Feedback from luminosity horizontal cells mediates depolarizing responses of chromaticity horizontal cells in the Xenopus retina

Diverse Cell Types, Circuits, and Mechanisms for Color Vision in the Vertebrate Retina

Synaptic interactions to extract information about wavelength, and thus color, begin in the vertebrate retina with three classes of light-sensitive cells: rod photoreceptors at low light levels, multiple types of cone photoreceptors that vary in spectral sensitivity, and intrinsically photosensitive ganglion cells that contain the photopigment melanopsin. When isolated from its neighbors, a photoreceptor confounds photon flux with wavelength and so by itself provides no information about color. The retina has evolved elaborate color opponent circuitry for extracting wavelength information by comparing the activities of different photoreceptor types broadly tuned to different parts of the visible spectrum. We review studies concerning the circuit mechanisms mediating opponent interactions in a range of species, from tetrachromatic fish with diverse color opponent cell types to common dichromatic mammals where cone opponency is restricted to a subset of specialized circuits. Distinct among mammals, primates have reinvented trichromatic color vision using novel strategies to incorporate evolution of an additional photopigment gene into the foveal structure and circuitry that supports high-resolution vision. Color vision is absent at scotopic light levels when only rods are active, but rods interact with cone signals to influence color perception at mesopic light levels. Recent evidence suggests melanopsin-mediated signals, which have been identified as a substrate for setting circadian rhythms, may also influence color perception. We consider circuits that may mediate these interactions. While cone opponency is a relatively simple neural computation, it has been implemented in vertebrates by diverse neural mechanisms that are not yet fully understood.

The role of nitric oxide in spectral information processing in the distal turtle retina

Chromaticity type horizontal cells (C-type HCs) are the first retinal neurons exhibiting spectral information processing in cold-blooded vertebrates. The simple input of hyperpolarizing responses of cone photoreceptors is transformed in the C-type HCs into spectral opponent output. Nitric oxide (NO), a known background neuromodulator in the distal retina, was tested here for its effects upon spectral information processing by C-type HCs in the retina of turtle. Photoresponses were intracellularly recorded from C-type HCs, using light stimuli of different wavelength, applied over backgrounds of different wavelengths, and changing retinal NO level. Raising retinal level of NO in darkness by adding the precursor for its synthesis (l-Arginine) augmented the depolarizing photoresponses elicited by long-wavelength light stimuli, and reduced the hyperpolarizing photoresponses elicited by short-wavelength light stimuli. Lowering retinal level of NO by l-NAME, an inhibitor of NO synthesis, induced the opposite effects. However, the total voltage range of operation remained constant regardless of the level of NO. Qualitatively similar effects were observed under background illuminations regardless of background strength and wavelength. Altering retinal level of NO exerted a small effect upon the null wavelength. Our findings are consistent with the known effects of NO upon turtle distal retinal neurons, with the addition of NO strengthening the negative feedback pathway from L-type horizontal cells onto medium-wavelength cones.

Colour in the eye of the beholder: receptor sensitivities and neural circuits underlying colour opponency and colour perception

Colour vision-the ability to discriminate spectral differences irrespective of variations in intensity-has two basic requirements: (1) photoreceptors with different spectral sensitivities, and (2) neural comparison of signals from these photoreceptors. Major progress has been made understanding the evolution of the basic stages of colour vision-opsin pigments, screening pigments, and the first neurons coding chromatic opponency, and similarities between mammals and insects point to general mechanisms. However, much work is still needed to unravel full colour pathways in various animals. While primates may have brain regions entirely dedicated to colour coding, animals with small brains, such as insects, likely combine colour information directly in parallel multisensory pathways controlling various behaviours.

Extracellular ATP hydrolysis inhibits synaptic transmission by increasing ph buffering in the synaptic cleft

A slow mechanism of retinal synaptic inhibition involves hydrolysis of ATP released from pannexin 1 channels (from the tips of horizontal cell dendrites); the resulting protons and phosphates acidify the synaptic cleft, which inhibits neurotransmitter release.

Neuronal computations strongly depend on inhibitory interactions. One such example occurs at the first retinal synapse, where horizontal cells inhibit photoreceptors. This interaction generates the center/surround organization of bipolar cell receptive fields and is crucial for contrast enhancement. Despite its essential role in vision, the underlying synaptic mechanism has puzzled the neuroscience community for decades. Two competing hypotheses are currently considered: an ephaptic and a proton-mediated mechanism. Here we show that horizontal cells feed back to photoreceptors via an unexpected synthesis of the two. The first one is a very fast ephaptic mechanism that has no synaptic delay, making it one of the fastest inhibitory synapses known. The second one is a relatively slow (τ≈200 ms), highly intriguing mechanism. It depends on ATP release via Pannexin 1 channels located on horizontal cell dendrites invaginating the cone synaptic terminal. The ecto-ATPase NTPDase1 hydrolyses extracellular ATP to AMP, phosphate groups, and protons. The phosphate groups and protons form a pH buffer with a pKa of 7.2, which keeps the pH in the synaptic cleft relatively acidic. This inhibits the cone Ca²⁺ channels and consequently reduces the glutamate release by the cones. When horizontal cells hyperpolarize, the pannexin 1 channels decrease their conductance, the ATP release decreases, and the formation of the pH buffer reduces. The resulting alkalization in the synaptic cleft consequently increases cone glutamate release. Surprisingly, the hydrolysis of ATP instead of ATP itself mediates the synaptic modulation. Our results not only solve longstanding issues regarding horizontal cell to photoreceptor feedback, they also demonstrate a new form of synaptic modulation. Because pannexin 1 channels and ecto-ATPases are strongly expressed in the nervous system and pannexin 1 function is implicated in synaptic plasticity, we anticipate that this novel form of synaptic modulation may be a widespread phenomenon.

At the first retinal synapse, specific cells—horizontal cells (HCs)—inhibit photoreceptors and help to organize the receptive fields of another retinal cell type, bipolar cells. This synaptic interaction is crucial for visual contrast enhancement. Here we show that horizontal cells feed back to photoreceptors via a very fast ephaptic mechanism and a relatively slow mechanism. The slow mechanism requires ATP release via Pannexin 1 (Panx1) channels that are located on HC dendrites near the site where photoreceptors release the neurotransmitter glutamate to HCs and bipolar cells. The released ATP is hydrolyzed to produce AMP, phosphate groups, and protons; these phosphates and protons form a pH buffer, which acidifies the synaptic cleft. This slow acidification inhibits presynaptic calcium channels and consequently reduces the neurotransmitter release of photoreceptors. This demonstrates a new way in which ATP release can be involved in synaptic modulation. Surprisingly, the action of ATP is not purinergic but is mediated via changes in the pH buffer capacity in the synaptic cleft. Given the broad expression of Panx1 channels in the nervous system and the suggestion that Panx1 function underlies stabilization of synaptic plasticity and is needed for learning, we anticipate that this mechanism will be more widespread than just occurring at the first retinal synapse.

Synaptic transmission from horizontal cells to cones is impaired by loss of connexin hemichannels

In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.

Contrast enhancement is a fundamental feature of our visual system, initiated at the first synaptic connections in the retina. These are the synapses between photoreceptors (rods and cones) and their targets, horizontal cells and bipolar cells. Horizontal cells receive input from many cones and subsequently send a feedback signal to photoreceptors. Bipolar cells, however, receive direct input from only a few photoreceptors, but also receive indirect inhibitory input from surrounding cones via the horizontal cell feedback pathway. This organization induces the classic center/surround organization of bipolar cells and is considered the first step in contrast enhancement. Exactly how horizontal cells send feedback signals to photoreceptors has remained a mystery, however. One hypothesis posits that connexin hemichannels are involved. In this study, we tested this hypothesis using mutant zebrafish that lack connexin hemichannels specifically in horizontal cells. Our electrophysiology experiments showed that feedback is indeed reduced in these mutants, confirming that connexin hemichannels play an important role in feedback from horizontal cells to cones. In addition, we find that these mutant fish have decreased contrast sensitivity at a behavioral level, illustrating that functionally relevant contrast enhancement begins at the first synapse of the visual system.

Hemichannel-mediated and pH-based feedback from horizontal cells to cones in the vertebrate retina

BACKGROUND

Recent studies designed to identify the mechanism by which retinal horizontal cells communicate with cones have implicated two processes. According to one account, horizontal cell hyperpolarization induces an increase in pH within the synaptic cleft that activates the calcium current (Ca(2+)-current) in cones, enhancing transmitter release. An alternative account suggests that horizontal cell hyperpolarization increases the Ca(2+)-current to promote transmitter release through a hemichannel-mediated ephaptic mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

To distinguish between these mechanisms, we interfered with the pH regulating systems in the retina and studied the effects on the feedback responses of cones and horizontal cells. We found that the pH buffers HEPES and Tris partially inhibit feedback responses in cones and horizontal cells and lead to intracellular acidification of neurons. Application of 25 mM acetate, which does not change the extracellular pH buffer capacity, does lead to both intracellular acidification and inhibition of feedback. Because intracellular acidification is known to inhibit hemichannels, the key experiment used to test the pH hypothesis, i.e. increasing the extracellular pH buffer capacity, does not discriminate between a pH-based feedback system and a hemichannel-mediated feedback system. To test the pH hypothesis in a manner independent of artificial pH-buffer systems, we studied the effect of interfering with the endogenous pH buffer, the bicarbonate/carbonic anhydrase system. Inhibition of carbonic anhydrase allowed for large changes in pH in the synaptic cleft of bipolar cell terminals and cone terminals, but the predicted enhancement of the cone feedback responses, according to the pH-hypothesis, was not observed. These experiments thus failed to support a proton mediated feedback mechanism. The alternative hypothesis, the hemichannel-mediated ephaptic feedback mechanism, was therefore studied experimentally, and its feasibility was buttressed by means of a quantitative computer model of the cone/horizontal cell synapse.

CONCLUSION

We conclude that the data presented in this paper offers further support for physiologically relevant ephaptic interactions in the retina.

Lateral gain control in the outer retina leads to potentiation of center responses of retinal neurons

The retina can function under a variety of adaptation conditions and stimulus paradigms. To adapt to these various conditions, modifications in the phototransduction cascade and at the synaptic and network levels occur. In this paper, we focus on the properties and function of a gain control mechanism in the cone synapse. We show that horizontal cells, in addition to inhibiting cones via a "lateral inhibitory pathway," also modulate the synaptic gain of the photoreceptor via a "lateral gain control mechanism." The combination of lateral inhibition and lateral gain control generates a highly efficient transformation. Horizontal cells estimate the mean activity of cones. This mean activity is subtracted from the actual activity of the center cone and amplified by the lateral gain modulation system, ensuring that the deviation of the activity of a cone from the mean activity of the surrounding cones is transmitted to the inner retina with high fidelity. Sustained surround illumination leads to an enhancement of the responses of transient ON/OFF ganglion cells to a flickering center spot. Blocking feedback from horizontal cells not only blocks the lateral gain control mechanism in the outer retina, but it also blocks the surround enhancement in transient ON/OFF ganglion cells. This suggests that the effects of the outer retinal lateral gain control mechanism are visible in the responses of ganglion cells. Functionally speaking, this result illustrates that horizontal cells are not purely inhibitory neurons but have a role in response enhancement as well.

Cobalt ions inhibit negative feedback in the outer retina by blocking hemichannels on horizontal cells

In goldfish, negative feedback from horizontal cells to cones shifts the activation function of the Ca2+ current of the cones to more negative potentials. This shift increases the amount of Ca2+ flowing into the cones, resulting in an increase in glutamate release. The increased glutamate release forms the basis of the feedback-mediated responses in second-order neurons, such as the surround-induced responses of bipolar cells and the spectral coding of horizontal cells. Low concentrations of Co2+ block these feedback-mediated responses in turtle retina. The mechanism by which this is accomplished is unknown. We studied the effects of Co2+ on the cone/horizontal network of goldfish retina and found that Co2+ greatly reduced the feedback-mediated responses in both cones and horizontal cells in a GABA-independent way. The reduction of the feedback-mediated responses is accompanied by a small shift of the Ca2+ current of the cones to positive potentials. We have previously shown that hemichannels on the tips of the horizontal cell dendrites are involved in the modulation of the Ca2+ current in cones. Both the absence of this Co2+-induced shift of the Ca2+ current in the absence of a hemichannel conductance and the sensitivity of Cx26 hemichannels to low concentrations of Co2+ are consistent with a role for hemichannels in negative feedback from horizontal cells to cones.

Light adaptation and color opponency of horizontal cells in the turtle retina

Chromaticity-type (C-type) horizontal cells of the turtle retina receive antagonistic inputs from cones of different spectral types, and therefore their response to background illumination is expected to reflect light adaptation of the cones and the interactions between their antagonistic inputs. Our goal was to study the behavior of C-type horizontal cells during background illumination and to evaluate the role of wavelength in background adaptation. The photoresponses of C-type horizontal cells were recorded intracellularly in the everted eyecup preparation of the turtleMauremys caspicaduring chromatic background illuminations. The voltage range of operation was either reduced or augmented, depending upon the wavelengths of the background and of the light stimuli, while the sensitivity to light was decreased by any background. The response–intensity curves were shifted to brighter intensities and became steeper as the background lights were made brighter regardless of wavelength. Comparing the effects of cone iso-luminant backgrounds on the Red/Green C-type horizontal cells indicated that background desensitization in these cells could not solely reflect background adaptation of cones but also depend upon response compression/expansion and changes in synaptic transmission. This leads to wavelength dependency of background adaptation in C-type horizontal cells, that is expressed as increased light sensitivity (smaller threshold elevation) and improved suprathreshold contrast detection when the wavelengths of the background and light stimuli were chosen to exert opponent effects on membrane potential.

Hemichannel-Mediated Inhibition in the Outer Retina

An essential feature of the first synapse in the retina is a negative feedback pathway from horizontal cells to cones. Here we show that at this synapse, connexin26 forms hemichannels on horizontal cell dendrites near the glutamate release site of the cones. Blocking these hemichannels hyperpolarizes horizontal cells, modulates the Ca2+ channels of the cones, and abolishes all feedback-mediated responses. We propose a feedback mechanism in which the activity of the Ca2+ channels and the subsequent glutamate release of the cones are modulated by a current through these hemichannels. Because the current through the hemichannels depends on the polarization of the horizontal cells, their activity modulates the output of the cones.

Photoreceptor classes and transmission at the photoreceptor synapse in the retina of the clawed frog, Xenopus laevis

The photoreceptor population in Xenopus consists of a green-sensitive rod (lambda(max) = 523 nm), a blue-sensitive rod (lambda(max) = 445 nm) and three classes of cone. The largest cone is red-sensitive (lambda(max) = 611 nm). The intermediate cone is presumed to be blue-sensitive based on physiological criteria, whereas the miniature cone may be UV-sensitive. Horizontal cells (HC) are of two sorts: axon-bearing and axonless. The axon-bearing HC is of the luminosity type and probably contacts all types of photoreceptor. The axonless HC is of the chromaticity type and contacts only intermediate (blue) cones and at least one type of rod. During development dendrites of HCs and bipolar neurons penetrate photoreceptor bases. A progressive maturation of HC and bipolar synapses with rods and cones occurs between tadpoles stages 37/8 and 46. Neighboring rods and cones are joined by gap junctions. During this same period, the outer segments are laid down and photopigments synthesized. A linear relation was found between the quantum capturing ability of the rod and its absolute threshold. Mature rods of the Xenopus retina release glutamate in a calcium-dependent manner. Glutamate release was found to be a linear function of calcium influx through L-type calcium channels. Both types of HC possess ionotropic glutamate receptors of the AMPA subtype.

Cloning and expression of a Xenopus short wavelength cone pigment

The short wavelength visual pigment from Xenopus responsible for vision in the blue/violet portion of the spectrum was characterized by sequence spectroscopic analysis. The amino acid sequence was deduced by sequencing clones isolated by reverse transcription PCR, from retinal cDNA and genomic libraries. The gene contains 5 exons spanning 8.4 kb of genomic DNA and produces an mRNA of 2.4 kb in length. The deduced amino acid sequence predicts a protein of 347 amino acids with 76-78% identity to other short wavelength opsins. The mRNA encoding the Xenopus violet pigment was detected using in situ hybridization in cones, comprising a few percent of the total photoreceptors in the adult retina. The Xenopus violet opsin cDNA, modified to contain an epitope from the carboxyl terminus of bovine rhodopsin, was expressed in COS1 cells by transient transfection and analysed by UV-visible absorption spectroscopy. The protein expressed in COS1 cells migrated at 34 kD and was glycosylated at a single site in the amino terminus, exhibiting a diffuse pattern on SDS PAGE similar to bovine rhodopsin expressed in COS1 cells. Following incubation with 11-cis retinal, a light-sensitive pigment was formed with the lambdamax=425+/-2 nm. A Schiff base linkage between retinal and the violet opsin was demonstrated by acid denaturation. Xenopus violet opsin was sensitive to hydroxylamine in the dark, reacting with a half-time of 5 min at room temperature. This is the first group S pigment for amphibians. The pigment was expressed and purified from COS1 cells in a form that has permitted for the first time determination of the extinction coefficient, reactivity to hydroxylamine and presence of a Schiff base.

Retinoic acid has light-adaptive effects on horizontal cells in the retina

Ambient light conditions affect the morphology of synaptic elements within the cone pedicle and modulate the spatial properties of the horizontal cell receptive field. We describe here that the effects of retinoic acid on these properties are similar to those of light adaptation. Intraorbital injection of retinoic acid into eyes of dark-adapted carp that subsequently were kept in complete darkness results in the formation of numerous spinules at the terminal dendrites of horizontal cells, a typical feature of light-adapted retinae. The formation of these spinules during light adaptation is impaired in the presence of citral, a competitive inhibitor of the dehydrogenase responsible for the generation of retinoic acid in vivo. Intracellularly recorded responses of horizontal cells from dark-adapted eyecup preparations superfused with retinoic acid reveal typical light-adapted spatial properties. Retinoic acid thus appears to act as a light-signaling modulator. Its activity appears not to be at the transcriptional level because its action was not blocked by actinomycin.

Amino acid neurotransmitters in the retina: a functional overview

Physiological and pharmacological mechanisms of glutamatergic, GABAergic and glycinergic synapses in the tiger salamander retina were studied. We used immunocytochemical and autoradiographic methods to study localizations of these neurotransmitters and their uptake transporters; and electrophysiological methods (intracellular, extracellular and whole cell patch electrode recordings) to study the light responses, miniature postsynaptic currents and neurotransmitter-induced postsynaptic currents in various retinal neurons. Our results are consistent with the following scheme: Glutamate is used by the photoreceptor and bipolar cell output synapses and the release of glutamate is largely mediated by calcium-dependent vesicular processes. The postsynaptic glutamate receptors in DBCs are L-AP4 receptors, in HBCs, HCs and ganglion cells are the kainate/AMPA and NMDA receptors. Subpopulations of HCs make GABAergic synapses on cones and gate chloride condunctance through GABAA receptors. GABAergic HCs do not make feedforward synapses on bipolar cell dendrites and the neurotransmitter identity of the HCs making feedforward synapses is unknown. Subpopulations of amacrine cells make GABAergic synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and GABA gates chloride conductances in theses cells. Glycinergic amacrine cells make synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and glycine opens postsynaptic chloride channels. Glycinergic interplexiform cells make synapses on bipolar cells in the outer retina and glycine released from these cells open chloride channels in bipolar cell dendrites.

Both high- and low voltage-activated calcium currents contribute to the light-evoked responses of luminosity horizontal cells in the Xenopus retina

We examined the contribution of two intrinsic voltage-dependent calcium channels to the light-evoked responses of a non-spiking retinal neuron, the horizontal cell (HC). HC's isolated from the Xenopus retina were studied by the whole cell version of the patch clamp. In a mixture of agents which suppressed Na- and K-dependent currents, we identified a transient, low voltage-activated Ca current suppressed by Ba2+ and blocked by Ni2+ (T-type) and a sustained, high voltage-activated, dihydropyridine-sensitive Ca current that was enhanced by Ba2+ (L-type). We made simultaneous intracellular recordings from rods and HC's in the intact, dark-adapted Xenopus retina. Under certain stimulus conditions, transient oscillations appeared in HC responses but were absent in rod light-evoked waveforms. One type of transient was seen at relatively hyperpolarized potentials (< -45 mV), was enhanced by Sr2+ and inhibited by Ni2+. It thus appears to depend on a T-type Ca-current. A second type of oscillation was seen to be superimposed on a prolonged depolarizing wave following light off in the HC and as spike-like depolarizations in rods. These oscillations were enhanced by Ba2+ and Sr2+, but blocked by the dihydropyridine, nifedipine, indicating their dependence on an L-type calcium conductance. All calcium-dependent oscillations were suppressed by 0.05-0.5 mM Co2+. Suppression of glutamate neurotransmission with CNQX or kynurenate, or glycine neurotransmission with strychnine, enhanced the HC oscillations.