In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2+-dependent adenylate cyclase.

The synaptic organization of GABA-immunoreactive (GABA-IR) amacrine cells in the inner plexiform layer (IPL) of salamander retina was studied with the use of postembedding immuno-electron microscopy. A total of 457 GABA-IR amacrine synapses, with identified postsynaptic elements, were analyzed on photomontages of electron micrographs covering 3,618 μm2 of the IPL. GABA-IR amacrine synapses were distributed throughout the IPL, with a small peak at the proximal margin of sublamina a. The majority of the output targets (81%) were GABA(−) neurons. Most of the contacts were simple synapses with one postsynaptic element identified as a process of an amacrine cell (55%), bipolar cell (19%) or ganglion cell (26%), and serial synapses were very rare. Of the 89 postsynaptic bipolar terminals, 63% participated in a reciprocal feedback synapse with the same presynaptic GABA-IR amacrine profile. There appeared to be no preference between GABA-IR amacrine contacts with rod- or cone-dominated bipolar cells (9.1% vs. 8.9%) or in the total number of amacrine synapses in sublaminas a and b (52% vs. 47%). The preponderance of amacrine cell input to bipolar cells in the OFF layer was derived from GABA-IR cells. These findings provide ultrastructural support to the existing physiological studies regarding the functional roles of the GABAergic amacrine cells in this species. Our results have added to the data base demonstrating that, in contrast to mammals, GABA-IR amacrine cells in amphibians and other nonmammals contact other amacrine cells more frequently, suggesting greater involvement of GABAergic amacrine cells in modulating lateral inhibitory pathways.

Photopigment optical density (OD) of middle-(M) and long-(L) wavelength-sensitive cones was determined to evaluate the hypothesis that reductions in the amount of photopigment are responsible for age-dependent sensitivity losses of the human cone pathways. Flicker thresholds were measured at the peak and tail of the photoreceptor's absorption spectrum as a function of the intensity of a bleaching background. Photopigment OD was measured at 0 (fovea), 2, 4, and 8 deg in the temporal retina by use of a 0.3-deg-diameter test spot. Seventy-two genetically characterized dichromats were studied so that the L- and M-cones could be analyzed separately. Subjects included 28 protanopes with M- but no L-cones and 44 deuteranopes with L- but no M-cones (all male, age range 12–29 and 55–83 years). Previous methods have not provided estimates of photopigment OD for separate cone classes in the foveola. In this study, it was found that foveolar cones are remarkably efficient, absorbing 78% of the available photons (OD = 0.65). Photopigment OD decreased exponentially with retinal eccentricity independently of age and cone type. Paradoxically, the OD of perifoveal cones increased significantly with age. Over the 70-year age range of our participants, the perifoveal M- and L-cones showed a 14% increase in capacity to absorb photons despite a 30% decrease in visual sensitivity over the same period.

Isolated dopaminergic amacrine (DA) cells in mouse retina fire rhythmic, spontaneous action potentials and respond to depolarizing current with trains of low-frequency action potentials. To investigate the roles of voltage-gated ion channels in these processes, the transient A-type K+ current (IK,A) and Ca2+ current (ICa) in isolated mouse DA cells were analyzed by voltage clamp. The IK,A activated at −60 mV and inactivated rapidly. ICa activated at around −30 mV and reached a peak at 10 mV without apparent inactivation. We also extended our previous computational model of the mouse DA cell to include the new electrophysiological data. The model consisted of a membrane capacitance in parallel with eight currents: Na+ transient (INa,T), Na+ persistent (INa,P), delayed rectifier potassium (IKdr), IK,A, calcium-dependent potassium (IK,Ca), L-type Ca2+ ICa, hyperpolarization-activated cation current (Ih), and a leak current (IL). Hodgkin-Huxley type equations were used to define the voltage- and time-dependent activation and inactivation. The simulations were implemented using the neurosimulator SNNAP. The model DA cell was spontaneously active from a wide range of initial membrane potentials. The spontaneous action potentials reached 35 mV at the peak and hyperpolarized to −76 mV between spikes. The spontaneous firing frequency in the model was 6 Hz. The model DA cell responded to prolonged depolarizing current injection by increasing its spiking frequency and eventually reaching a depolarization block at membrane potentials greater than −10 mV. The most important current for determining the firing rate was IK,A. When the amplitude of IK,A was decreased, the firing rate increased. ICa and IK,Ca also affected the width of action potentials but had only minor effects on the firing rate. Ih affected the firing rate slightly but did not change the waveform of the action potentials.

The development of spatial vision is relatively well documented in human and nonhuman primates. However, little is known about the development of sensitivity to motion. We measured the development of sensitivity to direction of motion, and the relationship between motion and contrast sensitivity in macaque monkeys as a function of age. Monkeys (Macaca nemestrina, aged between 10 days and 3 years) discriminated direction of motion in random-dot kinematograms. The youngest monkeys showed directionally selective orienting and the ability to integrate motion signals at large dot displacements and fast speeds. With age, coherence sensitivity improved for all spatial and temporal dot displacements tested. The temporal interval between the dots was far less important than the spatial offset in determining the animals' performance at all but the youngest ages. Motion sensitivity improved well beyond the end of the first postnatal year, when mid-spatial-frequency contrast sensitivity reached asymptote, and continued for at least 3 years. Sensitivity to contrast at high spatial frequencies also continued to develop beyond the end of the first year. We conclude that the development of motion sensitivity depends on mechanisms beyond the low-level filters presumed to limit acuity and contrast sensitivity, and most likely reflects the function of extrastriate visual areas.

Acetylcholine, acting through nicotinic acetylcholine receptors, mediates the response properties of many ganglion cells in the rabbit retina, including those that are directionally selective (DS; Ariel & Daw, 1982a,b). For example, Grzywacz et al. (1998) showed that cholinergic input is necessary for DS responses to drifting gratings, a form of textured stimulus. However, the identities and locations of the neuronal acetylcholine receptor (nAChR) subtypes that mediate this input are not clear (Keyser et al., 2000). We investigated the role of methyllycaconitine-sensitive, α7-like nAChRs in mediating DS responses to textured stimuli and apparent motion. We recorded extracellularly from On–Off DS ganglion cells in rabbit retina using everted eyecup preparations. Our data provide evidence that MLA-sensitive nAChRs are involved in mediating directionally selective responses to apparent motion and to a variety of complex, textured stimuli such as drifting square-wave gratings, transparent motion, and second-order motion.

We have used wild-type mice and mice possessing defects in specific retinal circuits in order to more clearly define functional circuits of the inner retina. The retina of the nob mouse lacks communication between photoreceptors and depolarizing bipolar cells (DBCs). Thus, all light driven activity in the nob mouse is mediated via remaining hyperpolarizing bipolar cell (HBC) circuits. Transducin null (Trα−/−) mice lack rod photoreceptor activity and thus remaining retinal circuits are solely generated via cone photoreceptor activity. Activation in inner retinal circuits in each of these mice was identified by monitoring light-induced expression of an immediate early gene, c-fos. The number of cells expressing c-fos in the inner retina was dependent upon stimulus intensity and was altered in a systematic fashion in mice with known retinal mutations. To determine whether c-fos is activated via circuits other than photoreceptors in the outer retina, we examined c-fos expression in tulp1−/− mice that lack photoreceptors in the outer retina; these mice showed virtually no c-fos activity following light exposure. Double-labeling immunohistochemical studies were carried out to more clearly define the population of c-fos expressing amacrine cells. Our results indicate that c-fos may be used to map functional circuits in the retina.

Contrast encoding for sinusoidal modulations of luminance contrast was investigated by intracellular recording in the intact salamander retina. In what appears to be the first study of this kind for vertebrate bipolar cells, responses of the central receptive-field mechanism of cone-driven cells to modulation of 3 Hz were analyzed quantitatively via both signal averaging and a Fast Fourier Transform (FFT) while the retina was light adapted to 20 cd/m2. Depolarizing and hyperpolarizing bipolar cells showed very similar encoding. Both responded with sinusoidal waveforms whose amplitude varied linearly with modulation depths ranging up to 7–8%. The slope of the modulation/response curve was very steep in this range. Thus, the contrast gain was high, reaching values of 6–7, and the half-maximal response was achieved at modulations of 9% or less. At modulations above ∼15%, the responses typically showed strong compressive nonlinearity and the waveform was increasingly distorted. At maximum modulation, the higher harmonics of the FFT constituted about 30% of the amplitude of the fundamental. Measurements were also made for cones and horizontal cells. Both cell types showed predominantly linear responses and low contrast gain, in marked contrast to bipolar cells. These results suggest that the high contrast gain and strong nonlinearity of bipolar cells largely arise postsynaptic to cone transmitter release. Further experiments were performed to compare responses to contrast steps versus those to sinusoidal modulation. In the linear range, we show that the contrast gains of cones and horizontal cells are low and virtually identical for both steps and sinusoidal modulations. In bipolar cells, on the other hand, the contrast gain is about two times greater for steps than that for the 3-Hz sine waves. These results suggest that mechanisms intrinsic to bipolar cells act like a high-pass filter with a short time constant to selectively emphasize contrast transients over slower changes in contrast.

Using a novel suite of computer-generated visual stimuli that mimicked components of optic flow, the visual responses of the tropical land crab, Cardisoma guanhumi, were investigated. We show that crabs are normally successful in distinguishing the rotational and translational components of the optic flow field, showing strong optokinetic responses to the former but not the latter. This ability was not dependant on the orientation of the crab, occurring both in “forwards-walking” and “sideways-walking” configurations. However, under conditions of low overall light intensity and/or low object/background contrast, the separation mechanism shows partial failure causing the crab to generate compensatory eye movements to translation, particularly in response to low-frequency (low-velocity) stimuli. Using this discovery, we then tested the ability of crabs to separate rotational and translational components in a combined rotation/translation flow field under different conditions. We demonstrate that, while crabs can successfully separate such a combined flow field under normal circumstances, showing compensatory eye movements only to the rotational component, they are unable to make this separation under conditions of low overall light intensity and low object/background contrast. Here, the responses to both flow-field components show summation when they are in phase, but, surprisingly, there is little reduction in the amplitude of responses to rotation when the translational component is in antiphase. Our results demonstrate that the crab's visual system finds separation of flow-field components a harder task than detection of movement, since the former shows partial failure at light intensities and/or object/background contrasts at which movement of the world around the crab is still generating high-gain optokinetic responses.

Although a number of global mechanisms have been proposed over the years that explain how crabs might separate the rotational and translational components of their optic flow field, there has been no evidence to date that local mechanisms such as motion parallax are used in this separation. We describe here a study that takes advantage of a recently developed suite of computer-generated visual stimuli that creates a three-dimensional world surrounding the crab in which we can simulate translational and rotational optic flow. We show that, while motion parallax is not the only mechanism used in flow-field separation, it does play a role in the recognition of translational optic flow fields in that, under conditions of low overall light intensity and low contrast ratio when crabs find the distinction between rotation and translation harder, smaller eye movements occur in response to translation when motion parallax cues are present than when they are absent. Thus, motion parallax is one of many cues that crabs use to separate rotational and translational optic flow by showing compensatory eye movements to only the former.

Light-evoked currents were recorded from rod bipolar cells in a dark-adapted mouse retinal slice preparation. Low-intensity light steps evoked a sustained inward current. Saturating light steps evoked an inward current with an initial peak that inactivated, with a time constant of about 60–70 ms, to a steady plateau level that was maintained for the duration of the step. The inactivation was strongest at hyperpolarized potentials, and absent at positive potentials. Inactivation was mediated by an increase in the intracellular calcium concentration, as it was abolished in cells dialyzed with 10 mM BAPTA, but was present in cells dialyzed with 1 mM EGTA. Moreover, responses to brief flashes of light were broader in the presence of intracellular BAPTA indicating that the calcium feedback actively shapes the time course of the light responses. Recovery from inactivation observed for paired-pulse stimuli occurred with a time constant of about 375 ms. Calcium feedback could act to increase the dynamic range of the bipolar cells, and to reduce variability in the amplitude and duration of the single-photon signal. This may be important for nonlinear processing at downstream sites of convergence from rod bipolar cells to AII amacrine cells. A model in which intracellular calcium rapidly binds to the light-gated channel and reduces the conductance can account for the results.

Significant variation in absolute dark-adapted thresholds is observed both within and between strains of mice with differing ocular pigmentation levels. Differences in threshold within a single strain are related to the Williams' photostasis effect, that is, photoreceptor rhodopsin levels are dependent upon ambient lighting conditions. To examine threshold differences among strains, we equalized rhodopsin levels by maintaining albino mice (c2J/c2J) at 2 × 10−4 cd/m2 (dim light) and black mice at 2 × 102 cd/m2 (bright light). This resulted in ocular rhodopsin levels for albino mice (albino—dim) of 494 ± 11 pmoles/eye and rhodopsin levels for black mice (black—bright) of 506 ± 25 pmoles/eye. For comparison, rhodopsin levels in black mice maintained in dim light are 586 ± 46 pmoles/eye and 217 ± 46 pmoles/eye in albino mice maintained in bright light. We found similar dark-adapted thresholds (6.38 log cd/m2vs. 6.47 log cd/m2)) in albino and black mice with equivalent rhodopsin determined with a water maze test. This suggests that dark-adapted thresholds are directly related to rhodopsin levels regardless of the level of ocular melanin. The number of photoreceptors, photoreceptor layer thickness, and outer segment length did not differ significantly between albino (dark) and black mice (bright). These results demonstrate that the visual sensitivity defect found in hypopigmented animals is secondary to abnormal rhodopsin regulation and that hypopigmented animals have either an improper input to the photostasis mechanism or that the photostasis mechanism is defective.

Mammalian retinas receive input from the posterior hypothalamus, and the neurotransmitter in this pathway is histamine. To determine whether histamine influences ganglion cells, we analyzed the effects of histamine on their maintained and light-evoked activity in vitro. In monkeys, histamine increased the maintained firing rate in 42% of ganglion cells, decreased it in 38%, and had no effect in 20%. When histamine and the HR3 agonist, methylhistamine, were applied to the same cells in succession, their effects were sometimes different, a finding suggesting that at least one other histamine receptor is present. In addition, the responses of some ganglion cells to full-field light stimuli were decreased by histamine and methylhistamine. In rats, the effects of histamine were somewhat different. Histamine increased the maintained firing rate of 82% of ganglion cells. Methylhistamine and the HR2 agonist, dimaprit, had the same effects as histamine. In some cells, histamine increased the light responses, but in others it decreased them. Histamine had no effect on ganglion cells in either species when synaptic transmission was blocked by low Ca2+/high Mg2+ Ames medium. Thus, the major effects of histamine were on the maintained activity of retinal ganglion cells. In both rats and monkeys, 80% or more of the ganglion cells were affected by histamine, and these responses were mediated by at least two of the histamine receptor subtypes.

Numerous reports have concluded that zebrafish (Danio rerio) possesses A1-based visual pigments in their rod and cone photoreceptors. In the present study, we investigated the possibility that zebrafish have a paired visual pigment system. We measured the spectral absorption characteristics of photoreceptors from zebrafish maintained in different temperature regimes and those treated with exogenous thyroid hormone using CCD-based microspectrophotometry. Rods from fish housed at 15°C and 28°C were not significantly different, having λmax values of 503 ± 5 nm (n = 106) and 504 ± 6 nm (n = 88), respectively. Thyroid hormone treatment (held at 28°C), however, significantly shifted the λmax of rods from 503 ± 5 nm (n = 194) to 527 ± 8 nm (n = 212). Cone photoreceptors in fish housed at 28°C (without thyroid hormone treatment) had λmax values of 361 ± 3 nm (n = 2) for ultraviolet-, 411 ± 5 nm (n = 18) for short-, 482 ± 6 nm (n = 9) for medium-, and 565 ± 10 nm (n = 14) for long-wavelength sensitive cones. Thyroid hormone treatment of fish held at 28°C significantly shifted the λmax of long-wavelength sensitive cones to 613 ± 11 nm (n = 20), substantially beyond that of the λmax of the longest possible A1-based visual pigment (∼580 nm). Thyroid hormone treatment produced smaller shifts of λmax in other cone types and increased the half-band width. All shifts in photoreceptor λmax values resulting from thyroid hormone treatment matched predictions for an A1- to A2-based visual pigment system. We therefore conclude that zebrafish possess a rhodopsin–porphyropsin interchange system that functions to spectrally tune rod and cone photoreceptors. We believe that these observations should be carefully considered during analysis of zebrafish spectral sensitivity.

The Publishers for Visual Neuroscience would like to say we are sorry for the errors that appeared on the Table of Contents for Volume 21, Number 5.