Light-induced c-fos expression in amacrine cells in the rabbit retina

The differential effects of blocking retinal orexin receptors on the expression of retinal c-fos and hypothalamic Vip, PACAP, Bmal1, and c-fos in Male Wistar Rats

Orexin A and B (OXA and OXB) and their receptors are expressed in the majority of retinal neurons in humans, rats, and mice. Orexins modulate signal transmission between the different layers of the retina. The suprachiasmatic nucleus (SCN) and the retina are central and peripheral components of the body's biological clocks; respectively. The SCN receives photic information from the retina through the retinohypothalamic tract (RHT) to synchronize bodily functions with environmental changes. In present study, we aimed to investigate the impact of inhibiting retinal orexin receptors on the expression of retinal Bmal1 and c-fos, as well as hypothalamic c-fos, Bmal1, Vip, and PACAP at four different time-points (Zeitgeber time; ZT 3, 6, 11, and ZT-0). The intravitreal injection (IVI) of OX1R antagonist (SB-334867) and OX2R antagonist (JNJ-10397049) significantly up-regulated c-fos expression in the retina. Additionally, compared to the control group, the combined injection of SB-334867 and JNJ-10397049 showed a greater increase in retinal expression of this gene. Moreover, the expression of hypothalamic Vip and PACAP was significantly up-regulated in both the SB-334867 and JNJ-10397049 groups. In contrast, the expression of Bmal1 was down-regulated. Furthermore, the expression of hypothalamic c-fos was down-regulated in all groups treated with SB-334867 and JNJ-10397049. Additionally, the study demonstrated that blocking these receptors in the retina resulted in alterations in circadian rhythm parameters such as mesor, amplitude, and acrophase. Finally, it affected the phase of gene expression rhythms in both the retina and hypothalamus, as identified through cosinor analysis and the zero-amplitude test. This study represents the initial exploration of how retinal orexin receptors influence expression of rhythmic genes in the retina and hypothalamus. These findings could provide new insights into how the retina regulates the circadian rhythm in both regions and illuminate the role of the orexinergic system expression within the retina.

Retinal VIP-amacrine cells: their development, structure, and function

Amacrine cells (ACs) are the most structurally and functionally diverse neuron type in the retina. Different ACs have distinct functions, such as neuropeptide secretion and inhibitory connection. Vasoactive intestinal peptide (VIP) -ergic -ACs are retina gamma-aminobutyric acid (GABA) -ergic -ACs that were discovered long ago. They secrete VIP and form connections with bipolar cells (BCs), other ACs, and retinal ganglion cells (RGCs). They have a specific structure, density, distribution, and function. They play an important role in myopia, light stimulated responses, retinal vascular disease and other ocular diseases. Their significance in the study of refractive development and disease is increasing daily. However, a systematic review of the structure and function of retinal VIP-ACs is lacking. We discussed the detailed characteristics of VIP-ACs from every aspect across species and providing systematic knowledge base for future studies. Our review led to the main conclusion that retinal VIP-ACs develop early, and although their morphology and distribution across species are not the same, they have similar functions in a wide range of ocular diseases based on their function of secreting neuropeptides and forming inhibitory connections with other cells.

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.

Updates on Genes and Genetic Mechanisms Implicated in Primary Angle-Closure Glaucoma

Primary angle-closure glaucoma (PACG) is estimated to affect over 30 million people worldwide by 2040 and is highly prevalent in the Asian population. PACG is more severe and carries three times the higher risk of blindness than primary open-angle glaucoma, thus representing a significant public health concern. High heritability and ethnic-specific predisposition to PACG suggest the involvement of genetic factors in disease development. In the recent past, genetic studies have led to the successful identification of several genes and loci associated with PACG across different ethnicities. The precise cellular and molecular roles of these multiple loci in the development and progression of PACG remains to be elucidated. Nonetheless, these studies have significantly increased our understanding of the emerging cellular processes and biological pathways that might provide more significant insights into the disease’s genetic etiology and may be valuable for future clinical applications. This review aims to summarize and update the current knowledge of PACG genetics analysis research.

Pharmacotherapeutic candidates for myopia: A review

This review provides insights into the mechanism underlying the pathogenesis of myopia and potential targets for clinical intervention. Although the etiology of myopia involves both environmental and genetic factors, recent evidence has suggested that the prevalence and severity of myopia appears to be affected more by environmental factors. Current pharmacotherapeutics are aimed at inhibiting environmentally induced changes in visual input and subsequent changes in signaling pathways during myopia pathogenesis and progression. Recent studies on animal models of myopia have revealed specific molecules potentially involved in the regulation of eye development. Among them, the dopamine receptor plays a critical role in controlling myopia. Subsequent studies have reported pharmacotherapeutic treatments to control myopia progression. In particular, atropine treatment yielded favorable outcomes and has been extensively used; however, current studies are aimed at optimizing its efficacy and confirming its safety. Furthermore, future studies are required to assess the efficacy of combinatorial use of low-dose atropine and contact lenses or orthokeratology.

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.

Cone ERG Changes During Light Adaptation in Two All-Cone Mutant Mice: Implications for Rod-Cone Pathway Interactions

Purpose

The b-wave of the cone ERG increases in amplitude and speed during the first few minutes of adaptation to a rod-suppressing background light. Earlier studies implicate rod pathway input to the cone pathway in these changes.

Methods

The timing and amplitude of the cone b-wave and isolated oscillatory potentials (OP) during the first 10 minutes of light adaptation in wild-type (WT) mice and two mutant lines without functional rods was examined: rhodopsin knockout (Rho-/-), lacking rod outer segments, and NRL knockout (Nrl-/-), in which rods are replaced by S-cones. Expression of the immediate-early gene c-fos, which is increased in the inner retina by light-induced activity, was evaluated by immunohistochemistry in dark- and light-adapted retinas.

Results

WT b-wave and OP amplitudes increased, and implicit times decreased during light adaptation. Subtracting OP did not alter b-wave changes. Rho-/- b-wave and OP amplitudes did not increase during adaptation. B-wave timing and amplitude and the timing of the major OP at 1 minute of adaptation were equivalent to WT at 10 minutes. The light-adapted ERG b-wave in Nrl-/- mice, which originates in both the rod and cone pathways, changed in absolute amplitude and timing similar to WT. C-fos expression was present in the inner retinas of dark-adapted Rho-/- but not WT or Nrl-/- mice.

Conclusions

Activity in the distal rod pathway produces changes in the cone ERG during light adaptation. Rods in Rho-/- mice constitutively activate this rod-cone pathway interaction. The rod pathway S-cones in Nrl-/- mice may maintain the WT interaction.

Cholinergic nervous system and glaucoma: From basic science to clinical applications

The cholinergic system has a crucial role to play in visual function. Although cholinergic drugs have been a focus of attention as glaucoma medications for reducing eye pressure, little is known about the potential modality for neuronal survival and/or enhancement in visual impairments. Citicoline, a naturally occurring compound and FDA approved dietary supplement, is a nootropic agent that is recently demonstrated to be effective in ameliorating ischemic stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, cerebrovascular diseases, memory disorders and attention-deficit/hyperactivity disorder in both humans and animal models. The mechanisms of its action appear to be multifarious including (i) preservation of cardiolipin, sphingomyelin, and arachidonic acid contents of phosphatidylcholine and phosphatidylethanolamine, (ii) restoration of phosphatidylcholine, (iii) stimulation of glutathione synthesis, (iv) lowering glutamate concentrations and preventing glutamate excitotoxicity, (v) rescuing mitochondrial function thereby preventing oxidative damage and onset of neuronal apoptosis, (vi) synthesis of myelin leading to improvement in neuronal membrane integrity, (vii) improving acetylcholine synthesis and thereby reducing the effects of mental stress and (viii) preventing endothelial dysfunction. Such effects have vouched for citicoline as a neuroprotective, neurorestorative and neuroregenerative agent. Retinal ganglion cells are neurons with long myelinated axons which provide a strong rationale for citicoline use in visual pathway disorders. Since glaucoma is a form of neurodegeneration involving retinal ganglion cells, citicoline may help ameliorate glaucomatous damages in multiple facets. Additionally, trans-synaptic degeneration has been identified in humans and experimental models of glaucoma suggesting the cholinergic system as a new brain target for glaucoma management and therapy.

Acute effects of sex steroids on visual processing in male goldfish

Elevations of sex steroids induced by social cues can rapidly modulate social behavior, but we know little about where they act within the nervous system to produce such effects. In male goldfish, testosterone (T) rapidly increases approach responses to the visual cues of females through its conversion to estradiol. Because aromatase is expressed in the retina, we tested if T can acutely influence retina responses to visual stimuli, and investigated the receptor mechanisms that may mediate such effects. Specifically, we measured FOS protein immunoreactivity to determine if T affects cellular responses to visual stimuli that include females, and used electrophysiology to investigate whether T can generally affect light sensitivity. We found that T acutely increased FOS responses to the simultaneous onset of light and the presence of female visual stimuli, both of which would normally be associated with early morning spawning, and increased electrophysiological responses to low intensity light pulses. Both effects were blocked by an estrogen receptor beta (ERβ) antagonist, indicating that T is likely being converted to estradiol (E2) and acting through an ERβ mediated mechanism to acutely modulate visual processing. Changes in sensory processing could subsequently influence approach behavior to increase reproductive success in competitive mating environments.

Long-term restoration of visual function in end-stage retinal degeneration using subretinal human melanopsin gene therapy

Optogenetic strategies to restore vision in patients who are blind from end-stage retinal degenerations aim to render remaining retinal cells light sensitive once photoreceptors are lost. Here, we assessed long-term functional outcomes following subretinal delivery of the human melanopsin gene (OPN4) in the rd1 mouse model of retinal degeneration using an adeno-associated viral vector. Ectopic expression of OPN4 using a ubiquitous promoter resulted in cellular depolarization and ganglion cell action potential firing. Restoration of the pupil light reflex, behavioral light avoidance, and the ability to perform a task requiring basic image recognition were restored up to 13 mo following injection. These data suggest that melanopsin gene therapy via a subretinal route may be a viable and stable therapeutic option for the treatment of end-stage retinal degeneration in humans.

The role of nitric oxide in spectral information processing in the distal turtle retina

Chromaticity type horizontal cells (C-type HCs) are the first retinal neurons exhibiting spectral information processing in cold-blooded vertebrates. The simple input of hyperpolarizing responses of cone photoreceptors is transformed in the C-type HCs into spectral opponent output. Nitric oxide (NO), a known background neuromodulator in the distal retina, was tested here for its effects upon spectral information processing by C-type HCs in the retina of turtle. Photoresponses were intracellularly recorded from C-type HCs, using light stimuli of different wavelength, applied over backgrounds of different wavelengths, and changing retinal NO level. Raising retinal level of NO in darkness by adding the precursor for its synthesis (l-Arginine) augmented the depolarizing photoresponses elicited by long-wavelength light stimuli, and reduced the hyperpolarizing photoresponses elicited by short-wavelength light stimuli. Lowering retinal level of NO by l-NAME, an inhibitor of NO synthesis, induced the opposite effects. However, the total voltage range of operation remained constant regardless of the level of NO. Qualitatively similar effects were observed under background illuminations regardless of background strength and wavelength. Altering retinal level of NO exerted a small effect upon the null wavelength. Our findings are consistent with the known effects of NO upon turtle distal retinal neurons, with the addition of NO strengthening the negative feedback pathway from L-type horizontal cells onto medium-wavelength cones.

Characterisation of light responses in the retina of mice lacking principle components of rod, cone and melanopsin phototransduction signalling pathways

Gnat(-/-), Cnga3(-/-), Opn4(-/-) triple knockout (TKO) mice lack essential components of phototransduction signalling pathways present in rods, cones and photosensitive retinal ganglion cells (pRGCs), and are therefore expected to lack all sensitivity to light. However, a number of studies have shown that light responses persist in these mice. In this study we use multielectrode array (MEA) recordings and light-induced c-fos expression to further characterise the light responses of the TKO retina. Small, but robust electroretinogram type responses are routinely detected during MEA recordings, with properties consistent with rod driven responses. Furthermore, a distinctive pattern of light-induced c-fos expression is evident in the TKO retina, with c-fos expression largely restricted to a small subset of amacrine cells that express disabled-1 (Dab1) but lack expression of glycine transporter-1 (GlyT-1). Collectively these data are consistent with the persistence of a novel light sensing pathway in the TKO retina that originates in rod photoreceptors, potentially a rare subset of rods with distinct functional properties, and which is propagated to an atypical subtype of AII amacrine cells. Furthermore, the minimal responses observed following UV light stimulation suggest only a limited role for the non-visual opsin OPN5 in driving excitatory light responses within the mouse retina.

Expression and light sensitivity of clock genes Per1 and Per2 and immediate-early gene c-fos within the retina of early postnatal Wistar rats

Mammalian retina contains a circadian clock that is composed of components similar to those of the master circadian clock within the suprachiasmatic nuclei of the hypothalamus. The aim of the present study was to elucidate whether, when, and where the transcripts of the clock genes Per1 and Per2 and the immediate early gene c-fos are spontaneously expressed and/or induced by light in the newborn rat retina. At postnatal day 1 (P1), P3, P5, and P10, Wistar rat pups were released into constant darkness, and a 30-minute light pulse was administered during the subjective day or during the first or second part of subjective night. Gene expression was determined 30 minutes, 1 hour, 2 hours, and 4 hours after the light pulse by in situ hybridization followed by emulsion autoradiography. Endogenous expression of Per1 was detected in the neuroblastic retina, and Per2 expression was detected in the inner part of the neuroblastic retina from birth. Light pulses induced c-fos expression in ganglion cells from P1. Until P5, the cells were localized in the dorsal part of the retina, but, at P10, they were already distributed across the entire retinal circumference. Light pulses also induced the expression of c-fos and Per1 in the retinal pigment epithelium until P3, but not afterward. Expression of the Per2 gene was not photoresponsive until P10. These data demonstrate that the rat retina is light-sensitive immediately after birth. During early postnatal development, the spatial distribution of spontaneous and light-induced gene expression within the retinal layers changes gradually.

Light-induced fos expression in intrinsically photosensitive retinal ganglion cells in melanopsin knockout (opn4) mice

Retinal ganglion cells that express the photopigment melanopsin are intrinsically photosensitive (ipRGCs) and exhibit robust synaptically driven ON-responses to light, yet they will continue to depolarize in response to light when all synaptic input from rod and cone photoreceptors is removed. The light-evoked increase in firing of classical ganglion cells is determined by synaptic input from ON-bipolar cells in the proximal sublamina of the inner plexiform layer. OFF-bipolar cells synapse with ganglion cell dendrites in the distal sublamina of the inner plexiform layer. Of the several types of ipRGC that have been described, M1 ipRGCs send dendrites exclusively into the OFF region of the inner plexiform layer where they stratify near the border of the inner nuclear layer. We tested whether M1 ipRGCs with dendrites restricted to the OFF sublamina of the inner plexiform layer receive synaptic ON-bipolar input by examining light-induced gene expression in vivo using melanopsin knockout mice. Mice in which both copies of the melanopsin gene (opn4) have been replaced with the tau-lacZ gene (homozygous tau-lacZ^+/+ knockin mice) are melanopsin knockouts (opn4^−/−) but M1 ipRGCs are specifically identified by their expression of β-galactosidase. Approximately 60% of M1 ipRGCs in Opn4^−/− mice exposed to 3 hrs of light expressed c-Fos; no β-galactosidase-positive RGCs expressed c-Fos in the dark. Intraocular application of L-AP4, a compound which blocks transmission of visual signals between photoreceptors and ON-bipolar cells significantly reduced light-evoked c-Fos expression in M1 ipRGCs compared to saline injected eyes (66% saline vs 27% L-AP4). The results are the first description of a light-evoked response in an ipRGC lacking melanopsin and provide in vivo confirmation of previous in vitro observations illustrating an unusual circuit in the retina in which ganglion cells sending dendrites to the OFF sublamina of the inner plexiform layer receive excitatory synaptic input from ON-bipolar cells.

Intraretinal signaling by ganglion cell photoreceptors to dopaminergic amacrine neurons

Retinal dopaminergic amacrine neurons (DA neurons) play a central role in reconfiguring retinal function according to prevailing illumination conditions, yet the mechanisms by which light regulates their activity are poorly understood. We investigated the means by which sustained light responses are evoked in DA neurons. Sustained light responses were driven by cationic currents and persisted in vitro and in vivo in the presence of L-AP4, a blocker of retinal ON-bipolar cells. Several characteristics of these L-AP4-resistant light responses suggested that they were driven by melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs), including long latencies, marked poststimulus persistence, and a peak spectral sensitivity of 478 nm. Furthermore, sustained DA neuron light responses, but not transient DA neuron responses, persisted in rod/cone degenerate retinas, in which ipRGCs account for virtually all remaining retinal phototransduction. Thus, ganglion-cell photoreceptors provide excitatory drive to DA neurons, most likely by way of the coramification of their dendrites and the processes of DA neurons in the inner plexiform layer. This unprecedented centrifugal outflow of ganglion-cell signals within the retina provides a novel basis for the restructuring of retinal circuits by light.

Novel signals mediating the functions of somatostatin: the emerging role of NO/cGMP

The neuropeptide somatostatin is a cyclic tetradecapeptide, which is widely distributed in the peripheral and central nervous system. It mediates a plethora of physiological actions and functions as a neurotransmitter, neuromodulator or trophic factor. Somatostatin activates six receptor subtypes that are expressed differentially in different tissues and are coupled to diverse signalling pathways. In order to elucidate the functional role of the individual receptor subtypes, many investigations focused on the assignment of each receptor to a particular signalling pathway. Signalling pathways involving enzyme (adenylate cyclase, phospholipases, phosphatases) and ion channel systems in native and recombinant receptor systems have been extensively studied. A one to one situation (receptor/pathway) has yet to be established, thus justifying the diverse actions of somatostatin. Recently, a NO/cGMP pathway has been shown to mediate the functions of somatostatin and its receptors. This review will present the findings that support the emerging role of NO/cGMP as a novel signal in SRIF's actions in retinal physiology and somatotroph release.

Somatostatin receptors (sst2) regulate cGMP production in rat retina

The present study investigated the effect of somatostatin in the regulation of cGMP levels in rat retina and the mechanisms involved in this process. Isolated rat retinas were treated alone or in the presence of somatostatin (0.01-10 microM), BIM23014 (sst2 agonist, 0.01-10 microM), L-796,778 (sst3 agonist, 10 microM), somatostatin (0.1 microM) in combination with CYN154806 (sst2 antagonist, 1 microM), N(G)-methyl-L-arginine acetate salt (NMMA, inhibitor of the nitric oxide synthase (NOS), 250 microM), orthovanadate (inhibitor of tyrosine phosphatase, SHP-1, 1 microM), and arginine alone (250 microM). cGMP levels were quantified by ELISA. Immunohistochemistry studies were performed for the detection of cGMP and nNOS, while Western blot analysis was employed for the detection of SHP-1. Somatostatin increased cGMP levels in a concentration-dependent manner. This increase was inhibited by CYN154806. BIM23014 increased cGMP levels only at the concentration of 10 microM, while L-796,778 had no effect. NMMA blocked completely the somatostatin stimulated increase of cGMP levels and nNOS was detected in rat retina. cGMP immunoreactivity was observed primarily in bipolar cells only of nitroprusside-treated retinas. SHP-1 inhibition by orthovanadate reduced the somatostatin effect in a statistically significant manner. These results suggest that a SRIF/SHP-1/NO/cGMP mechanism underlies the actions of somatostatin in the retina and in its influence of retinal circuitry.

Probing inner retinal circuits in the rod pathway: a comparison of c-fos activation in mutant mice

We have used wild-type mice and mice possessing defects in specific retinal circuits in order to more clearly define functional circuits of the inner retina. The retina of the nob mouse lacks communication between photoreceptors and depolarizing bipolar cells (DBCs). Thus, all light driven activity in the nob mouse is mediated via remaining hyperpolarizing bipolar cell (HBC) circuits. Transducin null (Tr alpha-/-) mice lack rod photoreceptor activity and thus remaining retinal circuits are solely generated via cone photoreceptor activity. Activation in inner retinal circuits in each of these mice was identified by monitoring light-induced expression of an immediate early gene, c-fos. The number of cells expressing c-fos in the inner retina was dependent upon stimulus intensity and was altered in a systematic fashion in mice with known retinal mutations. To determine whether c-fos is activated via circuits other than photoreceptors in the outer retina, we examined c-fos expression in tulp1-/- mice that lack photoreceptors in the outer retina; these mice showed virtually no c-fos activity following light exposure. Double-labeling immunohistochemical studies were carried out to more clearly define the population of c-fos expressing amacrine cells. Our results indicate that c-fos may be used to map functional circuits in the retina.

Fos-tau-LacZ mice expose light-activated pathways in the visual system

We have employed fos-tau-LacZ (FTL) transgenic mice to examine functional activation in the visual areas of the nervous system. The FTL mice express the marker gene lacZ in neurons and their processes following many different stimuli, and allow the imaging of activation from the level of the entire brain surface through individual neurons and their projections. Analysis of FTL expression in the retinas of mice following diurnal exposure to light shows that bipolar cells, specific classes of amacrine cells, ganglion cells, and a dense network of processes in the inner plexiform layer are functionally activated. In animals deprived of light, there is almost no activity in the retina. In the lateral geniculate nucleus (LGN), light exposure appears responsible for FTL expression in dorsal nuclei, but not for expression in the ventral nuclei or the intergeniculate leaflet. In the superficial layers of the superior colliculus, FTL expression is highly dependent on light exposure. Similarly, light exposure is required for FTL expression in primary visual cortex (area 17), but some expression remains in area 18 of dark-adapted animals. Finally, using mice with one or both eyes missing, we have determined which parts of the visual system are dependent on the presence of a functional connectivity from the eye. These data demonstrate the usefulness of the FTL mice to map functional activation within the entire visual system. Furthermore, we can capture visual activation in a conscious animal. Our findings give an insight into the architecture of activity within the retina and throughout the visual system.

Light-induced c-fos in melanopsin retinal ganglion cells of young and aged rodless/coneless (rd/rd cl) mice

Non-rod, non-cone ocular photoreceptors have been shown to mediate a range of irradiance detection tasks. The strongest candidates for these receptors are melanopsin-positive retinal ganglion cells (RGCs). To provide a more complete understanding of these receptors in vivo, we have utilized a mouse that lacks rod and cone photoreceptors (rd/rd cl) and compared these animals to congenic wild-types. Using real-time polymerase chain reaction and immunohistochemistry, we address the following. (1) Is Fos expression within these RGCs driven by an input from the rods/cones or is it the product of the intrinsic photosensitivity of these neurons? We demonstrate that most Fos expression across the entire retina is due to the rods/cones, but in the absence of these photoreceptors, light will induce Fos within melanopsin RGCs. (2) Could the reported age-related decline in circadian photosensitivity of rodents be linked to changes in the population of melanopsin RGCs? We show that old mice experience an approximately 40% reduction in melanopsin RGCs. (3) Does the loss of inner retinal neurons affect the responses of melanopsin RGCs? Aged (approximately 700 days) rd/rd cl mice lose most of their inner retina but retain the retinal ganglion cell layer. In these mice, the proportion of melanopsin RGCs that express Fos in response to light is significantly reduced. Collectively, our data suggest that melanopsin RGCs form a heterogeneous population of neurons, and that most of the light-induced c-fos expression within these cells is associated with the endogenous photosensitivity of these neurons.

Functional mapping of somatostatin receptors in the retina: a review

The peptide somatostatin is one of many neuroactive agents that influence retinal physiology. It is synthesized primarily in a subclass of amacrine cells and believed to function as a neurotransmitter, neuromodulator or trophic factor. The cloning of the somatostatin receptors (sst1-5) in the early nineties provided the appropriate tools for the study of ssts in many tissues, including the retina. In this review, emphasis is given to recent studies that have provided significant information on the functional role of somatostatin in retinal circuitry and the retinal pigment epithelium. The important role of somatostatin in retinal disease therapeutics is also discussed.

Demonstration of cholinergic ganglion cells in rat retina: expression of an alternative splice variant of choline acetyltransferase

Acetylcholine acts as a neurotransmitter in the retina. Although previous physiological studies have indicated that some retinal ganglion cells may be cholinergic, several immunohistochemical studies using antibodies to choline acetyltransferase (ChAT) have stained only amacrine cells but not ganglion cells. Recently, we identified a splice variant of ChAT mRNA, lacking exons 6-9, in rat peripheral nervous system. The encoded protein was designated as ChAT of a peripheral type (pChAT), against which an antiserum was raised. In the present study, we examined expression of pChAT in rat retina, both at the protein level by immunohistochemistry using the antiserum and at the mRNA level by RT-PCR. Immunohistochemistry revealed that although no positive neurons were found in untreated intact retinas, many neurons became immunoreactive for pChAT after intravitreal injection of colchicine. Damage of the optic nerve was also effective in disclosing positive cells. Such positive neurons were shown to be ganglion cells by double labeling with a retrograde tracer that had been injected into the contralateral superior colliculus. Western blot analysis and RT-PCR revealed a corresponding band to the pChAT protein and to the amplified pChAT gene fragment, respectively, in retinal samples. In addition, ChAT activity was definitely detected in retinofugal fibers of the optic nerve. These results indicate the presence of cholinergic ganglion cells in rat retina.

Static, but not optokinetic visual stimuli induce Fos expression in the retina and brain of retinal degeneration mice

The stimulated expression of the c-fos gene was used in this study to characterize the residual vision of retinal degeneration (rd) mice. Conventional immunohistochemistry was employed to detect Fos-positive nuclei in the rd mouse retina and brain after monocular application of static and dynamic photic stimuli. Diffuse light induced Fos-positive nuclei in the inner nuclear and ganglion cell layers of the stimulated retina and in the contralateral pretectal olivary nucleus. On the other hand, Fos-positive nuclei were rarely detected in the rd mouse retina and brain after stimulation with a slowly displaced random-dot pattern. These results suggest that the residual vision of the rd mouse involves mainly the circuits that are related to the processing of static photic stimuli.

Phase shifting the retinal circadian clock: xPer2 mRNA induction by light and dopamine

A circadian clock is located in the retinal photoreceptors of the African clawed frog Xenopus laevis. These photoreceptor clocks are thought to govern a wide variety of output rhythms, including melatonin release and gene expression. Both light and dopamine phase shift the retinal clock in a phase-dependent manner. Two homologs of the Drosophila period gene have been cloned in Xenopus, and one of these (xPer2) is acutely regulated by light. Light and dopamine induce xPer2 mRNA in a similar manner. In addition, the increase of xPer2 mRNA in response to light and dopamine is the same at all times of day tested. In contrast, xPer1 mRNA exhibits circadian oscillations but is relatively insensitive to phase-shifting treatments of light or dopamine. Our data suggest that xPer2 functions as the molecular link between the light/dark cycle and the circadian clock.

Regulation of alpha3 nicotinic acetylcholine receptor subunit mRNA levels by nerve growth factor and cyclic AMP in PC12 cells

To investigate the effects of nerve growth factor (NGF) and cyclic AMP (cAMP) on the level of the nicotinic acetylcholine receptor subunit alpha3 mRNA, we used PC12h cells, PC12 cells expressing dominant-negative Ras protein, and the parental PC12 cells. PC12h cells have NGF-responsive tyrosine hydroxylase activity. Expression of dominant-negative Ras protein prevents the signaling through the Ras-mitogen-activated protein kinase cascade. The morphological changes of the parental PC12 cells in response to NGF and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPTcAMP), a cell-penetrating cAMP analogue, were similar to those of PC12h cells. NGF up-regulated the alpha3 mRNA level in PC12h cells and down-regulated the alpha3 mRNA level in the parental PC12 cells. Expression of dominant-negative Ras protein and an inhibitor of mitogen-activated protein kinase kinase inhibited the effects of NGF on alpha3 mRNA level. CPTcAMP down-regulated the alpha3 mRNA level in all three PC12 cell lines. An inhibitor of protein kinase A inhibited the CPTcAMP-induced down-regulation of alpha3 mRNA. The alpha3 mRNA down-regulation required prolonged treatment with CPTcAMP even after cAMP response element binding protein phosphorylation was decreased. Membrane depolarization with high K+ had no effect on the alpha3 mRNA level in PC12h cells. Based on these results, we propose that at least two unknown effectors regulate alpha3 mRNA levels in PC12 cells.

Spatio-temporal analysis of light-induced Fos expression in the retina of rd mutant mice

Rd mutant mice are visually blind but they maintain the ability of synchronising their circadian rhythms to the external light-dark cycles. We used immunocytochemical procedures to detect light-induced Fos expression in the rd mice retina. We found that Fos is expressed in the rd retina in an unattenuated pattern through the entire life of the animal. Furthermore, we have found that cells expressing Fos are distributed throughout the whole retina, while opsin expression takes place only in the dorsal half of the retina in the 1-year old rd mice. Finally, we found that light induces Fos expression in the rd retina at the same levels during the subjective day as during the subjective night, whereas in the suprachiasmatic nucleus (SCN), Fos is stimulated by light only during the subjective night. Our results support the hypothesis that new, undiscovered photoreceptors are implicated in light perception for the circadian system.

Role of DNA 5-methylcytosine transferase in cell transformation by fos

The Fos and Jun oncoproteins form dimeric complexes that stimulate transcription of genes containing activator protein-1 regulatory elements. We found, by representational difference analysis, that expression of DNA 5-methylcytosine transferase (dnmt1) in fos-transformed cells is three times the expression in normal fibroblasts and that fos-transformed cells contain about 20 percent more 5-methylcytosine than normal fibroblasts. Transfection of the gene encoding Dnmt1 induced morphological transformation, whereas inhibition of dnmt1 expression or activity resulted in reversion of fos transformation. Inhibition of histone deacetylase, which associates with methylated DNA, also caused reversion. These results suggest that fos may transform cells through alterations in DNA methylation and in histone deacetylation.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Temporospatial characteristics of light-induced fos immunoreactivity in suprachiasmatic nuclei are not modified in Syrian hamsters treated neonatally with monosodium glutamate

Neonatal treatment of rodents by intraperitoneal injections of monosodium glutamate (MSG) destroys many retinal ganglion cells whose neurons belong to the circadian system; howertheless, adults always synchronize their locomotor activity rhythm (LAR) to the light/dark cycle. Recent studies have shown that light-induced phase shifts of LAR are associated with the c-fos induction in suprachiasmatic nuclei (SCN) of nocturnal rodents. In this study, the circadian system was analyzed in treated and control hamsters maintained in constant darkness and exposed to light at circadian times (CTs) 13 and 18 during subjective night, 1 and 6 h after the onset of LAR. The period of the LAR and delay (CT13) and advance (CT18) phase shifts of LAR were not significantly different between MSG-treated and control hamsters. Temporospatial variations of Fos induction after light exposure were similar in both MSG-treated and control hamsters although the total number of Fos immunoreactive (Fos-ir) nuclei in the SCN was always lower in treated hamsters. However, the decrease in Fos-ir was significant only for the caudal third of the SCN of treated hamsters, the part where retinal afferents are most dense. The effect of light exposure on Fos expression in SCN of MSG-treated and control hamsters was the same at CT13 and CT18: (1) Fos-ir nuclei were significantly more numerous at CT18 than at CT13 in the rostral SCN; (2) dorsal Fos-ir cells were observed in the SCN only at CT18; (3) a ventral subgroup expressed Fos protein in intermediate SCN only at CT13. This study demonstrates that MSG-treatment does not significantly modify the phase-shifting effects of light on either the LAR or the associated cellular activation.

Motion-sensitive neurons in the chick retina: a study using Fos immunohistochemistry

Fos immunohistochemistry was used to characterize neurons in the chick retina activated by optokinetic and stationary stimuli. Higher percentages of co-localization of Fos and the alpha5 subunit of the nicotinic acetylcholine receptor, and Fos and GABA were observed in retinal neurons after optokinetic compared to the stationary stimulation. These results indicate an involvement of the cholinergic and GABAergic circuitries in the motion detection by chick retinal cells.

AP-1, CREB and CBP transcription factors differentially regulate the tyrosine hydroxylase gene

The tyrosine hydroxylase (TH) gene encodes the rate-limiting enzyme in the biosynthesis of catecholamines. We have investigated the roles of two elements of the TH promoter, the TH-'Fat Specific Element' (TH-FSE) which binds the Fos-Jun complex, and the cAMP Response Element (CRE), which binds CREB and the co-activator protein, CREB Binding Protein (CBP) in regulating TH gene transcription. In PC12 cells, the TH-FSE was required for induction by NGF while the CRE was required for induction by cAMP. We show that both elements can function independently and contribute strongly to TH promoter basal activity in PC12 cells. We employed transient expression in the F9 teratocarcinoma cell line to vary experimentally the levels of the nuclear regulators implicated in TH control by the PC12 studies. In F9 cells, the TH promoter was strongly activated by Fos and Jun, and by PKA-stimulated CREB protein. In F9 and NIH3T3 cells, CBP, a co-activator which targets Fos-Jun and PKA-stimulated CREB, also induced the TH promoter. Immunohistochemical studies in rat brain regions enriched in dopaminergic neurons, including the midbrain and olfactory bulb (OB), suggest that Fos-Jun and CREB make differential contributions to TH gene activity in different tissues. Whereas changes in Fos protein levels parallel decreases in TH protein upon olfactory deprivation, CBP levels remain unchanged. This suggests that CRE-associated factors, including CBP, are not major regulators in the OB. In contrast, the presence of CREB and the absence of Fos immunoreactivity in midbrain dopaminergic cells suggests that the CRE is the primary regulator in this region.

Identification of c-fos related genes and their induction by neural activation in rainbow trout brain

A number of studies have shown that the induction of c-fos gene is an indicator of the responses of cells and tissues to the environmental stimuli. In the present study, using RT-PCR-based strategy, we isolated, from the brain of the rainbow trout, two partial cDNA clones (RT-fos1 and RT-fos2) that code proteins homologous to c-Fos proteins of higher vertebrates. Sequence analysis of the two clones indicated that the two rainbow trout clones are very similar to each other over the entire cloned region (88% amino acid identity) and showed moderate similarity to c-Fos proteins of higher vertebrates (40% amino acid identity with human c-Fos). Two functionally important domains (i.e. the leucine zipper and zinc finger) are highly conserved among all the vertebrate species analyzed, although the region between the two domains was highly variable between different species. Intraperitoneal administration of kainic acid, a stable agonist of glutamate receptors, transiently induced the mRNAs for both RT-fos1 and RT-fos2 in the rainbow trout brain. These results indicate that the expression pattern of the two clones can be utilized as a suitable anatomical marker for the increased neural activities in salmonid fish brain to investigate the higher order behavior such as the learning and imprinting of odors of the home river.

Fos expression in the retina of rd/rd mice during the light/dark cycle

An anti-Fos protein antiserum was used to elucidate the diurnal expression of Fos protein in the normal and degenerate rd/rd mice retina. We have found that Fos expression is stimulated in cells of both inner nuclear layer (INL) and ganglion cell layer (GCL) at the onset of light period and reaches its maximum after 2 h. After which, the number of stained nuclei decreases along the light/dark cycle until almost no reaction is observed at the end of dark period. This expression pattern was similar in both normal and rd/rd mice although degenerate retinas showed a much lower number of stained nuclei. Aged rd animals also show Fos expression in GCL and INL in response to light stimuli suggesting that severely degenerate retinas are still able to transduce light stimulus.

Light-induced c-fos mRNA expression in the suprachiasmatic nucleus and the retina of C3H/HeN mice

Light-induced expression of c-fos mRNA was studied over a circadian period (approximately 24 h) in C3H/HeN mice maintained in constant dark. This mouse strain expresses an rd mutation (retinal degeneration) which does not affect light-induced phase shifts of circadian rhythms. c-fos mRNA expression in the retina and the suprachiasmatic nucleus (SCN) after a light pulse (300 lux) was determined by in-situ hybridization autoradiography using a 35S-labeled c-fos riboprobe. Light induced the expression of c-fos mRNA in retino-recipient areas of the SCN. This response was dependent on the circadian time (CT) and was observed only during the subjective night (CT14-CT22) and early subjective day (CT2). However, the period of photosensitivity for c-fos induction extended 1 h over the period of photosensitivity for phase shifts in circadian behavior. In the retina of C3H/HeN mice, light-induced c-fos mRNA expression was observed in a small number of cells in the ganglion cell layer (approximately 0.2%) which may represent ganglion cells projecting to the SCN. A dependence of c-fos expression with the circadian time was observed in retinal ganglion cells, suggesting that retinal photosensitivity may also be controlled by a circadian oscillator. In conclusion, we demonstrated light-induced expression of the immediate early gene c-fos mRNA in both the retina and SCN of C3H/HeN mice expressing the rd mutation.