Threshold and chromatic sensitivity changes in fish cone horizontal cells following prolonged darkness

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Interactions of cone cannabinoid CB1 and dopamine D4 receptors increase day/night difference in rod-cone gap junction coupling in goldfish retina

KEY POINTS

Although cone and rod photoreceptor cells in the retina have a type of cannabinoid receptor called a CB1 receptor, little is known about how cannabinoids, the active component in marijuana, affect retinal function. Studies have shown that a circadian (24-h) clock in the retina uses dopamine receptors, which are also on photoreceptors, to regulate gap junctions (a type of cell-to-cell communication) between rods and cones, so that they are functional (open) at night but closed in the day. We show that CB₁ receptors have opposite effects on rod-cone gap junctions in day and night, decreasing communication in the day when dopamine receptors are active and increasing communication when dopamine receptors are inactive. CB₁ and dopamine receptors thus work together to enhance the day/night difference in rod-cone gap junction communication. The increased rod-cone communication at night due to cannabinoid CB₁ receptors may help improve night vision.

ABSTRACT

Cannabinoid CB₁ receptors and dopamine D₄ receptors in the brain form receptor complexes that interact but the physiological function of these interactions in intact tissue remains unclear. In vertebrate retina, rods and cones, which are connected by gap junctions, express both CB₁ and D₄ receptors. Because the retinal circadian clock uses cone D₄ receptors to decrease rod-cone gap junction coupling in the day and to increase it at night, we studied whether an interaction between cone CB₁ and D₄ receptors increases the day/night difference in rod-cone coupling compared to D₄ receptors acting alone. Using electrical recording and injections of Neurobiotin tracer into individual cones in intact goldfish retinas, we found that SR141716A (a CB₁ receptor antagonist) application alone in the day increased both the extent of rod-cone tracer coupling and rod input to cones, which reaches cones via open gap junctions. Conversely, SR141716A application alone at night or SR141716A application in the day following 30-min spiperone (a D₄ receptor antagonist) application decreased both rod-cone tracer coupling and rod input to cones. These results show that endogenous activation of cone CB₁ receptors decreases rod-cone coupling in the day when D₄ receptors are activated but increases it at night when D₄ receptors are not activated. Therefore, the D₄ receptor-dependent day/night switch in the effects of CB₁ receptor activation results in an enhancement of the day/night difference in rod-cone coupling. This synergistic interaction increases detection of very dim large objects at night and fine spatial details in the day.

Dopamine-Mediated Circadian and Light/Dark-Adaptive Modulation of Chemical and Electrical Synapses in the Outer Retina

The vertebrate retina, like most other brain regions, undergoes relatively slow alterations in neural signaling in response to gradual changes in physiological conditions (e.g., activity changes to rest), or in response to gradual changes in environmental conditions (e.g., day changes into night). As occurs elsewhere in the brain, the modulatory processes that mediate slow adaptation in the retina are driven by extrinsic signals (e.g., changes in ambient light level) and/or by intrinsic signals such as those of the circadian (24-h) clock in the retina. This review article describes and discusses the extrinsic and intrinsic modulatory processes that enable neural circuits in the retina to optimize their visual performance throughout day and night as the ambient light level changes by ~10 billion-fold. In the first synaptic layer of the retina, cone photoreceptor cells form gap junctions with rods and signal cone-bipolar and horizontal cells (HCs). Distinct extrinsic and intrinsic modulatory processes in this synaptic layer are mediated by long-range feedback of the neuromodulator dopamine. Dopamine is released by dopaminergic cells, interneurons whose cell bodies are located in the second synaptic layer of the retina. Distinct actions of dopamine modulate chemical and electrical synapses in day and night. The retinal circadian clock increases dopamine release in the day compared to night, activating high-affinity dopamine D₄ receptors on cones. This clock effect controls electrical synapses between rods and cones so that rod-cone electrical coupling is minimal in the day and robust at night. The increase in rod-cone coupling at night improves the signal-to-noise ratio and the reliability of very dim multi-photon light responses, thereby enhancing detection of large dim objects on moonless nights.Conversely, maintained (30 min) bright illumination in the day compared to maintained darkness releases sufficient dopamine to activate low-affinity dopamine D₁ receptors on cone-bipolar cell dendrites. This non-circadian light/dark adaptive process regulates the function of GABA_A receptors on ON-cone-bipolar cell dendrites so that the receptive field (RF) surround of the cells is strong following maintained bright illumination but minimal following maintained darkness. The increase in surround strength in the day following maintained bright illumination enhances the detection of edges and fine spatial details.

A Circadian Clock in the Retina Regulates Rod-Cone Gap Junction Coupling and Neuronal Light Responses via Activation of Adenosine A2A Receptors

Adenosine, a major neuromodulator in the central nervous system (CNS), is involved in a variety of regulatory functions such as the sleep/wake cycle. Because exogenous adenosine displays dark- and night-mimicking effects in the vertebrate retina, we tested the hypothesis that a circadian (24 h) clock in the retina uses adenosine to control neuronal light responses and information processing. Using a variety of techniques in the intact goldfish retina including measurements of adenosine overflow and content, tracer labeling, and electrical recording of the light responses of cone photoreceptor cells and cone horizontal cells (cHCs), which are post-synaptic to cones, we demonstrate that a circadian clock in the retina itself-but not activation of melatonin or dopamine receptors-controls extracellular and intracellular adenosine levels so that they are highest during the subjective night. Moreover, the results show that the clock increases extracellular adenosine at night by enhancing adenosine content so that inward adenosine transport ceases. Also, we report that circadian clock control of endogenous cone adenosine A_2A receptor activation increases rod-cone gap junction coupling and rod input to cones and cHCs at night. These results demonstrate that adenosine and A_2A receptor activity are controlled by a circadian clock in the retina, and are used by the clock to modulate rod-cone electrical synapses and the sensitivity of cones and cHCs to very dim light stimuli. Moreover, the adenosine system represents a separate circadian-controlled pathway in the retina that is independent of the melatonin/dopamine pathway but which nevertheless acts in concert to enhance the day/night difference in rod-cone coupling.

Circadian clock regulation of cone to horizontal cell synaptic transfer in the goldfish retina

Although it is well established that the vertebrate retina contains endogenous circadian clocks that regulate retinal physiology and function during day and night, the processes that the clocks affect and the means by which the clocks control these processes remain unresolved. We previously demonstrated that a circadian clock in the goldfish retina regulates rod-cone electrical coupling so that coupling is weak during the day and robust at night. The increase in rod-cone coupling at night introduces rod signals into cones so that the light responses of both cones and cone horizontal cells, which are post-synaptic to cones, become dominated by rod input. By comparing the light responses of cones, cone horizontal cells and rod horizontal cells, which are post-synaptic to rods, under dark-adapted conditions during day and night, we determined whether the daily changes in the strength of rod-cone coupling could account entirely for rhythmic changes in the light response properties of cones and cone horizontal cells. We report that although some aspects of the day/night changes in cone and cone horizontal cell light responses, such as response threshold and spectral tuning, are consistent with modulation of rod-cone coupling, other properties cannot be solely explained by this phenomenon. Specifically, we found that at night compared to the day the time course of spectrally-isolated cone photoresponses was slower, cone-to-cone horizontal cell synaptic transfer was highly non-linear and of lower gain, and the delay in cone-to-cone horizontal cell synaptic transmission was longer. However, under bright light-adapted conditions in both day and night, cone-to-cone horizontal cell synaptic transfer was linear and of high gain, and no additional delay was observed at the cone-to-cone horizontal cell synapse. These findings suggest that in addition to controlling rod-cone coupling, retinal clocks shape the light responses of cone horizontal cells by modulating cone-to-cone horizontal cell synaptic transmission.

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.

Regulation of photoreceptor gap junction phosphorylation by adenosine in zebrafish retina

Electrical coupling of photoreceptors through gap junctions suppresses voltage noise, routes rod signals into cone pathways, expands the dynamic range of rod photoreceptors in high scotopic and mesopic illumination, and improves detection of contrast and small stimuli. In essentially all vertebrates, connexin 35/36 (gene homologs Cx36 in mammals, Cx35 in other vertebrates) is the major gap junction protein observed in photoreceptors, mediating rod-cone, cone-cone, and possibly rod-rod communication. Photoreceptor coupling is dynamically controlled by the day/night cycle and light/dark adaptation, and is directly correlated with phosphorylation of Cx35/36 at two sites, serine110 and serine 276/293 (homologous sites in teleost fish and mammals, respectively). Activity of protein kinase A (PKA) plays a key role during this process. Previous studies have shown that activation of dopamine D4 receptors on photoreceptors inhibits adenylyl cyclase, down-regulates cAMP and PKA activity, and leads to photoreceptor uncoupling, imposing the daytime/light condition. In this study, we explored the role of adenosine, a nighttime signal with a high extracellular concentration at night and a low concentration in the day, in regulating photoreceptor coupling by examining photoreceptor Cx35 phosphorylation in zebrafish retina. Adenosine enhanced photoreceptor Cx35 phosphorylation in daytime, but with a complex dose-response curve. Selective pharmacological manipulations revealed that adenosine A2a receptors provide a potent positive drive to phosphorylate photoreceptor Cx35 under the influence of endogenous adenosine at night. A2a receptors can be activated in the daytime as well by micromolar exogenous adenosine. However, the higher affinity adenosine A1 receptors are also present and have an antagonistic though less potent effect. Thus, the nighttime/darkness signal adenosine provides a net positive drive on Cx35 phosphorylation at night, working in opposition to dopamine to regulate photoreceptor coupling via a push-pull mechanism. However, the lower concentration of adenosine present in the daytime actually reinforces the dopamine signal through action on the A1 receptor.

Adenosine and dopamine receptors coregulate photoreceptor coupling via gap junction phosphorylation in mouse retina

Gap junctions in retinal photoreceptors suppress voltage noise and facilitate input of rod signals into the cone pathway during mesopic vision. These synapses are highly plastic and regulated by light and circadian clocks. Recent studies have revealed an important role for connexin36 (Cx36) phosphorylation by protein kinase A (PKA) in regulating cell-cell coupling. Dopamine is a light-adaptive signal in the retina, causing uncoupling of photoreceptors via D4 receptors (D4R), which inhibit adenylyl cyclase (AC) and reduce PKA activity. We hypothesized that adenosine, with its extracellular levels increasing in darkness, may serve as a dark signal to coregulate photoreceptor coupling through modulation of gap junction phosphorylation. Both D4R and A2a receptor (A2aR) mRNAs were present in photoreceptors, inner nuclear layer neurons, and ganglion cells in C57BL/6 mouse retina, and showed cyclic expression with partially overlapping rhythms. Pharmacologically activating A2aR or inhibiting D4R in light-adapted daytime retina increased photoreceptor coupling. Cx36 among photoreceptor terminals, representing predominantly rod-cone gap junctions but possibly including some rod-rod and cone-cone gap junctions, was phosphorylated in a PKA-dependent manner by the same treatments. Conversely, inhibiting A2aR or activating D4R in daytime dark-adapted retina decreased Cx36 phosphorylation with similar PKA dependence. A2a-deficient mouse retina showed defective regulation of photoreceptor gap junction phosphorylation, fairly regular dopamine release, and moderately downregulated expression of D4R and AC type 1 mRNA. We conclude that adenosine and dopamine coregulate photoreceptor coupling through opposite action on the PKA pathway and Cx36 phosphorylation. In addition, loss of the A2aR hampered D4R gene expression and function.

Photoreceptor coupling is controlled by connexin 35 phosphorylation in zebrafish retina

Electrical coupling of neurons is widespread throughout the CNS and is observed among retinal photoreceptors from essentially all vertebrates. Coupling dampens voltage noise in photoreceptors and rod-cone coupling provides a means for rod signals to enter the cone pathway, extending the dynamic range of rod-mediated vision. This coupling is dynamically regulated by a circadian rhythm and light adaptation. We examined the molecular mechanism that controls photoreceptor coupling in zebrafish retina. Connexin 35 (homologous to Cx36 of mammals) was found at both cone-cone and rod-cone gap junctions. Photoreceptors showed strong Neurobiotin tracer coupling at night, extensively labeling the network of cones. Tracer coupling was significantly reduced in the daytime, showing a 20-fold lower diffusion coefficient for Neurobiotin transfer. The phosphorylation state of Cx35 at two regulatory phosphorylation sites, Ser110 and Ser276, was directly related to tracer coupling. Phosphorylation was high at night and low during the day. Protein kinase A (PKA) activity directly controlled both phosphorylation state and tracer coupling. Both were significantly increased in the day by pharmacological activation of PKA and significantly reduced at night by inhibition of PKA. The data are consistent with direct phosphorylation of Cx35 by PKA. We conclude that the magnitude of photoreceptor coupling is controlled by the dynamic phosphorylation and dephosphorylation of Cx35. Furthermore, the nighttime state is characterized by extensive coupling that results in a well connected cone network.

Retinal parallel pathways: seeing with our inner fish

The general organization of the vertebrate retina is highly conserved, in spite of structural variations that occur in different animal classes. The retinas of cyprinid fish, for example, differ in many aspects from those of primates. However, these differences are in the same order of magnitude as those found among mammalian species. Therefore, it is important to consider whether these changes are minor variations on the same theme or whether they lead to fundamentally different functions. In this light, we compare the retinal organization of teleost fish and mammals as regards parallel processing and discuss their many similarities.

Reciprocal Modulation of Calcium Dynamics at Rod and Cone Photoreceptor Synapses by Nitric Oxide

The abundance of nitric oxide (NO) synthesizing enzymes identified in the vertebrate retina highlight the importance of NO as a signaling molecule in this tissue. Here we describe opposing actions of NO on the rod and cone photoreceptor synapse. Depolarization-induced increases of calcium concentration in rods and cones were enhanced and inhibited, respectively, by the NO donor S-nitrosocysteine. NO suppressed calcium current in cones by decreasing the maximum conductance, whereas NO facilitated rod Ca channel activation. NO also activated a nonselective voltage-independent conductance in both rods and cones. Suppression of NO production in the intact retina with NG-nitro-l-arginine favored cone over rod driven postsynaptic signals, as would be expected if NO enhanced rod and suppressed cone synaptic activity. These findings may imply involvement of NO in regulating the strength of rod and cone pathways in the retina during different states of adaptation.

Retinal bipolar cell input mechanisms in giant danio. I. Electroretinographic analysis

Electroretinograms (ERGs) were recorded from the giant danio (Danio aequipinnatus) to study glutamatergic input mechanisms onto bipolar cells. Glutamate analogs were applied to determine which receptor types mediate synaptic transmission from rods and cones to on and off bipolar cells. Picrotoxin, strychnine, and tetrodotoxin were used to isolate the effects of the glutamate analogs to the photoreceptor-bipolar cell synapse. Under photopic conditions, the group III metabotropic glutamate receptor (mGluR) antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) only slightly reduced the b-wave, whereas the excitatory amino acid transporter (EAAT) blocker dl-threo-beta-benzyl-oxyaspartate (TBOA) removed most of it. Complete elimination of the b-wave required both antagonists. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) blocked the d-wave. Under scotopic conditions, rod and cone inputs onto on bipolar cells were studied by comparing the sensitivities of the b-wave to photopically matched green and red stimuli. The b-wave was >1 log unit more sensitive to the green than to the red stimulus under control conditions. In CPPG or l-AP4 (l-(+)-2-amino-4-phosphonobutyric acid, a group III mGluR agonist), the sensitivity of the b-wave to the green stimulus was dramatically reduced and the b-waves elicited by the 2 stimuli became nearly matched. The d-wave elicited by dim green stimuli, which presumably could be detected only by the rods, was eliminated by NBQX.

IN CONCLUSION

1) cone signals onto on bipolar cells involve mainly EAATs but also mGluRs (presumably mGluR6) to a lesser extent; 2) rods signal onto on bipolars by mainly mGluR6; 3) off bipolar cells receive signals from both photoreceptor types by AMPA/kainate receptors.

Inhibitory interaction of cannabinoid CB1 receptor and dopamine D2 receptor agonists on voltage-gated currents of goldfish cones

Dopamine is a light-adaptive signal that desensitizes the retina, while cannabinoids reportedly increase photosensitivity. The presynaptic membrane of goldfish retinal cones has dopamine D2 receptors and cannabinoid CB1 receptors. This work focused on whether dopamine D2 receptor agonist quinpirole and cannabinoid CB1 receptor agonist WIN 55212-2 (WIN) interacted to modulate voltage-dependent membrane currents of cones. A conventional patch-clamp method was used to record depolarization evoked whole-cell outward currents (Iout) and an inward calcium current (ICa) from the inner segment of cones in goldfish retinal slices. WIN had biphasic actions: low concentrations (<1 μM) increased the currentsviaGs, while higher concentrations (>1 μM) decreased the currentsviaGi/Go. Neither dopamine nor the D2 agonist quinpirole (1–20 μM) had a significant effect on eitherIoutorICa. Quinpirole at 50 μM had a mild suppressive (∼20%) effect onIout. However, quinpirole (<10 μM) completely blocked the enhancement of both currents seen with 0.7 μM WIN. The effect of quinpirole was blocked by sulpiride and by pertussis toxin, indicating that quinpirole was actingviaa D2 receptor-Gi/o coupled mechanism. The suppressive action of 50 μM quinpirole (∼20%) was not additive with the suppressive effect of 3 μM WIN (∼40%). D2 agonistsviaGi/o oppose the action of low concentrations of CB1 agonists actingviaGs to modulate cone membrane currents, suggesting a role in shaping the cone light response and/or sensitivity to changes in ambient light conditions. The nonadditive effect of high concentrations of WIN and quinpirole suggests that both decrease membrane currentsviathe same transduction pathway, Gi/Go protein kinase A (PKA).

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.

Light potentiation of horizontal cells in the isolated rabbit retina

When the retinas of some fishes and amphibians are dark-adapted the hyperpolarising response of horizontal cells (HCs) to a light stimulus is suppressed (= dark suppression) but is restored to that of light adaptation when a light stimulus is rapidly repeated (= light potentiation (LP). This phenomenon, which had not been previously demonstrated in a mammal, has been recorded from isolated rabbit retinas. The steady amount of LP of HCs after several light stimuli was expressed as the percentage of induced hyperpolarisation when that of the dark-adapted retina is taken as 100%. The LP was small (113+/-13%, n=14), when the HCs had stable and large responses (>15 mV), but varied greatly between HCs. Those HCs with slight LP (106+/-3% n=8) designated as x-HCs, mostly exhibited larger overshoots than did those HCs with stronger LP (123+/-14, n=6) and designated y-HCs. These mostly had a smaller overshoot. Other HCs (z-HCs) were unstable and were only slightly hyperpolarised by the first light stimulus after dark adaptation but showed the strongest LP (193+/-48%, n=7). These results could indicate a variable degree of light potentiation in normally-functioning HCs, which can be classified accordingly. It is possible, however, that the degree of LP is small in all normally-functioning HCs, but as the HCs in isolated mammalian retinal preparations deteriorate, the phenomenon of LP is progressively exaggerated. LP is not peculiar to HCs and can also occur in cells which depolarise in response to a light stimulus, and can be evident in the PIII component of the ERG.

The feedback pathway from horizontal cells to cones. A mini review with a look ahead

The feedback pathway from HCs to cones forms the basis of the surround responses of the bipolar cells and is essential for the spectral opponency of horizontal cells. The nature of this feedback pathway is an issue of debate. Three hypothesis are presented in literature: (1) a GABAA-ergic feedback pathway; (2) a GABA-independent feedback pathway that modulates the Ca-current in cones; and (3) an electrical feedback pathway. In this review the evidence for the various pathways will be discussed. The conclusion is that the available evidence favors the hypothesis that feedback modulates the Ca-current in the cones in a GABA independent way. An alternative role of GABA in the outer plexiform layer is discussed and finally the functional consequences of the negative feedback pathway from horizontal cells to cones are presented.

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.

Spatial and temporal properties of cat horizontal cells after prolonged dark adaptation

We studied the change of spatial and temporal response properties for cat horizontal (H-) cells during prolonged dark adaptation. H-cell responses were recorded intracellularly in the optically intact, in vivo eye. Spatial and temporal properties were first measured for light-adapted H-cells, followed by a period of dark adaptation, after which the same measurements were repeated. During dark adaptation threshold sensitivity was measured at regular intervals. Stable, long lasting recordings allowed us to measure changes of sensitivity and receptive field characteristics for adaptation periods up to 45 min. Although cat H-cells showed no signs of dark suppression or light sensitization, they remained insensitive in the scotopic range, even after prolonged dark adaptation. Absolute thresholds were in the low mesopic range. The sensitization was brought about by a shift from cone to rod input, and by substantial increases of both spatial and temporal integration upon dark adaptation. The length constant in the light-adapted state was on average about 4 deg. After dark adaptation it was up to a factor of three larger, with a median ratio of 1.85. Response delays, latencies and durations for (equal amplitude) threshold flash responses substantially increased during dark adaptation.

Spectral plasticity of H1 horizontal cells in carp retina: independent modulation by dopamine and light-adaptation

It was shown previously that the spectral sensitivity of luminosity/H1-type horizontal cells (HCs) in carp retinae reflects the absorption spectrum of red-sensitive cones for long wavelengths but can appear highly variable and "truncated' in the short-wavelength region of the spectrum. We have found that light-adaptation sharpened the red-sensitive spectral peak and decreased the blue/red response amplitude ratio (B/R ratio), mainly by decreasing the response to short-wavelength stimuli. The adaptation effect was more pronounced for red background light than for blue. During dark adaptation, the B/R ratio increased steadily. Exogenous dopamine (DA; 5 microM) changed the spectral response profile in a similar way to light-adaptation. However, the effect of light-adaptation in reducing the B/R ratio was still seen in retinae bathed in 5 microM DA. This effect of background adaptation was also recorded in retinae bathed in 37 microM haloperidol, as well as in retinae pretreated with 6-hydroxydopamine (i.e. DA-depleted). The results suggest that (i) short-wavelength-sensitive cones play a dynamic role in determining the spectral response profile of H1 HCs and (ii) spectral response characteristics are modulated independently by exogenous DA and an unknown endogenous neuromodulator which is activated by light-adaptation.

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.

Neurotensin induces calcium oscillations in cultured amacrine cells

The peptide, neurotensin, is found in a class of amacrine cells synapsing chiefly with other amacrine cells in the chicken retina (Li & Lam, 1990; Watt et al., 1991). To investigate the possible effects of neurotensin, we have used Ca2+ imaging to measure cytosolic Ca2+ concentrations in cultured chick amacrine cells. Following a delay of about 2 min, neurotensin (300 nM) induced oscillations in Ca2+ concentration that typically had a period of 2 min and peak values of about 300 nM when averaged over the cell body. The phospholipase C inhibitors U-73, 112 and 4'-bromophenacyl bromide terminated oscillations induced by neurotensin but the protein kinase inhibitors H7 and staurosporine did not inhibit oscillations, increasing their frequency instead. In the absence of external Ca2+, neurotensin induced only a single Ca2+ transient, much briefer than when external Ca2+ was present. Together these results suggest that neurotensin activates phospholipase C, thereby producing IP3 that triggers Ca2+ release from an internal store. Although this released Ca2+ contributes to periodic Ca2+ peaks, the majority of cytosolic Ca2+, even in the first peak, comes from Ca2+ influx across the plasmalemma.

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

The responsiveness of luminosity-type horizontal cells, recorded intracellularly from isolated hybrid bass retinas, decreased after superfusion for 2 h in constant darkness. Responsiveness was subsequently increased (light-sensitized) up to 10-fold after exposure to several short (approximately 0.5 min) periods of continuous illumination. The increase in horizontal cell responsiveness following light-sensitization was due to an increase of peak response amplitude rather than a reduction of peak response time. The increased responsiveness after light-sensitization was intensity-dependent with brighter sensitizing stimuli causing a greater increase than dimmer stimuli. The extent of LHC dark-suppression was affected by the time of day, being greater when induced during the night than during the day. However, there was no significant difference in horizontal cell responsiveness after light-sensitization in retinas studied during the night compared to those studied during the day. The responsiveness of light-sensitized horizontal cells from isolated hybrid bass retinas was found to be suppressed by relatively brief periods of darkness. The responsiveness of horizontal cells, that were first light-sensitized, decreased by more than 50% following only 5 min of darkness. Suppression of light-sensitized horizontal cell responsiveness after such a short time in the dark has not been described in other teleost retinas. The suppression of light-sensitized horizontal cell responsiveness in hybrid bass retinas may be rapid in comparison to other teleosts.