A circadian clock regulates the pH of the fish retina

pH in the vertebrate retina and its naturally occurring and pathological changes

This review summarizes the existing information on the concentration of H+ (pH) in vertebrate retinae and its changes due to various reasons. Special features of H+ homeostasis that make it different from other ions will be discussed, particularly metabolic production of H+ and buffering. The transretinal distribution of extracellular H+ concentration ([H+]o) and its changes under illumination and other conditions will be described in detail, since [H+]o is more intensively investigated than intracellular pH. In vertebrate retinae, the highest [H+]o occurs in the inner part of the outer nuclear layer, and decreases toward the RPE, reaching the blood level on the apical side of the RPE. [H+]o falls toward the vitreous as well, but less, so that the inner retina is acidic to the vitreous. Light leads to complex changes with both electrogenic and metabolic origins, culminating in alkalinization. There is a rhythm of [H+]o with H+ being higher during circadian night. Extracellular pH can potentially be used as a signal in intercellular volume transmission, but evidence is against pH as a normal controller of fluid transport across the RPE or as a horizontal cell feedback signal. Pathological and experimentally created conditions (systemic metabolic acidosis, hypoxia and ischemia, vascular occlusion, excess glucose and diabetes, genetic disorders, and blockade of carbonic anhydrase) disturb H+ homeostasis, mostly producing retinal acidosis, with consequences for retinal blood flow, metabolism and function.

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.

Multiple Roles of cAMP in Vertebrate Retina

cAMP is a key regulatory molecule that controls many important processes in the retina, including phototransduction, cell development and death, growth of neural processes, intercellular contacts, retinomotor effects, and so forth. The total content of cAMP changes in the retina in a circadian manner following the natural light cycle, but it also shows local and even divergent changes in faster time scales in response to local and transient changes in the light environment. Changes in cAMP might also manifest or cause various pathological processes in virtually all cellular components of the retina. Here we review the current state of knowledge and understanding of the regulatory mechanisms by which cAMP influences the physiological processes that occur in various retinal cells.

Review and Hypothesis: A Potential Common Link Between Glial Cells, Calcium Changes, Modulation of Synaptic Transmission, Spreading Depression, Migraine, and Epilepsy-H. Front Cell Neurosci 2021;15:693095. [PMID: 34539347 PMCID: PMC8446203 DOI: 10.3389/fncel.2021.693095]

There is significant evidence to support the notion that glial cells can modulate the strength of synaptic connections between nerve cells, and it has further been suggested that alterations in intracellular calcium are likely to play a key role in this process. However, the molecular mechanism(s) by which glial cells modulate neuronal signaling remains contentiously debated. Recent experiments have suggested that alterations in extracellular H+ efflux initiated by extracellular ATP may play a key role in the modulation of synaptic strength by radial glial cells in the retina and astrocytes throughout the brain. ATP-elicited alterations in H+ flux from radial glial cells were first detected from Müller cells enzymatically dissociated from the retina of tiger salamander using self-referencing H+-selective microelectrodes. The ATP-elicited alteration in H+ efflux was further found to be highly evolutionarily conserved, extending to Müller cells isolated from species as diverse as lamprey, skate, rat, mouse, monkey and human. More recently, self-referencing H+-selective electrodes have been used to detect ATP-elicited alterations in H+ efflux around individual mammalian astrocytes from the cortex and hippocampus. Tied to increases in intracellular calcium, these ATP-induced extracellular acidifications are well-positioned to be key mediators of synaptic modulation. In this article, we examine the evidence supporting H+ as a key modulator of neurotransmission, review data showing that extracellular ATP elicits an increase in H+ efflux from glial cells, and describe the potential signal transduction pathways involved in glial cell-mediated H+ efflux. We then examine the potential role that extracellular H+ released by glia might play in regulating synaptic transmission within the vertebrate retina, and then expand the focus to discuss potential roles in spreading depression, migraine, epilepsy, and alterations in brain rhythms, and suggest that alterations in extracellular H+ may be a unifying feature linking these disparate phenomena.

Diel pCO2 fluctuations alter the molecular response of coral reef fishes to ocean acidification conditions

Environmental partial pressure of CO₂ (pCO₂ ) variation can modify the responses of marine organisms to ocean acidification, yet the underlying mechanisms for this effect remain unclear. On coral reefs, environmental pCO₂  fluctuates on a regular day-night cycle. Effects of future ocean acidification on coral reef fishes might therefore depend on their response to this diel cycle of pCO₂ . To evaluate the effects on the brain molecular response, we exposed two common reef fishes (Acanthochromis polyacanthus and Amphiprion percula) to two projected future pCO₂  levels (750 and 1,000 µatm) under both stable and diel fluctuating conditions. We found a common signature to stable elevated pCO₂ for both species, which included the downregulation of immediate early genes, indicating lower brain activity. The transcriptional programme was more strongly affected by higher average pCO₂ in a stable treatment than for fluctuating treatments, but the largest difference in molecular response was between stable and fluctuating pCO₂ treatments. This indicates that a response to a change in environmental pCO₂ conditions is different for organisms living in a fluctuating than in stable environments. This differential regulation was related to steroid hormones and circadian rhythm (CR). Both species exhibited a marked difference in the expression of CR genes among pCO₂ treatments, possibly accommodating a more flexible adaptive approach in the response to environmental changes. Our results suggest that environmental pCO₂  fluctuations might enable reef fishes to phase-shift their clocks and anticipate pCO₂ changes, thereby avoiding impairments and more successfully adjust to ocean acidification conditions.

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 regulation in the retina: From molecules to network

The mammalian retina is the most unique tissue among those that display robust circadian/diurnal oscillations. The retina is not only a light sensing tissue that relays light information to the brain, it has its own circadian "system" independent from any influence from other circadian oscillators. While all retinal cells and retinal pigment epithelium (RPE) possess circadian oscillators, these oscillators integrate by means of neural synapses, electrical coupling (gap junctions), and released neurochemicals (such as dopamine, melatonin, adenosine, and ATP), so the whole retina functions as an integrated circadian system. Dysregulation of retinal clocks not only causes retinal or ocular diseases, it also impacts the circadian rhythm of the whole body, as the light information transmitted from the retina entrains the brain clock that governs the body circadian rhythms. In this review, how circadian oscillations in various retinal cells are integrated, and how retinal diseases affect daily rhythms.

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.

Identification of novel circadian transcripts in the zebrafish retina

High fecundity, transparent embryos for monitoring the rapid development of organs and the availability of a well-annotated genome has made zebrafish a model organism of choice for developmental biology and neurobiology. This vertebrate model, a favourite in chronobiology studies, shows striking circadian rhythmicity in behaviour. Here, we identify novel genes in the zebrafish genome, which are expressed in the zebrafish retina. We further resolve the expression pattern over time and tentatively assign specific novel transcripts to retinal bipolar cells of the inner nuclear layer. Using chemical ablation and free run experiments we segregate the transcripts that are rhythmic when entrained by light from those that show sustained oscillations in the absence of external cues. The transcripts reported here with rigorous annotation and specific functions in circadian biology provide the groundwork for functional characterisation of novel players in the zebrafish retinal clock.

Diel CO2 cycles reduce severity of behavioural abnormalities in coral reef fish under ocean acidification

Elevated CO₂ levels associated with ocean acidification (OA) have been shown to alter behavioural responses in coral reef fishes. However, all studies to date have used stable pCO₂ treatments, not considering the substantial diel pCO₂ variation that occurs in shallow reef habitats. Here, we reared juvenile damselfish, Acanthochromis polyacanthus, and clownfish, Amphiprion percula, at stable and diel cycling pCO₂ treatments in two experiments. As expected, absolute lateralization of A. polyacanthus and response to predator cue of Am. percula were negatively affected in fish reared at stable, elevated pCO₂ in both experiments. However, diel pCO₂ fluctuations reduced the negative effects of OA on behaviour. Importantly, in experiment two, behavioural abnormalities that were present in fish reared at stable 750 µatm CO₂ were largely absent in fish reared at 750 ± 300 µatm CO₂. Overall, we show that diel pCO₂ cycles can substantially reduce the severity of behavioural abnormalities caused by elevated CO₂. Thus, past studies may have over-estimated the impacts of OA on the behavioural performance of coral reef fishes. Furthermore, our results suggest that diel pCO₂ cycles will delay the onset of behavioural abnormalities in natural populations.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Proton feedback mediates the cascade of color-opponent signals onto H3 horizontal cells in goldfish retina

It has been postulated that horizontal cells (HCs) send feedback signals onto cones via a proton feedback mechanism, which generates the center-surround receptive field of bipolar cells, and color-opponent signals in many non-mammalian vertebrates. Here we used a strong pH buffer, HEPES, to reduce extracellular proton concentration changes and so determine whether protons mediate color-opponent signals in goldfish H3 (triphasic) HCs. Superfusion with 10mM HEPES-fortified saline elicited depolarization of H3 HCs' dark membrane potential and enhanced hyperpolarizing responses to blue stimuli, but suppressed both depolarization by yellow and orange and hyperpolarization by red stimuli. The response components suppressed by HEPES resembled the inverse of spectral responses of H2 (biphasic) HCs. These results are consistent with the Stell-Lightfoot cascade model, in which the HEPES-suppressed component of H3 HCs was calculated using light responses recorded experimentally in H1 (monophasic) and H2 HCs. Selective suppression of long- or long-+middle-wavelength cone signals by long-wavelength background enhanced the responses to short-wavelength stimuli. These results suggest that HEPES inhibited color opponent signals in H3 HCs, in which the source of opponent-color signals is primarily a feedback from H2 HCs and partly from H1 HCs onto short-wavelength cones, probably mediated by protons.

Inhibitory effect of somatostatin-14 on L-type voltage-gated calcium channels in cultured cone photoreceptors requires intracellular calcium

The inhibitory effects of somatostatin have been well documented for many physiological processes. The action of somatostatin is through G-protein-coupled receptor-mediated second-messenger signaling, which in turn affects other downstream targets including ion channels. In the retina, somatostatin is released from a specific class of amacrine cells. Here we report that there was a circadian phase-dependent effect of somatostatin-14 (SS14) on the L-type voltage-gated calcium channels (L-VGCCs) in cultured chicken cone photoreceptors, and our study reveals that this process is dependent on intracellular calcium stores. Application of 500 nM SS14 for 2 h caused a decrease in L-VGCC currents only during the subjective night but not the subjective day. We then explored the cellular mechanisms underlying the circadian phase-dependent effect of SS14. The inhibitory effect of SS14 on L-VGCCs was mediated through the pertussis-toxin-sensitive G-protein-dependent somatostatin receptor 2 (sst2). Activation of sst2 by SS14 further activated downstream signaling involving phospholipase C and intracellular calcium stores. Mobilization of intracellular Ca2+ was required for somatostatin induced inhibition of photoreceptor L-VGCCs, suggesting that somatostatin plays an important role in the modulation of photoreceptor physiology.

Strongly pH-buffered ringer's solution expands the receptive field size of horizontal cells in the carp retina

By intracellular recordings, we studied the effects of pH buffering on the size of the receptive field and the extent of dye coupling of horizontal cells (HCs) in the light-adapted carp retina. These parameters were compared between data obtained in fortified Ringer's solution and those obtained in control bicarbonate Ringer's of the same pH (7.60). In Ringer's fortified with 10 mM HEPES or 15 mM Tris, the dye-coupling ratio of HCs increased by 71% and 70%, respectively. These fortified Ringer's solutions also depolarized the dark membrane potential and increased the light-evoked response. The HC receptive field profile could be described by the exponential decline in peak response amplitude to a slit of light moved tangentially from the recording electrode. Thus, the receptive field size was determined as a space constant proportional to (gj/gm)(1/2), where gj and gm denote gap and non-gap-junctional conductances. The HEPES- or Tris-fortified Ringer's significantly increased the space constant by 43% and 41%, respectively. Since dye coupling was increased in the fortified Ringer's, it is likely that gj increased more than gm as a result of alkalinization of the cytosol. Since HEPES has an aminosulfonate moiety, it has been assumed to close the hemi-channels of connexin 26, but the pH-buffering effects were essentially the same as those of Tris that has no aminosulfonate moiety. Therefore, it is unlikely that the closure of connexin 26 hemichannels is responsible for the change in the receptive field size of HCs.

Glutamatergic mechanisms in the outer retina of larval zebrafish: analysis of electroretinogram b- and d-waves using a novel preparation

A new preparation is described for recording the electroretinogram (ERG) from larval zebrafish (5-8 days postfertilization) which has allowed the investigation of the pharmacology of cone photoreceptor inputs onto bipolar cells. By using a pharmacological cocktail to isolate the photoreceptors and bipolar cells from inhibitory influences, it was found that an excitatory amino acid transporter (EAAT) presumably linked to a Cl() channel mediates most of the synaptic transmission from the cone photoreceptors to the ON bipolar cells, although metabotropic glutamate receptors (presumably mGluR6) also make a small contribution. On the other hand, alpha-amino-3-hydroxy- 5-methyl-4-isoxazolepropionate (AMPA)/kainate receptors mediate synaptic transmission from cone photoreceptors to OFF bipolar cells. The glutamatergic input mechanisms underlying bipolar cell responses in the larval zebrafish are adultlike and similar to those in other teleost species.

Circadian proteomics of the mouse retina

The circadian clock in the retina regulates a variety of physiological phenomena such as disc shedding and melatonin release. Although these events are critical for retinal functions, it is almost unknown how the circadian clock controls the physiological rhythmicity. To gain insight into the processes, we performed a proteomic analysis using 2-DE to find proteins whose levels show circadian changes. Among 415 retinal protein spots, 11 protein spots showed circadian rhythmicity in their intensities. We performed MALDI-TOF MS and NanoLC-MS/MS analyses and identified proteins contained in the 11 spots. The proteins were related to vesicular transport, calcium-binding, protein degradation, metabolism, RNA-binding, and protein foldings, suggesting the clock-regulation of neurotransmitter release, transportation of the membrane proteins, calcium-binding capability, and so on. We also found a rhythmic phosphorylation of N-ethylmaleimide-sensitive fusion protein and identified one of the amino acid residues modified by phosphorylation. These findings provide a new perspective on the relationship between the physiological functions of the retina and the circadian clock system.

Role of eye movements in the retinal code for a size discrimination task

The concerted action of saccades and fixational eye movements are crucial for seeing stationary objects in the visual world. We studied how these eye movements contribute to retinal coding of visual information using the archer fish as a model system. We quantified the animal's ability to distinguish among objects of different sizes and measured its eye movements. We recorded from populations of retinal ganglion cells with a multielectrode array, while presenting visual stimuli matched to the behavioral task. We found that the beginning of fixation, namely the time immediately after the saccade, provided the most visual information about object size, with fixational eye movements, which consist of tremor and drift in the archer fish, yielding only a minor contribution. A simple decoder that combined information from <or=15 ganglion cells could account for the behavior. Our results support the view that saccades impose not just difficulties for the visual system, but also an opportunity for the retina to encode high quality "snapshots" of the environment.

Proton modulation of ion channels in isolated horizontal cells of the goldfish retina

Transient changes in extracellular pH (pH(o)) occur in the retina and may have profound effects on neurotransmission and visual processing due to the pH sensitivity of ion channels. The present study characterized the effects of acidification on the activity of membrane ion channels in isolated horizontal cells (HCs) of the goldfish retina using whole-cell patch-clamp recording. Currents recorded from HCs were characterized by prominent inward rectification at potentials negative to -80 mV, a negative slope conductance between -70 and -40 mV, a sustained inward current, and outward rectification positive to 40 mV. Inward currents were identified as those of inward rectifier K(+) (Kir) channels and Ca(2+) channels by their sensitivity to 10 mM Cs(+) or 20 microm Cd(2+), respectively. Both of these currents were reduced when pH(o) decreased from 7.8 to 6.8. Glutamate (1 mM)-activated currents were also identified, as were hemichannel currents that were enhanced by removal of extracellular Ca(2+) and application of 1 mM quinidine. Both glutamate-activated and hemichannel currents were suppressed by a similar reduction of pH(o). When all of these H(+)-inhibited currents were blocked, a small, sustained inward current at -60 mV increased following a decrease in pH(o) from 7.8 to 6.8. In addition, slope conductance between -70 and -20 mV increased during this acidification. Suppression of this H(+)-activated current by removal of extracellular Na(+), and an extrapolated E(rev) near E(Na), indicated that this current was carried predominantly by Na(+) ions.

Diurnal changes in exocytosis and the number of synaptic ribbons at active zones of an ON-type bipolar cell terminal

The number and morphology of synaptic ribbons at photoreceptor and bipolar cell terminals has been reported to change on a circadian cycle. Here we sought to determine whether this phenomenon exists at goldfish Mb-type bipolar cell terminals with the aim of exploring the role of ribbons in transmitter release. We examined the physiology and ultrastructure of this terminal around two time points: midday and midnight. Nystatin perforated-patch recordings of membrane capacitance (C(m)) revealed that synaptic vesicle exocytosis evoked by short depolarizations was reduced at night, even though Ca(2+) currents were larger. The efficiency of exocytosis (measured as the DeltaC(m) jump per total Ca(2+) charge influx) was thus significantly lower at night. The paired-pulse ratio remained unchanged, however, suggesting that release probability was not altered. Hence the decreased exocytosis likely reflects a smaller readily releasable vesicle pool at night. Electron microscopy of single sections from intact retinas averaged 65% fewer ribbons at night. Interestingly, the number of active zones did not change from day to night, only the probability of finding a ribbon at an active zone. Additionally, synaptic vesicle halos surrounding the ribbons were more completely filled at night when these on-type bipolar cells are more hyperpolarized. There was no change, however, in the physical dimensions of synaptic ribbons from day to night. These results suggest that the size of the readily releasable vesicle pool and the efficiency of exocytosis are reduced at night when fewer ribbons are present at bipolar cell terminal active zones.

Electrical Feedback in the Cone Pedicle: A Computational Analysis

One of the fundamental principles of neuroscience is that direct electrical interactions between neurons are not possible without specialized electrical contacts, gap junctions, because the transmembrane resistance of neurons is typically much higher than the resistance of the adjacent extracellular space. However it has been proposed that in the retina direct electrical interactions between cones and second-order neurons occur due to the specific morphology of the cone synaptic terminal. This electrical mechanism could potentially explain the phenomenon of “negative feedback” from horizontal cells to cones and the recent finding that the tips of horizontal cell dendrites contain hemichannels has rekindled interest in the idea. We quantitatively evaluated the possibility that hemichannels and/or glutamate channels mediate electrical feedback from horizontal cells to cones. The calculations show that it is unlikely that an electrical mechanism plays a significant functional role because 1) the necessity of preserving adequate cone-to-horizontal-cell synaptic transmission limits the extracellular space resistance and the horizontal-cell dendritic transmembrane resistances to values at which the effectiveness of electrical feedback is very low and its electrical effect on the cone presynaptic membrane is negligible, 2) electrical feedback is most effective in the dark and weaker during light adaptation, which contradicts the experimental data, and 3) electrical negative feedback is associated with much stronger electrical positive feedback from horizontal cells to cones, a phenomenon that has never been reported. Therefore it is likely that negative feedback from horizontal cells to cones is chemical in nature.

Mutations that affect the survival of selected amacrine cell subpopulations define a new class of genetic defects in the vertebrate retina

Amacrine neurons are among the most diverse cell classes in the vertebrate retina. To gain insight into mechanisms vital to the production and survival of amacrine cell types, we investigated a group of mutations in three zebrafish loci: kleks (kle), chiorny (chy), and bergmann (bgm). Mutants of all three genes display a severe loss of selected amacrine cell subpopulations. The numbers of GABA-expressing amacrine interneurons are sharply reduced in all three mutants, while cell loss in other amacrine cell subpopulations varies and some cells are not affected at all. To investigate how amacrine cell loss affects retinal function, we performed electroretinograms on mutant animals. While the kle mutation mostly influences the function of the inner nuclear layer, unexpectedly the chy mutant phenotype also involves a loss of photoreceptor cell activity. The precise ration and arrangement of amacrine cell subpopulations suggest that cell-cell interactions are involved in the differentiation of this cell class. To test whether defects of such interactions may be, at least in part, responsible for mutant phenotypes, we performed mosaic analysis and demonstrated that the loss of parvalbumin-positive amacrine cells in chy mutants is due to extrinsic (cell-nonautonomous) causes. The phenotype of another amacrine cell subpopulation, the GABA-positive cells, does not display a clear cell-nonautonomy in chy animals. These results indicate that environmental factors, possibly interactions among different subpopulations of amacrine neurons, are involved in the development of the amacrine cell class.

A circadian clock and light/dark adaptation differentially regulate adenosine in the mammalian retina

Although the purine adenosine acts as an extracellular neuromodulator in the mammalian CNS in both normal and pathological conditions and regulates sleep, the regulation of extracellular adenosine in the day and night is incompletely understood. To determine how extracellular adenosine is regulated, rabbit neural retinas were maintained by superfusion at different times of the regular light/dark and circadian cycles. The adenosine level in the superfusate, representing adenosine overflow from the retinas, and the adenosine level in retinal homogenates, representing adenosine content, were measured using HPLC with fluorescence detection in the absence or presence of blockers of adenosine transport and/or extracellular adenosine synthesis. We report that darkness, compared with illumination, increases the level of extracellular adenosine, and that a circadian clock also increases extracellular adenosine at night. In addition, we show that the darkness-evoked increase in the level of extracellular adenosine results primarily from an increase in the conversion of extracellular ATP into adenosine, but that the clock-induced increase at night results primarily from an increase in the accumulation of intracellular adenosine. We also show that a slightly hypoxic state increases adenosine content and overflow to an extent similar to that of the clock. Our findings demonstrate that the extracellular level of adenosine in the mammalian retina is differentially regulated by a circadian clock and the lighting conditions and is maximal at night under dark-adapted conditions. We conclude that adenosine is a neuromodulator involved in both circadian clock and dark-adaptive processes in the vertebrate retina.

Circadian clocks, clock networks, arylalkylamine N-acetyltransferase, and melatonin in the retina

Circadian clocks are self-sustaining genetically based molecular machines that impose approximately 24h rhythmicity on physiology and behavior that synchronize these functions with the solar day-night cycle. Circadian clocks in the vertebrate retina optimize retinal function by driving rhythms in gene expression, photoreceptor outer segment membrane turnover, and visual sensitivity. This review focuses on recent progress in understanding how clocks and light control arylalkylamine N-acetyltransferase (AANAT), which is thought to drive the daily rhythm in melatonin production in those retinas that synthesize the neurohormone; AANAT is also thought to detoxify arylalkylamines through N-acetylation. The review will cover evidence that cAMP is a major output of the circadian clock in photoreceptor cells; and recent advances indicating that clocks and clock networks occur in multiple cell types of the retina.

Neurotransmitter modulation of extracellular H+ fluxes from isolated retinal horizontal cells of the skate

Self-referencing H(+)-selective microelectrodes were used to measure extracellular H(+) fluxes from horizontal cells isolated from the skate retina. A standing H(+) flux was detected from quiescent cells, indicating a higher concentration of free hydrogen ions near the extracellular surface of the cell as compared to the surrounding solution. The standing H(+) flux was reduced by removal of extracellular sodium or application of 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting activity of a Na(+)-H(+) exchanger. Glutamate decreased H(+) flux, lowering the concentration of free hydrogen ions around the cell. AMPA/kainate receptor agonists mimicked the response, and the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) eliminated the effects of glutamate and kainate. Metabotropic glutamate agonists were without effect. Glutamate-induced alterations in H(+) flux required extracellular calcium, and were abolished when cells were bathed in an alkaline Ringer solution. Increasing intracellular calcium by photolysis of the caged calcium compound NP-EGTA also altered extracellular H(+) flux. Immunocytochemical localization of the plasmalemma Ca(2+)-H(+)-ATPase (PMCA pump) revealed intense labelling within the outer plexiform layer and on isolated horizontal cells. Our results suggest that glutamate modulation of H(+) flux arises from calcium entry into cells with subsequent activation of the plasmalemma Ca(2+)-H(+)-ATPase. These neurotransmitter-induced changes in extracellular pH have the potential to play a modulatory role in synaptic processing in the outer retina. However, our findings argue against the hypothesis that hydrogen ions released by horizontal cells normally act as the inhibitory feedback neurotransmitter onto photoreceptor synaptic terminals to create the surround portion of the centre-surround receptive fields of retinal neurones.

Retinal pH reflects retinal energy metabolism in the day and night

The extracellular pH of living tissue in the retina and elsewhere in the brain is lower than the pH of the surrounding milieu. We have shown that the pH gradient between the in vitro retina and the superfusion solution is regulated by a circadian (24-h) clock so that it is smaller in the subjective day than in the subjective night. We show here that the circadian changes in retinal pH result from a clock-mediated change in the generation of H+ that accompanies energy production. To demonstrate this, we suppressed energy metabolism and recorded the resultant reduction in the pH difference between the retina and superfusate. The magnitude of the reduction in the pH gradient correlated with the extent of energy metabolism suppression. We also examined whether the circadian-induced increase in acid production during the subjective night results from an increase in energy metabolism or from the selective activation of glycolysis compared with oxidative phosphorylation. We found that the selective suppression of either oxidative phosphorylation or glycolysis had almost identical effects on the dynamics and extent of H+ production during the subjective day and night. Thus the proportion of glycolysis and oxidative phosphorylation is maintained the same regardless of circadian time, and the pH difference between the tissue and superfusion solution can therefore be used to evaluate total energy production. We conclude that circadian clock regulation of retinal pH reflects circadian regulation of retinal energy metabolism.

Regulation and modulation of pH in the brain

The regulation of pH is a vital homeostatic function shared by all tissues. Mechanisms that govern H+ in the intracellular and extracellular fluid are especially important in the brain, because electrical activity can elicit rapid pH changes in both compartments. These acid-base transients may in turn influence neural activity by affecting a variety of ion channels. The mechanisms responsible for the regulation of intracellular pH in brain are similar to those of other tissues and are comprised principally of forms of Na+/H+ exchange, Na+-driven Cl-/HCO3- exchange, Na+-HCO3- cotransport, and passive Cl-/HCO3- exchange. Differences in the expression or efficacy of these mechanisms have been noted among the functionally and morphologically diverse neurons and glial cells that have been studied. Molecular identification of transporter isoforms has revealed heterogeneity among brain regions and cell types. Neural activity gives rise to an assortment of extracellular and intracellular pH shifts that originate from a variety of mechanisms. Intracellular pH shifts in neurons and glia have been linked to Ca2+ transport, activation of acid extrusion systems, and the accumulation of metabolic products. Extracellular pH shifts can occur within milliseconds of neural activity, arise from an assortment of mechanisms, and are governed by the activity of extracellular carbonic anhydrase. The functional significance of these compartmental, activity-dependent pH shifts is discussed.

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.

Lazy eyes zebrafish mutation affects Müller glial cells, compromising photoreceptor function and causing partial blindness

A behavioral assay based on the optokinetic reflex was used to screen chemically mutagenized zebrafish larvae for deficits in visual function. A homozygous recessive mutation, lazy eyes (lze), was isolated based on the observation that 5-day postfertilization (dpf) mutants displayed weaker and less frequent eye movements than wild-type fish in response to moving stripes. Electroretinographic (ERG) recordings revealed that mutants had severely reduced a- and b-wave amplitudes relative to wild-type fish, indicating outer retinal dysfunction. Retinal lamination and cellular differentiation were normal in the lze retina; however, mutant photoreceptor cells had small outer segments and pyknotic nuclei were occasionally observed in the outer retina and the marginal zone of lze. Cone, rod, amacrine, bipolar, and Müller cell marker analyses indicated that the typical lze retina contained fewer rod photoreceptors and fewer Müller cells than wild-type fish at 5 dpf. At 3 dpf, however, mutant retinas had normal numbers of rod photoreceptors and Müller cells, suggesting that the initial differentiation of these cell types occurred normally. Rod photoreceptor histology was normal at this early stage, but Müller cells were often hypertrophied, suggesting that they were unhealthy. Constant light rearing of mutant animals accelerated the Müller cell degeneration, severely worsened the visual deficit, but had no obvious affect on the photoreceptors. When ERG responses and Müller cell degeneration from the same mutant animals were analyzed, the extent of the Müller cell loss matched closely the degree to which ERG responses were reduced. In summary, the lze gene appears to be required for Müller cell viability and normal visual function. The lze mutant may be a model for the study of the involvement of Müller cells in photoreceptor development and function.

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.

Circadian rhythm of acidification in insect vas deferens regulated by rhythmic expression of vacuolar H+-ATPase

Recent studies have demonstrated that the peripheral tissues of vertebrates and invertebrates contain circadian clocks; however, little is known about their functions and the rhythmic outputs that they generate. To understand clock-controlled rhythms at the cellular level, we investigated a circadian clock located in the reproductive system of a male moth (the cotton leaf worm Spodoptera littoralis) that is essential for the production of fertile spermatozoa. Previous work has demonstrated that spermatozoa are released from the testes in a daily rhythm and are periodically stored in the upper vas deferens (UVD). In this paper, we demonstrate a circadian rhythm in pH in the lumen of the UVD, with acidification occurring during accumulation of spermatozoa in the lumen. The daily rhythm in pH correlates with a rhythmic increase in the expression of a proton pump, the vacuolar H+-ATPase (V-ATPase), in the apical portion of the UVD epithelium. Rhythms in pH and V-ATPase persist in light/dark cycles and constant darkness, but are abolished in constant light, a condition that disrupts clock function and renders spermatozoa infertile. Treatment with colchicine impairs the migration of V-ATPase-positive vesicles to the apical cell membrane and abates the acidification of the UVD lumen. Bafilomycin, a selective inhibitor of V-ATPase activity, also prevents the decline in luminal pH. We conclude that the circadian clock generates a rhythm of luminal acidification by regulating the levels and subcellular distribution of V-ATPase in the UVD epithelium. Our data provide the first evidence for circadian control of V-ATPase, the fundamental enzyme that provides the driving force for numerous secondary transport processes. They also demonstrate how circadian rhythms displayed by individual cells contribute to the synchrony of physiological processes at the organ level.

A null mutation in guanylate cyclase-1 alters the temporal dynamics and light entrainment properties of the iodopsin rhythm in cone photoreceptor cells

Guanylate cyclase-1 (GC1) plays a critical role in visual phototransduction and its absence severely compromises the ability of the photoreceptor cells to transduce light for vision. In this study we sought to determine if the absence of GC1 has any effect on light entrainment of the circadian oscillators located in these cells. We compared the rhythmic changes in transcript levels of iodopsin, a photoreceptor-specific gene whose expression is regulated by circadian oscillators, in retinas of normal chickens and GUCY1*B (*B) chickens that carry a null mutation in GC1. Our results show that iodopsin rhythms are present in *B retinas and that they can be entrained to light; however, the rise and fall of iodopsin transcript levels in *B retina under cyclic light conditions is significantly more rapid than that observed in normal retina, and under constant dark conditions, the phase of the iodopsin rhythm in *B retina is advanced by 6 h relative to that observed in normal retina. In addition, the rate of entrainment of the iodopsin rhythm in *B retina to a reversal of the light cycle is significantly slower than normal. The results of our study show that a functioning visual phototransduction cascade is not essential for light entrainment of the oscillators that drive the iodopsin rhythm in photoreceptor cells. We propose that the abnormal synthesis of cGMP in *B photoreceptors underlies the irregular iodopsin rhythms observed in post-hatch *B retina.

Circadian clock regulation of pH in the rabbit retina

Although it is generally accepted that the acid-base ratio of tissue, as represented by the pH, is strictly regulated to maintain normal function, recent studies in the mammalian nervous system have shown that neuronal activity can result in significant shifts in pH. In the mammalian retina, many cellular phenomena, including neuronal activity, are regulated by a circadian clock. We thus investigated whether a clock regulates retinal pH, using pH-sensitive microelectrodes to measure the extracellular pH (pH(o)) of the in vitro rabbit retina in the subjective day and night, that is, under conditions of constant darkness. These measurements demonstrated that a circadian clock regulates the pH(o) of the rabbit retina so that the pH(o) is lower at night than in the day. This day/night difference in retinal pH(o) was observed when the rabbits were maintained on a normal light/dark cycle and after they were maintained on a light/dark cycle that was phase-delayed by 9 hr. Continuous recordings of retinal pH(o) around subjective dusk indicated that the change from daytime to nighttime pH(o) is relatively fast and suggested that the clock that regulates pH(o) is located in the retina. The lowest pH(o) recorded in the retina in both the day and night was in the vicinity of the inner segments of photoreceptor cells, supporting the idea that photoreceptors serve as the primary source of protons. The circadian-induced shift in pH(o) was several times greater than light-induced pH(o) changes. These findings suggest that a circadian clock in the mammalian retina regulates retinal pH.

Circadian clock regulation of pH in the rabbit retina

Although it is generally accepted that the acid-base ratio of tissue, as represented by the pH, is strictly regulated to maintain normal function, recent studies in the mammalian nervous system have shown that neuronal activity can result in significant shifts in pH. In the mammalian retina, many cellular phenomena, including neuronal activity, are regulated by a circadian clock. We thus investigated whether a clock regulates retinal pH, using pH-sensitive microelectrodes to measure the extracellular pH (pH(o)) of the in vitro rabbit retina in the subjective day and night, that is, under conditions of constant darkness. These measurements demonstrated that a circadian clock regulates the pH(o) of the rabbit retina so that the pH(o) is lower at night than in the day. This day/night difference in retinal pH(o) was observed when the rabbits were maintained on a normal light/dark cycle and after they were maintained on a light/dark cycle that was phase-delayed by 9 hr. Continuous recordings of retinal pH(o) around subjective dusk indicated that the change from daytime to nighttime pH(o) is relatively fast and suggested that the clock that regulates pH(o) is located in the retina. The lowest pH(o) recorded in the retina in both the day and night was in the vicinity of the inner segments of photoreceptor cells, supporting the idea that photoreceptors serve as the primary source of protons. The circadian-induced shift in pH(o) was several times greater than light-induced pH(o) changes. These findings suggest that a circadian clock in the mammalian retina regulates retinal pH.