Origins of direction selectivity in the primate retina

From mouse to primate, there is a striking discontinuity in our current understanding of the neural coding of motion direction. In non-primate mammals, directionally selective cell types and circuits are a signature feature of the retina, situated at the earliest stage of the visual process. In primates, by contrast, direction selectivity is a hallmark of motion processing areas in visual cortex, but has not been found in the retina, despite significant effort. Here we combined functional recordings of light-evoked responses and connectomic reconstruction to identify diverse direction-selective cell types in the macaque monkey retina with distinctive physiological properties and synaptic motifs. This circuitry includes an ON-OFF ganglion cell type, a spiking, ON-OFF polyaxonal amacrine cell and the starburst amacrine cell, all of which show direction selectivity. Moreover, we discovered that macaque starburst cells possess a strong, non-GABAergic, antagonistic surround mediated by input from excitatory bipolar cells that is critical for the generation of radial motion sensitivity in these cells. Our findings open a door to investigation of a precortical circuitry that computes motion direction in the primate visual system.

Phototransduction in Retinal Ganglion Cells

The mammalian retina contains a small number of retinal ganglion cells that express melanopsin, a retinal based visual pigment, and generate a depolarizing response to light in the absence of rod and cone driven synaptic input; hence they are referred to as intrinsically photosensitive retinal ganglion cells (ipRGCs). They have been shown to be comprised of a number of sub-types and to provide luminance information that participates primarily in a variety of non-imaging forming visual functions. Here I review what is currently known about the cascade of events that couple the photoisomerization of melanopsin to the opening of a non-selective cation channel. While these events conform in a general sense to the prevailing model for invertebrate phototransduction, in which visual pigment signals through a G protein of the G_q class and a phospholipase C cascade to open a TRPC type ion channel, none of the molecular elements in the melanopsin transduction process have been unequivocally identified. This has given rise to the possibility that the underlying mechanism responsible for intrinsic photosensitivity is not same in all ipRGC sub-types and to the recognition that signal transduction in ipRGCs is more complex than originally thought.

Network oscillations drive correlated spiking of ON and OFF ganglion cells in the rd1 mouse model of retinal degeneration

Following photoreceptor degeneration, ON and OFF retinal ganglion cells (RGCs) in the rd-1/rd-1 mouse receive rhythmic synaptic input that elicits bursts of action potentials at ∼ 10 Hz. To characterize the properties of this activity, RGCs were targeted for paired recording and morphological classification as either ON alpha, OFF alpha or non-alpha RGCs using two-photon imaging. Identified cell types exhibited rhythmic spike activity. Cross-correlation of spike trains recorded simultaneously from pairs of RGCs revealed that activity was correlated more strongly between alpha RGCs than between alpha and non-alpha cell pairs. Bursts of action potentials in alpha RGC pairs of the same type, i.e. two ON or two OFF cells, were in phase, while bursts in dissimilar alpha cell types, i.e. an ON and an OFF RGC, were 180 degrees out of phase. This result is consistent with RGC activity being driven by an input that provides correlated excitation to ON cells and inhibition to OFF cells. A2 amacrine cells were investigated as a candidate cellular mechanism and found to display 10 Hz oscillations in membrane voltage and current that persisted in the presence of antagonists of fast synaptic transmission and were eliminated by tetrodotoxin. Results support the conclusion that the rhythmic RGC activity originates in a presynaptic network of electrically coupled cells including A2s via a Na(+)-channel dependent mechanism. Network activity drives out of phase oscillations in ON and OFF cone bipolar cells, entraining similar frequency fluctuations in RGC spike activity over an area of retina that migrates with changes in the spatial locus of the cellular oscillator.

Inhibitory inputs tune the light response properties of dopaminergic amacrine cells in mouse retina

Dopamine (DA) is a neuromodulator that in the retina adjusts the circuitry for visual processing in dim and bright light conditions. It is synthesized and released from retinal interneurons called dopaminergic amacrine cells (DACs), whose basic physiology is not yet been fully characterized. To investigate their cellular and input properties as well as light responses, DACs were targeted for whole cell recording in isolated retina using two-photon fluorescence microscopy in a mouse line where the dopamine receptor 2 promoter drives green fluorescent protein (GFP) expression. Differences in membrane properties gave rise to cell-to-cell variation in the pattern of resting spontaneous spike activity ranging from silent to rhythmic to periodic burst discharge. All recorded DACs were light sensitive and generated responses that varied with intensity. The threshold response to light onset was a hyperpolarizing potential change initiated by rod photoreceptors that was blocked by strychnine, indicating a glycinergic amacrine input onto DACs at light onset. With increasing light intensity, the ON response acquired an excitatory component that grew to dominate the response to the strongest stimuli. Responses to bright light (photopic) stimuli also included an inhibitory OFF response mediated by GABAergic amacrine cells driven by the cone OFF pathway. DACs expressed GABA (GABA(A)α1 and GABA(A)α3) and glycine (α2) receptor clusters on soma, axon, and dendrites consistent with the light response being shaped by dual inhibitory inputs that may serve to tune spike discharge for optimal DA release.

Eyecup scope--optical recordings of light stimulus-evoked fluorescence signals in the retina

Dendritic signals play an essential role in processing visual information in the retina. To study them in neurites too small for electrical recording, we developed an instrument that combines a multi-photon (MP) microscope with a through-the-objective high-resolution visual stimulator. An upright microscope was designed that uses the objective lens for both MP imaging and delivery of visual stimuli to functionally intact retinal explants or eyecup preparations. The stimulator consists of a miniature liquid-crystal-on-silicon display coupled into the optical path of an infrared-excitation laser-scanning microscope. A pair of custom-made dichroic filters allows light from the excitation laser and three spectral bands ('colors') from the stimulator to reach the retina, leaving two intermediate bands for fluorescence imaging. Special optics allow displacement of the stimulator focus relative to the imaging focus. Spatially resolved changes in calcium-indicator fluorescence in response to visual stimuli were recorded in dendrites of different types of mammalian retinal neurons.

A dendrite-autonomous mechanism for direction selectivity in retinal starburst amacrine cells

Detection of image motion direction begins in the retina, with starburst amacrine cells (SACs) playing a major role. SACs generate larger dendritic Ca(2+) signals when motion is from their somata towards their dendritic tips than for motion in the opposite direction. To study the mechanisms underlying the computation of direction selectivity (DS) in SAC dendrites, electrical responses to expanding and contracting circular wave visual stimuli were measured via somatic whole-cell recordings and quantified using Fourier analysis. Fundamental and, especially, harmonic frequency components were larger for expanding stimuli. This DS persists in the presence of GABA and glycine receptor antagonists, suggesting that inhibitory network interactions are not essential. The presence of harmonics indicates nonlinearity, which, as the relationship between harmonic amplitudes and holding potential indicates, is likely due to the activation of voltage-gated channels. [Ca(2+)] changes in SAC dendrites evoked by voltage steps and monitored by two-photon microscopy suggest that the distal dendrite is tonically depolarized relative to the soma, due in part to resting currents mediated by tonic glutamatergic synaptic input, and that high-voltage-activated Ca(2+) channels are active at rest. Supported by compartmental modeling, we conclude that dendritic DS in SACs can be computed by the dendrites themselves, relying on voltage-gated channels and a dendritic voltage gradient, which provides the spatial asymmetry necessary for direction discrimination.

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.

Functional polarity of dendrites and axons of primate A1 amacrine cells

The A1 cell is an axon-bearing amacrine cell of the primate retina with a diffusely stratified, moderately branched dendritic tree (approximately 400 microm diameter). Axons arise from proximal dendrites forming a second concentric, larger arborization (>4 mm diameter) of thin processes with bouton-like swellings along their length. A1 cells are ON-OFF transient cells that fire a brief high frequency burst of action potentials in response to light (Stafford & Dacey, 1997). It has been hypothesized that A1 cells receive local input to their dendrites, with action potentials propagating output via the axons across the retina, serving a global inhibitory function. To explore this hypothesis we recorded intracellularly from A1 cells in an in vitro macaque monkey retina preparation. A1 cells have an antagonistic center-surround receptive field structure for the ON and OFF components of the light response. Blocking the ON pathway with L-AP4 eliminated ON center responses but not OFF center responses or ON or OFF surround responses. Blocking GABAergic inhibition with picrotoxin increased response amplitudes without affecting receptive field structure. TTX abolished action potentials, with little effect on the sub-threshold light response or basic receptive field structure. We also used multi-photon laser scanning microscopy to record light-induced calcium transients in morphologically identified dendrites and axons of A1 cells. TTX completely abolished such calcium transients in the axons but not in the dendrites. Together these results support the current model of A1 function, whereby the dendritic tree receives synaptic input that determines the center-surround receptive field; and action potentials arise in the axons, which propagate away from the dendritic field across the retina.

Multiple phosphorylation sites confer reproducibility of the rod's single-photon responses

Although signals controlled by single molecules are expected to be inherently variable, rod photoreceptors generate reproducible responses to single absorbed photons. We show that this unexpected reproducibility-the consistency of amplitude and duration of rhodopsin activity-varies in a graded and systematic manner with the number but not the identity of phosphorylation sites on rhodopsin's C terminus. These results indicate that each phosphorylation site provides an independent step in rhodopsin deactivation and that collectively these steps tightly control rhodopsin's active lifetime. Other G protein cascades may exploit a similar mechanism to encode accurately the timing and number of receptor activation.

G-protein-coupled enzyme cascades have intrinsic properties that improve signal localization and fidelity

G-protein-coupled enzyme cascades are used by eukaryotic cells to detect external signals and transduce them into intracellular messages that contain biological information relevant to the cell's function. Since G-protein-coupled receptors that are designed to detect different kinds of external signals can generate the same kind of intracellular response, effective signaling requires that there are mechanisms to increase signal specificity and fidelity. Here we examine the kinetic equations for the initial three stages in a generic G-protein-coupled cascade and show that the physical properties of the transduction pathway result in two intrinsic features that benefit signaling. 1), The response to a single activated receptor is naturally confined to a localized spatial domain, which could improve signal specificity by reducing cross talk. 2), The peak of the response generated by such a signaling domain is limited. This saturation effect reduces trial-to-trial variability and increases signaling fidelity by limiting the response to receptors that remain active for longer than average. We suggest that this mechanism for reducing response fluctuations may be a contributing factor in making the single photon responses of vertebrate retinal rods so remarkably reproducible.

Directionally selective calcium signals in dendrites of starburst amacrine cells

The detection of image motion is fundamental to vision. In many species, unique classes of retinal ganglion cells selectively respond to visual stimuli that move in specific directions. It is not known which retinal cell first performs the neural computations that give rise to directional selectivity in the ganglion cell. A prominent candidate has been an interneuron called the 'starburst amacrine cell'. Using two-photon optical recordings of intracellular calcium concentration, here we find that individual dendritic branches of starburst cells act as independent computation modules. Dendritic calcium signals, but not somatic membrane voltage, are directionally selective for stimuli that move centrifugally from the cell soma. This demonstrates that direction selectivity is computed locally in dendritic branches at a stage before ganglion cells.

Recovery of visual functions in a mouse model of Leber congenital amaurosis

The visual process is initiated by the photoisomerization of 11-cis-retinal to all-trans-retinal. For sustained vision the 11-cis-chromophore must be regenerated from all-trans-retinal. This requires RPE65, a dominant retinal pigment epithelium protein. Disruption of the RPE65 gene results in massive accumulation of all-trans-retinyl esters in the retinal pigment epithelium, lack of 11-cis-retinal and therefore rhodopsin, and ultimately blindness. We reported previously (Van Hooser, J. P., Aleman, T. S., He, Y. G., Cideciyan, A. V., Kuksa, V., Pittler, S. J., Stone, E. M., Jacobson, S. G., and Palczewski, K. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 8623-8628) that in Rpe65-/- mice, oral administration of 9-cis-retinal generated isorhodopsin, a rod photopigment, and restored light sensitivity to the electroretinogram. Here, we provide evidence that early intervention by 9-cis-retinal administration significantly attenuated retinal ester accumulation and supported rod retinal function for more than 6 months post-treatment. In single cell recordings rod light sensitivity was shown to be a function of the amount of regenerated isorhodopsin; high doses restored rod responses with normal sensitivity and kinetics. Highly attenuated residual rod function was observed in untreated Rpe65-/- mice. This rod function is likely a consequence of low efficiency production of 11-cis-retinal by photo-conversion of all-trans-retinal in the retina as demonstrated by retinoid analysis. These studies show that pharmacological intervention produces long lasting preservation of visual function in dark-reared Rpe65-/- mice and may be a useful therapeutic strategy in recovering vision in humans diagnosed with Leber congenital amaurosis caused by mutations in the RPE65 gene, an inherited group of early onset blinding and retinal degenerations.

GCAP1 rescues rod photoreceptor response in GCAP1/GCAP2 knockout mice

Visual transduction in retinal photoreceptors operates through a dynamic interplay of two second messengers, Ca(2+) and cGMP. Ca(2+) regulates the activity of guanylate cyclase (GC) and the synthesis of cGMP by acting on a GC-activating protein (GCAP). While this action is critical for rapid termination of the light response, the GCAP responsible has not been identified. To test if GCAP1, one of two GCAPs present in mouse rods, supports the generation of normal flash responses, transgenic mice were generated that express only GCAP1 under the control of the endogenous promoter. Paired flash responses revealed a correlation between the degree of recovery of the rod a-wave and expression levels of GCAP1. In single cell recordings, the majority of the rods generated flash responses that were indistinguishable from wild type. These results demonstrate that GCAP1 at near normal levels supports the generation of wild-type flash responses in the absence of GCAP2.

Engineering aspects of enzymatic signal transduction: photoreceptors in the retina

Identifying the basic module of enzymatic amplification as an irreversible cycle of messenger activation/deactivation by a "push-pull" pair of opposing enzymes, we analyze it in terms of gain, bandwidth, noise, and power consumption. The enzymatic signal transduction cascade is viewed as an information channel, the design of which is governed by the statistical properties of the input and the noise and dynamic range constraints of the output. With the example of vertebrate phototransduction cascade we demonstrate that all of the relevant engineering parameters are controlled by enzyme concentrations and, from functional considerations, derive bounds on the required protein numbers. Conversely, the ability of enzymatic networks to change their response characteristics by varying only the abundance of different enzymes illustrates how functional diversity may be built from nearly conserved molecular components.

Cyclic AMP has no effect on the generation, recovery, or background adaptation of light responses in functionally intact rod outer segments: with implications about the function of phosducin

In retinal rods, light exposure decreases the total outer segment content of both cGMP and cAMP by about 50%. The functional role of the light-evoked change in cAMP is not known. It is postulated to trigger changes in the phosphorylation state of phosducin, a phosphoprotein that is phosphorylated in the dark by cAMP-dependent protein kinase (PKA) and dephosphorylated by basal phosphatase activity when PKA is inhibited by the light-evoked drop in cAMP. In biochemical studies, dephosphorylated phosducin binds to free beta gamma dimer of transducin (Tbeta gamma) and prevents the regeneration of heterotrimeric transducin by blocking the re-association of the beta gamma and alpha subunits. Phosducin's interaction with Tbeta gamma is blocked when it is phosphorylated on a single residue by PKA. To evaluate the effect of the light-evoked fall in cAMP, functionally intact isolated lizard rod outer segments were dialyzed in whole-cell voltage clamp with a standard internal solution and electrical light responses were recorded with and without adding cAMP to the dialysis solution. Since the total outer segment content of cAMP in darkness is approximately 5 microM, internal dialysis with solution containing a much higher concentration (100 microM) of cAMP (or 8-bromo-cAMP) will overcome the effects of a light-evoked decrease in its concentration by keeping cAMP-dependent processes fully activated. Neither cyclic nucleotide had any influence on the generation, light sensitivity, recovery, or background adaptation of the flash response. These results also argue against the participation of phosducin in the sequence of events that are responsible for these aspects of rod function. This does not exclude the possibility of phosducin being involved in adaptation caused by higher light levels than used in the present study, that is, bleaching adaptation, or in light-dependent processes other than phototransduction.

Longitudinal spread of second messenger signals in isolated rod outer segments of lizards

1. In vertebrate rods activation of the phototransduction cascade by light triggers changes in the concentrations of at least two diffusible intracellular second messengers (cGMP and Ca2+) whose actions depend on how far they spread from their site of production or entry. To address questions about their spatial spread, cell-attached patch current recording and fluorescence imaging of Calcium Green-dextran were used to measure the longitudinal spread of cGMP and Ca2+, respectively, in functionally intact isolated Gecko gecko lizard rod outer segments under whole-cell voltage clamp. 2. The light-evoked changes in cGMP and Ca2+ concentrations decayed with distance from a site of steady focal activation by two-photon absorption of 1064 nm light with similar decay lengths of approximately 3.5 microm. 3. These results can be understood on the basis of a quantitative model of coupled diffusible intracellular messengers, which is likely to have broad relevance for second messenger signalling pathways in general. 4. The decay length for the spread of adaptation from a site of steady local illumination was about 8 microm, i.e. substantially longer than the decay lengths measured for the spread of cGMP and Ca2+. There are a number of factors, however, that could broaden the apparent relationship between functional changes in the light response and the concentration of a diffusible messenger. For these reasons the measured decay length is an upper limit estimate of the spread of adaptation and does not rule out the possibility that Ca2+ and/or cGMP carry the adaptation signal.

Optical recording of light-evoked calcium signals in the functionally intact retina

Using two-photon excitation of fluorescent indicator dyes, we measured calcium concentration transients in retinal ganglion and amacrine cells without destroying the light sensitivity of the retina by maximally activating or bleaching the photoreceptors. This allowed an immediate assessment of the cellular morphology and study of the calcium signals evoked by visual stimuli. Calcium dynamics in individual dendritic processes could be examined for extensive periods without deterioration and with little apparent phototoxicity at excitation wavelengths of from 930 to 990 nm. Light-evoked increases in calcium were resolved in ganglion- and amacrine-cell neurites, making it possible to use optical recording to study the relationship between calcium signaling and retinal function.

Protein kinase C and IP3 in photoresponses of functionally intact rod outer segments: constraints about their role

Protein kinase C and polyphosphoinositide metabolism are reported to affect light-activated processes in cell free systems. To investigate their role in phototransduction under more physiological conditions the effects of nonhydrolyzable inositol trisphosphate (IP3) analogs as well as of protein kinase C and phospholipase C inhibitors on the characteristics of the electrical light response were studied. Rod outer segments were dialyzed in whole-cell voltage clamp and photoresponses in the presence and absence of the tested compounds were compared. None of the compounds influenced the light responses suggesting that neither IP3 nor protein kinase C participate in the phototransduction cascade. A number of different proposals about the participation of protein kinase C and inositol trisphosphate (IP3) in the phototransduction process based on a wide variety of in vitro experiments should therefore be reevaluated.

Effect of rhodopsin C-terminal peptide on photoresponses in functionally intact rod outer segments

The protein-protein interactions that underlie shut-off of the light-activated rhodopsin were studied using synthetic peptides derived from C-terminal region of the rhodopsin. The photoresponses were recorded in whole-cell voltage clamp from rod outer segments (ROS) that were internally dialyzed with an intracellular solution containing the synthetic peptides. This was the first time that synthetic peptides have been used in functionally intact ROS. None of the tested peptides promoted the shut-off of the photolyzed rhodopsin (R) by stimulating the binding of an activated arrestin to non-phosphorylated R, contrary to what was expected from in vitro experiments (Puig et al. FEBS Lett. 362: 185-188, 1995).

Arrestin with a single amino acid substitution quenches light-activated rhodopsin in a phosphorylation-independent fashion

Arrestins are members of a superfamily of regulatory proteins that participate in the termination of G protein-mediated signal transduction. In the phototransduction cascade of vertebrate rods, which serves as a prototypical G protein-mediated signaling pathway, the binding of visual arrestin is stimulated by phosphorylation of the C-terminus of photoactivated rhodopsin (Rh*). Arrestin is very selective toward light-activated phosphorhodopsin (P-Rh*). Previously we reported that a single amino acid substitution in arrestin, Arg175Gln, results in a dramatic increase in arrestin binding to Rh* [Gurevich, V. V., & Benovic, J. L. (1995) J. Biol. Chem. 270, 6010-6016]. Here we demonstrate that a similar mutant, arrestin(R175E), binds to light-activated rhodopsin independent of phosphorylation. Arrestin(R175E) binds with high affinity not only to P-Rh* and Rh* but also to light-activated truncated rhodopsin in which the C-terminus phosphorylation sites have been proteolytically removed. In an in vitro assay that monitored rhodopsin-dependent activation of cGMP phosphodiesterase (PDE), wild type arrestin quenched PDE response only when ATP was present to support rhodopsin phosphorylation. In contrast, as little as 30 nM arrestin(R175E) effectively quenched PDE activation in the absence of ATP. Arrestin(R175E) had no effect when the lifetime of Rh* no longer contributed to the time course of PDE activity, suggesting that it disrupts signal transduction at the level of rhodopsin-transducin interaction.

Ca2+ dependence of dark- and light-adapted flash responses in rod photoreceptors

Light adaptation is thought to be orchestrated by a Ca2+ feedback signal that desensitizes the response by speeding recovery. To evaluate the role of Ca2+ in adaptation, we compared the effect of lowered Ca2+ on response properties in darkness and during adaptation. Internal Ca2+ was reduced from its normal resting dark level (535 nM) by either background illumination or exposure to Ringer's solution containing low Ca2+ and/or cyclic GMP-gated channel blockers in darkness. Ca2+ reductions in light decreased the activation gain of the transduction process and speeded recovery kinetics, while equivalent Ca2+ reductions in darkness caused similar gain reduction without accelerating recovery. This indicates that adaptational changes in the response are not due purely to feedback effects on recovery.

The mechanisms of vertebrate light adaptation: speeded recovery versus slowed activation

Light adaptation in vertebrate photoreceptors is commonly attributed to a feedback mechanism that reduces the amplitude of the receptor potential by speeding the inactivation of the transduction cascade and hastening the recovery process. Recent studies have challenged this model and suggest instead that desensitization originates mainly from changes in the activation phase rather than the recovery phase of the response. This has important implications for understanding the molecular mechanisms that underlie the control of sensitivity in this G-protein-coupled, signal-transduction pathway.

The calcium feedback signal in the phototransduction cascade of vertebrate rods

Intracellular free Ca (Cai) was measured in functionally intact rod outer segments in darkness and during light responses using the fluorescent Ca indicator Indo-dextran. In darkness, Cai was 554 +/- 25 nM (n = 28) for -85 +/- 2 pA of circulating dark current (Id) and declined in saturating light to a minimum value of approximately 50 nM with a time course that paralleled the fall in Na:Ca,K exchange current. During a subsaturating flash response that reduced Id by 70%, Cai fell to a minimum of approximately 325 nM and recovered incompletely to a plateau of approximately 450 nM that lasted approximately 15 s after full recovery of Id. During a 60 s step that caused approximately 7-fold reduction in sensitivity of superimposed flash responses, Cai reached a steady-state level of approximately 252 nM.

Purification and physiological evaluation of a guanylate cyclase activating protein from retinal rods

In retinal rods light triggers a cascade of enzymatic reactions that increases cGMP hydrolysis and generates an electrical signal by causing closure of cGMP-gated ion channels in the photoreceptor outer segment. This leads to a decrease in internal Ca, which activates guanylate cyclase and promotes photoresponse recovery by stimulating the resynthesis of cGMP. We report here that the activation of guanylate cyclase by low Ca is mediated by an approximately 20-kDa protein purified from bovine rod outer segments by using DEAE-Sepharose, hydroxylapatite, and reverse-phase chromatographies. In a reconstituted system, this protein restores the Ca-sensitive regulation of guanylate cyclase and when dialyzed into functionally intact lizard rod outer segment decreases the sensitivity, time to peak, and recovery time of the flash response.

Visual transduction in dialysed detached rod outer segments from lizard retina

1. Properties of a new preparation for studying the physiology and biochemistry of phototransduction in retinal rods are described. Whole-cell voltage clamp was used to record the generation, maintenance and light-sensitivity of dark current in rod outer segments that had been isolated from the rest of the receptor cell by detachment at the connecting cilium. 2. Detached outer segments dialysed with standard internal solution supplemented with physiological amounts of ATP (5 mM) and GTP (1 mM) developed a standing inward dark current that was the sum of three components: approximately 91% light-sensitive current, approximately 6% Na(+)-Ca2+,K+ exchange current and approximately 3% leakage current. Light-sensitive dark current (mean amplitude approximately -63 pA) was suppressed transiently by brief flashes in an intensity-dependent manner. Light responses had the same kinetics, sensitivity and intensity-response relationship as those recorded from intact rods. 3. Dialysed outer segments differed from intact rods in that intense flashes evoked saturating responses that recovered incompletely to a plateau of reduced dark current caused by incomplete inactivation of the transduction cascade. Light sensitivity was reduced for a short time following an intense flash and then recovered despite persistent reduction of dark current. This suggests that there is no fixed relationship between dark current amplitude and light sensitivity. 4. Light-sensitive dark current faded rapidly when outer segments were not supplied with nucleotides. Outer segments dialysed with solution that contained cyclic GMP, but no ATP or GTP, supported dark current at a level that increased with [cyclic GMP]. When basal phosphodiesterase (PDE) activity is inhibited, 8 microM cyclic GMP supports a dark current of approximately 70 pA. 5. Light sensitivity decreased during recordings made with solution that contained only cyclic GMP, consistent with the inhibition of G protein activation by loss of GTP. After thorough nucleoside triphosphate depletion, however, intense illumination evoked a transient increase rather than a decrease in dark current, i.e. an inverted light response. This result suggests that isomerized rhodopsin may generate a signal that causes either inhibition of basal PDE activity or release of bound cyclic GMP. 6. Sustained Na(+)-Ca2+,K+ exchange current was recorded during steady illumination when Ca2+, but not when Mg2+, was added to the dialysis solution. Exchange current increased with the amount of added Ca2+ and saturated at approximately 18 pA when the dialysis solution contained > or = 10 mM Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of recoverin-like calcium-binding proteins on the photoresponse of retinal rods

The rod photoresponse is triggered by an enzyme cascade that stimulates cGMP hydrolysis. The resulting fall in cGMP leads to a decrease in Ca2+, which promotes photoresponse recovery by activating guanylate cyclase, causing cGMP resynthesis. In vitro biochemical studies suggest that Ca2+ activation of guanylate cyclase is medicated by recoverin, a 26 kd Ca(2+)-binding protein. To evaluate this, exogenous bovine recoverin and two other homologous Ca(2+)-binding proteins from chicken and Gecko retina were dialyzed into functionally intact Gecko rods using whole-cell recording. All three proteins prolonged the rising phase of the photoresponse without affecting the kinetics of response recovery. These results suggest that recoverin-like proteins affect termination of the transduction cascade, rather than mediate Ca(2+)-sensitive activation of guanylate cyclase.

Some unresolved issues in the physiology and biochemistry of phototransduction

A number of recent review articles have discussed what is known about the events responsible for generating the electrical light response in vertebrate photoreceptors. The similarity of the material covered and the unanimity of the conclusions drawn have given rise to the popular, but false, impression that visual transduction is understood fully. The purpose of the present review is to dispell this notion by focusing on some of the unresolved issues.

The influence of arrestin (48K protein) and rhodopsin kinase on visual transduction

The shutoff of the phototransduction cascade in retinal rods requires the inactivation of light-activated rhodopsin. The underlying mechanisms were studied in functionally intact detached rod outer segments by testing the effect of either sangivamycin, an inhibitor of rhodopsin kinase, or phytic acid, an inhibitor of 48K protein binding to phosphorylated rhodopsin, on light responses recorded in whole-cell voltage clamp. The results suggest that isomerized rhodopsin is inactivated fully by multiple phosphorylation and that the binding of 48K protein accelerates recovery by quenching partially phosphorylated rhodopsin. Higher concentrations of sangivamycin cause changes in the light response that cannot be explained by selective inhibition of rhodopsin kinase and suggest that other protein kinases are needed for normal rod function.

Distribution of membrane proteins in mechanically dissociated retinal rods

Solitary rods were isolated from frog retinas by mechanical dissociation. Typically, the rods cleave sclerad to the nucleus and consist of outer segments with attached partial inner segments with either tapered or rounded profiles. Light and electron microscopy reveal that the outer and inner segments of rods with tapered inner segments, like rods in the intact retina, are joined by a single connecting cilium. In contrast, the outer and inner segments of rods with rounded inner segments are fused, with no extracellular cleft between the two segments. Opsin distribution was studied in both unfused and fused rods by light and electron microscopic immunocytochemistry. Extensive surface labeling is restricted to the outer segments of tapered rods, as observed in vivo. In contrast, both inner and outer segments of rods with rounded inner segments (fused) label heavily with anti-opsin. Thus opsin, a mobile membrane protein, diffuses from the outer to the inner segment of fused rods. Segregated distribution of opsin in unfused rods suggests that the connecting cilium and/or its associated structures may normally act as a diffusion barrier between the outer and inner segments to mobile membrane proteins such as opsin. Immunofluorescence studies demonstrate that Na+/K+ ATPase is restricted in distribution to the inner segment and calycal processes of both fused and unfused isolated rods, as observed in vivo. Maintenance of its restricted distribution in fused cells indicates that Na+/K+ ATPase is not mobile and may be tethered in the surface membrane of the inner segment.

Intracellular biochemical manipulation of phototransduction in detached rod outer segments

Recent progress in understanding phototransduction has come primarily from studies on cell-free systems. To investigate the transduction process under physiological conditions, a fully functional preparation of retinal rod outer segments without attached inner segments was developed that allows electrical recording of light-sensitive current during intracellular dialysis with defined solutions. No light-sensitive current is recorded from detached outer segments dialyzed with nucleotide-free solutions, whereas cells detached from the retina into Ringer's solution containing 3-isobutyl-1-methyl-xanthine (a phosphodiesterase inhibitor) develop a light-sensitive inward dark current. This indicates that there is a basal level of cGMP-specific phosphodiesterase activity in the dark. Detached outer segments dialyzed with greater than or equal to 20 microM cGMP rapidly develop a light-suppressible current. A current of similar magnitude is generated more slowly during dialysis with a 50-fold greater concentration of GTP. Apparently, cGMP can be synthesized from GTP by guanylate cyclase in the outer segment. Cells dialyzed with cGMP alone show a reduced light sensitivity that is restored to normal by addition of 20 microM GTP. This action of GTP is antagonized by guanosine 5'-[beta-thio]diphosphate. These findings are in good agreement with biochemical evidence indicating that a GTP-binding protein (transducin) plays a pivotal role in the generation of responses to light. The recovery of photocurrent following a brief flash is delayed or abolished by dialysis with solutions that lack ATP or contain guanosine 5'-[gamma-thio]triphosphate, a nonhydrolyzable GTP analog. These results support the view that both GTP hydrolysis by activated transducin and ATP-dependent phosphorylation of a rhodopsin photoproduct are necessary for termination of the transduction process.

Light-evoked contraction of the photosensitive iris of the frog

Smooth muscle cells of the frog iris sphincter contain rhodopsin and contract in response to light. The mechanism of light-evoked contraction was studied with particular attention paid to the role of calcium. The contractile proteins of the sphincter smooth muscle cell can be activated by an increase in intracellular calcium. Light-evoked contraction, however, is not accompanied by a measurable change in membrane potential and occurs in the absence of extracellular Na+ and/or Ca2+, as well as in the presence of isotonic KCI. Maximum light-evoked tension is reduced by exposure to Ca2+-free solutions containing EGTA and high K+ and is restored by incubation in solutions containing Ca2+. The restored response, which persists after return to Ca2+-free solution, depends on the concentration of Ca2+ in the incubating solution and on the duration of the incubation. The results support the conclusion that light produces contraction of the iris sphincter by causing the release of Ca2+ from an intracellular storage site.

Patch-clamp recordings of the light-sensitive dark noise in retinal rods from the lizard and frog

In cell-attached recordings from rods in the intact lizard retina, light decreased a standing inward membrane current with a reversal potential approximately 60 mV more positive than the resting potential. The peak amplitude of saturating responses depended upon the area of recorded membrane and varied from cell to cell over approximately 100-fold range. Small patches of membrane gave variable responses to identical moderately intense flashes. Whole-cell voltage-clamp recordings were obtained on isolated frog rods with intact ellipsoids. Peak whole-cell photocurrent was related to flash intensity by a Michaelis equation with saturating response amplitudes ranging up to 30 pA in 0.1 mM-Ca2+ Ringer solution. In darkness the steady-state current-voltage relation, determined with whole-cell voltage clamp, showed outward rectification. Photocurrent had nearly constant amplitude between -80 and -10 mV, a mean reversal potential of +8 mV and recovered from flashes more slowly at positive holding potentials. Although it was not possible to resolve light-sensitive single-channel current events, power spectral analysis revealed both low- and high-frequency components of the light-sensitive noise in both cell-attached and whole-cell recordings. The low-frequency component was described by the product of two Lorentzians using time constants derived from the kinetics of the dim flash response. The high-frequency component of the light-sensitive noise was described by a single Lorentzian with a half-power frequency of 62 Hz in lizard and 212 Hz in frog. The half-power frequency was not appreciably affected by steady illumination. The Lorentzian nature of the noise suggests that the light-sensitive channel is a pore rather than a shuttle-type carrier. In cell-attached recordings the high-frequency component declined monotonically with increasing light intensity, suggesting that less than one-half of the channels are open in darkness. Furthermore, the ratio of the variance of the high-frequency noise to the mean photocurrent was independent of light intensity. Changing external Ca2+ from 0.1 to 0.5 mM reduced the ratio from 19.7 to 9.0 fA without a significant effect on the cut-off frequency of the noise. The results support the conclusion that the light-sensitive pore is opened by an internal transmitter that acts as an agonist and that both open and closed states of the pore may be blocked by external Ca2+. The conductance of the light-sensitive pore in the absence of external Ca2+ is estimated to be 1.25-2 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic and electrophysiological properties of retinal Müller (glial) cells of the turtle

The ionic and electrophysiological properties of Müller cells, the principal glial element of the vertebrate retina, were investigated. The membrane potential of enzymatically dissociated and in situ Müller cells was about -80 mV and depended on external K+ concentration in a manner that was described by the Goldman-Hodgkin-Katz equation with a Na+-K+ permeability ratio of 0.037. The current-voltage relation showed marked inward rectification, with the input resistance at the resting potential being about 30 M omega for dissociated cells and about 3 M omega for in situ cells. In situ Müller cells were found to be electrically coupled to each other which could explain their lower resistance. We conclude that Müller cells are similar to other types of glia. In spite of a finite Na+ permeability their membrane potential is determined mainly by K+, they are electrically inexcitable and form an electrically coupled network in the retina.

Selective uptake of lucifer yellow by retinal cells

When turtle retinae were incubated with the fluorescent dye, lucifer yellow, in the absence of Ca2+, the dye was selectively accumulated by cell bodies located in the inner nuclear layer (INL). The morphological features of the labeled cells suggested that they were bipolar cells. Other fluorescent dyes, Procion yellow and Primulin, were also taken up by somata in the INL, in the absence of external Ca2+, although the identity of the labeled cells was uncertain. As with turtle retina, lucifer yellow was accumulated predominantly by cell bodies in the INL of goldfish, frog, and rat retinae. Lucifer yellow uptake appeared to be independent of synaptic activity since dark-adaptation or aspartate treatment of retinae did not alter the dye uptake. Further, retinae from dystrophic (RCS) rats showed uptake similar to that seen in normal rat retinae. After uptake, most of the dye was found intracellularly as patches or vacuoles in the somata of the labeled cells. Dye uptake was not inhibited by removal of Na+ from the incubation medium. Further, prior treatment with metabolic inhibitors, cyanide and iodoacetate, or cytochalasin B, did not block the dye uptake.

Temporal and spatial characteristics of the voltage response of rods in the retina of the snapping turtle

1. In response to strong, large-field flashes the dark-adapted rods of Chelydra serpentina gave initial hyperpolarizing responses of 30-40 mV, declining rapidly to plateaus of 10-15 mV which lasted 20 sec or more.2. In the most sensitive cells the flash-sensitivity at 520 nm to a large illuminated area was 3-6 mV per photoisomerization (assuming an effective collecting area of 13.6 mum(2)).3. The initial response to a step of light agreed with that predicted by super-position from the flash response but even with very weak lights the step response fell below the predicted curve at times longer than about 2 sec.4. The step sensitivity defined from the initial peak of the response to a step of light was 2-6 mV photoisomerization(-1) sec, about 1000 times greater than the most sensitive cones in the turtle retina.5. The response to a small weakly illuminated spot (radius 21 mum) reached a peak later and lasted longer than the linear response to a weakly illuminated large area (radius 570 mum).6. The difference in sensitivity between large and small spots was reasonably consistent with the apparent space constant of the rod network obtained from the exponential decline of the flash response on either side of an illuminated strip.7. As others have found, strong flashes did not give an initial hyperpolarizing transient when the radius of the spot was less than about 50 mum.8. Experiments made by flashing long narrow strips of light onto the retina showed that the response spread a long way initially (lambda =... 70 mum) and then contracted down to a relatively small region (lambda =... 25 mum) at times of about 2 sec. When the line source was at some distance from the impaled rod the response reached a peak earlier and was shorter than when the source was close.9. The results in (8) can be explained quantitatively by assuming that delayed voltage-dependent conductance changes mimic an inductance and make the rod network behave like a high-pass filter with series resistance and parallel inductance.10. In sensitive rods, flash responses varied randomly with a variance which was about 1/30 of that expected in an isolated cell; this reduction in noise is satisfactorily explained by electrical coupling between rods.11. The variance peak usually occurred later than the potential peak of the rod response.12. The high-pass filter characteristics of the rod-network help to explain several puzzling features of the behaviour of rods, for example (1), (5), (7), (8) and (11) of this summary.13. The high-pass filter characteristics of the rod-network may help it to optimize the signal to noise ratio by integrating over a large area for rapid signals and over a small one for slow signals.

Electrical coupling between cones in turtle retina

1. The electrical coupling between cones of known spectral sensitivity in the peripheral part of the turtle's retina was studied by passing current through a micro-electrode inserted into one cone and recording with a second micro-electrode inserted into a neighbouring cone. 2. Spatial sensitivity profiles were determined by recording flash responses to a long narrow strip of light which was moved across the impaled cones in orthogonal directions. These measurements gave both the length constant lambda of electrical spread in the cone network and the separation of the two cones. 3. The cone separation determined from the spatial profiles agreed closely with that measured directly by injecting a fluorescent dye into two cones. 4. The length constant lambda varied from 18 to 39 micron with a mean of 25 micron for red-sensitive cones and 26 micron for green-sensitive cones. 5. The majority of cone pairs studied were electrically coupled provided they had the same spectral sensitivity and were separated by less than 60 micron: thirty-two out of thirty-six red-red pairs, two out of two green-green pairs, none out of eight red-green pairs: no blue cones were observed. 6. The strength of electrical coupling was expressed as a mutual resistance defined as the voltage in one cell divided by the current flowing into the other. Mutual resistances decreased from a maximum value of about 30 M omega at separations close to zero to 0.2 M omega, the lower limit of detectable coupling at separations of about 60 micron. Mutual resistances were always positive and were independent of which cell was directly polarized. The coupling seemed to be ohmic and any rectification or non-linearity probably arose in the cone membranes rather than in the coupling resistances. 7. The results were analysed in terms of the Lamb & Simon (1977) theories of square and hexagonal lattices, which approximate to the continuous sheet model except in the case of the cone to which current is applied. 8. The total membrane resistance of a single cone was estimated as 100--300 M omega and the connecting resistances as 100 M omega for a square array and 170 M omega for a hexagonal array. The input resistance of a cone in the network was 25--50 M omega. Lower values were often obtained but may be due to injury by the micro-electrodes. 9. The time constant of an isolated cone was estimated as about 20 msec and the capacity as about 100 pF. 10. Discrepancies between experimental findings and theoretical predictions of the hexagonal or square array models were tentatively attributed to an overestimate of lambda resulting from light scattering.

A surprising property of electrical spread in the network of rods in the turtle's retina

Flashing a localised stimulus onto a turtle's retina produces an intracellular potential wave which spreads through electrical connections from illuminated to unilluminated photoreceptors. The response in unilluminated rods (but not in cones) becomes faster as the distance from the source increases, perhaps because voltage-dependent permeability changes in the rod membrane make the coupled network behave like a high-pass filter.

Multiple light-evoked conductance changes in the photoreceptors of Hermissenda crassicornis

1. Light responses were recorded from the photoreceptors of Hermissenda crassicornis. The response to a flash is a complex potential change involving an initial depolarization, a hyperpolarization, and a depolarizing tail. None of the phases of the response are due to synaptic interactions.2. Polarization of the membrane by extrinsic current indicates that three separate conductance changes are associated with the response. The initial depolarization and hyperpolarization are accompanied by conductance increases and the tail with a conductance decrease. The initial depolarization has a positive reversal potential and the hyperpolarizing and tail phase have a reversal voltage more negative than resting potential.3. The different processes that give rise to the conductance changes have similar spectral sensitivities but are affected unequally by light adaptation. Strong light adaptation reduced the depolarizing phases more than the hyperpolarizing phase, so that following an adapting stimulus the cell responded to illumination with a pure hyperpolarization (isolated hyperpolarization).4. Removal of external Na(+) ions greatly reduced the initial depolarization. In Na(+)-free sea water the cell responds to dim flashes with a slow depolarization (isolated tail) that involves a conductance decrease, and has the same reversal potential as the hyperpolarizing response recorded from light adapted cells.5. The amplitude of the isolated hyperpolarization and tail varied inversely with the external K(+) concentration.6. It is concluded that in Hermissenda photoreceptors light initiates processes that result in three distinct permeability changes. Following a brief flash there is: a rapid and transient increase in Na(+) permeability that is responsible for the initial depolarization, a less rapid increase in K(+) permeability that is responsible for the hyperpolarizing phase, and a delayed decrease in K(+) permeability that gives rise to the depolarizing tail.

Responses of hair cells in the statocyst of Hermissenda

Responses to mechanical stimulation were recorded from hair cells in the statocyst of Hermissenda crassicornis. The response to a brief stimulus is a depolarizing wave which reaches peak in about 25 msec and decays slowly. 2. Hyperpolarization by extrinsic currents increases the amplitude of the response; depolarization decreases it and eventually reverses its polarity. It is inferred from these results that the primary outcome of the transduction process is an increase of membrane conductance and that the voltage change (generator potential) follows as a secondary event. 3. The features of the conductance change were reconstructed from the time course of the generator potential and the passive properties of the membrane. It was found that the increase of membrane conductance develops slowly and is roughly proportional to the energy delivered by the stimulus. 4. The time course of the conductance change required to reproduce the generator potential is similar to the output of a model involving a sequence of transformations. 5. The generator potential is sensitive to temperature, becoming faster as temperature is raised. This effect is reproduced by the model if the transition rates are assumed to be temperature-dependent, with a Q10 of about 2. 6. It is concluded that a chain of temperature-sensitive processes is interposed between the stimulus and the increase of membrane conductance.

Hair cell interactions in the statocyst of Hermissenda

Hair cells in the statocyst of Hermissenda crassicornis respond to mechanical stimulation with a short latency (<2 ms) depolarizing generator potential that is followed by hyperpolarization and inhibition of spike activity. Mechanically evoked hyperpolarization and spike inhibition were abolished by cutting the static nerve, repetitive mechanical stimulation, tetrodotoxin (TTX), and Co(++). Since none of these procedures markedly altered the generator potential it was concluded that the hyperpolarization is an inhibitory synaptic potential and not a component of the mechanotransduction process. Intracellular recordings from pairs of hair cells in the same statocyst and in statocysts on opposite sides of the brain revealed that hair cells are connected by chemical and/or electrical synapses. All chemical interactions were inhibitory. Hyperpolarization and spike inhibition result from inhibitory interactions between hair cells in the same and in opposite statocysts.