OFF pathway is preferentially suppressed by the activation of GABA(A) receptors in carp retina

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

Differential modulation by AMPA of signals from red- and green-sensitive cones in carp retinal luminosity-type horizontal cells

Intracellular recordings were made from luminosity-type horizontal cells (LHCs) in the isolated superfused carp retina and the effect of AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid), a glutamate receptor agonist, on these cells was studied. AMPA suppressed the responses of LHCs driven by red-sensitive (R-) cones whereas it potentiated the responses driven by green-sensitive (G-) cones. The AMPA effect could be completely blocked by GYKI 53655, a specific AMPA receptor antagonist, indicating the exclusive involvement of AMPA-preferring receptors. The AMPA effect persisted in the presence of picrotoxin (PTX) or dihydrokainic acid (DHK), suggesting that the feedback from LHCs onto cones and glutamate transporters on cones may not be involved. It is suggested that there may exist different AMPA receptor subtypes with distinct characteristics on LHCs, which mediate signal transfer from R-and G-cones to LHCs, respectively.

Electrophysiological evidence of GABAA and GABAC receptors on zebrafish retinal bipolar cells

To refine inhibitory circuitry models for ON and OFF pathways in zebrafish retina, GABAergic properties of zebrafish bipolar cells were studied with two techniques: whole cell patch responses to GABA puffs in retinal slice, and voltage probe responses in isolated cells. Retinal slices documented predominantly axon terminal responses; isolated cells revealed mainly soma-dendritic responses. In the slice, GABA elicited a conductance increase, GABA responses were more robust at axon terminals than dendrites, and Erev varied with [Cl(-)]in. Axon terminals of ON- and OFF-type cells were similarly sensitive to GABA (30-40 pA peak current); axotomized cells were unresponsive. Bicuculline-sensitive, picrotoxin-sensitive, and picrotoxin-insensitive components were identified. Muscimol was as effective as GABA; baclofen was ineffective. Isolated bipolar cells were either intact or axotomized. Even in cells without an axon, GABA or muscimol (but not baclofen) hyperpolarized dendritic and somatic regions, suggesting significant distal expression. Median fluorescence change for GABA was -0.22 log units (approximately -16 mV); median half-amplitude dose was 0.4 microM. Reduced [Cl(-)]out blocked GABA responses. GABA hyperpolarized isolated ON-bipolar cells; OFF-cells were either unresponsive or depolarized. Hyperpolarizing GABA responses in isolated cells were bicuculline and TPMPA insensitive, but blocked or partially blocked by picrotoxin or zinc. In summary, axon terminals contain bicuculline-sensitive GABAA receptors and both picrotoxin-sensitive and insensitive GABAC receptors. Dendritic processes express zinc- and picrotoxin-sensitive GABAC receptors.

GABAa and GABAc receptor mediated influences on the intensity-response functions of the b- and d-wave in the frog ERG

We assessed the contribution of GABAa and GABAc receptors to GABAergic effects on b- and d-wave in frog ERG in a wide range of light stimulation conditions. The amplitude of both b- and d-wave was increased during GABAa receptor blockade by bicuculline as well as during additional GABAc receptor blockade by picrotoxin. The effects of GABAa receptor blockade were more pronounced in light adaptation conditions. They strongly depended on stimulus intensity and showed considerable ON/OFF-response asymmetry. The effects of GABAc receptor blockade were more pronounced in dark adaptation conditions. They didn't vary much with stimulus intensity and showed little ON/OFF-response asymmetry.

Intrinsic ON responses of the retinal OFF pathway are suppressed by the ON pathway

Parallel ON and OFF pathways conduct visual signals from bipolar cells in the retina to higher centers in the brain. ON responses are thought to originate by exclusive use of metabotropic glutamate receptor 6 (mGluR6) expressed in retinal ON bipolar cells. Paradoxically, we find ON responses in retinal ganglion cells of mGluR6-null mice, but they occur at long latency. The long-latency ON responses are not blocked by metabotropic glutamate or cholinergic receptor antagonists and are not produced by activation of receptive field surrounds. We show that these longer-latency ON responses are initiated in the OFF pathways. Our results expose a previously unrecognized intrinsic property of OFF retinal pathways that generates responses to light onset. In mGluR6-null mice, long-latency ON responses are observed in the visual cortex, indicating that they can be conducted reliably to higher visual areas. In wild-type (WT) mice, APB (DL-2-amino-4-phosphonobutyric acid), an mGluR6 agonist, blocks normal, short-latency ON responses but unmasks longer-latency ones. We find that these potentially confusing ON responses in the OFF pathway are actively suppressed in WT mice via two pharmacologically separable retinal circuits that are activated by the ON system in the retina. Consequently, we propose that a major function of the signaling of the ON pathway to the OFF pathway is suppression of these mistimed, and therefore inappropriate, light-evoked responses.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

AMPA receptor is involved in transmission of cone signal to ON bipolar cells in carp retina

The present work focuses on characterization of glutamate receptor subtypes mediating cone signal transmission to ON bipolar cells (BCs) in the carp retina, using intracellular recording techniques. Glutamate (5 mM) hyperpolarized cone-dominant ON BCs, which was associated with a suppression of light responses, whereas Co(2+) (1 mM) depolarized these cells and suppressed their light responses. On the other hand, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) of 20 microM caused a membrane depolarization and blocked depolarizing light responses, L-2-amino-4-phosphonobutryic acid (l-AP4) was without effect. The effects of AMPA were reversed by coapplication of GYKI 52466, an AMPA receptor selective non-competitive antagonist, but persisted in the presence of picrotoxin and strychnine. For rod-dominant ON BCs, both l-AP4 and AMPA reversibly blocked depolarizing light responses, but with membrane potential changes of opposite polarities (hyperpolarization for l-AP4 and depolarization for AMPA). In the inner retina, AMPA depolarized transient ON-OFF amacrine cells and blocked both ON and OFF cone-driven depolarizing responses, but l-AP4 did not. These results suggest that AMPA receptors, but not l-AP4 receptors, are involved in synaptic transmission of cone signal to ON bipolar cells in carp retina.

Suppression by zinc of transient OFF responses of carp amacrine cells to red light is mediated by GABA(A) receptors

Modulation by Zn(2+) of ON and OFF responses of transient amacrine cells driven by red- and green-sensitive cones was investigated in isolated, superfused carp retina, using intracellular recording techniques. Zn(2+) selectively abolished the OFF response to red flash of the transient amacrine cells. This Zn(2+) effect was mimicked by GABA application and was blocked by bicuculline, indicating the involvement of GABA(A) receptors. Such differential modulation was observable neither in bipolar cells nor in sustained OFF amacrine cells. It is suggested that the Zn(2+) effect reported here might be due to a direct action of Zn(2+) on GABA(A) receptors of the transient amacrine cells.

Zn2+ differentially modulates signals from red- and short wavelength-sensitive cones to horizontal cells in carp retina

The effects of Zn2+ were studied while recording intracellularly from L-type horizontal cells (LHCs) in the isolated, superfused carp retina. In darkness, 25 microM Zn2+ hyperpolarized LHCs and potentiated responses of these cells to 500 nm flashes, but decreased those to 680 nm flashes. Zn2+ did not change photopic electroretinographic P III responses. The differential modulation by Zn2+ persisted when the Zn2+-induced membrane hyperpolarization was compensated by lowering Ca2+ concentration in the perfusate, but it was abolished in the presence of background illumination. Furthermore, the differential modulation no longer existed in the presence of bicuculline, suggesting the involvement of gamma-aminobutyric acid(A) (GABA(A)) receptors. We speculate that the differential modulation may be a consequence of multiple changes caused by Zn2+. Decreased glutamate release from the cone terminal by Zn2+ results in a reduction of cone signals. Zn2+ antagonizes GABA receptors on LHCs, leading to cone signal reduction. On the other hand, Zn2+ may reduce the strength of the negative feedback from LHCs to cones by downregulating the activity of GABA receptors on the cone terminal, which causes a potentiation of LHC light responses. Cone- or wavelength-relevance of the Zn2+-induced feedback strength change may account for the differential modulation.

Stimulus pattern related plasticity of synapses between cones and horizontal cells in carp retina

Stimulus pattern related synaptic plasticity in the luminosity-type horizontal cell (LHC) of isolated carp retina was investigated. The major findings were: (1) repetitive red flashes progressively strengthened the synaptic connection between red-cone and LHC, whereas weakened that between green-cone and LHC; (2) repetitive green flashes remarkably depressed the LHC's red response, but caused little changes in the cell's green response. A competitive depression between different cone signals is suggested.

GABA modulates color-opponent bipolar cells in carp retina

In the present work, the effects of gamma-aminobutyric acid (GABA) were examined while recording intracellularly from color-opponent bipolar cells in the isolated, superfused carp retina. Bath-applied GABA differentially suppressed light responses of R/G cells, which depolarized and hyperpolarized, respectively, in response to red and green flashes, and G/R cells, which showed the opposite responses. In either R/G or G/R cells, the response driven by input from red cones was invariably suppressed to a greater extent. Such effects of GABA were completely blocked by picrotoxin, but only partially by bicuculline, suggesting that the GABAC receptor, in addition to the GABAA receptor, may also be involved. It is postulated that GABA modulates the color-opponent bipolar cells either through activating GABA receptors on these cells directly or those on cone terminals indirectly.