Intensity-dependent, rapid activation of presynaptic metabotropic glutamate receptors at a central synapse

Light Controlled Signaling Initiated by Subretinal Semiconducting-Polymer Layer in Developing-Blind-Retina Mimics the Response of the Neonatal Retina

Optoelectronic semiconducting polymer material interfaced with a blind-developing chick-retina (E13-E18) in subretinal configuration reveals a response to full-field flash stimulus that resembles an elicited response from natural photoreceptors in a mature chick retina. The response manifests as evoked-firing of action potentials was recorded using a multi-electrode array in contact with the retinal ganglion layer. Characteristics of increasing features in the signal unfold during different retina-development stages and highlight the emerging network mediated pathways typically present in the vision process of the artificial photoreceptor interfaced retina.

Mechanisms creating transient and sustained photoresponses in mammalian retinal ganglion cells

Visual stimuli of different frequencies are encoded in the retina using transient and sustained responses. Zhao et al. describe the different strategies that are used by four types of retinal ganglion cells to shape photoresponse kinetics.

Retinal neurons use sustained and transient light responses to encode visual stimuli of different frequency ranges, but the underlying mechanisms remain poorly understood. In particular, although earlier studies in retinal ganglion cells (RGCs) proposed seven potential mechanisms, all seven have since been disputed, and it remains unknown whether different RGC types use different mechanisms or how many mechanisms are used by each type. Here, we conduct a comprehensive survey in mice and rats of 12 candidate mechanisms that could conceivably produce tonic rod/cone-driven ON responses in intrinsically photosensitive RGCs (ipRGCs) and transient ON responses in three types of direction-selective RGCs (TRHR+, Hoxd10+ ON, and Hoxd10+ ON-OFF cells). We find that the tonic kinetics of ipRGCs arises from their substantially above-threshold resting potentials, input from sustained ON bipolar cells, absence of amacrine cell inhibition of presynaptic ON bipolar cells, and mGluR7-mediated maintenance of light-evoked glutamatergic input. All three types of direction-selective RGCs receive input from transient ON bipolar cells, and each type uses additional strategies to promote photoresponse transience: presynaptic inhibition and dopaminergic modulation for TRHR+ cells, center/surround antagonism and relatively negative resting potentials for Hoxd10+ ON cells, and presynaptic inhibition for Hoxd10+ ON-OFF cells. We find that the sustained nature of ipRGCs’ rod/cone-driven responses depends neither on melanopsin nor on N-methyl-d-aspartate (NMDA) receptors, whereas the transience of the direction-selective cells’ responses is influenced neither by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor desensitization nor by glutamate uptake. For all cells, we further rule out spike frequency adaptation and intracellular Ca²⁺ as determinants of photoresponse kinetics. In conclusion, different RGC types use diverse mechanisms to produce sustained or transient light responses. Parenthetically, we find evidence in both mice and rats that the kinetics of light-induced mGluR6 deactivation determines whether an ON bipolar cell responds tonically or transiently to light.

The oscillation-like activity in bullfrog ON-OFF retinal ganglion cell

Oscillatory activity of retinal ganglion cell (RGC) has been observed in various species. It was reported such oscillatory activity is raised within large neural network and involved in retinal information coding. In the present research, we found an oscillation-like activity in ON-OFF RGC of bullfrog retina, and studied the mechanisms underlying the ON and OFF activities respectively. Pharmacological experiments revealed that the oscillation-like activity patterns in both ON and OFF pathways were abolished by GABA receptor antagonists, indicating GABAergic inhibition is essential for generating them. At the meantime, such activities in the ON and OFF pathways showed different responses to several other applied drugs. The oscillation-like pattern in the OFF pathway was abolished by glycine receptor antagonist or gap junction blocker, whereas that in the ON pathway was not affected. Furthermore, the blockade of the ON pathway by metabotropic glutamate receptor agonist led to suppression of the oscillation-like pattern in the OFF pathway. These results suggest that the ON pathway has modulatory effect on the oscillation-like activity in the OFF pathway. Therefore, the mechanisms underlying the oscillation-like activities in the ON and OFF pathways are different: the oscillation-like activity in the ON pathway is likely caused by GABAergic amacrine cell network, while that in the OFF pathway needs the contributions of GABAergic and glycinergic amacrine cell network, as well as gap junction connections.

Activation of group I metabotropic glutamate receptors regulates the excitability of rat retinal ganglion cells by suppressing Kir and I h

Group I metabotropic glutamate receptor (mGluR I) activation exerts a slow postsynaptic excitatory effect in the CNS. Here, the issues of whether and how this receptor is involved in regulating retinal ganglion cell (RGC) excitability were investigated in retinal slices using patch-clamp techniques. Under physiological conditions, RGCs displayed spontaneous firing. Extracellular application of LY367385 (10 µM)/MPEP (10 µM), selective mGluR1 and mGluR5 antagonists, respectively, significantly reduced the firing frequency, suggesting that glutamate endogenously released from bipolar cells constantly modulates RGC firing. DHPG (10 µM), an mGluR I agonist, significantly increased the firing and caused depolarization of the cells, which were reversed by LY367385, but not by MPEP, suggesting the involvement of the mGluR1 subtype. Intracellular Ca²⁺-dependent PI-PLC/PKC and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways mediated the DHPG-induced effects. In the presence of cocktail synaptic blockers (CNQX, D-AP5, bicuculline, and strychnine), which terminated the spontaneous firing in both ON and OFF RGCs, DHPG still induced depolarization and triggered the cells to fire. The DHPG-induced depolarization could not be blocked by TTX. In contrast, Ba²⁺, an inwardly rectifying potassium channel (Kir) blocker, and Cs⁺ and ZD7288, hyperpolarization-activated cation channel (I _h) blockers, mimicked the effect of DHPG. Furthermore, in the presence of Ba²⁺/ZD7288, DHPG did not show further effects. Moreover, Kir and I _h currents could be recorded in RGCs, and extracellular application of DHPG indeed suppressed these currents. Our results suggest that activation of mGluR I regulates the excitability of rat RGCs by inhibiting Kir and I _h.

Properties of a Glutamatergic Synapse Controlling Information Output from Retinal Bipolar Cells

One general categorization of retinal ganglion cells is to segregate them into tonically or phasically responding neurons, each conveying discrete aspects of the visual scene. Although best identified in the output signals of the retina, this distinction is initiated at the first synapse: between photoreceptors and the dendrites of bipolar cells. In this study we found that the output synapses of bipolar cells also contribute to separate these pathways. Both transient and sustained ganglion cells can produce maintained spike activity, but bipolar cell glutamate release exhibits a divergence that corresponds to the response characteristics of the ganglion cells. Comparing light intensity coding in the sustained and transient ON pathways revealed that they shared the intensity spectrum. The transient pathway had greater sensitivity but smaller dynamic range, and switched from intensity coding to event detection at light levels where sustained pathway sensitivity began to rise. The distinctive properties of the sustained pathway depended upon inhibition and shifted toward those of the transient pathway in the absence of inhibition. The transient system was comparatively unaffected by the loss of inhibition and this was due to the concomitant activation of perisynaptic NMDA receptors. Overall, the properties of bipolar cell dendritic and axon terminals both contribute to the formation of key aspects of the sustained/transient dichotomy normally associated with ganglion cells.

Disinhibitory recruitment of NMDA receptor pathways in retina

Glutamate release at bipolar to ganglion cell synapses activates NMDA and AMPA/kainic acid (KA) ionotropic glutamate receptors. Their relative strength determines the output signals of the retina. We found that this balance is tightly regulated by presynaptic inhibition that preferentially suppresses NMDA receptor (NMDAR) activation. In transient ON-OFF neurons, block of GABA and glycine feedback enhanced total NMDAR charge by 35-fold in the ON response and 9-fold in the OFF compared with a 1.7-fold enhancement of AMPA/KA receptors. Blocking only glycine receptors enhanced the NMDAR excitatory postsynaptic current 10-fold in the ON and 2-fold in the OFF pathway. Blocking GABA(A) or GABA(C) receptors (GABA(C)Rs or GABA(A)Rs) produced small changes in total NMDAR charge. When both GABA(A)Rs and GABA(C)Rs were blocked, the total NMDAR charge increased ninefold in the ON and fivefold in the OFF pathway. This exposed a strong GABA(C)R feedback to bipolar cells that was suppressed by serial amacrine cell synapses mediated by GABA(A)Rs. The results indicate that NMDAR currents are large but latent, held in check by dual GABA and glycine presynaptic inhibition. One example of this controlled NMDAR activation is the cross talk between ON and OFF pathways. Blocking the ON pathway increased NMDAR relative strength in the OFF pathway. Stimulus prolongation similarly increased the NMDAR relative strength in the OFF response. This NMDAR enhancement was produced by a diminution in GABA and glycine feedback. Thus the retinal network recruits NMDAR pathways through presynaptic disinhibition.

Extrasynaptic glutamate and inhibitory neurotransmission modulate ganglion cell participation during glutamatergic retinal waves

During the first 2 wk of mouse postnatal development, transient retinal circuits give rise to the spontaneous initiation and lateral propagation of depolarizations across the ganglion cell layer (GCL). Glutamatergic retinal waves occur during the second postnatal week, when GCL depolarizations are mediated by ionotropic glutamate receptors. Bipolar cells are the primary source of glutamate in the inner retina, indicating that the propagation of waves depends on their activation. Using the fluorescence resonance energy transfer-based optical sensor of glutamate FLII81E-1μ, we found that retinal waves are accompanied by a large transient increase in extrasynaptic glutamate throughout the inner plexiform layer. Using two-photon Ca(2+) imaging to record spontaneous Ca(2+) transients in large populations of cells, we found that despite this spatially diffuse source of depolarization, only a subset of neurons in the GCL and inner nuclear layer (INL) are robustly depolarized during retinal waves. Application of the glutamate transporter blocker dl-threo-β-benzyloxyaspartate (25 μM) led to a significant increase in cell participation in both layers, indicating that the concentration of extrasynaptic glutamate affects cell participation in both the INL and GCL. In contrast, blocking inhibitory transmission with the GABAA receptor antagonist gabazine and the glycine receptor antagonist strychnine increased cell participation in the GCL without significantly affecting the INL. These data indicate that during development, glutamate spillover provides a spatially diffuse source of depolarization, but that inhibitory circuits dictate which neurons within the GCL participate in retinal waves.

Differential modulation of retinal ganglion cell light responses by orthosteric and allosteric metabotropic glutamate receptor 8 compounds

To investigate the role of mGluR8 in modulating the synaptic responses of retinal ganglion cells, we used a recently identified positive allosteric modulator of mGluR8, AZ12216052 (AZ) and the mGluR8-specific orthosteric agonist (S)-3,4-dicarboxyphenylglycine (DCPG). These agents were applied to whole-cell voltage-clamped ganglion cells from an isolated, superfused mouse retina preparation. DCPG reduced OFF-ganglion cell excitatory currents, whereas AZ enhanced the peak excitatory currents in ON-, OFF-, and ON-OFF-ganglion cells. The effects on ganglion cell inhibitory currents were more varied. The effects of the allosteric modulator were stronger for bright stimuli than for dim stimuli, consistent with receptor stimulation by endogenous glutamate being stronger during bright light stimulation and with mGluR8 receptors mainly being localized away from glutamate release sites, immuno-labeled with VGLUT1. The differential sensitivity of ganglion cell light responses to DCPG and AZ supports multiple sites where mGluR8 modulates the light responses of ganglion cells.

Dopamine modulates the off pathway in light-adapted mouse retina

DL-2-Amino-4-phosphonobutyric acid (APB) is often used as a tool to block On pathways in studies of interactions between On and Off pathways in retinas. APB is an agonist of mGluR6 receptors and hyperpolarizes the On cone bipolar cells and rod bipolar cells. How APB affects Off responses of retinal ganglion cells (RGCs) in mouse retinas under dark and light adaptation is not clear. The light-evoked excitatory postsynaptic currents (light-evoked EPSCs) from Off and On-Off RGCs cells were recorded using whole-cell patch-clamp recording to assess how APB affects Off responses (light-evoked Off EPSCs) of RGCs in dark- and light-adapted mouse retinas. We found that APB differentially affected Off responses of RGCs in dark- and light-adapted mouse retinas. Under dark adaptation, while the APB-sensitive Off responses were blocked, APB increased the remaining Off responses (mainly from the secondary rod Off pathways) via removal of inhibition from On pathways to Off pathways. Under light adaptation, APB decreased Off responses. Glycinergic and GABAergic antagonists did not prevent the APB-induced reduction of Off responses of RGCs; however, a dopaminergic type 1 receptor (D(1)) blocker (SCH 23390) and a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker (ZD 7288) prevented the APB-induced reduction of Off responses of RGCs under light adaptation. The results indicated afunctional circuit: On cone bipolar cells to Off cone bipolar cells via D(1) receptors and HCN channels.

"mGlu Receptors in the Retina" - WIREs Membrane Transport and Signaling

Glutamate, a key neurotransmitter in the vertebrate retina, acts via ionotropic and metabotropic receptors. Retina expresses mRNA for all metabotropic glutamate receptors and proteins for all but mGluR3. Every retinal cell class expresses one or more of these receptors. In general, these receptors are present presynaptically and serve to modulate synaptic transmission. While mGluRs on the photoreceptor terminal act as autoreceptors to titer glutamate levels, those on horizontal cell processes seem to shape the light response. Similarly, autoreceptors on bipolar axon terminals modulate glutamate release and the receptors on amacrine and ganglion cells modulate feedforward signals by modulating K+ or Ca2+ current to fine tune light responses. Since most of the mGluR sub-types are present in amacrine and ganglion cells that belong to many cell types, the pathways downstream of mGluRs are highly diverse with primarily modulatory effects. An exception to most mGluRs which have modulatory function is mGluR6 because it plays a key role in the feedforward transmission from photoreceptors to ON bipolar cells and is also required for the correct localization of the synaptic proteins in the dendritic tips. In humans, mutations in the gene encoding mGluR6 cause autosomal recessive night blindness. In addition, mGluRs appear to play a trophic role in development and after retinal damage, suggesting potential future therapeutic implications.

Cross inhibition from ON to OFF pathway improves the efficiency of contrast encoding in the mammalian retina

The retina is divided into parallel and mostly independent ON and OFF pathways, but the ON pathway "cross" inhibits the OFF pathway. Cross inhibition was thought to improve signal processing by the OFF pathway, but its effect on contrast encoding had not been tested experimentally. To quantify the effect of cross inhibition on the encoding of contrast, we presented a dark flash to an in vitro preparation of the mammalian retina. We then recorded excitatory currents, inhibitory currents, membrane voltages, and spikes from OFF α-ganglion cells. The recordings were subjected to an ideal observer analysis that used Bayesian methods to determine how accurately the recordings detected the dark flash. We found that cross inhibition increases the detection accuracy of currents and membrane voltages. Yet these improvements in encoding do not fully reach the spike train, because cross inhibition also hyperpolarizes the OFF α-cell below spike threshold, preventing small signals in the membrane voltages at low contrast from reaching the spike train. The ultimate effect of cross inhibition is to increase the accuracy with which the spike train detects moderate contrast, but reduce the accuracy with which it detects low contrast. In apparent compensation for the loss of accuracy at low contrast, cross inhibition, by hyperpolarizing the OFF α-cell, reduces the number of spikes required to detect the dark flash and thereby increases encoding efficiency.

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.

Selective activation of mGluR8 receptors modulates retinal ganglion cell light responses

Extracellular and whole-cell light-evoked responses of mouse retinal ganglion cells were recorded in the presence of the mGluR8 selective agonist, (S)-3,4-dicarboxy-phenylglycine (DCPG). Off-light responses were reversibly reduced in the presence of DCPG in wild-type but not in mGluR8-deficient retinas. On-responses were only marginally modulated by DCPG. During Off-responses, DCPG suppressed both excitatory and inhibitory synaptic conductances suggesting that mGluR8 receptor activity reduces glutamate release from bipolar cell terminals and possibly also the release of an inhibitory neurotransmitter from amacrine cell processes.

Slow excitation of cultured rat retinal ganglion cells by activating group I metabotropic glutamate receptors

As in many CNS neurons, retinal ganglion cells (RGCs) receive fast synaptic activation through postsynaptic ionotropic receptors. However, the potential role of postsynaptic group I metabotropic glutamate receptors (mGluRs) in these neurons is unknown. In this study we first demonstrated that the selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) increased intracellular calcium concentration in neurons within the ganglion cell layer of the rat retina. This prompted us to use an immunopanned-RGC and cortical astroglia coculture preparation to explore the effect of group I mGluR activation on the electrophysiological properties of cultured RGCs. Using perforated patch-clamp recordings in current-clamp configuration, we found that application of DHPG increased spontaneous spiking and depolarized the resting membrane potential of RGCs. This boosting effect was attributed to an increase in membrane resistance due to blockade of a background K(+) conductance. Further experiments showed that the group I mGluR-sensitive K(+) conductance was not blocked by 3 mM Cs(+), but was sensitive to acidification. Pharmacological studies indicated that the effect of DHPG on RGCs was mediated by the mGluR1 rather than the mGluR5 receptor subtype. Our results suggest a facilitatory role for group I mGluR activation in modulating RGC excitability in the mammalian inner retina.

A computational study on the role of gap junctions and rod Ih conductance in the enhancement of the dynamic range of the retina

Recent works suggest that one of the roles of gap junctions in sensory systems is to enhance their dynamic range by avoiding early saturation in the first processing stages. In this work, we use a minimal conductance-based model of the ON rod pathways in the vertebrate retina to study the effects of electrical synaptic coupling via gap junctions among rods and among AII amacrine cells on the dynamic range of the retina. The model is also used to study the effects of the maximum conductance of rod hyperpolarization activated current I_h on the dynamic range of the retina, allowing a study of the interrelations between this intrinsic membrane parameter with those two retina connectivity characteristics. Our results show that for realistic values of I_h conductance the dynamic range is enhanced by rod-rod coupling, and that AII-AII coupling is less relevant to dynamic range amplification in comparison with receptor coupling. Furthermore, a plot of the retina output response versus input intensity for the optimal parameter configuration is well fitted by a power law with exponent . The results are consistent with predictions of more theoretical works and suggest that the earliest expression of gap junctions along the rod pathways, together with appropriate values of rod I_h conductance, has the highest impact on vertebrate retina dynamic range enhancement.

Complex temporal response patterns with a simple retinal circuit

The retina can respond to a wide array of features in the visual input. It was recently reported that the retina can even recognize complicated temporal input patterns and signal violations in the patterns. When a sequence of flashes was presented, ganglion cells exhibited a variety of firing profiles and many cells showed an "omitted stimulus response" (OSR), in which they fired strongly if a flash in the sequence was omitted. We examined the synaptic origins of the OSR by recording excitatory synaptic currents from ganglion cells in the salamander retina in response to periodic flash sequences. Consistent with previous spike recordings, ganglion cells exhibited an OSR in their current response and the OSR shifted in time with a change in flash frequency such that it could predict when the next flash should have occurred. Although the behavior may seem sophisticated, we show that a simple linear-nonlinear model with a spike threshold can account for the OSR in on ganglion cells and that the variety of complex firing profiles seen in other ganglion cells can be explained by adding contributions from the off pathway. We discuss the physiological and simulation results and their implications for understanding retinal mechanisms of visual information processing.

Presynaptic glutamate receptors: physiological functions and mechanisms of action

Glutamate acts on postsynaptic glutamate receptors to mediate excitatory communication between neurons. The discovery that additional presynaptic glutamate receptors can modulate neurotransmitter release has added complexity to the way we view glutamatergic synaptic transmission. Here we review evidence of a physiological role for presynaptic glutamate receptors in neurotransmitter release. We compare the physiological roles of ionotropic and metabotropic glutamate receptors in short- and long-term regulation of synaptic transmission. Furthermore, we discuss the physiological conditions that are necessary for their activation, the source of the glutamate that activates them, their mechanisms of action and their involvement in higher brain function.

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.

How robust is a neural circuit?

Design in engineering begins with the problem of robustness-by what factor should intrinsic capacity exceed normal demand? Here we consider robustness for a neural circuit that crosses the retina from cones to ganglion cells. The circuit's task is to represent the visual scene at many successive stages, each time by modulating a stream of stochastic events: photoisomerizations, then transmitter quanta, then spikes. At early stages, the event rates are high to achieve some critical signal-to-noise ratio and temporal bandwidth, which together set the information rate. Then neural circuits concentrate the information and repackage it, so that nearly the same total information can be represented by modulating far lower event rates. This is important for spiking because of its high metabolic cost. Considering various measurements at the outer and inner retina, we conclude that the "safety factors" are about 2-10, similar to other tissues.

Distribution of group-III metabotropic glutamate receptors in the retina

In the brain and the retina metabotropic glutamate receptors (mGluRs) modulate synaptic transmission; in particular, L-2-amino-4-phosphonobutyrate-sensitive group-III mGluRs are generally presynaptic and provide negative feedback of neurotransmitter release. We performed a comparative immunohistochemical analysis of the distribution of all group-III mGluRs in the mouse retina. mGluR6 expression was limited to the outer plexiform layer. Discrete, punctate immunolabeling, exclusively in the inner plexiform layer (IPL), was observed for each of the remaining group-III mGluRs. mGluR4 immunostaining was most abundant in IPL sublamina 1; mGluR7 immunoreactivity was organized in four bands, corresponding to sublaminae 1-4; and mGluR8 was localized in two broad bands, one each in the OFF and ON layers of the IPL. mGluR8 immunoreactivity was evident in the OFF plexus of cholinergic amacrine cell processes. Surprisingly, we found little overlap between group-III mGluR immunolabeling and that for the vesicular glutamate transporter VGLUT1. Instead, we found that mGluR4 and mGluR7 were located close to bipolar cell ribbons. No compensatory changes in the distribution of group-III mGluRs, or of several other markers also showing a stratified localization in the IPL, were observed in genetically engineered mice lacking either mGluR4, mGluR8, or both mGluR4 and mGluR8. The unique pattern of expression of each receptor suggests that they have distinct functions in the retina, and their asymmetric distribution in the ON and OFF layers of the IPL suggests distinct roles in the processing of light-ON and light-OFF stimuli.

Modeling the pre- and post-synaptic components involved in the synaptic modification between cones and horizontal cells in carp retina

In retinal synapses between cones and luminosity type horizontal cells (LHC), it was previously found in this laboratory that repetitive red flashes progressively strengthened the LHC's response to red flash, whereas weakened the LHC's response to green flash; repetitive green flash remarkably depressed the LHC's red response, but caused little changes in the cell's green response. However, the detailed mechanisms underlying these phenomena are not entirely clear. In the present study, based on an ion-channel model described mainly in the form of Hodgkin-Huxley equations, possible mechanisms of the short-term synaptic modification are investigated. The simulation results suggest that: (1) the auto-enhancement effect might be induced by the Ca2+-dependent process on the post-synaptic AMPA receptors, which could lead to changes of the ionic channel's properties; (2) the asymmetric response to red- and green-flashes and the mutual-chromatic suppression effects might be attributed to the regulatory effects on the presynaptic glutamate release.

Ca2+-dependent mechanisms of presynaptic control at central synapses

Classically, a high-power association relates the neurotransmitter release probability to the concentration of presynaptic Ca2+. Activated by the action potential waveform, voltage-gated Ca2+ channels mediate Ca2+entry into presynaptic terminals. Inside the terminal, Ca2+ ions rapidly bind to endogenous intracellular buffers and could trigger Ca2+ release from internal Ca2+ stores. The resulting space-time profile of free Ca2+ determines the time course and probability of neurotransmitter release through the interaction with molecular release triggers strategically located in the vicinity of release sites. Following a rapid concentration transient, excess Ca2+ has to be removed from the cytosol through the process involving Ca2+ uptake by the endoplasmatic reticulum stores, sequestration by mitochondria, and/or extrusion into the extracellular medium. The ongoing synaptic activity could affect any of the multiple factors that shape presynaptic Ca2+ dynamics, thus arbitrating use-dependent modification of the neurotransmitter release probability. Here we present an overview of major players involved in Ca2+-dependent presynaptic regulation of neurotransmitter release and discuss the relationships arising between their actions.

Presynaptic Mechanism for Slow Contrast Adaptation in Mammalian Retinal Ganglion Cells

Visual neurons, from retina to cortex, adapt slowly to stimulus contrast. Following a switch from high to low contrast, a neuron rapidly decreases its responsiveness and recovers over 5-20 s. Cortical adaptation arises from an intrinsic cellular mechanism: a sodium-dependent potassium conductance that causes prolonged hyperpolarization. Spiking can drive this mechanism, raising the possibility that the same mechanism exists in retinal ganglion cells. We found that adaptation in ganglion cells corresponds to a slowly recovering afterhyperpolarization (AHP), but, unlike in cortical cells, this AHP is not primarily driven by an intrinsic cellular property: spiking was not sufficient to generate adaptation. Adaptation was strongest following spatial stimuli tuned to presynaptic bipolar cells rather than the ganglion cell; it was driven by a reduced excitatory conductance, and it persisted while blocking GABA and glycine receptors, K((Ca)) channels, or mGluRs. Thus, slow adaptation arises from reduced glutamate release from presynaptic (nonspiking) bipolar cells.

Immunolocalization of metabotropic glutamate receptors 1 and 5 in the synaptic layers of the chicken retina

We have examined the distribution of metabotropic glutamate receptors (mGluRs) 1 and 5 in the adult chicken retina using preembedding immuno-electronmicroscopy. Immunoreactivity for mGluRs 1 and 5 was found in both the outer plexiform layer (OPL) and the inner plexiform layer (IPL). For mGluR1, OPL labeling was observed at cone pedicles and horizontal and bipolar cell processes. In the IPL, mGluR1 labeling could be found on bipolar cell terminals, as well as postsynaptic processes, including amacrine cell processes. Neither presynaptic nor postsynaptic elements were labeled at rod synapses. For mGluR5, OPL labeling was associated with cone pedicles as well as bipolar and horizontal cell processes. As for mGluR1, rod synapses were unlabeled. In the IPL, labeling for mGluR5 was found on bipolar cell terminals and amacrine cell processes. The presynaptic expression of these receptors in the OPL was confirmed at the light level by double-labeling experiments with SV2. The distributions of mGluRs 1 and 5 indicate that they have the potential to regulate function in both synaptic layers. Furthermore, the similar expression patterns for these two receptors indicate that they might be co-expressed and thus have the potential to interact functionally.

Structure and function of ribbon synapses

Sensory neurons with short conduction distances can use nonregenerative, graded potentials to modulate transmitter release continuously. This mechanism can transmit information at much higher rates than spiking. Graded signaling requires a synapse to sustain high rates of exocytosis for relatively long periods, and this capacity is the special virtue of ribbon synapses. Vesicles tethered to the ribbon provide a pool for sustained release that is typically fivefold greater than the docked pool available for fast release. The current article, which is part of the TINS Synaptic Connectivity series, reviews recent evidence for this fundamental computational strategy and its underlying cell biology.

Localization of NMDA receptor subunits and mapping NMDA drive within the mammalian retina

Glutamate is a major neurotransmitter in the retina and other parts of the central nervous system, exerting its influence through ionotropic and metabotropic receptors. One ionotropic receptor, the N-methyl-D-aspartate(NMDA) receptor, is central to neural shaping, but also plays a major role during neuronal development and in disease processes. We studied the distribution pattern of different subunits of the NMDA receptor within the rat retina including quantifying the pattern of labelling for all the NRI splice variants, the NR2A and NR2B subunits. The labelling pattern for the subunits was confined predominantly in the outer two-thirds of the inner plexiform layer. We also wanted to probe NMDA receptor function using an organic cation, agmatine (AGB); a marker for cation channel activity. Although there was an NMDA concentration-dependent increase in AGB labelling of amacrine cells and ganglion cells, we found no evidence of functional NMDA receptors on horizontal cells in the peripheral rabbit retina, nor in the visual streak where the type A horizontal cell was identified by GABA labelling. Basal AGB labelling within depolarizing bipolar cells was also noted. This basal bipolar cell AGB labelling was not modulated by NMDA and was completely abolished by the use of L-2-amino-4-phosphono-butyric acid,which is known to hyperpolarize retinal depolarizing bipolar cells. AGB is therefore not only useful as a probe of ligand-gated drive, but can also identify neurons that have constitutively open cationic channels. In combination,the NMDA receptor subunit distribution pattern and the AGB gating experiments strongly suggests that this ionotropic glutamate receptor is functional in the cone-driven pathway of the inner retina.

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.

Responses of rabbit retinal ganglion cells to electrical stimulation with an epiretinal electrode

Rational selection of electrical stimulus parameters for an electronic retinal prosthesis requires knowledge of the electrophysiological responses of retinal neurons to electrical stimuli. In this study, we examined the effects of cathodal and anodal current pulses on the extracellularly recorded responses of OFF and ON rabbit retinal ganglion cells (RGCs) in an in vitro preparation. Current pulses (1 msec duration), delivered by a 125 microm electrode placed on the inner retinal surface within the receptive field of a RGC, produced both short-latency (< or =5 msec) and long-latency (8-60 msec) responses. The long-latency responses, but not the short-latency responses, were abolished upon application of the glutamate receptor antagonists CNQX and NBQX, thus indicating that the long-latency responses of RGCs are due to activation of presynaptic neurons in the retina. The latency of the long-latency response depended upon the polarity of the stimulus. For OFF RGCs, the average latency was 11 msec for a cathodal stimulus and 24 msec for an anodal stimulus. For ON RGCs, the average latency was 25 msec for a cathodal stimulus and 16 msec for an anodal stimulus. The threshold current also depended upon the polarity of the stimulus, at least for OFF RGCs. The average threshold current for evoking a long-latency response in OFF RGCs was 10 microA for a cathodal stimulus and 21 microA for an anodal stimulus. In ON RGCs, the average threshold current was 13 microA for a cathodal stimulus and 15 microA for an anodal stimulus.

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.

Depression of GABAergic input to identified hippocampal neurons by group III metabotropic glutamate receptors in the rat

The release of GABA in synapses is modulated by presynaptic metabotropic glutamate receptors (mGluRs). We tested whether GABA release to identified hippocampal neurons is influenced by group III mGluR activation using the agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) on inhibitory postsynaptic currents (IPSCs) evoked in CA1 interneurons and pyramidal cells. In interneurons, characterized with biocytin and immunolabelling for somatostatin, evoked IPSCs were depressed by 50 micro m L-AP4 (activating mGluR4 and 8) to 68 +/- 6% of control, but they were rarely depressed in pyramidal cells (96 +/- 4% of control). At 300-500 micro m concentration (activating mGluR4, 7 and 8), L-AP4 depressed IPSCs in both interneurons (to 70 +/- 6%) and pyramidal cells (to 67 +/- 4%). The change in trial-to-trial variability and in paired-pulse depression indicated a presynaptic action. In interneurons, the degree of IPSC depression was variable (to 9-87%), and a third of IPSCs were not affected by L-AP4. The L-AP4-evoked IPSC depression was blocked by LY341495. The depression of IPSCs was similar in O-LM cells and other interneurons. The lack of cell-type selectivity and the similar efficacy of different concentrations of L-AP4 suggest that several group III mGluRs are involved in the depression of IPSCs. Electron microscopic immunocytochemistry confirmed that mGluR4, mGluR7a and mGluR8a occur in the presynaptic active zone of GABAergic terminals on interneurons, but not on those innervating pyramidal cells. The high variability of L-AP4-evoked IPSC suppression is in line with the selective expression of presynaptic mGluRs by several distinct types of GABAergic neuron innervating each interneuron type.

Retinal bipolar cell input mechanisms in giant danio. I. Electroretinographic analysis

Electroretinograms (ERGs) were recorded from the giant danio (Danio aequipinnatus) to study glutamatergic input mechanisms onto bipolar cells. Glutamate analogs were applied to determine which receptor types mediate synaptic transmission from rods and cones to on and off bipolar cells. Picrotoxin, strychnine, and tetrodotoxin were used to isolate the effects of the glutamate analogs to the photoreceptor-bipolar cell synapse. Under photopic conditions, the group III metabotropic glutamate receptor (mGluR) antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) only slightly reduced the b-wave, whereas the excitatory amino acid transporter (EAAT) blocker dl-threo-beta-benzyl-oxyaspartate (TBOA) removed most of it. Complete elimination of the b-wave required both antagonists. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) blocked the d-wave. Under scotopic conditions, rod and cone inputs onto on bipolar cells were studied by comparing the sensitivities of the b-wave to photopically matched green and red stimuli. The b-wave was >1 log unit more sensitive to the green than to the red stimulus under control conditions. In CPPG or l-AP4 (l-(+)-2-amino-4-phosphonobutyric acid, a group III mGluR agonist), the sensitivity of the b-wave to the green stimulus was dramatically reduced and the b-waves elicited by the 2 stimuli became nearly matched. The d-wave elicited by dim green stimuli, which presumably could be detected only by the rods, was eliminated by NBQX.

IN CONCLUSION

1) cone signals onto on bipolar cells involve mainly EAATs but also mGluRs (presumably mGluR6) to a lesser extent; 2) rods signal onto on bipolars by mainly mGluR6; 3) off bipolar cells receive signals from both photoreceptor types by AMPA/kainate receptors.

Desensitization of the mGluR6 transduction current in tiger salamander On bipolar cells

Light depolarizes retinal On bipolar cells, opening the cation-selective channels that are responsible for producing the synaptic current. In this study, the basic features of light-induced signals were mimicked by bathing slices of salamander retina with an agonist for the mGluR6 receptor that is expressed on the dendrites of On cells, and then displacing the agonist with the mGluR6 antagonist (RS)-a-cyclopropyl-4-phosphonophenylglycine (CPPG). The transduction current that is activated by this protocol rapidly shuts off, or desensitizes. Desensitization was highly correlated with the concentration and the type of Ca2+ buffer that was dialysed into the cell: When Ca2+ buffering was minimized by dialysing cells with 0.5 mM EGTA, the steady-state response was reduced to approximately 40% of the peak response. Buffering with 10 mM EGTA reduced desensitization, while BAPTA completely eliminated it. Removing external Ca2+ also prevented desensitization, suggesting that entry of Ca2+ through the transduction channel provides the trigger. The time course of desensitization was measured by using a voltage jump protocol to rapidly increase Ca2+ influx, and could be fitted with a single time constant on the order of 1 s, in good agreement with previously published rates of desensitization to steps of light in this species. It is proposed that Ca(2+)-dependent shut-off of the On bipolar cell transduction current may contribute to the conversion of sustained to transient light responses that predominate in the inner retina.

Synaptic cleft acidification and modulation of short-term depression by exocytosed protons in retinal bipolar cells

The release of vesicular protons during exocytosis causes a feedback inhibition of Ca2+ channels in photoreceptor terminals; however, the effect of this inhibition on subsequent exocytosis has not been studied. Here we show that a similar L-type Ca2+ channel inhibition occurs in bipolar cell terminals in slices of goldfish retina, and we investigate the effect that this has on subsequent exocytosis with membrane capacitance measurements. We find that transient Ca2+ current inhibition is correlated with exocytosis and modulated by the concentration of extracellular pH buffer. Ca2+ current inhibition is negligible in acutely dissociated terminals, demonstrating the importance of an intact synaptic cleft. The sensitivity of bipolar cell Ca2+ currents to extracellular pH was assessed: channel conductance is reduced and activation is shifted to more positive potentials by acidification. The effect of Ca2+ current inhibition on subsequent exocytosis was investigated by measuring paired-pulse depression. Under conditions in which there is a large amount of inhibition of Ca2+ influx, the degree of paired-pulse depression is significantly reduced. Finally, we show that under physiological (bicarbonate) buffering conditions, pronounced Ca2+ current inhibition occurs after exocytosis ( approximately 60% peak inhibition), which can decrease subsequent exocytosis during single depolarizations. We estimate that exocytosis is accompanied by a transient change in synaptic cleft pH from 7.5 to approximately 6.9. We suggest that this effect serves as an activity-dependent modulator of exocytosis at ribbon-type synapses where a large and compact coterie of vesicles can fuse at each active zone.

Metabotropic glutamate receptor-mediated hetero-synaptic interaction of red- and green-cone inputs to LHC of carp retina

The role of presynaptic metabotropic glutamate receptor (mGluR) on the interaction of red- and green-cone signals was investigated in luminosity-type horizontal cell (LHC) of isolated carp retina. It was found that a dim red background could enhance LHC's light response to green stimulus, and a dim green background was also able to increase the cell's response to red flash. Such mutual color enhancement was eliminated by application of groups II and III mGluR antagonist (S)-methyl-4-carboxyphenyl-glycine (MCPG). Furthermore, inhibition of glutamate uptake by using D-aspartate (D-Asp) or DL-threo-beta-hydroxy-aspartic acid (THA) completely blocked the mutual enhancement of color signals in LHC. However, the GABAergic feedback pathway in the outer retina was unnecessarily involved.

Timing of quantal release from the retinal bipolar terminal is regulated by a feedback circuit

In isolation, a presynaptic terminal generally releases quanta according to Poisson statistics, but in a circuit its release statistics might be shaped by synaptic interactions. We monitored quantal glutamate release from retinal bipolar cell terminals (which receive GABA-ergic feedback from amacrine cells) by recording spontaneous EPSCs (sEPSCs) in their postsynaptic amacrine and ganglion cells. In about one-third of these cells, sEPSCs were temporally correlated, arriving in brief bursts (10-55 ms) more often than expected from a Poisson process. Correlations were suppressed by antagonizing the GABA(C) receptor (expressed on bipolar terminals), and correlations were induced by raising extracellular calcium or osmolarity. Simulations of the feedback circuit produced "bursty" release when the bipolar cell escaped intermittently from inhibition. Correlations of similar duration were present in the light-evoked sEPSCs and spike trains of sluggish-type ganglion cells. These correlations were suppressed by antagonizing GABA(C) receptors, indicating that glutamate bursts from bipolar terminals induce spike bursts in ganglion cells.

Different circuits for ON and OFF retinal ganglion cells cause different contrast sensitivities

The theory of "parallel pathways" predicts that, except for a sign reversal, ON and OFF ganglion cells are driven by a similar presynaptic circuit. To test this hypothesis, we measured synaptic inputs to ON and OFF cells as reflected in the subthreshold membrane potential. We made intracellular recordings from brisk-transient (Y) cells in the in vitro guinea pig retina and show that ON and OFF cells in fact express significant asymmetries in their synaptic inputs. An ON cell receives relatively linear input that modulates a single excitatory conductance; whereas an OFF cell receives rectified input that modulates both inhibitory and excitatory conductances. The ON pathway, blocked by L-AP-4, tonically inhibits an OFF cell at mean luminance and phasically inhibits an OFF cell during a light increment. Our results suggest that basal glutamate release is high at ON but not OFF bipolar terminals, and inhibition between pathways is unidirectional: ON --> OFF. These circuit asymmetries explain asymmetric contrast sensitivity observed in spiking behavior.

Differential modulation of light-evoked on- and off-EPSCs by paired-pulse stimulation in salamander retinal ganglion cells

Short-term plasticity of On- and Off-EPSPs, and its potential role in regulation of signal processing was studied in salamander retinal On-Off ganglion cells by whole-cell recording. Paired-pulse light stimulation resulted in a depression of On-, and an enhancement of Off-EPSCs. Recovery from depression and enhancement was exponential and complete by 20 s. Paired-pulse enhancement, but not depression, was abolished with increasing stimulus duration. Blockade of On-EPSC by L-2-amino-4-phosphonobutyrate (AP-4), an agonist at group III mGluRs, significantly increased Off-EPSCs evoked by short (<2 s) duration conditioning light stimuli, resulting in a reversal of the paired-pulse enhancement to depression. The acetylcholinesterase inhibitor eserine reduced Off-EPSC1 and increased the ratio of enhancement. An opposite effect was observed in the presence of the nACh receptor antagonist d-tubocurarine. AP-7, an antagonist of NMDA receptors attenuated the enhancement of Off-EPSCs. In current clamp mode paired-pulse stimulation resulted in a modulation of light evoked, as well as the depolarization-induced spike firing pattern of ganglion cells. The present study suggests that paired light stimulation differently modulates On and Off EPSPs, and the light-evoked spike firing pattern of On-Off ganglion cells.

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.

Metabotropic glutamate receptors provide intrinsic modulation of the lamprey locomotor network

Spinal networks generate the motor pattern underlying locomotion. These are subject to modulatory systems that influence their operation and thereby result in a flexible network organization. In this review, we have summarized the mechanisms by which the different metabotropic glutamate receptor subtypes fine-tune the cellular and synaptic properties and thus underlie intrinsic modulation of the activity of the locomotor network in the lamprey.

Activation of mGluR5 modulates GABA(A) receptor function in retinal amacrine cells

Amacrine cells in the vertebrate retina receive glutamatergic input from bipolar cells and make synapses onto bipolar cells, ganglion cells, and other amacrine cells. Recent studies indicate that amacrine cells express metabotropic glutamate receptors (mGluRs) and that signaling within the inner plexiform layer (IPL) of the retina might be modulated by mGluR activity. This study tests the hypothesis that activation of mGluR5 modulates GABA(A) receptor function in retinal amacrine cells. Whole cell voltage-clamp recordings were combined with pharmacology to establish the identity of the ionotropic GABA receptors expressed in primary cultures of chick amacrine cells and to determine how mGluR5 activity affected the behavior of those receptors. Application of GABA (20 microM) produced currents that reversed at -58.2 +/- 0.9 mV, near the predicted Cl(-) reversal potential of -59 mV. The GABA(A) receptor antagonist, bicuculline (50 microM), completely blocked the GABA-gated currents. cis-4-Aminocrotonic acid (CACA; 100 microM), a GABA(C) receptor agonist, produced small currents that were not blocked by the GABA(C) antagonist, (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA; 20 microM), but were completely blocked by bicuculline. These results indicate that cultured amacrine cells express GABA(A) receptors exclusively. Activating mGluR5 with (RS)-2-chloro-5-hydroxyphenylglycine (CHPG; 300 microM) enhanced GABA-gated currents by 10.0 +/- 1.5%. Buffering internal Ca(2+) with BAPTA (10 mM) blocked the CHPG-dependent enhancement. Activation of PKC with the cell-permeable PKC activators (-)-7-octylindolactam V, phorbol 12-myristate 13 acetate (PMA), or 1-oleoyl-2-acetyl-sn-glycerol (OAG) also enhanced GABA-gated currents in a dose-dependent manner. Preactivation of PKC occluded the mGluR5-dependent enhancement, and inhibition of Ca-dependent PKC isotypes with Gö6976 (35 nM) suppressed the effects of mGluR5 activation, suggesting that mGluR5 and PKC are part of the same pathway. To determine if mGluR5-dependent enhancement occurred at synaptic GABA(A) receptors, postsynaptic currents were recorded in the presence of CHPG. On average, the mean amplitudes of the quantal events were increased by about 18% when mGluR5 was activated. These results indicate that activation of mGluR5 enhances GABA-gated current in cultured amacrine cells in a manner that is both Ca(2+)- and PKC-dependent. These results support the possibility that glutamate released from bipolar cells can modulate the function of GABAergic amacrine cells and alter signaling in the inner plexiform layer.

Calcium-induced transitions between the spontaneous miniature outward and the transient outward currents in retinal amacrine cells

Spontaneous miniature outward currents (SMOCs) occur in a subset of retinal amacrine cells at membrane potentials between -60 and -40 mV. At more depolarized potentials, a transient outward current (I(to)) appears and SMOCs disappear. Both SMOCs and the I(to) are K(+) currents carried by BK channels. They both arise from Ca(2+) influx through high voltage-activated (HVA) Ca(2+) channels, which stimulates release of internal Ca(2+) from caffeine- and ryanodine-sensitive stores. An increase in Ca(2+) influx resulted in an increase in SMOC frequency, but also led to a decline in SMOC mean amplitude. This reduction showed a temporal dependence: the effect being greater in the latter part of a voltage step. Thus, Ca(2+) influx, although required to generate SMOCs, also produced a negative modulation of their amplitudes. Increasing Ca(2+) influx also led to a decline in the first latency to SMOC occurrence. A combination of these effects resulted in the disappearance of SMOCs, along with the concomitant appearance of the I(to) at high levels of Ca(2+) influx. Therefore, low levels of Ca(2+) influx, arising from low levels of activation of the HVA Ca(2+) channels, produce randomly occurring SMOCs within the range of -60 to -40 mV. Further depolarization leads to greater activation of the HVA Ca(2+) channels, larger Ca(2+) influx, and the disappearance of discontinuous SMOCs, along with the appearance of the I(to). Based on their characteristics, SMOCs in retinal neurons may function as synaptic noise suppressors at quiescent glutamatergic synapses.

A metabotropic glutamate receptor regulates transmitter release from cone presynaptic terminals in carp retinal slices

The role of group III metabotropic glutamate receptors (mGluRs) in photoreceptor-H1 horizontal cell (HC) synaptic transmission was investigated by analyzing the rate of occurrence and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in H1 HCs uncoupled by dopamine in carp retinal slices. Red light steps or the application of 100 microM cobalt reduced the sEPSC rate without affecting their peak amplitude, which is consistent with hyperpolarization or the suppression of Ca(2+) entry into cone synaptic terminals reducing vesicular transmitter release. Conversely, postsynaptic blockade of H1 HC AMPA receptors by 500 nM CNQX reduced the amplitude of sEPSCs without affecting their rate. This analysis of sEPSCs represents a novel methodology for distinguishing between presynaptic and postsynaptic sites of action. The selective agonist for group III mGluRs, l-2-amino-4-phosphonobutyrate (L-APB or L-AP4; 20 microM), reduced the sEPSC rate with a slight reduction in amplitude, which is consistent with a presynaptic action on cone synaptic terminals to reduce transmitter release. During L-APB application, recovery of sEPSC rate occurred with 500 microM (s)-2-methyl-2-amino-4-phosphonobutyrate (MAP4), a selective antagonist of group III mGluR, and with 200 microM 4-aminopyridine (4-AP), a blocker of voltage-dependent potassium channels. Whole-cell recordings from cones in the retinal slice showed no effect of L-APB on voltage-activated Ca(2+) conductance. These results suggest that the activation of group III mGluRs suppresses transmitter release from cone presynaptic terminals via a 4-AP-sensitive pathway. Negative feedback, operating via mGluR autoreceptors, may limit excessive glutamate release from cone synaptic terminals.

Decreased plasma and cerebrospinal fluid glutamine concentrations in a patient with bialaphos poisoning

A 47-year-old Japanese woman undergoing maintenance hemodialysis (HD) was admitted to our hospital because of poisoning with the herbicide bialaphos. Respiratory arrest and loss of consciousness ensued rapidly, accompanied by convulsions and nystagmus. Treatment with HD and direct hemoperfusion, followed by HD alone, effectively removed bialaphos and its chief toxic metabolite (L-AMPB) from the circulation (bialaphos decreased from 0.33 to < 0.05 microg/ml and L-AMPB from 14 to 0.86 microg/ml). The glutamate concentration improved gradually after the removal of bialaphos and L-AMPB from plasma (plasma glutamate concentration: 250.4 nmol/l on day 5 to 120.6 nmol/l on day 26). Decreased glutamine concentration in cerebrospinal fluid was demonstrated for the first time as well as in plasma, indicating glutamine synthetase inhibition not only in plants but also in humans by bialaphos poisoning.