Béla Völgyi

Connexin36 Is Essential for Transmission of Rod-Mediated Visual Signals in the Mammalian Retina

To examine the functions of electrical synapses in the transmission of signals from rod photoreceptors to ganglion cells, we generated connexin36 knockout mice. Reporter expression indicated that connexin36 was present in multiple retinal neurons including rod photoreceptors, cone bipolar cells, and AII amacrine cells. Disruption of electrical synapses between adjacent AIIs and between AIIs and ON cone bipolars was demonstrated by intracellular injection of Neurobiotin. In addition, extracellular recording in the knockout revealed the complete elimination of rod-mediated, on-center responses at the ganglion cell level. These data represent direct proof that electrical synapses are critical for the propagation of rod signals across the mammalian retina, and they demonstrate the existence of multiple rod pathways, each of which is dependent on electrical synapses.

The diverse functional roles and regulation of neuronal gap junctions in the retina

Electrical synaptic transmission through gap junctions underlies direct and rapid neuronal communication in the CNS. The diversity of functional roles that electrical synapses have is perhaps best exemplified in the vertebrate retina, in which gap junctions are formed by each of the five major neuron types. These junctions are dynamically regulated by ambient illumination and by circadian rhythms acting through light-activated neuromodulators such as dopamine and nitric oxide, which in turn activate intracellular signalling pathways in the retina.The networks formed by electrically coupled neurons are plastic and reconfigurable, and those in the retina are positioned to play key and diverse parts in the transmission and processing of visual information at every retinal level.

Tracer coupling patterns of the ganglion cell subtypes in the mouse retina

It is now clear that electrical coupling via gap junctions is prevalent across the retina, expressed by each of the five main neuronal types. With the introduction of mutants in which selective gap junction connexins are deleted, the mouse has recently become an important model for studying the function of coupling between retinal neurons. In this study we examined the tracer‐coupling pattern of ganglion cells by injecting them with the gap junction‐permanent tracer Neurobiotin to provide, for the first time, a comprehensive survey of ganglion cell coupling in the wildtype mouse retina. Murine ganglion cells were differentiated into 22 morphologically distinct subtypes based on soma‐dendritic parameters. Most (16/22) ganglion cell subtypes were tracer‐coupled to neighboring ganglion and/or amacrine cells. The amacrine cells coupled to ganglion cells displayed either polyaxonal or wide‐field morphologies with extensive arbors. We found that different subtypes of ganglion cells were never coupled to one another, indicating that they subserved independent electrical networks. Finally, we found that the tracer‐coupling patterns of the 22 ganglion cell populations were largely stereotypic across the 71 retinas studied. Our results indicate that electrical coupling is extensive in the inner retina of the mouse, suggesting that gap junctions play essential roles in visual information processing. J. Comp. Neurol. 512:664–687, 2009.

Convergence and segregation of the multiple rod pathways in mammalian retina

Using a multidisciplinary approach, we demonstrate that three different pathways are responsible for the transmission of rod signals across the mouse retina. Each pathway serves a primarily nonoverlapping range of stimulus intensities, with ganglion cells receiving either segregated or convergent inputs. For both on-center (ON) and off-center (OFF) ganglion cells, the primary rod pathway carries signals with the lowest threshold, whereas the secondary rod pathway is less sensitive by ∼1 log unit. In addition, OFF signaling uses a tertiary rod pathway that is ∼1 log unit less sensitive than the secondary. Although some ganglion cells received rod inputs exclusively from one of the pathways, others showed convergent inputs. Using pharmacological and genetic approaches, we defined classes of ON and OFF ganglion cells for which the scotopic inputs derive only from the primary pathway or from both primary and secondary pathways. In addition, we observed a class of OFF ganglion cell receiving mixed input from primary and tertiary pathways. Interestingly, OFF ganglion cells receiving convergent inputs from all three rod pathways or from the secondary and tertiary pathways together were never observed. Overall, our data show a complex arrangement of convergence and segregation of rod inputs to ganglion cells in the mammalian retina.

Cadherin-6 Mediates Axon-Target Matching in a Non-Image-Forming Visual Circuit

Neural circuits consist of highly precise connections among specific types of neurons that serve a common functional goal. How neurons distinguish among different synaptic targets to form functionally precise circuits remains largely unknown. Here, we show that during development, the adhesion molecule cadherin-6 (Cdh6) is expressed by a subset of retinal ganglion cells (RGCs) and also by their targets in the brain. All of the Cdh6-expressing retinorecipient nuclei mediate non-image-forming visual functions. A screen of mice expressing GFP in specific subsets of RGCs revealed that Cdh3-RGCs which also express Cdh6 selectively innervate Cdh6-expressing retinorecipient targets. Moreover, in Cdh6-deficient mice, the axons of Cdh3-RGCs fail to properly innervate their targets and instead project to other visual nuclei. These findings provide functional evidence that classical cadherins promote mammalian CNS circuit development by ensuring that axons of specific cell types connect to their appropriate synaptic targets.

Morphology and physiology of the polyaxonal amacrine cells in the rabbit retina

We examined the morphology and physiological response properties of the axon‐bearing, long‐range amacrine cells in the rabbit retina. These so‐called polyaxonal amacrine cells all displayed two distinct systems of processes: (1) a dendritic field composed of highly branched and relatively thick processes and (2) a more extended, often sparsely branched axonal arbor derived from multiple thin axons emitted from the soma or dendritic branches. However, we distinguished six morphological types of polyaxonal cells based on differences in the fine details of their soma/dendritic/axonal architecture, level of stratification within the inner plexiform layer (IPL), and tracer coupling patterns. These morphological types also showed clear differences in their light‐evoked response activity. Three of the polyaxonal amacrine cell types showed on‐off responses, whereas the remaining cells showed on‐center responses; we did not encounter polyaxonal cells with off‐center physiology. Polyaxonal cells respected the on/off sublamination scheme in that on‐off cells maintained dendritic/axonal processes in both sublamina a and b of the IPL, whereas processes of on‐center cells were restricted to sublamina b. All polyaxonal amacrine cell types displayed large somatic action potentials, but we found no evidence for low‐amplitude dendritic spikes that have been reported for other classes of amacrine cell. The center‐receptive fields of the polyaxonal cells were comparable to the diameter of their respective dendritic arbors and, thus, were significantly smaller than their extensive axonal fields. This correspondence between receptive and dendritic field size was seen even for cells showing extensive homotypic and/or heterotypic tracer coupling to neighboring neurons. These data suggest that all polyaxonal amacrine cells are polarized functionally into receptive dendritic and transmitting axonal zones. J. Comp. Neurol. 440:109–125, 2001.

Function and plasticity of homologous coupling between AII amacrine cells.

The AII amacrine cells are critical elements in the primary rod pathway of the mammalian retina, acting as an obligatory conduit of rod signals to both on- and off-center ganglion cells. In addition to the chemical synaptic circuitry they subserve, AII cells form two types of electrical synapses corresponding to gap junctions formed between neighboring AII cells as well as junctions formed between AII cells and on-center cone bipolar cells. Our recent results indicate that coupling between AII cells and cone bipolar cells forms an obligatory synapse for transmission of scotopic visual signals to on-center ganglion cells. In contrast, AII-AII cell coupling acts to maintain the sensitivity of the primary rod pathway by allowing for summation of synchronous activity and the attenuation of asynchronous background noise. Further, the conductance of AII-AII cell gap junctions is highly dynamic, regulated by ambient light conditions, thereby preserving the fidelity of rod signaling over the scotopic operating range from starlight to twilight.

Morphology and tracer coupling pattern of alpha ganglion cells in the mouse retina.

Alpha cells are a type of ganglion cell whose morphology appears to be conserved across a number of mammalian retinas. In particular, alpha cells display the largest somata and dendritic arbors at a given eccentricity and tile the retina as independent on‐ (ON) and off‐center (OFF) subtypes. Mammalian alpha cells also express a variable tracer coupling pattern, which often includes homologous (same cell type) coupling to a few neighboring alpha cells and extensive heterologous (different cell type) coupling to two to three amacrine cell types. Here, we use the gap junction‐permeant tracer Neurobiotin to determine the architecture and coupling pattern of alpha cells in the mouse retina. We find that alpha cells show the same somatic and dendritic architecture described previously in the mammal. However, alpha cells show varied tracer coupling patterns related to their ON and OFF physiologies. ON alpha cells show no evidence of homologous tracer coupling but are coupled heterologously to at least two types of amacrine cell whose somata lie within the ganglion cell layer. In contrast, OFF alpha cells are coupled to one another in circumscribed arrays as well as to two to three types of amacrine cell with somata occupying the inner nuclear layer. We find that homologous coupling between OFF alpha cells is unaltered in the connexin36 (Cx36) knockout (KO) mouse retina, indicating that it is not dependent on Cx36. However, a subset of the heterologous coupling of ON alpha cells and all the heterologous coupling of OFF alpha cells are eliminated in the KO retina, suggesting that Cx36 comprises most of the junctions made with amacrine cells. J. Comp. Neurol. 492:66–77, 2005.

A Unique Role for Kv3 Voltage-Gated Potassium Channels in Starburst Amacrine Cell Signaling in Mouse Retina

Direction-selective retinal ganglion cells show an increased activity evoked by light stimuli moving in the preferred direction. This selectivity is governed by direction-selective inhibition from starburst amacrine cells occurring during stimulus movement in the opposite or null direction. To understand the intrinsic membrane properties of starburst cells responsible for direction-selective GABA release, we performed whole-cell recordings from starburst cells in mouse retina. Voltage-clamp recordings revealed prominent voltage-dependent K+ currents. The currents were mostly blocked by 1 mm TEA, activated rapidly at voltages more positive than -20 mV, and deactivated quickly, properties reminiscent of the currents carried by the Kv3 subfamily of K+ channels. Immunoblots confirmed the presence of Kv3.1 and Kv3.2 proteins in retina and immunohistochemistry revealed their expression in starburst cell somata and dendrites. The Kv3-like current in starburst cells was absent in Kv3.1-Kv3.2 knock-out mice. Current-clamp recordings showed that the fast activation of the Kv3 channels provides a voltage-dependent shunt that limits depolarization of the soma to potentials more positive than -20 mV. This provides a mechanism likely to contribute to the electrical isolation of individual starburst cell dendrites, a property thought essential for direction selectivity. This function of Kv3 channels differs from that in other neurons where they facilitate high-frequency repetitive firing. Moreover, we found a gradient in the intensity of Kv3.1b immunolabeling favoring proximal regions of starburst cells. We hypothesize that this Kv3 channel gradient contributes to the preference for centrifugal signal flow in dendrites underlying direction-selective GABA release from starburst amacrine cells

Feedback inhibition in the inner plexiform layer underlies the surround-mediated responses of AII amacrine cells in the mammalian retina

Intracellular recordings were made from narrow‐field, bistratified AII amacrine cells in the isolated, superfused retina‐eyecup of the rabbit. Pharmacological agents were applied to neurons to dissect the synaptic pathways subserving AII cells so as to determine the circuitry generating their off‐surround responses. Application of the GABA antagonists, picrotoxin, bicuculline and 1,2,5,6‐tetrahydropyridine‐4‐yl methylphosphinic acid (TPMPA) all increased the on‐centre responses of AII amacrine cells, but attenuated the off‐surround activity. At equal concentrations, picrotoxin was approximately twice as effective as bicuculline or TPMPA in modifying the response activity of AII amacrine cells. These results indicate that the mechanism underlying surround inhibition of AII amacrine cells includes activation of both GABAA and GABAC receptors in an approximately equal ratio. Application of the GABA antagonists also increased the size of on‐centre receptive fields of AII amacrine cells. Again, picrotoxin was most effective, producing, on average, a 54 % increase in the size of the receptive field, whereas bicuculline and TPMPA produced comparable 34 and 33 % increases, respectfully. Application of the voltage‐gated sodium channel blocker TTX produced effects on AII amacrine cells qualitatively similar to those of the GABA blockers. Intracellular application of the chloride channel blocker 4,4′‐dinitro‐stilbene‐2,2′‐disulphonic acid (DNDS) abolished the direct effects of GABA on AII amacrine cells. Moreover, DNDS increased the amplitude of both the on‐centre and off‐surround responses. The failure of DNDS to block the off‐surround activity indicates that it is not mediated by direct GABAergic inhibition. Taken together, our results suggest that surround receptive fields of AII amacrine cells are generated indirectly by the GABAergic, reciprocal feedback synapses from S1/S2 amacrine cells to the axon terminals of rod bipolar cells.