Membrane properties of an unusual intrinsically oscillating, wide-field teleost retinal amacrine cell

Potential contributions of the intrinsic retinal oscillations recording using non-invasive electroretinogram to bioelectronics

Targeted electric signal use for disease diagnostics and treatment is emerging as a healthcare game-changer. Besides arrhythmias, treatment-resistant epilepsy and chronic pain, blindness, and perhaps soon vision loss, could be among the pathologies that benefit from bioelectronic medicine. The electroretinogram (ERG) technique has long demonstrated its role in diagnosing eye diseases and early stages of neurodegenerative diseases. Conspicuously, ERG applications are all based on light-induced responses. However, spontaneous, intrinsic activity also originates in retinal cells. It is a hallmark of degenerated retinas and its alterations accompany obesity and diabetes. To the extent that variables extracted from the resting activity of the retina measured by ERG allow the predictive diagnosis of risk factors for type 2 diabetes. Here, we provided a comparison of the baseline characteristics of intrinsic oscillatory activity recorded by ERGs in mice, rats, and humans, as well as in several rat strains, and explore whether zebrafish exhibit comparable activity. Their pattern was altered in neurodegenerative models including the cuprizone-induced demyelination model in mice as well as in the Royal College of Surgeons (RCS-/-) rats. We also discuss how the study of their properties may pave the way for future research directions and treatment approaches for retinopathies, among others.

Preventable risk factors for type 2 diabetes can be detected using noninvasive spontaneous electroretinogram signals

Given the ever-increasing prevalence of type 2 diabetes and obesity, the pressure on global healthcare is expected to be colossal, especially in terms of blindness. Electroretinogram (ERG) has long been perceived as a first-use technique for diagnosing eye diseases, and some studies suggested its use for preventable risk factors of type 2 diabetes and thereby diabetic retinopathy (DR). Here, we show that in a non-evoked mode, ERG signals contain spontaneous oscillations that predict disease cases in rodent models of obesity and in people with overweight, obesity, and metabolic syndrome but not yet diabetes, using one single random forest-based model. Classification performance was both internally and externally validated, and correlation analysis showed that the spontaneous oscillations of the non-evoked ERG are altered before oscillatory potentials, which are the current gold-standard for early DR. Principal component and discriminant analysis suggested that the slow frequency (0.4-0.7 Hz) components are the main discriminators for our predictive model. In addition, we established that the optimal conditions to record these informative signals, are 5-minute duration recordings under daylight conditions, using any ERG sensors, including ones working with portative, non-mydriatic devices. Our study provides an early warning system with promising applications for prevention, monitoring and even the development of new therapies against type 2 diabetes.

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.

μ-Opioid Receptor Activation Directly Modulates Intrinsically Photosensitive Retinal Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) encode light intensity and trigger reflexive responses to changes in environmental illumination. In addition to functioning as photoreceptors, ipRGCs are post-synaptic neurons in the inner retina, and there is increasing evidence that their output can be influenced by retinal neuromodulators. Here we show that opioids can modulate light-evoked ipRGC signaling, and we demonstrate that the M1, M2 and M3 types of ipRGCs are immunoreactive for μ-opioid receptors (MORs) in both mouse and rat. In the rat retina, application of the MOR-selective agonist DAMGO attenuated light-evoked firing ipRGCs in a dose-dependent manner (IC50 < 40 nM), and this effect was reversed or prevented by co-application of the MOR-selective antagonists CTOP or CTAP. Recordings from solitary ipRGCs, enzymatically dissociated from retinas obtained from melanopsin-driven fluorescent reporter mice, confirmed that DAMGO exerts its effect directly through MORs expressed by ipRGCs. Reduced ipRGC excitability occurred via modulation of voltage-gated potassium and calcium currents. These findings suggest a potential new role for endogenous opioids in the mammalian retina and identify a novel site of action-MORs on ipRGCs-through which opioids might exert effects on reflexive responses to environmental light.

Retinal Remodeling: Concerns, Emerging Remedies and Future Prospects

Deafferentation results not only in sensory loss, but also in a variety of alterations in the postsynaptic circuitry. These alterations may have detrimental impact on potential treatment strategies. Progressive loss of photoreceptors in retinal degenerative diseases, such as retinitis pigmentosa and age-related macular degeneration, leads to several changes in the remnant retinal circuitry. Müller glial cells undergo hypertrophy and form a glial seal. The second- and third-order retinal neurons undergo morphological, biochemical and physiological alterations. A result of these alterations is that retinal ganglion cells (RGCs), the output neurons of the retina, become hyperactive and exhibit spontaneous, oscillatory bursts of spikes. This aberrant electrical activity degrades the signal-to-noise ratio in RGC responses, and thus the quality of information they transmit to the brain. These changes in the remnant retina, collectively termed “retinal remodeling”, pose challenges for genetic, cellular and bionic approaches to restore vision. It is therefore crucial to understand the nature of retinal remodeling, how it affects the ability of remnant retina to respond to novel therapeutic strategies, and how to ameliorate its effects. In this article, we discuss these topics, and suggest that the pathological state of the retinal output following photoreceptor loss is reversible, and therefore, amenable to restorative strategies.

Origins of spontaneous activity in the degenerating retina

Sensory deafferentation resulting from the loss of photoreceptors during retinal degeneration (rd) is often accompanied by a paradoxical increase in spontaneous activity throughout the visual system. Oscillatory discharges are apparent in retinal ganglion cells in several rodent models of rd, indicating that spontaneous activity can originate in the retina. Understanding the biophysical mechanisms underlying spontaneous retinal activity is interesting for two main reasons. First, it could lead to strategies that reduce spontaneous retinal activity, which could improve the performance of vision restoration strategies that aim to stimulate remnant retinal circuits in blind patients. Second, studying emergent network activity could offer general insights into how sensory systems remodel upon deafferentation. Here we provide an overview of the work describing spontaneous activity in the degenerating retina, and outline the current state of knowledge regarding the cellular and biophysical properties underlying spontaneous neural activity.

Inputs underlying the ON-OFF light responses of type 2 wide-field amacrine cells in TH::GFP mice

In the mammalian retina, two types of catecholaminergic amacrine cells have been described. Although dopaminergic type 1 cells are well characterized, the physiology of type 2 cells is, so far, unknown. To target type 2 cells specifically, we used a transgenic mouse line that expresses green fluorescent protein under the control of the tyrosine hydroxylase promoter. Type 2 cells are GABAergic and have an extensive dendritic arbor, which stratifies in the middle of the inner plexiform layer. Our data suggest that type 2 cells comprise two subpopulations with identical physiological properties: one has its somata located in the inner nuclear layer and the other in the ganglion cell layer. Immunostaining with bipolar cell markers suggested that type 2 cells receive excitatory inputs from type 3 OFF and type 5 ON bipolar cells. Consistently, patch-clamp recordings showed that type 2 cells are ON-OFF amacrine cells. Blocking excitatory inputs revealed that different rod and cone pathways are active under scotopic and mesopic light conditions. Blockade of inhibitory inputs led to membrane potential oscillations in type 2 cells, suggesting that GABAergic and glycinergic amacrine cells strongly influence type 2 cell signaling. Among the glycinergic amacrine cells, we identified the VGluT3-immunoreactive amacrine cell as a likely candidate. Collectively, light responses of type 2 cells were remarkably uniform over a wide range of light intensities. These properties point toward a general function of type 2 cells that is maintained under scotopic and mesopic conditions.

An intrinsic neural oscillator in the degenerating mouse retina

The loss of photoreceptors during retinal degeneration (RD) is known to lead to an increase in basal activity in remnant neural networks. To identify the source of activity, we combined two-photon imaging with patch-clamp techniques to examine the physiological properties of morphologically identified retinal neurons in a mouse model of RD (rd1). Analysis of activity in rd1 ganglion cells revealed sustained oscillatory (∼10 Hz) synaptic activity in ∼30% of all classes of cells. Oscillatory activity persisted after putative inputs from residual photoreceptor, rod bipolar cell, and inhibitory amacrine cell synapses were pharmacologically blocked, suggesting that presynaptic cone bipolar cells were intrinsically active. Examination of presynaptic rd1 ON and OFF bipolar cells indicated that they rested at relatively negative potentials (less than -50 mV). However, in approximately half the cone bipolar cells, low-amplitude membrane oscillation (∼5 mV, ∼10 Hz) were apparent. Such oscillations were also observed in AII amacrine cells. Oscillations in ON cone bipolar and AII amacrine cells exhibited a weak apparent voltage dependence and were resistant to blockade of synaptic receptors, suggesting that, as in wild-type retina, they form an electrically coupled network. In addition, oscillations were insensitive to blockers of voltage-gated Ca(2+) channels (0.5 mm Cd(2+) and 0.5 mm Ni(2+)), ruling out known mechanisms that underlie oscillatory behavior in bipolar cells. Together, these results indicate that an electrically coupled network of ON cone bipolar/AII amacrine cells constitutes an intrinsic oscillator in the rd1 retina that is likely to drive synaptic activity in downstream circuits.

Zebrafish Tg(7.2mab21l2:EGFP)ucd2 transgenics reveal a unique population of retinal amacrine cells

PURPOSE

Amacrine cells constitute a diverse, yet poorly characterized, cell population in the inner retina. Here, the authors sought to characterize the morphology, molecular physiology, and electrophysiology of a subpopulation of EGFP-expressing retinal amacrine cells identified in a novel zebrafish transgenic line.

METHODS

After 7.2 kb of the zebrafish mab21l2 promoter was cloned upstream of EGFP, it was used to create the Tg(7.2mab21l2:EGFP)ucd2 transgenic line. Transgenic EGFP expression was analyzed by fluorescence microscopy in whole mount embryos, followed by detailed analysis of EGFP-expressing amacrine cells using fluorescence microscopy, immunohistochemistry, and electrophysiology.

RESULTS

A 7.2-kb fragment of the mab21l2 promoter region is sufficient to drive transgene expression in the developing lens and tectum. Intriguingly, EGFP was also observed in differentiated amacrine cells. EGFP-labeled amacrine cells in Tg(7.2mab21l2:EGFP)ucd2 constitute a novel GABA- and glycine-negative amacrine subpopulation. Morphologically, EGFP-expressing cells stratify in sublamina 1 to 2 (type 1 OFF) or sublamina 3 to 4 (type 1 ON) or branch diffusely (type 2). Electrophysiologically, these cells segregate into amacrine cells with somas in the vitreal part of the INL and linear responses to current injection or, alternatively, amacrine cells with somas proximal to the IPL and active oscillatory voltage signals. CONCLUSIONS; The novel transgenic line Tg(7.2mab21l2:EGFP)ucd2 uncovers a unique subpopulation of retinal amacrine cells.

A central pacemaker that underlies the production of seasonal and sexually dimorphic social signals: anatomical and electrophysiological aspects

Our long-term goal is to approach the understanding of the anatomical and physiological bases for communication signal diversity in gymnotiform fishes as a model for vertebrate motor pattern generation. Brachyhypopomus gauderio emits, in addition to its electric organ discharge (EOD) at basal rate, a rich repertoire of rate modulations. We examined the structure of the pacemaker nucleus, responsible for the EOD rate, to explore whether its high output signal diversity was correlated to complexity in its neural components or regional organization. We confirm the existence of only two neuron types and show that the previously reported dorsal-caudal segregation of these neurons is accompanied by rostral-caudal regionalization. Pacemaker cells are grouped dorsally in the rostral half of the nucleus, and relay cells are mainly ventral and more abundant in the caudal half. Relay cells are loosely distributed from the center to the periphery of the nucleus in correlation to somata size. Our findings support the hypothesis that regional organization enables a higher diversity of rate modulations, possibly offering distinct target areas to modulatory inputs. Since no anatomical or electrophysiological seasonal or sexual differences were found, we explored these aspects from a functional point of view in a companion article.

Neuronal functional diversity and collective behaviors: a scientific case

A major issue in today's neuroscience is how the brain complex and highly flexible organization emerges from its individual components. Robustness of neuronal properties with weak linkages between regulatory processes are suggested to account for the adaptive, tunable, multistable dynamics, the coding schemes and the complexity of neuronal functional (sub)systems. Interneurons and neurotransmitter diversity, resonance phenomena due to properties of the cell or network, time/frequency-dependent activation of dedicated neuronal assemblies, code- and frequency-specific oscillations interact in determining the brain functional setup and operations. Despite the scientific relevance, comprehensive theories are not yet available, but the scenario--however incomplete and incompletely characterized--is promising and warrants further investigation.

Neuronal functional diversity and collective behaviors

A major question in today's neuroscience is how the brain's complex operations and organization emerge from individual components. The robustness of neuronal properties with flexible linkages between regulatory processes conceivably accounts for the adaptive, tunable, multistable dynamics; the coding schemes; and the complexity of neuronal functional (sub)systems. Interneurons and neurotransmitter diversity, resonance phenomena due to properties of the cell, time/frequency-dependent activation of dedicated neuronal assemblies, and code- and frequency-specific oscillations interact in determining the brain functional setup and operations. Such an arrangement would also provide the functional requirements for access to neural mechanisms, dedicated neuronal circuitry and the proper timing allowing for the selective differentiation among cortical neurons due to performing in different tasks. No comprehensive theory or systematic methodological approach appears yet conceivable. The scenario, however incomplete and incompletely characterized, is nevertheless promising and warrants further investigation.

Synaptic regulation of the light-dependent oscillatory currents in starburst amacrine cells of the mouse retina

Responses of on-center starburst amacrine cells to steady light stimuli were recorded in the dark-adapted mouse retina. The response to spots of dim white light appear to show two components, an initial peak that correspond to the onset of the light stimulus and a series of oscillations that ride on top of the initial peak relaxation. The frequency of oscillations during light stimulation was three time higher than the frequency of spontaneous oscillations recorded in the dark. The light-evoked responses in starburst cells were exclusively dependent on the release of glutamate likely from presynaptic bipolar axon terminals and the binding of glutamate to AMPA/kainate receptors because they were blocked by 6-cyano-7-nitroquinoxalene-2,3-dione. The synaptic pathway responsible for the light responses was blocked by AP4, an agonist of metabotropic glutamate receptors that hyperpolarize on-center bipolar cells on activation. Light responses were inhibited by the calcium channel blockers cadmium ions and nifedipine, suggesting that the release of glutamate was calcium dependent. The oscillatory component of the response was specifically inhibited by blocking the glutamate transporter with d-threo-beta-benzyloxyaspartic acid, suggesting that glutamate reuptake is necessary for the oscillatory release. GABAergic antagonists bicuculline, SR 95531, and picrotoxin increased the amplitude of the initial peak while they inhibit the frequency of oscillations. TTX had a similar effect. Strychnine, the blocker of glycine receptors did not affect the initial peak but strongly decreased the oscillations frequency. These inhibitory inputs onto the bipolar axon terminals shape and synchronize the oscillatory component.

Response properties of a unique subtype of wide-field amacrine cell in the rabbit retina

We studied the morphology and physiology of a unique wide-field amacrine cell in the rabbit retina. These cells displayed a stereotypic dendritic morphology consisting of a large, circular and monostratified arbor that often extended over 2 mm. Their responses contained both somatic and dendritic sodium spikes suggesting active propagation of synaptic signals within the dendritic arbor. This idea is supported by the enormous size of their ON-OFF receptive fields. Interestingly, these cells exhibited separate ON and OFF receptive fields that, while concentric, were vastly different in size. Whereas the ON receptive field of these cells extended nearly 2 mm, the OFF receptive field was typically 75% smaller. Blockade of voltage-gated sodium channels with QX-314 dramatically reduced the large ON receptive field, but had little effect on the smaller OFF receptive field. These results indicate a spatial disparity in the location of on- and off-center bipolar cell inputs to the dendritic arbor of wide-field amacrine cells. In addition, the active propagation of signals suggests that synaptic inputs are integrated both locally and globally within the dendritic arbor.

Glycine receptors are functionally expressed on bullfrog retinal cone photoreceptors

Using immunocytochemical and whole cell recording techniques, we examined expression of glycine receptors on bullfrog retinal cone photoreceptors. Immunofluorescence double labeling experiments conducted on retinal sections and isolated cell preparations showed that terminals and inner segments of cones were immunoreactive to both alpha1 and beta subunits of glycine receptors. Moreover, application of glycine induced a sustained inward current from isolated cones, which increased in amplitude in a dose-dependent manner, with an EC50 (concentration of glycine producing half-maximal response) of 67.3+/-4.9 microM, and the current was blocked by the glycine receptor antagonist strychnine, but not 5,7-dichlorokynurenic acid (DCKA) of 200 microM, a blocker of the glycine recognition site at the N-methyl-D-aspartate (NMDA) receptor. The glycine-induced current reversed in polarity at a potential close to the calculated chloride equilibrium potential, and the reversal potential was changed as a function of the extracellular chloride concentration. These results suggest that strychnine-sensitive glycine receptors are functionally expressed in bullfrog cones, which may mediate signal feedback from glycinergic interplexiform cells to cones in the outer retina.

A high frequency resonance in the responses of retinal ganglion cells to rapidly modulated stimuli: a computer model

Brisk Y-type ganglion cells in the cat retina exhibit a high frequency resonance (HFR) in their responses to large, rapidly modulated stimuli. We used a computer model to test whether negative feedback mediated by axon-bearing amacrine cells onto ganglion cells could account for the experimentally observed properties of HFRs. Temporal modulation transfer functions (tMTFs) recorded from model ganglion cells exhibited HFR peaks whose amplitude, width, and locations were qualitatively consistent with experimental data. Moreover, the wide spatial distribution of axon-mediated feedback accounted for the observed increase in HFR amplitude with stimulus size. Model phase plots were qualitatively similar to those recorded from Y ganglion cells, including an anomalous phase advance that in our model coincided with the amplification of low-order harmonics that overlapped the HFR peak. When axon-mediated feedback in the model was directed primarily to bipolar cells, whose synaptic output was graded, or else when the model was replaced with a simple cascade of linear filters, it was possible to produce large HFR peaks but the region of anomalous phase advance was always eliminated, suggesting the critical involvement of strongly non-linear feedback loops. To investigate whether HFRs might contribute to visual processing, we simulated high frequency ocular tremor by rapidly modulating a naturalistic image. Visual signals riding on top of the imposed jitter conveyed an enhanced representation of large objects. We conclude that by amplifying responses to ocular tremor, HFRs may selectively enhance the processing of large image features.

Light-induced changes in spike synchronization between coupled ON direction selective ganglion cells in the mammalian retina

Although electrical coupling via gap junctions is prevalent among ganglion cells in the vertebrate retina, there have been few direct studies of their influence on the light-evoked signaling of these cells. Here, we describe the pattern and function of coupling between the ON direction selective (DS) ganglion cells, a unique subtype whose signals are transmitted to the accessory optic system (AOS) where they initiate the optokinetic response. ON DS cells are coupled indirectly via gap junctions made with a subtype of polyaxonal amacrine cell. This coupling underlies synchronization of the spontaneous and light-evoked spike activity of neighboring ON DS cells. However, we find that ON DS cell pairs show robust synchrony for all directions of stimulus movement, except for the null direction. Null stimulus movement evokes a GABAergic inhibition that temporally shifts firing of ON DS cell neighbors, resulting in a desynchronization of spike activity. Thus, detection of null stimulus movement appears key to the direction selectivity of ON DS cells, evoking both an attenuation of spike frequency and a desynchronization of neighbors. We posit that active desynchronization reduces summation of synaptic potentials at target AOS cells and thus provides a secondary mechanism by which ON DS cell ensembles can signal direction of stimulus motion to the brain.

Spontaneous oscillatory activity of starburst amacrine cells in the mouse retina

Using patch-clamp techniques, we investigated the characteristics of the spontaneous oscillatory activity displayed by starburst amacrine cells in the mouse retina. At a holding potential of -70 mV, oscillations appeared as spontaneous, rhythmic inward currents with a frequency of approximately 3.5 Hz and an average maximal amplitude of approximately 120 pA. Application of TEA, a potassium channel blocker, increased the amplitude of oscillatory currents by >70% but reduced their frequency by approximately 17%. The TEA effects did not appear to result from direct actions on starburst cells, but rather a modulation of their synaptic inputs. Oscillatory currents were inhibited by 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX), an antagonist of AMPA/kainate receptors, indicating that they were dependent on a periodic glutamatergic input likely from presynaptic bipolar cells. The oscillations were also inhibited by the calcium channel blockers cadmium and nifedipine, suggesting that the glutamate release was calcium dependent. Application of AP4, an agonist of mGluR6 receptors on on-center bipolar cells, blocked the oscillatory currents in starburst cells. However, application of TEA overcame the AP4 blockade, suggesting that the periodic glutamate release from bipolar cells is intrinsic to the inner plexiform layer in that, under experimental conditions, it can occur independent of photoreceptor input. The GABA receptor antagonists picrotoxin and bicuculline enhanced the amplitude of oscillations in starburst cells prestimulated with TEA. Our results suggest that this enhancement was due to a reduction of a GABAergic feedback inhibition from amacrine cells to bipolar cells and the resultant increased glutamate release. Finally, we found that some ganglion cells and other types of amacrine cell also displayed rhythmic activity, suggesting that oscillatory behavior is expressed by a number of inner retinal neurons.

Voltage-clamp analysis and computational model of dopaminergic neurons from mouse retina

Isolated dopaminergic amacrine (DA) cells in mouse retina fire rhythmic, spontaneous action potentials and respond to depolarizing current with trains of low-frequency action potentials. To investigate the roles of voltage-gated ion channels in these processes, the transient A-type K+ current (IK,A) and Ca2+ current (ICa) in isolated mouse DA cells were analyzed by voltage clamp. The IK,A activated at −60 mV and inactivated rapidly. ICa activated at around −30 mV and reached a peak at 10 mV without apparent inactivation. We also extended our previous computational model of the mouse DA cell to include the new electrophysiological data. The model consisted of a membrane capacitance in parallel with eight currents: Na+ transient (INa,T), Na+ persistent (INa,P), delayed rectifier potassium (IKdr), IK,A, calcium-dependent potassium (IK,Ca), L-type Ca2+ ICa, hyperpolarization-activated cation current (Ih), and a leak current (IL). Hodgkin-Huxley type equations were used to define the voltage- and time-dependent activation and inactivation. The simulations were implemented using the neurosimulator SNNAP. The model DA cell was spontaneously active from a wide range of initial membrane potentials. The spontaneous action potentials reached 35 mV at the peak and hyperpolarized to −76 mV between spikes. The spontaneous firing frequency in the model was 6 Hz. The model DA cell responded to prolonged depolarizing current injection by increasing its spiking frequency and eventually reaching a depolarization block at membrane potentials greater than −10 mV. The most important current for determining the firing rate was IK,A. When the amplitude of IK,A was decreased, the firing rate increased. ICa and IK,Ca also affected the width of action potentials but had only minor effects on the firing rate. Ih affected the firing rate slightly but did not change the waveform of the action potentials.

Light-evoked oscillatory discharges in retinal ganglion cells are generated by rhythmic synaptic inputs

In the visual system, optimal light stimulation sometimes generates gamma-range (ca. 20 approximately 80 Hz) synchronous oscillatory spike discharges. This phenomenon is assumed to be related to perceptual integration. Applying a planar multi-electrode array to the isolated frog retina, Ishikane et al. demonstrated that dimming detectors, off-sustained type ganglion cells, generate synchronous oscillatory spike discharges in response to diffuse dimming illumination. In the present study, applying the whole cell current-clamp technique to the isolated frog retina, we examined how light-evoked oscillatory spike discharges were generated in dimming detectors. Light-evoked oscillatory ( approximately 30 Hz) spike discharges were triggered by rhythmic ( approximately 30 Hz) fluctuations superimposed on a depolarizing plateau potential. When a suprathreshold steady depolarizing current was injected into a dimming detector, only a few spikes were evoked at the stimulus onset. However, repetitive spikes were triggered by a gamma-range sinusoidal current superimposed on the steady depolarizing current. Thus the light-evoked rhythmic fluctuations are likely to be generated presynaptically. The light-evoked rhythmic fluctuations were suppressed not by intracellular application of N-(2,6-dimethyl-phenylcarbamoylmethyl)triethylammonium bromide (QX-314), a Na(+) channel blocker, to the whole cell clamped dimming detector but by bath-application of tetrodotoxin to the retina. The light-evoked rhythmic fluctuations were suppressed by a GABA(A) receptor antagonist but potentiated by a GABA(C) receptor antagonist, whereas these fluctuations were little affected by a glycine receptor antagonist. Because amacrine cells are spiking neurons and because GABA is one of the main transmitters released from amacrine cells, amacrine cells may participate in generating rhythmically fluctuated synaptic input to dimming detectors.

Development of Transient Outward Currents Coupled With Ca2+-Induced Ca2+Release Mediates Oscillatory Membrane Potential in Ascidian Muscle Cells

Isolated ascidian Halocynthia roretzi blastomeres of the muscle lineage exhibit muscle cell-like excitability on differentiation despite the arrest of cell cleavage early in development. This characteristic provides a unique opportunity to track changes in ion channel expression during muscle cell differentiation. Here, we show that the intrinsic membrane property of ascidian cleavage-arrested muscle-type cells becomes oscillatory by expressing transient outward currents ( Ito) activated by Ca2+-induced Ca2+release (CICR) in a maturation-dependent manner. In current-clamp mode, most day 4 (72 h after fertilization) cleavage-arrested muscle cells exhibited an oscillatory membrane potential of –20 mV at 15 Hz, whereas most day 3 (48 h after fertilization) cells exhibited a spiking pattern. In voltage-clamp mode, the day 4 cells exhibited prominent transient outward currents that were not present in day 3 cells. Itowas abolished by the application of 10 mM caffeine, implying that CICR was involved in Itoactivation. Itowas based on K+efflux and sensitive to tetraethylammonium and some Ca2+-activated K+channel inhibitors. We found a 60-pS single channel conductance that was activated by local Ca2+release in ascidian muscle cell. Voltage-clamp recording with an oscillatory waveform as a command pulse showed that CICR-activated K+currents were activated during the falling phase of the membrane potential oscillation. These results suggest that developmental expression of CICR-activated K+current plays a role in the maturation of larval locomotion by modifying the intrinsic membrane excitability of muscle cells.

L-type calcium channels mediate transmitter release in isolated, wide-field retinal amacrine cells

Transmitter release in neurons is triggered by intracellular Ca2+ increase via the opening of voltage-gated Ca2+ channels. Here we investigated the voltage-gated Ca2+ channels in wide-field amacrine cells (WFACs) isolated from the white-bass retina that are functionally coupled to transmitter release. We monitored transmitter release through the measurement of the membrane capacitance (Cm). We found that 500-ms long depolarizations of WFACs from −70 mV to 0 mV elicited about a 6% transient increase in the Cm or membrane surface area. This Cm jump could be eliminated either by intracellular perfusion with 10 mM BAPTA or by extracellular application of 4 mM cobalt. WFACs possess N-type and L-type voltage-gated Ca2+ channels. Depolarization-evoked Cm increases were unaffected by the specific N-type channel blocker ω-conotoxin GVIA, but they were markedly reduced by the L-type blocker diltiazem, suggesting a role for the L-type channel in synaptic transmission. Further supporting this notion, in WFACs the synaptic protein syntaxin always colocalized with the pore-forming subunit of the retinal specific L-type channels (CaV1.4 or α1F), but never with that of the N-type channels (CaV2.2 or α1B).

Spontaneous activity of dopaminergic retinal neurons

Dopaminergic local circuit neurons in the retina (DA cells) show robust, spontaneous, tetrodotoxin-sensitive pacemaking. To investigate the mechanism underlying this behavior, we characterized the sodium current and a subset of the potassium currents in the cells in voltage-clamp experiments. We found that there is a persistent component of the sodium current in DA cells which activates at more depolarized potentials than the transient component of the current. The transient component was completely inactivated at -50 mV, but DA cells remained able to fire spontaneous action potentials when potassium channels were partially blocked and the membrane potential remained above -40 mV. Based on these electrophysiological data, we developed a reduced computer model that reproduced the major features of DA cells. In simulations at the physiological resting potential, the persistent component of the sodium current was both necessary and sufficient to account for spontaneous activity, and the major contribution of the transient component of the sodium current was to initiate the depolarization of the model cell during the interspike interval. When tonic inhibition was simulated by lowering the input impedance of the model cell, the transient component played a larger role.

A model of high-frequency oscillatory potentials in retinal ganglion cells

High-frequency oscillatory potentials (HFOPs) have been recorded from ganglion cells in cat, rabbit, frog, and mudpuppy retina and in electroretinograms (ERGs) from humans and other primates. However, the origin of HFOPs is unknown. Based on patterns of tracer coupling, we hypothesized that HFOPs could be generated, in part, by negative feedback from axon-bearing amacrine cells excited via electrical synapses with neighboring ganglion cells. Computer simulations were used to determine whether such axon-mediated feedback was consistent with the experimentally observed properties of HFOPs. (1) Periodic signals are typically absent from ganglion cell PSTHs, in part because the phases of retinal HFOPs vary randomly over time and are only weakly stimulus locked. In the retinal model, this phase variability resulted from the nonlinear properties of axon-mediated feedback in combination with synaptic noise. (2) HFOPs increase as a function of stimulus size up to several times the receptive-field center diameter. In the model, axon-mediated feedback pooled signals over a large retinal area, producing HFOPs that were similarly size dependent. (3) HFOPs are stimulus specific. In the model, gap junctions between neighboring neurons caused contiguous regions to become phase locked, but did not synchronize separate regions. Model-generated HFOPs were consistent with the receptive-field center dynamics and spatial organization of cat alpha cells. HFOPs did not depend qualitatively on the exact value of any model parameter or on the numerical precision of the integration method. We conclude that HFOPs could be mediated, in part, by circuitry consistent with known retinal anatomy.

Ionic mechanisms mediating oscillatory membrane potentials in wide-field retinal amacrine cells

Particular types of amacrine cells of the vertebrate retina show oscillatory membrane potentials (OMPs) in response to light stimulation. Historically it has been thought the oscillations arose as a result of circuit properties. In a previous study we found that in some amacrine cells, the ability to oscillate was an intrinsic property of the cell. Here we characterized the ionic mechanisms responsible for the oscillations in wide-field amacrine cells (WFACs) in an effort to better understand the functional properties of the cell. The OMPs were found to be calcium (Ca2+) dependent; blocking voltage-gated Ca2+ channels eliminated the oscillations, whereas elevating extracellular Ca2+ enhanced them. Strong intracellular Ca2+ buffering (10 mM EGTA or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid) eliminated any attenuation in the OMPs as well as a Ca2+-dependent inactivation of the voltage-gated Ca2+ channels. Pharmacological and immunohistochemical characterization revealed that WFACs express L- and N-type voltage-sensitive Ca2+ channels. Block of the L-type channels eliminated the OMPs, but omega-conotoxin GVIA did not, suggesting a different function for the N-type channels. The L-type channels in WFACs are functionally coupled to a set of calcium-dependent potassium (K(Ca)) channels to mediate OMPs. The initiation of OMPs depended on penitrem-A-sensitive (BK) K(Ca) channels, whereas their duration is under apamin-sensitive (SK) K(Ca) channel control. The Ca2+ current is essential to evoke the OMPs and triggering the K(Ca) currents, which here act as resonant currents, enhances the resonance as an amplifying current, influences the filtering characteristics of the cell membrane, and attenuates the OMPs via CDI of the L-type Ca2+ channel.

Intracellular calcium release resulting from mGluR1 receptor activation modulates GABAA currents in wide-field retinal amacrine cells: a study with caffeine

The modulatory action of calcium (Ca2+) released from intracellular stores on GABAA receptor-mediated current was investigated in wide-field amacrine cells isolated from the teleost, Morone chrysops, retina. Caffeine, ryanodine or inositol 1,4,5-triphosphate (IP3) markedly inhibited the GABAA current by elevating [Ca2+]i. The inhibition resulted from the activation of a Ca2+--> Ca2+/calmodulin --> calcineurin cascade. Long (>12 s) exposure to glutamate mimicked the caffeine effect, i.e. it inhibited the GABAA current by elevating [Ca2+]i through mGluR1 receptor activation and consequent IP3 generation. This pathway provides a 'timed' disinhibitory mechanism to potentiate excitatory postsynaptic potentials in wide-field amacrine cells. It occurs as a result of the suppression of GABA-mediated conductances as a function of the duration of presynaptic excitatory input activity. This is much like some forms of long-term potentiation in the central nervous system. In a local retinal circuit this will selectively accentuate particular excitatory inputs to the wide-field amacrine cell. Similar to other neural systems, we suggest that activity-dependent postsynaptic disinhibition is an important feature of the signal processing in the inner retina.