Electrophysiologically, color-opponent retinal bipolar cells respond with opposite polarities to stimulation with different wavelengths of light. The origin of these different polarities in the same bipolar cell has always been a mystery. Here we show that an intracellularly recorded and HRP-injected, red-ON, blue/green-OFF bipolar cell of the turtle retina made invaginating (ribbon associated) synapses exclusively with L-cones. Non-invaginating synapses resembling wide-cleft basal junctions were made exclusively with M-cones. Input from S-cones was not seen. From these results we suggest sign-inverting transmission from L-cones at invaginating synapses via metabotropic glutamate receptors, and sign-conserving transmission from M-cones at wide-cleft basal junctions via ionotropic receptors. To explain the pronounced blue sensitivity of the bipolar cell, computer simulations were performed using a sign-conserving input from a yellow/blue chromaticity-type (H3) horizontal cell. The response properties of the red-ON, blue/green-OFF bipolar cell could be quantitatively reproduced by this means. The simulation also explained the asymmetry in L- and M-cone inputs to the bipolar cell as found in the ultrastructural analysis and assigned a putative role to H3 horizontal cells in color processing in the turtle retina.

Electrical coupling between H2 horizontal cell pairs isolated from the hybrid bass retina was studied using dual whole-cell, voltage-clamp technique. Voltage-dependent inactivation of junctional currents in response to steps in transjunctional voltage (Vj) over a range of ±100 mV was characterized for 89 cell pairs. Approximately one-quarter of the pairs exhibited strongly voltage-dependent junctions (>50% reduction in junctional current at ±100 mV), another quarter of the pairs exhibited voltage-independent junctional current (<5% reduction at ±100 mV), and the remainder of the pairs exhibited intermediate values for voltage inactivation. We focused on further characterizing the Vj-independent junctions of horizontal cells, which have not been described previously in detail. When Lucifer Yellow dye was included in one recording pipette, pairs exhibiting Vj-independent coupling showed no (9/12), or limited (3/12), passage of dye. Vj-independent coupling was markedly less sensitive to the modulators SNP (100–300 μM, −9% reduction in coupling) and dopamine (100–300 μM, −6%) than were Vj-dependent junctions (−45% and −44%). However, simultaneous application of both SNP and dopamine significantly reduced Vj-independent coupling (−56%). Both Vj-independent and Vj-dependent junctions were blocked by DMSO (1–2%), but Vj-independent junctions were not blocked by heptanol. Single-channel junctional conductances of Vj-independent junctions range from 112–180 pS, versus 50–60 pS for Vj-dependent junctions. The results reveal that Vj-independent coupling in a subpopulation of horizontal cells from the hybrid bass retina is mediated by cellular junctions with physiological and pharmacological characteristics distinct from those previously described in fish horizontal cells.

The avian ciliary ganglion (CG) controls choroidal blood flow by its choroidal neurons, and pupil constriction and accommodation by its ciliary neurons. It was previously reported that both choroidal and ciliary neurons label positively for NADPH diaphorase (NADPHd), a marker for nitric oxide synthase (NOS). To assess if this labeling is preganglionic or postganglionic and to determine if it is attributable to neuronal NOS (nNOS), we studied pigeon CG using NADPHd histochemistry and nNOS immunohistochemistry (IHC). Short-duration staining times by NADPHd histochemistry yielded intense labeling of structures that appeared to be the cap-like endings on ciliary neurons and the boutonal endings on choroidal neurons that arise from the nucleus of Edinger-Westphal (EW), and light or no postganglionic perikaryal staining. The light postganglionic staining that was observed tended to be localized to ciliary neurons. Consistent with this, NADPHd+ nerve fibers were observed in the postganglionic ciliary nerves but rarely in the postganglionic choroidal nerves. These same staining times yielded robust staining of neurons in the orbital pterygopalatine microganglia network, which are known to be nNOS+. Diffuse staining of CG perikarya was observed with longer staining durations, and this staining tended to mask the preganglionic labeling. Preganglionic NADPHd+ staining in CG with short staining times was blocked by the NOS inhibitors iodonium diphenyl (IDP) and dichlorophenol-indophenol (DPIP), but the diffuse postganglionic staining observed with the longer staining times was not completely blocked. Labeling of CG sections for substance P (SP) by IHC (which labels EW-originating preganglionic endings in CG) and subsequently for NADPHd confirmed that NADPHd was localized to preganglionic endings on CG neurons. Immunohistochemical double labeling for nNOS and SP or enkephalin further confirmed that nNOS is found in boutonal and cap-like endings in the CG. Two studies were then carried out to demonstrate that the nNOS+ preganglionic endings in CG arise from EW. First, NADPHd+ and nNOS+ neurons were observed in EW in pigeons treated with colchicine to enhance perikaryal labeling. Second, NADPHd+ and nNOS+ preganglionic endings were eliminated from CG ipsilateral to an EW lesion. These various results indicate that NOS is present in EW-arising preganglionic endings on choroidal and ciliary neurons in avian CG. NOS also appears to be found in some ciliary neurons, but its presence in choroidal neurons is currently uncertain.

Amacrine cells that respond transiently to maintained illumination are thought to mediate transient inhibitory input to ganglion cells. The excitation of these transient amacrine cells is thought to be limited by inhibitory feedback to bipolar cells. We investigated the possibility that desensitizing AMPA and/or kainate (KA) receptors on amacrine cells might also limit the duration of amacrine cell excitation. To determine how these receptors might affect amacrine cell input and output, we made whole-cell recordings from amacrine and ganglion cells in the salamander retinal slice. The specific AMPA receptor antagonist GYKI-53655 blocked non-NMDA receptor-mediated amacrine cell excitatory postsynaptic currents (EPSCs) and kainate puff-elicited currents, indicating that AMPA, and not KA, receptors mediated the responses. Cyclothiazide, an agent that reduces AMPA receptor desensitization, increased the amplitude and duration of amacrine cell EPSCs. To measure the output of transient amacrine cells, we recorded glycinergic inhibitory postsynaptic currents (IPSCs) from ganglion cells, and found that these were also enhanced by cyclothiazide. Thus, prolongation of amacrine cell AMPA receptor activation enhanced amacrine cell output. Current responses elicited by puffing glycine onto ganglion cell dendrites were not affected by cyclothiazide, indicating that the enhancement of glycinergic IPSCs was not due to a direct effect on glycine receptors. These data suggest that rapid AMPA receptor desensitization and/or deactivation limits glycinergic amacrine cell excitation and the resulting inhibitory synaptic output.

Adult Japanese quail have an endogenous circadian clock located in their eyes that has been shown to regulate melatonin biosynthesis. We investigated if a circadian oscillator is present in cultures of dispersed embryonic quail retina. Melatonin release in retinal cell culture is modulated by the light cycle, indicating that there are functional photoreceptors in culture. However, when cultures were placed in constant darkness no rhythm of melatonin was observed, indicating that at this period of development the circadian oscillator does not influence melatonin release. To explore further the question of whether a circadian oscillator is present in embryonic cell culture, we examined expression of iodopsin, the red visual pigment. Iodopsin mRNA is expressed in a circadian rhythm with peak levels occurring late in the afternoon (ZT 9). Analysis indicates that the clock influence is at the level of gene transcription. These results suggest that a clock is not “hooked up” to melatonin release embryonically or that a different oscillator regulates photopigment expression versus melatonin release.

The patchy pattern of retrograde labeling produced by injections of anatomical tracers into the lateral suprasylvian (LS) visual area was compared to the cytochrome oxidase (CO) blobs in cat visual cortex. Following large injections of anatomical tracers in LS, retrograde labeling formed an irregular lattice of patches with a spacing of slightly less than 1 mm in area 17, and slightly greater than 1 mm in area 18. By comparing labeling in alternate serial sections, patches of LS-projecting cells in both areas were found to align with CO blobs. The conclusion of alignment between CO blob columns and patches of LS-projecting cells was confirmed by a quantitative analysis which showed a significant correlation between the local density of LS-projecting cells in reconstructions of charted cells and the intensity of CO staining in the CO-reacted sections. As for areas 17 and 18, labeling in other afferent areas of LS was also patchy with a spacing on the order of 1 mm except for area 19 where we found patches of LS-projecting cells with a larger spacing, roughly 2 mm. No matching fluctuations in CO density could be discerned in area 19, however. In conjunction with recent evidence that CO blob columns in cats receive strong input from Y-cells of the lateral geniculate nucleus (Boyd & Matsubara, 1996; Shoham, et al., 1996), these data support the hypothesis (Shipp & Grant, 1991) that the patches of LS-projecting cells correspond to Y-cell input columns. As a relationship between the CO architecture and certain classes of efferent cells has previously been shown in primates, these findings show new similarities between CO blobs in different mammalian species.

Population-based methods were used to study labeled retinal ganglion cells from the cane toad Bufo marinus and the treefrog Litoria moorei, two visually competent bufonoid neobatrachians with contrasting habitats. In both, cells with large somata and thick dendrites formed distinct types with independent mosaics. The αa, αab, and αc mosaics of Bufo in all major respects resembled those of ranids, studied previously, and could be provisionally matched to the same functional classes. As in other frogs, some αa cells were displaced and many α-cells of all types were asymmetric, but within each type all variants belonged to one mosaic. Nearest-neighbor analyses and spatial correlograms confirmed that all three mosaics were regular and independent. In Litoria, monostratified αa cells were not found. Instead, two bistratified types were present, distinguished individually by soma size and dendritic caliber and collectively by membership of independent mosaics: the larger (∼0.8% of all ganglion cells) was termed α1ab and the smaller (∼2.2%) α2ab. An αc cell type was also present, although too inconstantly labeled for mosaic analysis. Nearest-neighbor analyses and spatial correlograms confirmed that the two αab mosaics were regular and independent. Densities, proportions, soma sizes, and mosaic statistics are tabulated for each species. The emergence of a consensus pattern of α-cell types in fishes and frogs, from which this treefrog partly diverges, offers new possibilities for studying correlations between function, phylogeny, ecology, and neuronal form.

Research has shown that adult zebrafish have a complex visual system, with two possible opponent mechanisms. Anatomically, zebrafish retina develops in a sequential manner and is immature at hatching. The purpose of the present study was to assess zebrafish retinal development using the electroretinogram (ERG). ERG responses to visual stimuli were obtained from 4–5, 6–8, 13–15, and 21–24 days postfertilization (dpf) zebrafish. Individual waveforms were assessed and compared across the four age groups. Spectral-sensitivity functions were calculated for the a- and b-wave components of the ERG response. Results showed that the ERG waveforms and spectral-sensitivity functions varied with age. While the 21–24 dpf subjects had an ERG waveform that was similar to that of adults, the younger subjects did not. Although there were modest differences in the a-wave spectral sensitivity, substantial differences were found in the b-wave spectral sensitivities across the ages. There was a consistent strong response to ultraviolet wavelengths, while across the remaining parts of the spectrum, there was a gradual increase in sensitivity with age. Also, the 21–24 dpf subjects appear to have adult-like U- and S-cone functions, but were missing the L-M and the M-S opponent mechanisms found in the adult. These results support the findings of the anatomical studies and demonstrate that the zebrafish is a useful model for examining the development of retinal function.

We analyzed postsynaptic potentials and dye-labeled morphology of tectal neurons responding to electrical stimulation of the optic nerve and of the nucleus isthmi in a reptile, Gekko gekko, in order to compare with previously reported interactions between the optic tectum and the nucleus isthmi in amphibians and birds. The results indicate that isthmic stimulation exerts inhibitory and excitatory actions on tectal cells, similar to dual isthmotectal actions in amphibians. It appears that dual actions of the isthmotectal pathway in amphibians and reptiles are shared by two subdivisions of the nucleus isthmi in birds. The morphology of tectal cells responding to isthmic stimulation is generally similar to that of tectoisthmic projecting neurons, but they differ particularly in that some tectoisthmic cells bear numerous varicosities whereas cells receiving isthmic afferents do not. Thus, it is likely that at least some tectoisthmic cells may not be in the population of tectal cells that can be affected by isthmic stimulation. Forty-four percent of injections resulted in dye-coupled labeling, suggesting extensive electrical connections between tectal cells in reptiles.

Local electroretinograms (ERGs) were recorded in the parietal eye of Xantusia vigilis. The responses to monochromatic light under dark- and light-adapted conditions were studied. We found that two antagonistic chromatic mechanisms dominate the overall response. With the electrode tip in the lumen of the eye, light stimulation under dark-adapted conditions evoked responses of negative polarity with maximum sensitivity to green light. Intense green background illumination saturated the green-sensitive mechanism, and superposition of a blue stimulus then elicited responses of opposite polarity, driving the potentials back toward the dark resting level. The spectral sensitivities of the two chromatic mechanisms were determined using chromatic adaptation. The lower threshold, green-sensitive mechanism has a maximum sensitivity at 495 nm while the antagonistic mechanism, with its maximal spectral sensitivity at 430 nm, is at least 2 log units less sensitive. The polarity of the ERG recording inverts as the electrode traverses the photoreceptor layer, suggesting that the photoreceptors are the major source of the ERG. This result was confirmed with intracellular recordings from photoreceptors, glial, and lens cells. The glial and lens cells of the parietal eye respond to local changes in [K+]o. Intracellular recordings of the responses of these cells to light stimuli follow time courses similar to changes in extracellular potassium concentrations measured with K+-specific electrodes. These results suggest that the glial and lens cell membranes are highly permeable to potassium and, therefore, the electrical responses of these cells are evoked by changes in [K+]o elicited by light stimulation of the photoreceptors. Nevertheless, the major component of the parietal eye ERG is the photoreceptor signal. A circuit model of the ERG sources is presented.

Vertebrate retinal photoreceptors periodically shed membrane from their outer segment distal tips; this material is phagocytosed and degraded by the retinal pigmented epithelium. Both a circadian oscillator and the daily light–dark cycle affect disk shedding, and the effects of both may be mediated by melatonin. To clarify melatonin's role in this process, we asked whether endogenous melatonin is required for rhythmic disk shedding in mouse retina. We analyzed disk shedding in two mouse strains: C3H, which produce melatonin in retina and pineal under the control of circadian oscillators, and C57BL/6, which do not produce melatonin. In cyclic light, both strains exhibited a robust cycle of disk phagosome content in the pigmented epithelium. Peak shedding occurred just after dawn, and trough levels occurred during the middle of the dark phase. In constant darkness, mice exhibited circadian rhythms of locomotor activity, the characteristics of which were similar between strains. Both strains also exhibited rhythmic disk shedding in constant darkness, although amplitudes of the rhythms were damped. Exogenous melatonin delivered once per day failed to reestablish high-amplitude cyclic shedding in mice held in constant darkness. Our results show that, while disk shedding in cyclic light is robustly rhythmic, neither rhythmic production of melatonin nor the circadian oscillator responsible for rhythmic locomotor activity is sufficient to drive high-amplitude rhythmic shedding in constant darkness. More importantly, melatonin is required neither for cyclic changes in the rate of disk shedding in cyclic light, nor for the circadian rhythm of disk shedding in constant darkness.

Human symmetry detection in dense patterns exhibits a spatial integration range that becomes narrower with distance of the symmetry axis from the fovea. This narrowing violates the general properties of eccentricity that have been found for all previous visual cortical areas, tasks, and assessment techniques. This reverse eccentricity scaling may, in conjunction with the long-range matching properties for symmetry described in Tyler and Hardage (1996), imply that symmetry is processed by a specialized cortical area with non-retinotopic neural architecture.

Ascorbic acid (AA), a neuromodulator in the vertebrate CNS, is released from glutamatergic neurons in exchange with glutamate uptake and, in turn, modulates the release of both glutamate and dopamine. We have reported that voltage-gated K+ currents (IK(V)) in ON-mixed rod/cone bipolar cells (Mb) were suppressed 60% by 100–200 μM AA when added to an ascorbate-free solution. However, as the in vivo [AA]o in retina is about 200 μM, we studied the effects of changes in [AA]o on IK(V) when [AA]o was varied around a baseline concentration of 200 μM. Whole-cell currents were recorded with patch-clamp methods from goldfish Mb cells in retinal slices, bathed in a solution containing 200 μM AA. We found that (1) IK(V) was enhanced (180 ± 36%, n = 9) by increases of [AA]o less than 40 μM with an average latency of 8 min. (2) However, IK(V) was suppressed without an appreciable latent period by two conditions: increases more than 40 μM [AA]o and decreases by any amount greater than 10 μM. (3) Effects of Δ[AA]o on IK(V) were blocked by a D1 dopamine receptor antagonist, SCH 23390, but not by a D2 receptor antagonist, spiperone. Increased concentrations of a D1 agonist (SKF 38390) and dopamine had similar concentration-dependent effects on IK(V) as did AA, even in the presence of 200 μM ascorbate. Ascorbate has complicated concentration-dependent effects on IK(V) of Mb cells in vitro that were mediated by D1 dopamine receptors, suggesting that dopamine and ascorbate may be involved reciprocally in modulating IK(V), with consequences on the transmission of rod signals to the inner retina.

The Cavalieri method and the optical fractionator were employed to estimate the volume and neuron numbers, respectively, of the dorsal lateral geniculate nucleus (dLGN) in seven adult male pigtail monkeys (Macaca nemestrina). Unbiased estimates were selectively obtained for the parvocellular (P), magnocellular (M), and interlaminar plus superficial (I + S) layers of the nucleus. The dLGN had a mean volume of 56.5 mm3, and contained on average 1.79 million neurons. The P layers contributed 64% of the volume and 83% of the neurons in the dLGN; the corresponding proportions for the other layers were 13% and 9% (M), and 23% and 8% (I + S). Interindividual variability was large for neuron counts, which varied within a two-fold range, and lower for volume estimates. Since no published data are available for the pigtail dLGN, the present results are compared with quantitative studies of the dLGN in other macaque species, placing special emphasis in the discussion of the methodologies used.

In previous work, we have shown that sudden image displacements well outside the classical receptive field modulate the visual sensitivity of LGN relay cells. Here we report the effect of image displacements on the response versus contrast function. The stimuli consisted of a central spot of optimal size and polarity (contrast range: 3–98%), flashed alone or in the presence of a peripheral annulus (radii: 5–15 deg) containing a low spatial-frequency grating displaced at saccade-like velocities (shift). The most consistent effect of the shift on the response to a central spot was to reduce the responsiveness of Y relay cells and, to a lesser extent, of X relay cells. The reduction in responsiveness was primarily a divisive rather than a subtractive effect and could be modelled by assuming that a greater contrast was required to produce a given excitatory response. In the absence of a central spot, remote motion had inhibitory effects on the firing rates of the majority of relay cells, but its effect on retinal ganglion cells was mainly excitatory. When the shifting grating covered the classical receptive field and its periphery, facilitatory effects or suppressive effects, depending on the spatial phase of the pattern, were observed in both retinal and geniculate cells. Remote motion strongly suppresses the responsiveness of relay cells to stimuli within the classical receptive field. This suppressive effect involves intrageniculate processing and is primarily associated with a reduction in contrast gain. It is likely that shift suppression contributes to the loss of visual sensitivity observed in saccadic suppression.

Glycine is a major inhibitory neurotransmitter in the mammalian retina and has been shown to influence the responses of ganglion cells. Midget and parasol ganglion cells serve distinct physiological roles in the primate retina and show differences in their response characteristics to light stimuli. In the present study, we addressed the question of whether the expression of glycine receptors differs in midget and parasol ganglion cells. Ganglion cells in the retinae of marmoset and macaque monkeys were injected with Neurobiotin in a live in vitro retinal whole-mount preparation. Retinal pieces were then processed with an antibody against the α1 subunit of the glycine receptor. Strong punctate immunoreactivity indicative of synaptic localization is present in the ON and OFF sublamina of the inner plexiform layer. Many of the immunoreactive puncta coincide with the dendrites of both midget and parasol ganglion cells. Immunoreactive puncta are present on distal and proximal dendrites of ON and OFF cells. These results suggest that ON and OFF midget and parasol cells do not differ with respect to the distribution of the α1 subunit of the glycine receptor.

The physiological function of GABA transporters and GABA receptors in retinal horizontal cells (HCs) under dark- and light-adapted conditions were studied by whole-cell voltage clamp and intracellular recording techniques in retinal slices and whole-mounted isolated retinas of the larval tiger salamander. Puff application of GABA in picrotoxin elicited a NO-711 (a potent GABA transporter blocker)-sensitive inward current that did not exhibit a reversal potential in the physiological range, consistent with the idea that these HCs contain electrogenic GABA transporters. Application of GABA in NO-711 elicited a chloride current in HCs; about half of the current was suppressed by bicuculline or I4AA (a GABAC receptor antagonist), and the remaining half was suppressed by bicuculline + I4AA or picrotoxin. In whole-mount retinas, NO-711, bicuculline, I4AA, or picrotoxin hyperpolarized the HCs and enhanced the light responses under dark-adapted conditions, and blocked the time-dependent recovery of HC membrane potential and light responses during background illumination. Based on the parallel conductance model, GABA released in darkness mediates a chloride conductance about three times greater than the leak conductance or the glutamate-gated cation conductance. About half of this chloride conductance is mediated by GABAA receptors, and the other half is mediated by GABAC receptors. These results suggest that GABA released from HCs through the NO-711-sensitive GABA transporters activates GABAA and GABAC receptors, resulting in chloride conductance increase which leads to a HC depolarization and reduction of the light response. Additionally, GABA transporters also mediate GABA release in background light that is responsible for the recovery of HC membrane potential and light responses.

The following is a correction for the article “Spectral sensitivity of macaque monkeys measured with ERG flicker photometry,” by Gerald H. Jacobs and Jess F. Deegan II, that appeared in Visual Neuroscience, 1997, 14, 921–928. As a result of a transcription error the spectral sensitivity values included in Table 1 were incorrect.