Estimating the contributions of NMDA and non-NMDA currents to EPSPs in retinal ganglion cells

A synaptic signature for ON- and OFF-center parasol ganglion cells of the primate retina

In the primate retina, parasol ganglion cells contribute to the primary visual pathway via the magnocellular division of the lateral geniculate nucleus, display ON and OFF concentric receptive field structure, nonlinear spatial summation, and high achromatic temporal-contrast sensitivity. Parasol cells may be homologous to the alpha-Y cells of nonprimate mammals where evidence suggests that N-methyl-D-aspartate (NMDA) receptor-mediated synaptic excitation as well as glycinergic disinhibition play critical roles in contrast sensitivity, acting asymmetrically in OFF- but not ON-pathways. Here, light-evoked synaptic currents were recorded in the macaque monkey retina in vitro to examine the circuitry underlying parasol cell receptive field properties. Synaptic excitation in both ON and OFF types was mediated by NMDA as well as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate receptors. The NMDA-mediated current-voltage relationship suggested high Mg2+ affinity such that at physiological potentials, NMDA receptors contributed ∼20% of the total excitatory conductance evoked by moderate stimulus contrasts and temporal frequencies. Postsynaptic inhibition in both ON and OFF cells was dominated by a large glycinergic "crossover" conductance, with a relatively small contribution from GABAergic feedforward inhibition. However, crossover inhibition was largely rectified, greatly diminished at low stimulus contrasts, and did not contribute, via disinhibition, to contrast sensitivity. In addition, attenuation of GABAergic and glycinergic synaptic inhibition left center-surround and Y-type receptive field structure and high temporal sensitivity fundamentally intact and clearly derived from modulation of excitatory bipolar cell output. Thus, the characteristic spatial and temporal-contrast sensitivity of the primate parasol cell arises presynaptically and is governed primarily by modulation of the large AMPA/kainate receptor-mediated excitatory conductance. Moreover, the negative feedback responsible for the receptive field surround must derive from a nonGABAergic mechanism.

NR2C and NR2D subunits of NMDA receptors in frog and turtle retina

Glutamate NMDA (N-methyl-D-aspartate) receptors are widely distributed in the central nervous system where they are involved in cognitive processes, motor control and many other functions. They are also well studied in the retina, which may be regarded as a biological model of the nervous system. However, little is known about NR2C and NR2D subunits of NMDA receptors, which have some specific features as compared to other subunits. Consequently the aim of the present study was to investigate their distribution in frog (Rana ridibunda) and turtle (Emys orbicularis) retinas which possess mixed and cone types of retina respectively. The experiments were performed using an indirect immunofluorescence method. Four antibodies directed to NR2C and NR2D subunits of NMDA receptor, as well as three antibodies directed to different splice variants of NR1 subunit, which is known to be obligatory for proper functioning of the receptor, were applied. All antibodies caused well expressed labeling in frog and turtle retinas. The NR2C and NR2D subunits were localized in glial Müller cells, while the NR1 subunit had both neuronal and glial localization. Our results show that glial NMDA receptors differ from neuronal ones in their subunit composition. The functional significance of the NMDA receptors and their NR2C and NR2D subunits, in particular for the neuron-glia interactions, is discussed.

Imperfect space clamp permits electrotonic interactions between inhibitory and excitatory synaptic conductances, distorting voltage clamp recordings

The voltage clamp technique is frequently used to examine the strength and composition of synaptic input to neurons. Even accounting for imperfect voltage control of the entire cell membrane ("space clamp"), it is often assumed that currents measured at the soma are a proportional indicator of the postsynaptic conductance. Here, using NEURON simulation software to model somatic recordings from morphologically realistic neurons, we show that excitatory conductances recorded in voltage clamp mode are distorted significantly by neighboring inhibitory conductances, even when the postsynaptic membrane potential starts at the reversal potential of the inhibitory conductance. Analogous effects are observed when inhibitory postsynaptic currents are recorded at the reversal potential of the excitatory conductance. Escape potentials in poorly clamped dendrites reduce the amplitude of excitatory or inhibitory postsynaptic currents recorded at the reversal potential of the other conductance. In addition, unclamped postsynaptic inhibitory conductances linearize the recorded current-voltage relationship of excitatory inputs comprising AMPAR and NMDAR-mediated components, leading to significant underestimation of the relative contribution by NMDARs, which are particularly sensitive to small perturbations in membrane potential. Voltage clamp accuracy varies substantially between neurons and dendritic arbors of different morphology; as expected, more reliable recordings are obtained from dendrites near the soma, but up to 80% of the synaptic signal on thin, distant dendrites may be lost when postsynaptic interactions are present. These limitations of the voltage clamp technique may explain how postsynaptic effects on synaptic transmission could, in some cases, be attributed incorrectly to presynaptic mechanisms.

AMPA and Kainate Receptors in Turtle Retina: An Immunocytochemical Study

1. Glutamate is one of the main neurotransmitters in the retina. Its effects are mediated by a large number of ionotropic and metabotropic receptors.2. The distribution of ionotropic AMPA receptor subunits GluR1-4, kainate receptor subunits GluR5-7 and KA2, as well as delta receptors 1-2 was studied in turtle retina. Indirect immunofluorescence was used to localize the different receptor subunits viewed using light microscopy.3. Results show that all subunits, with exception of GluR1 and GluR5, are widely distributed in the turtle retina.4. They are mainly located in the both plexiform layers of the retina where punctate staining, a sign for synaptic localization, is observed.5. The vast majority of the subunits possess specific pattern of staining that allow to suppose that they are involved in different retinal circuits.6. It can be assumed that the GluR2/3 and GluR6/7 subunits are expressed on the dendrites of a subpopulation of bipolar cells that are immunopositive for alpha-isoform of protein kinase C (PKCalpha). The GluR2/3 and GluR6/7 subunits are most probably used by the same PKCalpha immunopositive bipolar cells in their synaptic contacts with the third-order retinal neurons, the amacrine and ganglion cells.

Non-NMDA receptors in frog retina: an immunocytochemical study

Glutamate is one of the main neurotransmitters in the retina. Its effects are mediated by a large number of ionotropic and metabotropic membrane receptors. The distribution of ionotropic AMPA receptor subunits GluR1-4, kainate receptor subunits GluR5-7 and KA2, delta receptors 1-2, as well as the metabotropic receptor mGluR6 were studied in the frog retina. Indirect immunofluorescence was used to localize the different receptor subunits. Results showed that all subunits, with the exception of GluR1 and GluR5, are widely distributed in the retina. They are mainly located in both plexiform layers: the outer (OPL) and the inner one (IPL), where punctate labelling, a sign of synaptic localization, is observed. The metabotropic receptor mGluR6 is localised only in the OPL. The AMPA receptor subunit GluR4 is localised on the glial Müller cells of the retina. The vast majority of the subunits possess specific patterns of labelling that indicate that they are involved with different retinal functions. The significance of the AMPA receptors and involvement of glia in modulation of synaptic transmission are discussed.

Form and Function of on-off Amacrine Cells in the Amphibian Retina

on-off amacrine cells were studied with whole cell recording techniques and intracellular staining methods using intact retina-eyecup preparations of the tiger salamander ( Ambystoma tigrinum) and the mudpuppy ( Necturus maculosus). Morphological characterization of these cells included three-dimensional reconstruction methods based on serial optical sections obtained with a confocal microscope. Some cells had their detailed morphology digitized with a computer-assisted tracing system and converted to compartmental models for computer simulations. The dendrites of on-off amacrine cells have spines and numerous varicosities. Physiological recordings confirmed that on-off amacrine cells generate both large- and small-amplitude impulses attributed, respectively, to somatic and dendritic generation sites. Using a multichannel model for impulse generation, computer simulations were carried out to evaluate how impulses are likely to propagate throughout these structures. We conclude that the on-off amacrine cell is organized with multifocal dendritic impulse generating sites and that both dendritic and somatic impulse activity contribute to the functional repertoire of these interneurons: locally generated dendritic impulses can provide regional activation, while somatic impulse activity results in rapid activation of the entire dendritic tree.

Glutamate-induced Ca2+ influx in third-order neurons of salamander retina is regulated by the actin cytoskeleton

Ligand-gated ion channels (ionotropic receptors) link to the cortical cytoskeleton via specialized scaffold proteins and thereby to appropriate signal transduction pathways in the cell. We studied the role of filamentous actin in the regulation of Ca influx through glutamate receptor-activated channels in third-order neurons of salamander retina. Staining by Alexa-Fluor 488-phalloidin, to visualize polymerized actin, we show localization of filamentous actin in neurites, and the membrane surrounding the cell soma. With Ca(2+) imaging we found that in dissociated neurons, depolymerization of filamentous actin by latrunculin A, or cytochalasin D significantly reduced glutamate-induced intracellular Ca(2+) accumulation to 53+/-7% of control value. Jasplakinolide, a stabilizer of filamentous actin, by itself slightly increased the glutamate-induced Ca(2+) signal and completely attenuated the inhibitory effect when applied in combination with actin depolymerizing agents. These results indicate that in salamander retinal neurons the actin cytoskeleton regulates Ca(2+) influx through ionotropic glutamate receptor-activated channels, suggesting regulatory roles for filamentous actin in a number of Ca(2+)-dependent physiological and pathological processes.

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.

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.

Mapping glutamatergic drive in the vertebrate retina with a channel-permeant organic cation

Patterns of neuronal excitation in complex populations can be mapped anatomically by activating ionotropic glutamate receptors in the presence of 1-amino-4-guanidobutane (AGB), a channel-permeant guanidinium analogue. Intracellular AGB signals were trapped with conventional glutaraldehyde fixation and were detected by probing registered serial thin sections with anti-AGB and anti-amino acid immunoglobulins, revealing both the accumulated AGB and the characteristic neurochemical signatures of individual cells. In isolated rabbit retina, both glutamate and the ionotropic glutamate receptor agonists alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA), kainic acid (KA), and N-methyl-D-aspartic acid (NMDA) activated permeation of AGB into retinal neurons in dose-dependent and pharmacologically specific modes. Horizontal cells and bipolar cells were dominated by AMPA/KA receptor activation with little or no evidence of NMDA receptor involvement. Strong NMDA activation of AGB permeation was restricted to subsets of the amacrine and ganglion cell populations. Threshold agonist doses for the most responsive cell groups (AMPA, 300 nm; KA, 2 microM; NMDA, 63 microm; glutamate, 1 mM) were similar to values obtained from electrophysiological and neurotransmitter release measures. The threshold for activation of AGB permeation by exogenous glutamate was shifted to <200 microM in the presence of the glutamate transporter antagonist dihydrokainate, indicating substantial spatial buffering of extracellular glutamate levels in vitro. Agonist-activated permeation of AGB into neurons persisted under blockades of Na+ -dependent transporters, voltage-activated Ca2+ and Na+ channels, and ionotropic gamma-aminobutyric acid and glycine receptors. Cholinergic agonists evoked no permeation.