The relationship between light-evoked synaptic excitation and spiking behaviour of salamander retinal ganglion cells

Efficient Coding by Midget and Parasol Ganglion Cells in the Human Retina

In humans, midget and parasol ganglion cells account for most of the input from the eyes to the brain. Yet, how they encode visual information is unknown. Here, we perform large-scale multi-electrode array recordings from retinas of treatment-naive patients who underwent enucleation surgery for choroidal malignant melanomas. We identify robust differences in the function of midget and parasol ganglion cells, consistent asymmetries between their ON and OFF types (that signal light increments and decrements, respectively) and divergence in the function of human versus non-human primate retinas. Our computational analyses reveal that the receptive fields of human midget and parasol ganglion cells divide naturalistic movies into adjacent spatiotemporal frequency domains with equal stimulus power, while the asymmetric response functions of their ON and OFF types simultaneously maximize stimulus coverage and information transmission and minimize metabolic cost. Thus, midget and parasol ganglion cells in the human retina efficiently encode our visual environment.

"Silent" NMDA Synapses Enhance Motion Sensitivity in a Mature Retinal Circuit

Retinal direction-selective ganglion cells (DSGCs) have the remarkable ability to encode motion over a wide range of contrasts, relying on well-coordinated excitation and inhibition (E/I). E/I is orchestrated by a diverse set of glutamatergic bipolar cells that drive DSGCs directly, as well as indirectly through feedforward GABAergic/cholinergic signals mediated by starburst amacrine cells. Determining how direction-selective responses are generated across varied stimulus conditions requires understanding how glutamate, acetylcholine, and GABA signals are precisely coordinated. Here, we use a combination of paired patch-clamp recordings, serial EM, and large-scale multi-electrode array recordings to show that a single high-sensitivity source of glutamate is processed differentially by starbursts via AMPA receptors and DSGCs via NMDA receptors. We further demonstrate how this novel synaptic arrangement enables DSGCs to encode direction robustly near threshold contrasts. Together, these results reveal a space-efficient synaptic circuit model for direction computations, in which "silent" NMDA receptors play critical roles.

Pathway-Specific Maturation, Visual Deprivation, and Development of Retinal Pathway

One of the fundamental features of the visual system is the segregation of neural circuits that process increments and decrements of luminance into ON and OFF pathways. In mature retina, the dendrites of retinal ganglion cells (RGCs) in the inner plexiform layer (IPL) of retina are separated into ON or OFF sublaminaspecific stratification. At an early developmental stage, however, the dendrites of most RGCs are ramified throughout the IPL. The maturation of RGC ON/OFF dendritic stratification requires neural activities mediated by afferent inputs from bipolar and amacrine cells. The synchronized spontaneous burst activities in early postnatal developing retina regulate RGC dendritic filopodial movements and the maintenance or elimination of dendritic processes. After eye opening, visual experience further remodels and consolidates the retinal neural circuit into mature forms. Several neurotransmitter systems, including glutamatergic, acetylcholinergic, GAB Aergic, and glycinergic systems, might act together to modulate the RGC dendritic refinement. In addition, both the bipolar cells and cholinergic amacrine cells may provide laminar cues for the maturation of RGC dendritic stratification.

A Central Role for Mixed Acetylcholine/GABA Transmission in Direction Coding in the Retina

A surprisingly large number of neurons throughout the brain are endowed with the ability to co-release both a fast excitatory and inhibitory transmitter. The computational benefits of dual transmitter release, however, remain poorly understood. Here, we address the role of co-transmission of acetylcholine (ACh) and GABA from starburst amacrine cells (SACs) to direction-selective ganglion cells (DSGCs). Using a combination of pharmacology, optogenetics, and linear regression methods, we estimated the spatiotemporal profiles of GABA, ACh, and glutamate receptor-mediated synaptic activity in DSGCs evoked by motion. We found that ACh initiates responses to motion in natural scenes or under low-contrast conditions. In contrast, classical glutamatergic pathways play a secondary role, amplifying cholinergic responses via NMDA receptor activation. Furthermore, under these conditions, the network of SACs differentially transmits ACh and GABA to DSGCs in a directional manner. Thus, mixed transmission plays a central role in shaping directional responses of DSGCs.

Physiological and morphological characterization of ganglion cells in the salamander retina

Retinal ganglion cells (RGCs) integrate visual information from the retina and transmit collective signals to the brain. A systematic investigation of functional and morphological characteristics of various types of RGCs is important to comprehensively understand how the visual system encodes and transmits information via various RGC pathways. This study evaluated both physiological and morphological properties of 67 RGCs in dark-adapted flat-mounted salamander retina by examining light-evoked cation and chloride current responses via voltage-clamp recordings and visualizing morphology by Lucifer yellow fluorescence with a confocal microscope. Six groups of RGCs were described: asymmetrical ON-OFF RGCs, symmetrical ON RGCs, OFF RGCs, and narrow-, medium- and wide-field ON-OFF RGCs. Dendritic field diameters of RGCs ranged 102-490 μm: narrow field (<200 μm, 31% of RGCs), medium field (200-300 μm, 45%) and wide field (>300 μm, 24%). Dendritic ramification patterns of RGCs agree with the sublamina A/B rule. 34% of RGCs were monostratified, 24% bistratified and 42% diffusely stratified. 70% of ON RGCs and OFF RGCs were monostratified. Wide-field RGCs were diffusely stratified. 82% of RGCs generated light-evoked ON-OFF responses, while 11% generated ON responses and 7% OFF responses. Response sensitivity analysis suggested that some RGCs obtained separated rod/cone bipolar cell inputs whereas others obtained mixed bipolar cell inputs. 25% of neurons in the RGC layer were displaced amacrine cells. Although more types may be defined by more refined classification criteria, this report is to incorporate more physiological properties into RGC classification.

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.

NMDA receptor contributions to visual contrast coding

In the retina, it is not well understood how visual processing depends on AMPA- and NMDA-type glutamate receptors. Here we investigated how these receptors contribute to contrast coding in identified guinea pig ganglion cell types in vitro. NMDA-mediated responses were negligible in ON alpha cells but substantial in OFF alpha and delta cells. OFF delta cell NMDA receptors were composed of GluN2B subunits. Using a novel deconvolution method, we determined the individual contributions of AMPA, NMDA, and inhibitory currents to light responses of each cell type. OFF alpha and delta cells used NMDA receptors for encoding either the full contrast range (alpha), including near-threshold responses, or only a high range (delta). However, contrast sensitivity depended substantially on NMDA receptors only in OFF alpha cells. NMDA receptors contribute to visual contrast coding in a cell-type-specific manner. Certain cell types generate excitatory responses using primarily AMPA receptors or disinhibition.

Dendritic spikes amplify the synaptic signal to enhance detection of motion in a simulation of the direction-selective ganglion cell

The On-Off direction-selective ganglion cell (DSGC) in mammalian retinas responds most strongly to a stimulus moving in a specific direction. The DSGC initiates spikes in its dendritic tree, which are thought to propagate to the soma with high probability. Both dendritic and somatic spikes in the DSGC display strong directional tuning, whereas somatic PSPs (postsynaptic potentials) are only weakly directional, indicating that spike generation includes marked enhancement of the directional signal. We used a realistic computational model based on anatomical and physiological measurements to determine the source of the enhancement. Our results indicate that the DSGC dendritic tree is partitioned into separate electrotonic regions, each summing its local excitatory and inhibitory synaptic inputs to initiate spikes. Within each local region the local spike threshold nonlinearly amplifies the preferred response over the null response on the basis of PSP amplitude. Using inhibitory conductances previously measured in DSGCs, the simulation results showed that inhibition is only sufficient to prevent spike initiation and cannot affect spike propagation. Therefore, inhibition will only act locally within the dendritic arbor. We identified the role of three mechanisms that generate directional selectivity (DS) in the local dendritic regions. First, a mechanism for DS intrinsic to the dendritic structure of the DSGC enhances DS on the null side of the cell's dendritic tree and weakens it on the preferred side. Second, spatially offset postsynaptic inhibition generates robust DS in the isolated dendritic tips but weak DS near the soma. Third, presynaptic DS is apparently necessary because it is more robust across the dendritic tree. The pre- and postsynaptic mechanisms together can overcome the local intrinsic DS. These local dendritic mechanisms can perform independent nonlinear computations to make a decision, and there could be analogous mechanisms within cortical circuitry.

The On-Off direction-selective ganglion cell (DSGC) found in mammalian retinas generates a directional signal, responding most strongly to a stimulus moving in a specific direction. The DSGC initiates spikes in its dendritic tree which are thought to propagate to the soma and brain with high probability. Both dendritic and somatic spikes in the DSGC display strong directional tuning, whereas postsynaptic potentials (PSPs) recorded in the soma are only weakly directional, indicating that postsynaptic spike generation markedly enhances the directional signal. We constructed a realistic computational model to determine the source of the enhancement. Our results indicate that the DSGC dendritic tree is partitioned into separate computational regions. Within each region, the local spike threshold produces nonlinear amplification of the preferred response over the null response on the basis of PSP amplitude. The simulation results showed that inhibition acts locally within the dendritic arbor and will not stop dendritic spikes from propagating. We identified the role of three mechanisms that generate direction selectivity in the local dendritic regions, which suggests the origin of the previously described “non-direction-selective region,” and also suggests that the known DS in the synaptic inputs is apparently necessary for robust DS across the dendritic tree.

Ectopic expression of multiple microbial rhodopsins restores ON and OFF light responses in retinas with photoreceptor degeneration

By expressing channel rhodopsin-2 (ChR2) in inner retinal neurons, previous studies have demonstrated restoration of ON responses in the retina after the death of rod and cone photoreceptors. In this study, we report that the expression of halorhodopsin (HaloR), a light-driven chloride pump, can effectively restore OFF responses in inner retinal neurons of mice with retinal degeneration. We show that HaloR-expressing retinal ganglion cells respond to light with rapid hyperpolarization and suppression of spike activity. After termination of the light stimulus, their membrane potential exhibits a rapid rebound overshoot with robust sustained or transient spike firing. Furthermore, we show that coexpression of ChR2/HaloR in retinal ganglion cells can produce ON, OFF, and even ON-OFF responses, depending on the wavelength of the light stimulus. Our results suggest that the expression of multiple microbial rhodopsins such as ChR2 and HaloR is a possible strategy to restore both ON and OFF light responses in the retina after the death of rod and cone photoreceptors.

Dendritic excitability and neuronal morphology as determinants of synaptic efficacy

The ability to trigger neuronal spiking activity is one of the most important functional characteristics of synaptic inputs and can be quantified as a measure of synaptic efficacy (SE). Using model neurons with both highly simplified and real morphological structures (from a single cylindrical dendrite to a hippocampal granule cell, CA1 pyramidal cell, spinal motoneuron, and retinal ganglion neurons) we found that SE of excitatory inputs decreases with the distance from the soma and active nonlinear properties of the dendrites can counterbalance this global effect of attenuation. This phenomenon is frequency dependent, with a more prominent gain in SE observed at lower levels of background input-output neuronal activity. In contrast, there are no significant differences in SE between passive and active dendrites under higher frequencies of background activity. The influence of the nonuniform distribution of active properties on SE is also more prominent at lower background frequencies. In models with real morphologies, the effect of active dendritic conductances becomes more dramatic and inverts the SE relationship between distal and proximal locations. In active dendrites, distal synapses have higher efficacy than that of proximal ones because of arising dendritic spiking in thin branches with high-input resistance. Lower levels of dendritic excitability can make SE independent of the distance from the soma. Although increasing dendritic excitability may boost SE of distal synapses in real neurons, it may actually reduce overall SE. The results are robust with respect to morphological variation and biophysical properties of the model neurons. The model of CA1 pyramidal cell with realistic distributions of dendritic conductances demonstrated important roles of hyperpolarization-activated (h-) current and A-type K(+) current in controlling the efficacy of single synaptic inputs and overall SE differently in basal and apical dendrites.

Glutamatergic input is coded by spike frequency at the soma and proximal dendrite of AII amacrine cells in the mouse retina

In the mammalian retina, AII amacrine cells play a crucial role in scotopic vision. They transfer rod signals from rod bipolar cells to the cone circuit, and divide these signals into the ON and OFF pathways at the discrete synaptic layers. AII amacrine cells have been reported to generate tetrodotoxin (TTX)-sensitive repetitive spikes of small amplitude. To investigate the properties of the spikes, we performed whole-cell patch-clamping of AII amacrine cells in mouse retinal slices. The spike frequency increased in proportion to the concentration of glutamate puffer-applied to the arboreal dendrite and to the intensity of the depolarizing current injection. The spike activity was suppressed by L-2-amino-4-phosphonobutyric acid, a glutamate analogue that hyperpolarizes rod bipolar cells, puffer-applied to the outer plexiform layer. Therefore, it is most likely that the spike frequency generated by AII amacrine cells is dependent on the excitatory glutamatergic input from rod bipolar cells. Gap junction blockers reduced the range of intensity of input with which spike frequency varies. Application of TTX to the soma and the proximal dendrite of AII amacrine cells blocked the voltage-gated Na(+) current significantly more than application to the arboreal dendrite, indicating that the Na(+) channels are mainly localized in these regions. Our results suggest that the intensity of the glutamatergic input from rod bipolar cells is coded by the spike frequency at the soma and the proximal dendrite of AII amacrine cells, raising the possibility that the spikes could contribute to the OFF pathway to enhance release of neurotransmitter.

Normal and rebound impulse firing in retinal ganglion cells

Given that the action potential output of retinal ganglion cells (RGCs) determines the nature of the visual information that is transmitted from the retina, an understanding of their intrinsic impulse firing characteristics is critical for an appreciation of the overall processing of visual information. Recordings from RGCs within an isolated whole-mount retina preparation showed that their normal impulse firing from the resting membrane potential (RMP) was linearly correlated in its frequency with the stimulus intensity. In addition to describing the relationship between the magnitude of the current injection and the resulting impulse frequency (F/I relationship), we have characterized the properties of individual action potentials when they are elicited from the RMP. In contrast, hyperpolarizing below the RMP revealed that RGCs displayed a time dependent anomalous rectification, manifested by the appearance of a depolarizing sag in their voltage response. When an adequate period of hyperpolarization was terminated, a fast phasic period of “rebound excitation” was observed, characterized by a brief phasic burst of impulse activity. When compared to equivalent action potential firing evoked by depolarizing from the RMP, rebound spiking was associated with a lower threshold and shorter latency for impulse activation as well as a prominent, phasic, burst-like doublet, or triplet of impulses. The rebound action potential had a more positive voltage overshoot and displayed a higher peak rate of rise in its upstroke than those correspondingly generated by depolarizing current pulses from the RMP. Blocking sodium spikes with TTX confirmed that the preceding hyperpolarization led to the recruitment and subsequent generation of a transient depolarizing voltage overshoot, which we have termed the net depolarizing overshoot (NDO). We propose that the NDO boosts the generation of sodium spikes by triggering rebound spikes on its upstroke and crest, thus accounting for the observed voltage dependent change in the firing pattern of RGCs.

Network variability limits stimulus-evoked spike timing precision in retinal ganglion cells

Visual, auditory, somatosensory, and olfactory stimuli generate temporally precise patterns of action potentials (spikes). It is unclear, however, how the precision of spike generation relates to the pattern and variability of synaptic input elicited by physiological stimuli. We determined how synaptic conductances evoked by light stimuli that activate the rod bipolar pathway control spike generation in three identified types of mouse retinal ganglion cells (RGCs). The relative amplitude, timing, and impact of excitatory and inhibitory input differed dramatically between On and Off RGCs. Spikes evoked by repeated somatic injection of identical light-evoked synaptic conductances were more temporally precise than those evoked by light. However, the precision of spikes evoked by conductances that varied from trial to trial was similar to that of light-evoked spikes. Thus, the rod bipolar pathway modulates different RGCs via unique combinations of synaptic input, and RGC temporal variability reflects variability in the input this circuit provides.

Visual experience and maturation of retinal synaptic pathways

The retinal synaptic network continues its maturational refinement after eye opening in mammals. This synaptic refinement is reflected in changes of retinal neuron synaptic activity and connectivity. In mature retina, the dendrites of retinal ganglion cells (RGCs) in the inner plexiform layer (IPL) of retina are separated into ON or OFF sublamina. At early developmental stage, however, the dendrites of most RGCs are ramified throughout the IPL. Recently we found that the postnatal maturational processes converting bistratified ON-OFF responsive RGCs to monostratified ON and OFF responsive RGCs depend upon visual stimulation after eye opening.

Dissociation of retinal ganglion cells without enzymes

We describe here methods for dissociating retinal ganglion cells from adult goldfish and rat without proteolytic enzymes, and show responses of ganglion cells isolated this way to step-wise voltage changes and fluctuating current injections. Taking advantage of the laminar organization of vertebrate retinas, photoreceptors and other cells were lifted away from the distal side of freshly isolated goldfish retinas, after contact with pieces of membrane filter. Likewise, cells were sliced away from the distal side of freshly isolated rat retinas, after these adhered to a membrane filter. The remaining portions of retina were incubated in an enzyme-free, low Ca2+ solution, and triturated. After aliquots of the resulting cell suspension were plated, ganglion cells could be identified by dye retrogradely transported via the optic nerve. These cells showed no obvious morphological degeneration for several days of culture. Perforated-patch whole-cell recordings showed that the goldfish ganglion cells spike tonically in response to depolarizing constant current injections, that these spikes are temporally precise in response to fluctuating current injections, and that the largest voltage-gated Na+ currents of these cells were larger than those of ganglion cells isolated with a neutral protease.

Spike generator limits efficiency of information transfer in a retinal ganglion cell

The quality of the signal a retinal ganglion cell transmits to the brain is important for preception because it sets the minimum detectable stimulus. The ganglion cell converts graded potentials into a spike train with a selective filter but in the process adds noise. To explore how efficiently information is transferred to spikes, we measured contrast detection threshold and increment threshold from graded potential and spike responses of brisk-transient ganglion cells. Intracellular responses to a spot flashed over the receptive field center of the cell were recorded in an intact mammalian retina maintained in vitro at 37 degrees C. Thresholds were measured in a single-interval forced-choice procedure with an ideal observer. The graded potential gave a detection threshold of 1.5% contrast, whereas spikes gave 3.8%. The graded potential also gave increment thresholds approximately twofold lower and carried approximately 60% more gray levels. Increment threshold "dipped" below the detection threshold at a low contrast (<5%) but increased rapidly at higher contrasts. The magnitude of the "dipper" for both graded potential and spikes could be predicted from a threshold nonlinearity in the responses. Depolarization of the cell by current injection reduced the detection threshold for spikes but also reduced the range of contrasts they can transmit. This suggests that contrast sensitivity and dynamic range are related in an essential trade-off.

Light deprivation suppresses the light response of inner retina in both young and adult mouse

The retinal synaptic network continues its development after birth in mammals. Recent studies show that postnatal development of retinal circuitry depends on visual stimulation. We sought to determine whether there is a time period during which the retina shows evidence of increased plasticity. We examined the effects of light deprivation on the retinal light response of mouse retina using electroretinogram (ERG) measurements. Our results showed that dark rearing mice from birth to postnatal day (P) 30, 60, and 90 suppressed the amplitudes of oscillatory potentials (OPs) and the magnitudes of suppression were age independent. In addition, dark-rearing-produced suppression of OP amplitudes can be completely reversed in both young and adult mice by returning them to cyclic light/dark conditions for 1 to 2 weeks. However, the recovery time course was age dependent with younger animals needing a longer time to achieve a full recovery. Furthermore, dark rearing of P60 mice raised under cyclic light/dark conditions for 30 days resulted in a similar magnitude of suppression of OP amplitudes as in age-matched mice dark reared from birth. These findings demonstrate that both the normal developmental changes and the maintenance of mature inner retinal light response in adult animals require visual stimulation. These results indicate a degree of activity-dependent plasticity in mouse retina that has not been previously described.

Ganglion cell contributions to the rat full-field electroretinogram

The purpose of this study was to determine what contributions are made to the rat full-field electroretinogram (ERG) by ganglion cells (GCs). To that end, the ERG was assessed longitudinally following optic nerve transection (ONTx). Additional studies were conducted using intravitreal injections of pharmacologically active substances. The ERG was recorded simultaneously from both eyes of anaesthetized adult Brown-Norway rats (ketamine: xylazine: acepromazine, 55: 5: 1 mg kg(-1)) using custom silver chloride electrodes. Stimuli were brief, white xenon discharges delivered via a Ganzfeld under dark-adapted and light-adapted conditions (150 cd m(-2)). ERGs were obtained 1, 2, 3, 4 and 9 weeks after ONTx (n = 8) or sham (n = 8) operations. ONTx reduced both positive and negative components of the scotopic threshold response (pSTR and nSTR). Scotopic ERG responses to brighter flashes, including a-waves, b-waves and oscillatory potentials (OPs) were unaffected by ONTx. ONTx reduced the photopic b-wave and OPs. TTX (6 microM) reduced the pSTR and nSTR, but not the scotopic a-wave, b-wave or OPs. TTX had dramatic effects on the photopic ERG, surpassing the effects of ONTx. TTX application 9 weeks post-ONTx had little additional effect on the STR. Inhibition of inner retinal responses using GABA (10 mM) or NMDA (0.8 mM) reduced the nSTR substantially. Similar results were obtained with antagonists of AMPA/KA ionotropic glutamate receptors 6-cyano-7-nitroquinoxaline-2,3(1H,4H)-dione (CNQX, 0.2 mM) or cis-2,3-piperidinedicarboxylic acid (PDA, 5 mm); however, both also reduced the scotopic b-wave by approximately 40 %. By contrast, the NMDA receptor antagonist D(-)-2-amino-7-phosphonoheptanoic acid (D-AP7, 0.2 mM) had no effect alone, but the combination of D-AP7 and CNQX completely abolished the STR. The results of this study indicate that: (1) both pSTR and nSTR components in the rat depend directly upon intact GC responses, and that amacrine cell contributions to these components are relatively small; (2) scotopic ERG response components to brighter flashes receive little influence from GCs; (3) the rat photopic ERG also reflects GC signals and may serve as an additional useful test of GC function; (4) TTX had dramatic effects on the rat photopic ERG that were not attributable to GC currents, but rather to voltage-gated sodium currents in amacrine or interplexiform cells; (5) a small residual negative STR persisted after ONTx that was likely to be generated by graded responses of third-order retinal cells, most likely amacrine cells.

D-serine and serine racemase are present in the vertebrate retina and contribute to the physiological activation of NMDA receptors

d-serine has been proposed as an endogenous modulator of N-methyl-d-aspartate (NMDA) receptors in many brain regions, but its presence and function in the vertebrate retina have not been characterized. We have detected d-serine and its synthesizing enzyme, serine racemase, in the retinas of several vertebrate species, including salamanders, rats, and mice and have localized both constituents to Müller cells and astrocytes, the two major glial cell types in the retina. Physiological studies in rats and salamanders demonstrated that, in retinal ganglion cells, d-serine can enhance excitatory currents elicited by the application of NMDA, as well as the NMDA receptor component of light-evoked synaptic responses. Application of d-amino acid oxidase, which degrades d-serine, reduced the magnitude of NMDA receptor-mediated currents, raising the possibility that endogenous d-serine serves as a ligand for setting the sensitivity of NMDA receptors under physiological conditions. These observations raise exciting new questions about the role of glial cells in regulating the excitability of neurons through release of d-serine.

Synaptically released glutamate activates extrasynaptic NMDA receptors on cells in the ganglion cell layer of rat retina

NMDA and AMPA receptors (NMDARs and AMPARs) are colocalized at most excitatory synapses in the CNS. Consequently, both receptor types are activated by a single quantum of transmitter and contribute to miniature and evoked EPSCs. However, in amphibian retina, miniature EPSCs in ganglion cell layer neurons are mediated solely by AMPARs, although both NMDARs and AMPARs are activated during evoked EPSCs. One explanation for this discrepancy is that NMDARs are located outside of the synaptic cleft and are activated only when extrasynaptic glutamate levels increase during coincident release from multiple synapses. Alternatively, NMDARs may be segregated at synapses that either are not spontaneously active or yield miniature EPSCs that are too small to detect. In this study, we examined excitatory, glutamatergic synaptic inputs to neurons in the ganglion cell layer of acute slices of rat retina. EPSCs, elicited by electrically stimulating presynaptic bipolar cells, exhibited both NMDAR- and AMPAR-mediated components. However, spontaneous EPSCs exhibited only an AMPAR-mediated component. The effects of low-affinity, competitive receptor antagonists indicated that NMDARs encounter less glutamate than AMPARs during an evoked synaptic response. Reducing glutamate uptake or changing the probability of release preferentially affected the NMDAR component in evoked EPSCs; reducing uptake revealed an NMDAR component in spontaneous EPSCs. These results indicate that NMDARs are located extrasynaptically and that glutamate transporters prevent NMDAR activation by a transmitter released from a single vesicle and limit their activation during evoked responses.

Synaptically released glutamate activates extrasynaptic NMDA receptors on cells in the ganglion cell layer of rat retina

NMDA and AMPA receptors (NMDARs and AMPARs) are colocalized at most excitatory synapses in the CNS. Consequently, both receptor types are activated by a single quantum of transmitter and contribute to miniature and evoked EPSCs. However, in amphibian retina, miniature EPSCs in ganglion cell layer neurons are mediated solely by AMPARs, although both NMDARs and AMPARs are activated during evoked EPSCs. One explanation for this discrepancy is that NMDARs are located outside of the synaptic cleft and are activated only when extrasynaptic glutamate levels increase during coincident release from multiple synapses. Alternatively, NMDARs may be segregated at synapses that either are not spontaneously active or yield miniature EPSCs that are too small to detect. In this study, we examined excitatory, glutamatergic synaptic inputs to neurons in the ganglion cell layer of acute slices of rat retina. EPSCs, elicited by electrically stimulating presynaptic bipolar cells, exhibited both NMDAR- and AMPAR-mediated components. However, spontaneous EPSCs exhibited only an AMPAR-mediated component. The effects of low-affinity, competitive receptor antagonists indicated that NMDARs encounter less glutamate than AMPARs during an evoked synaptic response. Reducing glutamate uptake or changing the probability of release preferentially affected the NMDAR component in evoked EPSCs; reducing uptake revealed an NMDAR component in spontaneous EPSCs. These results indicate that NMDARs are located extrasynaptically and that glutamate transporters prevent NMDAR activation by a transmitter released from a single vesicle and limit their activation during evoked responses.

Non-linear, high-gain and sustained-to-transient signal transmission from rods to amacrine cells in dark-adapted retina of Ambystoma

In darkness, On-Off amacrine cells (ACs) of the tiger salamander retina exhibited large spontaneous transient depolarizing potentials (sTDPs) with average peak amplitude of 5.05 +/- 2.5 mV and average frequency of 0.42 +/- 0.25 s(-1). Under voltage-clamp conditions the cell displayed large spontaneous postsynaptic currents (sPSCs) with average peak amplitude of 98 +/- 39 pA and average frequency of 0.45 +/- 0.22 s(-1). To a light step, ACs gave rise to a transient 'On' response at the light onset and a transient 'Off' response at light offset, followed by a train of TDPs ('After' response). Near the response threshold (0.3 activated rhodopsin molecules per rod per second), light-evoked TDPs (leTDPs) of similar amplitude and kinetics as the large sTDPs observed in darkness were seen, and about half of these leTDPs elicited a regenerative potential (RP). Brighter light steps gave rise to more leTDPs and higher rates of RPs in the On, Off and After responses. Within the linear response range of the rods, the AC response was non-linear, with the highest gain (676 +/- 429) near the dark potential. The amplitude of Off responses increased with the duration of the light step, and ACs may use this to encode speeds of moving stimuli: the faster the light object moves, the smaller the AC Off response. Moreover, the number of leTDPs in the AC After response increased with light intensity, and the onset of the After response coincides with bipolar cell tail response recovery. One possible origin of the large sTDPs and leTDPs is the spontaneous and depolarization-induced regenerative calcium potentials (RCaPs) in bipolar cell synaptic terminals. RCaPs in bipolar cell synaptic terminals cause transient glutamate release that results in the sTDPs in darkness, and leTDPs in On, Off and After responses in ACs.

Heterogeneous intrinsic firing properties of vertebrate retinal ganglion cells

Retinal ganglion cells (RGCs) use their characteristic firing patterns to encode various aspects of visual information and carry them to the brain. It has been thought that the firing pattern of an RGC's light response is determined primarily by the time course and spatiotemporal interaction of the synaptic inputs. However, it is unclear whether there is a difference in intrinsic firing properties among RGCs that could contribute to the cell-to-cell distinction of the light response firing pattern. We investigated the intrinsic firing properties of isolated goldfish RGCs, minimizing cytoplasmic disturbance with a perforated-patch, whole-cell recording technique. In response to a 1-s depolarizing current step, the majority of the examined RGCs (n = 84) displayed sustained firing that lasted over 800 ms (n = 24; tonic RGCs) or transient firing accommodated within 200 ms of the step onset (n = 47; phasic RGCs). Tonic and phasic RGCs also differed in their firing frequency-current intensity dynamics. There was a significant difference in the soma sizes of phasic and tonic RGCs, indicating that some parts of these groups originate from distinct morphological subtypes. In the presence of extracellular Ba(2+) (1 mM), phasic RGCs displayed sustained firing and firing frequency-current intensity dynamics similar to those of tonic RGCs. Thus a Ba(2+)-sensitive ion current (I(Ba-s)) underlies the firing characteristics of phasic RGCs. Under voltage-clamp conditions, I(Ba-s) was identified as a low-threshold, noninactivating voltage-dependent K(+) current. Because of its slow kinetics (time constant of activation, approximately 100 ms), I(Ba-s) may confer a gradually increasing hyperpolarizing driving force during maintained excitatory stimulus, which eventually would result in firing accommodation. These findings suggest that RGCs have heterogeneous intrinsic firing properties that could aid synaptic inputs in shaping light responses.

Relative contributions of bipolar cell and amacrine cell inputs to light responses of ON, OFF and ON-OFF retinal ganglion cells

Light-evoked postsynaptic currents (lePSCs) were recorded from ON, OFF and ON-OFF ganglion cells in dark-adapted salamander retinal slices under voltage clamp conditions, and the cell morphology was examined using Lucifer yellow fluorescence with confocal microscopy. The current-voltage relations of the lePSCs in all three types of ganglion cells are approximately linear within the cells' physiological range. The average chloride/cation conductance ratio (Deltag(Cl)(NR)/Deltag(C)(NR)) of the lePSCs is near 3, suggesting that ganglion cell light responses are associated with a greater postsynaptic conductance change at the amacrine-ganglion cell inhibitory synapses than at the bipolar-ganglion cell excitatory synapses. By comparing the charge transfer of lePSCs in normal Ringer's and in picrotoxin+strychnine+Imidazole-4-acidic acid, we found that the GABAergic and glycinergic amacrine-bipolar cell feedback synapses decreased the light-induced glutamatergic vesicle release from bipolar cells to all ganglion cells, and the degree of release reduction varied widely from ganglion cell to ganglion cell, with a range of 3-28 fold.

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

Synaptic signals from retinal bipolar cells were monitored by measuring EPSCs in ganglion cells voltage-clamped at -70 mV. Spontaneous EPSCs were strongly suppressed by l-2-amino-4-phosphonobutyrate (AP-4), an agonist at group III metabotropic glutamate receptors (mGluRs). Agonists of group I or II mGluRs were ineffective. AP-4 also suppressed ganglion cell EPSCs evoked by bipolar cell stimulation using potassium puffs, sucrose puffs, or zaps of current (0.5-1 microA). In addition, AP-4 suppressed Off EPSCs evoked by dim-light stimuli. This indicates that group III mGluRs mediate a direct suppression of bipolar cell transmitter release. An mGluR antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked the action of AP-4. When bipolar cells were weakly stimulated, AP-4 produced a large suppression of the EPSC, but CPPG alone had little effect. Conversely, when bipolar cells were strongly stimulated, CPPG produced an enhancement of the EPSC, but AP-4 alone had little effect. This indicates that endogenous feedback regulates bipolar cell transmitter release and that the dynamic range of the presynaptic metabotropic autoreceptor is similar to that of the postsynaptic ionotropic receptor. Furthermore, the feedback is rapid and intensity-dependent. Hence, concomitant activation of presynaptic and postsynaptic glutamate receptors shapes the responses of ganglion cells.

Vertical interactions across ten parallel, stacked representations in the mammalian retina

The mammalian visual system analyses the world through a set of separate spatio-temporal channels. The organization of these channels begins in the retina, where the precise laminations of both the axon terminals of bipolar cells and the dendritic arborizations of ganglion cells suggests the presence of a vertical stack of neural strata at the inner plexiform layer (IPL). Conversely, many inhibitory amacrine cell classes are multiply or diffusely stratified, indicating that they might convey information between strata. On the basis of the diverse stratification and physiological properties of ganglion cells, it was suggested that the IPL contains a parallel set of representations of the visual world embodied in the strata and conveyed to higher centres by the classes of ganglion cells whose dendrites ramify at that stratum. Here we show that each stratum receives unique and substantively different excitatory and inhibitory neural inputs that are integrated to form at least ten different, parallel space-time spiking outputs. The response properties of these strata are ordered in the time domain. Inhibition through GABAC receptors extracts spatial edges in neural representations and seems to separate the functional properties of the strata. We describe a new form of neuronal interaction that we call 'vertical inhibition' that acts not laterally, but between strata.

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

Synaptic signals from retinal bipolar cells were monitored by measuring EPSCs in ganglion cells voltage-clamped at -70 mV. Spontaneous EPSCs were strongly suppressed by l-2-amino-4-phosphonobutyrate (AP-4), an agonist at group III metabotropic glutamate receptors (mGluRs). Agonists of group I or II mGluRs were ineffective. AP-4 also suppressed ganglion cell EPSCs evoked by bipolar cell stimulation using potassium puffs, sucrose puffs, or zaps of current (0.5-1 microA). In addition, AP-4 suppressed Off EPSCs evoked by dim-light stimuli. This indicates that group III mGluRs mediate a direct suppression of bipolar cell transmitter release. An mGluR antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked the action of AP-4. When bipolar cells were weakly stimulated, AP-4 produced a large suppression of the EPSC, but CPPG alone had little effect. Conversely, when bipolar cells were strongly stimulated, CPPG produced an enhancement of the EPSC, but AP-4 alone had little effect. This indicates that endogenous feedback regulates bipolar cell transmitter release and that the dynamic range of the presynaptic metabotropic autoreceptor is similar to that of the postsynaptic ionotropic receptor. Furthermore, the feedback is rapid and intensity-dependent. Hence, concomitant activation of presynaptic and postsynaptic glutamate receptors shapes the responses of ganglion cells.

Origin of transient and sustained responses in ganglion cells of the retina

Phasic and tonic light responses provide a fundamental division of visual information that is thought to originate in the inner retina. However, evidence presented here indicates that this duality originates in the outer retina. In response to a steady light stimulus, the temporal responses of On-bipolar cells fell into two groups. In one group, the light response peaked and then rapidly declined (tau approximately 400 msec) close to the resting membrane potential. At light offset, these cells exhibited a transient afterhyperpolarization. In the second group of On-bipolar cells, the light response declined 10-fold more slowly and reached a steady depolarization that was approximately 40% of the peak response. These neurons had a slowly decaying afterhyperpolarization at light offset. A metabotropic glutamate antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked light responses in both types of On-bipolar cell. CPPG only slightly depolarized transient On-bipolar cells, whereas sustained On-bipolar cells were significantly depolarized. Inorganic calcium channel blockers disclosed that these distinct On-bipolar responses were inherent to the bipolar cell and not attributable to synaptic feedback. CPPG had distinct effects on sustained and transient ganglion cells, similar to its action on bipolar cells. The antagonist depolarized and blocked the light responses of sustained ganglion cells. In transient ganglion cells, CPPG suppressed the On light response but did not depolarize the cell or block the Off light response. These results suggest that transient and sustained light responses in ganglion cells result from selective bipolar cell input and that these two fundamental visual channels originate at the dendritic terminals of bipolar cells.

Deactivation, recovery from inactivation, and modulation of extra-synaptic ion currents in fish retinal ganglion cells

As is shown magnificently by Heron Island's reef, the visual environment of many fishes includes various light intensities, hues and shapes that can change on large and small scales in space and time. Several articles in this issue address why fishes are sensitive to some of these properties, and how fishes and other aquatic species have acquired or fostered these sensitivities. This article discusses contributions of extrasynaptic ion currents, in a specific population of neurons, to the detection of ambient light levels, the appearance of certain visual stimuli and the disappearance of others.

Origin of transient and sustained responses in ganglion cells of the retina

Phasic and tonic light responses provide a fundamental division of visual information that is thought to originate in the inner retina. However, evidence presented here indicates that this duality originates in the outer retina. In response to a steady light stimulus, the temporal responses of On-bipolar cells fell into two groups. In one group, the light response peaked and then rapidly declined (tau approximately 400 msec) close to the resting membrane potential. At light offset, these cells exhibited a transient afterhyperpolarization. In the second group of On-bipolar cells, the light response declined 10-fold more slowly and reached a steady depolarization that was approximately 40% of the peak response. These neurons had a slowly decaying afterhyperpolarization at light offset. A metabotropic glutamate antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked light responses in both types of On-bipolar cell. CPPG only slightly depolarized transient On-bipolar cells, whereas sustained On-bipolar cells were significantly depolarized. Inorganic calcium channel blockers disclosed that these distinct On-bipolar responses were inherent to the bipolar cell and not attributable to synaptic feedback. CPPG had distinct effects on sustained and transient ganglion cells, similar to its action on bipolar cells. The antagonist depolarized and blocked the light responses of sustained ganglion cells. In transient ganglion cells, CPPG suppressed the On light response but did not depolarize the cell or block the Off light response. These results suggest that transient and sustained light responses in ganglion cells result from selective bipolar cell input and that these two fundamental visual channels originate at the dendritic terminals of bipolar cells.

Light-evoked excitatory synaptic currents of X-type retinal ganglion cells

The excitatory amino acid receptor (EAAR) types involved in the generation of light-evoked excitatory postsynaptic currents (EPSCs) were examined in X-type retinal ganglion cells. Using isolated and sliced preparations of cat and ferret retina, the light-evoked EPSCs of X cells were isolated by adding picrotoxin and strychnine to the bath to remove synaptic inhibition. N-methyl-D-aspartate (NMDA) receptors contribute significantly to the light-evoked EPSCs of ON- and OFF-X cells at many different holding potentials. An NMDA receptor contribution to the EPSCs was observable when retinal synaptic inhibition was either normally present or pharmacologically blocked. NMDA receptors formed 80% of the peak light-evoked EPSC at a holding potential of -40 mV; however, even at -80 mV, 20% of the light-evoked EPSC was NMDA-mediated. An alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor-mediated component to the light-evoked EPSCs predominated at a holding potential of -80 mV. The light-evoked EPSC was blocked by the AMPA receptor-selective antagonist GYKI52466 (50-100 microM). The AMPA receptor-mediated EPSC component had a linear current-voltage relation. AMPA receptors form the main non-NMDA EAAR current on both ON- and OFF- X ganglion cell dendrites. When synaptic transmission was blocked by the addition of Cd(2+) to the Ringer, application of kainate directly to ganglion cells evoked excitatory currents that were strongly blocked by GYKI52466. Experiments using selective EAAR modulators showed the AMPA receptor-selective modulator cyclothiazide potentiated glutamate-evoked currents on X cells, while the kainate receptor-selective modulator concanavalin A (ConA) had no effect on kainate-evoked currents. Whereas the present study confirms the general notion that AMPA EAAR-mediated currents are transient and NMDA receptor-mediated currents are sustained, current-voltage relations of the light-evoked EPSC at different time points showed the contributions of these two receptor types significantly overlap. Both NMDA and AMPA EAARs can transmit transient and sustained visual signals in X ganglion cells, suggesting that much signal shaping occurs presynaptically in bipolar cells.

Rate of quantal excitation to a retinal ganglion cell evoked by sensory input

To determine the rate and statistics of light-evoked transmitter release from bipolar synapses, intracellular recordings were made from ON-alpha ganglion cells in the periphery of the intact, superfused, cat retina. Sodium channels were blocked with tetrodotoxin to prevent action potentials. A light bar covering the receptive field center excited the bipolar cells that contact the alpha cell and evoked a transient then a sustained depolarization. The sustained depolarization was quantified as change in mean voltage (Deltav), and the increase in voltage noise that accompanied it was quantified as change in voltage variance (Deltasigma(2)). As light intensity increased, Deltav and Deltasigma(2) both increased, but their ratio held constant. This behavior is consistent with Poisson arrival of transmitter quanta at the ganglion cell. The response component attributable to glutamate quanta from bipolar synapses was isolated by application of 6-cyano-7-nitroquinoxaline (CNQX). As CNQX concentration increased, the signal/noise ratio of this response component (Deltav(CNQX)/Deltasigma(CNQX)) held constant. This is also consistent with Poisson arrival and justified the application of fluctuation analysis. Two different methods of fluctuation analysis applied to Deltav(CNQX) and Deltasigma(CNQX) produced similar results, leading to an estimate that a just-maximal sustained response was caused by approximately 3,700 quanta s(-1). The transient response was caused by a rate that was no more than 10-fold greater. Because the ON-alpha cell at this retinal locus has approximately 2,200 bipolar synapses, one synapse released approximately 1.7 quanta s(-1) for the sustained response and no more than 17 quanta s(-1) for the transient. Consequently, within the ganglion cell's integration interval, here calculated to be approximately 16 ms, a bipolar synapse rarely releases more than one quantum. Thus for just-maximal sustained and transient depolarizations, the conductance modulated by a single bipolar cell synapse is limited to the quantal conductance ( approximately 100 pS at its peak). This helps preserve linear summation of quanta. The Deltav/Deltasigma(2) ratio remained constant even as the ganglion cell's response saturated, which suggested that even at the peak of sensory input, summation remains linear, and that saturation occurs before the bipolar synapse.

Three levels of lateral inhibition: A space-time study of the retina of the tiger salamander

The space-time patterns of activity generated across arrays of retinal neurons can provide a sensitive measurement of the effects of neural interactions underlying retinal activity. We measured the excitatory and inhibitory components associated with these patterns at each cellular level in the retina and further dissected inhibitory components pharmacologically. Using perforated and loose patch recording, we measured the voltages, currents, or spiking at 91 lateral positions covering approximately 2 mm in response to a flashed 300-microm-wide bar. First, we showed how the effect of well known lateral inhibition at the outer retina, mediated by horizontal cells, evolved in time to compress the spatial representation of the stimulus bar at ON and OFF bipolar cell bodies as well as horizontal cells. Second, we showed, for the first time, how GABA(C) receptor mediated amacrine cell feedback to bipolar terminals compresses the spatial representation of the stimulus bar at ON bipolar terminals over time. Third, we showed that a third spatiotemporal compression exists at the ganglion cell layer that is mediated by feedforward amacrine cells via GABA(A) receptors. These three inhibitory mechanisms, via three different receptor types, appear to compensate for the effects of lateral diffusion of activity attributable to dendritic spread and electrical coupling between retinal neurons. As a consequence, the width of the final representation at the ganglion cell level approximates the dimensions of the original stimulus bar.

A method for activating neurons using endogenous synaptic waveforms

Neuronal input-output functions are traditionally studied using rectangular or ramp waveforms of injected current. These waveforms are easy to produce and responses to them easy to quantify; thus they have been central to our understanding of the roles that membrane properties play in controlling repetitive firing. However, since smooth rectangular step and ramp waveforms lack the dynamic features of endogenous synaptic input, their use has the potential to underemphasize the importance of input patterns in controlling physiological patterns of neuronal output. To activate neurons with current waveforms that replicate natural synaptic input, we developed a method for acquiring, digitally manipulating and reinjecting endogenous synaptic currents. We demonstrate, by applying this technique to phrenic motoneurons (PMNs) in rhythmically-active in vitro preparations from neonatal rats, that stimulation of neurons with endogenous current waveforms produces responses that mimic those produced by spontaneous synaptic inputs. Acquired waveforms can be reinjected repeatedly to produce consistent responses, and can also be amplified or filtered prior to reinjection to yield a range of information including standard descriptors of firing behavior such as frequency/current plots. This technique provides a valuable tool for analysing characteristics of the synaptic waveform important in generating neuronal output and how synaptic factors interact with membrane properties to control repetitive firing.

Three levels of lateral inhibition: A space-time study of the retina of the tiger salamander

The space-time patterns of activity generated across arrays of retinal neurons can provide a sensitive measurement of the effects of neural interactions underlying retinal activity. We measured the excitatory and inhibitory components associated with these patterns at each cellular level in the retina and further dissected inhibitory components pharmacologically. Using perforated and loose patch recording, we measured the voltages, currents, or spiking at 91 lateral positions covering approximately 2 mm in response to a flashed 300-microm-wide bar. First, we showed how the effect of well known lateral inhibition at the outer retina, mediated by horizontal cells, evolved in time to compress the spatial representation of the stimulus bar at ON and OFF bipolar cell bodies as well as horizontal cells. Second, we showed, for the first time, how GABA(C) receptor mediated amacrine cell feedback to bipolar terminals compresses the spatial representation of the stimulus bar at ON bipolar terminals over time. Third, we showed that a third spatiotemporal compression exists at the ganglion cell layer that is mediated by feedforward amacrine cells via GABA(A) receptors. These three inhibitory mechanisms, via three different receptor types, appear to compensate for the effects of lateral diffusion of activity attributable to dendritic spread and electrical coupling between retinal neurons. As a consequence, the width of the final representation at the ganglion cell level approximates the dimensions of the original stimulus bar.

The network-selective actions of quinoxalines on the neurocircuitry operations of the rabbit retina

We examined the contribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxalole-4-propionic acid (AMPA)/kainate (KA) receptors to the light-responses of rabbit retinal neurons. In the outer retina, bath application of the AMPA/KA receptor antagonists 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and 2,3,dihydroxy-6-nitro-7-sulfamoyl-benzo-f-quinoxaline (NBQX) blocked the light-responses of horizontal cells. Application of quinoxalines enhanced ON-bipolar cell light-responses, and was associated with a hyperpolarization of their resting potentials. In the inner retina, application of both AMPA/KA and NMDA antagonists to AII amacrine-like cells only partially blocked their light-responses. Their residual responses may reflect electrical coupling to neighboring ON-center cone bipolar cells, and can inhibit OFF-center ganglion cells. ON-sustained ganglion cells were highly sensitive to the quinoxalines, which reduced their light-evoked firing, while the firing of ON-transient cells remained as NMDA-mediated light-responses. Quinoxalines had differential effects on the firing rates of ON- and OFF-center ganglion cells: ON-cells were reduced, while OFF-cells were increased. In contrast, firing rates of ON-OFF ganglion cells were not excited by NBQX, and showed a recovered light-response mediated by NMDA receptors. The receptive field surround was lost in ganglion cells. For comparison, NMDA antagonists had only moderate effects on all ganglion cell light-responses. Our results indicate that NMDA and AMPA/KA receptors both contribute to ganglion cell light-responses. However, AMPA/KA receptors also significantly effect the light-response of neurons presynaptic to retinal ganglion cells, altering the observed pharmacology at the ganglion cell level.

Multiple types of spontaneous excitatory synaptic currents in salamander retinal ganglion cells

Spontaneous and light-evoked excitatory postsynaptic currents (sEPSCs and leEPSCs) in retinal ganglion cells of the larval tiger salamander were recorded under voltage clamp conditions from living retinal slices. sEPSCs were isolated from the spontaneous inhibitory postsynaptic currents (sIPSCs) by application of 100 M picrotoxin+1 microM strychnine. In addition to the previously reported sEPSCs [K. Matsui, N. Hosoi, M. Tachibana, Excitatory synaptic transmission in the inner retina: pair recordings of bipolar cells and neurons of the ganglion cell layer, J. Neurosci. 18 (1998) 4500-4510; W.R. Taylor, E. Chen, D.R. Copenhagen, Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells, J. Physiol. 486 (1995) 207-221] [which are equivalent to our fast AMPA receptor-mediated sEPSCs (fAMPAsEPSCs)], we found another type of AMPA receptor-mediated sEPSC with slower rise and decay time courses and larger peak amplitudes (sAMPAsEPSCs), and the NMDA receptor-mediated sEPSCs (NMDAsEPSCs) in ON-OFF ganglion cells. The frequency of all three types of sEPSCs is greatly reduced by cobalt (with zero calcium) and increased by hyperosmotic solution, suggesting that these events are mediated by calcium-dependent exocytosis of glutamatergic synaptic vesicles. The amplitude histograms of sEPSCs do not show multiple peaks, suggesting that larger events are not discrete multiples of elementary events, or quanta, of similar neurotransmitter contents, as in the neuromuscular junction [P. Fatt, B. Katz, Spontaneous subthreshold activity at motor nerve endings, J. Physiol. 117 (1952) 109-128]. The average I-V relations of the fAMPAsEPSCs and sAMPAsEPSCs were outward rectified with reversal potentials at -12.2 mV and -10.8 mV, and that of the NMDAsEPSCs was N-shaped with a reversal potential at -5.8 mV. The average conductance increase associated with a single fAMPAsEPSC, a single sAMPAsEPSC, and a single NMDAsEPSC were 163. 26+/-51.02 pS, 233.33+/-163.64 pS, and 37.5+/-50.0 pS at -110 mV; 241.67+/-22.92 pS, 444.90+/-469.94 pS, and 25.93+/-70.37 pS at -60 mV; and 440.48+/-183.33 pS, 1,192.68+/-651.22 pS, and 517.71+/-238. 24 pS at +30 mV, respectively. The average frequency of the three sEPSCs at +30 mV were 15 Hz, 3.7 Hz and 3.6 Hz, respectively. The rise time (time to peak) of fAMPAsEPSCs was 1.5+/-1.05 ms and the decay time could be fitted with a single exponential with an average time constant of 3.4+/-4.1 ms. The rise and decay time course of the sAMPAsEPSCs and NMDAsEPSCs were much slower and sawtooth-shaped, and each 'sawtooth' had time course and amplitude similar to those of individual fAMPAsEPSCs. We propose that each fAMPAsEPSC is mediated by single or synchronized multiples of glutamatergic synaptic vesicles from bipolar cells, and each sAMPAsEPSC or NMDAsEPSC is mediated by larger clusters of synaptic vesicles triggered by spontaneous calcium spikes in bipolar cell axon terminals [J. Burrone, L. Lagnado, Electrical resonance and calcium influx in the synaptic terminal of depolarizing bipolar cells from the goldfish retina, J. Physiol. 505 (1997) 571-584; D. Zenisek, G. Matthews, Calcium action potentials in retinal bipolar neurons, Vis. Neurosci. 15 (1998) 69-75].

Voltage-dependent properties of dendrites that eliminate location-dependent variability of synaptic input

We examined the hypothesis that voltage-dependent properties of dendrites allow for the accurate transfer of synaptic information to the soma independent of synapse location. This hypothesis is motivated by experimental evidence that dendrites contain a complex array of voltage-gated channels. How these channels affect synaptic integration is unknown. One hypothesized role for dendritic voltage-gated channels is to counteract passive cable properties, rendering all synapses electrotonically equidistant from the soma. With dendrites modeled as passive cables, the effect a synapse exerts at the soma depends on dendritic location (referred to as location-dependent variability of the synaptic input). In this theoretical study we used a simplified three-compartment model of a neuron to determine the dendritic voltage-dependent properties required for accurate transfer of synaptic information to the soma independent of synapse location. A dendrite that eliminates location-dependent variability requires three components: 1) a steady-state, voltage-dependent inward current that together with the passive leak current provides a net outward current and a zero slope conductance at depolarized potentials, 2) a fast, transient, inward current that compensates for dendritic membrane capacitance, and 3) both alpha amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid- and N-methyl-D-aspartate-like synaptic conductances that together permit synapses to behave as ideal current sources. These components are consistent with the known properties of dendrites. In addition, these results indicate that a dendrite designed to eliminate location-dependent variability also actively back-propagates somatic action potentials.

Neuronal discriminator formed by metabotropic gamma-aminobutyric acid receptors

Neuronal discriminator formed by metabotropic gamma-aminobutyric acid receptors. J. Neurophysiol. 80: 3365-3368, 1998. Neurotransmitters function in one of two modes, promoting either inhibition or excitation. However, the metabotropic gamma-aminobutyric acid receptor (GABABR) system can switch between these modes. In the presence of a small excitatory stimulus, the GABABR mediates a shunting inhibition that suppresses excitation. However, in the presence of a strong excitatory stimulus, the GABABR potentiates the response. This bipartite action is accomplished by linking the GABABR to two electrogenic mechanisms; one activates an outward current and another reduces an outward current. As a consequence, the GABABR serves as a discriminator that reduces the influence of weak signals while augmenting responses to strong signals. In retinal ganglion cells, this mechanism acts to promote the communication of phasic information.

Interactions of inhibition and excitation in the light-evoked currents of X type retinal ganglion cells

The excitatory and inhibitory conductances driving the light-evoked currents (LECs) of cat and ferret ON- and OFF-center X ganglion cells were examined in sliced and isolated retina preparations using center spot stimulation in tetrodotoxin (TTX)-containing Ringer. ON-center X ganglion cells showed an increase in an excitatory conductance reversed positive to +20 mV during the spot stimulus. At spot offset, a transient inhibitory conductance was activated on many cells that reversed near ECl. OFF-center X ganglion cells showed increases in a sustained inhibitory conductance that reversed near ECl during spot stimulation. At spot offset, an excitatory conductance was activated that reversed positive to +20 mV. The light-evoked current kinetics of ON- and OFF-center X cells to spot stimulation did not significantly differ in form from their Y cell counterparts in TTX Ringer. When inhibition was blocked, current-voltage relations of the light-evoked excitatory postsynaptic currents (EPSCs) of both ON- and OFF-X cells were L-shaped and reversed near 0 mV. The EPSCs averaged between 300 and 500 pA at -80 mV. The metabotropic glutamate receptor agonist 2-amino-4-phosphonobutyric acid (APB), was used to block ON-center bipolar cell function. The LECs of ON-X ganglion cells were totally blocked in APB at all holding potentials. APB caused prominent reductions in the dark holding current and synaptic noise of ON-X cells. In contrast, the LECs of OFF-X ganglion cells remained in APB. An increase in the dark holding current was observed. The excitatory amino acid receptor antagonist combination of D-amino-5-phosphono-pentanoic acid (D-AP5) and 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxalinedione (NBQX) was used to block ionotropic glutamate receptor retinal neurotransmission. The LECs of all ON-X ganglion cells were totally blocked, and their holding currents were reduced similar to the actions of APB. For OFF-X ganglion cells, the antagonist combination always blocked the excitatory current at light-OFF; however, in many cells, the inhibitory current at light-ON remained. ON-center X ganglion cells receive active excitation during center illumination, and a transient inhibition at light-OFF. In contrast OFF-center X ganglion cells experience a sustained active inhibition during center illumination, and a shorter increase in excitation at light-offset. Cone bipolar cells provide a resting level of glutamate release on X ganglion cells on which their light-evoked currents are superimposed [corrected].

Characterization of spontaneous inhibitory synaptic currents in salamander retinal ganglion cells

Spontaneous and light-evoked postsynaptic currents (sPSCs and lePSCs, respectively) in retinal ganglion cells of the larval tiger salamander were recorded under voltage-clamp conditions from living retinal slices. The focus of this study is to characterize the spontaneous inhibitory PSCs (sIPSCs) and their contribution to the light-evoked inhibitory PSCs (leIPSCs) in ON-OFF ganglion cells. sIPSCs were isolated from spontaneous excitatory PSCs (sEPSCs) by application of 10 microM 6,7-dinitroquinoxaline-2,3-dione (DNQX) + 50 microM 2-amino-5-phosphonopentanoic acid (AP5). In approximately 70% of ON-OFF ganglion cells, bicuculline (or picrotoxin) completely blocks sIPSCs, suggesting all sIPSCs in these cells are mediated by GABAergic synaptic vesicles and gamma-aminobutyric acid-A (GABAA) receptors (GABAergic sIPSCs, or GABAsIPSCs). In the remaining 30% of - ganglion cells, bicuculline (or picrotoxin) blocks 70-98% of the sIPSCs, and the remaining 2-30% are blocked by strychnine (glycinergic sIPSCs, or GLYsIPSCs). GABAsIPSCs occur randomly with an exponentially distributed interval probability density function, and they persist without noticeable rundown over time. The GABAsIPSC frequency is greatly reduced by cobalt, consistent with the idea that they are largely mediated by calcium-dependent vesicular release. GABAsIPSCs in DNQX + AP5 are tetrodotoxin (TTX) insensitive, suggesting that amacrine cells that release GABA under these conditions do not generate spontaneous action potentials. The average GABAsIPSCs exhibited linear current-voltage relation with a reversal potential near the chloride equilibrium potential, and an average peak conductance of 319.67 +/- 252.83 (SD) pS. For GLYsIPSCs, the average peak conductance increase is 301.68 +/- 94.34 pS. These parameters are of the same order of magnitude as those measured in inhibitory miniature postsynaptic currents (mIPSCs) associated with single synaptic vesicles in the CNS. The amplitude histograms of GABAsIPSCs did not exhibit multiple peaks, suggesting that the larger events are not discrete multiples of elementary events (or quanta). We propose that each GABAsIPSC or GLYsIPSC in retinal ganglion cells is mediated by a single or synchronized multiple of synaptic vesicles with variable neurotransmitter contents. In a sample of 16 ON-OFF ganglion cells, the average peak leIPSC (held at 0 mV) at the light onset is 509.0 +/- 233.85 pA and that at the light offset is 529.0 +/- 339.88 pA. The approximate number of GABAsIPSCs and GLYsIPSCs required to generate the average light responses, calculated by the ratio of the charge (area under current traces) of the leIPSCs to that of the average single sIPSCs, is 118 +/- 52 for the light onset, and 132 +/- 76 for the light offset.

Refractoriness and neural precision

The response of a spiking neuron to a stimulus is often characterized by its time-varying firing rate, estimated from a histogram of spike times. If the cell's firing probability in each small time interval depends only on this firing rate, one predicts a highly variable response to repeated trials, whereas many neurons show much greater fidelity. Furthermore, the neuronal membrane is refractory immediately after a spike, so that the firing probability depends not only on the stimulus but also on the preceding spike train. To connect these observations, we investigated the relationship between the refractory period of a neuron and its firing precision. The light response of retinal ganglion cells was modeled as probabilistic firing combined with a refractory period: the instantaneous firing rate is the product of a "free firing rate, " which depends only on the stimulus, and a "recovery function," which depends only on the time since the last spike. This recovery function vanishes for an absolute refractory period and then gradually increases to unity. In simulations, longer refractory periods were found to make the response more reproducible, eventually matching the precision of measured spike trains. Refractoriness, although often thought to limit the performance of neurons, may in fact benefit neuronal reliability. The underlying free firing rate derived by allowing for the refractory period often exceeded the observed firing rate by an order of magnitude and was found to convey information about the stimulus over a much wider dynamic range. Thus, the free firing rate may be the preferred variable for describing the response of a spiking neuron.

Synchronous period-doubling in flicker vision of salamander and man

Periodic flashes of light have long served to probe the temporal properties of the visual system. Here we show that during rapid flicker of high contrast and intensity the eye reports to the brain only every other flash of light. In this regime, retinal ganglion cells of the salamander fire spikes on alternating flashes. Neurons across the entire retina are locked to the same flashes. The effect depends sharply on contrast and flash frequency. It results from a period-doubling bifurcation in retinal processing, and a simple model of nonlinear feedback reproduces the phenomenon. Pharmacological studies indicate that the critical feedback interactions require only cone photoreceptors and bipolar cells. Analogous period-doubling is observed in the human visual system. Under bright full-field flicker, the electroretinogram (ERG) shows a regime of period-doubling between 30 and 70 Hz. In visual evoked potentials from the occiput, the subharmonic component is even stronger. By analyzing the accompanying perceptual effects, we find that retinal period-doubling begins in the periphery of the visual field, and that it is the cause of a long mysterious illusory flicker pattern.

Refractoriness and neural precision

The response of a spiking neuron to a stimulus is often characterized by its time-varying firing rate, estimated from a histogram of spike times. If the cell's firing probability in each small time interval depends only on this firing rate, one predicts a highly variable response to repeated trials, whereas many neurons show much greater fidelity. Furthermore, the neuronal membrane is refractory immediately after a spike, so that the firing probability depends not only on the stimulus but also on the preceding spike train. To connect these observations, we investigated the relationship between the refractory period of a neuron and its firing precision. The light response of retinal ganglion cells was modeled as probabilistic firing combined with a refractory period: the instantaneous firing rate is the product of a "free firing rate, " which depends only on the stimulus, and a "recovery function," which depends only on the time since the last spike. This recovery function vanishes for an absolute refractory period and then gradually increases to unity. In simulations, longer refractory periods were found to make the response more reproducible, eventually matching the precision of measured spike trains. Refractoriness, although often thought to limit the performance of neurons, may in fact benefit neuronal reliability. The underlying free firing rate derived by allowing for the refractory period often exceeded the observed firing rate by an order of magnitude and was found to convey information about the stimulus over a much wider dynamic range. Thus, the free firing rate may be the preferred variable for describing the response of a spiking neuron.

Response sensitivity and voltage gain of the rod- and cone-bipolar cell synapses in dark-adapted tiger salamander retina

Response sensitivity and voltage gain of the rod- and cone-bipolar cell synapses in dark-adapted tiger salamander retina. J. Neurophysiol. 78: 2662-2673, 1997. Rods, cones, and bipolar cells were recorded in superfused, flat-mounted isolated retinas of the larval tiger salamander, Ambystoma tigrinum, under dark-adapted conditions. Voltage responses of 24 rods, 15 cones, and 41 bipolar cells in dark-adapted retinas to 500 nm light steps of various intensities were listed and fitted with hyperbolic functions, and their step sensitivities and relative sensitivities (log sigma) were estimated. In the linear response-intensity ranges, the step sensitivity of rods, SS(rod), is -1.0 mV photon-1 micron2 s or 0.034 mV Rh*-1 s rod and that of the cones, SS(cone), is approximately 0. 00146 mV photon-1 micron2 s or 0.000048 mV Rh*-1 s rod. The rod and cone responses were relatively homogenous with little variations in response amplitude and sensitivity. In contrast, bipolar cell responses were heterogenous with large variations in response amplitude and sensitivity. The maximum response amplitude of bipolar cells varied from 5 to 25 mV, and the relative response sensitivity (log sigma) varied >6 log units (-8.11 to -2.32). The step sensitivity of bipolar cells in the linear response-intensity range varied from 0.0000438 to 51.82 mV photon-1 micron2 s. Bipolar cells in dark-adapted tiger salamander retinas fell into two groups according to their relative sensitivities with very few cells falling in the intermediate light intensity region. The mixed bipolar cells (DBCM and HBCM) exhibited relative response sensitivity ranged from -8.11 to -5.54, and step sensitivity ranged from 1.22 to 51.82 mV photon-1 micron2 s. The cone-driven bipolar cells (DBCC and HBCC) exhibited relative response sensitivity ranged from -3.45 to -2.32, and step sensitivity ranged from 0.0000438 to 0. 00201 mV photon-1 micron2 sec. The chord voltage gain of the rod-DBCM or rod-HBCM synapses near the rod dark membrane potential ranged from 1.14 to 48.43 and that of the cone-DBCC or cone-HBCC synaptic gain near the cone dark membrane potential ranged from 0.03 to 1.38. The highest voltage gains were found near the rod or cone dark membrane potentials. By the use of linear subtraction method, we studied the synaptic inputs from cones to five mixed bipolar cells, and the voltage gains of the cone synapses in each of the bipolar cells were very close to the voltage gain of the rod synapses. This result suggests that although the responses of mixed bipolar cells are mediated mainly by rods when lights of short and medium wavelengths are used, their responses to long wavelength lights (>650 nm) are mediated by both rods and cones with comparable synaptic gains. Functional roles of the mixed and cone-driven bipolar cells in information processing in dark-adapted retinas are discussed.