To study processing of UV stimuli in the retina of the turtle, Trachemys dorbignii, we recorded intracellular responses to spectral light from 89 cells: 54 horizontal (47 monophasic, five (R/G) biphasic and two (Y/B) triphasic), 14 bipolar, 12 amacrine, and nine ganglion cells. Spectral sensitivities were measured with monochromatic flashes or with the dynamic constant response method in dark or chromatic adapted states. Stray light and second-order harmonics were also measured. (1) All cells responded to UV stimuli, although none had maximum sensitivity in the UV. (2) Most horizontal, bipolar, and amacrine cells had red-peaked spectral sensitivities. (3) Red adaptation of all monophasic horizontal cells indicated a single red input, except one that had additional peaks in the blue and UV. (4) Responses of biphasic and triphasic horizontal cells to UV light were always hyperpolarizing. Opposition between hyperpolarizing and depolarizing responses at long wavelengths indicates that UV responses were not due to the beta band of red receptors. (5) An unstained spectrally opponent bipolar cell hyperpolarized in the center to green light and antagonistically depolarized in the surround to UV, blue, and green flashes, but hyperpolarized to red. (6) All dark-adapted amacrine cells were red-peaked monophasic cells, but red adaptation broadened their spectral-sensitivity curves or displaced their peaks. An A15, an A18, and an A24 wide-field amacrine cell were stained. (7) A G15 bistratified ganglion cell is shown here for the first time to be spectrally opponent. This UVB/RG cell depolarized to UV and blue and hyperpolarized to red and green. It differs from previously reported turtle ganglion cells in being color opponent in the entire field, not only in the surround, and in showing spatial opponency.

Previous studies in the rabbit retina have shown that drugs which block AMPA glutamate receptors abolish directional selectivity in ON–OFF directionally selective (DS) ganglion cells. The effects of activation of AMPA receptors on the directionally selective responses of these ganglion cells had not been studied. In the present study, extracellular recordings of the responses of ON–OFF DS ganglion cells to a moving bar of light were made in an in vitro rabbit retinal preparation. In control solution, bath application of AMPA (7–10 μM) abolished the light responses of most ON–OFF DS ganglion cells. On washout of AMPA, the light responses rapidly returned; however, the cells temporarily lost the ability to discriminate the direction of the moving bar of light. That is, the cells responded equally to movement in the preferred and null directions. Pretreatment of retinas with the glycine receptor antagonist strychnine (1–2 μM) did not alter the effects of AMPA. On the other hand, in retinas pretreated with the GABAA receptor antagonist SR95531 (0.2–0.25 μM), AMPA did not abolish the light responses of ON–OFF DS ganglion cells but instead abolished directional selectivity in these cells by bringing out a response to movement in the null direction. This finding suggests that an AMPA-induced GABA efflux from cells in the retina was responsible for the suppression of the light responses by AMPA. In control solution, application of the selective AMPA receptor agonist (S)-5-fluorowillardiine (2–3 μM) only temporarily abolished the light responses of ON–OFF DS ganglion cells. As the light responses returned, it was clear that directional selectivity had been abolished by (S)-5-fluorowillardiine. In control solution, blocking AMPA receptor desensitization with cyclothiazide (80–100 μM) greatly reduced the light responses of ON–OFF DS ganglion cells. As the light responses slowly returned on washout of cyclothiazide, directional selectivity was clearly reduced although not abolished. In retinas pretreated with SR95531, application of cyclothiazide abolished directional selectivity. Diazoxide (700–1000 μM), another blocker of AMPA receptor desensitization, abolished directional selectivity in ON–OFF DS ganglion cells without the need of adding SR95531 to the bathing solution. It is concluded that, in the rabbit retina, AMPA receptors play an important role in generating directional selectivity in ON–OFF DS ganglion cells. Moreover, excessive activation of AMPA receptors greatly compromises the mechanism for directional selectivity in ON–OFF DS ganglion cells.

Three 5-HT receptors have been implicated in retinal processing but positive identification of the receptors and the localization of receptor subtypes in the retina have not been achieved. In this study, molecular techniques were used to identify one class of 5-HT receptor—5-HT2a—in the retina, and immunohistochemical techniques were used to localize the receptor in the retinal network. Reverse transcription polymerase chain reaction (RT-PCR) techniques were used to identify a segment of the rabbit 5-HT2a gene; a 422 base fragment was identified, cloned, and sequenced. The fragment shows a high degree (ca. 90%) of nucleotide sequence identity with the 5-HT2a receptor gene from other mammals. 5-HT2a immunoreactivity was seen in both the inner and outer plexiform (synaptic) layers of the retina. Using cell-type-specific markers, the 5-HT2a immunoreactivity was shown to be on the terminals of photoreceptor and rod bipolar cells. This association of 5-HT2a receptors with these two synapses suggests that serotonin may be a modulator of synaptic function in the retina.

We have examined the distribution of the glycine transporter glyt-1 in retinae of macaques, cats, rabbits, rats, and chickens. In all species, all glycine-containing amacrine cells expressed immunoreactivity for glyt-1, though the intensity of immunoreactivity for glyt-1 did not appear to directly correlate with the intensity of immunoreactivity for glycine in individual cells. A small subpopulation of glycine-immunoreactive displaced amacrine cells or ganglion cells also expressed glyt-1 in retinae from macaques, cats, chickens, and rats but not in retinae from rabbits. In addition, in all species examined, some displaced amacrine cells also contained glycine but did not express glyt-1. In monkeys, cats, and rats, populations of cells which we interpret as being glycine-containing interplexiform cells expressed glyt-1; these cells lacked a content of glutamate, suggesting they are not bipolar cells. The glycine-containing bipolar cells did not express glyt-1, suggesting that these cells probably acquired their content of glycine by other means such as via gap junctional connections with glycine-containing amacrine cells.

Teleost fish retinas can regenerate in vivo in adulthood. Retinal and visual function was assessed in adult goldfish following comprehensive retinal destruction by intraocular injection of ouabain. Electroretinograms (ERGs) and the dorsal light reflex (DLR) were used to evaluate the return of visual function. ERGs were detectable in regenerating eyes 50 to 70 days following ouabain injection. Amplitudes of both a- and b-waves increased steadily through day 210 following ouabain treatment, at which time a-wave amplitude was 90% and b-wave amplitude approached 50% of the contralateral control eye. The progressive gain observed in the a-wave was attributed to photoreceptor regeneration. The increase in b-wave amplitude was attributed to an increase in the number of inner nuclear layer cells and the number and efficacy of neuronal connections to or within the inner retina. The photopic spectral sensitivity of the b-wave in regenerating retina closely matched the intrafish control retina, suggesting that the relative numbers of cone photoreceptors was normal in regeneration. The recovery of the DLR (indicated by improved postural balance during regeneration) paralleled electrophysiological gains during retinal regeneration. Fish displayed a marked longitudinal body imbalance toward the control eye following retinal destruction. Improvement in equilibrium was correlated with increasing b-wave amplitudes. When the b-wave reached 50% of control amplitude (30 weeks), normal posture was restored. The return of the ERG indicates that photoreceptors and their synaptic connections must be functional in regenerating retina. Failure of the retina to regenerate produced an abnormal DLR that persisted through 30 weeks and ERGs were not measurable. The return of normal equilibrium indicates that the regenerating retina can establish central connections to the brain, and that the regenerated connections can mediate functional visual behavior.

The retinas of adult teleost fish can regenerate following injury, but little is known about the neuronal integration of the visual scene that is performed by the regenerated retina. Using goldfish retinal ganglion cells (RGCs) as the experimental system, an evaluation of dendritic arbor structure and passive electrotonic properties was developed, the aim being to quantitatively test the hypothesis that native and regenerated RGC dendritic arbors have similar structural and modeled electrotonic attributes. Fractal dimension was chosen as the descriptor of RGC dendritic arbor complexity, and the arbors' transfer function magnitudes were estimated using an electrically passive, equivalent-circuit analysis. For both native and regenerated RGCs, arbors qualitatively judged to be simple tended to have lower fractal dimension values than arbors judged to be more complex. All cells had similar cut-off frequencies, and for random stimulation of greater than 25% of an RGC's population of dendritic tips, there was a positive correlation between fractal dimension and transfer function magnitude. Some regenerated RGCs had abnormally long primary dendrites, but neither the distributions of fractal dimension values, nor the estimated transfer function magnitudes, were significantly different between native and regenerated RGCs. The results appear to support the hypothesis that structural and modeled electrotonic attributes of regenerated goldfish RGCs are similar to those of native RGCs, suggesting that regenerated RGCs may restore normal visual function.

Excitatory amino acid (EAA)-induced production of inositolphosphates (IPs) was studied in primary cultures of chick retinal pigment epithelium (RPE) following in vitro incorporation of [3H] myo-inositol. Glutamic acid (L-glu) significantly increased [3H]-IPs accumulation (215%). L-glu agonists stimulated [3H]IPs accumulation in the following order of efficiency: N-methyl-D-aspartate (NMDA) ≥ L-glu > quisqualate ≥ kainate > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD). Stimulation was dependent on external Ca2+. The NMDA-induced response was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo-cyclohepten-5,10-imine maleate (MK-801) and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and was decreased by the L-Ca2+-channel blockers verapamil and nifedipine as well as by dantrolene. The metabotropic glutamate receptor (mGluR) antagonist (+)-α-methyl-4-carboxyphenylglycine (+)MCPG inhibited 3,5-dihydroxyphenylglycine (DHPG) and ACPD-induced stimulation, which demonstrates the presence in RPE of mGluRs 1 and/or 5, as well as NMDA receptors coupled directly, or through the influx of external Ca2+, to phospholipase C activation. L-glu agonists showed no effect either on basal level of intracellular cyclic adenosine monophosphate, nor on forskolin- or carbachol-induced stimulation of adenylyl cyclase. Since L-glu is released from the retina upon illumination, and receptors for this compound are present in RPE, the activation of the inositide pathway could be involved in the regulation of retina-RPE interaction, which is essential for the visual process.

Immunohistochemistry for choline acetyltransferase (ChAT) revealed an extensive network of cholinergic fibers in the tectum of amphibians. The distribution of ChAT immunoreactive fibers was not restricted to superficial retinocipient layers, but also included deep tectal layers. The aim of the present study was to investigate the origin of the cholinergic inputs to the tectum of amphibians. For that purpose, application of retrograde tracers in the tectum of the anuran Rana perezi and the urodele Pleurodeles waltl was combined with ChAT immunohistochemistry. Double-labeled cells were found primarily in the nucleus isthmi of both species. The cholinergic isthmotectal projection is bilateral and topographically arranged and all retrogradely labeled cells found in this nucleus were ChAT immunoreactive. Remarkably, abundant cholinergic cells in two tegmental nuclei, the pedunculopontine tegmental nucleus (anurans) and the laterodorsal tegmental nucleus (anurans and urodeles), were demonstrated to provide additional cholinergic innervation to the tectum. We compare the present results with previously reported studies in amphibians and other vertebrates, and discuss the possible functional significance of the cholinergic innervation of the amphibian tectum.

Patch-clamp recordings were made from ganglion cells in an in vitro dark-adapted rabbit retina preparation. Cells were stimulated by images generated on a computer monitor and focussed onto the photoreceptors. Excitatory inward currents were recorded in response to spot stimuli centered on the somas of the recorded cells. Center illumination of on-brisk-transient cells produced large transient excitatory postsynaptic currents (EPSCs) which were invariably followed by a small steady-state inward component. Illumination of a centered annulus failed to elicit the transient EPSC. Simultaneous illumination of the annulus and the center spot blocked the large transient EPSC, consistent with activation of an inhibitory surround. Application of tetrodotoxin (TTX), which blocks sodium-dependent action potentials, also blocked the surround inhibition in ON-brisk transient cells as well as some other classes of ganglion cells. It is concluded that, in some ganglion cell classes, the surround is generated largely through the activity of spiking neurons, and it is suggested that the amacrine cells in the inner plexiform layer are involved.

Mammalian retinae generally contain low numbers of short-wavelength-sensitive cones (S-cones) and higher numbers of middle- to long-wavelength-sensitive cones (M-cones). Some recent studies found topographic differences between the different photoreceptor types and in some instances between photoreceptors and ganglion cells. To investigate this question further, we constructed topographical maps of the different photoreceptors found in an Australian marsupial, the tammar wallaby. We used two polyclonal antibodies that have been shown to label S-cones (JH455) or M-cones (JH492) in a range of mammals. In the tammar wallaby, the antisera clearly distinguish two cone types. JH455 recognizes a small subset of cones (S-cones) with a density of less than 500 cells/mm2 in the ventral retina. Their density increases towards the dorsal retina to about 1600–2000 cells/mm2. JH492 recognizes all remaining cones (M-cones), but also faintly labels most cone cells recognized by JH455. The distribution of M-cones, unlike that of the S-cones, shows a clear horizontal streak of high cell density through the central retina, just like the ganglion cells. Unlike the ganglion cells, however, the M-cones do not peak in the temporal retina but show a very broad peak (12,000–18,000 cells/mm2) in the central or even slightly nasal retina. Based on our findings, the retina of the tammar can be divided into three distinct regions: firstly, the dorsal retina, which has a low ganglion and low cone cell density but a high percentage of S-cones (30%), is thought to provide good spectral sensitivity; secondly, the central horizontal band of retina, which has a high ganglion and high cone cell density and therefore provides good spatial resolution; and thirdly, the ventral retina, which has a low ganglion cell but high cone cell density with few S-cones (5%) and is therefore thought to have a high contrast sensitivity but low acuity.

We report results of numerical simulations for a model of generation of orientation selectivity in macaque striate cortex. In contrast to previous models, where the initial orientation bias is generated by convergent geniculate input to simple cells and subsequently sharpened by lateral circuits, our approach is based on anisotropic intracortical excitatory connections which provide both the initial orientation bias and its subsequent amplification. Our study shows that the emerging response properties are similar to the response properties that are observed experimentally, hence the hypothesis of an intracortical generation of orientation bias is a sensible alternative to the notion of an afferent bias by convergent geniculocortical projection patterns. In contrast to models based on an afferent orientation bias, however, the “intracortical hypothesis” predicts that orientation tuning gradually evolves from an initially nonoriented response and a complete loss of orientation tuning when the recurrent excitation is blocked, but new experiments must be designed to unambiguously decide between both hypotheses.

Receptive fields of retinal ganglion cells in turtle have excitatory and inhibitory components that are balanced along the dimensions of wavelength, functional ON and OFF responses, and spatial assignments of center and surround. These components were analyzed by spectral light adaptations and by the glutamate agonist, 2-amino-4-phosphonobutyric acid (APB). Extracellular recordings to stationary and moving spots of light were used to map changes in receptive fields. ON spike counts minus OFF spike counts, derived from flashed stationary light spots, quantified functional shifts by calculating normalized mean response modulations. The data show that receptive fields are not static, but rather are dynamic arrangements which depend on linked, antagonistic balances among the three dimensions of wavelength, ON and OFF response functions, and center/surround areas.

We have studied the morphology and physiology of retinal ganglion cells of a short-wavelength-sensitive cone (SWS-cone) pathway in dichromatic and trichromatic New World anthropoids, the capuchin monkey (Cebus apella) and tufted-ear marmoset (Callithrix jacchus). In Old World anthropoids, in which males and females are both trichromats, blue-ON/yellow-OFF retinal ganglion cells have excitatory SWS-cone and inhibitory middle- and long-wavelength-sensitive (MWS- and LWS-) cone inputs, and have been anatomically identified as small-field bistratified ganglion cells (SB-cells) (Dacey & Lee, 1994). Among retinal ganglion cells of New World monkeys, we find SB-cells which have very similar morphology to such cells in macaque and human; for example, the inner dendritic tree is larger and denser than the outer dendritic tree. We also find blue-on retinal ganglion cells of the capuchin to have physiological responses strongly resembling such cells of the macaque monkey retina; for example, responses were more sustained, with a gentler low frequency roll-off than MC-cells, and no evidence of contrast gain control. There was no difference between dichromatic and trichromatic individuals. The results support the view that SWS-cone pathways are similarly organized in New and Old World primates, consistent with the hypothesis that these pathways form a phylogenetically ancient color system.

The purpose of the present experiments was to evaluate the contribution of the glutamate-glutamine cycle in retinal glial (Müller) cells to photoreceptor cell synaptic transmission. Dark-adapted isolated rat retinas were superfused with oxygenated bicarbonate-buffered media. Recordings were made of the b-wave of the electroretinogram as a measure of light-induced photoreceptor to ON-bipolar neuron transmission. L-methionine sulfoximine (1–10 mM) was added to superfusion media to inhibit glutamine synthetase, a Müller cell specific enzyme, by more than 99% within 5–10 min, thereby disrupting the conversion of glutamate to glutamine in the Müller cells. Threo-hydroxyaspartic acid and D-aspartate were used to block glutamate transporters. The amplitude of the b-wave was well maintained for 1–2 h provided 0.25 mM glutamate or 0.25 mM glutamine was included in the media. Without exogenous glutamate or glutamine the amplitude of the b-wave declined by about 70% within 1 h. Inhibition of glutamate transporters led to a rapid (2–5 min) reversible loss of the b-wave in the presence and absence of the amino acids. In contrast, inhibition of glutamine synthetase did not alter significantly either the amplitude of the b-wave in the presence of glutamate or glutamine or the rate of decline of the b-wave found in the absence of these amino acids. Excellent recovery of the b-wave was found when 0.25 mM glutamate was resupplied to L-methionine sulfoximine–treated retinas. The results suggest that in the isolated rat retina uptake of released glutamate into photoreceptors plays a more important role in transmitter recycling than does uptake of glutamate into Müller cells and its subsequent conversion to glutamine.

The retinal ganglion cells (RGCs) of the primate form at least two classes—M and P—that differ fundamentally in their functional properties. M cells have temporal-frequency response characteristics distinct from P cells (Benardete et al., 1992; Lee et al., 1994). In this paper, we elaborate on the temporal-frequency responses of M cells and focus in detail on the contrast gain control (Shapley & Victor, 1979a,b). Earlier data showed that the temporal-frequency response of M cells is altered by the level of stimulus contrast (Benardete et al., 1992). Higher contrast shifts the peak of the frequency-response curve to higher temporal frequency and produces a phase advance. In this paper, by fitting the data to a linear filter model, the effect of contrast on the temporal-frequency response is subsumed into a change in a single parameter in the model. Furthermore, the model fits are used to predict the response of M cells to steps of contrast, and these predictions demonstrate the dynamic effect of contrast on the M cells' response. We also present new data concerning the spatial organization of the contrast gain control in the primate and show that the signal that controls the contrast gain must come from a broadly distributed network of small subunits in the surround of the M-cell receptive field.

The technique of current source-density analysis was applied to several components of the light-evoked field potentials (electroretinogram) from the retina of the superfused eyecup of rabbit. The depth distributions of the major current sources and sinks were: b-wave—sink at outer plexiform layer, source at inner plexiform layer; M-wave—sink at inner plexiform layer, source at retinal surface; and slow PIII—source near outer plexiform layer, sink at retinal surface. These distributions, along with the sensitivities of these responses to certain pharmacological agents, support earlier studies that Müller cells generate the M-wave and slow PIII, but that depolarizing bipolar cells directly generate the b-wave.

We have investigated the morphology of the NOS-like immunoreactive neurons and their synaptic connectivity in the rat retina by immunocytochemistry using antisera against nitric oxide synthase (NOS). In the present study, several types of amacrine cells were labeled with anti-NOS antisera. Type 1 cells had large somata located in the inner nuclear layer (INL) with long and sparsely branched processes ramifying mainly in stratum 3 of the inner plexiform layer (IPL). Somata of type 2 cells with smaller diameters were also located in the INL. Their fine processes branched mostly in stratum 3 of the IPL. A third population showing NOS-like immunoreactivity was a class of displaced amacrine cells in the ganglion cell layer (GCL). Their soma size was similar to that of the type 1 cells; however, their processes stratified mainly in strata 4 and 5 of the IPL. Labeled neurons were evenly distributed throughout the retina, and the mean densities were 57.0 ± 9.7 cells/mm2 for the type 1 cells, 239.3 ± 43.4 cells/mm2 for the type 2 cells and 121.2 ± 27.5 cells/mm2 cells for the displaced amacrine cells. The synaptic connectivity of NOS-like immunoreactive amacrine cells was identified in the IPL by electron microscopy. NOS-labeled amacrine cell processes received synaptic input from other amacrine cell processes and bipolar cell axon terminals in all strata of the IPL. The most frequent postsynaptic targets of NOS-immunoreactive amacrine cells were other amacrine cell processes. Ganglion cell dendrites were also postsynaptic to NOS-like immunoreactive neurons in both sublaminae of the IPL. Synaptic outputs onto bipolar cells were observed in sublamina b of the IPL. In addition, a few synaptic contacts between labeled cell processes were observed. Our results suggest that NOS immunoreactive cells may be modulated by other amacrine cells and ON cone bipolar cells, and act preferentially on other amacrine cells.

The electroretinogram (ERG) of the rhodopsin knockout (rho−/−) mouse of Humphries et al. (1997) (Humphries et al., 1997) was studied for evidence of light-evoked rod activity and to describe the cone function. The rho−/− retina develops normal numbers of rod and cone nuclei, but the rods have no outer segments, and no rhodopsin is found by immunohistochemistry. The dark-adapted ERG threshold was elevated 4.7 log units above wild-type (WT) control mice, indicating that any residual rod responses were reduced >50,000-fold, consistent with a complete functional knockout. The dark-adapted rho−/− ERG had a cone waveform, and the spectral sensitivity peaked near 510 nm for both dark-adapted and light-adapted conditions, without evidence of a Purkinje shift. The light-adapted ERG b-wave amplitude of young rho−/− mice was the same as WT. The amplitude remained steady up to postnatal day P47, but thereafter it declined to only 1–2% by P80 when no cone outer segments remained. Cone b-wave threshold of dark-adapted rho−/− mice was −1.07 ± 0.39 log cd-s/m2 (n = 17), which is 1.27 log units more sensitive than light-adapted thresholds against a rod-suppressing Ganzfeld background of 1.61 log scotopic cd/m2. This indicates that dark-adapted WT responses to still dimmer stimuli are exclusively rod driven with minimal cone intrusion. Above this cone threshold intensity, the dark-adapted b-wave of WT will be a summation of rod and cone responses. Threshold versus intensity (TVI) studies gave no evidence of a rod influence on the mouse cone b-wave.