Circadian rhythm of Period1 clock gene expression in NOS amacrine cells of the mouse retina

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

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.

Core circadian clock genes Per1 and Per2 regulate the rhythm in photoreceptor outer segment phagocytosis

Retinal photoreceptors undergo daily renewal of their distal outer segments, a process indispensable for maintaining retinal health. Photoreceptor outer segment (POS) phagocytosis occurs as a daily peak, roughly about 1 hour after light onset. However, the underlying cellular and molecular mechanisms which initiate this process are still unknown. Here we show that, under constant darkness, mice deficient for core circadian clock genes (Per1 and Per2) lack a daily peak in POS phagocytosis. By qPCR analysis, we found that core clock genes were rhythmic over 24 hours in both WT and Per1, Per2 double mutant whole retinas. More precise transcriptomics analysis of laser capture microdissected WT photoreceptors revealed no differentially expressed genes between time points preceding and during the peak of POS phagocytosis. In contrast, we found that microdissected WT retinal pigment epithelium (RPE) had a number of genes that were differentially expressed at the peak phagocytic time point compared to adjacent ones. We also found a number of differentially expressed genes in Per1, Per2 double mutant RPE compared to WT ones at the peak phagocytic time point. Finally, based on STRING analysis, we found a group of interacting genes that potentially drive POS phagocytosis in the RPE. This potential pathway consists of genes such as: Pacsin1, Syp, Camk2b, and Camk2d among others. Our findings indicate that Per1 and Per2 are necessary clock components for driving POS phagocytosis and suggest that this process is transcriptionally driven by the RPE.

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.

Circadian organization of the mammalian retina: from gene regulation to physiology and diseases

The retinal circadian system represents a unique structure. It contains a complete circadian system and thus the retina represents an ideal model to study fundamental questions of how neural circadian systems are organized and what signaling pathways are used to maintain synchrony of the different structures in the system. In addition, several studies have shown that multiple sites within the retina are capable of generating circadian oscillations. The strength of circadian clock gene expression and the emphasis of rhythmic expression are divergent across vertebrate retinas, with photoreceptors as the primary locus of rhythm generation in amphibians, while in mammals clock activity is most robust in the inner nuclear layer. Melatonin and dopamine serve as signaling molecules to entrain circadian rhythms in the retina and also in other ocular structures. Recent studies have also suggested GABA as an important component of the system that regulates retinal circadian rhythms. These transmitter-driven influences on clock molecules apparently reinforce the autonomous transcription-translation cycling of clock genes. The molecular organization of the retinal clock is similar to what has been reported for the SCN although inter-neural communication among retinal neurons that form the circadian network is apparently weaker than those present in the SCN, and it is more sensitive to genetic disruption than the central brain clock. The melatonin-dopamine system is the signaling pathway that allows the retinal circadian clock to reconfigure retinal circuits to enhance light-adapted cone-mediated visual function during the day and dark-adapted rod-mediated visual signaling at night. Additionally, the retinal circadian clock also controls circadian rhythms in disk shedding and phagocytosis, and possibly intraocular pressure. Emerging experimental data also indicate that circadian clock is also implicated in the pathogenesis of eye disease and compelling experimental data indicate that dysfunction of the retinal circadian system negatively impacts the retina and possibly the cornea and the lens.

Circadian phase-dependent effect of nitric oxide on L-type voltage-gated calcium channels in avian cone photoreceptors

Nitric oxide (NO) plays an important role in phase-shifting of circadian neuronal activities in the suprachiasmatic nucleus and circadian behavior activity rhythms. In the retina, NO production is increased in a light-dependent manner. While endogenous circadian oscillators in retinal photoreceptors regulate their physiological states, it is not clear whether NO also participates in the circadian regulation of photoreceptors. In this study, we demonstrate that NO is involved in the circadian phase-dependent regulation of L-type voltage-gated calcium channels (L-VGCCs). In chick cone photoreceptors, the L-VGCCα1 subunit expression and the maximal L-VGCC currents are higher at night, and both Ras-mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (Erk) and Ras-phosphatidylinositol 3 kinase (PI3K)-protein kinase B (Akt) are part of the circadian output pathways regulating L-VGCCs. The NO-cGMP-protein kinase G (PKG) pathway decreases L-VGCCα1 subunit expression and L-VGCC currents at night, but not during the day, and exogenous NO donor or cGMP decreases the phosphorylation of Erk and Akt at night. The protein expression of neural NO synthase (nNOS) is also under circadian control, with both nNOS and NO production being higher during the day. Taken together, NO/cGMP/PKG signaling is involved as part of the circadian output pathway to regulate L-VGCCs in cone photoreceptors. In cone photoreceptors, the protein expression of neural nitric oxide synthase (nNOS) and NO production are under circadian control. NO-cGMP-protein kinase G (PKG) signaling serves in the circadian output pathway to regulate the circadian rhythms of L-type voltage-gated calcium channels (L-VGCCs) in part through regulating the phosphorylation states of extracellular-signal-regulated kinase (Erk) and protein kinase B (Akt).

Age-dependent morphology of NADPH diaphorase-positive amacrine cells in the mouse retina

NADPH diaphorase-positive amacrine cells (NAC) were studied in retinal whole mount preparation of mice, ranging from 1 day to 30 months of age. Following a peak in number and size during early development at postnatal day 14, their number and distribution remained well preserved up to senescence. Functional considerations include immunological, vascular and neuro-modulating aspects.

Morphology and immunoreactivity of retrogradely double-labeled ganglion cells in the mouse retina

PURPOSE

To examine the specificity and reliability of a retrograde double-labeling technique that was recently established for identification of retinal ganglion cells (GCs) and to characterize the morphology of displaced (d)GCs (dGs).

METHODS

A mixture of the gap-junction-impermeable dye Lucifer yellow (LY) and the permeable dye neurobiotin (NB) was applied to the optic nerve stump for retrograde labeling of GCs and the cells coupled with them. A confocal microscope was adopted for morphologic observation.

RESULTS

GCs were identified by LY labeling, and they were all clearly labeled by NB. Cells coupled to GCs contained a weak NB signal but no LY. LY and NB revealed axon bundles, somas and dendrites of GCs. The retrogradely identified GCs numbered approximately 50,000 per retina, and they constituted 44% of the total neurons in the ganglion cell layer (GCL). Somas of retrogradely identified dGs were usually negative for glycine, ChAT (choline acetyltransferase), bNOS (brain-type nitric oxidase), GAD (glutamate decarboxylase), and glial markers, and occasionally, they were weakly GABA-positive. dGs averaged 760 per retina and composed 1.7% of total GCs. Sixteen morphologic subtypes of dGs were encountered, three of which were distinct from known GCs. dGs sent dendrites to either sublaminas of the IPL, mostly sublamina a.

CONCLUSIONS

The retrograde labeling is reliable for identification of GCs. dGs participate in ON and OFF light pathways but favor the OFF pathway. ChAT, bNOS, glycine, and GAD remain reliable AC markers in the GCL. GCs may couple to GABAergic ACs, and the gap junctions likely pass NB and GABA.

Slice preparation, organotypic tissue culturing and luciferase recording of clock gene activity in the suprachiasmatic nucleus

A central circadian (~24 hr) clock coordinating daily rhythms in physiology and behavior resides in the suprachiasmatic nucleus (SCN) located in the anterior hypothalamus. The clock is directly synchronized by light via the retina and optic nerve. Circadian oscillations are generated by interacting negative feedback loops of a number of so called "clock genes" and their protein products, including the Period (Per) genes. The core clock is also dependent on membrane depolarization, calcium and cAMP ¹. The SCN shows daily oscillations in clock gene expression, metabolic activity and spontaneous electrical activity. Remarkably, this endogenous cyclic activity persists in adult tissue slices of the SCN ^2-4. In this way, the biological clock can easily be studied in vitro, allowing molecular, electrophysiological and metabolic investigations of the pacemaker function.

The SCN is a small, well-defined bilateral structure located right above the optic chiasm ⁵. In the rat it contains ~8.000 neurons in each nucleus and has dimensions of approximately 947 μm (length, rostrocaudal axis) x 424 μm (width) x 390 μm (height) ⁶. To dissect out the SCN it is necessary to cut a brain slice at the specific level of the brain where the SCN can be identified. Here, we describe the dissecting and slicing procedure of the SCN, which is similar for mouse and rat brains. Further, we show how to culture the dissected tissue organotypically on a membrane ⁷, a technique developed for SCN tissue culture by Yamazaki et al.⁸. Finally, we demonstrate how transgenic tissue can be used for measuring expression of clock genes/proteins using dynamic luciferase reporter technology, a method that originally was used for circadian measurements by Geusz et al.⁹. We here use SCN tissues from the transgenic knock-in PERIOD2::LUCIFERASE mice produced by Yoo et al.¹⁰. The mice contain a fusion protein of PERIOD (PER) 2 and the firefly enzyme LUCIFERASE. When PER2 is translated in the presence of the substrate for luciferase, i.e. luciferin, the PER2 expression can be monitored as bioluminescence when luciferase catalyzes the oxidation of luciferin. The number of emitted photons positively correlates to the amount of produced PER2 protein, and the bioluminescence rhythms match the PER2 protein rhythm in vivo¹⁰. In this way the cyclic variation in PER2 expression can be continuously monitored real time during many days. The protocol we follow for tissue culturing and real-time bioluminescence recording has been thoroughly described by Yamazaki and Takahashi ¹¹.

Electroretinography of wild-type and Cry mutant mice reveals circadian tuning of photopic and mesopic retinal responses

Attempts to understand circadian organization in the mammalian retina have concentrated increasingly on the mouse. However, rather little is known regarding circadian control of retinal light responses in this species. Here, the authors address this deficit using electroretinogram (ERG) recordings in C57BL/6 mice to evaluate rhythmicity in the wild-type retina and to identify the consequences of circadian clock loss in Cry1(- /-)Cry2(-/-) mice. They observe a circadian rhythm in the ERG waveform under light-adapted, cone-isolating conditions in wild-type mice, with b-wave speed and amplitude and the total power of oscillatory potentials all enhanced during the day. Wild types also exhibited a circadian dependence to ERG amplitude under dark-adapted conditions, but only when the flash stimulus was sufficiently bright to lie within the response range of cones. Cry1(-/ -)Cry2(-/-) mice lacked rhythmicity but retained superficially normal ERGs under all conditions suggesting that circadian clocks are dispensable for general retinal function. However, clock loss was associated with subtle abnormalities in retinal responses, with the amplitude of cone and mixed rod + cone ERGs constitutively enhanced. These data suggest that circadian clocks drive a fundamental fine-tuning of retinal pathways that is particularly apparent under conditions in which vision relies upon either cones alone or mixed rod + cone photoreception.

An autonomous circadian clock in the inner mouse retina regulated by dopamine and GABA

The influence of the mammalian retinal circadian clock on retinal physiology and function is widely recognized, yet the cellular elements and neural regulation of retinal circadian pacemaking remain unclear due to the challenge of long-term culture of adult mammalian retina and the lack of an ideal experimental measure of the retinal circadian clock. In the current study, we developed a protocol for long-term culture of intact mouse retinas, which allows retinal circadian rhythms to be monitored in real time as luminescence rhythms from a PERIOD2::LUCIFERASE (PER2::LUC) clock gene reporter. With this in vitro assay, we studied the characteristics and location within the retina of circadian PER2::LUC rhythms, the influence of major retinal neurotransmitters, and the resetting of the retinal circadian clock by light. Retinal PER2::LUC rhythms were routinely measured from whole-mount retinal explants for 10 d and for up to 30 d. Imaging of vertical retinal slices demonstrated that the rhythmic luminescence signals were concentrated in the inner nuclear layer. Interruption of cell communication via the major neurotransmitter systems of photoreceptors and ganglion cells (melatonin and glutamate) and the inner nuclear layer (dopamine, acetylcholine, GABA, glycine, and glutamate) did not disrupt generation of retinal circadian PER2::LUC rhythms, nor did interruption of intercellular communication through sodium-dependent action potentials or connexin 36 (cx36)-containing gap junctions, indicating that PER2::LUC rhythms generation in the inner nuclear layer is likely cell autonomous. However, dopamine, acting through D1 receptors, and GABA, acting through membrane hyperpolarization and casein kinase, set the phase and amplitude of retinal PER2::LUC rhythms, respectively. Light pulses reset the phase of the in vitro retinal oscillator and dopamine D1 receptor antagonists attenuated these phase shifts. Thus, dopamine and GABA act at the molecular level of PER proteins to play key roles in the organization of the retinal circadian clock.

The circadian clock in the mammalian retina regulates many retinal functions, and its output modulates the central circadian clock in the brain. Details about the cellular location and neural regulation of the mammalian retinal circadian clock remain unclear, however, largely due to the difficulty of maintaining long-term culture of adult mammalian retina and the lack of an ideal experimental measure of the retinal clock. We have circumvented these limitations by developing a protocol for long-term culture of intact mouse retinas to monitor circadian rhythms of clock gene expression in real time. Using this protocol, we have localized expression of molecular retinal circadian rhythms to the inner nuclear layer. We find molecular retinal rhythms generation is independent of many forms of signaling from photoreceptors and ganglion cells, or major forms of neural communication within the inner nuclear layer, and have characterized light-induced resetting of the retinal clock. Retinal dopamine and GABA, although not necessary for the generation of molecular retinal rhythms, were revealed to regulate the phase and amplitude of retinal molecular rhythms, respectively, with dopamine participating in light-induced resetting. Our data indicate that dopamine and GABA play prominent roles in the organization of the retinal circadian clock.

Long-term culture of mouse retinas reveals a circadian clock in the inner retina that can be reset by light and is regulated by the neurotransmitters dopamine and GABA.

Cholecystokinin-A receptors regulate photic input pathways to the circadian clock

Daily behaviors are strongly dominated by internally generated circadian rhythms, but the underlying mechanisms remain unclear. In mammals, photoentrainment of behaviors to light-dark cycles involves signaling from both intrinsically photosensitive retinal ganglion cells and classic photoreceptor pathways to the suprachiasmatic nucleus (SCN). How classic photoreceptor pathways work with the photosensitive ganglion cells, however, is not fully understood. Although cholecystokinin (CCK) peptide has been shown to be present in a variety of vertebrate retinas, its function at a systems level is also unknown. In the present study we examined a possible role of CCK-A receptors in photoentrainment using CCK-A receptor knockout mice. The lacZ reporter gene within a gene-knockout cassette revealed precise localization of CCK-A receptors in the circadian clock system. We demonstrated that CCK-A receptors were located predominately on glycinergic amacrine cells but were rarely found on SCN neurons. Moreover, Ca(2+) imaging analysis demonstrated that the CCK-A agonist, CCK-8 sulfate (CCK-8s), mobilized intracellular Ca(2+) in amacrine cells but not glutamate-receptive SCN neurons. Furthermore, light pulse-induced mPer1/mPer2 gene expression in SCN, behavioral phase shifts, and the pupillary reflex were significantly reduced in CCK-A receptor knockout mice. These data indicate a novel function of CCK-A receptors in the nonimage-forming photoreception presumably via amacrine cell-mediated signal transduction pathways.

Impact of melatonin receptors on pCREB and clock-gene protein levels in the murine retina

In several mammalian species, the retina is capable of synthesizing melatonin and contains an autonomous circadian clock that relies on interlocking transcriptional/translational feedback loops involving several clock genes, such as Per1 and Cry2. Our previous investigations have shown remarkable differences in retinae of melatonin-deficient (C57BL) and melatonin-proficient (C3H) mice with regard to the protein levels of PER1, CRY2, and phosphorylated (p) cyclic AMP response element binding protein (CREB). To elucidate the melatonin receptor type possibly responsible for these differences, we have performed immunocytochemical analyses for PER1, CRY2, and pCREB in retinae of melatonin-proficient wild type (WT) mice and mice with targeted deletions of the MT1 receptor (MelaaBB) or the MT1 and MT2 receptors (Melaabb) at four different time points. Immunoreactions for PER1, CRY2 and pCREB were localized to the nuclei of cells in the inner nuclear layer (INL) and ganglion cell layer (GC) of all strains. Surprisingly, in MelaaBB and Melaabb the day/night rhythm of pCREB, PER1, and CRY2 levels was not abolished, but the maxima and minima of PER1 were 180 degrees out of phase as compared to the WT. These data suggest that MT1 and MT2 melatonin receptors are not necessary to maintain rhythmic changes in clock-gene protein levels in the murine retina, but, as shown for PER1, appear to be involved in internal synchronization.

Cytoplasmic localization of mPER1 clock protein isoforms in the mouse retina

The mammalian Period1 gene is rhythmically expressed and its proteins are found within the nucleus of the cells of the suprachiasmatic nuclei (SCN), the central circadian pacemaker in mammals; however, whether the target of the PER1 proteins is also the nucleus in the retinal peripheral clock cells is yet to be determined. Using an anti-PER1 protein antibody in Western blot analyses, we found three isoforms (75, 110 and 140kDa) in extracts of the SCN, as well as in other different parts of the brain, whereas just two isoforms (75 and 110kDa) were detected in the retinal extracts. We have observed that PER1 immunolabelling has a cytoplasmic location in many cells of the ganglion cell layer and in a few cells in the inner nuclear layer of the mouse retina. This cellular location was seen in any of the tissue samples taken at 4h intervals, either in the day/night cycle or in constant darkness, of both wild type and rd mice. Unlike this situation, PER1 isoforms were nuclear proteins in the SCN cells as well as in other parts of the brain of the same animals. No circadian changes were found for these clock proteins in the neural retina. These findings suggest that PER1 proteins play roles in the retina different from those established in the SCN.

Clock gene expression in the retina of melatonin-proficient (C3H) and melatonin-deficient (C57BL) mice

In several mammalian species, the retina contains an autonomous circadian clock and is capable of synthesizing melatonin. The function of circadian clocks depends on interlocking transcriptional/translational feedback loops involving several clock genes. Here we investigated the expression of two clock genes (Per1, Cry2) and the level of phosphorylated (p) cyclic AMP response element binding protein (CREB) in retinae of melatonin-deficient (C57BL) with an intact retina and melatonin-proficient (C3H) mice with degenerated outer nuclear layer. RNase protection assay and in situ hybridization revealed in both strains a rhythm in transcript levels for Per1 with a peak at zeitgeber time (ZT) 08, but not for Cry2. Immunoreactions for PER1, CRY2 and pCREB were localized to the nuclei of cells in the inner nuclear layer (INL) and ganglion cell layer (GC) of both strains and to the outer nuclear layer of C57BL. In C3H, protein levels of PER1 and CRY2 followed a clear day/night rhythm in the INL and the GC with a peak at the end of the day (ZT14). pCREB levels peaked at the beginning of the day. Noteably, in melatonin-deficient C57BL mice, protein levels of PER1, CRY2 and pCREB did not show significant changes over a 16L/8D cycle. These data suggest that melatonin influences PER1 and CRY2 protein levels via post-transcriptional mechanisms and also plays a role in rhythmic regulation of pCREB levels in the mammalian retina.

Expression of nicotinamide adenine dinucleotide phosphate-diaphorase in the retina of postnatal golden hamsters deprived of light stimulation

Nicotinamide Adenine Dinucleotide Phosphate-Diaphorase (NADPH-d) expressing neurons in the retina of golden hamsters have been identified to be a subset of amacrine cells that provide a major source of Nitric Oxide (NO) in retina. This subset of amacrine cells in mouse retina was recently proved to contain the circadian clock gene Per1 (D.Q. Zhang, T. Zhou, G.X. Ruan, D.G. McMahon, Circadian rhythm of Period 1 clock gene expression in NOS amacrine cells of the mouse retina, Brain Res., 1050 (2005) 101-109). However, it remains unknown whether these clock-related NADPH-d amacrine cells can be regulated by light stimulation and thus synchronized to ambient day/night cycle. A previous study has reported that NADPH-d expressing amacrine cells in postnatal hamsters exhibited a surge after eye-opening (D. Tay, Y.C. Diao, Y.M. Xiao, K.F. So, Postnatal development of nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in the retina of the golden hamster, J. Comp. Neurol., 446 (2002) 342-348) suggesting a possible effect of light on the NADPH-d amacrine cells. In order to further reveal the relationship between NADPH-d amacrine cells and light stimulation, the present study focuses on the changes of the expression of NADPH-d in the retina of postnatal hamsters reared in completely deprived light conditions. Prior to eye opening, P12 hamster pups were subjected to either bilateral eyelid suturing or dark rearing. On P28 a subgroup of light deprived hamsters was returned to lighting conditions and the expression of NADPH-d activities in the retina was assessed. In hamsters reared in the 12:12 light-dark cycle, the number of NADPH-d amacrine cells in the ganglion cell layer (GCL) increased right after eye-opening and reached the adult level gradually. However, hamsters subjected to both bilateral eyelid suturing and dark rearing, the number of NADPH-d amacrine cells in GCL was maintained at a low level but increased again upon returning to the 12:12 light-dark condition. In contrast, the number of NADPH-d expressing amacrine cells in the inner nuclear layer (INL) remained low and unaltered regardless of the lighting environment. This study demonstrates that there are two subpopulations of NADPH-d expressing amacrine cells with respect to different locations in the retina of hamsters. Different from those in INL, the NADPH-d amacrine cells in GCL of postnatal hamsters are dependent on the lighting environment implicating that these clock-related amacrine cells and the production of NO might be under a modulation of light stimulation.

Circadian organization of the mammalian retina

The mammalian retina contains an endogenous circadian pacemaker that broadly regulates retinal physiology and function, yet the cellular origin and organization of the mammalian retinal circadian clock remains unclear. Circadian clock neurons generate daily rhythms via cell-autonomous autoregulatory clock gene networks, and, thus, to localize circadian clock neurons within the mammalian retina, we have studied the cell type-specific expression of six core circadian clock genes in individual, identified mouse retinal neurons, as well as characterized the clock gene expression rhythms in photoreceptor degenerate rd mouse retinas. Individual photoreceptors, horizontal, bipolar, dopaminergic (DA) amacrines, catecholaminergic (CA) amacrines, and ganglion neurons were identified either by morphology or by a tyrosine hydroxylase (TH) promoter-driven red fluorescent protein (RFP) fluorescent reporter. Cells were collected, and their transcriptomes were subjected to multiplex single-cell RT-PCR for the core clock genes Period (Per) 1 and 2, Cryptochrome (Cry) 1 and 2, Clock, and Bmal1. Individual horizontal, bipolar, DA, CA, and ganglion neurons, but not photoreceptors, were found to coordinately express all six core clock genes, with the lowest proportion of putative clock cells in photoreceptors (0%) and the highest proportion in DA neurons (30%). In addition, clock gene rhythms were found to persist for >25 days in isolated, cultured rd mouse retinas in which photoreceptors had degenerated. Our results indicate that multiple types of retinal neurons are potential circadian clock neurons that express key elements of the circadian autoregulatory gene network and that the inner nuclear and ganglion cell layers of the mammalian retina contain functionally autonomous circadian clocks.