Rod vision: pathways and processing in the mammalian retina

Approach sensitivity in the retina processed by a multifunctional neural circuit

The detection of approaching objects, such as looming predators, is necessary for survival. Which neurons and circuits mediate this function? We combined genetic labeling of cell types, two-photon microscopy, electrophysiology and theoretical modeling to address this question. We identify an approach-sensitive ganglion cell type in the mouse retina, resolve elements of its afferent neural circuit, and describe how these confer approach sensitivity on the ganglion cell. The circuit's essential building block is a rapid inhibitory pathway: it selectively suppresses responses to non-approaching objects. This rapid inhibitory pathway, which includes AII amacrine cells connected to bipolar cells through electrical synapses, was previously described in the context of night-time vision. In the daytime conditions of our experiments, the same pathway conveys signals in the reverse direction. The dual use of a neural pathway in different physiological conditions illustrates the efficiency with which several functions can be accommodated in a single circuit.

The significance of neuronal and glial cell changes in the rat retina during oxygen-induced retinopathy

Retinopathy of prematurity is a devastating vascular disease of premature infants. A number of studies indicate that retinal function is affected in this disease. Using the rat model of oxygen-induced retinopathy, it is possible to explore more fully the complex relationship between neuronal, glial and vascular pathology in this condition. This review examines the structural and functional changes that occur in the rat retina following oxygen-induced retinopathy. We highlight that vascular pathology in rats is characterized by aberrant growth of blood vessels into the vitreous at the expense of blood vessel growth into the body of the retina. Moreover, amino acid neurochemistry, a tool for examining neuronal changes in a spatially complete manner reveals widespread changes in amacrine and bipolar cells. In addition, neurochemical anomalies within inner retinal neurons are highly correlated with the absence of retinal vessels. The key cell types that link blood flow with neuronal function are macroglia. Macroglia cells, which in the retina include astrocytes and Müller cells, are affected by oxygen-induced retinopathy. Astrocyte loss occurs in the peripheral retina, while Müller cells show signs of reactive gliosis that is highly localized to regions that are devoid of intraretinal blood vessels. Finally, we propose that treatments, such as blockade of the renin-angiotensin system, that not only targets pathological angiogenesis, but that also promotes re-vascularization of the retina are likely to prove important in the treatment of those with retinopathy of prematurity.

Rod bipolar cells in the retina of the capuchin monkey (Cebus apella): Characterization and distribution

Rod bipolar cells in Cebus apella monkey retina were identified by an antibody against the alpha isoform of protein kinase C (PKCα), which has been shown to selectively identify rod bipolars in two other primates and various mammals. Vertical sections were used to confirm the identity of these cells by their characteristic morphology of dendrites and axons. Their topographic distribution was assessed in horizontal sections; counts taken along the dorsal, ventral, nasal, and temporal quadrants. The density of rod bipolar cells increased from 500 to 2900 cells/mm2 at 1 mm from the fovea to reach a peak of 10,000–12,000 cells/mm2 at 4 mm, approximately 5 deg of eccentricity, and then gradually decreased toward retinal periphery to values of 5000 cells/mm2 or less. Rod to rod bipolar density ratio remained between 10 and 20 across most of the retinal extension. The number of rod bipolar cells per retina was 6,360,000 ± 387,433 (mean ± s.d., n = 6). The anti-PKCα antibody has shown to be a good marker of rod bipolar cells of Cebus, and the cell distribution is similar to that described for other primates. In spite of the difference in the central retina, the density variation of rod bipolar cells in the Cebus and Macaca as well as the convergence from rod to rod bipolar cells are generally similar, suggesting that both retinae stabilize similar sensitivity (as measured by rod density) and convergence.

The composition of the inner nuclear layer of the cat retina

The cellular composition of the inner nuclear layer (INL) is largely conserved among mammals. Studies of rabbit, monkey, and mouse retinas have shown that bipolar, amacrine, Müller, and horizontal cells make up constant fractions of the INL (42, 35, 20, and 3%, respectively); these proportions remain relatively constant at all retinal eccentricities. The purpose of our study was to test whether the organization of cat retina is similar to that of other mammalian retinas. Fixed retinas were embedded in plastic, serially sectioned at a thickness of 1 microm, stained, and imaged at high power in the light microscope. Bipolar, amacrine, Müller, and horizontal cells were classified and counted according to established morphological criteria. Additional sets of sections were processed for protein kinase C and calretinin immunoreactivity to determine the relative fraction of rod bipolar and AII amacrine cells. Our results show that the organization of INL in the cat retina contains species-specific alterations in the composition of the INL tied to the large fraction of rod photoreceptors. Compared with other mammalian retinas, cat retinas show an expansion of the rod pathway with rod bipolar cells accounting for about 70% of all bipolar cells and AII cells accounting for nearly a quarter of all amacrine cells. Our results suggest that evolutionary pressures in cats over time have refined their retinal organization to suit its ecological niche.

Ectopic expression of multiple microbial rhodopsins restores ON and OFF light responses in retinas with photoreceptor degeneration

By expressing channel rhodopsin-2 (ChR2) in inner retinal neurons, previous studies have demonstrated restoration of ON responses in the retina after the death of rod and cone photoreceptors. In this study, we report that the expression of halorhodopsin (HaloR), a light-driven chloride pump, can effectively restore OFF responses in inner retinal neurons of mice with retinal degeneration. We show that HaloR-expressing retinal ganglion cells respond to light with rapid hyperpolarization and suppression of spike activity. After termination of the light stimulus, their membrane potential exhibits a rapid rebound overshoot with robust sustained or transient spike firing. Furthermore, we show that coexpression of ChR2/HaloR in retinal ganglion cells can produce ON, OFF, and even ON-OFF responses, depending on the wavelength of the light stimulus. Our results suggest that the expression of multiple microbial rhodopsins such as ChR2 and HaloR is a possible strategy to restore both ON and OFF light responses in the retina after the death of rod and cone photoreceptors.

High-sensitivity rod photoreceptor input to the blue-yellow color opponent pathway in macaque retina

Small bistratified cells (SBCs) in the primate retina carry a major blue-yellow opponent signal to the brain. Here we show that SBCs also carry signals from rod photoreceptors, with the same sign as S cone input. SBCs exhibited robust responses under low scotopic conditions (<0.01 P*/rod/s). Physiological and anatomical experiments indicated that this rod input arose from the AII amacrine cell mediated rod pathway. Rod and cone signals were both present in SBCs at mesopic light levels. We discuss three implications of these findings. First, more retinal circuits than previously thought may multiplex rod and cone signals, efficiently exploiting the limited number of optic nerve fibers. Second, signals from AII amacrine cells may diverge to most or all of the <20 RGC types in the peripheral primate retina. Third, rod input to SBCs may be the substrate for behavioral biases toward perception of blue at mesopic light levels.

The diverse functional roles and regulation of neuronal gap junctions in the retina

Electrical synaptic transmission through gap junctions underlies direct and rapid neuronal communication in the CNS. The diversity of functional roles that electrical synapses have is perhaps best exemplified in the vertebrate retina, in which gap junctions are formed by each of the five major neuron types. These junctions are dynamically regulated by ambient illumination and by circadian rhythms acting through light-activated neuromodulators such as dopamine and nitric oxide, which in turn activate intracellular signalling pathways in the retina.The networks formed by electrically coupled neurons are plastic and reconfigurable, and those in the retina are positioned to play key and diverse parts in the transmission and processing of visual information at every retinal level.

Parvalbumin-immunoreactive amacrine cells of macaque retina

A number of authors have observed amacrine cells containing high levels of immunoreactive parvalbumin in primate retinas. The experiments described here were designed to identify these cells morphologically, to determine their neurotransmitter, to record their light responses, and to describe the other cells that they contact. Macaque retinas were fixed in paraformaldehyde and labeled with antibodies to parvalbumin and one or two other markers, and this double- and triple-labeled material was analyzed by confocal microscopy. In their morphology and dendritic stratification patterns, the parvalbumin-positive cells closely resembled the knotty type 2 amacrine cells described using the Golgi method in macaques. They contained immunoreactive glycine transporter, but not immunoreactive gamma-aminobutyric acid, and therefore, they use glycine as their neurotransmitter. Their spatial density was relatively high, roughly half that of AII amacrine cells. They contacted lobular dendrites of AII cells, and they are expected to be presynaptic to AII cells based on earlier ultrastructural studies. They also made extensive contacts with axon terminals of OFF midget bipolar cells whose polarity cannot be predicted with certainty. A macaque amacrine cell of the same morphological type depolarized at the onset of increments in light intensity, and it was well coupled to other amacrine cells. Previously, we described amacrine cells like these that contacted OFF parasol ganglion cells and OFF starburst amacrine cells. Taken together, these findings suggest that one function of these amacrine cells is to inhibit the transmission of signals from rods to OFF bipolar cells via AII amacrine cells. Another function may be inhibition of the OFF pathway following increments in light intensity.

Uniform signal redundancy of parasol and midget ganglion cells in primate retina

The collective representation of visual space in high resolution visual pathways was explored by simultaneously measuring the receptive fields of hundreds of ON and OFF midget and parasol ganglion cells in isolated primate retina. As expected, the receptive fields of all four cell types formed regular mosaics uniformly tiling the visual scene. Surprisingly, comparison of all four mosaics revealed that the overlap of neighboring receptive fields was nearly identical, for both the excitatory center and inhibitory surround components of the receptive field. These observations contrast sharply with the large differences in the dendritic overlap between the parasol and midget cell populations, revealing a surprising lack of correspondence between the anatomical and functional architecture in the dominant circuits of the primate retina.

Chromatic perceptive field sizes measured at 10 degrees eccentricity along the horizontal and vertical meridians

The different hemifields in the retina are known to vary in photoreceptor density as well as in the number of photoreceptors converging onto one ganglion cell. The effect of these differences among the retinal hemifields at 10 degrees retinal eccentricity was investigated using a color-naming procedure to derive perceptive field sizes for the hue terms of blue, green, yellow, and red. Color-naming data were obtained under two conditions: (1) after a bleach condition, chosen to minimize rod contribution, and (2) after 30 min dark adaptation, chosen to maximize rod contribution. Perceptive field sizes measured in the bleach condition were consistent with degree of neural convergence of cones to ganglion cells across the retina rather than differences in cone density. Rod densities relative to cone densities correlated with the size of perceptive fields in the no-bleach condition, i.e., the greater the rod:cone ratio, the larger the perceptive field.

Physiology and morphology of color-opponent ganglion cells in a retina expressing a dual gradient of S and M opsins

Most mammals are dichromats, having short-wavelength-sensitive (S) and middle-wavelength-sensitive (M) cones. Smaller terrestrial species commonly express a dual gradient in opsins, with M opsin concentrated superiorly and declining inferiorly, and vice-versa for S opsin. Some ganglion cells in these retinas combine S- and M-cone inputs antagonistically, but no direct evidence links this physiological opponency with morphology; nor is it known whether opponency varies with the opsin gradients. By recording from >3000 ganglion cells in guinea pig, we identified small numbers of color-opponent cells. Chromatic properties were characterized by responses to monochromatic spots and/or spots produced by mixtures of two primary lights. Superior retina contained cells with strong S+/M- and M+/S- opponency, whereas inferior retina contained cells with weak opponency. In superior retina, the opponent cells had well-balanced M and S weights, while in inferior retina the weights were unbalanced, with the M weights being much weaker. The M and S components of opponent cell receptive fields had approximately the same diameter. Opponent cells injected with Lucifer yellow restricted their dendrites to the ON stratum of the inner plexiform layer and provided sufficient membrane area (approximately 2.1 x 10(4) microm(2)) to collect approximately 3.9 x 10(3) bipolar synapses. Two bistratified cells studied were nonopponent. The apparent decline in S/M opponency from superior to inferior retina is consistent with the dual gradient and a model where photoreceptor signals in both superior and inferior retina are processed by the same postreceptoral circuitry.

The retina in Parkinson's disease

As a more complete picture of the clinical phenotype of Parkinson's disease emerges, non-motor symptoms have become increasingly studied. Prominent among these non-motor phenomena are mood disturbance, cognitive decline and dementia, sleep disorders, hyposmia and autonomic failure. In addition, visual symptoms are common, ranging from complaints of dry eyes and reading difficulties, through to perceptual disturbances (feelings of presence and passage) and complex visual hallucinations. Such visual symptoms are a considerable cause of morbidity in Parkinson's disease and, with respect to visual hallucinations, are an important predictor of cognitive decline as well as institutional care and mortality. Evidence exists of visual dysfunction at several levels of the visual pathway in Parkinson's disease. This includes psychophysical, electrophysiological and morphological evidence of disruption of retinal structure and function, in addition to disorders of 'higher' (cortical) visual processing. In this review, we will draw together work from animal and human studies in an attempt to provide an insight into how Parkinson's disease affects the retina and how these changes might contribute to the visual symptoms experienced by patients.

Neuronal and glial cell expression of angiotensin II type 1 (AT1) and type 2 (AT2) receptors in the rat retina

The bio-active peptide, angiotensin II (Ang II), has been suggested to exert a neuromodulatory effect on inner retinal neurons. In this study, we examined the distribution of angiotensin receptors (ATRs) in the developing and mature rat retina and optic nerve using immunofluorescence immunocytochemistry. Double-labeling experiments were performed with established markers to identify different retinal cell populations. In adult retinae, ATRs were observed on neurons involved in "ON" pathways of neurotransmission. Angiotensin II type 1 receptors (AT(1)Rs) were expressed by a sub-population of "ON" cone bipolar cells that also labeled for G alpha(0) and islet-1. Extra-neuronal expression of AT(1)Rs was evident on retinal astrocytes, Müller cells and blood vessels. Immunoreactivity for the angiotensin II type 2 receptor (AT(2)R) was observed on conventional and displaced GABAergic amacrine cells. Co-localization studies showed that AT(2)R-expressing amacrine cells constituted at least two separate sub-populations. Cell counts revealed that all wide-field amacrine cells expressing protein kinase C-alpha were also AT(2)R-positive; a further subset of amacrine cells expressing AT(2)Rs and stratifying in sublamina "b" of the inner plexiform layer (IPL) was identified. Developmental expression of AT(1)Rs was dynamic, involving multiple inner neuronal classes. At postnatal day 8 (P8), AT(1)R immunoreactivity was observed on putative ganglion cells. The characteristic bipolar cell labeling observed in adults was not evident until P13. In contrast, AT(2)Rs were detected as early as P2 and localized specifically to amacrine cells from this age onward. These data provide further evidence for the potential role of angiotensin II in the modulation of retinal neurons and glia. The differential pattern of expression of these receptors across these cell types is similar to that observed in the brain and suggests that a similar functional role for Ang II may also exist within the retina.

Meclofenamic acid blocks electrical synapses of retinal AII amacrine and on-cone bipolar cells

Gap junction channels constitute specialized intercellular contacts that can serve as electrical synapses. In the rod pathway of the retina, electrical synapses between AII amacrine cells express connexin 36 (Cx36) and electrical synapses between AII amacrines and on-cone bipolar cells express Cx36 on the amacrine side and Cx36 or Cx45 on the bipolar side. For physiological investigations of the properties and functions of these electrical synapses, it is highly desirable to have access to potent pharmacological blockers with selective and reversible action. Here we use dual whole cell voltage-clamp recordings of pairs of AII amacrine cells and pairs of AII amacrine and on-cone bipolar cells in rat retinal slices to directly measure the junctional conductance (G(j)) between electrically coupled cells and to study the effect of the drug meclofenamic acid (MFA) on G(j). Consistent with previous tracer coupling studies, we found that MFA reversibly blocked the electrical synapse currents in a concentration-dependent manner, with complete block at 100 muM. Whereas MFA evoked a detectable decrease in G(j) within minutes of application, the time to complete block of G(j) was considerably longer, typically 20-40 min. After washout, G(j) recovered to 20-90% of the control level, but the time to maximum recovery was typically >1 h. These results suggest that MFA can be a useful drug to investigate the physiological functions of electrical synapses in the rod pathway, but that the slow kinetics of block and reversal might compromise interpretation of the results and that explicit monitoring of G(j) is desirable.

Synaptic vesicle dynamics in mouse rod bipolar cells

To better understand synaptic signaling at the mammalian rod bipolar cell terminal and pave the way for applying genetic approaches to the study of visual information processing in the mammalian retina, synaptic vesicle dynamics and intraterminal calcium were monitored in terminals of acutely isolated mouse rod bipolar cells and the number of ribbon-style active zones quantified. We identified a releasable pool, corresponding to a maximum of 7 s. The presence of a smaller, rapidly releasing pool and a small, fast component of refilling was also suggested. Following calcium channel closure, membrane surface area was restored to baseline with a time constant that ranged from 2 to 21 s depending on the magnitude of the preceding Ca2+ transient. In addition, a brief, calcium-dependent delay often preceded the start of onset of membrane recovery. Thus, several aspects of synaptic vesicle dynamics appear to be conserved between rod-dominant bipolar cells of fish and mammalian rod bipolar cells. A major difference is that the number of vesicles available for release is significantly smaller in the mouse rod bipolar cell, both as a function of the total number per neuron and on a per active zone basis.

Electrical Synapses Between AII Amacrine Cells: Dynamic Range and Functional Consequences of Variation in Junctional Conductance

AII amacrine cells form a network of electrically coupled interneurons in the mammalian retina and tracer coupling studies suggest that the junctional conductance ( Gj) can be modulated. However, the dynamic range of Gjand the functional consequences of varying Gjover the dynamic range are unknown. Here we use whole cell recordings from pairs of coupled AII amacrine cells in rat retinal slices to provide direct evidence for physiological modulation of Gj, appearing as a time-dependent increase from about 500 pS to a maximum of about 3,000 pS after 30–90 min of recording. The increase occurred in recordings with low- but not high-resistance pipettes, suggesting that it was related to intracellular washout and perturbation of a modulatory system. Computer simulations of a network of electrically coupled cells verified that our recordings were able to detect and quantify changes in Gjover a large range. Dynamic-clamp electrophysiology, with insertion of electrical synapses between AII amacrine cells, allowed us to finely and reversibly control Gjwithin the same range observed for physiologically coupled cells and to examine the quantitative relationship between Gjand steady-state coupling coefficient, synchronization of subthreshold membrane potential fluctuations, synchronization and transmission of action potentials, and low-pass filter characteristics. The range of Gjvalues over which signal transmission was modulated depended strongly on the specific functional parameter examined, with the largest range observed for action potential transmission and synchronization, suggesting that the full range of Gjvalues observed during spontaneous run-up of coupling could represent a physiologically relevant dynamic range.

Glycine receptor-mediated synaptic transmission regulates the maturation of ganglion cell synaptic connectivity

It is well documented that neuronal activity is required for the developmental segregation of retinal ganglion cell (RGC) synaptic connectivity with ON and OFF bipolar cells in mammalian retina. Our recent study showed that light deprivation preferentially blocked the developmental RGC dendritic redistribution from the center to sublamina a of the inner plexiform layer (IPL). To determine whether OFF signals in visual stimulation are required for OFF RGC dendritic development, the light-evoked responses and dendritic stratification patterns of RGCs in Spastic mutant mice, in which the OFF signal transmission in the rod pathway is largely blocked due to a reduction of glycine receptor (GlyR) expression, were quantitatively studied at different ages and rearing conditions. The dendritic distribution in the IPL of these mice was indistinguishable from wildtype controls at the age of postnatal day (P)12. However, the adult Spastic mutants had altered RGC light-evoked synaptic inputs from ON and OFF pathways, which could not be mimicked by pharmacologically blocking of glycinergic synaptic transmission on age-matched wildtype animals. Spastic mutation also blocked the developmental redistribution of RGC dendrites from the center to sublamina a of the IPL, which mimicked the effects induced by light deprivation on wildtype animals. Moreover, light deprivation of the Spastic mutants had no additional impact on the RGC dendritic distribution and light response patterns. We interpret these results as that visual stimulation regulates the maturation of RGC synaptic activity and connectivity primarily through GlyR-mediated synaptic transmission.

Nicotinic acetylcholine receptor subunits in rhesus monkey retina

PURPOSE

The purpose of this study was to detect and establish the cellular localizations of nicotinic acetylcholine receptor (nAChR) subunits in Rhesus monkey retina.

METHODS

Retinas were dissected from the eyes of monkeys killed after unrelated experiments. RNA was extracted and analyzed by RT-PCR, using primers designed against human sequences of alpha3-alpha7, alpha9, and beta2-beta4 nAChR subunits. The RT-PCR products were separated by gel electrophoresis and sequenced. Frozen sections of postmortem fixed monkey eyes were immunolabeled with well-characterized and specific monoclonal antibodies against the alpha3, alpha4, alpha6, alpha7, beta2, or beta4 nAChR subunits and visualized with fluorescence labeling.

RESULTS

Products of the predicted size for the alpha3-alpha7, alpha9, and beta2-beta4 nAChR subunits were detected by RT-PCR in Rhesus monkey retina. Homology between transcripts from monkey retina and human nucleotide sequences ranged from 93 to 99%. Immunohistochemical studies demonstrated that neurons in various cell layers of monkey retina expressed alpha3, alpha4, alpha7, or beta2 nAChR subunits and cells with the morphology of microglia were immunoreactive for the alpha6 or beta4 nAChR subunits.

CONCLUSIONS

nAChR subunits are expressed in the monkey retina and localize to diverse retinal neurons as well as putative microglia. Besides mediating visual processing, retinal nAChRs may influence refractive development and ocular pathologies such as neovascularization.

The circadian clock in the retina controls rod-cone coupling

Although rod and cone photoreceptor cells in the vertebrate retina are anatomically connected or coupled by gap junctions, a type of electrical synapse, rod-cone electrical coupling is thought to be weak. Using tracer labeling and electrical recording in the goldfish retina and tracer labeling in the mouse retina, we show that the retinal circadian clock, and not the retinal response to the visual environment, controls the extent and strength of rod-cone coupling by activating dopamine D(2)-like receptors in the day, so that rod-cone coupling is weak during the day but remarkably robust at night. The results demonstrate that circadian control of rod-cone electrical coupling serves as a synaptic switch that allows cones to receive very dim light signals from rods at night, but not in the day. The increase in the strength and extent of rod-cone coupling at night may facilitate the detection of large dim objects.

Distinct expressions of contrast gain control in parallel synaptic pathways converging on a retinal ganglion cell

Visual neurons adapt to increases in stimulus contrast by reducing their response sensitivity and decreasing their integration time, a collective process known as 'contrast gain control.' In retinal ganglion cells, gain control arises at two stages: an intrinsic mechanism related to spike generation, and a synaptic mechanism in retinal pathways. Here, we tested whether gain control is expressed similarly by three synaptic pathways that converge on an OFF alpha/Y-type ganglion cell: excitatory inputs driven by OFF cone bipolar cells; inhibitory inputs driven by ON cone bipolar cells; and inhibitory inputs driven by rod bipolar cells. We made whole-cell recordings of membrane current in guinea pig ganglion cells in vitro. At high contrast, OFF bipolar cell-mediated excitatory input reduced gain and shortened integration time. Inhibitory input was measured by clamping voltage near 0 mV or by recording in the presence of ionotropic glutamate receptor (iGluR) antagonists to isolate the following circuit: cone --> ON cone bipolar cell --> AII amacrine cell --> OFF ganglion cell. At high contrast, this input reduced gain with no effect on integration time. Mean luminance was reduced 1000-fold to recruit the rod bipolar pathway: rod --> rod bipolar cell --> AII cell --> OFF ganglion cell. The spiking response, measured with loose-patch recording, adapted despite essentially no gain control in synaptic currents. Thus, cone bipolar-driven pathways adapt differently, with kinetic effects confined to the excitatory OFF pathway. The ON bipolar-mediated inhibition reduced gain at high contrast by a mechanism that did not require an iGluR. Under rod bipolar-driven conditions, ganglion cell firing showed gain control that was explained primarily by an intrinsic property.

Synaptic activity, visual experience and the maturation of retinal synaptic circuitry

A fundamental feature of the synaptic organization of retina is the laminar-specific structure, in which specific types of retinal neurons form highly selective synapses to transfer distinct synaptic signals. In mature vertebrate retina, the dendrites of most retinal ganglion cells (RGCs) are narrowly stratified and ramified in specific strata of the inner plexiform layer (IPL) of retina to synapse with distinct subtypes of bipolar cells (BCs). However, little is known of how retinal neurons form this laminar-specific synaptic structure during development. Recent studies showed that the formation of retinal synaptic circuitry is regulated by both gene expression and neuronal activity. Here I will briefly discuss the recent advances in our understanding of how synaptic activity modulates the maturation of RGC synaptic connections.

Functional circuitry of visual adaptation in the retina

The visual system continually adjusts its sensitivity, or 'adapts', to the conditions of the immediate environment. Adaptation increases responses when input signals are weak, to improve the signal-to-noise ratio, and decreases responses when input signals are strong, to prevent response saturation. Retinal ganglion cells adapt primarily to two properties of light input: the mean intensity and the variance of intensity over time (contrast). This review focuses on cellular mechanisms for contrast adaptation in mammalian retina. High contrast over the ganglion cell's receptive field centre reduces the gain of spiking responses. The mechanism for gain control arises partly in presynaptic bipolar cell inputs and partly in the process of spike generation. Following strong contrast stimulation, ganglion cells exhibit a prolonged after-hyperpolarization, driven primarily by suppression of glutamate release from presynaptic bipolar cells. Ganglion cells also adapt to high contrast over their peripheral receptive field. Long-range adaptive signals are carried by amacrine cells that inhibit the ganglion cell directly, causing hyperpolarization, and inhibit presynaptic bipolar terminals, reducing gain of their synaptic output. Thus, contrast adaptation in ganglion cells involves multiple synaptic and intrinsic mechanisms for gain control and hyperpolarization. Several forms of adaptation in ganglion cells originate in presynaptic bipolar cells.

Disinhibition combines with excitation to extend the operating range of the OFF visual pathway in daylight

Cone signals divide into parallel ON and OFF bipolar cell pathways, which respond to objects brighter or darker than the background and release glutamate onto the corresponding type of ganglion cell. It is assumed that ganglion cell excitatory responses are driven by these bipolar cell synapses. Here, we report an additional mechanism: OFF ganglion cells were driven in part by the removal of synaptic inhibition (disinhibition). The disinhibition played a relatively large role in driving responses at low contrasts. The disinhibition persisted in the presence of CNQX and d-AP-5. Furthermore, the CNQX/d-AP-5-resistant response was blocked by l-AP-4, meclofenamic acid, quinine, or strychnine but not by bicuculline. Thus, the disinhibition circuit was driven by the ON pathway and required gap junctions and glycine receptors but not ionotropic glutamate or GABA(A) receptors. These properties implicate the AII amacrine cell, better known for its role in rod vision, as a critical circuit element through the following pathway: cone --> ON cone bipolar cell --> AII cell --> OFF ganglion cell. Rods could also drive this circuit through their gap junctions with cones. Thus, to light decrement, AII cells, driven by electrical synapses with ON cone bipolar cells, would hyperpolarize and reduce glycine release to excite OFF ganglion cells. To light increment, the AII circuit would directly inhibit OFF ganglion cells. These results show a new role for disinhibition in the retina and suggest a new role for the AII amacrine cell in daylight vision.

Low-level human equivalent gestational lead exposure produces supernormal scotopic electroretinograms, increased retinal neurogenesis, and decreased retinal dopamine utilization in rats

BACKGROUND

Postnatal lead exposure in children and animals produces alterations in the visual system primarily characterized by decreases in the rod-mediated (scotopic) electroretinogram (ERG) amplitude (subnormality). In contrast, low-level gestational Pb exposure (GLE) increases the amplitude of scotopic ERGs in children (supernormality).

OBJECTIVES

The goal of this study was to establish a rat model of human equivalent GLE and to determine dose-response effects on scotopic ERGs and on retinal morphology, biochemistry, and dopamine metabolism in adult offspring.

METHODS

We exposed female Long-Evans hooded rats to water containing 0, 27 (low), 55 (moderate), or 109 (high) ppm of Pb beginning 2 weeks before mating, throughout gestation, and until postnatal day (PND) 10. We measured maternal and litter indices, blood Pb concentrations (BPb), retinal Pb concentrations, zinc concentrations, and body weights. On PND90, we performed the retinal experiments.

RESULTS

Peak BPb concentrations were < 1, 12, 24, and 46 microg/dL in control, low-, moderate- and high-level GLE groups, respectively, at PNDs 0-10. ERG supernormality and an increased rod photoreceptor and rod bipolar cell neurogenesis occurred with low- and moderate-level GLE. In contrast, high-level GLE produced ERG subnormality, rod cell loss, and decreased retinal Zn levels. GLE produced dose-dependent decreases in dopamine and its utilization.

CONCLUSIONS

Low- and moderate-level GLE produced persistent scotopic ERG supernormality due to an increased neurogenesis of cells in the rod signaling pathway and/or decreased dopamine utilization, whereas high-level GLE produced rod-selective toxicity characterized by ERG subnormality. The ERG is a differential and noninvasive biomarker of GLE. The inverted U-shaped dose-response curves reveal the sensitivity and vulnerability of the developing retina to GLE.

Ultraweak signals can cause synaptic depression and adaptation

Synaptic depression at conventional synapses is usually caused by strong or prolonged stimuli, like tetanic bursts of afferent fiber discharge at high frequencies. In this issue of Neuron, Dunn and Rieke report that, in the retina, even the weakest stimuli, single photons, can lead to synaptic depression at ribbon-type synapses and adaptation of neuronal output to ambient light levels.

Light-activated channels targeted to ON bipolar cells restore visual function in retinal degeneration

Genetically encoded optical neuromodulators create an opportunity for circuit-specific intervention in neurological diseases. One of the diseases most amenable to this approach is retinal degeneration, where the loss of photoreceptors leads to complete blindness. To restore photosensitivity, we genetically targeted a light-activated cation channel, channelrhodopsin-2, to second-order neurons, ON bipolar cells, of degenerated retinas in vivo in the Pde6b(rd1) (also known as rd1) mouse model. In the absence of 'classical' photoreceptors, we found that ON bipolar cells that were engineered to be photosensitive induced light-evoked spiking activity in ganglion cells. The rescue of light sensitivity was selective to the ON circuits that would naturally respond to increases in brightness. Despite degeneration of the outer retina, our intervention restored transient responses and center-surround organization of ganglion cells. The resulting signals were relayed to the visual cortex and were sufficient for the animals to successfully perform optomotor behavioral tasks.

Development of presynaptic inhibition onto retinal bipolar cell axon terminals is subclass-specific

Synaptic integration is modulated by inhibition onto the dendrites of postsynaptic cells. However, presynaptic inhibition at axonal terminals also plays a critical role in the regulation of neurotransmission. In contrast to the development of inhibitory synapses onto dendrites, GABAergic/glycinergic synaptogenesis onto axon terminals has not been widely studied. Because retinal bipolar cells receive subclass-specific patterns of GABAergic and glycinergic presynaptic inhibition, they are a good model for studying the development of inhibition at axon terminals. Here, using whole cell recording methods and transgenic mice in which subclasses of retinal bipolar cells are labeled, we determined the temporal sequence and patterning of functional GABAergic and glycinergic input onto the major subclasses of bipolar cells. We found that the maturation of GABAergic and glycinergic synapses onto the axons of rod bipolar cells (RBCs), on-cone bipolar cells (ON-CBCs) and off-cone bipolar cells (OFF-CBCs) were temporally distinct: spontaneous chloride-mediated currents are present in RBCs earlier in development compared with ON- and OFF-CBC, and RBCs receive GABAergic and glycinergic input simultaneously, whereas in OFF-CBCs, glycinergic transmission emerges before GABAergic transmission. Because on-CBCs show little inhibitory activity, GABAergic and glycinergic events could not be pharmacologically distinguished for these bipolar cells. The balance of GABAergic and glycinergic input that is unique to RBCs and OFF-CBCs is established shortly after the onset of synapse formation and precedes visual experience. Our data suggest that presynaptic modulation of glutamate transmission from bipolar cells matures rapidly and is differentially coordinated for GABAergic and glycinergic synapses onto distinct bipolar cell subclasses.

Regulation of ON bipolar cell activity

Synaptic transmission from photoreceptors to all types of ON bipolar cells is primarily mediated by the mGluR6 receptor. This receptor, which is apparently expressed uniquely in the nervous system by ON bipolar cells, couples negatively to a nonselective cation channel. This arrangement results in a sign reversal at photoreceptor/ON bipolar cell synapse, which is necessary in order to establish parallel ON and OFF pathways in the retina. The synapse is an important target for second messenger molecules that are known to modulate synaptic transmission elsewhere in the nervous system, second messengers that act on a time scale ranging from milliseconds to minutes. This review focuses on two of these molecules, Ca2+ and cGMP, summarizing our current knowledge of how they modulate gain at the photoreceptor/ON bipolar cell synapse, as well as their proposed sites of action within the mGluR6 cascade. The implications of plasticity at this synapse for retinal function will also be examined.

Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Although the neural retina appears as a relatively uniform tissue when viewed from its surface, it is in fact highly patterned along its anterior-posterior and dorso-ventral axes. The question of how and when such patterns arise has been the subject of intensive investigations over several decades. Most studies aimed at understanding retinal pattern formation have used the retinotectal map, the ordered projections of retinal ganglion cells to the brain, as a functional readout of the pattern. However, other cell types are also topographically organized in the retina. The most commonly recognized example of such a topographic cellular organization is the differential distribution of photoreceptor types across the retina. Photoreceptor patterns are highly species-specific and may represent an important adaptation to the visual niche a given species occupies. Nevertheless, few studies have addressed this functional readout of pattern to date and our understanding of its development has remained superficial. Here, we review recent advances in understanding the molecular cascades that control regionalization of the eye anlage, relate these findings to the development of photoreceptor patterns and discuss common and unique strategies involved in both aspects of retinal pattern formation.

Signals and noise in an inhibitory interneuron diverge to control activity in nearby retinal ganglion cells

Information about sensory stimuli is represented by spatiotemporal patterns of neural activity. The complexity of the central nervous system, however, frequently obscures the origin and properties of signals and noise that underlie these activity patterns. We minimized this constraint by examining mechanisms governing correlated activity in mouse retinal ganglion cells (RGCs) under conditions in which light-evoked responses traverse a specific circuit, the rod bipolar pathway. Signals and noise in this circuit produced correlated synaptic input to neighboring On and Off RGCs. Temporal modulation of light intensity did not alter the degree to which noise in the input to nearby RGCs was correlated, and action potential generation in individual RGCs was largely insensitive to differences in network noise generated by dynamic and static light stimuli. Together, these features enable noise in shared circuitry to diminish simultaneous action potential generation in neighboring On and Off RGCs under a variety of conditions.

Dendritic impulse collisions and shifting sites of action potential initiation contract and extend the receptive field of an amacrine cell

We evaluated the contributions of somatic and dendritic impulses to the receptive field dimensions of amacrine cells in the amphibian retina. For this analysis, we used the NEURON simulation program with a multicompartmental, multichannel model of an On-Off amacrine cell with a three-dimensional structure obtained through computer tracing techniques. Simulated synaptic inputs were evenly spaced along the dendritic branches and organized into eight annuli of increasing radius. The first set of simulations activated each ring progressively to simulate an area summation experiment, while a second approach activated each annulus individually. Both sets of simulations were done with and without the presence of Na channels in the dendrites and soma. Unexpectedly, the receptive field dimensions observed in the area summation simulations was often smaller than that predicted from the summation of the annular simulations. Collisions of action potentials moving in opposite directions in the dendrites largely accounted for this contraction in receptive field size for the area summation studies. The presence of dendritic Na channels increased the size of the receptive field beyond that achieved in their absence and allowed the physiological size of the receptive field to approximate the physical dimensions of the dendritic tree. This receptive field augmentation was the result of impulse generating ability in the dendrites which enhanced the signal observed at the soma. These simulations provide a plausible mechanistic explanation for physiological recordings from amacrine cells that show similar phenomena.

Optimization of single-photon response transmission at the rod-to-rod bipolar synapse

Our ability to see in dim light is limited by the statistics of light absorption in rod photoreceptors and the faithful transmission of the light-evoked signals through the retina. This article reviews the physiological mechanisms at the synapse between rods and rod bipolar cells, the first relay in a pathway that mediates vision near absolute threshold.

Responses of suprachiasmatic nucleus neurons to light and dark adaptation: relative contributions of melanopsin and rod-cone inputs

The circadian oscillator in the suprachiasmatic nucleus (SCN) is entrained to the environmental light/dark cycle through photic information conveyed from the retina. The vast majority of projections to the SCN arise from melanopsin-expressing ganglion cells that are intrinsically light sensitive and that receive inputs from both rods and cones. To investigate the relative contributions of the different photoreceptive systems in shaping the photic signal influencing the circadian clock, we analyzed neuronal responses of single SCN neurons using extracellular electrophysiological recordings under different conditions of light adaptation. In the majority of neurons (78%), the spike rate is increased by light stimulation whereas the remainder are light-inhibited. The neuronal response to light is composed of several components distinguished by their temporal dynamics and degree of alteration after previous light exposure. SCN neurons display a sustained response to light followed by persistence of the response after light offset. These responses are sluggish and relatively unaffected by previous light exposures. Neurons also respond with a brisk, excitatory ON response and often an OFF response that is either excitatory or inhibitory. ON-OFF responses are transient and strongly reduced by previous bright white light exposure. Furthermore, two types of neuronal response patterns can be distinguished by the presence or absence of a slow-transient component that follows the transient ON response. The transient ON-OFF components express light adaptation properties characteristic of retinal channels involving cones, whereas the sustained and persistent components are consistent with in vitro response properties reported for melanopsin-expressing ganglion cells.

Narrow and wide field amacrine cells fire action potentials in response to depolarization and light stimulation

Action potentials in amacrine cells are important for lateral propagation of signals across the inner retina, but it is unclear how many subclasses of amacrine cells contain voltage-gated sodium channels or can fire action potentials. This study investigated the ability of amacrine cells with narrow (< 200 μm) and wide (> 200 μm) dendritic fields to fire action potentials in response to depolarizing current injections and light stimulation. The pattern of action potentials evoked by current injections revealed two distinct classes of amacrine cells; those that responded with a single action potential (single-spiking cells) and those that responded with repetitive action potentials (repetitive-spiking cells). Repetitive-spiking cells differed from single-spiking cells in several regards: Repetitive-spiking cells were more often wide field cells, while single-spiking cells were more often narrow field cells. Repetitive-spiking cells had larger action potential amplitudes, larger peak voltage-gated NaV currents lower action potential thresholds, and needed less current to induce action potentials. However, there was no difference in the input resistance, holding current or time constant of these two classes of cells. The intrinsic capacity to fire action potentials was mirrored in responses to light stimulation; single-spiking amacrine cells infrequently fired action potentials to light steps, while repetitive-spiking amacrine cells frequently fired numerous action potentials. These results indicate that there are two physiologically distinct classes of amacrine cells based on the intrinsic capacity to fire action potentials.

Different mechanisms generate maintained activity in ON and OFF retinal ganglion cells

Neuronal discharge is driven by either synaptic input or cell-autonomous intrinsic pacemaker activity. It is commonly assumed that the resting spike activity of retinal ganglion cells (RGCs), the output cells of the retina, is driven synaptically, because retinal photoreceptors and second-order cells tonically release neurotransmitter. Here we show that ON and OFF RGCs generate maintained activity through different mechanisms: ON cells depend on tonic excitatory input to drive resting activity, whereas OFF cells continue to fire in the absence of synaptic input. In addition to spontaneous activity, OFF cells exhibit other properties of pacemaker neurons, including subthreshold oscillations, burst firing, and rebound excitation. Thus, variable weighting of synaptic mechanisms and intrinsic properties underlies differences in the generation of maintained activity in these parallel retinal pathways.

Glutamatergic input is coded by spike frequency at the soma and proximal dendrite of AII amacrine cells in the mouse retina

In the mammalian retina, AII amacrine cells play a crucial role in scotopic vision. They transfer rod signals from rod bipolar cells to the cone circuit, and divide these signals into the ON and OFF pathways at the discrete synaptic layers. AII amacrine cells have been reported to generate tetrodotoxin (TTX)-sensitive repetitive spikes of small amplitude. To investigate the properties of the spikes, we performed whole-cell patch-clamping of AII amacrine cells in mouse retinal slices. The spike frequency increased in proportion to the concentration of glutamate puffer-applied to the arboreal dendrite and to the intensity of the depolarizing current injection. The spike activity was suppressed by L-2-amino-4-phosphonobutyric acid, a glutamate analogue that hyperpolarizes rod bipolar cells, puffer-applied to the outer plexiform layer. Therefore, it is most likely that the spike frequency generated by AII amacrine cells is dependent on the excitatory glutamatergic input from rod bipolar cells. Gap junction blockers reduced the range of intensity of input with which spike frequency varies. Application of TTX to the soma and the proximal dendrite of AII amacrine cells blocked the voltage-gated Na(+) current significantly more than application to the arboreal dendrite, indicating that the Na(+) channels are mainly localized in these regions. Our results suggest that the intensity of the glutamatergic input from rod bipolar cells is coded by the spike frequency at the soma and the proximal dendrite of AII amacrine cells, raising the possibility that the spikes could contribute to the OFF pathway to enhance release of neurotransmitter.

Modeling the role of mid-wavelength cones in circadian responses to light

Nonvisual responses to light, such as photic entrainment of the circadian clock, involve intrinsically light-sensitive melanopsin-expressing ganglion cells as well as rod and cone photoreceptors. However, previous studies have been unable to demonstrate a specific contribution of cones in the photic control of circadian responses to light. Using a mouse model that specifically lacks mid-wavelength (MW) cones we show that these photoreceptors play a significant role in light entrainment and in phase shifting of the circadian oscillator. The contribution of MW cones is mainly observed for light exposures of short duration and toward the longer wavelength region of the spectrum, consistent with the known properties of this opsin. Modeling the contributions of the various photoreceptors stresses the importance of considering the particular spectral, temporal, and irradiance response domains of the photopigments when assessing their role and contribution in circadian responses to light.

Environmental enrichment effects on development of retinal ganglion cell dendritic stratification require retinal BDNF

A well-known developmental event of retinal maturation is the progressive segregation of retinal ganglion cell (RGC) dendrites into a and b sublaminae of the inner plexiform layer (IPL), a morphological rearrangement crucial for the emergence of the ON and OFF pathways. The factors regulating this process are not known, although electrical activity has been demonstrated to play a role. Here we report that Environmental Enrichment (EE) accelerates the developmental segregation of RGC dendrites and prevents the effects exerted on it by dark rearing (DR). Development of RGC stratification was analyzed in a line of transgenic mice expressing plasma-membrane marker green fluorescent protein (GFP) under the control of Thy-1 promoter; we visualized the a and b sublaminae of the IPL by using an antibody selectively directed against a specific marker of cholinergic neurons. EE precociously increases Brain Derived Neurotrophic Factor (BDNF) in the retina, in parallel with the precocious segregation of RGC dendrites; in addition, EE counteracts retinal BDNF reduction in DR retinas and promotes a normal segregation of RGC dendrites. Blocking retinal BDNF by means of antisense oligos blocks EE effects on the maturation of RGC dendritic stratification. Thus, EE affects the development of RGC dendritic segregation and retinal BDNF is required for this effect to take place, suggesting that BDNF could play an important role in the emergence of the ON and OFF pathways.

Neuronal expression of P2X3 purinoceptors in the rat retina

P2X3 purinoceptors are involved in fast, excitatory neurotransmission in the nervous system, and are expressed predominantly within sensory neurons. In this study, we examined the cellular and synaptic localization of the P2X3 receptor subunit in the retina of the rat using immunofluorescence immunohistochemistry and pre-embedding immunoelectron microscopy. In addition, we investigated the activity of ecto-ATPases in the inner retina using an enzyme cytochemical method. The P2X3 receptor subunit was expressed in the soma of a subset of GABA immunoreactive amacrine cells, some of which also expressed protein kinase C-alpha. In addition, punctate immunoreactivity was observed within both the inner and outer plexiform layers of the retina. Double labeling studies showed that P2X3 receptor puncta were associated with both rod and cone bipolar cell axon terminals in the inner plexiform layer. Ultrastructural studies indicated that P2X3 receptor subunits were expressed on putative A17 amacrine cells at sites of reciprocal synaptic input to the rod bipolar cell axon terminal. Moreover, we observed P2X3 immunolabeling on amacrine cell processes that were associated with cone bipolar cell axon terminals and other conventional synapses. In the outer retina, P2X3 immunoreactivity was observed on specialized junctions made by putative interplexiform cells. Ecto-ATPase activity was localized to the inner plexiform layer on the extracellular side of all plasma membranes, but was not apparent in the ganglion cell layer or the inner nuclear layer, suggesting that ATP dephosphorylation occurs exclusively in synaptic regions of the inner retina. These data provide further evidence that purines participate in retinal transmission, particularly within the rod pathway.

Specific alterations of tyrosine hydroxylase immunopositive cells in the retina of NT-4 knock out mice

To assess the effect of NT-4 deprivation on maturation of retinal circuitry, we investigated a mouse with targeted deletion of the gene encoding nt-4 (nt-4(-/-)). In particular, we studied neurons immunostained by an antibody recognizing tyrosine hydroxylase (TH), the rate limiting enzyme for dopamine (DA) synthesis. We found that TH immunopositive processes were altered in the retina of nt-4(-/-). Alteration of TH immunopositive processes in nt-4(-/-) mice resulted in changes of DA turnover, as assessed by high-pressure liquid chromatography measurements. These findings suggest that retinal NT-4 plays a role in the morphological maturation of dopaminergic retinal cells.

Temporal response properties of the primary and secondary rod-signaling pathways in normal and Gnat2 mutant mice

Multiple signaling pathways have been proposed for rod vision in the mammalian retina. The primary and secondary rod pathways have been characterized in humans with the scotopic 15-Hz flicker electroretinogram (ERG). The purpose of this study was to determine whether the response properties of these pathways in the mouse are similar to those of humans. C57BL/6J and Gnat2(cpfl3) mutant mice lacking functional cones were used in these experiments. Standard ERG recording techniques were employed. Response functions were obtained for a range of flash intensities (-4.7logcd-s/m(2) to -0.2logcd-s/m(2)) and temporal modulation frequencies (1-30Hz). The mouse intensity-response functions to 15-Hz flickering stimuli possessed the same features as that of humans - a local amplitude minimum and a rapid phase change in the intensity region where the primary and secondary pathways are mutually inhibitory. However, the secondary pathway in the mouse did not achieve the same level of sensitivity as previously shown for humans, suggesting inter-species differences in post-receptoral signal processing. In Gnat2(cpfl3) mutant mice, the secondary pathway was completely abolished. Measurements of temporal acuity indicated that the primary and secondary rod pathways could mediate temporal frequencies as high as 30 and 50Hz, respectively. The response functions for mice are similar to those of humans, although the evidence suggests that the primary rod pathway dominates all rod-mediated signal processing in the mouse. Nevertheless, these results demonstrate the feasibility of measuring non-invasively the performance characteristics of the primary and secondary rod retinal pathways in the mouse and provide a mechanism for testing hypotheses about the action of disease where post-receptoral cells are differentially affected.

Network variability limits stimulus-evoked spike timing precision in retinal ganglion cells

Visual, auditory, somatosensory, and olfactory stimuli generate temporally precise patterns of action potentials (spikes). It is unclear, however, how the precision of spike generation relates to the pattern and variability of synaptic input elicited by physiological stimuli. We determined how synaptic conductances evoked by light stimuli that activate the rod bipolar pathway control spike generation in three identified types of mouse retinal ganglion cells (RGCs). The relative amplitude, timing, and impact of excitatory and inhibitory input differed dramatically between On and Off RGCs. Spikes evoked by repeated somatic injection of identical light-evoked synaptic conductances were more temporally precise than those evoked by light. However, the precision of spikes evoked by conductances that varied from trial to trial was similar to that of light-evoked spikes. Thus, the rod bipolar pathway modulates different RGCs via unique combinations of synaptic input, and RGC temporal variability reflects variability in the input this circuit provides.

Expression of alpha 7 nicotinic acetylcholine receptors by bipolar, amacrine, and ganglion cells of the rabbit retina

Cholinergic agents affect the light responses of many ganglion cells (GCs) in the mammalian retina by activating nicotinic acetylcholine receptors (nAChRs). Whereas retinal neurons that express beta2 subunit-containing nAChRs have been characterized in the rabbit retina, expression patterns of other nAChR subtypes remain unclear. Therefore, we evaluated the expression of alpha7 nAChRs in retinal neurons by means of single-, double-, and triple-label immunohistochemistry. Our data demonstrate that, in the rabbit retina, several types of bipolar cells, amacrine cells, and cells in the GC layer express alpha7 nAChRs. At least three different populations of cone bipolar cells exhibited alpha7 labeling, whereas glycine-immunoreactive amacrine cells comprised the majority of alpha7-positive amacrine cells. Some GABAergic amacrine cells also displayed alpha7 immunoreactivity; alpha7 labeling was never detected in rod bipolar cells or rod amacrine cells (AII amacrine cells). Our data suggest that activation of alpha7 nAChRs by acetylcholine (ACh) or choline may affect glutamate release from several types of cone bipolar cells, modulating GC responses. ACh-induced excitation of inhibitory amacrine cells might cause either inhibition or disinhibition of other amacrine and GC circuits. Finally, ACh may act on alpha7 nAChRs expressed by GCs themselves.

Chromatic properties of horizontal and ganglion cell responses follow a dual gradient in cone opsin expression

In guinea pig retina, immunostaining reveals a dual gradient of opsins: cones expressing opsin sensitive to medium wavelengths (M) predominate in the upper retina, whereas cones expressing opsin sensitive to shorter wavelengths (S) predominate in the lower retina. Whether these gradients correspond to functional gradients in postreceptoral neurons is essentially unknown. Using monochromatic flashes, we measured the relative weights with which M, S, and rod signals contribute to horizontal cell responses. For a background that produced 4.76 log10 photoisomerizations per rod per second (Rh*/rod/s), mean weights in superior retina were 52% (M), 2% (S), and 46% (rod). Mean weights in inferior retina were 9% (M), 50% (S), and 41% (rod). In superior retina, cone opsin weights agreed quantitatively with relative pigment density estimates from immunostaining. In inferior retina, cone opsin weights agreed qualitatively with relative pigment density estimates, but quantitative comparison was impossible because individual cones coexpress both opsins to varying and unquantifiable degrees. We further characterized the functional gradients in horizontal and brisk-transient ganglion cells using flickering stimuli produced by various mixtures of blue and green primary lights. Cone weights for both cell types resembled those obtained for horizontal cells using monochromatic flashes. Because the brisk-transient ganglion cell is thought to mediate behavioral detection of luminance contrast, our results are consistent with the hypothesis that the dual gradient of cone opsins assists achromatic contrast detection against different spectral backgrounds. In our preparation, rod responses did not completely saturate, even at background light levels typical of outdoor sunlight (5.14 log10 Rh*/rod/s).

Protein kinase A-mediated phosphorylation of connexin36 in mouse retina results in decreased gap junctional communication between AII amacrine cells

Gap junctions in AII amacrine cells of mammalian retina participate in the coordination of the rod and cone signaling pathway involved in visual adaptation. Upon stimulation by light, released dopamine binds to D(1) receptors on AII amacrine cells leading to increased intracellular cAMP (cyclic adenosine monophosphate) levels. AII amacrine cells express the gap junctional protein connexin36 (Cx36). Phosphorylation of Cx36 has been hypothesized to regulate gap junctional activity of AII amacrine cells. However, until now in vivo phosphorylation of Cx36 has not been reported. Indeed, it had been concluded that Cx36 in bovine retina is not phosphorylated, but in vitro phosphorylation for Cx35, the bass ortholog of Cx36, had been shown. To clarify this experimental discrepancy, we examined protein kinase A (PKA)-induced phosphorylation of Cx36 in mouse retina as a possible mechanism to modulate the extent of gap junctional coupling. The cytoplasmic domains of Cx36 and the total Cx36 protein were phosphorylated in vitro by PKA. Mass spectroscopy revealed that all four possible PKA consensus motifs were phosphorylated; however, domains point mutated at the sites in question showed a prevalent usage of Ser-110 and Ser-293. Additionally, we demonstrated that Cx36 was phosphorylated in cultured mouse retina. Furthermore, activation of PKA increased the level of phosphorylation of Cx36. cAMP-stimulated, PKA-mediated phosphorylation of Cx36 protein was accompanied by a decrease of tracer coupling between AII amacrine cells. Our results link increased phosphorylation of Cx36 to down-regulation of permeability through gap junction channels mediating light adaptation in the retina.

DARPP-32 involvement in the photic pathway of the circadian system

The multifunctional regulator of protein kinases and phosphatases dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) is an important molecular target of the dopamine signaling pathway. In the present study, we investigated the possible involvement of DARPP-32 regulation in the circadian system using DARPP-32 knock-out (KO) mice. These mice showed normal entrainment to a 12 h light/dark cycle and free run in constant darkness with a period similar to that of wild-type controls. After light exposure, however, the behavioral phase-delay response and the expression of light-induced clock gene mPer2 were attenuated in the DARPP-32 KO mice. Attenuated phase delays were also seen in animals bearing a point mutation in DARPP-32 at the PKA (Thr34) but not at the casein kinase I (Ser130) phosphorylation site. We next examined DARPP-32 expression in the retina and intergeniculate leaflet (IGL), both of which convey light information to the suprachiasmatic nucleus (SCN), the locus of a master circadian clock, and in the SCN itself. DARPP-32 was expressed in the retina but not in the IGL or the SCN. The results indicate that DARPP-32 is involved in the retinal pathway transmitting photic information that resets the circadian clock.

Functional properties of spontaneous IPSCs and glycine receptors in rod amacrine (AII) cells in the rat retina

AII amacrine cells play a crucial role in retinal signal transmission under scotopic conditions. We have used rat retinal slices to investigate the functional properties of inhibitory glycine receptors on AII cells by recording spontaneous IPSCs (spIPSCs) in whole cells and glycine-evoked responses in outside-out patches. Glycinergic spIPSCs displayed fast kinetics with an average 10-90% rise time of approximately 500 mus, and a decay phase best fitted by a double-exponential function with tau(fast) approximately 4.8 ms (97.5% amplitude contribution) and tau(slow) approximately 33 ms. Decay kinetics were voltage dependent. Ultrafast application of brief ( approximately 2-5 ms) pulses of glycine (3 mm) to patches, evoked responses with fast deactivation kinetics best fitted with a double-exponential function with tau(fast) approximately 4.6 ms (85% amplitude contribution) and tau(slow) approximately 17 ms. Double-pulse experiments indicated recovery from desensitization after a 100-ms pulse of glycine with a double-exponential time course (tau(fast) approximately 71 ms and tau(slow) approximately 1713 ms). Non-stationary noise analysis of spIPSCs and patch responses, and directly observed channel gating yielded similar single-channel conductances ( approximately 41 to approximately 47 pS). In addition, single-channel gating occurred at approximately 83 pS. These results suggest that the fast glycinergic spIPSCs in AII cells are probably mediated by alpha1beta heteromeric receptors with a contribution from alpha1 homomeric receptors. We hypothesize that glycinergic synaptic input may target the arboreal dendrites of AII cells, and could serve to shunt excitatory input from rod bipolar cells and transiently uncouple the transcellular current through electrical synapses between AII cells and between AII cells and ON-cone bipolar cells.

Dark adaptation of human rod bipolar cells measured from the b-wave of the scotopic electroretinogram

To examine the dark adaptation of human rod bipolar cells in vivo, we recorded ganzfeld ERGs to (a) a family of flashes of increasing intensity, (b) dim test flashes presented on a range of background intensities, and (c) dim test flashes presented before, and up to 40 min after, exposure to intense illumination eliciting bleaches from a few per cent to near total. The dim flash ERG was characterized by a prominent b-wave response generated principally by rod bipolar cells. In the presence of background illumination the response reached peak earlier and desensitized according to Weber's Law. Following bleaching exposures, the response was initially greatly desensitized, but thereafter recovered slowly with time. For small bleaches, the desensitization was accompanied by acceleration, in much the same way as for real light. Following a near-total bleach, the response was unrecordable for >10 min, but after approximately 23 min half-maximal sensitivity was reached, and full sensitivity was restored between approximately 35 and 40 min. With smaller bleaches, recovery commenced earlier. We converted the post-bleach measurements of desensitization into 'equivalent background intensities' using a Crawford transformation. Across the range of bleaching levels, the results were described by a prominent 'S2' component (0.24 decades min(-1)) together with a smaller and slower 'S3' component (0.06 decades min(-1)), as is found for dark adaptation of the scotopic visual system. We attribute the S2 component to the presence of unregenerated opsin, and we speculate that the S3 component results from ion channel closure by all-trans retinal.

Controlling the gain of rod-mediated signals in the Mammalian retina

Effective sensory processing requires matching the gain of neural responses to the range of signals encountered. For rod vision, gain controls operate at light levels at which photons arrive rarely at individual rods, light levels too low to cause adaptation in rod phototransduction. Under these conditions, adaptation within a conserved pathway in mammalian retina maintains sensitivity as light levels change. To relate retinal signals to behavioral work on detection at low light levels, we measured how background light affects the gain and noise of primate ganglion cells. To determine where and how gain is controlled, we tracked rod-mediated signals across the mouse retina. These experiments led to three main conclusions: (1) the primary site of adaptation at low light levels is the synapse between rod bipolar and AII amacrine cells; (2) cellular noise after the gain control is nearly independent of background intensity; and (3) at low backgrounds, noise in the circuitry, rather than rod noise or fluctuations in arriving photons, limits ganglion cell sensitivity. This work provides physiological insights into the rich history of experiments characterizing how rod vision avoids saturation as light levels increase.

Flicker VEPs reflecting multiple rod and cone pathways

In an attempt to determine whether the relative contributions of magno-mediated and parvo-mediated inputs to the cortex are significantly altered in the transition from cone to rod vision, VEPs were recorded at different luminance levels (photopic to scotopic) for 2Hz square-wave, isochromatic flicker. The VEP mass response appears capable of reflecting major parvo-mediated contributions even at luminance levels for which responses from individual cells in the parvocellular pathway are reported to be weak. Our findings suggest that parvo-mediated responses are the dominant source of high-contrast isochromatic flicker VEPs at all light levels.