Complementary roles of two excitatory pathways in retinal directional selectivity

The two major excitatory synapses onto ON-OFF directionally selective (DS) ganglion cells of the rabbit retina appear to be nicotinic cholinergic and NMDA glutamatergic. Blockade of either of these synapses with antagonists does not eliminate directional selectivity. This suggests that these synapses may have complementary roles in the computation of the direction of motion. To test this hypothesis, quantitative features of the DS cell excitatory pathways were determined by collecting responses, under nicotinic and/or NMDA blockade, to a sweeping bar, hyperacute apparent motions, or a drifting sinusoidal grating. Sweeping bar responses were reduced, but directional selectivity not eliminated, by blockade of either excitatory path, as previously shown (Cohen & Miller, 1995; Kittila & Massey, 1997). However, residual responses under combined blockades were not statistically significantly DS. NMDA blockade reduced responses more than nicotinic blockade for each protocol, and shifted hyperacute motion thresholds to higher values. This supported the notion that glutamate provides the main excitatory drive to DS cells, that is, the one responsible for contrast sensitivity. In turn, nicotinic, but not NMDA blockade eliminated directional selectivity to a drifting low spatial-frequency sinusoidal grating in these cells. This suggested that acetylcholine (ACh) is the main excitatory input with regards to directional selectivity for some textured stimuli, that is, those with multiple peaks in their spatial luminance profile. Moreover, nicotinic blockade raised the low temporal-frequency cutoff of the grating responses, consistent with the proposal that preferred-direction facilitation, which is temporally sustained, is dependent on the cholinergic input. These different properties of the NMDA and nicotinic pathways are consistent with a recently proposed two-asymmetric-pathways model of directional selectivity.     

Identification of glutamate receptor subtypes mediating inputs to bipolar cells and ganglion cells in the tiger salamander retina

1. The effects of glutamate receptor agonists and antagonists on bipolar cells and ganglion cells were studied with the use of intracellular and extracellular recording in the superfused, isolated, flat-mounted tiger salamander retina. The goal of the experiments was to correlate glutamate receptor subtypes with their localization at specific synaptic sites in the tiger salamander retina. The drugs tested were the kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the N-methyl-D-aspartate (NMDA) receptor antagonist 3-(C+/-)-2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid (CPP) and L-2-amino-4-phosphonobutyrate (L-AP4). 2. The light responses of hyperpolarizing bipolar cells were suppressed by 20 microM CNQX, whereas L-AP4 had no effect on their light responses. In contrast, 20 microM CNQX had no effect on depolarizing bipolar cells, whereas L-AP4 abolished the light responses of these cells. 3. The light offset responses of OFF and ON-OFF ganglion cells were completely blocked by concentrations of CNQX as low as 5 microM. The light onset responses of ON-OFF ganglion cells were blocked when the concentration of CNQX was raised to 20 microM. In addition, 30 microM CPP partially blocked the light onset responses of ON-OFF ganglion cells but had a lesser effect on the light offset responses. 4. Twenty micromolars of CNQX blocked a transient component, and 20 microM CPP blocked a sustained component of the light response of sustained-ON ganglion cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of retinoic acid (all-trans and 9-cis) on tumor progression in small-cell lung carcinoma

ON direction-selective (DS) ganglion cells were identified by electrophysiological recordings in DAPI labeled, isolated rabbit retinas. Their responses to a flashing spot were sustained. Their responses to moving stimuli were strong in the preferred direction and weak in the null direction. Injection of the recorded cells with Lucifer yellow revealed that the cells had a distinct dendritic morphology, consistent with that described previously (Buhl & Peichl, 1986; Amthor et al., 1989; Famiglietti, 1992a). When neighboring cells were injected, an extensive dendritic co-fasciculation was observed. The pattern of fasciculation restricts the possible synaptic connections of the ON DS cell.     

Blockade of glutamate-mediated activity in the developing retina perturbs the functional segregation of ON and OFF pathways

The dendrites of ganglion cells initially ramify throughout the inner plexiform layer of the developing retina before becoming stratified into ON or OFF sublaminae. This ontogenetic event is thought to depend on glutamate-mediated afferent activity, because treating the developing retina with the glutamate analog 2-amino-4-phosphonobutyrate (APB), which hyperpolarizes ON cone bipolar cells and rod bipolar cells, thereby preventing their release of glutamate, effectively arrests the dendritic stratification process. To assess the functional consequences of this manipulation, extracellular recordings were made from single cells in the A laminae of the dorsal lateral geniculate nucleus and from the optic tract in mature cats that had received intraocular injections of APB during the first postnatal month. Such recordings revealed that stimulation of the APB-treated eye evoked both ON as well as OFF discharges in 37% of the cells tested. (As expected, when the normal eye was activated, virtually all cells yielded only ON or OFF responses.) The proportion of ON-OFF cells found here corresponds closely to the incidence of multistratified dendrites observed previously in anatomical studies of APB-treated cat retinas. This suggests that the ganglion cells with multistratified dendrites receive functional inputs from ON as well as OFF cone bipolar cells. This interpretation is further supported by the finding that the proportion of ON-OFF cells was very similar in the geniculate layer innervated by the treated eye and in the optic tract. The cells activated by the APB-treated eye were also found not to show response suppression when flashing stimuli of increasing size were used. This suggests that exposing the developing retina to APB perturbs the neural circuitry mediating the antagonistic center-surround organization found in normal receptive fields. The functional changes evident after treating the developing retina with APB suggest that it should now be feasible to assess how the segregation of ON and OFF retinal pathways relates to organizational features at higher levels of the visual system, such as orientation selectivity in cortical cells.     

Neuronal encoding of sound direction in the auditory midbrain of the rainbow trout

Acoustical stimulation causes displacement of the sensory hair cells relative to the otoliths of the fish inner ear. The swimbladder, transforming the acoustical pressure component into displacement, also contributes to the displacement of the hair cells. Together, this (generally) yields elliptical displacement orbits. Alternative mechanisms of fish directional hearing are proposed by the phase model, which requires a temporal neuronal code, and by the orbit model, which requires a spike density code. We investigated whether the directional selective response of auditory neurons in the midbrain torus semicircularis (TS; homologous to the inferior colliculus) is based on spike density and/or temporal encoding. Rainbow trout were mounted on top of a vibrating table that was driven in the horizontal plane to simulate sound source direction. Rectilinear and elliptical (or circular) motion was applied at 172 Hz. Generally, responses to rectilinear and elliptical/circular stimuli (irrespective of direction of revolution) were the same. The response of auditory neurons was either directionally selective (DS units, n = 85) or not (non-DS units, n = 106). The average spontaneous discharge rate of DS units was less than that of non-DS units. Most DS units (70%) had spontaneous activities < 1 spike per second. Response latencies (mode at 18 ms) were similar for both types of units. The response of DS units is transient (19%), sustained (34%), or mixed (47%). The response of 75% of the DS units synchronized to stimulus frequency, whereas just 23% of the non-DS responses did. Synchronized responses were measured at stimulus amplitudes as low as 0.5 nm (at 172 Hz), which is much lower than for auditory neurons in the medulla of the trout, suggesting strong convergence of VIIIth nerve input. The instant of firing of 42% of the units was independent of stimulus direction (shift <15 degrees), but for the other units, a direction dependent phase shift was observed. In the medial TS spatial tuning of DS units is in the rostrocaudal direction, whereas in the lateral TS all preferred directions are present. On average, medial DS units have a broader directional selectivity range, are less often synchronized, and show a smaller shift of the instant of firing as a function of stimulus direction than lateral DS units. DS response characteristics are discussed in relation to different hypotheses. We conclude that the results are more in favor of the phase model.     

ON-OFF amacrine cells in cat retina

We studied the morphology, photic responses, and synaptic connections of ON-OFF amacrine cells in the cat retina by penetrating them with intracellular electrodes, staining them with horseradish peroxidase, and examining them with the electron microscope. In a sample of seven cells, we found two different morphological types: the A19, which ramifies narrowly in stratum 2 (sublamina a) of the inner plexiform layer, and the A22, which ramifies mostly in stratum 4 (sublamina b) but extends some dendrites to sublamina a. Both of these cell types have axon-like processes that extend > 800 microns from the conventional dendritic arbor. ON-OFF amacrine cells in our sample had receptive fields (1.7 +/- 0.3 mm diameter) that were broader than their dendritic arbors (425 +/- 35 microns diameter) and that extended over the region of axon-like processes. In addition, we found many features in common with ON-OFF amacrine cells in poikilotherm vertebrates: a broad receptive field without surround antagonism, two sizes of spike-like events, narrow dynamic range (1 log unit intensity), and excitatory postsynaptic potentials at light on and light off. Two A19 amacrine cells were examined in the electron microscope: most synaptic inputs (93 and 76%, respectively) to either cell were from amacrine cells, with minor inputs from cone bipolar cells. Synaptic outputs were to bipolar, amacrine, and ganglion cells, including the OFF-alpha cell.     

The receptive field of the primate P retinal ganglion cell, II: Nonlinear dynamics

The ganglion cells of the primate retina include two major anatomical and functional classes: P cells which project to the four parvocellular layers of the lateral geniculate nucleus (LGN), and M cells which project to the two magnocellular layers. The characteristics of the P-cell receptive field are central to understanding early form and color vision processing (Kaplan et al., 1990; Schiller & Logothetis, 1990). In this and in the following paper, P-cell dynamics are systematically analyzed in terms of linear and nonlinear response properties. Stimuli that favor either the center or the surround of the receptive field were produced on a CRT and modulated with a broadband signal composed of multiple m-sequences (Benardete et al., 1992b; Benardete & Victor, 1994). The first-order responses were calculated and analyzed in this paper (part I). The findings are: (1) The first-order responses of the center and surround depend linearly on contrast. (2) The dynamics of the center and surround are well described by a bandpass filter model. The most significant difference between center and surround dynamics is a delay of approximately 8 ms in the surround response. (3) In the LGN, these responses are attenuated and delayed by an additional 1-5 ms. (4) The spatial transfer function of the P cell in response to drifting sine gratings at three temporal frequencies was measured. This independent method confirmed the delay between the (first-order) responses of the center and surround. This delay accounts for the dependence of the spatial transfer function on the frequency of stimulation.     

GABA-induced inactivation of functionally characterized sites in cat visual cortex (area 18): effects on direction selectivity

1. Microiontophoresis of gamma-aminobutyric acid was used to reversibly inactivate small sites of defined orientation and direction specificity at a horizontal distance of 400-700 microns from single cells recorded in cat area 18. There was extensive or complete overlap between the receptive fields of cells at the recording and inactivation sites. A cell's directionality index [DI: 1 - (response to nonpreferred direction/response to preferred direction)], the response to the preferred direction, and orientation tuning width (measured at half the maximum response) were compared before and during inactivation of either iso-orientation sites (where the orientation preference was within 22.5 degrees) or cross-orientation sites (where it differed by 45-90 degrees). 2. During iso-orientation inactivation, 40 (73%) of 55 cells showed a significant (> 0.20) change in DI; the mean change in DI for these cells was 0.59. An additional cell showed a marked increase in response to the preferred direction that did not result in a change in DI. With one exception, the effects occurred in the absence of a significant (> 25%) change in orientation tuning width. 3. In most cases, the results were broadly predictable in the sense that iso-orientation inactivation predominantly affected a cell's response to the direction of motion of an optimally oriented bar that was closest to the preferred direction at the inactivation site: viz., a decrease in response to the preferred direction and an increase in response to the preferred or nonpreferred direction. 4. It is argued that the decreases in response were due to a reduction in the strength of intracortical iso-orientation excitatory connections made primarily between cells with similar direction preferences, whereas the increases in response involved a loss of iso-orientation inhibition. 5. In cases where remote inactivation caused an increase in response to the nonpreferred direction, comparable effects could be elicited when a mask left exposed only the excitatory subregion of the receptive field in S cells or the most responsive part of the excitatory discharge region in C cells. This implies extensive or complete spatial overlap between the profiles of excitation and inhibition in a cell's nonpreferred direction. 6. During cross-orientation inactivation, a significant change in DI was seen in only 14 (19%) of 73 cells and, with one exception, these changes were accompanied by increases in response to non-optimal orientations and significant broadening of orientation tuning. The effects of cross-orientation inactivation on directionality were presumably due to the loss of cross-orientation inhibition, which contributes primarily to orientation tuning. 7. Inactivation of the same site could cause an increase in response to the nonpreferred direction in cells recorded at iso-orientation sites and an increase in response to nonoptimal orientations and broadening of orientation tuning in cells recorded at cross-orientation sites. This is consistent with the notion that a single inhibitory neuron can contribute to the directionality or orientation tuning of different target cells depending on their location in the orientation map. 8. The results provide evidence for a major contribution of intrinsic mechanisms to the orientation tuning and direction selectivity of cells in cat area 18. It is proposed that two different intracortical processes are involved in the enhancement of orientation and direction selectivity: 1) suppression of responses to nonoptimal orientations and directions as a result of cross-orientation inhibition and iso-orientation inhibition; and 2) facilitation of responses to optimal orientations/directions via iso-orientation excitatory connections.     

Responses to acetylcholine of ganglion cells in an isolated mammalian retina

1. Rabbit retinas were isolated and superfused with a physiological medium. Ganglion cell activity was recorded during stimulation with focused light, and receptive fields were mapped. Receptive fields were identical to those found in vivo and did not change during a 6-h incubation. After the receptive field of a ganglion cell had been identified, acetylcholine or related agents were introduced singly or in combination into the medium, and their effect on the cell's spontaneous and light-evoked activity was observed. 2. Ganglion cells with on-center or directionally selective receptive fields were excited when ACh was added to the medium. The response to exogenous ACh was prevented by cholinergic antagonists. 3. These cells' spontaneous activity and response to light were enhanced by anticholinesterase and depressed by cholinergic antagonists. Antagonists varied in their ability to block the light-evoked response, with dihydro-beta-erythroidine the most effective. 4. Thresholds for ACh or the related agents were low, ranging from 1 to 40 muM; their effects were rapidly and completely reversed when the retina was returned to control medium. 5. In retinas incubated in medium containing 20 mM Mg2+ and 0.2 mM Ca2+, ganglion cells lost completely both their spontaneous and light-evoked activity, but retained their ability to generate action potentials in response to elevated K+. Ganglion cell activity rapidly returned to normal when the retina was returned to medium containing normal electrolytes. On-center and directionally selective cells were excited by ACh in retinas where synaptic transmission had been inhibited by 20 mM Mg2+ and 0.2 mM Ca2+. 6. The responses of on-center and directionally selective cells to ACh, to anticholinesterase, and to cholinergic antagonists in control medium indicate that the retina contains one or more synapses using ACh as a neurotransmitter. The response to ACh in retinas exposed to 20 mM Mg2+ and 0.2 mM Ca2+ suggests that at least one such synapse in on the ganglion cell itself. 7. Off-center cells were inhomogenous in their response to ACh. Although some responded just as the other classes of cell, the majority responded quite weakly and a subgroup was encountered which was entirely unaffected by even 1 mM ACh, by levels of physostigmine which inactivate virtually all retinal acetyl-cholinesterase, or by high concentrations of cholinergic antagonists. Only 2 of 20 off-cells tested in the presence of 20 mM Mg2+ and 0.2 mM Ca2+ were excited by ACh. Apparently ACh is not a primary transmitter for most off-cells.     

Desensitizing glutamate receptors shape excitatory synaptic inputs to tiger salamander retinal ganglion cells

AMPA/kainate (KA) receptors mediate a component of ganglion cell excitatory postsynaptic currents (EPSCs). We investigated whether desensitization at these receptors contribute to the shape of transient EPSCs in ON-OFF ganglion cells. Whole-cell, voltage-clamp recordings were made from ganglion cells in the retinal slice or in isolation. EPSCs were evoked by either stimulating the slice with light or puffing K+ at the outer plexiform layer (OPL). The AMPA/KA receptor-mediated component of the EPSCs was isolated by including NMDA receptor antagonists in the bath. Strychnine and picrotoxin blocked inhibitory inputs. In isolated ganglion cells, cyclothiazide (10 microM), which blocks desensitization in non-NMDA receptors, enhanced both the amplitude and the duration of currents evoked by puffs of AMPA or glutamate. EPSCs evoked by K(+)-puffs in the OPL were also enhanced by cyclothiazide (30 microM). When AMPA/KA receptors were blocked with NBQX (10 microM), no enhancement of the EPSCs by cyclothiazide was observed, indicating that cyclothiazide did not act presynaptically. Cyclothiazide also enhanced the amplitude and duration of both the ON and OFF light-evoked (L-) EPSCs recorded in ON-OFF ganglion cells. Current-voltage relationships showed the enhancement was not voltage dependent. When control and enhanced responses where normalized, it was observed that the rate of desensitization of both the ON and OFF L-EPSCs was decreased by cyclothiazide. Cyclothiazide selectively enhanced the AMPA/KA receptor-mediated component of ganglion cells EPSCs, suggesting that desensitization of AMPA/KA receptors shape transient L-EPSCs.     

Localization of nitric oxide synthase, NADPH diaphorase and soluble guanylyl cyclase in adult rabbit retina

Nitric oxide (NO) acts as a neuronal messenger which activates soluble guanylyl cyclase (SGC) in neighboring cells and produces a wide range of physiological effects in the central nervous system (CNS). Using immunocytochemical and histochemical stains, we have characterized the NO/SGC system in the rabbit retina and to a lesser extent, in monkey retina. Based on staining patterns observed with an antibody to nitric oxide synthase (NOS) type I and a histochemical marker for NADPH diaphorase, a metabolic intermediate required for NOS activity, three major classes of neurons appear to generate NO in the rabbit retina. These include two subclasses of sparsely distributed wide field amacrine cells, rod and cone photoreceptors, and a subpopulation of ganglion cells. Equivalent cell populations were labeled in monkey retina. An antibody to SGC (tested only in rabbit retina), labeled large arrays of cone photoreceptors in the outer nuclear layer, both amacrine and bipolar cells in the inner nuclear layer (INL), as well as populations of neurons in the ganglion cell layer. These data suggest that the ability to generate NO is restricted to relatively few neurons in the inner retina and to photoreceptor cells in the outer retina; while presumptive target cells, containing pools of SGC, are widespread and form contiguous fields across the inner and outer nuclear layers (ONL) as well as the ganglion cell layer.     

Pattern-reversal electroretinogram in response to chromatic stimuli: II. Monkey

We have recorded steady-state PERGs from five macaque monkeys in response to red-green plaid patterns reversed sinusoidally in contrast. The patterns had either a pure luminance contrast (red-black, green-black, yellow-black), pure red-green color contrast, or a variable amount of luminance and color contrast. By varying the relative luminance of the red-to-total luminance (color ratio) of red-green patterns, a value could be obtained at which the PERG amplitude was either minimum or locally maximum, and the phase was most lagged. This value was very similar to that producing equiluminance in human observers, and was considered to be equiluminance for the monkey. The phase of the PERG to chromatic stimulus was systematically lagged compared with that of luminance stimuli, by an amount corresponding to about 10-20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 80 ms for color contrast compared with 63 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. As a function of temporal frequency, the chromatic PERG function was clearly low-pass with a cutoff around 15 Hz, whereas that to luminance was double-peaked and extended to higher temporal frequencies, around 30 Hz. For both luminance and chromatic stimuli, the amplitude of PERGs increases with increasing stimulus contrast. By summing vectorially the luminance and chromatic responses of appropriate contrasts, we were able to predict with accuracy the response as a function of color ratio. In two monkeys, the optic chiasm was sectioned sagittally causing total degeneration of ganglion cells in the nasal retina, without affecting the temporal retina (verified by histology). In these animals, there was a strong response to both luminance and chromatic patterns in the temporal retinae, but none to either type of pattern in the nasal retinae, suggesting that the PERG to both luminance and chromatic stimuli arises from the inner-retinal layers. Electrophysiological studies suggest that the PERG to chromatic stimuli is probably associated with the activity of P-cells. P-cells may also make a major contribution to the PERG of luminance stimuli, although M-cells may also participate. The above findings on normal monkeys all agree with those reported in the accompanying paper for humans (Morrone et al., 1994), so similar conclusions can probably be extended to human PERG.     

The intrinsic temporal properties of alpha and beta retinal ganglion cells are equivalent

BACKGROUND: Mammalian retinal ganglion cells have been traditionally classified on the basis of morphological and functional criteria, but as yet little is known about the intrinsic membrane properties of these neurons. This study has investigated these properties by making patch-clamp recordings from morphologically identified ganglion cells in the intact retina. RESULTS: The whole-cell configuration of the patch-clamp technique was used to assess the temporal tuning characteristics of alpha and beta cells, the two most extensively studied ganglion cell classes. Fourier analysis was used to examine discharge patterns in response to sinusoidal currents of different frequencies (1-50 Hz). With few exceptions, neurons responded in a stereotypic fashion to changes in temporal modulation, with their output initially increasing and then decreasing as a function of stimulus frequency. Moreover, peak responses in both cell classes were obtained at equivalent temporal frequencies. At high stimulus rates, response probability decreased, but the spikes remained phase-locked to the stimulus cycle, thereby enabling populations of cells to convey temporal information. A small number of ganglion cells did not show an appreciable decrease in output as a function of stimulus frequency, but these cells were not confined to either ganglion cell class. CONCLUSIONS: These findings provide the first evidence that the intrinsic temporal properties of alpha and beta cells are alike. Furthermore, the responses obtained to direct current injections were strikingly similar to those described previously with temporally modulated visual stimuli, suggesting that intrinsic membrane properties may shape the visual responses of alpha and beta cells to a larger degree than has been commonly assumed.     

Retinal direction selectivity after targeted laser ablation of starburst amacrine cells

Directionally selective retinal ganglion cells respond strongly when a stimulus moves in their preferred direction, but respond little or not at all when it moves in the opposite direction. This selectivity represents a classic paradigm of computation by neural microcircuits, but its cellular mechanism remains obscure. The directionally selective ganglion cells receive many synapses from a type of amacrine cell termed 'starburst' because of its regularly spaced, evenly radiating dendrites. Starburst amacrine cells have a synaptic asymmetry that has been proposed as the source of the directional response in the ganglion cells. Here we report experiments that make this unlikely, and offer an alternative concept of the function of starburst cells. We labelled starburst cells in living retinas, then killed them by targeted laser ablation while recording from individual directionally selective ganglion cells. Ablating starburst cells revealed no asymmetric contribution to the ganglion cell response. Instead of being direction discriminators, the starburst cells appear to potentiate generically the responses of ganglion cells to moving stimuli. The origin of direction selectivity probably lies with another type of amacrine cell.     

The push-pull action of dopamine on spatial tuning of the monkey retina: the effects of dopaminergic deficiency and selective D1 and D2 receptor ligands on the pattern electroretinogram

Retinal dopamine depletion in monkeys using either systemic MPTP or 6-OHDA results in attenuated electroretinographic (ERG) responses to peak spatial frequency stimuli. Diverse dopamine receptors have been identified in the primate retina. ERG studies performed using Haloperidol (a mixed antagonist), L-Sulpiride (D2 antagonist) and CY 208-243 (a D1 agonist) cause spatial frequency dependent diverse effects. 'Tuning' of the normal spatial contrast response PERG, was quantified by dividing the amplitude of the response at the peak spatial frequency with the amplitude to the low spatial frequency response yielding a number greater than one. Tuning for the pharmacological experiments was defined by dividing the actual amplitude obtained at the normal peak response with the actual amplitude at the low spatial frequency response. The PERG spatial contrast response function is discussed as the envelope output of retinal ganglion cells or the average or 'equivalent' retinal ganglion cell. However, we postulate the existence of two dopamine sensitive pathways with different weights for two classes of ganglion cells. It is inferred that D1 receptors are primarily affecting the 'surround' organization of ganglion cells with large centers, while D2 post-synaptic receptors contribute to 'center' response amplification of ganglion cells with smaller centers. These inferences are consistent with some lower vertebrate data. It is also inferred that low affinity D2 autoreceptors may be involved in the D1 'surround' pathway. An understanding of the logic performed by retinal D1 and D2 receptors may be useful to discern the functional role of diverse dopamine receptors in DA circuits elsewhere in the CNS.     

Kinetics of the light-induced proton translocation associated with the pH-dependent formation of the metarhodopsin I/II equilibrium of bovine rhodopsin

The kinetics of the formation of the metaII (MII) state of bovine rhodopsin was investigated by time-resolved electrical and absorption measurements with rod outer segment (ROS) fragments. Photoexcitation leads to proton transfer in the direction from the cytosolic to the intradiscal side of the membrane, probably from the Schiff base to the acceptor glutamate 113. Two components of comparable amplitude are required to describe the charge movement with exponential times of 1.1 (45%) and 3.0 ms (55%) (pH 7.8, 22 degreesC, 150 mM KCl). The corresponding activation energies are 86 and 123 kJ/mol, respectively (150 mM KCl). The time constants and amplitudes depend strongly on pH. Between pH 7.1 and 3.8 the kinetics becomes much faster, with the faster and slower components accelerating by factors of about 8 and 2, respectively. Complementary single-flash absorption experiments at 380 nm and 10 degreesC show that the formation of MII also occurs with two components with similar time constants and pH dependence. This suggests that both signals monitor the same molecular events. The pH dependence of the two apparent time constants and amplitudes of the optical data can be described well over the pH range 4-7.5 by two coupled equilibria between MI and two isochromic MII species MIIa and MIIb: MI MIIa(380) MIIb(380), with k0 proportional to the proton concentration. This model implies that deprotonation of the Schiff base and proton uptake are tightly coupled in ROS membranes. Models with k2 proportional to the proton concentration cannot describe the data. Photoreversal of MII by blue flashes (420 nm) leads to proton transfer in a direction opposite to that of the signal associated with MII formation. In this transition the Schiff base is reprotonated, most likely from glutamate 113. At pH 7.3, 150 mM KCl, 22 degreesC, this electrical charge reversal has an exponential time constant of about 30 ms and is about 10 times slower than the forward charge motion.     

Responses of MT and MST neurons to one and two moving objects in the receptive field

To test the effects of complex visual motion stimuli on the responses of single neurons in the middle temporal visual area (MT) and the medial superior temporal area (MST) of the macaque monkey, we compared the response elicited by one object in motion through the receptive field with the response of two simultaneously presented objects moving in different directions through the receptive field. There was an increased response to a stimulus moving in a direction other than the best direction when it was paired with a stimulus moving in the best direction. This increase was significant for all directions of motion of the non-best stimulus and the magnitude of the difference increased as the difference in the directions of the two stimuli increased. Similarly, there was a decreased response to a stimulus moving in a non-null direction when it was paired with a stimulus moving in the null direction. This decreased response in MT did not reach significance unless the second stimulus added to the null direction moved in the best direction, whereas in MST the decrease was significant when the second stimulus direction differed from the null by 90 degrees or more. Further analysis showed that the two-object responses were better predicted by taking the averaged response of the neuron to the two single-object stimuli than by summation, multiplication, or vector addition of the responses to each of the two single-object stimuli. Neurons in MST showed larger modulations than did neurons in MT with stimuli moving in both the best direction and in the null direction and the average better predicted the two-object response in area MST than in area MT. This indicates that areas MT and MST probably use a similar integrative mechanisms to create their responses to complex moving visual stimuli, but that this mechanism is further refined in MST. These experiments show that neurons in both MT and MST integrate the motion of all directions in their responses to complex moving stimuli. These results with the motion of objects were in sound agreement with those previously reported with the use of random dot patterns for the study of transparent motion in MT and suggest that these neurons use similar computational mechanisms in the processing of object and global motion stimuli.     

A-type horizontal cells of the superior edge of the linear visual streak of the rabbit retina have oriented, elongated dendritic trees

The horizontal cells of the rabbit retina have been studied by light microscopy of Golgi-impregnated whole-mount retinas. The two types of horizontal cell of the rabbit retina are similar to the horizontal cells of the cat retina in most respects. However, the majority of the A-type horizontal cells of the rabbit have asymmetrical dendritic fields compared to the circular, symmetrical dendritic fields of this cell type in the cat. The A-type horizontal cells of the superior edge of the linear visual streak in the rabbit retina are the most strikingly asymmetric and most of them are elongated and oriented in a direction approximately parallel to the linear visual streak. Like H1 axon terminals of the turtle retina the oriented, elongated A-type horizontal cells of the rabbit visual streak region may play a role in the neurocircuitry which underlies orientation sensitive ganglion cells.     

Manipulation of autoimmune diseases with T-suppressor cells: lessons from experimental SLE and EAE

Electrical spike activity of ganglion cells has been recorded extracellularly in the teleost (roach) retina, and effects of a variety of tachykinins studied at a working concentration of 1 microM. Application of substance P mostly caused a slow and prolonged increase in background activity. In contrast, the response to carbachol was very brisk and short-lasting. Substance P and physalaemin predominantly induced an enhancement of 'On' and 'Off' components of light-evoked responses, whilst eledoisin and neurokinin A were mostly inhibitory. All effects were independent of chromatic and spatial aspects of the responses. Interestingly, in the presence of a tachykinin antagonist, 'Spantide' [D-Arg1,D-Pro2, D-Trp7.9, Leu11]SP, the profile of the effect of substance P reversed, inhibitory actions becoming much more common. Taken together, the results suggest that a tachykinin system utilising two subtypes of the receptor may be active in the roach retina and these may be involved in differential control of visual sensitivity.     

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.