change in an isoform-specific manner. Additionally Kv1 channels are found in the outer and inner plexiform layer (OPL, IPL) and in the inner nuclear layer (INL) at different postnatal stages. In adult retina the Kv 1.3 channel localizes to the inner and outer segments of cones. In contrast, Kv1.4 is highly expressed in the outer retina at P8. In adult retina Kv1.4 occurs in rod inner segments (RIS) near the connecting cilium where it colocalizes with synapse associated protein SAP 97. By using confocal laser scanning microscopy we showed a differential localization of Kv1.1-1.6 to cholinergic amacrine and rod bipolar cells of the INL of the adult retina.

Histamine receptors in mammalian retinas

Mammalian retinas are innervated by histaminergic axons that originate from perikarya in the posterior hypothalamus. To identify the targets of these retinopetal axons, we localized histamine receptors (HR) in monkey and rat retinas by light and electron microscopy. In monkeys, puncta containing HR3 were found at the tips of ON-bipolar cell dendrites in cone pedicles and rod spherules, closer to the photoreceptors than the other neurotransmitter receptors. This is the first ultrastructural localization of any histamine receptor and the first direct evidence that HR3 is present on postsynaptic membranes in the central nervous system. In rat retinas, most HR1 were localized to dopaminergic amacrine cells. The differences in histamine receptor localization may reflect the differences in the activity patterns of the two species.

Heterogeneous expression of voltage-dependent Na+ and K+ channels in mammalian retinal bipolar cells

Retinal bipolar cells show heterogeneous expression of voltage-dependent Na+ and K+ currents. We used whole-cell patch-clamp recordings to investigate the possible roles of these currents in the response properties of bipolar cells in rats. Isolated bipolar cells showed robust spontaneous regenerative activity, but the regenerative potential of rod bipolar cells reached a more depolarized level than that of cone bipolar cells. In both isolated cells and cells in retinal slices, the membrane depolarization evoked by current injection was apparently capped. The evoked membrane potential was again more depolarized in rod bipolar cells than in cone bipolar cells. Application of tetraethylammonium and 4-aminopyridine shifted the spontaneous regenerative potential as well as the evoked potential to a more depolarized level. In addition, a subclass of cone bipolar cells showed a prominent spike in the initial phase of the voltage response when the cells were depolarized from a relatively negative membrane potential. The spike was mediated mainly by tetrodotoxin-sensitive Na+ current. The presence of the spike sped up the response kinetics and enhanced the peak membrane potential. Results of this study raise the possibility that voltage-dependent K+ currents may play a role in defining different membrane operating ranges of rod and cone bipolar cells and that voltage-dependent Na+ currents may enhance the response kinetics and amplitude of certain cone bipolar cells.

Voltage-dependent K(+) channels in pancreatic beta cells: role, regulation and potential as therapeutic targets

Insulin secretion from pancreatic islet beta cells is acutely regulated by a complex interplay of metabolic and electrogenic events. The electrogenic mechanism regulating insulin secretion from beta cells is commonly referred to as the ATP-sensitive K(+) (K(ATP)) channel dependent pathway. Briefly, an increase in ATP and, perhaps more importantly, a decrease in ADP stimulated by glucose metabolism depolarises the beta cell by closing K(ATP) channels. Membrane depolarisation results in the opening of voltage-dependent Ca(2+) channels, and influx of Ca(2+) is the main trigger for insulin secretion. Repolarisation of pancreatic beta cell action potential is mediated by the activation of voltage-dependent K(+) (Kv) channels. Various Kv channel homologues have been detected in insulin secreting cells, and recent studies have shown a role for specific Kv channels as modulators of insulin secretion. Here we review the evidence supporting a role for Kv channels in the regulation of insulin secretion and discuss the potential and the limitations for beta-cell Kv channels as therapeutic targets. Furthermore, we review recent investigations of mechanisms regulating Kv channels in beta cells, which suggest that Kv channels are active participants in the regulation of beta-cell electrical activity and insulin secretion.

Relation between potassium-channel kinetics and the intrinsic dynamics in isolated retinal bipolar cells

Characterization of the intrinsic dynamics of isolated retinal bipolar cells by a whole-cell patch-clamp technique combined with estimation of effective impulse responses across a range of mean injected currents reveals strikingly adaptive behavior. At resting potential, bipolar cells' effective impulse response is slow, high gain, and low pass. Depolarization speeds up response, decreases gain, and, in most cells, induces bandpass behavior. This adaptive behavior involves two K(+) currents. The delayed-rectifier accounts for the observed gain reduction, speed increase, and bandpass behavior. The A-channel further shortens the impulse responses but suppresses bandpass features. Computer simulations of model neurons with a delayed-rectifier and varying A-channel conductances reveal that impulse responses largely reflect the flux of electrical charge through the two K(+) channels. The A-channel broadens the frequency response and preempts the action of the delayed-rectifier, thereby reducing the associated bandpass features. Admixtures of the two K(+) channels produce the observed variety of dynamics of retinal bipolar cells.

Early loss of synaptic protein PSD-95 from rod terminals of rhodopsin P347L transgenic porcine retina

Retinitis pigmentosa (RP), a type of retinal degeneration involving first rod and then slow cone photoreceptor degeneration, can be caused by any of a number of mutations in different genes. In the cases of mutations affecting rod-specific genes such as rhodopsin, it is unclear how the mutations may cause degeneration of cones. We have used the porcine retina, which is rod-dominated and has an abundance of cones, to study the mutation-induced changes in both rod and cone photoreceptors. Like patients with the same mutation, rhodopsin P347L transgenic swine manifest rod-cone degeneration. In addition, the rod bipolar cells fail to form synaptic connections with rods; instead, they form ectopic synapses with cones. The mechanisms that prevent the formation of the rod-rod bipolar cell synaptic connection are not known. We used specific antibodies and immunocytochemistry to show that the synaptic protein, PSD-95, is present in both normal and transgenic porcine retinas. During neonatal development, however, PSD-95 is lost from rod terminals in the transgenic swine. This loss is virtually complete (90%) by postnatal day 5, at a time when greater than 80% of rod cell bodies still remain. Furthermore, the remaining rods retain their outer segments and their gross morphology appears relatively normal. In contrast, PSD-95 expression continues in cone terminals, even in 10-month-old transgenic swine, where the rods have all disappeared and the cones show signs of severe degeneration. These results suggest that loss of PSD-95 may not be a general consequence of the deteriorating cell. Rather, the very early and selective loss of PSD-95 from the rod terminals may be causally related to the absence of rod-rod bipolar cell synapses in the rhodopsin P347L transgenic retina.

Voltage-gated potassium channels in retinal ganglion cells of trout: a combined biophysical, pharmacological, and single-cell RT-PCR approach

Retinal ganglion cells of young mature trout were acutely isolated by tissue printing and analyzed with the whole-cell mode of the patch-clamp technique in combination with single-cell RT-PCR. All cells either exhibited spontaneous spiking activity or could be induced to fire trains of action potentials by current injection. Depolarizing voltage steps elicited a TTX-sensitive sodium inward current and a complex outward current that could be subdivided into a calcium-dependent component that was sensitive to 100 nM iberiotoxin as well as three major types of voltage-sensitive currents: 1) a high-threshold (-20 mV) noninactivating current that was highly sensitive to submicromolar TEA and quinine, resembling recombinant mammalian Kv3.1 channels; 2) a low-threshold DTX-sensitive current, matching mammalian Kv1; and 3) a fast-inactivating transient current that was highly sensitive to TEA (3 mM) but resistant to alpha-DTX (1 microM) and quinine (0.1 mM). By multiplex single-cell RT-PCR, coexpression of multiple transcripts encoding Shaker-related channel genes of trout (termed Tsha1-Tsha4) as well as two Shaw-related channels (termed Traw1 and Traw2) could be demonstrated in individual cells.

Kv4.2 mRNA abundance and A-type K(+) current amplitude are linearly related in basal ganglia and basal forebrain neurons

A-type K(+) currents are key determinants of repetitive activity and synaptic integration. Although several gene families have been shown to code for A-type channel subunits, recent studies have suggested that Kv4 family channels are the principal contributors to A-type channels in the somatodendritic membrane of mammalian brain neurons. If this hypothesis is correct, there should be a strong correlation between Kv4 family mRNA and A-type channel protein or aggregate channel currents. To test this hypothesis, quantitative single-cell reverse transcription-PCR analysis of Kv4 family mRNA was combined with voltage-clamp analysis of A-type K(+) currents in acutely isolated neurons. These studies revealed that Kv4.2 mRNA abundance was linearly related to A-type K(+) current amplitude in neostriatal medium spiny neurons and cholinergic interneurons, in globus pallidus neurons, and in basal forebrain cholinergic neurons. In contrast, there was not a significant correlation between estimates of Kv4.1 or Kv4.3 mRNA abundance and A-type K(+) current amplitudes. These results argue that Kv4.2 subunits are major constituents of somatodendritic A-type K(+) channels in these four types of neuron. In spite of this common structural feature, there were significant differences in the voltage dependence and kinetics of A-type currents in the cell types studied, suggesting that other determinants may create important functional differences between A-type K(+) currents.

Developmental changes in voltage-activated potassium currents of rat retinal ganglion cells

Ca2(+)-independent voltage-activated potassium currents were investigated during the differentiation of rat retinal ganglion cells. Whole-cell patch-clamp recordings of Ca2(+)-independent voltage-activated potassium currents and their individual current components, i.e. a sustained, tetraethylammonium-sensitive current, a transient, 4-aminopyridine-sensitive current, and a slowly decaying current that was blocked by Ba2+, revealed distinct ontogenetic modifications in current densities and in activation and inactivation parameters. All three current types were expressed simultaneously at embryonic day 17/18 and were present in all retinal ganglion cells thereafter without showing any significant changes until the end of the first postnatal week. Ca2(+)-independent voltage-activated potassium current densities then increased strongly from postnatal day 8 onwards. Tetraethylammonium-sensitive current density increased about eightfold from 74 pA/pF in embryonic stages to 586 pA/pF in adult cells, whereas the transient potassium currents blocked by 4-aminopyridine increased only about 2.5-fold from 174 pA/pF to 442 pA/pF. The Ba2(+)-sensitive current increased simultaneously from 35 pA/pF to 332 pA/pF. The much higher increase in the sustained current components during retinal ganglion cell differentiation accounted for the changes in decay kinetics of Ca2(+)-independent voltage-activated potassium current observed in later postnatal stages. Alterations in current densities were paralleled by pronounced changes in current kinetics. From postnatal day 8 onwards, activation of Ca2(+)-independent voltage-activated potassium current was right-shifted for about 10 mV owing to a shift in tetraethylammonium-sensitive current-activation, whereas activation of other K+ components remained unaltered. Tetraethylammonium-sensitive current steady-state inactivation was incomplete at all developmental stages. About 50% of the tetraethylammonium-sensitive current elicited by a depolarization to +36 mV did not inactivate after prepulse potentials positive to -10 mV. In contrast, transient potassium current blocked by 4-aminopyridine almost fully inactivated during embryonic stages, whereas in adult retinal ganglion cells about 40% of this current component did not inactivate after prepulse potentials positive to -20 mV. Parallel investigation of the resting membrane potential during retinal ganglion cells differentiation showed an exponential increase from -3 mV at embryonic day 15/16 when no voltage-activated ion currents were expressed to a final value of -58 mV at postnatal day 8. These results show that fundamental potassium current modifications occur relatively late in retinal ganglion cell development and only after the resting potential is at its final value.

Immunocytochemical localization of the postsynaptic density protein PSD-95 in the mammalian retina

Synapse-associated proteins are the scaffold for the selective aggregation of ion channels at synapses; they provide the link to cytoskeletal elements and possibly are involved with the regulation of synaptic efficacy by electrical activity. The localization of the postsynaptic density protein PSD-95 was studied in different mammalian retinae (rat, monkey, and tree shrew) by using immunocytochemical methods. Immunofluorescence for PSD-95 was most prominent in the outer plexiform layer (OPL). The axon terminals of rods and cones, the rod spherules and cone pedicles, were strongly labeled. Electron microscopy, using preembedding immunocytochemistry, showed PSD-95 localized presynaptically within the photoreceptor terminals. Distinct PSD-95 labeling was also present in the inner plexiform layer (IPL). It had a punctate appearance suggesting the synaptic clustering of PSD-95 in the IPL. Electron microscopy showed that PSD-95 was concentrated in processes that were postsynaptic at bipolar cell ribbon synapses (dyads). As a rule, only one of the two postsynaptic members of the dyad was labeled for PSD-95. Double-labeling experiments were performed for PSD-95 and for SAP 102 or PSD-93, respectively, two other members of the family of synapse-associated proteins. All three were found to be colocalized in the synaptic hot spots in the IPL. In the OPL, however, PSD-95 and PSD-93 were found presynaptically, whereas SAP 102 was located postsynaptically at photoreceptor synapses. Double-labeling experiments also were performed for PSD-95 and for the NR1 subunit of the NMDA receptor. They were found to be colocalized in synaptic hot spots in the IPL.

Differential distribution of Shaker-like and Shab-like K+-channel subunits in goldfish retina and retinal bipolar cells

The distributions of Shaker subfamily Kv1.1 and Kv1.2 and Shab subfamily Kv2.1 subunits of voltage-gated K+ channels were determined in the retina and ON bipolar cells of goldfish by using double-label light and electron microscopic immunocytochemistry. All labeling to be described was blocked by preabsorption of the primary antibodies with antigen. The retina was labeled throughout with all three antibodies. However, labeling was densest in the inner plexiform layer for Kv1.1, more concentrated in the outer nuclear layer for Kv2.1, and uniform throughout for Kv1.2. All ON mixed rod/cone (mb) and cone (cb) bipolar somata and the proximal portions of their axons and dendrites were labeled for anti-Kv1.1, Kv1.2, and Kv2.1. Labeling of axons rarely extended over the mb axon terminal. Only Kv1.2 antibodies labeled mb bipolar cell dendrites in the outer plexiform layer. No evidence for Kv1.1, 1.2, or 2.1 antibody labeling of OFF bipolar cells was found. Ultrastructurally, Kv1.2-immunoreactivity was associated with the plasma membrane of bipolar cell bodies and with dendrites that make narrow-cleft junctions with cone terminals (ON-type). Kv immunoreactivity was not found associated with presynaptic membranes in the inner plexiform layer and was found only rarely with membranes, postsynaptic to an amacrine cell process. Although both Shaker and Shab subfamilies include delayed rectifiers, their activation properties differ, suggesting differential modulation of K+ conductances in bipolar cells based not only on the presence or absence of rod photoreceptor input but also whether the bipolar cells are of the ON or OFF type.