[Show abstract][Hide abstract] ABSTRACT: Presynaptic inhibition onto axons regulates neuronal output, but how such inhibitory synapses develop and are maintained in vivo remains unclear. Axon terminals of glutamatergic retinal rod bipolar cells (RBCs) receive GABAA and GABAC receptor-mediated synaptic inhibition. We found that perturbing GABAergic or glutamatergic neurotransmission does not prevent GABAergic synaptogenesis onto RBC axons. But, GABA release is necessary for maintaining axonal GABA receptors. This activity-dependent process is receptor subtype specific: GABAC receptors are maintained, whereas GABAA receptors containing α1, but not α3, subunits decrease over time in mice with deficient GABA synthesis. GABAA receptor distribution on RBC axons is unaffected in GABAC receptor knockout mice. Thus, GABAA and GABAC receptor maintenance are regulated separately. Although immature RBCs elevate their glutamate release when GABA synthesis is impaired, homeostatic mechanisms ensure that the RBC output operates within its normal range after eye opening, perhaps to regain proper visual processing within the scotopic pathway.
[Show abstract][Hide abstract] ABSTRACT: Excitatory amino acid transporters (EAATs) terminate signaling in the CNS by clearing released glutamate. Glutamate also evokes an EAAT-mediated Cl(-) current, but its role in CNS signaling is poorly understood. We show in mouse retina that EAAT-mediated Cl(-) currents that were evoked by light inhibit rod pathway signaling. EAATs reside on rod bipolar cell axon terminals where GABA and glycine receptors also mediate light-evoked inhibition. We found that the mode of inhibition depended on light intensity. Dim light evoked GABAergic and glycinergic inhibition with rapid kinetics and a large spatial extent. Bright light evoked predominantly EAAT-mediated inhibition with slow kinetics and a small spatial extent. The switch to EAAT-mediated signaling in bright light supplements receptor-mediated signaling to expand the dynamic range of inhibition and contributes to the transition from rod to cone signaling by suppressing rod pathway signaling in bright light conditions.
Journal of Neuroscience 03/2012; 32(13):4360-71. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The visual system is highly sensitive to dynamic features in the visual scene. However, it is not known how or where this enhanced sensitivity first occurs. We investigated this phenomenon by studying interactions between excitatory and inhibitory synapses in the second synaptic layer of the mouse retina. We found that these interactions showed activity-dependent changes that enhanced signaling of dynamic stimuli. Excitatory signaling from cone bipolar cells to ganglion cells exhibited strong synaptic depression, attributable to reduced glutamate release from bipolar cells. This depression was relieved by amacrine cell inhibitory feedback that activated presynaptic GABA(C) receptors. We found that the balance between excitation and feedback inhibition depended on stimulus frequency; at short interstimulus intervals, excitation was enhanced, attributable to reduced inhibitory feedback. This dynamic interplay may enrich visual processing by enhancing retinal responses to closely spaced temporal events, representing rapid changes in the visual environment.
Journal of Neuroscience 10/2011; 31(42):15102-12. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A fundamental question of cell signaling biology is how faint external signals produce robust physiological responses. One universal mechanism relies on signal amplification via intracellular cascades mediated by heterotrimeric G-proteins. This high amplification system allows retinal rod photoreceptors to detect single photons of light. Although much is now known about the role of the α-subunit of the rod-specific G-protein transducin in phototransduction, the physiological function of the auxiliary βγ-complex in this process remains a mystery. Here, we show that elimination of the transducin γ-subunit drastically reduces signal amplification in intact mouse rods. The consequence is a striking decline in rod visual sensitivity and severe impairment of nocturnal vision. Our findings demonstrate that transducin βγ-complex controls signal amplification of the rod phototransduction cascade and is critical for the ability of rod photoreceptors to function in low light conditions.
Journal of Neuroscience 06/2011; 31(22):8067-77. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bipolar cells (BCs) are critical relay neurons in the retina that are organized into parallel signaling pathways. The three main signaling pathways in the mammalian retina are the rod, ON cone, and OFF cone BCs. Rod BCs mediate incrementing dim light signals from rods, and ON cone and OFF cone BCs mediate incrementing and decrementing brighter light signals from cones, respectively. The outputs of BCs are shaped by inhibitory inputs from GABAergic and glycinergic amacrine cells in the inner plexiform layer, mediated by three distinct types of inhibitory receptors: GABA(A), GABA(C), and glycine receptors. The three main BC pathways receive distinct forms of inhibition from these three receptors that shape their light-evoked inhibitory signals. Rod BC inhibition is dominated by slow GABA(C) receptor inhibition, while OFF cone BCs are dominated by glycinergic inhibition. The inhibitory inputs to BCs are also shaped by serial inhibitory connections between GABAergic amacrine cells that limit the spatial profile of BC inhibition. We discuss our recent studies on how inhibitory inputs to BCs are shaped by receptor expression, receptor properties, and neurotransmitter release properties and how these affect the output of BCs.
[Show abstract][Hide abstract] ABSTRACT: While connections between inhibitory interneurons are common circuit elements, it has been difficult to define their signal processing roles because of the inability to activate these circuits using natural stimuli. We overcame this limitation by studying connections between inhibitory amacrine cells in the retina. These interneurons form spatially extensive inhibitory networks that shape signaling between bipolar cell relay neurons to ganglion cell output neurons. We investigated how amacrine cell networks modulate these retinal signals by selectively activating the networks with spatially defined light stimuli. The roles of amacrine cell networks were assessed by recording their inhibitory synaptic outputs in bipolar cells that suppress bipolar cell output to ganglion cells. When the amacrine cell network was activated by large light stimuli, the inhibitory connections between amacrine cells unexpectedly depressed bipolar cell inhibition. Bipolar cell inhibition elicited by smaller light stimuli or electrically activated feedback inhibition was not suppressed because these stimuli did not activate the connections between amacrine cells. Thus the activation of amacrine cell circuits with large light stimuli can shape the spatial sensitivity of the retina by limiting the spatial extent of bipolar cell inhibition. Because inner retinal inhibition contributes to ganglion cell surround inhibition, in part, by controlling input from bipolar cells, these connections may refine the spatial properties of the retinal output. This functional role of interneuron connections may be repeated throughout the CNS.
Journal of Neurophysiology 11/2009; 103(1):25-37. · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Synaptic integration is modulated by inhibition onto the dendrites of postsynaptic cells. However, presynaptic inhibition at axonal terminals also plays a critical role in the regulation of neurotransmission. In contrast to the development of inhibitory synapses onto dendrites, GABAergic/glycinergic synaptogenesis onto axon terminals has not been widely studied. Because retinal bipolar cells receive subclass-specific patterns of GABAergic and glycinergic presynaptic inhibition, they are a good model for studying the development of inhibition at axon terminals. Here, using whole cell recording methods and transgenic mice in which subclasses of retinal bipolar cells are labeled, we determined the temporal sequence and patterning of functional GABAergic and glycinergic input onto the major subclasses of bipolar cells. We found that the maturation of GABAergic and glycinergic synapses onto the axons of rod bipolar cells (RBCs), on-cone bipolar cells (ON-CBCs) and off-cone bipolar cells (OFF-CBCs) were temporally distinct: spontaneous chloride-mediated currents are present in RBCs earlier in development compared with ON- and OFF-CBC, and RBCs receive GABAergic and glycinergic input simultaneously, whereas in OFF-CBCs, glycinergic transmission emerges before GABAergic transmission. Because on-CBCs show little inhibitory activity, GABAergic and glycinergic events could not be pharmacologically distinguished for these bipolar cells. The balance of GABAergic and glycinergic input that is unique to RBCs and OFF-CBCs is established shortly after the onset of synapse formation and precedes visual experience. Our data suggest that presynaptic modulation of glutamate transmission from bipolar cells matures rapidly and is differentially coordinated for GABAergic and glycinergic synapses onto distinct bipolar cell subclasses.
Journal of Neurophysiology 08/2008; 100(1):304-16. · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mutations in the NYX gene that encodes the protein nyctalopin cause congenital stationary night blindness type 1. In no b-wave (nob) mice, a mutation in Nyx results in a functional phenotype that includes the absence of the electroretinogram b-wave and abnormal spontaneous and light-evoked activity in retinal ganglion cells (RGCs). In contrast, there is no morphological abnormality in the retina at either the light or electron microscopic levels. These functional deficits suggest that nyctalopin is required for normal synaptic transmission between retinal photoreceptors and depolarizing bipolar cells (DBCs). However, the synaptic etiology and, specifically, the exact location and function of nyctalopin, remain uncertain. We show that nob DBCs fail to respond to exogenous application of the photoreceptor neurotransmitter, glutamate, thus demonstrating a postsynaptic deficit in photoreceptor to bipolar cell communication. To determine if postsynaptic expression of nyctalopin is necessary and sufficient to rescue the nob phenotype, we constructed transgenic mice that expressed an EYFP-nyctalopin fusion protein on the dendritic tips of the DBCs. Immunohistochemical and ultrastructural studies verified that fusion protein expression was limited to the DBC dendritic tips. Fusion gene expression in nob mice restored normal outer and inner visual function as determined by the electroretinogram and RGC spontaneous and evoked responses. Together, our data show that nyctalopin expression on DBC dendrites is required for normal function of the murine retina.
Journal of Neurophysiology 12/2007; 98(5):3023-33. · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Diverse retinal outputs are mediated by ganglion cells that receive excitatory input from distinct classes of bipolar cells (BCs). These classes of BCs separate visual signals into rod, ON and OFF cone pathways. Although BC signalling is a major determinant of the ganglion cell-mediated retinal output, it is not fully understood how light-evoked, presynaptic inhibition from amacrine cell inputs shapes BC outputs. To determine whether differences in presynaptic inhibition uniquely modulate BC synaptic output to specific ganglion cells, we assessed the inhibitory contributions of GABA(A), GABA(C) and glycine receptors across the BC pathways. Here we show that different proportions of GABA(A) and GABA(C) receptor-mediated inhibition determined the kinetics of GABAergic presynaptic inhibition across different BC classes. Large, slow GABA(C) and small, fast GABA(A) receptor-mediated inputs to rod BCs prolonged light-evoked inhibitory postsynaptic currents (L-IPSCs), while smaller GABA(C) and larger GABA(A) receptor-mediated contributions produced briefer L-IPSCs in ON and OFF cone BCs. Glycinergic inhibition also varied across BC class. In the rod-dominant conditions studied here, slow glycinergic inputs dominated L-IPSCs in OFF cone BCs, attributable to inputs from the rod pathway via AII amacrine cells, while rod and ON cone BCs received little and no glycinergic input, respectively. As these large glycinergic inputs come from rod signalling pathways, in cone-dominant conditions L-IPSCs in OFF cone bipolar cells will probably be dominated by GABA(A) receptor-mediated input. Thus, unique presynaptic receptor combinations mediate distinct forms of inhibition to selectively modulate BC outputs, enhancing the distinctions among parallel retinal signals.
The Journal of Physiology 08/2007; 582(Pt 2):569-82. · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Members of the carbonic anhydrase (CA) family play an important role in the regulation of pH, CO(2), ion, and water transport. CA IV and CA XIV are membrane-bound isozymes expressed in the eye. CA IV immunostaining is limited to the choriocapillaris overlying the retina, whereas CA XIV is expressed within the retina in Müller glial cells and retinal pigment epithelium. Here, we have characterized the physiological and morphological phenotype of the CA IV-null, CA XIV-null, and CA IV/CA XIV-double-null mouse retinas. Flash electroretinograms performed at 2, 7, and 10 months of age showed that the rod/cone a-wave, b-wave, and cone b-wave were significantly reduced (26-45%) in the CA XIV-null mice compared with wild-type littermates. Reductions in the dark-adapted response were not progressive between 2 and 10 months, and no differences in retinal morphology were observed between wild-type and CA XIV-null mice. Müller cells and rod bipolar cells had a normal appearance. Retinas of CA IV-null mice showed no functional or morphological differences compared with normal littermates. However, CA IV/CA XIV double mutants showed a greater deficit in light response than the CA XIV-null retina. Our results indicate that CA XIV, which regulates extracellular pH and pCO(2), plays an important part in producing a normal retinal light response. A larger functional deficit in the CA IV/CA XIV double mutants suggests that CA IV can also contribute to pH regulation, at least in the absence of CA XIV.
Proceedings of the National Academy of Sciences 06/2007; 104(20):8514-9. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The retinal network increases its sensitivity in low-light conditions to detect small visual inputs and decreases its sensitivity in bright-light conditions to prevent saturation. However, the cellular mechanisms that adjust visual signaling in the retinal network are not known. Here, we show that voltage-gated sodium channels in bipolar cells dynamically control retinal light sensitivity. In dim conditions, sodium channels amplified light-evoked synaptic responses mediated by cone pathways. Conversely, in bright conditions, sodium channels were inactivated by dopamine released from amacrine cells, and they did not amplify synaptic inputs, minimizing signal saturation. Our findings demonstrate that bipolar cell sodium channels mediate light adaptation by controlling retinal signaling gain.
Journal of Neuroscience 05/2007; 27(17):4756-64. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Synaptic inhibition is determined by the properties of postsynaptic receptors, neurotransmitter release, and clearance, but little is known about how these factors shape sensation-evoked inhibition. The retina is an ideal system to investigate inhibition because it can be activated physiologically with light, and separate inhibitory pathways can be assayed by recording from rod bipolar cells that possess distinct glycine, GABA(A), and GABA(C) receptors (R). We show that receptor properties differentially shape spontaneous IPSCs, whereas both transmitter release and receptor properties shape light-evoked (L) IPSCs. GABA(C)R-mediated IPSCs decayed the slowest, whereas glycineR- and GABA(A)R-mediated IPSCs decayed more rapidly. Slow GABA(C)Rs determined the L-IPSC decay, whereas GABA(A)Rs and glycineRs, which mediated rapid onset responses, determined the start of the L-IPSC. Both fast and slow inhibitory inputs distinctly shaped the output of rod bipolar cells. The slow GABA(C)Rs truncated glutamate release, making the A17 amacrine cell L-EPSCs more transient, whereas the fast GABA(A)R and glycineRs reduced the initial phase of glutamate release, limiting the peak amplitude of the L-EPSC. Estimates of transmitter release time courses suggested that glycine release was more prolonged than GABA release. The time course of GABA release activating GABA(C)Rs was slower than that activating GABA(A)Rs, consistent with spillover activation of GABA(C)Rs. Thus, both postsynaptic receptor and transmitter release properties shape light-evoked inhibition in retina.
Journal of Neuroscience 10/2006; 26(37):9413-25. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sensory information is thought to be modulated by presynaptic inhibition. Although this form of inhibition is a well-studied phenomenon, it is still unclear what role it plays in shaping sensory signals in intact circuits. By visually stimulating the retinas of transgenic mice lacking GABAc receptor-mediated presynaptic inhibition, we found that this inhibition regulated the dynamic range of ganglion cell (GC) output to the brain. Presynaptic inhibition acted differentially upon two major retinal pathways; its elimination affected GC responses to increments, but not decrements, in light intensity across the visual scene. The GC dynamic response ranges were different because presynaptic inhibition limited glutamate release from ON, but not OFF, bipolar cells, which modulate the extent of glutamate spillover and activation of perisynaptic NMDA receptors at ON GCs. Our results establish a role for presynaptic inhibitory control of spillover in determining sensory output in the CNS.
[Show abstract][Hide abstract] ABSTRACT: Rod bipolar cells relay visual signals evoked by dim illumination from the outer to the inner retina. GABAergic and glycinergic amacrine cells contact rod bipolar cell terminals, where they modulate transmitter release and contribute to the receptive field properties of third order neurones. However, it is not known how these distinct inhibitory inputs affect rod bipolar cell output and subsequent retinal processing. To determine whether GABA(A), GABA(C) and glycine receptors made different contributions to light-evoked inhibition, we recorded light-evoked inhibitory postsynaptic currents (L-IPSCs) from rod bipolar cells mediated by each pharmacologically isolated receptor. All three receptors contributed to L-IPSCs, but their relative roles differed; GABA(C) receptors transferred significantly more charge than GABA(A) and glycine receptors. We determined how these distinct inhibitory inputs affected rod bipolar cell output by recording light-evoked excitatory postsynaptic currents (L-EPSCs) from postsynaptic AII and A17 amacrine cells. Consistent with their relative contributions to L-IPSCs, GABA(C) receptor activation most effectively reduced the L-EPSCs, while glycine and GABA(A) receptor activation reduced the L-EPSCs to a lesser extent. We also found that GABAergic L-IPSCs in rod bipolar cells were limited by GABA(A) receptor-mediated inhibition between amacrine cells. We show that GABA(A), GABA(C) and glycine receptors mediate functionally distinct inhibition to rod bipolar cells, which differentially modulated light-evoked rod bipolar cell output. Our findings suggest that modulating the relative proportions of these inhibitory inputs could change the characteristics of rod bipolar cell output.
The Journal of Physiology 05/2006; 572(Pt 1):215-25. · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Illumination of the receptive-field surround reduces the sensitivity of a retinal ganglion cell to centre illumination. The steady, antagonistic receptive-field surround of retinal ganglion cells is classically attributed to the signalling of horizontal cells in the outer plexiform layer (OPL). However, amacrine cell signalling in the inner plexiform layer (IPL) also contributes to the steady receptive-field surround of the ganglion cell. We examined the contributions of these two forms of presynaptic lateral inhibition to ganglion cell light sensitivity by measuring the effects of surround illumination on EPSCs evoked by centre illumination. GABA(C) receptor antagonists reduced inhibition attributed to dim surround illumination, suggesting that this inhibition was mediated by signalling to bipolar cell axon terminals. Brighter surround illumination further reduced the light sensitivity of the ganglion cell. The bright surround effects on the EPSCs were insensitive to GABA receptor blockers. Perturbing outer retinal signalling with either carbenoxolone or cobalt blocked the effects of the bright surround illumination, but not the effects of dim surround illumination. We found that the light sensitivities of presynaptic, inhibitory pathways in the IPL and OPL were different. GABA(C) receptor blockers reduced dim surround inhibition, suggesting it was mediated in the IPL. By contrast, carbenoxolone and cobalt reduced bright surround, suggesting it was mediated by horizontal cells in the OPL. Direct amacrine cell input to ganglion cells, mediated by GABA(A) receptors, comprised another surround pathway that was most effectively activated by bright illumination. Our results suggest that surround activation of lateral pathways in the IPL and OPL differently modulate the sensitivity of the ganglion cell to centre illumination.
The Journal of Physiology 07/2005; 565(Pt 2):517-35. · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Retinal bipolar cells are slow potential neurons that respond to photoreceptor inputs with graded potentials and do not fire action potentials. We found that transient ON bipolar cells recorded in retinal slices possess voltage-gated sodium channels located on either their dendrites or somas. The sodium currents in these neurons did not generate spikes but enhanced voltage responses evoked by visual stimulation, which selectively boosted transmission to transient ganglion cells. In contrast, sodium currents were not found in sustained ON bipolar cells, and light responses in sustained bipolar cells and ganglion cells were not affected by TTX. The presence of sodium channels in transient ON bipolar cells contributed to the separation of transient and sustained signals by selectively enhancing the responses of ON transient ganglion cells to light. Our results suggest that bipolar cell sodium channels augment transient signals and contribute to the temporal segregation of visual information.
Journal of Neuroscience 03/2005; 25(7):1856-65. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The retina is a layered structure that processes information in two stages. The outer plexiform layer (OPL) comprises the first stage and is where photoreceptors, bipolar cells, and horizontal cells interact synaptically. This is the synaptic layer where ON and OFF responses to light are formed, as well as the site where receptive field center and surround organization is first thought to occur. The inner plexiform layer (IPL) is where the second stage of synaptic interactions occurs. This synaptic layer is where subsequent visual processing occurs that may contribute to the formation of transient responses, which may underlie motion and direction sensitivity. In addition, synaptic interactions in the IPL may also contribute to the classical ganglion cell receptive field properties. This chapter will focus on the synapse and network properties at the IPL that sculpt light-evoked ganglion cell responses. These include synaptic mechanisms that may shape ganglion cell responses like desensitizing glutamate receptors and transporters, which remove glutamate from the synapse. Recent work suggests that inhibitory signaling at the IPL contributes to the surround receptive field organization of ganglion cells. A component of this amacrine cell inhibitory signaling is mediated by GABAC receptors, which are found on bipolar cell axon terminals in the IPL. Pharmacological experiments show that a component of the ganglion cell surround signal is mediated by these receptors, indicating that the ganglion cell center and surround receptive field organization is not formed entirely in the outer plexiform layer, as earlier thought.
Progress in brain research 02/2005; 147:205-18. · 4.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Inhibition at bipolar cell axon terminals regulates excitatory signaling to ganglion cells and is mediated, in part, by GABAC receptors. We investigated GABAC receptor-mediated inhibition using pharmacological approaches and genetically altered mice that lack GABAC receptors. Responses to applied GABA showed distinct time courses in various bipolar cell classes, attributable to different proportions of GABAA and GABAC receptors. The elimination of GABAC receptors in GABAC null mice reduced and shortened GABA-activated currents and light-evoked inhibitory synaptic currents (L-IPSCs) in rod bipolar cells. ERG measurements and recordings from the optic nerve showed that inner retinal function was altered in GABAC null mice. These data suggest that GABAC receptors determine the time course and extent of inhibition at bipolar cell terminals that, in turn, modulates the magnitude of excitatory transmission from bipolar cells to ganglion cells.
Vision Research 12/2004; 44(28):3289-96. · 2.14 Impact Factor