Multireceptor GABAergic regulation of synaptic communication in amphibian retina

Retinal GABAergic Alterations in Adults with Autism Spectrum Disorder

Alterations in γ-aminobutyric acid (GABA) have been implicated in sensory differences in individuals with autism spectrum disorder (ASD). Visual signals are initially processed in the retina, and in this study, we explored the hypotheses that the GABA-dependent retinal response to light is altered in individuals with ASD. Light-adapted electroretinograms were recorded from 61 adults (38 males and 23 females; n = 22 ASD) in response to three stimulus protocols: (1) the standard white flash, (2) the standard 30 Hz flickering protocol, and (3) the photopic negative response protocol. Participants were administered an oral dose of placebo, 15 or 30 mg of arbaclofen (STX209, GABAB agonist) in a randomized, double-blind, crossover order before the test. At baseline (placebo), the a-wave amplitudes in response to single white flashes were more prominent in ASD, relative to typically developed (TD) participants. Arbaclofen was associated with a decrease in the a-wave amplitude in ASD, but an increase in TD, eliminating the group difference observed at baseline. The extent of this arbaclofen-elicited shift significantly correlated with the arbaclofen-elicited shift in cortical responses to auditory stimuli as measured by using an electroencephalogram in our prior study and with broader autistic traits measured with the autism quotient across the whole cohort. Hence, GABA-dependent differences in retinal light processing in ASD appear to be an accessible component of a wider autistic difference in the central processing of sensory information, which may be upstream of more complex autistic phenotypes.

Quantifying the effect of light activated outer and inner retinal inhibitory pathways on glutamate release from mixed bipolar cells

Inhibition mediated by horizontal and amacrine cells in the outer and inner retina, respectively, are fundamental components of visual processing. Here, our purpose was to determine how these different inhibitory processes affect glutamate release from ON bipolar cells when the retina is stimulated with full-field light of various intensities. Light-evoked membrane potential changes (ΔV_m ) were recorded directly from axon terminals of intact bipolar cells receiving mixed rod and cone inputs (Mbs) in slices of dark-adapted goldfish retina. Inner and outer retinal inhibition to Mbs was blocked with bath applied picrotoxin (PTX) and NBQX, respectively. Then, control and pharmacologically modified light responses were injected into axotomized Mb terminals as command potentials to induce voltage-gated Ca²⁺ influx (Q^Ca ) and consequent glutamate release. Stimulus-evoked glutamate release was quantified by the increase in membrane capacitance (ΔC_m ). Increasing depolarization of Mb terminals upon removal of inner and outer retinal inhibition enhanced the ΔV_m /Q^Ca ratio equally at a given light intensity and inhibition did not alter the overall relation between Q^Ca and ΔC_m . However, relative to control, light responses recorded in the presence of PTX and PTX + NBQX increased ΔC_m unevenly across different stimulus intensities: at dim stimulus intensities predominantly the inner retinal GABAergic inhibition controlled release from Mbs, whereas the inner and outer retinal inhibition affected release equally in response to bright stimuli. Furthermore, our results suggest that non-linear relationship between Q^Ca and glutamate release can influence the efficacy of inner and outer retinal inhibitory pathways to mediate Mb output at different light intensities.

GABAergic neurotransmission and retinal ganglion cell function

Ganglion cells are the output retinal neurons that convey visual information to the brain. There are ~20 different types of ganglion cells, each encoding a specific aspect of the visual scene as spatial and temporal contrast, orientation, direction of movement, presence of looming stimuli; etc. Ganglion cell functioning depends on the intrinsic properties of ganglion cell's membrane as well as on the excitatory and inhibitory inputs that these cells receive from other retinal neurons. GABA is one of the most abundant inhibitory neurotransmitters in the retina. How it modulates the activity of different types of ganglion cells and what is its significance in extracting the basic features from visual scene are questions with fundamental importance in visual neuroscience. The present review summarizes current data concerning the types of membrane receptors that mediate GABA action in proximal retina; the effects of GABA and its antagonists on the ganglion cell light-evoked postsynaptic potentials and spike discharges; the action of GABAergic agents on centre-surround organization of the receptive fields and feature related ganglion cell activity. Special emphasis is put on the GABA action regarding the ON-OFF and sustained-transient ganglion cell dichotomy in both nonmammalian and mammalian retina.

Generation of recombinant guinea pig antibody fragments to the human GABAC receptor

To generate monoclonal antibodies to the human ρ1 GABA(C) receptor, a ligand-gated chloride ion channel that is activated by the neurotransmitter γ-aminobutyric acid (GABA), we recovered the immunoglobulin variable heavy chain (V(H)) and light chain (V(L)) regions of a guinea pig immunized with a 14-mer peptide segment of the N-terminal extracellular domain of the ρ1 subunit. Oligonucleotide primers were designed and used to amplify the V(H) and V(L) regions of guinea pig RNA by the reverse transcriptase polymerase chain reaction. The amplified and cloned V(H) and V(L) regions were transferred together into a phagemid vector, yielding a library of 5×10(6) members, which displayed chimeric fragments of antigen binding (Fabs) with guinea pig variable and human constant regions fused to protein III of M13 bacteriophage. Through affinity selection of this phage-display library with the biotinylated 14-mer peptide segment of GABA(C), we isolated four different antibody fragments that bound specifically to the immunogenic peptide. Phage particles displaying two of these antibodies, but not negative controls, bound selectively to the surface of neuroblastoma cells expressing the ρ1 GABA(C) receptor. Such antibody fragments will be useful in future studies involving targeting of specific neural tissues that express the GABA(C) receptor.

GABA(B) receptor feedback regulation of bipolar cell transmitter release

GABAergic amacrine cell feedback to bipolar cells in retina has been described, activating both GABA(A) and GABA(C) receptors. We explored whether metabotropic GABA(B) receptors also participate in this feedback pathway. CGP55845, a potent GABA(B) receptor antagonist, was employed to determine the endogenous role of these receptors. Ganglion cell EPSCs and IPSCs were monitored to measure the output of bipolar and amacrine cells. Using the tiger salamander slice preparation, we found that GABA(B) receptor pathways regulate bipolar cell release directly and indirectly. In the direct pathway, the GABA(B) receptor antagonist reduces EPSC amplitude, indicating that GABA(B) receptors cause enhanced glutamate release from bipolar cells to one set of ganglion cells. In the indirect pathway, the GABA(B) receptor antagonist reduces EPSC amplitude in another set of ganglion cells. The indirect pathway is only evident when GABA(A) receptors are inhibited, and is blocked by a glycine receptor antagonist. Thus, this second feedback pathway involves direct glycine feedback to the bipolar cell and this glycinergic amacrine cell is suppressed by GABAergic amacrine cells, through both GABA(A) and GABA(B) but not GABA(C) receptors. Overall, GABA(B) receptors do contribute to feedback regulation of bipolar cell transmitter release. However, unlike the ionotropic GABA receptor pathways, the metabotropic GABA receptor pathways act to enhance bipolar cell transmitter release. Furthermore, there are three discrete subsets of bipolar cell output regulated by GABA(B) receptor feedback (direct, indirect and null), implying three distinct, non-overlapping bipolar cell to ganglion cell circuits.

Role of neurotransmitter receptors in mediating light-evoked responses in retinal interplexiform cells

Interplexiform (IP) cells are a long-neglected group of retinal neurons the function of which is yet to be determined. Anatomical study indicates that IP cells are located in the inner nuclear layer, juxtaposed with the third-order neurons. However, the synaptic transmission of IP cells in the inner retina is poorly understood. Using whole cell patch-clamp and pharmacological techniques, we extensively studied synaptic receptors in IP cells. The IP cells in amphibian retinal slices were identified by electrical and morphological properties with voltage-clamp recording and Lucifer yellow dialysis. We find that light-evoked excitatory postsynaptic currents (L-EPSCs) are mediated by AMPA and N-methyl-d-aspartate receptors in IP cells. Although both receptors contributed to the amplitude and kinetics of L-EPSCs, AMPA receptor desensitization substantially shaped L-EPSCs in the neurons, similar to those found in the third-order neurons. The light-evoked inhibitory postsynaptic currents (L-IPSCs) in IP cells were primarily mediated by strychnine-sensitive glycine receptors with a small component of GABA(C) receptors. GABA(C) receptor rho2 subunits were detected in IP cells with single-cell RT-PCR assays. Expression of GABA(C) receptors is one of the special features for IP cells, distinct from most of the third-order neurons that depend on GABA(A) and glycine receptors to relay the inhibitory signals. However, GABA(A) receptors in IP cells acted like nonsynaptic receptors that were activated by exogenous GABA application. Furthermore, L-IPSCs in IP cells were inhibited by the serial inhibitions between amacrine cells in the inner retina. In addition, application of neurotransmitters on the axon terminals of IP cells had no significant current generated in the cells, indicating that the synaptic inputs of IP cells are mainly from the inner retina. This study demonstrates the important role that light signals are encoded by both experiment of inhibitory receptors in IP cells.

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.

Synaptic inhibition by glycine acting at a metabotropic receptor in tiger salamander retina

Glycine is the lone fast neurotransmitter for which a metabotropic pathway has not been identified. In retina, we found a strychnine-insensitive glycine response in bipolar and ganglion cells. This glycine response reduced high voltage-activated calcium current. It was G-protein mediated and protein kinase A dependent. The EC(50) of the metabotropic glycine response is 3 mum, an order of magnitude lower than the ionotropic glycine receptor in the same retina. The bipolar cell glutamatergic input to ganglion cells was suppressed by metabotropic glycine action. The synaptic output of about two-thirds of bipolar cells and calcium current in two-thirds of ganglion cells are sensitive to the action of glycine at metabotropic receptors, suggesting this signal regulates specific synaptic pathways in proximal retina. This study resolves the curious absence of a metabotropic glycine pathway in the nervous system and reveals that the major fast inhibitory neurotransmitters, GABA and glycine, both activate G-protein-coupled pathways as well.

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.

GABA(C) receptors in retina and brain

The expression of GABA(C) receptors has long been regarded as a specific property of bipolar cells in the inner retina where they control the information transfer from bipolar to retinal ganglion cells. A number of recent anatomical and physiological studies, however, have provided evidence that GABA(C) receptors are also expressed in many brain structures apart from the retina. The presence of GABA(C) receptors in many GABAergic neurons suggests that this receptor type may be involved in the regulation of local inhibition. This chapter focuses on the distribution of GABA(C) receptors and their possible function in various brain areas.

Contribution of the GABAergic pathway(s) to the correlated activities of chicken retinal ganglion cells

In the present study, the spatiotemporal pattern of chicken retinal ganglion cells' firing activity in response to full-field white light stimulation was investigated. Cross-correlation analysis showed that ganglion cells of sustained subtype fired in precise synchrony with their adjacent neurons of the same subtype (delay lag within 2 ms, narrow correlation). On the other hand, the activities of neighboring ganglion cells of transient subtype were correlated with distributed time lags (10-30 ms, medium correlation). Pharmacological studies demonstrated that the intensity of the medium correlations could be strengthened when exogenous GABA was applied and attenuated when GABA receptors were blocked by picrotoxin. Meanwhile, the GABAergic modulation on the narrow correlations was not consistent. These results suggest that, in the chicken retina, GABAergic pathway(s) are likely involved in the formation of medium correlations between ganglion cells. Neurons might fire at a lower rate but with higher level of synchronization to improve the efficiency of information transmission, with the mechanism involving the GABAergic inhibitory input.

Binding of muscimol-conjugated quantum dots to GABAC receptors

Functionalization of highly fluorescent CdSe/ZnS core-shell nanocrystals (quantum dots, qdots) is an emerging technology for labeling cell surface proteins. We have synthesized a conjugate consisting of approximately 150-200 muscimols (a GABA receptor agonist) covalently joined to the qdot via a poly(ethylene glycol) (PEG) linker (approximately 78 ethylene glycol units) and investigated the binding of this muscimol-PEG-qdot conjugate to homomeric rho1 GABAC receptors expressed in Xenopus oocytes. GABAC receptors mediate inhibitory synaptic signaling at multiple locations in the central nervous system (CNS). Binding of the conjugate was analyzed quantitatively by determining the fluorescence intensity of the oocyte surface membrane in relation to that of the surrounding incubation medium. Upon 5- to 10-min incubation with muscimol-PEG-qdots (34 nM in qdot concentration), GABAC-expressing oocytes exhibited a fluorescent halo at the surface membrane that significantly exceeded the fluorescence of the incubation medium. This halo was absent following muscimol-PEG-qdot treatment of oocytes lacking GABAC receptors. Incubation of the oocyte with free muscimol (100 microM-5 mM), PEG-muscimol (500 microM), or GABA (100 microM - 5 mM) substantially reduced or eliminated the fluorescence halo produced by muscimol-PEG-qdots, and the removal of GABA or free muscimol led to a recovery of muscimol-PEG-qdot binding. Unconjugated qdots and PEG-qdots that lacked conjugated muscimol neither exhibited significant binding activity nor diminished the subsequent binding of muscimol-PEG-qdots. The results indicate that muscimol joined to qdots via a long-chain PEG linker exhibits specific binding activity at the ligand-binding pocket of expressed GABAC receptors, despite the presence of both the long PEG linker and the sterically bulky qdot.

Functional segregation of synaptic GABAA and GABAC receptors in goldfish bipolar cell terminals

The transmission of light responses to retinal ganglion cells is regulated by inhibitory input from amacrine cells to bipolar cell (BC) synaptic terminals. GABA(A) and GABA(C) receptors in BC terminals mediate currents with different kinetics and are likely to have distinct functions in limiting BC output; however, the synaptic properties and localization of the receptors are currently poorly understood. By recording endogenous GABA receptor currents directly from BC terminals in goldfish retinal slices, I show that spontaneous GABA release activates rapid GABA(A) receptor miniature inhibitory postsynaptic currents (mIPSCs) (predominant decay time constant (tau(decay)), 1.0 ms) in addition to a tonic GABA(C) receptor current. The GABA(C) receptor antagonist (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) has no effect on the amplitude or kinetics of the rapid GABA(A) mIPSCs. In addition, inhibition of the GAT-1 GABA transporter, which strongly regulates GABA(C) receptor currents in BC terminals, fails to reveal a GABA(C) component in the mIPSCs. These data suggest that GABA(A) and GABA(C) receptors are highly unlikely to be synaptically colocalized. Using non-stationary noise analysis of the mIPSCs, I estimate that GABA(A) receptors in BC terminals have a single-channel conductance (gamma) of 17 pS and that an average of just seven receptors mediates a quantal event. From noise analysis of the tonic current, GABA(C) receptor gamma is estimated to be 4 pS. Identified GABA(C) receptor mIPSCs exhibit a slow decay (tau(decay), 54 ms) and are mediated by approximately 42 receptors. The distinct properties and localization of synaptic GABA(A) and GABA(C) receptors in BC terminals are likely to facilitate their specific roles in regulating the transmission of light responses in the retina.

Characterization of the binding of [3H]CGP54626 to GABAB receptors in the male bullfrog (Rana catesbeiana)

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate brain. GABA activates both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors in mammals. Whether non-mammalian vertebrates possess receptors with similar characteristics is not well understood. We used a mammalian GABA(B)-specific antagonist to determine the pharmacology of putative receptors in the brain of an anuran amphibian, the male bullfrog (Rana catesbeiana). Receptor binding assays with the antagonist [(3)H]CGP54626 revealed a single class of high affinity binding sites (with a K(D) of 2.97 nM and a B(max) of 2619 fmol/mg protein). Binding was time- and temperature-dependent, saturable and specific. Specific binding of [(3)H]CGP54626 was inhibited by several mammalian GABA(B) receptor agonists and antagonists. The rank order potency of agonists was: GABA = SKF97541 > (R)-Baclofen > 3-APPA. The rank order for antagonists was: CGP54626 = CGP55845 > CGP52432 > CGP35348. The GABA(A) receptor ligands muscimol and SR95531 had very low affinity for [(3)H]CGP54626 binding sites, while bicuculline compounds had no affinity. Binding of GABA was positively modulated by CGP7930. Taurine did not allosterically modulate GABA binding but did inhibit [(3)H]CGP54626 binding in a linear fashion. Bullfrog brain thus possesses binding sites with significant similarity to mammalian GABA(B) receptors. These receptors differ from mammalian receptors, however, in dissociation kinetics, ligand specificity and allosteric modulation.

Gycine and GABA interact to regulate the nitric oxide/cGMP signaling pathway in the turtle retina

Nitric oxide (NO) is a free radical that is important in retinal signal transduction and cyclic guanosine monophosphate (cGMP) is a critical downstream messenger of NO. The NO/cGMP signaling pathway has been shown to modulate neurotransmitter release and gap junction coupling in horizontal cells and amacrine cells, and increase the gain of the light response in photoreceptors. However, many of the mechanisms controlling the production of NO and cGMP remain unclear. Previous studies have shown activation of NO/cGMP production in response to stimulation with N-methyl-d-aspartate (NMDA) or nicotine, and the differential modulation of cGMP production by GABA(A) and GABA(C) receptors (GABA(A)Rs and GABA(C)Rs). This study used cGMP immunocytochemistry and NO imaging to investigate how the inhibitory GABAergic and glycinergic systems modulate the production of NO and cGMP. Our data show that blocking glycine receptors (GLYR) with strychnine (STRY) produced moderate increases in cGMP-like immunoreactivity (cGMP-LI) in select types of amacrine and bipolar cells, and strong increases in NO-induced fluorescence (NO-IF). TPMPA, a selective GABACR antagonist, greatly reduced the increases in cGMP-LI stimulated by STRY, but did not influence the increase in NO-IF stimulated by STRY. Bicuculline (BIC), a GABA(A)R antagonist, however, enhanced the increases in both the cGMP-LI and NO-IF stimulated by STRY. CNQX, a selective antagonist for alpha-Amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid hydrobromide/kainic acid (AMPA/KA) receptors, eliminated both the increases in cGMP-LI and NO-IF stimulated by STRY, while MK801, a selective antagonist for NMDA receptors, slightly increased the cGMP-LI and slightly decreased the NO-IF stimulated by STRY. Finally, double labeling of NO-stimulated cGMP and either GLY or GABA indicated that cGMP predominantly colocalized with GLY. Taken together, these findings support the hypothesis that GLY and GABA interact in the regulation of the NO/cGMP signaling pathway, where GLY primarily inhibits NO production and GABA has a greater effect on cGMP production. Such interacting inhibitory pathways could shape the course of signal transduction of the NO/cGMP pathway under different physiological situations.

GABA(A), GABA(C) and glycine receptor-mediated inhibition differentially affects light-evoked signalling from mouse retinal rod bipolar cells

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.

C-type natriuretic peptide modulates glutamate receptors on cultured rat retinal amacrine cells

C-type natriuretic peptide, widely distributed in the CNS, may work as a neuromodulator. In this work, we investigated modulation by C-type natriuretic peptide of functional properties of glutamate receptors in rat retinal GABAergic amacrine cells in culture. Immunocytochemical data revealed that natriuretic peptide receptor-B was strongly expressed on the membrane of cultured GABAergic amacrine cells. By whole cell recording techniques we further identified the glutamate receptor expressed on the GABAergic amacrine cells as an AMPA-preferring subtype. Incubation with C-type natriuretic peptide suppressed the AMPA receptor-mediated current of these cells in a dose-dependent manner by decreasing the efficacy and apparent affinity for glutamate. The effect of C-type natriuretic peptide was reversed by HS-142-1, a guanylyl cyclase-coupled natriuretic peptide receptor-A/B antagonist. Meanwhile, the selective natriuretic peptide receptor-C agonist cANF did not change the glutamate current. In conjunction with the immunocytochemical data, these results suggest that the C-type natriuretic peptide effect may be mediated by natriuretic peptide receptor-B. Furthermore, incubation of retinal cultures in the C-type natriuretic peptide-containing medium elevated cGMP immunoreactivity in the GABAergic amacrine cells, and the C-type natriuretic peptide effect on the glutamate current was mimicked by application of 8-Br-cGMP. It is therefore concluded that C-type natriuretic peptide may modulate the glutamate current by increasing the intracellular concentration of cGMP in these cells via activation of natriuretic peptide receptor-B.

Pharmacological properties of motion vision in goldfish measured with the optomotor response

In goldfish, the retinal pathways involved in motion coding have been demonstrated to have an L-cone dominated action spectrum (S. Schaerer, C. Neumeyer, Motion detection in goldfish investigated with the optomotor response is "color blind", Vision Res. 36 (1996) 4025-4034). The neurotransmitters involved in retinal motion coding mechanisms, and the relevance of these retinal motion coding mechanisms for motion perception, are little investigated in fish. In this study, the optomotor response was used to investigate the effect of antagonists on different receptor types for acetylcholine (ACh), GABA, for the dopamine D2-receptor (D2-R) - which is known to modulate the action spectrum in motion coding (C. Mora-Ferrer, K. Behrend, Dopaminergic modulation of photopic temporal transfer properties in goldfish retina investigated with the ERG, Vision Res. 44 (2004) 2067-2081) - and of an agonist for against the mGluR6-receptor (mGluR6) on goldfish motion vision in the photopic range. Blockade of nicotinic ACh-R, GABAa-R and both GABAa- and GABAc-R eliminated the optomotor response completely. Neither a muscarinic ACH-R antagonist, a D2-R antagonist or a mGluR6-agonist affected goldfish motion vision. The pharmacological profile of the goldfish optomotor response resembles the pharmacological profile of direction-selective ganglion cells (DS-GC) described for vertebrate retinas in electrophysiological experiments, e.g. (S. Weng, W. Sun, S. He, Identification of ON-OFF direction-selective ganglion cells in the mouse retina, J. Physiol. 562 (2005) 915-923). This indicates that cells with direction-selective receptive field properties exist in the goldfish retina. It is proposed that these cells provide the input for the full field motion perception in goldfish.

Synthesis and characterization of an electroactive surface that releases gamma-aminobutyric acid (GABA)

We report the synthesis and characterization of a new electroactive surface capable of releasing the neurotransmitter gamma-aminobutyric acid (GABA) upon reduction. The GABA was anchored to an alkanethiol via electrochemically active quinone (abbreviation, TM-GABA). The quinone unit, upon reduction to the hydroquinone, cyclizes to release GABA into solution. The half-life is 99 s. The self-assembled monolayer (SAM) of TM-GABA on gold was prepared and characterized with several surface sensitive techniques. X-ray photoelectron spectroscopy (XPS) explored the SAM formation of TM-GABA on Au surfaces. Cyclic voltammograms showed the ability to electrochemically control the quinone unit at the distal end of the chain. GABA was selectively released upon electrochemical reduction at a potential of -700 mV. The functional GABA terminal group was detected by surface plasmon resonance measurements of anti-GABA antibody binding.

Long-term plasticity mediated by mGluR1 at a retinal reciprocal synapse

The flow of information across the retina is controlled by reciprocal synapses between bipolar cell terminals and amacrine cells. However, the synaptic delays and properties of plasticity at these synapses are not known. Here we report that glutamate release from goldfish Mb-type bipolar cell terminals can trigger fast (delay of 2-3 ms) and transient GABA(A) IPSCs and a much slower and more sustained GABA(C) feedback. Synaptically released glutamate activated mGluR1 receptors on amacrine cells and, depending on the strength of presynaptic activity, potentiated subsequent feedback. This poststimulus enhancement of GABAergic feedback lasted for up to 10 min. This form of mGluR1-mediated long-term synaptic plasticity may provide retinal reciprocal synapses with adaptive capabilities.

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.

Synaptic organization of GABAergic amacrine cells in the salamander retina

The synaptic organization of GABA-immunoreactive (GABA-IR) amacrine cells in the inner plexiform layer (IPL) of salamander retina was studied with the use of postembedding immuno-electron microscopy. A total of 457 GABA-IR amacrine synapses, with identified postsynaptic elements, were analyzed on photomontages of electron micrographs covering 3,618 microm2 of the IPL. GABA-IR amacrine synapses were distributed throughout the IPL, with a small peak at the proximal margin of sublamina a. The majority of the output targets (81%) were GABA(-) neurons. Most of the contacts were simple synapses with one postsynaptic element identified as a process of an amacrine cell (55%), bipolar cell (19%) or ganglion cell (26%), and serial synapses were very rare. Of the 89 postsynaptic bipolar terminals, 63% participated in a reciprocal feedback synapse with the same presynaptic GABA-IR amacrine profile. There appeared to be no preference between GABA-IR amacrine contacts with rod- or cone-dominated bipolar cells (9.1% vs. 8.9%) or in the total number of amacrine synapses in sublaminas a and b (52% vs. 47%). The preponderance of amacrine cell input to bipolar cells in the OFF layer was derived from GABA-IR cells. These findings provide ultrastructural support to the existing physiological studies regarding the functional roles of the GABAergic amacrine cells in this species. Our results have added to the data base demonstrating that, in contrast to mammals, GABA-IR amacrine cells in amphibians and other nonmammals contact other amacrine cells more frequently, suggesting greater involvement of GABAergic amacrine cells in modulating lateral inhibitory pathways.

Inner and outer retinal pathways both contribute to surround inhibition of salamander ganglion cells

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.

Synaptic mechanisms that shape visual signaling at the inner retina

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.

GABAC receptor-mediated inhibition in the retina

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.

Inwardly rectifying potassium channels in rat retinal ganglion cells

Inwardly rectifying potassium channels (Kir channels) are important for neuronal signalling and membrane excitability. In the present work we characterized, for the first time, Kir channels in rat retinal ganglion cells (RGCs), the output neurons in the retina, using immunocytochemical and patch-clamp techniques. Various subunits of Kir channels (Kir1.1, 2.1, 2.3, 3.1, 3.2 and 3.3) were expressed in RGCs, but with distinct subcellular localization. Kir1.1 was mainly expressed in axons of RGCs. Kir2.1 and Kir2.3 were both present in somata of RGCs. Whereas staining for Kir3.1 was profoundly present in an endoplasmic reticulum-like structure and Kir3.2 was strongly expressed in the cytoplasm and the cytomembrane of somata, dendrites and axons of RGCs, faint, sparse labelling for Kir3.3 was seen in the cytomembrane. Immunoreactivity for Kir4.1 and Kir4.2 was not detectable in RGCs. Whole-cell currents mediated by Kir channels were recorded in isolated RGCs and they differed from hyperpolarization-activated currents (I(h)) by showing full activation in < 10 ms, no inactivation, and being significantly suppressed by 300 microM Ba2+. Unlike in retinal horizontal cells and bipolar cells, these currents were mainly mediated by G-protein-coupled Kir3 (GIRK) channels, as demonstrated by the fact that GDP(beta)S and GTP(gamma)S included in the pipette solution markedly decreased and increased the currents, respectively. Furthermore, the GIRK channels were probably coupled to GABA(B) receptors, because baclofen considerably increased the Kir currents and the increased currents were suppressed by Ba2+. These characteristics of the Kir currents provide more versatility for signalling of RGCs.

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.

Ionotropic GABA receptors with mixed pharmacological properties of GABAA and GABAC receptors

Ionotropic gamma-aminobutyric acid (GABA) receptors form a large family of molecular isoforms with distinct properties. We have characterized a distinct new type of GABA receptors in CA1 pyramidal cells in rat hippocampal slices. Somatic application of GABA induced currents which were partially suppressed by (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), a specific antagonist of GABA(C) receptors. This sensitivity was enhanced when we evoked the currents by the GABA(C) receptor agonist cis-4-aminocrotonic acid (CACA). However, both GABA- and CACA-evoked currents were sensitive towards bicuculline and thus lack the defining feature of GABA(C) receptors, which are insensitive towards this antagonist. Spontaneous miniature post-synaptic currents (mIPSCs) revealed a similar pharmacological behaviour. We conclude that juvenile CA1 pyramidal cells express a fraction of ionotropic GABA receptors with mixed pharmacological properties of both, GABA(A) and GABA(C) receptors.

Dopaminergic modulation of photopic temporal transfer properties in goldfish retina investigated with the ERG

The influence of dopamine (DA) through either D1- or D2-dopamine receptors (D1-/D2-R) onto temporal transfer properties of the retina has been investigated using the ERG. Single flash responses and flicker responses were measured in the vitreous under photopic illumination conditions after application of either D1-/D2-R agonists or antagonists. All DA-R drugs did change the single flash responses, but only blockade of D2-R or activation of D1-R also changed the temporal transfer properties. In the Bode plot the gain characteristic was changed and thereby the upper limit frequency reduced. The action of DA is discussed on the base of a membrane resonance model in the outer retina versus a feed-forward inhibition model in the inner retina.

GABA(A) and GABA(C) receptor antagonists increase retinal cyclic GMP levels through nitric oxide synthase

The nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signal transduction pathway plays a role in every retinal cell type. Previous studies have shown that excitatory glutamatergic synaptic pathways can increase cGMP-like immunoreactivity (cGMP-LI) in retina through stimulation of NO production, but little is known about the role of synaptic inhibition in the modulation of cGMP-LI. Gamma-amino-n-butyric acid (GABA) plays critical roles in modulating excitatory synaptic pathways in the retina. Therefore, we used GABA receptor antagonists to explore the role of GABAergic inhibitory synaptic pathways on the modulation of the NO/cGMP signal-transduction system. Cyclic GMP immunocytochemistry was used to investigate the effects of the GABA receptor antagonists bicuculline, picrotoxin, and (1,2,5,6-tetrahyropyridin-4-yl) methylphosphinic acid (TPMPA) on levels of cGMP-LI. Cyclic GMP-LI was strongly increased in response to the GABA(A) receptor antagonist bicuculline, while the GABA(C) receptor antagonist TPMPA had little effect on cGMP-LI. The GABA(A)/GABA(C) receptor antagonist, picrotoxin, caused a moderate increase in cGMP-LI, which was mimicked by the combination of bicuculline and TPMPA. The nitric oxide synthase inhibitor, S-methyl-L-thiocitrulline (SMTC), blocked the increased cGMP-LI in response to stimulation with either bicuculline or picrotoxin. Treatments with either of the glutamate receptor antagonists (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) partially blocked the increases in cGMP-LI seen in response to bicuculline, but a combination of MK-801 and CNQX completely eliminated these increases. These results suggest that inhibitory synaptic pathways involving both types of GABA receptors work through excitatory glutamatergic receptors to regulate the NO/cGMP signal-transduction pathway in retina.

Scotopic threshold response changes after vigabatrin therapy in a child without visual field defects: a new electroretinographic marker of early damage?

Vigabatrin (VGB) has been widely used in patients affected by drug-resistant epilepsy and West syndrome. Following reports of visual field loss associated with vigabatrin therapy, some authors have investigated retinal electrophysiologic variables to identify early electrophysiologic markers and pathogenetic mechanisms of retinal damage. There are no previous reports of a scotopic threshold response (STR) reduction associated with vigabatrin therapy. A 13-year-old male child was submitted to a complete electroretinographic study before and after the start of vigabatrin therapy. Of the electroretinographic responses analyzed, only the scotopic threshold response was altered. The scotopic threshold response is a corneal-negative wave in the electroretinogram (ERG) of a fully dark-adapted eye. In cat, this response has been shown to be mediated by K+ spatial buffer currents that flow from proximal to distal retina in retinal glia as a result of elevated concentration of K+ in proximal retina following depolarization of local neurons in response to light onset. The prospective nature of the study in a previously untreated patient on vigabatrin monotherapy allows us to speculate on the underlying pathogenetic mechanisms and level of action of vigabatrin therapy-related retinal damage. If the predictive value of the scotopic threshold response changes is documented, this ERG response could be used to perform a preliminary evaluation of drugs, which modify gamma-aminobutyric acid (GABA) receptors and/or GABA levels.

Studies on the mechanisms of action of picrotoxin, quercetin and pregnanolone at the GABA rho 1 receptor

1. The mechanisms of action of antagonists of the gamma-aminobutyric acid C (GABA(C)) receptor picrotoxin, quercetin and pregnanolone were studied. 2. Ionic currents (chloride), mediated through human homomeric GABA rho(1) receptors expressed in Xenopus oocytes, were recorded by two-electrode voltage clamp. 3. Dose-response (D-R) curves and kinetic measurements of GABA rho(1) currents were carried out in the presence or absence of antagonists. Use-dependent actions were also evaluated. 4. Picrotoxin, quercetin and pregnanolone exerted noncompetitive actions. 5. IC(50) values measured at the EC(50) for GABA (1 microM) were as follows: picrotoxin 0.6+/-0.1 microM (Hill coefficient n=1.0+/-0.2); quercetin 4.4+/-0.4 microM (n=1.5+/-0.2); pregnanolone 2.1+/-0.5 microM (n=0.8+/-0.1). 6. These antagonists produced changes only in the slope of the linear current-voltage relationships, which was indicative of voltage-independent effects. 7. The effect of picrotoxin on GABA rho(1) currents was use-dependent, strongly relied on agonist concentration and showed a slow onset and offset. The mechanism was compatible with an allosteric inhibition and receptor activation was a prerequisite for antagonism. 8. The effect of quercetin was use-independent, showed relatively fast onset and offset, and resulted in a slowed time course of the GABA-evoked currents. 9. The effect of pregnanolone was use-independent, presented fast onset and a very slow washout, and did not affect current activation. 10. All the antagonists accelerated the time course of deactivation of the GABA rho(1) currents.

Presynaptic effects of group III metabotropic glutamate receptors on excitatory synaptic transmission in the retina

Metabotropic glutamate receptors (mGluRs) are located in both plexiform layers in the retina and may modulate transmission between photoreceptors and bipolar cells and between bipolar and ganglion cells. We investigated whether mGluR activation modulates excitatory synaptic input to bipolar cells and ganglion cells in the salamander retinal slice preparation. The group III mGluR agonist L-2-amino-4-phosphonobutyric acid (AP4) inhibited monosynaptic excitatory postsynaptic currents (EPSCs) in ganglion cells evoked by electrical stimuli, whereas group I and group II agonists had no significant effect. AP4 reduced the frequency but not the amplitude of ganglion cell miniature EPSCs, suggesting a presynaptic action at bipolar cell terminals. AP4 also reduced ganglion cell EPSCs evoked by the offset of a light stimulus, suggesting that group III mGluRs modulate release from OFF bipolar cells. Comparison of light-evoked EPSCs in OFF bipolar cells and ganglion cells indicated that AP4 reduced ganglion cell EPSCs by acting primarily at bipolar cell terminals, and to a lesser extent at photoreceptor terminals. The group II/III mGluR antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) blocked the effect of AP4 at bipolar cell terminals, consistent with localization of group III mGluRs at these sites. However, CPPG did not increase EPSCs at light offset, indicating that activation of group III mGluRs by synaptic glutamate does not play a large role in modulating transmission from bipolar cells to ganglion cells.

GABA transporters regulate inhibition in the retina by limiting GABA(C) receptor activation

Inhibition is mediated by two classes of ionotropic receptors in the retina, GABA(A) and GABA(C) receptors. We used the GABA transport blocker NO-711 to examine the role of GABA transporters in shaping synaptic responses mediated by these two receptors in the salamander retinal slice preparation. Focal applications (puffs) of GABA onto GABA(C) receptors on bipolar cells terminals or GABA(A) receptors on ganglion cells elicited currents that were enhanced by NO-711, demonstrating the presence of transporters in the inner plexiform layer (IPL). IPSCs were evoked in bipolar and ganglion cells by puffing kainate into the IPL. NO-711 enhanced the IPSCs only in bipolar cells, suggesting that, when GABA uptake was blocked, the GABA(C) receptors were more strongly activated by spillover transmission than the GABA(A) receptors on ganglion cells. NO-711 enhanced the light-evoked IPSCs mediated by GABA(C) receptors on bipolar cell axon terminals, which resulted in reduced transmission between bipolar and ganglion cells. NO-711 also shifted the intensity-response relationship of the ganglion cell, reducing its sensitivity to light. Surround illumination has been shown by others to produce similar shifts in ganglion cell light sensitivity. Our results show that GABA transporters limit the extent of inhibitory transmission at the inner retina during light-evoked signal processing.

Glutamate receptors in the rod pathway of the mammalian retina

Rod bipolar (RB) cells of the mammalian retina release glutamate in a graded, light-dependent fashion from 20 to 40 ribbon synapses (dyads). At the dyads, two classes of amacrine cells, the AI and AII cells, are the postsynaptic partners. We examined the glutamate receptors (GluRs) that are expressed by AI and AII cells using immunocytochemistry with specific antibodies against GluR subunits. Sections of macaque monkey and rabbit retina were examined by confocal microscopy. AII amacrine cells were selectively labeled for calretinin, and AI cells in rabbits were labeled for 5-HT uptake. Thus, double- and triple-labeling for these markers and GluR subunits was possible. Electron microscopy using postembedding immunocytochemistry and double-labeling was applied to show the synaptic expression of GluRs. We also studied the synaptic localization of the two postsynaptic density proteins PSD-95 and glutamate receptor-interacting protein (GRIP). We found that AII amacrine cells express the AMPA receptor subunits GluR2/3 and GluR4 at the RB cell dyads, and they are clustered together with PSD-95. In contrast, AI amacrine cells express the delta1/2 subunits that appear to be associated with kainate receptor subunits and to be clustered together with GRIP. The RB cell dyad is therefore a synapse that initiates two functionally and molecularly distinct pathways: a "through conducting" pathway based on AMPA receptors and a modulatory pathway mediated by a combination of delta1/2 subunits and kainate receptors.

Modulation of excitatory synaptic transmission by GABA(C) receptor-mediated feedback in the mouse inner retina

In many vertebrate CNS synapses, the neurotransmitter glutamate activates postsynaptic non-N-methyl-D-aspartate (NMDA) and NMDA receptors. Since their biophysical properties are quite different, the time course of excitatory postsynaptic currents (EPSCs) depends largely on the relative contribution of their activation. To investigate whether the activation of the two receptor subtypes is affected by the synaptic interaction in the inner plexiform layer (IPL) of the mouse retina, we analyzed the properties of the light-evoked responses of ON-cone bipolar cells and ON-transient amacrine cells in a retinal slice preparation. ON-transient amacrine cells were whole cell voltage-clamped, and the glutamatergic synaptic input from bipolar cells was isolated by a cocktail of pharmacological agents (bicuculline, strychnine, curare, and atropine). Direct puff application of NMDA revealed the presence of functional NMDA receptors. However, the light-evoked EPSC was not significantly affected by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), but suppressed by 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) or 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). These results indicate that the light-evoked EPSC is mediated mainly by AMPA receptors under this condition. Since bipolar cells have GABA(C) receptors at their terminals, it has been suggested that bipolar cells receive feedback inhibition from amacrine cells. Application of (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), a specific blocker of GABA(C) receptors, suppressed both the GABA-induced current and the light-evoked feedback inhibition observed in ON-cone bipolar cells and enhanced the light-evoked EPSC of ON-transient amacrine cells. In the presence of TPMPA, the light-evoked EPSC of amacrine cells was composed of AMPA and NMDA receptor-mediated components. Our results suggest that photoresponses of ON-transient amacrine cells in the mouse retina are modified by the activation of presynaptic GABA(C) receptors, which may control the extent of glutamate spillover.

Glutamate receptors in the rod pathway of the mammalian retina

Rod bipolar (RB) cells of the mammalian retina release glutamate in a graded, light-dependent fashion from 20 to 40 ribbon synapses (dyads). At the dyads, two classes of amacrine cells, the AI and AII cells, are the postsynaptic partners. We examined the glutamate receptors (GluRs) that are expressed by AI and AII cells using immunocytochemistry with specific antibodies against GluR subunits. Sections of macaque monkey and rabbit retina were examined by confocal microscopy. AII amacrine cells were selectively labeled for calretinin, and AI cells in rabbits were labeled for 5-HT uptake. Thus, double- and triple-labeling for these markers and GluR subunits was possible. Electron microscopy using postembedding immunocytochemistry and double-labeling was applied to show the synaptic expression of GluRs. We also studied the synaptic localization of the two postsynaptic density proteins PSD-95 and glutamate receptor-interacting protein (GRIP). We found that AII amacrine cells express the AMPA receptor subunits GluR2/3 and GluR4 at the RB cell dyads, and they are clustered together with PSD-95. In contrast, AI amacrine cells express the delta1/2 subunits that appear to be associated with kainate receptor subunits and to be clustered together with GRIP. The RB cell dyad is therefore a synapse that initiates two functionally and molecularly distinct pathways: a "through conducting" pathway based on AMPA receptors and a modulatory pathway mediated by a combination of delta1/2 subunits and kainate receptors.