Voltage-gated sodium channels contribute to the b-wave of the rodent electroretinogram by mediating input to rod bipolar cell GABAc receptors

Contribution of chemical and electrical transmission to the low delta-like intrinsic retinal oscillation in mice: A role for daylight-activated neuromodulators

Basal electroretinogram (ERG) oscillations have shown predictive value for modifiable risk factors for type 2 diabetes. However, their origin remains unknown. Here, we seek to establish the pharmacological profile of the low delta-like (δ1) wave in the mouse because it shows light sensitivity in the form of a decreased peak frequency upon photopic exposure. Applying neuropharmacological drugs by intravitreal injection, we eliminated the δ1 wave using lidocaine or by blocking all chemical and electrical synapses. The δ1 wave was insensitive to the blockade of photoreceptor input, but was accelerated when all inhibitory or ionotropic inhibitory receptors in the retina were antagonized. The sole blockade of GABAA, GABAB, GABAC, and glycine receptors also accelerated the δ1 wave. In contrast, the gap junction blockade slowed the δ1 wave. Both GABAA receptors and gap junctions contribute to the light sensitivity of the δ1 wave. We further found that the day light-activated neuromodulators dopamine and nitric oxide donors mimicked the effect of photopic exposure on the δ1 wave. All drug effects were validated through light flash-evoked ERG responses. Our data indicate that the low δ-like intrinsic wave detected by the non-photic ERG arises from an inner retinal circuit regulated by inhibitory neurotransmission and nitric oxide/dopamine-sensitive gap junction-mediated communication.

Synaptotagmins 1 and 7 in vesicle release from rods of mouse retina

Synaptotagmins are the primary Ca2+ sensors for synaptic exocytosis. Previous work suggested synaptotagmin-1 (Syt1) mediates evoked vesicle release from cone photoreceptor cells in the vertebrate retina whereas release from rods may involve another sensor in addition to Syt1. We found immunohistochemical evidence for syntaptotagmin-7 (Syt7) in mouse rod terminals and so performed electroretinograms (ERG) and single-cell recordings using mice in which Syt1 and/or Syt7 were conditionally removed from rods and/or cones. Synaptic release was measured in mouse rods by recording presynaptic anion currents activated during glutamate re-uptake and from exocytotic membrane capacitance changes. Deleting Syt1 from rods reduced glutamate release evoked by short depolarizing steps but not long steps whereas deleting Syt7 from rods reduced release evoked by long but not short steps. Deleting both sensors completely abolished depolarization-evoked release from rods. Effects of various intracellular Ca2+ buffers showed that Syt1-mediated release from rods involves vesicles close to ribbon-associated Ca2+ channels whereas Syt7-mediated release evoked by longer steps involves more distant release sites. Spontaneous release from rods was unaffected by eliminating Syt7. While whole animal knockout of Syt7 slightly reduced ERG b-waves and oscillatory potentials, selective elimination of Syt7 from rods had no effect on ERGs. Furthermore, eliminating Syt1 from rods and cones abolished ERG b-waves and additional elimination of Syt7 had no further effect. These results show that while Syt7 contributes to slow non-ribbon release from rods, Syt1 is the principal sensor shaping rod and cone inputs to bipolar cells in response to light flashes.

Review: Use of Electrophysiological Techniques to Study Visual Functions of Aquatic Organisms

The light environments of natural water sources have specific characteristics. For the majority of aquatic organisms, vision is crucial for predation, hiding from predators, communicating information, and reproduction. Electroretinography (ERG) is a diagnostic method used for assessing visual function. An electroretinogram records the comprehensive potential response of retinal cells under light stimuli and divides it into several components. Unique wave components are derived from different retinal cells, thus retinal function can be determined by analyzing these components. This review provides an overview of the milestones of ERG technology, describing how ERG is used to study visual sensitivity (e.g., spectral sensitivity, luminous sensitivity, and temporal resolution) of fish, crustaceans, mollusks, and other aquatic organisms (seals, sea lions, sea turtles, horseshoe crabs, and jellyfish). In addition, it describes the correlations between visual sensitivity and habitat, the variation of visual sensitivity as a function of individual growth, and the diel cycle changes of visual sensitivity. Efforts to identify the visual sensitivity of different aquatic organisms are vital to understanding the environmental plasticity of biological evolution and for directing aquaculture, marine fishery, and ecosystem management.

Electrophysiologic Characterization of Developing Human Embryonic Stem Cell-Derived Photoreceptor Precursors

Purpose

Photoreceptor precursor cells (PRPs) differentiated from human embryonic stem cells can serve as a source for cell replacement therapy aimed at vision restoration in patients suffering from degenerative diseases of the outer retina, such as retinitis pigmentosa and AMD. In this work, we studied the electrophysiologic maturation of PRPs throughout the differentiation process.

Methods

Human embryonic stem cells were differentiated into PRPs and whole-cell recordings were performed for electrophysiologic characterization at days 0, 30, 60, and 90 along with quantitative PCR analysis to characterize the expression level of various ion channels, which shape the electrophysiologic response. Finally, to characterize the electrically induced calcium currents, we employed calcium imaging (rhod4) to visualize intracellular calcium dynamics in response to electrical activation.

Results

Our results revealed an early and steady presence (approximately 100% of responsive cells) of the delayed potassium rectifier current. In contrast, the percentage of cells exhibiting voltage-gated sodium currents increased with maturation (from 0% to almost 90% of responsive cells at 90 days). Moreover, calcium imaging revealed the presence of voltage-gated calcium currents, which play a major role in vision formation. These results were further supported by quantitative PCR analysis, which revealed a significant and continuous (3- to 50-fold) increase in the expression of various voltage-gated channels concomitantly with the increase in the expression of the photoreceptor marker CRX.

Conclusions

These results can shed light on the electrophysiologic maturation of neurons in general and PRP in particular and can form the basis for devising and optimizing cell replacement-based vision restoration strategies.

Voltage-gated sodium channel-dependent retroaxonal modulation of photoreceptor function during post-natal development in mice

Juvenile (postnatal day 16) mice lacking Na_v 1.6 channels (null-mutant Scn8a^dmu ) have reduced photoreceptor function, which is unexpected given that Na_v channels have not been detected in mouse photoreceptors and do not contribute appreciably to photoreceptor function in adults. We demonstrate that acute block of Na_v channels with intravitreal TTX in juvenile (P16) wild-type mice has no effect on photoreceptor function. However, reduced light activity by prolonged dark adaptation from P8 caused significant reduction in photoreceptor function at P16. Injecting TTX into the retrobulbar space at P16 to specifically block Na_v channels in the optic nerve also caused a reduction in photoreceptor function comparable to that seen at P16 in null-mutant Scn8a mice. In both P16 null-mutant Scn8a^dmu and retrobulbar TTX-injected wild-type mice, photoreceptor function was restored following intravitreal injection of the TrkB receptor agonist 7,8-dihydroxyflavone, linking Na_v -dependent retrograde transport to TrkB-dependent neurotrophic factor production pathways as a modulatory influence of photoreceptor function at P16. We also found that in Scn8a^dmu mice, photoreceptor function recovers by P22-25 despite more precarious general health of the animal. Retrobulbar injection of TTX in the wild type still reduced the photoreceptor response at this age but to a lesser extent, suggesting that Na_v -dependent modulation of photoreceptor function is largely transient, peaking soon after eye opening. Together, these results suggest that the general photosensitivity of the retina is modulated following eye opening by retrograde transport through activity-dependent retinal ganglion cell axonal signaling targeting TrkB receptors.

Angiopoietin-1 Knockout Mice as a Genetic Model of Open-Angle Glaucoma

Purpose

A leading cause of blindness worldwide, glaucoma is often caused by elevated intraocular pressure (IOP) due to impaired aqueous humor outflow from the anterior chamber through Schlemm's canal (SC) and the trabecular meshwork. Despite the large clinical burden, glaucoma research and drug development are hindered by a limited selection of preclinical models that accurately recapitulate human disease. Here, we propose that Angpt1 conditional knockout mice may provide one such model. Angiopoietin/TEK (ANGPT/TEK) signaling is crucial for SC formation and integrity in mice and humans, and mice lacking TEK or its ligand ANGPT1 develop a hypomorphic SC insufficient for normal aqueous humor outflow.

Methods

We used a comprehensive histology and physiology approach to characterize the glaucoma phenotype of Angpt1 inducible knockout mice, especially focusing on retina morphology and function.

Results

Angpt1 deletion resulted in persistent ocular hypertension beginning in the first month after birth and leading to decreased visual acuity with age due to glaucomatous neuropathy. In the neural retina, we identified marked and specific loss of the retinal ganglion cells, whereas other retinal neurons exhibited largely normal morphology and patterning. Electroretinogram recordings demonstrated reduced scotopic threshold response, further indicating loss of retinal ganglion cell function.

Conclusions

These findings highlight the potential of Angpt1 conditional knockout mice as a valuable new glaucoma model.

Translational Relevance

Currently, few reliable, rapid-onset genetic glaucoma models are available, and Angpt1 knockout mice will provide an additional tool for studies of IOP-induced neural damage, mechanisms of disease progression, and novel treatment strategies.

Voltage-gated Sodium Channels in Sensory Information Processing

Objective & Background:

Voltage-gated sodium channels (VGSCs) and potassium channels are critical in the generation of action potentials in the nervous system. VGSCs and potassium channels play important roles in the five fundamental senses of vision, audition, olfaction, taste and touch. Dysfunctional VGSCs are associated with clinical sensory symptoms, such as hyperpselaphesia, parosphresia, and so on.

Conclusion:

This short review highlights the recent advances in the study of VGSCs in sensory information processing and discusses the potential role of VGSCs to serve as pharmacological targets for the treatment of sensory system diseases.

Revisiting cannabinoid receptor 2 expression and function in murine retina

The cannabinoid receptor CB2 plays a significant role in the regulation of immune function whereas neuronal expression remains a subject of contention. Multiple studies have described CB2 in retina and a recent study showed that CB2 deletion altered retinal visual processing. We revisited CB2 expression using immunohistochemistry and a recently developed CB2-eGFP reporter mouse. We examined the consequence of acute vs. prolonged CB2 deactivation on the electroretinogram (ERG) responses. We also examined lipidomics in CB2 knockout mice and potential changes in microglia using Scholl analysis. Consistent with a published report, in CB2 receptor knockout mice see an increased ERG scotopic a-wave, as well as stronger responses in dark adapted cone-driven ON bipolar cells and, to a lesser extent cone-driven ON bipolar cells early in light adaptation. Significantly, however, acute block with CB2 antagonist, AM630, did not mimic the results observed in the CB2 knockout mice whereas chronic (7 days) block did. Immunohistochemical studies show no CB2 in retina under non-pathological conditions, even with published antibodies. Retinal CB2-eGFP reporter signal is minimal under baseline conditions but upregulated by intraocular injection of either LPS or carrageenan. CB2 knockout mice see modest declines in a broad spectrum of cannabinoid-related lipids. The numbers and morphology of microglia were unaltered. In summary minimal CB2 expression is seen in healthy retina. CB2 appears to be upregulated under pathological conditions. Previously reported functional consequences of CB2 deletion are an adaptive response to prolonged blockade of these receptors. CB2 therefore impacts retinal signaling but perhaps in an indirect, potentially extra-ocular fashion.

Dopamine-Dependent Sensitization of Rod Bipolar Cells by GABA Is Conveyed through Wide-Field Amacrine Cells

The vertebrate retina has the remarkable ability to support visual function under conditions of limited illumination, including the processing of signals evoked by single photons. Dim-light vision is regulated by several adaptive mechanisms. The mechanism explored in this study is responsible for increasing the light sensitivity and operational range of rod bipolar cells, the retinal neurons operating immediately downstream of rod photoreceptors. This sensitization is achieved through the sustained dopamine-dependent GABA release from other retinal neurons. Our goals were to identify the cell type responsible for the GABA release and the site of its modulation by dopamine. Previous studies have suggested the involvement of amacrine and/or horizontal cells. We now demonstrate, using mice of both sexes, that horizontal cells do not participate in this mechanism. Instead, sustained GABA input is provided by a subpopulation of wide-field amacrine cells, which stimulate the GABA_C receptors at rod bipolar cell axons. We also found that dopamine does not act directly on either of these cells. Rather, it suppresses inhibition imposed on these wide-field cells by another subpopulation of upstream GABAergic amacrine cells, thereby sustaining the GABA_C receptor activation required for rod bipolar cell sensitization.SIGNIFICANCE STATEMENT The vertebrate retina has an exquisite ability to adjust information processing to ever-changing conditions of ambient illumination, from bright sunlight to single-photon counting under dim starlight. Operation under each of these functional regimes requires an engagement of specific adaptation mechanisms. Here, we describe a mechanism optimizing the performance of the dim-light channel of vision, which consists of sensitizing rod bipolar cells by a sustained GABAergic input originating from a population of wide-field amacrine cells. Wide-field amacrine cells span large segments of the retina, making them uniquely equipped to normalize and optimize response sensitivity across distant receptive fields and preclude any bias toward local light-intensity fluctuations.

Retinal A2A and A3 adenosine receptors modulate the components of the rat electroretinogram

Adenosine is a neuromodulator present in various areas of the central nervous system, including the retina. Adenosine may serve a neuroprotective role in the retina, based on electroretinogram (ERG) recordings from the rat retina. Our purpose was to assess the role of A2A and A3 adenosine receptors in the generation and modulation of the rat ERG. The flash ERG was recorded with corneal electrodes from Sprague Dawley rats. Agonists and antagonists for A2A and A3 receptors, and adenosine were injected (5 µl) into the vitreous. The effects on the components of the single flash scotopic and photopic ERGs were examined, and ERG flicker. Adenosine (0.5 mM) increased the mean amplitudes of the scotopic ERG a-waves (68 ± 8 to 97 ± 14 µV, P = 0.042), and b-waves (236 ± 38 µV to 305 ± 42 µV). A2A agonist CGS21680 (2 mM) reduced the mean amplitude of the ERG b-wave, from 298 ± 21 µV in response to the brightest stimulus to 212 ± 19 µV (P = 0.005), and mean scotopic oscillatory potentials (OPs) from 100 ± 9 µV to 47 ± 11 µV (P = 0.023). ZM241385 [4 mM], an A2A antagonist, decreased the scotopic b-wave of the ERG. A3 agonist 2-CI-IB-MECA (0.5 mM) increased the a-wave, while decreasing the scotopic and photopic ERG b-waves, and the scotopic OPs. A3 antagonist VUF5574 (1 mM) increased the mean amplitude of the scotopic a-wave (66 ± 8 to 140 ± 29 µV, P = 0.046) and b-wave (224 ± 20 to 312 ± 39 µV, P = 0.0037). No significant effects on ERG flicker were found. We conclude that retinal neurons containing A2A and/or A3 adenosine receptors contribute to the generation of the ERG a- and b-waves and OPs.

Contribution of GABAa, GABAc and glycine receptors to rat dark-adapted oscillatory potentials in the time and frequency domain

Retinal oscillatory potentials (OPs) consist of a series of relatively high-frequency rhythmic wavelets, superimposed onto the ascending phase of the b-wave of the electroretinogram (ERG). However, the origin of OPs is uncertain and methods of measurement of OPs are diverse. In this study, we first isolated OPs from the rat ERG and fitted them with Gabor functions and found that the envelope of the OP contained information about maximum amplitude and time-to-peak to enable satisfactory quantification of the later OPs. And the OP/b-wave ratio should be evaluated to exclude an effect of the b-wave on the OPs. Next, we recorded OPs after intravitreal injection of 2-amino-4-phosphonobutyric acid (APB), tetrodotoxin (TTX), γ-aminobutyric acid (GABA), strychnine (STR), SR95531 (SR), isoguvacine (ISO), (1,2,5,6-tetrahydropyridin-4-yl) methylphosphinic acid (TPMPA) and GABA+TPMPA. We showed that GABA and APB only removed the later OPs, when compared to control eyes. TTX delayed the peak time, and STR, SR and ISO reduced the amplitude of OPs. TPMPA delayed the peak time but increased the ratio of OPs to b-wave. Furthermore, administration of combined GABA and TPMPA caused the later OPs to increase in amplitude with time, compared with those after delivery of GABA alone. Finally, we observed that GABAc and glycine receptors contributed to a low-frequency component of the OPs, while GABAa contributed to both components. These results suggest that the early components of the OPs are mainly generated by the photoreceptors, whilst the later components are mainly regulated by GABAa, GABAc and glycine receptors.

Contribution of Nav1.8 sodium channels to retinal function

We examined the contribution of the sodium channel isoform Na_v1.8 to retinal function using the specific blocker A803467. We found that A803467 has little influence on the electroretinogram (ERG) a- and b-waves, but significantly reduces the oscillatory potentials (OPs) to 40-60% of their original amplitude, with significant changes in implicit time in the rod-driven range. To date, only two cell types were found in mouse to express Na_v1.8; the starburst amacrine cells (SBACs), and a subtype of retinal ganglion cells (RGCs). When we recorded light responses from ganglion cells using a multielectrode array we found significant and opposing changes in two physiological groups of RGCs. ON-sustained cells showed significant decreases while transient ON-OFF cells showed significant increases. The effects on ON-OFF transient cells but not ON-sustained cells disappeared in the presence of an inhibitory cocktail. We have previously shown that RGCs have only a minor contribution to the OPs (Smith et al., 2014), therefore suggesting that SBACs might be a significant contributor to this ERG component. Targeting SBACs with the cholinergic neurotoxin ethylcholine mustard aziridinium (AF64A) caused a reduction in the amplitude of the OPs similar to A803467. Our results, both using the ERG and MEA recordings from RGCs, suggest that Na_v1.8 plays a role in modulating specific aspects of the retinal physiology and that SBACs are a fundamental cellular contributor to the OPs in mice, a clear demonstration of the dichotomy between ERG b-wave and OPs.

Polysialylated Neural Cell Adhesion Molecule Protects Against Light-Induced Retinal Degeneration

Purpose

We previously demonstrated that neural cell adhesion molecule (NCAM) plays an important role in supporting the survival of injured retinal ganglion cells. In the current study, we used light-induced retinal degeneration (LIRD) as a model to investigate whether NCAM plays a functional role in neuroprotection and whether NCAM influences p75^NTR signaling in modulating retinal cell survival.

Methods

Retinas from wild-type (WT) and NCAM deficient (−/−) mice were tested by electroretinogram before and after LIRD, and changes in the protein expressions of NCAM, polysialic acid (PSA)-NCAM, p75^NTR, and active caspase 3 were measured by immunoblot from 0 to 4 days after light induction. The effects of NCAM and PSA-NCAM on p75^NTR were examined by intraocular injections of the p75^NTR function-blocking antibody and/or the removal of PSA with endoneuraminidase-N prior to LIRD.

Results

In WT mice, low levels of active caspase 3 activation were detected on the first day, followed by increases up to 4 days after LIRD. Conversely, in NCAM^−/− mice, higher cleaved caspase 3 levels along with rapid reductions in electroretinogram amplitudes were found earlier at day 1, followed by reduced levels by day 4. The removal of PSA prior to LIRD induced earlier onset of retinal cell death, an effect delayed by the coadministration of endoneuraminidase-N and the p75^NTR function-blocking antibody antiserum.

Conclusions

These results indicate that NCAM protects WT retinas from LIRD; furthermore, the protective effect of NCAM is, at least in part, attributed to its effects on p75^NTR.

Short Communication: Contribution of Voltage-Gated Sodium Channels to the Rabbit Cone Electroretinograms

PURPOSE

Although the rabbit eye is of a similar size to the human eye, our limited understanding of the differences in retinal physiology to other species hinders its use in retinal research. The role of voltage-gated sodium channels (Nav) in the propagation of excitatory potentials along bipolar cells remains unclear, as conflicting data have been reported in the rabbit. The present study assesses the relative contributions of Nav to the scotopic and photopic flash ERGs as well as the wavelength-dependence of Nav blockade on the rabbit flicker ERG.

MATERIALS AND METHODS

Tetrodotoxin (TTX, 1 μM) was injected into the vitreous cavity of Chinchilla bastard rabbits. Scotopic ERGs were evoked by white flashes ranging from 10(-5) to 10 cds m(-2), photopic ERGs on a background of 25 cdm(-2) using flash intensities of 0.032-25 cds m(-2). Flicker ERGs (3-50 Hz) were elicited by blue, green and yellow stimuli at 2.34 cds m(-2) on a white background of 30 cdm(-2).

RESULTS

The a- and b-waves of the scotopic ERG were unaffected by intravitreal injection of the Nav blocker TTX. In contrast, the b-wave, but not the a-wave, of the photopic ERG was selectively blocked by TTX. The reduction by TTX of the flicker ERG was greater for blue than for green and yellow stimuli.

DISCUSSION

The data suggest that Nav selectively contribute to the generation of the photopic b-wave in the rabbit, indicating that they play an important role in the propagation of excitatory signals on bipolar cells in the cone, but not rod pathways. Importantly, the present study resolves conflicting previous reports into the role of Nav in the retinal function of the rabbit.

Effect of NCAM on aged-related deterioration in vision

The neural cell adhesion molecule (NCAM) is involved in developmental processes and age-associated cognitive decline; however, little is known concerning the effects of NCAM in the visual system during aging. Using anatomical, electrophysiological, and behavioral assays, we analyzed age-related changes in visual function of NCAM deficient (-/-) and wild-type mice. Anatomical analyses indicated that aging NCAM -/- mice had fewer retinal ganglion cells, thinner retinas, and fewer photoreceptor cell layers than age-matched controls. Electroretinogram testing of retinal function in young adult NCAM -/- mice showed a 2-fold increase in a- and b-wave amplitude compared with wild-type mice, but the retinal activity dropped dramatically to control levels when the animals reached 10 months. In behavioral tasks, NCAM -/- mice had no visual pattern discrimination ability and showed premature loss of vision as they aged. Together, these findings demonstrate that NCAM plays significant roles in the adult visual system in establishing normal retinal anatomy, physiology and function, and in maintaining vision during aging.

RdgB2 is required for dim-light input into intrinsically photosensitive retinal ganglion cells

A subset of retinal ganglion cells is intrinsically photosensitive (ipRGCs) and contributes directly to the pupillary light reflex and circadian photoentrainment under bright-light conditions. ipRGCs are also indirectly activated by light through cellular circuits initiated in rods and cones. A mammalian homologue (RdgB2) of a phosphoinositide transfer/exchange protein that functions in Drosophila phototransduction is expressed in the retinal ganglion cell layer. This raised the possibility that RdgB2 might function in the intrinsic light response in ipRGCs, which depends on a cascade reminiscent of Drosophila phototransduction. Here we found that under high light intensities, RdgB2(-/-) mutant mice showed normal pupillary light responses and circadian photoentrainment. Consistent with this behavioral phenotype, the intrinsic light responses of ipRGCs in RdgB2(-/-) were indistinguishable from wild-type. In contrast, under low-light conditions, RdgB2(-/-) mutants displayed defects in both circadian photoentrainment and the pupillary light response. The RdgB2 protein was not expressed in ipRGCs but was in GABAergic amacrine cells, which provided inhibitory feedback onto bipolar cells. We propose that RdgB2 is required in a cellular circuit that transduces light input from rods to bipolar cells that are coupled to GABAergic amacrine cells and ultimately to ipRGCs, thereby enabling ipRGCs to respond to dim light.

In vivo electroretinographic studies of the role of GABAC receptors in retinal signal processing

All three classes of receptors for the inhibitory neurotransmitter GABA (GABAR) are expressed in the retina. This study investigated roles of GABAR, especially GABACR (GABA(A)-ρ), in retinal signaling in vivo by studying effects on the mouse electroretinogram (ERG) of genetic deletion of GABACR versus pharmacological blockade using receptor antagonists. Brief full-field flash ERGs were recorded from anesthetized GABACR(-/-) mice, and WT C57BL/6 (B6) mice, before and after intravitreal injection of GABACR antagonists, TPMPA, 3-APMPA, or the more recently developed 2-AEMP; GABAAR antagonist, SR95531; GABABR antagonist, CGP, and agonist, baclofen. Intravitreal injections of TPMPA and SR95531 were also made in Brown Norway rats. The effect of 2-AEMP on GABA-induced current was tested directly in isolated rat rod bipolar cells, and 2-AEMP was found to preferentially block GABACR in those cells. Maximum amplitudes of dark (DA) and light-adapted (LA) ERG b-waves were reduced in GABACR(-/-) mice, compared to B6 mice, by 30-60%; a-waves were unaltered and oscillatory potential amplitudes were increased. In B6 mice, after injection of TPMPA (also in rats), 3-APMPA or 2-AEMP, ERGs became similar to ERGs of GABACR(-/-) mice. Blockade of GABAARs and GABABRs, or agonism of GABABRs did not alter B6 DA b-wave amplitude. The negative scotopic threshold response (nSTR) was slightly less sensitive in GABACR(-/-) than in B6 mice, and unaltered by 2-AEMP. However, amplitudes of nSTR and photopic negative response (PhNR), both of which originate from inner retina, were enhanced by TPMPA and 3-APMPA, each of which has GABAB agonist properties, and further increased by baclofen. The finding that genetic deletion of GABACR, the GABACR antagonist 2-AEMP, and other antagonists all reduced ERG b-wave amplitude, supports a role for GABACR in determining the maximum response amplitude of bipolar cells contributing to the b-wave. GABACR antagonists differed in their effects on nSTR and PhNR; antagonists with GABAB agonist properties enhanced light-driven responses whereas 2-AEMP did not.

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.

Possible roles of glutamate transporter EAAT5 in mouse cone depolarizing bipolar cell light responses

A remarkable feature of neuronal glutamate transporters (EAATs) is their dual functions of classical carriers and ligand-gated chloride (Cl(-)) channels. Cl(-) conductance is rapidly activated by glutamate in subtype EAAT5, which mediates light responses in depolarizing bipolar cells (DBC) in retinae of lower vertebrates. In this study, we examine whether EAAT5 also mediates the DBC light response in mouse. We took advantage of an infrared illuminated micro-injection system, and studied the effects of the EAAT blocker (TBOA) and a glutamate receptor agonist (LAP4) on the mouse electroretinogram (ERG) b-wave responses. Our results showed that TBOA and LAP4 shared similar temporal patterns of inhibition: both inhibited the ERG b-wave shortly after injection and recovered with similar time courses. TBOA inhibited the b-wave completely at mesopic light intensity with an IC50 value about 1 log unit higher than that of LAP4. The inhibitory effects of TBOA and LAP4 were found to be additive in the photopic range. Furthermore, TBOA alone inhibited the b-wave in the cone operative range in knockout mice lacking DBCRs at a low concentration that did not alter synaptic glutamate clearance activity. It also produced a stronger inhibition than that of LAP4 on the cone-driven b-wave measured with a double flash method in wildtype mice. These electrophysiological data suggest a significant role for EAAT5 in mediating cone-driven DBC light responses. Our immunohistochemistry data indicated the presence of postsynaptic EAAT5 on some DBCCs and some DBCRs, providing an anatomical basis for EAAT5's role in DBC light responses.