Dark-suppression and light-sensitization of horizontal cell responses in the hybrid bass retina

A Life in Vision

I was drawn into research in George Wald's laboratory at Harvard, where as an undergraduate and graduate student, I studied vitamin A deficiency and dark adaptation. A chance observation while an assistant professor at Harvard led to the major research of my career-to understand the functional organization of vertebrate retinas. I started with a retinal circuit analysis of the primate retina with Brian Boycott and intracellular retinal cell recordings in mudpuppies with Frank Werblin. Subsequent pharmacology studies with Berndt Ehinger primarily with fish focused on dopamine and neuromodulation. Using zebrafish, we studied retinal development, neuronal connectivity, and the effects of genetic mutations on retinal structure and function. Now semi-retired, I have returned to primate retinal circuitry, undertaking a connectomic analysis of the human fovea in Jeffrey Lichtman's laboratory.

Electrical activation of degenerated photoreceptors in blind mouse retina elicited network-mediated responses in different types of ganglion cells

Electrical (e-) stimulation is explored in schemes to rescue the vision of blind people, e.g. those affected by Retinitis Pigmentosa (RP). We e-activated subretinally the surviving degenerated photoreceptors (d-Phrs) of the rd1 mouse (RP model) and evoked visual responses in the blind retina. The e-stimulation was applied with a single platinum/iridium electrode. The d-Phrs (calcium-imaging) and ganglion cells (GC) activity (MEA-recording) were recorded in simultaneous multilayer recordings. The findings of this study confirm that the d-Phrs responded to e-stimulation and modulated the retinal network-activity. The application of blockers revealed that the synaptic interactions were dependent on voltage-gated calcium channels and mediated by the transmitters glutamate and GABA. Moreover, the gap junctions coupled networks promoted the lateral-spread of the e-evoked activity in the outer (~60 µm) and inner (~120 µm) retina. The activated GCs were identified as subtypes of the ON, OFF and ON-OFF classes. In conclusion, d-Phrs are the ideal interface partners for implants to elicit enhanced visual responses at higher temporal and spatial resolution. Furthermore, the retina's intact circuity at the onset of complete blindness makes it a tempting target when considering the implantation of implants into young patients to provide a seamless transition from blinding to chip-aided vision.

Gap Junctions Contribute to Differential Light Adaptation across Direction-Selective Retinal Ganglion Cells

Direction-selective ganglion cells (DSGCs) deliver signals from the retina to multiple brain areas to indicate the presence and direction of motion. Delivering reliable signals in response to motion is critical across light levels. Here we determine how populations of DSGCs adapt to changes in light level, from moonlight to daylight. Using large-scale measurements of neural activity, we demonstrate that the population of DSGCs switches encoding strategies across light levels. Specifically, the direction tuning of superior (upward)-preferring ON-OFF DSGCs becomes broader at low light levels, whereas other DSGCs exhibit stable tuning. Using a conditional knockout of gap junctions, we show that this differential adaptation among superior-preferring ON-OFF DSGCs is caused by connexin36-mediated electrical coupling and differences in effective GABAergic inhibition. Furthermore, this adaptation strategy is beneficial for balancing motion detection and direction estimation at the lower signal-to-noise ratio encountered at night. These results provide insights into how light adaptation impacts motion encoding in the retina.

Dopamine D1 receptors regulate the light dependent development of retinal synaptic responses

Retinal synaptic connections and function are developmentally regulated. Retinal synaptic activity plays critical roles in the development of retinal synaptic circuitry. Dopamine receptors have been thought to play important roles in the activity-dependent synaptic plasticity in central nervous system. The primary goal of this study is to determine whether dopamine D1 receptor regulates the activity-dependent development of retinal light responsiveness. Accordingly, we recorded electroretinogram from wild type mice and mice with genetic deletion of D1 dopamine receptor (D1-/- mice) raised under cyclic light conditions and constant darkness. Our results demonstrated that D1-/- mice have reduced amplitudes of all three major components of electroretinogram in adulthood. When the relative strength of the responses is considered, the D1-/- mice have selective reduction of the amplitudes of a-wave and oscillatory potentials evoked by low-intermediate intensities of lights. During postnatal development, D1-/- mice have increased amplitude of b-wave at the time of eye-opening but reduced developmental increase of the amplitude of b-wave after eye opening. Light deprivation from birth significantly reduced the amplitudes of b-wave and oscillatory potentials, increased the outer retinal light response gain and altered the light response kinetics of both a- and b-waves of wild type mice. In D1-/- mice, the effect of dark rearing on the amplitude of oscillatory potentials was diminished and dark rearing induced effects on the response gain of outer retina and the kinetics of a-wave were reversed. These results demonstrated roles of dopamine D1 receptor in the activity-dependent functional development of mouse retina.

The light-induced reduction of horizontal cell receptive field size in the goldfish retina involves nitric oxide

Horizontal cells of the vertebrate retina have large receptive fields as a result of extensive gap junction coupling. Increased ambient illumination reduces horizontal cell receptive field size. Using the isolated goldfish retina, we have assessed the contribution of nitric oxide to the light-dependent reduction of horizontal cell receptive field size. Horizontal cell receptive field size was assessed by comparing the responses to centered spot and annulus stimuli and from the responses to translated slit stimuli. A period of steady illumination decreased the receptive field size of horizontal cells, as did treatment with the nitric oxide donor (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (100 μM). Blocking the endogenous production of nitric oxide with the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (1 mM), decreased the light-induced reduction of horizontal cell receptive field size. These findings suggest that nitric oxide is involved in light-induced reduction of horizontal cell receptive field size.

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.

Tracer coupling between fish rod horizontal cells: modulation by light and dopamine but not the retinal circadian clock

Horizontal cells are second order neurons that receive direct synaptic input from photoreceptors. In teleosts horizontal cells can be divided into two categories, cone-connected and rod-connected. Although the anatomy and physiology of fish cone horizontal cells have been extensively investigated, less is known about rod horizontal cells. This study was undertaken to determine whether light and/or the circadian clock regulate gap junctional coupling between goldfish rod horizontal cells. We used fine-tipped, microelectrode intracellular recording to monitor rod horizontal cells under various visual stimulation conditions, and tracer (biocytin) iontophoresis to visualize their morphology and evaluate the extent of coupling. Under dark-adapted conditions, rod horizontal cells were extensively coupled to cells of like-type (homologous coupling) with an average of approximately 120 cells coupled. Under these conditions, no differences were observed between day, night, the subjective day, and subjective night. In addition, under dark-adapted conditions, application of the dopamine D2-like agonist quinpirole (1 microM), the D2-like antagonist spiperone (10 microM), or the D1-like antagonist SCH23390 (10 microM) had no effect on rod horizontal cell tracer coupling. In contrast, the extent of tracer coupling was reduced by approximately 90% following repetitive light (photopic range) stimulation of the retina or application of the D1-agonist SKF38393 (10 microM) during the subjective day and night. We conclude that similarly to cone horizontal cells, rod horizontal cells are extensively coupled to one another in darkness and that the extent of coupling is dramatically reduced by bright light stimulation or dopamine D1-receptor activation. However, in contrast to cone horizontal cells whose light responses are under the control of the retinal clock, the light responses of rod horizontal cells under dark-adapted conditions were similar during the day, night, subjective day, and subjective night thus demonstrating that they are not under the influence of the circadian clock.

Absence of circadian clock regulation of horizontal cell gap junctional coupling reveals two dopamine systems in the goldfish retina

In fish and other vertebrate retinas, although dopamine release is regulated by both light and an endogenous circadian (24-hour) clock, light increases dopamine release to a greater extent than the clock. The clock increases dopamine release during the subjective day so that D2-like receptors are activated. It is not known, however, whether the retinal clock also activates D1 receptors, which display a much lower sensitivity to dopamine in intact tissue. Because activation of the D1 receptors on fish cone horizontal (H1) cells uncouples the gap junctions between the cells, we studied whether the clock regulates the extent of biocytin tracer coupling in the goldfish retina. Tracer coupling between H1 cells was extensive under dark-adapted conditions (low scotopic range) and similar in the subjective day, subjective night, day, and night. An average of approximately 180 cells were coupled in each dark-adapted condition. However, bright light stimulation or application of the D1 agonist SKF38393 (10 microM) dramatically reduced H1 cell coupling. The D2 agonist quinpirole (1 microM) or application of the D1 antagonist SCH23390 (10 microM) and/or the D2 antagonist spiperone (10 microM) had no effect on H1 cell coupling in dark-adapted retinas. These observations demonstrate that H1 cell gap junctional coupling and thus D1 receptor activity are not affected by endogenous dopamine under dark-adapted conditions. The results suggest that two different dopamine systems are present in the goldfish retina. One system is controlled by an endogenous clock that activates low threshold D2-like receptors in the day, whereas the second system is controlled by light and involves activation of higher threshold D1 receptors.

A circadian clock in the fish retina regulates dopamine release via activation of melatonin receptors

Although many biochemical, morphological and physiological processes in the vertebrate retina are controlled by a circadian (24 h) clock, the location of the clock and how the clock alters retinal function are unclear. For instance, several observations have suggested that dopamine, a retinal neuromodulator, may play an important role in retinal rhythmicity but the link between dopamine and a clock located within or outside the retina remains to be established. We found that endogenous dopamine release from isolated goldfish retinae cultured in continuous darkness for 56 h clearly exhibited a circadian rhythm with high values during the subjective day. The continuous presence of melatonin (1 nM) in the culture medium abolished the circadian rhythm of dopamine release and kept values constantly low and equal to the night-time values. The selective melatonin antagonist luzindole (1 microM) also abolished the dopamine rhythm but the values were high and equal to the daytime values. Melatonin application during the late subjective day introduced rod input and reduced cone input to fish cone horizontal cells, a state usually observed during the subjective night. In contrast, luzindole application during the subjective night decreased rod input and increased cone input. Prior application of dopamine or spiperone, a selective dopamine D(2)-like antagonist, blocked the above effects of melatonin and luzindole, respectively. These findings indicate that a circadian clock in the vertebrate retina regulates dopamine release by the activation of melatonin receptors and that endogenous melatonin modulates rod and cone pathways through dopamine-mediated D(2)-like receptor activation.

Dopamine mediates circadian clock regulation of rod and cone input to fish retinal horizontal cells

A circadian (24-hour) clock regulates the light responses of fish cone horizontal cells, second order neurones in the retina that receive synaptic contact from cones and not from rods. Due to the action of the clock, cone horizontal cells are driven by cones in the day, but primarily driven by rods at night. We show here that dopamine, a retinal neurotransmitter, acts as a clock signal for the day by increasing cone input and decreasing rod input to cone horizontal cells. The amount of endogenous dopamine released from in vitro retinae was greater during the subjective day than the subjective night. Application of dopamine or quinpirole, a dopamine D(2)-like agonist, during the subjective night increased cone input and eliminated rod input to the cells, a state usually observed during the subjective day. In contrast, application of spiperone, a D(2)-like antagonist, or forskolin, an activator of adenylyl cyclase, during the subjective day reduced cone input and increased rod input. SCH23390, a D(1) antagonist, had no effect. Application of R(p)-cAMPS, an inhibitor of cAMP-dependent protein kinase, or octanol, an alcohol that uncouples gap junctions, during the night increased cone input and decreased rod input. Because D(2)-like receptors are on photoreceptor cells, but not horizontal cells, the results suggest that the clock-induced increase in dopamine release during the day activates D(2)-like receptors on photoreceptor cells. The resultant decrease in intracellular cyclic AMP and protein kinase A activation then mediates the increase in cone input and decrease in rod input.

Long-term light history modulates the light response kinetics of luminosity (L)-type horizontal cells in the roach retina

We have examined the effects of prolonged periods of darkness on the responses of luminosity-type horizontal cells (L-HCs) in the freshwater cyprinid, Rutilus rutilus. Two groups of retinae were compared, those recorded after 10 min dark adaptation (SA) and those recorded after 3 h dark adaptation (LA). The results suggest that long-term light history does not modify the general responsiveness of the L-HCs in this species. However, there are apparent changes in the receptive field of the cells and modifications to the kinetics of the light-evoked response. The kinetics changes involve both a delay in the onset of light response and a selective effect on the hyperpolarizing light-ON response. Thus the mean time constant (tau) for the SA cells was 32.4+/-2.39 ms (n=62), whilst that for the LA cells was 53.4+/-3.03 ms (n=61). These effects occur in the absence of changes in the relative spectral sensitivity or threshold sensitivity of the HCs. The results suggest that in some vertebrate retinae, prolonged darkness (light-history) may regulate long-term plasticity in the kinetics of the cone-HC pathway.

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.

Dark- and light-induced changes in coupling between horizontal cells in mammalian retina

Retinal horizontal cells exhibit large receptive fields derived from their extensive electrical coupling by means of gap junctions. The conductance of these gap junctions seems to be regulated by dopamine acting through a cAMP-mediated cascade. There is now abundant evidence that extracellular dopamine levels vary with changes in ambient light intensity, suggesting that changes in the dark/light adaptational state of the retina can modulate coupling between horizontal cells. We studied this question in the mammalian retina by determining the effects of ambient light levels, in the form of changing background light intensity, on the coupling profiles of A- and B-type horizontal cells in the rabbit. Changes in coupling were assessed by measurements of the space constants of the syncytium formed by horizontal cells and the intercellular spread of the biotinylated tracer Neurobiotin. Our results indicate that dark-adapted horizontal cells show relatively weak coupling. However, presentation of background lights as dim as one-quarter log unit above rod threshold resulted in increases in both the averaged extent of tracer coupling and space constants of A- and B-type horizontal cells. Coupling expanded further as background light intensities were increased by 1-1.5 log units, after which additional light adaptation brought about an uncoupling of cells. Coupling reached its minimum at light intensities about 3 log units above rod threshold, after which, with further light adaptation, it stabilized at levels close to those seen in dark-adapted retinas. Our results indicate that electrical coupling between mammalian horizontal cells is modulated dramatically by changes in the adaptational state of the retina: coupling is maximized under dim ambient light conditions and diminishes as the retina is dark or light adapted from this level.

Modulation of endogenous dopamine release in the fish retina by light and prolonged darkness

The effect of light stimuli and prolonged darkness on the release of endogenous dopamine was measured in the white perch and hybrid bass retinas. Isolated retinas were superfused and released dopamine was measured using extraction and high-pressure liquid chromatography separation techniques. Potassium-induced release did not depend on the background illumination nor on the period of previous darkness. Steady white light did not affect release, but flickering light of 2 Hz increased release about two-fold. During prolonged darkness, the release of dopamine increased steadily over the test period of 2 h, but only if the experiments were performed at night. During the day such an increase was not observed. The increased release during prolonged darkness at night was turned off by a short period of steady white light. The release patterns obtained from the white perch and the hybrid bass were similar. However, the hybrid bass retina showed much lower levels of dopamine than did the white perch retina.

A circadian clock regulates rod and cone input to fish retinal cone horizontal cells

In the vertebrate retina, the light responses of post-receptor neurons depend on the ambient or background illumination. Using intracellular recording, we have found that a circadian clock regulates the light responses of dark-adapted fish cone horizontal cells. Goldfish were maintained on a 12-hr light/12-hr dark cycle. At different times of the day or night, retinas were superfused in darkness for 90 min ("prolonged darkness"), following which horizontal cells were impaled without the aid of any light flashes. In some of the experiments, fish were kept in constant darkness for 3-48 hr prior to surgery. After prolonged darkness during the night, but not during the day, the light responses of L-type cone horizontal cells resembled those of rod horizontal cells with respect to threshold, waveform, intensity-response functions, and spectral sensitivity. Following light sensitization during the night and day, the light responses of rod and cone horizontal cells were clearly different with respect to threshold, waveform, intensity-response functions, and spectral sensitivity. Under conditions of constant darkness for two full light/dark cycles, average responses of cone horizontal cells to a bright light stimulus during the subjective day were greater than during the subjective night. Prior reversal of the light/dark cycle reversed the 24-hr rhythm of cone horizontal cell responses to bright lights. In addition, following one full cycle of constant darkness, average cone horizontal cell spectral sensitivity during the subjective night closely matched that of rod horizontal cells, whereas average cone horizontal cell spectral sensitivity during the subjective day was similar to that of red (625 nm) cones. These results indicate that the effects of dark adaptation depend on the time of day and are regulated by a circadian clock so that cone input to cone horizontal cells predominates in the day and rod input predominates in the night.