Synaptic transmission at N-methyl-D-aspartate receptors in the proximal retina of the mudpuppy

Electoretinographic evidence of retinal ganglion cell-dependent function in schizophrenia

Schizophrenia is a complex disorder that is diagnosed mainly with clinical observation and evaluation. Recent studies suggest that many people with schizophrenia have abnormalities in the function of the N-methyl-d-aspartate receptor (NMDAR). The retina is part of the central nervous system and expresses the NMDAR, raising the possibility of the early detection of NMDAR-related schizophrenia by detecting differences in retinal function. As a first-step, we used two non-invasive outpatient tests of retinal function, the photopic negative response (PhNR) of the light-adapted flash-electroretinogram (PhNR-fERG) and the pattern ERG (PERG), to test individuals with schizophrenia and controls to determine if there were measurable differences between the two populations. The PhNR-fERG showed that males with schizophrenia had a significant increase in the variability of the overall response, which was not seen in the females with schizophrenia. Additionally at the brightest flash strength, there were significant increases in the PhNR amplitude in people with schizophrenia that were maximal in controls. Our results show measurable dysfunction of retinal ganglion cells (RGCs) in schizophrenia using the PhNR-fERG, with a good deal of variability in the retinal responses of people with schizophrenia. The PhNR-fERG holds promise as a method to identify individuals more at risk for developing schizophrenia, and may help understand heterogeneity in etiology and response to treatment.

Cellular Signaling in Müller Glia: Progenitor Cells for Regenerative and Neuroprotective Responses in Pharmacological Models of Retinal Degeneration

Retinal degenerative diseases are a leading cause of visual impairment or blindness. There are many therapies for delaying the progression of vision loss but no curative strategies currently. Stimulating intrinsic neuronal regeneration is a potential approach to therapy in retinal degenerative diseases. In contrast to stem cells, as embryonic/pluripotent stem cell-derived retinal progenitor cell or mesenchymal stem cells, Müller glia provided an endogenous cellular source for regenerative therapy in the retina. Müller glia are a major component of the retina and considerable evidence suggested these cells can be induced to produce the lost neurons in several species. Understanding the specific characteristic of Müller glia to generate lost neurons will inspire an attractive and alternative therapeutic strategy for treating visual impairment with regenerative research. This review briefly provides the different signal transduction mechanisms which are underlying Müller cell-mediated neuroprotection and neuron regeneration and discusses recent advances about regeneration from Müller glia-derived progenitors.

The 1980s: D-AP5, LTP and a Decade of NMDA Receptor Discoveries

In the 1960s and 70s, biochemical and pharmacological evidence was pointing toward glutamate as a synaptic transmitter at a number of distinct receptor classes, known as NMDA and non-NMDA receptors. The field, however, lacked a potent and highly selective antagonist to block these putative postsynaptic receptors. So, the discoveries in the early 1980s of d-AP5 as a selective NMDA receptor antagonist and of its ability to block synaptic events and plasticity were a major breakthrough leading to an explosion of knowledge about this receptor subtype. During the next 10 years, the role of NMDA receptors was established in synaptic transmission, long-term potentiation, learning and memory, epilepsy, pain, among others. Hints at pharmacological heterogeneity among NMDA receptors were followed by the cloning of separate subunits. The purpose of this review is to recognize the important contributions made in the 1980s by Graham L. Collingridge and other key scientists to the advances in our understanding of the functions of NMDA receptors throughout the central nervous system.

NMDA Receptors Contribute to Retrograde Synaptic Transmission from Ganglion Cell Photoreceptors to Dopaminergic Amacrine Cells

Recently, a line of evidence has demonstrated that the vertebrate retina possesses a novel retrograde signaling pathway. In this pathway, phototransduction is initiated by the photopigment melanopsin, which is expressed in a small population of retinal ganglion cells. These ganglion cell photoreceptors then signal to dopaminergic amacrine cells (DACs) through glutamatergic synapses, influencing visual light adaptation. We have previously demonstrated that in Mg²⁺-containing solution, α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptors mediate this glutamatergic transmission. Here, we demonstrate that removing extracellular Mg²⁺ enhances melanopsin-based DAC light responses at membrane potentials more negative than −40 mV. Melanopsin-based responses in Mg²⁺-free solution were profoundly suppressed by the selective N-methyl-D-aspartate (NMDA) receptor antagonist D-AP5. In addition, application of NMDA to the retina produced excitatory inward currents in DACs. These data strongly suggest that DACs express functional NMDA receptors. We further found that in the presence of Mg²⁺, D-AP5 reduced the peak amplitude of melanopsin-based DAC responses by ~70% when the cells were held at their resting membrane potential (−50 mV), indicating that NMDA receptors are likely to contribute to retrograde signal transmission to DACs under physiological conditions. Moreover, our data show that melanopsin-based NMDA-receptor-mediated responses in DACs are suppressed by antagonists specific to either the NR2A or NR2B receptor subtype. Immunohistochemical results show that NR2A and NR2B subunits are expressed on DAC somata and processes. These results suggest that DACs express functional NMDA receptors containing both NR2A and NR2B subunits. Collectively, our data reveal that, along with AMPA receptors, NR2A- and NR2B-containing NMDA receptors mediate retrograde signal transmission from ganglion cell photoreceptors to DACs.

Effects of picrotoxin on light adapted frog electroretinogram are not due entirely to its action in proximal retina

In order to evaluate the site of action of picrotoxin (antagonist of ionotropic GABA receptors) on the electroretinographic (ERG) b- and d-waves, in this study we compared its effects on the intensity-response function of the ERG waves in intact light adapted frog eyecup preparations with its effects in eyecups, where the activity of proximal neurons was blocked by 1 mMN-methyl-d-aspartate (MNDA). Picrotoxin markedly enhanced the b- and d-wave amplitude and slowed the time course of the responses at all stimulus intensities in the intact eyecups. Perfusion with NMDA alone caused significant enhancement of the b-wave amplitude and diminution of the d-wave amplitude without altering their time course in the entire intensity range. When picrotoxin was applied in combination with NMDA, an enhancement of the b-wave amplitude and slowing of its time course were observed at all stimulus intensities. The increase of the b-wave amplitude was significantly higher than that seen in NMDA group. Combined application of picrotoxin and NMDA caused an enhancement of the d-wave amplitude at the lower stimulus intensities and its diminution at the higher ones, while the d-wave time course was delayed over the entire intensity range. The results obtained indicate that a part of picrotoxin effects on the amplitude and time course of the photopic ERG b- and d-waves are due to its action in the distal frog retina.

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.

Glycine transporter 1 modulates GABA release from amacrine cells by controlling occupancy of coagonist binding site of NMDA receptors

The occupancy of coagonist binding sites of NMDA receptors (NMDARs) by glycine or d-serine has been thought to mediate NMDAR-dependent excitatory signaling, as simultaneous binding of glutamate and a coagonist is obligatory for NMDAR activation. Amacrine cells (ACs) mediating GABAergic feedback inhibition of mixed bipolar cells (Mbs) in the goldfish retina have been shown to express NMDARs. Here we studied whether NMDAR-mediated GABAergic inhibitory currents (IGABA) recorded from the axon terminals of Mbs are influenced by experimental manipulations altering retinal glycine and d-serine levels. Feedback IGABA in Mb axon terminals was triggered by focal NMDA application or by synaptically released glutamate from depolarized Mb terminals. In both cases, blocking the coagonist binding sites of NMDARs eliminated the NMDAR-dependent IGABA, demonstrating that coagonist binding is critical in mediating NMDAR activity-triggered GABA release. Glycine transporter 1 (GLYT1) inhibition increased IGABA, indicating that coagonist binding sites of NMDARs on ACs providing GABAergic feedback inhibition to Mbs were not saturated. Focal glycine application, in the presence of the ionotropic glycine receptor blocker strychnine, triggered a GLYT1-dependent current in ACs, suggesting that GLYT1 expressed by putative glycinergic ACs controls the saturation level of NMDARs' coagonist sites. External d-serine also increased NMDAR activation-triggered IGABA in Mbs, further substantiating that the coagonist sites were unsaturated. Together, our findings demonstrate that coagonist modulation of glutamatergic input to GABAergic ACs via NMDARs is strongly reflected in the AC neuronal output (i.e., transmitter release) and thus is critical in GABAergic signal transfer function in the inner retina.

Functional expression of ionotropic glutamate receptors in the rabbit retinal ganglion cells

It has been known that retinal ganglion cells (RGCs) with distinct morphologies have different physiological properties. It was hypothesized that different functions of RGCs may in part result from various expressions of N-methyl-d-aspartate (NMDA), α-amino-3-hydroxyl-5-methyl-isoxazole-4-propinoic acid (AMPA), and kainic acid (KA) receptors on their dendrites. In the present study, we aimed to characterize the functional expression of AMPA and NMDA receptors of morphologically identified RGCs in the wholemount rabbit retina. The agmatine (AGB) activation assay was used to reveal functional expression of ionotropic glutamate receptors after the RGCs were targeted by injecting Neurobiotin. To examine the excitability of these glutamate receptors in an agonist specific manner, the lower concentrations of AMPA (2 μM) and NMDA (100 μM) were chosen to examine G7 (ON-OFF direction selective ganglion cells) and G11 (alpha ganglion cells) types of RGCs. We found that less than 40% of G7 type RGCs had salient AGB activation when incubated with 2 μM AMPA or 100 μM NMDA. The G11 type RGCs also showed similar activation frequencies, except that all of the OFF subtype examined had no AGB permeation under the same AMPA concentration. These results suggest that RGCs with large somata (G7 and G11 types) may express various heterogeneous functional ionotropic glutamate receptors, thus in part rendering their functional diversity.

Light-evoked synaptic activity of retinal ganglion and amacrine cells is regulated in developing mouse retina

Recent studies have shown a continued maturation of visual responsiveness and synaptic activity of retina after eye opening, including the size of receptive fields of retinal ganglion cells (RGCs), light-evoked synaptic output of RGCs, bipolar cell spontaneous synaptic inputs to RGCs, and the synaptic connections between RGCs and ON and OFF bipolar cells. Light deprivation retarded some of these age-dependent changes. However, many other functional and morphological features of RGCs are not sensitive to visual experience. To determine whether light-evoked synaptic responses of RGCs undergo developmental change, we directly examined the light-evoked synaptic inputs from ON and OFF synaptic pathways to RGCs in developing retinas, and found that both light-evoked excitatory and inhibitory synaptic currents decreased, but not increased, with age. We also examined the light-evoked synaptic inputs from ON and OFF synaptic pathways to amacrine cells in developing retinas and found that the light-evoked synaptic input of amacrine cells is also downregulated in developing mouse retina. Different from the developmental changes of RGC spontaneous synaptic activity, dark rearing has little effect on the developmental changes of light-evoked synaptic activity of both RGCs and amacrine cells. Therefore, we concluded that the synaptic mechanisms mediating spontaneous and light-evoked synaptic activity of RGCs and amacrine cells are likely to be different.

Neurotransmitter-specific identification and characterization of neurons in the all-cone retina of Anolis carolinensis II: Glutamate and aspartate

Immunocytochemical and autoradiographic methods were used to identify neurons in the pure cone retina of the lizard (Anolis carolinensis) that are likely to employ glutamate (GLU) or aspartate (ASP) as a neurotransmitter.

GLU immunocytochemistry demonstrated high levels of endogenous GLU in all cone types and numerous bipolar cells. Moderate GLU levels were found in horizontal and ganglion cells. Müller cells and most amacrine cells had very low GLU levels. GLU immunoreactivity (GLU-IR) in the cones was present from the inner segment to the synaptic pedicle. A large spherical cell type with moderate GLU-IR was identified in the proximal inner plexiform layer (IPL). These cells also contain ASP and have been tentatively identified as amacrine cells. Uptake of [3H]-L-GLU labeled all retinal layers. All cone types and Müller cells sequestered [3H]-D-ASP, a substrate specific for the GLU transporter.

Anti-ASP labeling was observed in cones, horizontal cells, amacrine cells, and cells in the ganglion cell layer. ASP immunoreactivity (ASP-IR) in the cones was confined to the inner segment. One ASP-containing pyriform amacrine cell subtype ramifying in IPL sublamina b was identified.

Analysis of GLU-IR, ASP-IR, and GABA-IR on serial sections indicated that there were two distinct populations of horizontal cells in the Anolis retina: one containing GABA-IR, GLU-IR, and ASP-IR; and another type containing only GLU-IR and ASP-IR. Light GLU-IR was frequently found in GABA-containing amacrine cells but ASP-IR was not.

The distinct distributions of GLU and ASP may indicate distinctly different roles for these amino acids. GLU, not ASP, is probably the major neurotransmitter in the cone-biploar-ganglion cell pathway of the Anolis retina. Both GLU and ASP are present in horizontal cells and specific subpopulations of amacrine cells, but it is unclear if GLU or ASP have a neurotransmitter role in these cells.

GABA inhibits ACh release from the rabbit retina: A direct effect or feedback to bipolar cells?

The cholinergic amacrine cells of the rabbit retina may be labeled with [3H]-Ch and the activity of the cholinergic population monitored by following the release of [3H]-ACh. We have tested the effect of muscimol, a potent GABAA agonist, on (1) the light-evoked release of ACh, presumably mediated via bipolar cells, which are known to have a direct input to the cholinergic amacrine cells and (2) ACh release produced by exogenous glutamate analogs that probably have a direct effect on cholinergic amacrine cells. Muscimol blocked the light-evoked release of ACh with an IC50 of 1.0 μM. In contrast, ACh release produced by nonsaturating doses of kainate or NMDA was not reduced even by 100 μM muscimol. Thus, we have been unable to demonstrate a direct effect of GABA on the cholinergic amacrine cells.

GABA antagonists, such as picrotoxin, caused a large increase in the base release and potentiated the light-evoked release of ACh. Both these effects were abolished by DNQX, a kainate antagonist that blocks the input to cholinergic amacine cells from bipolar cells. DNQX blocked the effects of picrotoxin even when controls showed that the mechanism of ACh release was still functional. Together, these results imply that the dominant site for the GABA-mediated inhibition of ACh release is on the bipolar cell input to the cholinergic amacrine cells. This is consistent with previous anatomical and physiological evidence that bipolar cells receive negative feedback from GABA amacrine cells.

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.

Differential synaptic organization of GABAergic bipolar cells and non-GABAergic (glutamatergic) bipolar cells in the tiger salamander retina

The synaptic organizations of gamma-aminobutyric acid-immunoreactive (GABA-IR, GABAergic) and non-GABA-IR (non-IR, glutamatergic) bipolar cells in salamander retina were compared by postembedding immunoelectron microscopy. A total of 238 presynaptic bipolar cell synapses were studied; 61 were GABA-IR and 177 were non-IR. Both groups were similar in that (1). they made asymmetrical ribbon synapses as well as asymmetrical non-ribbon synapses; (2). they made ribbon synapses at dyads, triads, and monads; and (3). the vast majority of ribbon synapses ( approximately 90%) were with dyads. The differences were that synapses of GABA-IR bipolar cells had a higher proportion of (1). direct contact with ganglion cells, (2). non-ribbon synapses, (3). output to GABA-IR amacrine cells, and (4). output in sublamina a. Overall, the output of GABA-IR ribbons was equally split between amacrine and ganglion cell processes, whereas for non-IR ribbons, it was approximately 2:1 in favor of amacrine cells. The ribbon:non-ribbon synapse ratio was approximately 1.2:1 (33:28) for GABA-IR but approximately 2:1 (118:59) for non-IR terminals. Thus, GABA-IR bipolar cells made more direct contacts with ganglion cells and used a higher proportion of non-ribbon synapses. GABA-IR dyads were more likely to contact GABA-IR amacrine profiles (52% vs. 38%). Finally, GABA-IR ribbon synapses were more common in sublamina a than sublamina b (2:1), whereas non-IR synapses were equally present in sublaminas a and b. This differential targeting of ganglion cells and amacrine cells in the OFF vs. ON layers indicates a difference in the role of bipolar cells in the generation of receptive field properties, depending on whether or not they use GABA as well as glutamate for their transmitter.

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.

Intracellular calcium reduces light-induced excitatory post-synaptic responses in salamander retinal ganglion cells

The whole-cell patch clamp technique was used to study the effect of intracellular Ca2+ on light-evoked EPSCs in on-off ganglion cells in salamander retinal slices. Both AMPA and NMDA receptors contributed to the light-evoked responses. In the presence of strychnine and picrotoxin, ganglion cells responded to light onset and offset with transient inward currents at -70 mV. These currents were reduced by 35 +/- 3 % when the light stimulus was preceded by a depolarizing step from -70 to 0 mV. The inhibitory effect of depolarization on light-evoked EPSCs was strongly reduced in the presence of 10 mM BAPTA. The degree of EPSC inhibition by the prepulse holding potential followed the current-voltage relationship of the Ca2+ current found in the ganglion cell. In the presence of the NMDA receptor antagonist AP-7, glutamate-dependent current was nearly abolished when high Ca2+ was substituted for high Na+ solution. The release of Ca2+ from internal stores by caffeine or inositol trisphosphate reduced the EPSCs by 36 +/- 5 and 38 +/- 11 %, respectively, and abolished the inhibitory effect of depolarization. The inhibitory effect of depolarization on EPSCs was reduced 5-fold in the presence of AP-7, but was not reduced by the AMPA receptor antagonist CNQX. Neither inhibition of Ca2+-calmodulin-dependent enzymes, nor inhibition of protein kinase A or C had any significant effect on the depolarization-induced inhibition of EPSCs. Our data suggest that elevation of [Ca2+]i, through voltage-gated channels or by release from intracellular stores, reduced primarily the NMDA component of the light-evoked EPSCs.

Anatomical and electrophysiological evidence for GABAergic bipolar cells in tiger salamander retina

Our previous work showed that about 12% of bipolar cells in salamander retina synthesize and take up gamma-aminobutyric acid (GABA), are GABA transporter (GAT)-immunoreactive, and respond with a GAT current to extracellularly applied GABA, suggesting that these bipolar cells use GABA, in addition to glutamate, as a neurotransmitter. Further support for this idea was obtained in this study by use of immunogold electron microscopy and whole-cell patch clamp electrophysiology. Ultrastructural analysis showed that amacrine cell and ganglion cell processes were postsynaptic to GABA-immunoreactive synapses made by bipolar cell axon terminals. Whole-cell recordings were obtained from amacrine and ganglion cells in response to activation of bipolar cells by puffing KCl at their dendrites in the outer plexiform layer. Inhibitory postsynaptic currents were observed in several third order neurons, even after blocking the excitatory postsynaptic responses, generated in the inner plexiform layer, with a combined application of NMDA and non-NMDA receptor antagonists, AP-5 and CNQX. These ultrastructural and electrophysiological data support our previous neurochemical results, and suggest that the retinal through-information pathway in salamander includes both inhibitory GABAergic as well as excitatory glutamatergic synaptic mechanisms.

Retinal ganglion cell dysfunction induced by hypoxia and glutamate: potential neuroprotective effects of beta-blockers

The objective of this study was to examine the effects of hypoxia, glutamate, and beta-blockers on the electrical activities of retinal ganglion cells. Single-unit extracellular and whole-cell voltage clamp recording techniques were used to record electrical activities from ganglion cells in the tiger salamander retina. This was performed under physiologic conditions, hypoxia, or elevated exogenous or endogenous glutamate levels. Light-evoked spike activities, glutamate-induced currents, and voltage-gated sodium and calcium currents were measured in the presence of the beta-1 selective antagonist betaxolol or the nonselective antagonist timolol. Hypoxia resulted in suppressing or blocking the OFF responses in the majority of ON-OFF ganglion cells tested, whereas the ON responses were only slightly affected. The presence of increased glutamate had similar findings and demonstrated an increase in the spontaneous firing rate of retinal ganglion cells. Betaxolol (2-50 microM) reduced the rate of spontaneous firing of retinal ganglion cells induced by glutamate. At 2 to 50 microM, betaxolol reversibly reduced the voltage-gated sodium currents and calcium currents in retinal ganglion cells. Timolol (up to 100 microM) did not demonstrate any detectable action on these currents. The physiologic responses of retinal ganglion cells to hypoxia or elevated glutamate levels in this animal model appear to be very similar. Although short-term exposure to hypoxia and glutamate used in this study exerts reversible actions on ganglion cells and does not induce permanent cell damage, such initial physiologic actions are likely to be precursors of permanent cell damage. Thus, hypoxia and elevated glutamate levels in the retina may represent a final pathway in diseases affecting retinal ganglion cells, such as glaucoma. Similar damage could result from different factors, such as decreased perfusion-induced ischemia or anomalous neuronal processing of glutamate. Betaxolol exerts its primary neuronal actions on retinal ganglion cells. It reversibly blocked voltage-gated calcium current and reduced the spontaneous firing rate by suppressing glutamate-gated currents and sodium currents in ganglion cells. These actions may protect ganglion cells from damage caused by ischemia or elevated glutamate levels.

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].

AMPA-preferring receptors mediate excitatory synaptic inputs to retinal ganglion cells

Pharmacological studies were performed to determine whether alpha-amino-3-hydroxy-5-methyl-4-isoazoleprionic acid (AMPA)- and/or kainate (KA)-preferring receptors mediate excitatory synaptic inputs to tiger salamander retinal ganglion cells. Excitatory postsynaptic currents (EPSCs), evoked either by light or by stimulating bipolar cells with puffs of K+, were measured using whole cell recording techniques in the tiger salamander retinal slice. The AMPA/KA component of the EPSCs was isolated by including antagonists of glycine-, gamma-aminobutyric acid (GABA)- and NMDA-receptors in the bath. The AMPA-preferring receptor antagonists, 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI-52466) and 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4 - dihydro-5H-2,3-benzodiazepine (GYKI-53665), reduced light-evoked EPSCs and K+ puff-evoked EPSCs amplitudes in a concentration-dependent manner. The IC50 values for GYKI-52466 were 3.6 and 4.2 microM for the light- and puff-evoked responses, respectively. The more potent GYKI-53665 had IC50 values of 0.7 microM for both the light- and puff evoked responses. KA activates both KA- and AMPA-preferring receptors. KA-evoked currents were completely blocked by 10-40 microM GYKI-53665, indicating that little or no excitatory synaptic current was mediated by KA-preferring receptors. Concanavalin A, a compound that preferentially potentiates responses mediated by KA-preferring receptors, did not enhance either EPSCs or glutamate-evoked responses. By contrast, cyclothiazide, which selectively enhances AMPA-preferring receptor mediated responses, was found to enhance both EPSCs and glutamate-evoked currents. Our results indicate that the non-NMDA component of ganglion cell EPSCs is mediated by AMPA-preferring receptors and not significantly by KA-preferring receptors.

Immunocytochemical localization of polyamines in the tiger salamander retina

The polyamines spermine and spermidine are present in neural tissue, but their functions there are not well understood. Recent work suggests that the NMDA subtype of glutamate receptors, other glutamate receptor subtypes, and certain K(+)-channels, are neural targets for polyamines. To better understand the neuron-specific roles of polyamines, we have developed antibodies that interact with spermine and spermidine in aldehyde-fixed tissue and used these antibodies in immunocytochemical studies to determine the cellular localization of these polyamines in the tiger salamander retina. The affinity-purified, polyclonal antibodies were highly specific for spermine and spermidine, exhibiting < 1% cross reactivity with putrescine, and virtually no cross-reactivity with GABA, arginine, lysine, or glutaraldehyde. Polyamine labeling was most abundant in cells in the inner half of the inner nuclear layer and in the ganglion cell layer. Some cells in the outer half of the inner nuclear layer are labeled, and there was some labeling in both synaptic layers. Double-labeling experiments indicated (1) all GABAergic amacrine cells were polyamine-positive; and (2) all ganglion cells (identified by back-filling after microinjections of rhodamine in the optic nerve) were polyamine-positive. These results are consistent with a role for polyamines as modulators of NMDA receptor function and channel function in the inner retina.

Glutamate-, GABA-, and GAD-immunoreactivities co-localize in bipolar cells of tiger salamander retina

The presence of inhibitory bipolar cells in salamander retina was investigated by a comparative analysis of the distribution of glutamate- and GABA-immunoreactivities (GLU-IR; GABA-IR) using a postembedding immunocytochemical method. GLU-IR was found in virtually all photoreceptors, bipolar cells and ganglion cells, neuronal elements that transfer information vertically through the retina. GLU-IR also was found in numerous amacrine cells in the mid and proximal inner nuclear layer as well as in the cytoplasm of horizontal cells, while the nucleus of horizontal cells was either lightly labeled or not labeled at all. GLU-IR was found in the outer plexiform layer and intensely in the inner plexiform layer, in which there was no apparent sublamination. Forty-seven percent of Type IB bipolar cells in the distal inner nuclear layer and 13% of the displaced bipolar cells were GABA-IR. All bipolar cells were also GLU-IR, indicating that GABA-IR bipolar cells were a subset of GLU-IR bipolar cells rather than a separate population. About 12% of the Type IB bipolar cells were moderately GABA-IR and likely comprised a GABAergic subtype. GLU-IR levels in the presumed GABAergic bipolar cells were higher than in other purely GLU-IR bipolar cells suggesting that these GABA-IR bipolar cells are glutamatergic as well. All of the displaced bipolar cells were only lightly GABA-IR, indicating that displaced bipolar cells comprise a more homogeneous class of glutamatergic cell than orthotopic bipolar cells. GAD-IR co-localized with GABA-IR in orthotopic but not displaced bipolar cells, further supporting the idea that some orthotopic bipolar cells are GABAergic. A small proportion of bipolar cells in salamander retina contain relatively high levels of both GABA and glutamate. Co-release of these substances by bipolar cells could contribute to the "push-pull" modulation of ganglion cell responses.

Colocalization of glutamate and glycine in bipolar cell terminals of the human retina

Human retinae from surgical specimens rapidly fixed in a glutaraldehyde/formaldehyde mixture were subjected to postembedding, immunogold immunocytochemistry of glutamate and glycine, and subsequently analysed in an electron microscope. The two amino acids were visualised in the same tissue sections by the use of two different gold particle sizes. All bipolar cell perikarya and terminals showed significant glutamate labelling with mean gold particle densities 3-4 times higher than those of the retinal, non-neural pigment epithelial and Müller cells. Bipolar cell terminals displayed significantly higher glutamate labelling density than the bipolar cell bodies, as would be expected of glutamatergic neurons. A subpopulation of the glutamate-immunolabelled bipolar cell bodies (18%) and terminals (32%) also exhibited strong glycine labelling (7-8 times that of pigment epithelial and Müller cells). These glutamate-glycine positive terminals established contacts with amacrine cell processes and ganglion cell dendrites and were localised almost exclusively at between 44% and 88% depth of the inner plexiform layer, indicating that they belong to the "ON" cone bipolar system. This subpopulation of terminals was endowed with significantly higher glycine labelling density than the glycine positive bipolar cell bodies. These results show that human bipolar cell terminals colocalise glutamate and glycine and provide the first direct demonstration of an enrichment of these two amino acids in the same presynaptic element.

The contribution of NMDA and non-NMDA receptors to the light-evoked input-output characteristics of retinal ganglion cells

To examine how light-evoked excitatory synaptic inputs to retinal ganglion cells are transformed into output patterns of activity, action potentials were recorded with cell-attached patch-clamp techniques, and then EPSCs and EPSPs were recorded from the same cell in the whole-cell configuration. AP7, an NMDA antagonist, reduced the light-evoked peak spike frequency 36% +/- 21% (mean +/- SD) and reduced the EPSC amplitude, indicating a major role for NMDA receptors in the light response. CNQX, a non-NMDA receptor antagonist, reduced the light-evoked peak spike frequency 28% +/- 22%. CNQX also caused a voltage- and magnesium-dependent delay in spike onset. AP7 and CNQX, however, did not differ significantly in their effect on the EPSC time course, indicating that postsynaptic cellular properties are responsible for the delay observed in the presence of CNQX. These results show that the NMDA receptor contribution to the excitatory response is increased as the cell is depolarized from rest by non-NMDA input.

Two types of glutamate receptors differentially excite amacrine cells in the tiger salamander retina

1. Excitatory inputs to amacrine cells in the salamander retinal slice preparation were examined using whole-cell patch pipette voltage-clamp techniques. In strychnine (500 nM) and bicuculline (100 microM), two types of amacrine cell were easily distinguished by their light-evoked excitatory responses: transient and sustained. 2. In transient amacrine cells the current-voltage (I-V) relation for the peak light-evoked current was non-linear with a negative slope region between -50 and -70 mV. Responses reversed near +10 mV and were prolonged at more positive holding potentials. 3. In DL-2-amino-phosphonoheptanoate (AP7, 30 microM), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, both the negatively sloped region of the light I-V relation and the prolongation of the response at positive potentials were eliminated. In 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 2 microM), a selective non-NMDA receptor antagonist, light-evoked currents at the most hyperpolarized holding potentials were eliminated. At potentials positive to -85 mV the light-evoked currents lacked a fast onset. The light I-V relation in CNQX had a negative slope region between -35 and -80 mV. 4. With synaptic transmission blocked, kainate evoked responses in transient cells with a resultant I-V relation that was nearly linear, whereas glutamate and NMDA elicited responses with non-linear I-V relations. 5. Light-evoked currents in sustained amacrine cells had a nearly linear I-V relation and reversed near +10 mV. AP7 at a concentration of 30 microM did not affect the light-evoked currents in sustained cells, but 2 microM-CNQX eliminated all light-evoked currents in these cells. 6. With synaptic transmission blocked, sustained amacrine cells responded only to glutamate and kainate, not NMDA. The resultant I-V relations were linear. 7. We conclude that the light-evoked responses of transient amacrine cells are mediated by concomitant activation of both non-NMDA and NMDA receptors whereas the responses of sustained amacrine cells are mediated only by non-NMDA receptors. Furthermore, these data provide supportive evidence that the primary light-evoked excitatory neurotransmitter activating amacrine cells is glutamate.

N-methyl-D-aspartate-induced neurotoxicity in the adult rat retina

The present study provides evidence that the adult mammalian retina is highly sensitive to the excitotoxic action of NMDA. In particular, we have investigated the effects of a single intravitreal injection of different doses of N-methyl-D-aspartate (NMDA) (2-200 nmoles) on the adult rat retina. Morphological evaluation of transverse sections of retinae demonstrated a dose-dependent loss of cells in the ganglion cell layer (GCL) and a reduction in the thickness of the inner plexiform layer. No obvious alterations were noted in the more distal retinal layers. Quantitative analyses of Nissl-stained whole-mounted retinae revealed that administration of 20 nmoles of NMDA resulted in a 70% loss of cells with a soma diameter greater than 8 microns (presumed retinal ganglion cells); a 20% loss of cells with a soma diameter smaller than 8 microns (presumed displaced amacrine cells) was also observed. In addition, NMDA produced a dose-dependent decrease of retinal choline acetyltransferase (ChAT) activity, suggesting that NMDA affects cholinergic amacrine cells as well. MK-801, a non-competitive NMDA antagonist, completely prevented the NMDA-induced loss of cells in the GCL and blocked, in a dose-dependent manner, the NMDA-induced decrease of ChAT activity. The excitotoxic action of NMDA observed in these experiments is thus likely mediated through the NMDA receptor subtype. This "in vivo" model may be utilized to identify potential drugs that antagonize or limit the deleterious effects consequent to NMDA receptor overstimulation in the central nervous system.

Patterns of glutamate-like immunoreactive bipolar cell axons in the retina of the marine teleost, the dragonet

Oblique 1 microns-sections through the dorsal inner plexiform layer of the light-adapted dragonet retina were processed for postembedding, silver-enhanced immunogold labeling after incubation with a glutamate-specific antiserum. Light microscopy showed strongly immunolabeled boutons grouped into distinct square patterns. These patterns were compared with the successive grids of bipolar axonal boutons revealed by electron microscope analysis of serial, oblique sections through the entire dorsal inner plexiform layer. With one exception, all types of patterned bipolar synaptic boutons could be clearly identified in the immunoreactive staining pattern. The elevated levels of endogenous glutamate in most bipolar synaptic boutons suggest that the large majority of bipolar cell types use glutamate as their neurotransmitter. However, some bipolar synaptic boutons displaying low levels of glutamate indicate that a small number of bipolar cells may contain another neuroactive substance.

Concomitant activation of two types of glutamate receptor mediates excitation of salamander retinal ganglion cells

1. Cells in the ganglion cell layer of salamander retinal slices were voltage clamped using patch pipettes. Light elicited transient excitatory postsynaptic currents (EPSCs) in on-off ganglion cells and sustained EPSCs in on ganglion cells. Light-evoked inhibitory postsynaptic currents in these cells could be blocked by 100 microM-bicuculline methobromide and 500 nM-strychnine. 2. In the presence of external Cd2+, at a concentration that blocked light-evoked synaptic inputs, N-methyl-D-aspartate (NMDA) and the non-NMDA-receptor agonists, quisqualate and kainate, gated conductances in both on-off and on ganglion cells. The current-voltage (I-V) curve for the conductance elicited by NMDA had a negative slope between -40 and -70 mV and a reversal potential near 0 mV. The I-V curves for the non-NMDA-receptor-mediated conductances were nearly linear and also had reversal potentials near 0 mV. 3. I-V curves were measured at an early time point near the peak of transient EPSCs and at a later time point during the decay phase of the responses. The late I-V curve had a negative slope below -40 mV. The early I-V curve had a positive slope over the entire voltage range but the slope was greater at positive than at negative potentials. The evoked current reversed near 0 mV at both time points. 4. The region of negative slope of the late I-V curve was eliminated when Mg2+ was removed from the external saline. A slowly decaying component of transient EPSCs was eliminated in 20 microM-DL-2-amino-7-phosphonoheptanoate (AP7), an NMDA-receptor antagonist. 5. Application of 1 microM-6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA-receptor antagonist at this concentration, blocked a fast component of transient EPSCs. 6. Our results demonstrate that the synaptic inputs to on-off ganglion cells have two components: a slower NMDA-receptor-mediated component having a time-to-peak of 110 +/- 45 ms and an e-fold decay time of 209 +/- 35 ms at -31 mV (mean +/- S.D., n = 5), and a faster non-NMDA-receptor-mediated component having a time-to-peak of 28 +/- 10 ms and an e-fold decay time of 43 +/- 20 ms at -31 mV (n = 8). 7. A similar analysis of sustained EPSCs of on ganglion cells showed that these currents resulted from sustained activation of both NMDA and non-NMDA receptors.

Identification of kainic and quisqualic acid receptors on inner retinal cells of the salamander Ambystoma mexicanum

The presence of kainic (KA) and quisqualic acid (QA) receptors on inner retinal neurones of the axolotl Ambystoma mexicanum has been studied using intracellular recording techniques. In the presence of CoCl2, which blocks neurotransmitter release, KA and QA depolarized the membrane. The minimum concentration of KA that induced a response was 1 microM and a maximum response was obtained with 10 microM (EC50: 3 microM). The operating range of QA was between 0.5 and 5 microM with an EC50 of 1 microM. These data show that inner retinal cells of the axolotl are sensitive to KA and QA. Cis-2,3-piperidine dicarboxylic acid (PDA, 3 mM) completely blocked responses to 5 microM KA, but not those induced by 2 microM QA. This suggests that the KA- and QA-sensitive receptors on inner retinal cells of the salamander are pharmacologically different and that PDA can be a valuable tool in distinguishing KA- and QA-sensitive receptors on these neurones.

Synaptic and voltage-gated currents in interplexiform cells of the tiger salamander retina

We have correlated the membrane properties and synaptic inputs of interplexiform cells (IPCs) with their morphology using whole-cell patch-clamp and Lucifer yellow staining in retinal slices. Three morphological types were identified: (a) a bistratified IPC with descending processes ramifying in both sublaminas a and b of the inner plexiform layer (IPL), and an ascending process that branched in the outer plexiform layer (OPL) and originated from the soma, (b) another bistratified IPC with descending processes ramifying in both sublaminas a and b, and an ascending process that branched in the OPL and originated directly from IPC processes in the IPL, and (c) a monostratified IPC with a descending process ramifying over large lateral extents within the most distal stratum of the IPL, and sending an ascending process to the OPL with little branching. Similar voltage-gated currents were measured in all three types including: (a) a transient inward sodium current, (b) an outward potassium current, and (c) an L-type calcium current. All cells generated multiple spikes with frequency increasing monotonically with the magnitude of injected current. The IPCs that send their descending processes into both sublaminas of the IPL (bistratified) receive excitatory synaptic inputs at both light ON and OFF that decay with a time constant of approximately 1.3 s. Slowly decaying excitation at both ON and OFF suggests that bistratified IPCs may spike continuously in the presence of a dynamic visual environment.

Centre components of cone-driven retinal ganglion cells: differential sensitivity to 2-amino-4-phosphonobutyric acid

1. Changes in different components of the cone-driven centre responses of cat retinal ganglion cells were studied before and during local application of 2-amino-4-phosphonobutyric acid (APB). Responses were elicited with bar stimuli whose luminance was above ('brighter') or below ('dimmer') the photopic background luminance. The bars were centrally located, and were similar in width to the receptive field centre. 2. APB acted differently on the on- and off-centre cells. For on-centre X and Y cells, all components of the responses to bright and dim bars were diminished by APB. For the off-centre X and Y cells. APB reduced all components except the transient increase in firing rate when the bright bar was turned off or when the dim bar was turned on. 3. These results suggest that the centre response mechanism of off-centre X and Y cells comprises APB-sensitive and APB-resistant components. The APB-sensitive component is more sustained and responds to both brightening and dimming stimuli. The APB-resistant component is more transient and responds primarily to dimming stimuli. For on-centre X and Y cells only APB-sensitive components could be demonstrated. 4. Experiments with stationary sinusoidal gratings modulated at 0.5-10 Hz showed that responses of off-centre cells were more affected by APB at low than at high temporal frequencies, confirming that the APB-sensitive pathway is responsible for more of the low temporal frequency responses. As expected from the responses to bar stimuli, APB had a uniform effect at all temporal frequencies in on-centre cells. 5. For off-centre cells, the APB-sensitive component is probably derived from input from depolarizing bipolar cells, and the APB-resistant component is derived from hyperpolarizing bipolar input, although one or both pathways could also involve amacrine cells. The combination of these pathways increases the range of temporal frequencies to which the cell can respond and also increases the range of response amplitudes. 6. The lack of differential effects on on-centre cells may have several explanations. The most likely explanations are that only depolarizing bipolars contribute significantly to the centre responses of these cells under the conditions of these experiments, or that there is an APB-sensitive synapse somewhere in the retina besides the one from cones to depolarizing bipolars.

Glutamate receptors of ganglion cells in the rabbit retina: evidence for glutamate as a bipolar cell transmitter

1. Intracellular and extracellular recordings were obtained from ganglion cells in the rabbit retina. The effects of glutamate analogues and antagonists were studied using a perfusion method for drug application. 2. Kainate (KA) excited all ganglion cells directly and caused a large increase in firing rate. N-Methyl-DL-aspartate (NMDLA) also excited ganglion cells but it was less potent and caused burst firing. 3. Quisqualate (QQ) and (RS)-2-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) excited many ganglion cells and were approximately as potent as KA. Less frequently, QQ and AMPA had inhibitory effects possibly due to polysynaptic action. 4. General glutamate antagonists such as cis-2,3-piperidine dicarboxylic acid (PDA) and kynurenic acid blocked the light input to all ganglion cells. PDA and kynurenic acid blocked the effects of KA and NMDLA, but not carbachol, indicating that they act as glutamate antagonists in the rabbit retina. Kynurenic acid did not block the excitatory action of QQ, even though light responses were abolished. 5. Amacrine cells were depolarized by KA or QQ and less potently by NMDLA. Their light-evoked responses were blocked by PDA. 6. We conclude that the light input to ganglion cells in the rabbit retina is predominantly mediated by KA receptors. This is consistent with the idea that 'on' and 'off' bipolar cells are excitatory and release glutamate.

Bipolar cells in the turtle retina are strongly immunoreactive for glutamate

Strong glutamate immunoreactivity was observed by both light and electron microscopy in bipolar cells of the turtle (Pseudemys scripta elegans) retina after postembedding immunohistochemistry. Virtually all bipolar cells showed strong labeling, on average 18 times that of the Müller (glial) cells. The data suggest that both on- and off-center bipolar cells are glutamatergic. Photoreceptors were also labeled, but with a labeling intensity about half that of the bipolar cells. Other types of retinal neurons showed less immunoreactivity, except for a small population of strongly labeled amacrine cells.

Localization of putative GABAergic neurons in the larval tiger salamander retina by immunocytochemical and autoradiographic methods

Putative GABAergic neurons in the larval tiger salamander retina were localized by a comparative analysis of glutamate decarboxylase immunoreactivity (GAD-IR), GABA-like immunoreactivity (GABA-IR), and high-affinity 3H-GABA uptake at the light microscopical level. Preliminary data showed that all GAD-IR neurons were double labeled for GABA-IR. However, because the weak somatic labeling with GAD-IR, we could not determine if the converse were true. Neurons commonly labeled with GABA-IR and 3H-GABA uptake include horizontal cells, type I (outer) and type II (inner) bipolar cells, type I (inner) and type II (outer) amacrine cells, and cell bodies in the ganglion cell layer (GCL). In addition, interplexiform cells were identified with GABA-IR. The presence of GABA-IR ganglion cells was indicated by GABA-IR fibers in the optic fiber layer and optic nerve as well as by a GABA-IR cell in the GCL that included a labeled axon. The percentage of labeled somas in the inner nuclear layer (INL) compared to all cells in each layer was similar for the two methods: 30% in INL 1 (outer layer of somas), 15% in INL 2 (middle layer), 43-52% in INL 3 (inner layer), and about 21-26% in the GCL. Labeled processes were found in three bands in the inner plexiform layer, with the densest band located in the most proximal part. Postembedding labeling of 1-micron Durcupan resin sections for GABA-IR showed the same general pattern as obtained with 10-microns cryostat sections, with additional staining, however, of type II (inner) bipolar cell Landolt's clubs. Extensive colocalization of labeling was indicated, and we conclude that GABA-IR can serve as a valid and reliable marker for GABA-containing neurons in this retina and suggest that GABA serves as a transmitter for horizontal cells, several types of amacrine cell, a type of interplexiform cell, and perhaps a small percentage of type I and type II bipolar cells and ganglion cells.

Ultrastructural localization of N-acetylaspartylglutamate in synaptic vesicles of retinal neurons

Localization of N-acetylaspartylglutamate (NAAG) in a variety of central and peripheral neurons, as well as its receptor-mediated activation of membrane conductance have led to speculation that this peptide has a role in chemical neurotransmission. We previously identified NAAG in retinal neurons of several species, including the grass frog, and now have determined its ultrastructural distribution within the plexiform layers of this amphibian retina. NAAG immunoreactivity was localized within vesicles in synaptic endings of presumptive amacrine and bipolar neurons in the inner plexiform layer. Additionally, the peptide was present in vesicles within ribbon synapses in the outer plexiform layer, a result suggestive of release from photoreceptor cells. These data support the hypothesis that NAAG is secreted at points of synaptic contact between neurons, including retinal amacrine, bipolar and photoreceptor cells.

The cellular basis for suppressive rod-cone interaction

The response to spatially focal flicker is enhanced by dim, spatially diffuse, rod-stimulating backgrounds. This effect is called suppressive rod-cone interaction (SRCI) as it reflects a tonic, suppressive influence of dark-adapted rods upon cone pathways which is removed by selective rod-light adaptation. SRCI is observed in amphibian retina with intracellular recordings from most cone-driven cells including the cones themselves, and is most obvious using stimuli flickering at frequencies too rapid for rods to follow. SRCI is blocked by glutamate analogs which selectively block the photic response of horizontal cells (HCs). In the presence of these agents, flicker responses from bipolar cells and cones are enhanced to levels normally seen only with selective rod-light adaptation. In the HCs themselves, SRCI is similarly blocked by lead chloride which blocks rod-, but not cone-related activity. In amphibian and cat HCs and in human observers, SRCI is limited by a space constant of very similar value (between 100 and 150 microns). We suggest that SRCI in all three species is mediated by HCs: in amphibians, SRCI must at least partially reflect rod-modulation of HC feedback onto cones.

Responses mediated by excitatory amino acid receptors in solitary retinal ganglion cells from rat

1. The pharmacological properties of excitatory amino acid responses on ganglion cells dissociated from the rat retina were examined with the use of the whole-cell voltage-clamp technique. 2. L-Glutamate at a concentration of 50 microM produced inward non-desensitizing currents at negative holding potentials in nearly every cell tested (83%, n = 18) In physiological solutions, L-glutamate responses reversed at approximately -9 mV, and higher concentrations of this agonist introduced a desensitizing component to the response. 3. At negative holding potentials, kainate (25-125 microM) produced inward currents in all of the cells tested (n = 37). These currents never desensitized, even at high agonist concentrations, and reversed near -6 mV. Currents induced by 50 microM-kainate were reversibly antagonized by kynurenate (100-300 microM) but not by 100 microM-2-amino-5-phosphonovalerate (APV). 4. Quisqualate generated smaller, non-desensitizing currents in only 50% of the cells tested (n = 38). Quisqualate responses reversed in polarity near -4 mV and were maximal at an agonist dose of 25 microM, with higher concentrations introducing a rapidly desensitizing component without a detectable increase in amplitude. Currents produced by quisqualate at a concentration of 50 microM were not antagonized by either 750 microM-kynurenate or 100 microM-APV. 5. N-Methyl-D-aspartate (NMDA) produced inward currents at negative holding potentials in 68% of the cells tested (n = 31), but only when magnesium was excluded from the extracellular medium. NMDA currents were non-desensitizing at agonist concentrations of up to 200 microM, with higher concentrations introducing a rapidly desensitizing component. NMDA (200 microM) responses were blocked by APV (100 microM) and kynurenate (300 microM) and reversed near -1 mV. 6. Responses generated by kainate (50-125 microM) were antagonized by quisqualate (30-250 microM). This antagonism occurred even in cells having no measurable response to quisqualate alone, suggesting the possibility that quisqualate may be acting both as an agonist, in the 50% of the cells that have the quisqualate-specific receptor, and as an antagonist, at the kainate-specific site on all cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Subtle actions of 2-amino-4-phosphonobutyrate (APB) on the Off pathway in the mudpuppy retina

The principal action of 2-amino-4-phosphonobutyrate (APB) applied to the retina is the elimination of the On-bipolar cell light response. In this study, we report that APB also has more subtle effects; it sometimes enhances Off responses of Off bipolar cells and On, Off, and On-Off inner retina neurons. Possible mechanisms for this enhancement are considered.

Push-pull effect of surround illumination on excitatory and inhibitory inputs to mudpuppy retinal ganglion cells

1. Changes in membrane potential and conductance were measured in on-centre and off-centre ganglion cells during the responses to illumination of different portions of the receptive field. 2. In on-centre ganglion cells the sustained depolarizing response to steady illumination of the receptive field centre was associated with a net increase in conductance. In the presence of centre illumination, stimulation of the surround with an annulus of light caused a hyperpolarization and a net decrease in conductance, and the reversal potential of the light-evoked response was shifted in a negative direction. In the absence of centre illumination the same annular stimulus caused a hyperpolarization and a net increase in conductance. 3. In off-centre ganglion cells the sustained hyperpolarizing response to centre illumination was associated with a net increase in conductance. In the presence of centre illumination, stimulation of the surround with an annulus caused a depolarization and a net decrease in conductance, and the reversal potential of the light-evoked response was shifted in a positive direction. In the absence of centre illumination the same annulus caused a depolarization and a net increase in conductance. 4. The results indicate that illumination of the receptive field surround can affect both the excitatory and inhibitory sustained inputs to a given ganglion cell in a 'push-pull' manner, by decreasing the synaptic input that was increased by centre illumination and increasing the synaptic input of opposite sign. The relative effect of a given surround illumination on these two inputs, and hence the sign and magnitude of the net conductance change, varied with the amount of centre illumination.

N-acetylaspartylglutamate identified in the rat retinal ganglion cells and their projections in the brain

N-Acetylaspartylglutamate like immunoreactivity (NAAG-L) was identified in retinal ganglion cell bodies and their axons. The presence of the dipeptide in ganglion cell projection areas, the lateral geniculate nucleus (LGN) and superior colliculus (SC), was confirmed following NAAG purification from these tissues by a high-performance liquid chromatographic method. NAAG-L was identified in the optic tract as well as within fibers and puncta in the LGN and SC. The hypothesis that NAAG is present within ganglion cell axons in the brain was tested by unilateral enucleation which resulted in loss of NAAG and NAAG-L within the contralateral LGN and SC.

Cellular localization of N-acetylaspartylglutamate in amphibian retina and spinal sensory ganglia

Antisera were produced against N-acetylaspartylglutamate (NAAG) and were used to localize the molecule within the retina and spinal sensory ganglia of Rana pipiens. NAAG immunoreactivity (IR) in the retina was confined to a subpopulation of amacrine and bipolar cells. The dipeptide was present in the perikarya of these cells and their neurites which terminated in two discrete bands of the inner plexiform layer. Some NAAG-IR was also present in the outer plexiform layer and the inner segment layer. In spinal ganglia, a subpopulation of relatively large sensory neuron cell bodies expressed NAAG-IR. These data are consistent with the hypothesis that this dipeptide has a function which is specific to discrete subclasses of neurons. In the amphibian retina, the NAAG distribution can be related to the reported involvement of the N-methyl-D-aspartate receptor in neurotransmission at the level of amacrine and ganglion cells.

Excitatory amino acids have different effects on horizontal cells in eyecup and isolated retina

Horizontal cells in the mudpuppy eyecup responded to continuous superfusion with L-glutamate, L-aspartate, kainate and quisqualate with a transient depolarization and reduction of the light evoked responses. However, in isolated retina preparations, in which these substances were applied to the photoreceptor side of the retina, the effects were sustained as long as the agonists were present. These results suggest that the transient action of these agonists in eyecup preparations was due to the rapid development of an intraretinal diffusion barrier, and are consistent with the hypothesis that photoreceptors release an excitatory amino acid transmitter.