Rod electrical coupling is controlled by a circadian clock and dopamine in mouse retina

Advances in the study of the influence of photoreceptors on the development of myopia

This review examines the pivotal role of photoreceptor cells in ocular refraction development, focusing on dopamine (DA) as a key neurotransmitter. Contrary to the earlier view favoring cone cells, recent studies have highlighted the substantial contributions of both rod and cone cells to the visual signaling pathways that influence ocular refractive development. Notably, rod cells appeared to play a central role. Photoreceptor cells interact intricately with circadian rhythms, color vision pathways, and other neurotransmitters, all of which are crucial for the complex mechanisms driving the development of myopia. This review emphasizes that ocular refractive development results from a coordinated interplay between diverse cell types, signaling pathways, and neurotransmitters. This perspective has significant implications for unraveling the complex mechanisms underlying myopia and aiding in the development of more effective prevention and treatment strategies.

When microscopy and electrophysiology meet connectomics-Steve Massey's contribution to unraveling the structure and function of the rod/cone gap junction

Electrical synapses, formed of gap junctions, are ubiquitous components of the central nervous system (CNS) that shape neuronal circuit connectivity and dynamics. In the retina, electrical synapses can create a circuit, control the signal-to-noise ratio in individual neurons, and support the coordinated neuronal firing of ganglion cells, hence, regulating signal processing at the network, single-cell, and dendritic level. We, the authors, and Steve Massey have had a long interest in gap junctions in retinal circuits, in general, and in the network of photoreceptors, in particular. Our combined efforts, based on a wide array of techniques of molecular biology, microscopy, and electrophysiology, have provided fundamental insights into the molecular structure and properties of the rod/cone gap junction. Yet, a full understanding of how rod/cone coupling controls circuit dynamics necessitates knowing its operating range. It is well established that rod/cone coupling can be greatly reduced or eliminated by bright-light adaptation or pharmacological treatment; however, the upper end of its dynamic range has long remained elusive. This held true until Steve Massey's recent interest for connectomics led to the development of a new strategy to assess this issue. The effort proved effective in establishing, with precision, the connectivity rules between rods and cones and estimating the theoretical upper limit of rod/cone electrical coupling. Comparing electrophysiological measurements and morphological data indicates that under pharmacological manipulation, rod/cone coupling can reach the theoretical maximum of its operating range, implying that, under these conditions, all the gap junction channels present at the junctions are open. As such, channel open probability is likely the main determinant of rod/cone coupling that can change momentarily in a time-of-day- and light-dependent manner. In this article we briefly review our current knowledge of the molecular structure of the rod/cone gap junction and of the mechanisms behind its modulation, and we highlight the recent work led by Steve Massey. Steve's contribution has been critical toward asserting the modulation depth of rod/cone coupling as well as elevating the rod/cone gap junction as one of the most suitable models to examine the role of electrical synapses and their plasticity in neural processing.

Extrinsic and Intrinsic Factors Determine Expression Levels of Gap Junction-Forming Connexins in the Mammalian Retina

Gap junctions (GJs) are not static bridges; instead, GJs as well as the molecular building block connexin (Cx) proteins undergo major expression changes in the degenerating retinal tissue. Various progressive diseases, including retinitis pigmentosa, glaucoma, age-related retinal degeneration, etc., affect neurons of the retina and thus their neuronal connections endure irreversible changes as well. Although Cx expression changes might be the hallmarks of tissue deterioration, GJs are not static bridges and as such they undergo adaptive changes even in healthy tissue to respond to the ever-changing environment. It is, therefore, imperative to determine these latter adaptive changes in GJ functionality as well as in their morphology and Cx makeup to identify and distinguish them from alterations following tissue deterioration. In this review, we summarize GJ alterations that take place in healthy retinal tissue and occur on three different time scales: throughout the entire lifespan, during daily changes and as a result of quick changes of light adaptation.

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.

Connexins Biology in the Pathophysiology of Retinal Diseases

Connexins (Cx) are a family of transmembrane proteins that form gap junction intercellular channels that connect neighboring cells. These channels allow the passage of ions and other biomolecules smaller than 1 kDa, thereby synchronizing the cells both electrically and metabolically. Cxs are expressed in all retinal cell types and the diversity of Cx isoforms involved in the assembly of the channels provides a functional syncytium required for visual transduction. In this chapter, we summarize the status of current knowledge regarding Cx biology in retinal tissues and discuss how Cx dysfunction is associated with retinal disease pathophysiology. Although the contribution of Cx deficiency to retinal degeneration is not well understood, recent findings present Cx as a potential therapeutic target. Therefore, we will briefly discuss pharmacological approaches and gene therapies that are being explored to modulate Cx function and fight sight-threatening eye diseases.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

Analytical methods for assessing retinal cell coupling using cut-loading

Electrical coupling between retinal neurons contributes to the functional complexity of visual circuits. “Cut-loading” methods allow simultaneous assessment of cell-coupling between multiple retinal cell-types, but existing analysis methods impede direct comparison with gold standard direct dye injection techniques. In the current study, we both improved an existing method and developed two new approaches to address observed limitations. Each method of analysis was applied to cut-loaded dark-adapted Guinea pig retinae (n = 29) to assess coupling strength in the axonless horizontal cell type (‘a-type’, aHCs). Method 1 was an improved version of the standard protocol and described the distance of dye-diffusion (space constant). Method 2 adjusted for the geometric path of dye-transfer through cut-loaded cells and extracted the rate of dye-transfer across gap-junctions in terms of the coupling coefficient (k_j). Method 3 measured the diffusion coefficient (De) perpendicular to the cut-axis. Dye transfer was measured after one of five diffusion times (1–20 mins), or with a coupling inhibitor, meclofenamic acid (MFA) (50–500μM after 20 mins diffusion). The standard protocol fits an exponential decay function to the fluorescence profile of a specified retina layer but includes non-specific background fluorescence. This was improved by measuring the fluorescence of individual cell soma and excluding from the fit non-horizontal cells located at the cut-edge (p<0.001) (Method 1). The space constant (Method 1) increased with diffusion time (p<0.01), whereas Methods 2 (p = 0.54) and 3 (p = 0.63) produced consistent results across all diffusion times. Adjusting distance by the mean cell-cell spacing within each tissue reduced the incidence of outliers across all three methods. Method 1 was less sensitive to detecting changes induced by MFA than Methods 2 (p<0.01) and 3 (p<0.01). Although the standard protocol was easily improved (Method 1), Methods 2 and 3 proved more sensitive and generalisable; allowing for detailed assessment of the tracer kinetics between different populations of gap-junction linked cell networks and direct comparison to dye-injection techniques.

Analysis of rod/cone gap junctions from the reconstruction of mouse photoreceptor terminals

Electrical coupling, mediated by gap junctions, contributes to signal averaging, synchronization, and noise reduction in neuronal circuits. In addition, gap junctions may also provide alternative neuronal pathways. However, because they are small and especially difficult to image, gap junctions are often ignored in large-scale 3D reconstructions. Here, we reconstruct gap junctions between photoreceptors in the mouse retina using serial blockface-scanning electron microscopy, focused ion beam-scanning electron microscopy, and confocal microscopy for the gap junction protein Cx36. An exuberant spray of fine telodendria extends from each cone pedicle (including blue cones) to contact 40-50 nearby rod spherules at sites of Cx36 labeling, with approximately 50 Cx36 clusters per cone pedicle and 2-3 per rod spherule. We were unable to detect rod/rod or cone/cone coupling. Thus, rod/cone coupling accounts for nearly all gap junctions between photoreceptors. We estimate a mean of 86 Cx36 channels per rod/cone pair, which may provide a maximum conductance of ~1200 pS, if all gap junction channels were open. This is comparable to the maximum conductance previously measured between rod/cone pairs in the presence of a dopamine antagonist to activate Cx36, suggesting that the open probability of gap junction channels can approach 100% under certain conditions.

Genetic elimination of rod/cone coupling reveals the contribution of the secondary rod pathway to the retinal output

In the retina, signals originating from rod and cone photoreceptors can reach retinal ganglion cells (RGCs)-the output neurons-through different pathways. However, little is known about the exact sensitivities and operating ranges of these pathways. Previously, we created rod- or cone-specific Cx36 knockout (KO) mouse lines. Both lines are deficient in rod/cone electrical coupling and therefore provide a way to selectively remove the secondary rod pathway. We measured the threshold of the primary rod pathway in RGCs of wild-type mice. Under pharmacological blockade of the primary rod pathway, the threshold was elevated. This secondary component was removed in the Cx36 KOs to unmask the threshold of the third rod pathway, still below cone threshold. In turn, the cone threshold was estimated by several independent methods. Our work defines the functionality of the secondary rod pathway and describes an additive contribution of the different pathways to the retinal output.

Visual pigment-deficient cones survive and mediate visual signaling despite the lack of outer segments

Rhodopsin and cone opsins are essential for light detection in vertebrate rods and cones, respectively. It is well established that rhodopsin is required for rod phototransduction, outer segment disk morphogenesis, and rod viability. However, the roles of cone opsins are less well understood. In this study, we adopted a loss-of-function approach to investigate the physiological roles of cone opsins in mice. We showed that cones lacking cone opsins do not form normal outer segments due to the lack of disk morphogenesis. Surprisingly, cone opsin-deficient cones survive for at least 12 mo, which is in stark contrast to the rapid rod degeneration observed in rhodopsin-deficient mice, suggesting that cone opsins are dispensable for cone viability. Although the mutant cones do not respond to light directly, they maintain a normal dark current and continue to mediate visual signaling by relaying the rod signal through rod-cone gap junctions. Our work reveals a striking difference between the role of rhodopsin and cone opsins in photoreceptor viability.

Circadian regulation of vertebrate cone photoreceptor function

Eukaryotes generally display a circadian rhythm as an adaption to the reoccurring day/night cycle. This is particularly true for visual physiology that is directly affected by changing light conditions. Here we investigate the influence of the circadian rhythm on the expression and function of visual transduction cascade regulators in diurnal zebrafish and nocturnal mice. We focused on regulators of shut-off kinetics such as Recoverins, Arrestins, Opsin kinases, and Regulator of G-protein signaling that have direct effects on temporal vision. Transcript as well as protein levels of most analyzed genes show a robust circadian rhythm-dependent regulation, which correlates with changes in photoresponse kinetics. Electroretinography demonstrates that photoresponse recovery in zebrafish is delayed in the evening and accelerated in the morning. Functional rhythmicity persists in continuous darkness, and it is reversed by an inverted light cycle and disrupted by constant light. This is in line with our finding that orthologous gene transcripts from diurnal zebrafish and nocturnal mice are often expressed in an anti-phasic daily rhythm.

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

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.

Loss of the K+ channel Kv2.1 greatly reduces outward dark current and causes ionic dysregulation and degeneration in rod photoreceptors

Vertebrate retinal photoreceptors signal light by suppressing a circulating “dark current” that maintains their relative depolarization in the dark. This dark current is composed of an inward current through CNG channels and NCKX transporters in the outer segment that is balanced by outward current exiting principally from the inner segment. It has been hypothesized that K_v2.1 channels carry a predominant fraction of the outward current in rods. We examined this hypothesis by comparing whole cell, suction electrode, and electroretinographic recordings from K_v2.1 knockout (K_v2.1^−/−) and wild-type (WT) mouse rods. Single cell recordings revealed flash responses with unusual kinetics, and reduced dark currents that were quantitatively consistent with the measured depolarization of the membrane resting potential in the dark. A two-compartment (outer and inner segment) physiological model based on known ionic mechanisms revealed that the abnormal K_v2.1^−/− rod photoresponses arise principally from the voltage dependencies of the known conductances and the NCKX exchanger, and a highly elevated fraction of inward current carried by Ca²⁺ through CNG channels due to the aberrant depolarization. K_v2.1^−/− rods had shorter outer segments than WT and dysmorphic mitochondria in their inner segments. Optical coherence tomography of knockout animals demonstrated a slow photoreceptor degeneration over a period of 6 mo. Overall, these findings reveal that K_v2.1 channels carry 70–80% of the non-NKX outward dark current of the mouse rod, and that the depolarization caused by the loss of K_v2.1 results in elevated Ca²⁺ influx through CNG channels and elevated free intracellular Ca²⁺, leading to progressive degeneration.

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.

Illuminating and Sniffing Out the Neuromodulatory Roles of Dopamine in the Retina and Olfactory Bulb

In the central nervous system, dopamine is well-known as the neuromodulator that is involved with regulating reward, addiction, motivation, and fine motor control. Yet, decades of findings are revealing another crucial function of dopamine: modulating sensory systems. Dopamine is endogenous to subsets of neurons in the retina and olfactory bulb (OB), where it sharpens sensory processing of visual and olfactory information. For example, dopamine modulation allows the neural circuity in the retina to transition from processing dim light to daylight and the neural circuity in the OB to regulate odor discrimination and detection. Dopamine accomplishes these tasks through numerous, complex mechanisms in both neural structures. In this review, we provide an overview of the established and emerging research on these mechanisms and describe similarities and differences in dopamine expression and modulation of synaptic transmission in the retinas and OBs of various vertebrate organisms. This includes discussion of dopamine neurons’ morphologies, potential identities, and biophysical properties along with their contributions to circadian rhythms and stimulus-driven synthesis, activation, and release of dopamine. As dysregulation of some of these mechanisms may occur in patients with Parkinson’s disease, these symptoms are also discussed. The exploration and comparison of these two separate dopamine populations shows just how remarkably similar the retina and OB are, even though they are functionally distinct. It also shows that the modulatory properties of dopamine neurons are just as important to vision and olfaction as they are to motor coordination and neuropsychiatric/neurodegenerative conditions, thus, we hope this review encourages further research to elucidate these mechanisms.

Dopaminergic Modulation of Signal Processing in a Subset of Retinal Bipolar Cells

The retina and the olfactory bulb are the gateways to the visual and olfactory systems, respectively, similarly using neural networks to initiate sensory signal processing. Sensory receptors receive signals that are transmitted to neural networks before projecting to primary cortices. These networks filter sensory signals based on their unique features and adjust their sensitivities by gain control systems. Interestingly, dopamine modulates sensory signal transduction in both systems. In the retina, dopamine adjusts the retinal network for daylight conditions (“light adaptation”). In the olfactory system, dopamine mediates lateral inhibition between the glomeruli, resulting in odorant signal decorrelation and discrimination. While dopamine is essential for signal discrimination in the olfactory system, it is not understood whether dopamine has similar roles in visual signal processing in the retina. To elucidate dopaminergic effects on visual processing, we conducted patch-clamp recording from second-order retinal bipolar cells, which exhibit multiple types that can convey different temporal features of light. We recorded excitatory postsynaptic potentials (EPSPs) evoked by various frequencies of sinusoidal light in the absence and presence of a dopamine receptor 1 (D₁R) agonist or antagonist. Application of a D₁R agonist, SKF-38393, shifted the peak temporal responses toward higher frequencies in a subset of bipolar cells. In contrast, a D₁R antagonist, SCH-23390, reversed the effects of SKF on these types of bipolar cells. To examine the mechanism of dopaminergic modulation, we recorded voltage-gated currents, hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and low-voltage activated (LVA) Ca²⁺ channels. SKF modulated HCN and LVA currents, suggesting that these channels are the target of D₁R signaling to modulate visual signaling in these bipolar cells. Taken together, we found that dopamine modulates the temporal tuning of a subset of retinal bipolar cells. Consequently, we determined that dopamine plays a role in visual signal processing, which is similar to its role in signal decorrelation in the olfactory bulb.

Molecular and functional architecture of the mouse photoreceptor network

Mouse photoreceptors are electrically coupled via gap junctions, but the relative importance of rod/rod, cone/cone, or rod/cone coupling is unknown. Furthermore, while connexin36 (Cx36) is expressed by cones, the identity of the rod connexin has been controversial. We report that FACS-sorted rods and cones both express Cx36 but no other connexins. We created rod- and cone-specific Cx36 knockout mice to dissect the photoreceptor network. In the wild type, Cx36 plaques at rod/cone contacts accounted for more than 95% of photoreceptor labeling and paired recordings showed the transjunctional conductance between rods and cones was ~300 pS. When Cx36 was eliminated on one side of the gap junction, in either conditional knockout, Cx36 labeling and rod/cone coupling were almost abolished. We could not detect direct rod/rod coupling, and cone/cone coupling was minor. Rod/cone coupling is so prevalent that indirect rod/cone/rod coupling via the network may account for previous reports of rod coupling.

A G-Protein Coupled Receptor and Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness in the world. The risk of AMD increases with age and is most common among the white population. Here, we discuss the convergence of factors related to race, pigmentation, and susceptibility to AMD, where the primary defect occurs in retinal support cells, the retinal pigment epithelium (RPE). We explore whether the observed racial bias in AMD incidence is related to innate differences in the basal level of pigmentation between races, and whether the pigmentation pathway activity in the RPE might protect from retinal degeneration. More specifically, we explore whether the downstream signaling activity of GPR143, a G-protein coupled receptor in the pigmentation pathway, might underly the racial bias of AMD and be a target to prevent the disease. Lastly, we summarize the past findings of a large retrospective study that investigated the relationship between the stimulation of GPR143 with L-DOPA, the pigmentation pathway, and AMD, to potentially help develop new ways to prevent or treat AMD. The reader of this review will come to understand the racial bias of AMD, which is related to the function of the RPE.

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.

Paired Recording to Study Electrical Coupling Between Photoreceptors in Mouse Retina

Gap junction-mediated electrical coupling between retinal photoreceptors is an important determinant of photoreceptor function. Yet, quantitative measurements of the junctional conductance between coupled photoreceptors are required to fully assess the effects of coupling on visual performance. Such measurements have been obtained in salamander and other lower vertebrate retinas but are difficult to acquire in mammalian retinas, in part because of the much smaller size of photoreceptors in mammals. Here, we describe in detail a dual whole-cell patch-clamp technique we recently developed to measure the junctional conductance between photoreceptor pairs in the mouse retina. With this method, electrical coupling strength between mouse photoreceptors can be estimated with high accuracy and its impact on retinal processing of visual information further evaluated.

Mice Reach Higher Visual Sensitivity at Night by Using a More Efficient Behavioral Strategy

Circadian clocks predictively adjust the physiology of organisms to the day/night cycle. The retina has its own clock, and many diurnal changes in its physiology have been reported. However, their implications for retinal functions and visually guided behavior are largely unresolved. Here, we study the impact of diurnal rhythm on the sensitivity limit of mouse vision. A simple photon detection task allowed us to link well-defined retinal output signals directly to visually guided behavior. We show that visually guided behavior at its sensitivity limit is strongly under diurnal control, reaching the highest sensitivity and stability at night. The diurnal differences in visual sensitivity did not arise in the retina, as assessed by spike recordings from the most sensitive retinal ganglion cell types: ON sustained, OFF sustained, and OFF transient alpha ganglion cells. Instead, we found that mice, as nocturnal animals, use a more efficient search strategy for visual cues at night. Intriguingly, they can switch to the more efficient night strategy even at their subjective day after first having performed the task at night. Our results exemplify that the shape of visual psychometric functions depends robustly on the diurnal state of the animal, its search strategy, and even its diurnal history of performing the task. The results highlight the impact of the day/night cycle on high-level sensory processing, demonstrating a direct diurnal impact on the behavioral strategy of the animal.

Rod Photoreceptors Signal Fast Changes in Daylight Levels Using a Cx36-Independent Retinal Pathway in Mouse

Temporal contrast detected by rod photoreceptors is channeled into multiple retinal rod pathways that ultimately connect to cone photoreceptor pathways via Cx36 gap junctions or via chemical synapses. However, we do not yet understand how the different rod pathways contribute to the perception of temporal contrast (changes in luminance with time) at mesopic light levels, where both rods and cones actively respond to light. Here, we use a forced-choice, operant behavior assay to investigate rod-driven, temporal contrast sensitivity (TCS) in mice of either sex. Transgenic mice with desensitized cones (GNAT2 ^cpfl3 line) were used to identify rod contributions to TCS in mesopic lights. We found that at low mesopic lights (400 photons/s/μm² at the retina), control and GNAT2 ^cpfl3 mice had similar TCS. Surprisingly, at upper mesopic lights (8000 photons/s/μm²), GNAT2 ^cpfl3 mice exhibited a relative reduction in TCS to low (<12 Hz) while maintaining normal TCS to high (12-36 Hz) temporal frequencies. The rod-driven responses to high temporal frequencies developed gradually over time (>30 min). Furthermore, the TCS of GNAT2 ^cpfl3 and GNAT2 ^cpfl3 ::Cx36^-/- mice matched closely, indicating that transmission of high-frequency signals (1) does not require the rod-cone Cx36 gap junctions as has been proposed in the past; and (2) a Cx36-independent rod pathway(s) (e.g., direct rod to OFF cone bipolar cell synapses and/or glycinergic synapses from AII amacrine cells to OFF ganglion cells) is sufficient for fast, mesopic rod-driven vision. These findings extend our understanding of the link between visual circuits and perception in mouse.SIGNIFICANCE STATEMENT The contributions of specific retinal pathways to visual perception are not well understood. We found that the temporal processing properties of rod-driven vision in mice change significantly with light level. In dim lights, rods relay relatively slow temporal variations. However, in daylight conditions, rod pathways exhibit high sensitivity to fast but not to slow temporal variations, whereas cone-driven responses supplement the loss in rod-driven sensitivity to slow temporal variations. Our findings highlight the dynamic interplay of rod- and cone-driven vision as light levels rise from night to daytime levels. Furthermore, the fast, rod-driven signals do not require the rod-to-cone Cx36 gap junctions as proposed in the past, but rather, can be relayed by alternative Cx36-independent rod pathways.

Ocular Clocks: Adapting Mechanisms for Eye Functions and Health

Vision is a highly rhythmic function adapted to the extensive changes in light intensity occurring over the 24-hour day. This adaptation relies on rhythms in cellular and molecular processes, which are orchestrated by a network of circadian clocks located within the retina and in the eye, synchronized to the day/night cycle and which, together, fine-tune detection and processing of light information over the 24-hour period and ensure retinal homeostasis. Systematic or high throughput studies revealed a series of genes rhythmically expressed in the retina, pointing at specific functions or pathways under circadian control. Conversely, knockout studies demonstrated that the circadian clock regulates retinal processing of light information. In addition, recent data revealed that it also plays a role in development as well as in aging of the retina. Regarding synchronization by the light/dark cycle, the retina displays the unique property of bringing together light sensitivity, clock machinery, and a wide range of rhythmic outputs. Melatonin and dopamine play a particular role in this system, being both outputs and inputs for clocks. The retinal cellular complexity suggests that mechanisms of regulation by light are diverse and intricate. In the context of the whole eye, the retina looks like a major determinant of phase resetting for other tissues such as the retinal pigmented epithelium or cornea. Understanding the pathways linking the cell-specific molecular machineries to their cognate outputs will be one of the major challenges for the future.

Dopaminergic modulation of retinal processing from starlight to sunlight

Neuromodulators such as dopamine, enable context-dependent plasticity of neural circuit function throughout the central nervous system. For example, in the retina, dopamine tunes visual processing for daylight and nightlight conditions. Specifically, high levels of dopamine release in the retina tune vision for daylight (photopic) conditions, while low levels tune it for nightlight (scotopic) conditions. This review covers the cellular and circuit-level mechanisms within the retina that are altered by dopamine. These mechanisms include changes in gap junction coupling and ionic conductances, both of which are altered by the activation of diverse types of dopamine receptors across diverse types of retinal neurons. We contextualize the modulatory actions of dopamine in terms of alterations and optimizations to visual processing under photopic and scotopic conditions, with particular attention to how they differentially impact distinct cell types. Finally, we discuss how transgenic mice and disease models have shaped our understanding of dopaminergic signaling and its role in visual processing. Cumulatively, this review illustrates some of the diverse and potent mechanisms through which neuromodulation can shape brain function.

Circadian Regulation of Mitochondrial Dynamics in Retinal Photoreceptors

Energy expenditure and metabolism in the vertebrate retina are under circadian control, as we previously reported that the overall retinal ATP content and various signaling molecules related to metabolism display daily or circadian rhythms. Changes in the fission and fusion process of mitochondria, the major organelles producing ATP, in retinal photoreceptors are largely dependent on light exposure, but whether mitochondrial dynamics in photoreceptors and retinal neurons are under circadian control is not clear. Herein, we investigated the possible roles of circadian oscillators in regulating mitochondrial dynamics, mitophagy, and redox states in the chicken retina and mammalian photoreceptors. After entrainment to 12:12-h light-dark (LD) cycles for several days followed by free-running in constant darkness (DD), chicken embryonic retinas and cone-derived 661W cells were collected in either LD or DD at 6 different zeitgeber time (ZT) or circadian time (CT) points. The protein expression of mitochondrial dynamin-related protein 1 (DRP1), mitofusin 2 (MFN2), and PTEN-induced putative kinase 1 (PINK1) displayed daily rhythms, but only DRP1 was under circadian control in the chicken retinas and cultured 661W cells. In addition, cultured chicken retinal cells responded to acute oxidative stress differently from 661W cells. Using pMitoTimer as a mitochondrial redox indicator, we found that the mitochondrial redox states were more affected by light exposure than regulated by circadian oscillators. Thus, this study demonstrates that the influence of cyclic lights might outweigh the circadian regulation of complex mitochondrial dynamics in light-sensing retinal cells.

Rods contribute to the light-induced phase shift of the retinal clock in mammals

While rods, cones, and intrinsically photosensitive melanopsin-containing ganglion cells (ipRGCs) all drive light entrainment of the master circadian pacemaker of the suprachiasmatic nucleus, recent studies have proposed that entrainment of the mouse retinal clock is exclusively mediated by a UV-sensitive photopigment, neuropsin (OPN5). Here, we report that the retinal circadian clock can be phase shifted by short duration and relatively low-irradiance monochromatic light in the visible part of the spectrum, up to 520 nm. Phase shifts exhibit a classical photon dose-response curve. Comparing the response of mouse models that specifically lack middle-wavelength (MW) cones, melanopsin, and/or rods, we found that only the absence of rods prevented light-induced phase shifts of the retinal clock, whereas light-induced phase shifts of locomotor activity are normal. In a “rod-only” mouse model, phase shifting response of the retinal clock to light is conserved. At shorter UV wavelengths, our results also reveal additional recruitment of short-wavelength (SW) cones and/or OPN5. These findings suggest a primary role of rod photoreceptors in the light response of the retinal clock in mammals.

The mammalian retina contains a circadian clock that plays a crucial role in adapting retinal physiology and visual function to light/dark changes. In addition, the retina coordinates rhythmic behavior and physiology by providing visual input to the master hypothalamic clock in the suprachiasmatic nucleus through a network of retinal photoreceptor cells involving rods, cones, and intrinsically photosensitive melanopsin-containing ganglion cells (ipRGCs). In contrast, recent studies argue that none of these photoreceptors are involved in light responses of the retinal clock and propose that photoresponses are exclusively mediated by the UV-sensitive photopigment neuropsin (OPN5). Our study demonstrates that rods are required to phase shift the retinal clock, while melanopsin and middle-wavelength (MW) cones influence the intrinsic period of the clock.

Rod Photoreceptor Activation Alone Defines the Release of Dopamine in the Retina

Retinal dopamine is released by a specialized subset of amacrine cells in response to light and has a potent influence on how the retina responds to, and encodes, visual information. Here, we address the critical question of which retinal photoreceptor is responsible for coordinating the release of this neuromodulator. Although all three photoreceptor classes-rods, cones, and melanopsin-containing retinal ganglion cells (mRGCs)-have been shown to provide electrophysiological inputs to dopaminergic amacrine cells (DACs), we show here that the release of dopamine is defined only by rod photoreceptors. Remarkably, this rod signal coordinates both a suppressive signal at low intensities and drives dopamine release at very bright light intensities. These data further reveal that dopamine release does not necessarily correlate with electrophysiological activity of DACs and add to a growing body of evidence that rods define aspects of retinal function at very bright light levels.

Light-dependent pathways for dopaminergic amacrine cell development and function

Retinal dopamine is a critical modulator of high acuity, light-adapted vision and photoreceptor coupling in the retina. Dopaminergic amacrine cells (DACs) serve as the sole source of retinal dopamine, and dopamine release in the retina follows a circadian rhythm and is modulated by light exposure. However, the retinal circuits through which light influences the development and function of DACs are still unknown. Intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged as a prime target for influencing retinal dopamine levels because they costratify with DACs in the inner plexiform layer and signal to them in a retrograde manner. Surprisingly, using genetic mouse models lacking specific phototransduction pathways, we find that while light influences the total number of DACs and retinal dopamine levels, this effect does not require ipRGCs. Instead, we find that the rod pathway is a critical modulator of both DAC number and retinal dopamine levels.

Dopamine maintains network synchrony via direct modulation of gap junctions in the crustacean cardiac ganglion

The Large Cell (LC) motor neurons of the crab cardiac ganglion have variable membrane conductance magnitudes even within the same individual, yet produce identical synchronized activity in the intact network. In a previous study we blocked a subset of K+ conductances across LCs, resulting in loss of synchronous activity (Lane et al., 2016). In this study, we hypothesized that this same variability of conductances makes LCs vulnerable to desynchronization during neuromodulation. We exposed the LCs to serotonin (5HT) and dopamine (DA) while recording simultaneously from multiple LCs. Both amines had distinct excitatory effects on LC output, but only 5HT caused desynchronized output. We further determined that DA rapidly increased gap junctional conductance. Co-application of both amines induced 5HT-like output, but waveforms remained synchronized. Furthermore, DA prevented desynchronization induced by the K+ channel blocker tetraethylammonium (TEA), suggesting that dopaminergic modulation of electrical coupling plays a protective role in maintaining network synchrony.

Plasticity of Retinal Gap Junctions: Roles in Synaptic Physiology and Disease

Electrical synaptic transmission via gap junctions underlies direct and rapid neuronal communication in the central nervous system. The diversity of functional roles played by electrical synapses is perhaps best exemplified in the vertebrate retina, in which gap junctions are expressed by each of the five major neuronal types. These junctions are highly plastic; they are dynamically regulated by ambient illumination and circadian rhythms acting through light-activated neuromodulators. The networks formed by electrically coupled neurons provide plastic, reconfigurable circuits positioned to play key and diverse roles in the transmission and processing of visual information at every retinal level. Recent work indicates gap junctions also play a role in the progressive cell death and aberrant activity seen in various pathological conditions of the retina. Gap junctions thus form potential targets for novel neuroprotective therapies in the treatment of neurodegenerative retinal diseases such as glaucoma and ischemic retinopathies.

Flexible Neural Hardware Supports Dynamic Computations in Retina

The ability of the retina to adapt to changes in mean light intensity and contrast is well known. Classically, however, adaptation is thought to affect gain but not to change the visual modality encoded by a given type of retinal neuron. Recent findings reveal unexpected dynamic properties in mouse retinal neurons that challenge this view. Specifically, certain cell types change the visual modality they encode with variations in ambient illumination or following repetitive visual stimulation. These discoveries demonstrate that computations performed by retinal circuits with defined architecture can change with visual input. Moreover, they pose a major challenge for central circuits that must decode properties of the dynamic visual signal from retinal outputs.

Design principles of electrical synaptic plasticity

Essentially all animals with nervous systems utilize electrical synapses as a core element of communication. Electrical synapses, formed by gap junctions between neurons, provide rapid, bidirectional communication that accomplishes tasks distinct from and complementary to chemical synapses. These include coordination of neuron activity, suppression of voltage noise, establishment of electrical pathways that define circuits, and modulation of high order network behavior. In keeping with the omnipresent demand to alter neural network function in order to respond to environmental cues and perform tasks, electrical synapses exhibit extensive plasticity. In some networks, this plasticity can have dramatic effects that completely remodel circuits or remove the influence of certain cell types from networks. Electrical synaptic plasticity occurs on three distinct time scales, ranging from milliseconds to days, with different mechanisms accounting for each. This essay highlights principles that dictate the properties of electrical coupling within networks and the plasticity of the electrical synapses, drawing examples extensively from retinal networks.

Homeostatic Plasticity Mediated by Rod-Cone Gap Junction Coupling in Retinal Degenerative Dystrophic RCS Rats

Rod-cone gap junctions open at night to allow rod signals to pass to cones and activate the cone-bipolar pathway. This enhances the ability to detect large, dim objects at night. This electrical synaptic switch is governed by the circadian clock and represents a novel form of homeostatic plasticity that regulates retinal excitability according to network activity. We used tracer labeling and ERG recording in the retinae of control and retinal degenerative dystrophic RCS rats. We found that in the control animals, rod-cone gap junction coupling was regulated by the circadian clock via the modulation of the phosphorylation of the melatonin synthetic enzyme arylalkylamine N-acetyltransferase (AANAT). However, in dystrophic RCS rats, AANAT was constitutively phosphorylated, causing rod-cone gap junctions to remain open. A further b/a-wave ratio analysis revealed that dystrophic RCS rats had stronger synaptic strength between photoreceptors and bipolar cells, possibly because rod-cone gap junctions remained open. This was despite the fact that a decrease was observed in the amplitude of both a- and b-waves as a result of the progressive loss of rods during early degenerative stages. These results suggest that electric synaptic strength is increased during the day to allow cone signals to pass to the remaining rods and to be propagated to rod bipolar cells, thereby partially compensating for the weak visual input caused by the loss of rods.

Dopamine: A Modulator of Circadian Rhythms in the Central Nervous System

Circadian rhythms are daily rhythms that regulate many biological processes – from gene transcription to behavior – and a disruption of these rhythms can lead to a myriad of health risks. Circadian rhythms are entrained by light, and their 24-h oscillation is maintained by a core molecular feedback loop composed of canonical circadian (“clock”) genes and proteins. Different modulators help to maintain the proper rhythmicity of these genes and proteins, and one emerging modulator is dopamine. Dopamine has been shown to have circadian-like activities in the retina, olfactory bulb, striatum, midbrain, and hypothalamus, where it regulates, and is regulated by, clock genes in some of these areas. Thus, it is likely that dopamine is essential to mechanisms that maintain proper rhythmicity of these five brain areas. This review discusses studies that showcase different dopaminergic mechanisms that may be involved with the regulation of these brain areas’ circadian rhythms. Mechanisms include how dopamine and dopamine receptor activity directly and indirectly influence clock genes and proteins, how dopamine’s interactions with gap junctions influence daily neuronal excitability, and how dopamine’s release and effects are gated by low- and high-pass filters. Because the dopamine neurons described in this review also release the inhibitory neurotransmitter GABA which influences clock protein expression in the retina, we discuss articles that explore how GABA may contribute to the actions of dopamine neurons on circadian rhythms. Finally, to understand how the loss of function of dopamine neurons could influence circadian rhythms, we review studies linking the neurodegenerative disease Parkinson’s Disease to disruptions of circadian rhythms in these five brain areas. The purpose of this review is to summarize growing evidence that dopamine is involved in regulating circadian rhythms, either directly or indirectly, in the brain areas discussed here. An appreciation of the growing evidence of dopamine’s influence on circadian rhythms may lead to new treatments including pharmacological agents directed at alleviating the various symptoms of circadian rhythm disruption.

Ocular Photoreception for Circadian Rhythm Entrainment in Mammals

Circadian rhythms are self-sustained, approximately 24-h rhythms of physiology and behavior. These rhythms are entrained to an exactly 24-h period by the daily light-dark cycle. Remarkably, mice lacking all rod and cone photoreceptors still demonstrate photic entrainment, an effect mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells utilize melanopsin (OPN4) as their photopigment. Distinct from the ciliary rod and cone opsins, melanopsin appears to function as a stable photopigment utilizing sequential photon absorption for its photocycle; this photocycle, in turn, confers properties on ipRGCs such as sustained signaling and resistance from photic bleaching critical for an irradiance detection system. The retina itself also functions as a circadian pacemaker that can be autonomously entrained to light-dark cycles. Recent experiments have demonstrated that another novel opsin, neuropsin (OPN5), is required for this entrainment, which appears to be mediated by a separate population of ipRGCs. Surprisingly, the circadian clock of the mammalian cornea is also light entrainable and is also neuropsin-dependent for this effect. The retina thus utilizes a surprisingly broad array of opsins for mediation of different light-detection tasks.

Characteristics and plasticity of electrical synaptic transmission

Electrical synapses are an omnipresent feature of nervous systems, from the simple nerve nets of cnidarians to complex brains of mammals. Formed by gap junction channels between neurons, electrical synapses allow direct transmission of voltage signals between coupled cells. The relative simplicity of this arrangement belies the sophistication of these synapses. Coupling via electrical synapses can be regulated by a variety of mechanisms on times scales ranging from milliseconds to days, and active properties of the coupled neurons can impart emergent properties such as signal amplification, phase shifts and frequency-selective transmission. This article reviews the biophysical characteristics of electrical synapses and some of the core mechanisms that control their plasticity in the vertebrate central nervous system.

Direct Evidence for Daily Plasticity of Electrical Coupling between Rod Photoreceptors in the Mammalian Retina

Rod photoreceptors are electrically coupled through gap junctions. Coupling is a key determinant of their light response properties, but whether rod electrical coupling is dynamically regulated remains elusive and controversial. Here, we have obtained direct measurements of the conductance between adjacent rods in mouse retina and present evidence that rod electrical coupling strength is dependent on the time of day, the lighting conditions, and the mouse strain. Specifically, we show in CBA/Ca mice that under circadian conditions, the rod junctional conductance has a median value of 98 pS during the subjective day and of 493 pS during the subjective night. In C57BL/6 mice, the median junctional conductance between dark-adapted rods is ∼140 pS, regardless of the time in the circadian cycle. Adaptation to bright light decreases the rod junctional conductance to ∼0 pS, regardless of the time of day or the mouse strain. Together, these results establish the high degree of plasticity of rod electrical coupling over the course of the day. Estimates of the rod coupling strength will provide a foundation for further investigations of rod interactions and the role of rod coupling in the ability of the visual system to anticipate, assimilate, and respond to the daily changes in ambient light intensity.

SIGNIFICANCE STATEMENT

Many cells in the CNS communicate via gap junctions, or electrical synapses, the regulation of which remains largely unknown. Here, we show that the strength of electrical coupling between rod photoreceptors of the retina is regulated by the time of day and the lighting conditions. This mechanism may help us understand some key aspects of day and night vision as well as some visual malfunctions.

The Retina and Other Light-sensitive Ocular Clocks

Ocular clocks, first identified in the retina, are also found in the retinal pigment epithelium (RPE), cornea, and ciliary body. The retina is a complex tissue of many cell types and considerable effort has gone into determining which cell types exhibit clock properties. Current data suggest that photoreceptors as well as inner retinal neurons exhibit clock properties with photoreceptors dominating in nonmammalian vertebrates and inner retinal neurons dominating in mice. However, these differences may in part reflect the choice of circadian output, and it is likely that clock properties are widely dispersed among many retinal cell types. The phase of the retinal clock can be set directly by light. In nonmammalian vertebrates, direct light sensitivity is commonplace among body clocks, but in mice only the retina and cornea retain direct light-dependent phase regulation. This distinguishes the retina and possibly other ocular clocks from peripheral oscillators whose phase depends on the pace-making properties of the hypothalamic central brain clock, the suprachiasmatic nuclei (SCN). However, in mice, retinal circadian oscillations dampen quickly in isolation due to weak coupling of its individual cell-autonomous oscillators, and there is no evidence that retinal clocks are directly controlled through input from other oscillators. Retinal circadian regulation in both mammals and nonmammalian vertebrates uses melatonin and dopamine as dark- and light-adaptive neuromodulators, respectively, and light can regulate circadian phase indirectly through dopamine signaling. The melatonin/dopamine system appears to have evolved among nonmammalian vertebrates and retained with modification in mammals. Circadian clocks in the eye are critical for optimum visual function where they play a role fine tuning visual sensitivity, and their disruption can affect diseases such as glaucoma or retinal degeneration syndromes.

Parallel Inhibition of Dopamine Amacrine Cells and Intrinsically Photosensitive Retinal Ganglion Cells in a Non-Image-Forming Visual Circuit of the Mouse Retina

An inner retinal microcircuit composed of dopamine (DA)-containing amacrine cells and melanopsin-containing, intrinsically photosensitive retinal ganglion cells (M1 ipRGCs) process information about the duration and intensity of light exposures, mediating light adaptation, circadian entrainment, pupillary reflexes, and other aspects of non-image-forming vision. The neural interaction is reciprocal: M1 ipRGCs excite DA amacrine cells, and these, in turn, feed inhibition back onto M1 ipRGCs. We found that the neuropeptide somatostatin [somatotropin release inhibiting factor (SRIF)] also inhibits the intrinsic light response of M1 ipRGCs and postulated that, to tune the bidirectional interaction of M1 ipRGCs and DA amacrine cells, SRIF amacrine cells would provide inhibitory modulation to both cell types. SRIF amacrine cells, DA amacrine cells, and M1 ipRGCs form numerous contacts. DA amacrine cells and M1 ipRGCs express the SRIF receptor subtypes sst(2A) and sst4 respectively. SRIF modulation of the microcircuit was investigated with targeted patch-clamp recordings of DA amacrine cells in TH-RFP mice and M1 ipRGCs in OPN4-EGFP mice. SRIF increases K(+) currents, decreases Ca(2+) currents, and inhibits spike activity in both cell types, actions reproduced by the selective sst(2A) agonist L-054,264 (N-[(1R)-2-[[[(1S*,3R*)-3-(aminomethyl)cyclohexyl]methyl]amino]-1-(1H-indol-3-ylmethyl)-2-oxoethyl]spiro[1H-indene-1,4'-piperidine]-1'-carboxamide) in DA amacrine cells and the selective sst4 agonist L-803,087 (N(2)-[4-(5,7-difluoro-2-phenyl-1H-indol-3-yl)-1-oxobutyl]-L-arginine methyl ester trifluoroacetate) in M1 ipRGCs. These parallel actions of SRIF may serve to counteract the disinhibition of M1 ipRGCs caused by SRIF inhibition of DA amacrine cells. This allows the actions of SRIF on DA amacrine cells to proceed with adjusting retinal DA levels without destabilizing light responses by M1 ipRGCs, which project to non-image-forming targets in the brain.

Dopamine D1 receptor expression is bipolar cell type-specific in the mouse retina

In the retina, dopamine is a key molecule for daytime vision. Dopamine is released by retinal dopaminergic amacrine cells and transmits signaling either by conventional synaptic or by volume transmission. By means of volume transmission, dopamine modulates all layers of retinal neurons; however, it is not well understood how dopamine modulates visual signaling pathways in bipolar cells. Here we analyzed Drd1a-tdTomato BAC transgenic mice and found that the dopamine D1 receptor (D1R) is expressed in retinal bipolar cells in a type-dependent manner. Strong tdTomato fluorescence was detected in the inner nuclear layer and localized to type 1, 3b, and 4 OFF bipolar cells and type 5-2, XBC, 6, and 7 ON bipolar cells. In contrast, type 2, 3a, 5-1, 9, and rod bipolar cells did not express Drd1a-tdTomato. Other interneurons were also found to express tdTomato including horizontal cells and a subset (25%) of AII amacrine cells. Diverse visual processing pathways, such as color or motion-coded pathways, are thought to be initiated in retinal bipolar cells. Our results indicate that dopamine sculpts bipolar cell performance in a type-dependent manner to facilitate daytime vision. J. Comp. Neurol. 524:2059-2079, 2016. © 2015 Wiley Periodicals, Inc.

Circadian and light-driven regulation of rod dark adaptation

Continuous visual perception and the dark adaptation of vertebrate photoreceptors after bright light exposure require recycling of their visual chromophore through a series of reactions in the retinal pigmented epithelium (RPE visual cycle). Light-driven chromophore consumption by photoreceptors is greater in daytime vs. nighttime, suggesting that correspondingly higher activity of the visual cycle may be required. However, as rod photoreceptors are saturated in bright light, the continuous turnover of their chromophore by the visual cycle throughout the day would not contribute to vision. Whether the recycling of chromophore that drives rod dark adaptation is regulated by the circadian clock and light exposure is unknown. Here, we demonstrate that mouse rod dark adaptation is slower during the day or after light pre-exposure. This surprising daytime suppression of the RPE visual cycle was accompanied by light-driven reduction in expression of Rpe65, a key enzyme of the RPE visual cycle. Notably, only rods in melatonin-proficient mice were affected by this daily visual cycle modulation. Our results demonstrate that the circadian clock and light exposure regulate the recycling of chromophore in the RPE visual cycle. This daily melatonin-driven modulation of rod dark adaptation could potentially protect the retina from light-induced damage during the day.

Feedback-induced glutamate spillover enhances negative feedback from horizontal cells to cones

KEY POINTS

In the retina, horizontal cells feed back negatively to cone photoreceptors. Glutamate released from cones can spill over to neighbouring cones. Here we show that cone glutamate release induced by negative feedback can also spill over to neighbouring cones. This glutamate activates the glutamate transporter-associated chloride current in these neighbouring cones, which leads to a change in their membrane potential and thus modulates their output. In this way, feedback-induced glutamate spillover enhances negative feedback from horizontal cells to cones, thus forming an additional feedback pathway. This effect will be particularly prominent in cones that are strongly hyperpolarized by light.

ABSTRACT

Inhibition in the outer retina functions via an unusual mechanism. When horizontal cells hyperpolarize the activation potential of the Ca(2+) current of cones shifts to more negative potentials. The underlying mechanism consists of an ephaptic component and a Panx1/ATP-mediated component. Here we identified a third feedback component, which remains active outside the operating range of the Ca(2+) current. We show that the glutamate transporters of cones can be activated by glutamate released from their neighbours. This pathway can be triggered by negative feedback from horizontal cells to cones, thus providing an additional feedback pathway. This additional pathway is mediated by a Cl(-) current, can be blocked by either removing the gradient of K(+) or by adding the glutamate transporter blocker TBOA, or low concentrations of Zn(2+) . These features point to a glutamate transporter-associated Cl(-) current. The pathway has a delay of 4.7 ± 1.7 ms. The effectiveness of this pathway in modulating the cone output depends on the equilibrium potential of Cl(-) (ECl ) and the membrane potential of the cone. Because estimates of ECl show that it is around the dark resting membrane potential of cones, the activation of the glutamate transporter-associated Cl(-) current will be most effective in changing the membrane potential during strong hyperpolarization of cones. This means that negative feedback would particularly be enhanced by this pathway when cones are hyperpolarized. Spatially, this pathway does not reach further than the direct neighbouring cones. The consequence is that this feedback pathway transmits information between cones of different spectral type.