Browsing by Author "Vigh, Jozsef, advisor"
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Item Open Access A retinal contribution to chronic opioid-induced sleep/wake dysfunction(Colorado State University. Libraries, 2023) Bergum, Nikolas, author; Vigh, Jozsef, advisor; Myers, Brent, committee member; Hentges, Shane, committee member; Chanda, Soham, committee memberLight is among the most important environmental factors that regulate mammalian sleep/circadian behaviors. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) transmit environmental light information to key sleep/circadian centers in the brain through a process known as photoentrainment. Interestingly, past studies have revealed that ipRGCs express µ-opioid receptors (MORs), the primary molecular target for opioid analgesics. Furthermore, MOR agonists can directly inhibit ipRGC firing. Therefore, we hypothesize that opioid drugs acting on MORs expressed by ipRGCs could disrupt ipRGC-mediated regulation sleep/wake rhythms in response to environmental light/dark cycles. To test this idea, we need to confirm that morphine reaches the mouse retina following systemic delivery. To accomplish this, tissue (retina, brain and serum) was collected from mice following intraperitoneal morphine administration. Importantly, results from this study show that systemically administered morphine selectively accumulates in the mouse retina, but not the serum or the brain. To test the role that MORs expressed by ipRGCs play in opioid-induced dysregulation of sleep/circadian behaviors, we used mini-telemetry devices to assess how chronic morphine alters their sleep/wake behavior in mice. Importantly, we performed these experiments in wildtype mice along with mice lacking MORs exclusively in ipRGCs (McKO). Results from these studies reveal that McKO animals exhibit decreased morphine-induced locomotion compared to controls, which implicates MORs expressed by ipRGCs as a mediator of opioid-induced sleep-wake alterations. Finally, we tested whether ipRGCs developed cellular tolerance to MOR agonists following chronic exposure to morphine. The lack of cellular tolerance development at the level of solitary ipRGCs provides a potential cellular correlate for the persistent sleep/wake dysfunction commonly attributed to chronic opioid exposure. Taken together, these findings support the idea that opioid accumulation in the eye persistently activate MORs on ipRGCs, continuously altering the ability of ipRGCs to transmit light information to the brain's sleep/wake circuitry. This alteration in photic input to the brain could underlie some of the sleep/wake problems associated with long-term opioid use.Item Open Access Inducible photoreceptor degeneration model in goldfish(Colorado State University. Libraries, 2011) Varland, Dezaray D., author; Vigh, Jozsef, advisor; Gionfriddo, Juliet, committee member; Ishii, Douglas, committee member; Madl, James, committee member; Tobet, Stuart, committee memberPhotoreceptor degenerative diseases are among the leading causes of vision loss and there is presently no known cure. The future success of biological and prosthetic vision rescue approaches following photoreceptor loss remains questionable, due to the morphological and functional changes occurring in the remaining retinal circuitry. In the current study we sought to establish a chemically-induced photoreceptor degenerative model in goldfish, based on the ability of teleost to regenerate their retina following damage. N-methyl-N-nitrosourea (MNU) was chosen to chemically induce the photoreceptor degeneration, because it has been found to be potent, and selective in mammalian studies. We hypothesized that MNU would induce selective and complete photoreceptor loss in the goldfish retina as well as the consequent morphological changes observed in mammalian retinas. Under anesthesia, fish received a direct, intraocular injection of MNU into the posterior chamber of one eye whereas the contralateral eye served as sham-injected control. The effects of MNU were determined by standard immunohistochemical methods using known, well-established molecular markers of retinal cells. The MNU induced unilateral, selective, and dose-dependent photoreceptor degeneration: up to ~60% of photoreceptors lost the injected eye of the goldfish within 7 days, followed by nearly complete regeneration by ~50 days post-injection. Repeated MNU treatments did not increase the magnitude of degeneration, but delayed the regeneration. Unlike in mammals, MNU did not destroy all of the photoreceptors in fish. The incomplete photoreceptor degeneration together with the quick regeneration may be responsible for preventing the development of chronic morphological and functional consequences. However, the regeneration observed after MNU treatment is promising. Inducing total photoreceptor degeneration in fish retina, possibly by combining MNU with other factors shown to destroy photoreceptors (i.e. strong light) could provide an all-encompassing natural model for studying the potential of stem cell-based vision rescue approaches after photoreceptor loss.Item Open Access Mu-opioid system in the mammalian retina(Colorado State University. Libraries, 2013) Gallagher, Shannon K., author; Vigh, Jozsef, advisor; Clapp, Tod R., committee member; Gionfriddo, Juliet R., committee member; Hentges, Shane T., committee member; Partin, Kathryn M., committee memberUntil recently, the most solid evidence suggesting a role for endogenous opioids in mammalian visual processing has been the existence of μ-opioid receptors (MORs) in the retina. Nonetheless, in most reports the location of these receptors has been limited to retinal regions rather than specific cell-types. Reports on expression of endogenous opioids in the adult mammalian retina were missing, and even in juveniles have been sparse. Additionally, our knowledge of the possible physiological functions of opioid signaling in the retina is based on only a handful of studies using exogenous opioids. For example, the recent resurgence in retinal opioid research has focused on the somewhat controversial role of δ-opioid receptors in neuroprotection. The purpose of this work was to identify if the endogenous opioid peptide preferred by MORs, β-endorphin, is present in the mammalian retina, and to determine its possible influence on the light-evoked signaling of retinal neurons that express MORs. We have identified through use of transgenic mice, in situ hybridization and immunohistochemistry (IHC) that the cholinergic "Starburst" amacrine cells express β-endorphin. Using IHC we've shown that multiple neuronal cell types in the mouse retina possess MORs, including dopaminergic amacrine cells and intrinsically photosensitive retinal ganglion cells (ipRGCs). ipRGCs play a central role in mammalian non-image forming vision. Neuromodulatory processes that are capable of altering ipRGCs activity are likely to have profound consequences on light-mediated behavior and/or disease. Using IHC, we found that M1-M3 types of ipRGCs are MOR+ in both mouse and rat. Using electrophysiological techniques we found that DAMGO, a MOR selective agonist, dramatically reduces both duration and rate of light-evoked firing from rat and mouse ipRGCs. Our study is the first to demonstrate opioid modulation of light-evoked activity of neurons in mammalian retina. These findings demonstrate a new role for endogenous opioids in the mammalian retina and provide a novel site of action--MORs on ipRGCs--through which exogenous, systemically applied, opioids could exert an effect on light-mediated behaviors.Item Open Access Opioid bioavailability in the retina modulates sleep/wake behavior(Colorado State University. Libraries, 2023) Berezin, Casey-Tyler, author; Vigh, Jozsef, advisor; Hentges, Shane, committee member; Bailey, Susan, committee member; Montgomery, Tai, committee memberOpioids, such as the prototypical opioid morphine, primarily exert their analgesic and rewarding effects through μ-opioid receptors (MORs), and it has been shown that acute morphine treatment negatively impacts sleep/wake behavior through MORs. Continued opioid administration exacerbates sleep/wake disturbances, such that approximately 80% of chronic users report persistent sleep disturbances, including insomnia and daytime sleepiness. These opioid-induced sleep disturbances (OISD) are associated with negative outcomes such as increased drug craving, relapse and negative affect (i.e. anxiety/depression), as well as hyperalgesia in chronic pain patients. Therefore, therapeutic interventions for OISD are expected to improve the outcomes of long-term opioid use in general. However, the neuronal population(s) and cellular mechanism(s) mediating sleep/wake changes in response to chronic opioid treatment are not currently known. Although many neuronal populations in the brain contribute to sleep/wake behavior, the photoentrainment of sleep/wake cycles to environmental light/dark cycles is uniquely regulated by intrinsically photosensitive retinal ganglion cells (ipRGCs). Importantly, light-evoked firing by ipRGCs is modulated by activation of their MORs, and endogenous MOR activation has been shown to modulate an ipRGC-driven behavior, the pupillary light reflex. Additionally, opioids (e.g. morphine) are routinely detected in the human eye to diagnose overdose-related deaths, and morphine accumulates in the mouse retina upon chronic systemic administration. Therefore, the central hypothesis addressed in the current work is that the bioavailability of opioids (i.e. endogenous opioid peptides and/or opioid drugs) in the retina contributes to the modulation of sleep/wake behavior. In Chapter 2, we show that endogenous opioid peptides in the retina contribute to healthy sleep/wake behavior. Using a mouse model with a cell-specific knockout of MORs expressed by ipRGCs (McKO), we show that endogenous activation of these MORs is important for maintaining activity and suppressing slow-wave sleep during the mouse's active period. Concurrent work showed that the MORs expressed by ipRGCs also contribute to morphine-driven changes in sleep/wake behavior during a chronic treatment paradigm. Thus, both endogenous opioid peptides and opioid drugs modulate sleep/wake behavior via the activation of the MORs expressed by ipRGCs. Systemically applied opioid drugs penetrate the blood-retina barrier and accumulate in the retina. Thus, ipRGCs may be liable to opioid modulation throughout chronic opioid treatment, contributing to the development and persistence of OISD. In Chapter 3, we investigate the relationship between opioid transporter expression at the blood-retina barrier and morphine deposition in the retina. We show that low expression of the opioid extruder permeability glycoprotein (P-gp) may allow morphine to persist in the retina, compared to the brain where P-gp expression is high and morphine is more quickly cleared from the tissue. In Chapter 4, we discuss how upregulation of P-gp at the blood-retina barrier via the non-steroidal anti-inflammatory drug diclofenac may reduce opioid bioavailability in the retina, thereby alleviating OISD. This work demonstrates a previously unknown contribution of ipRGC signaling to opioid-modulated sleep/wake changes, and identifies a potential therapeutic target for OISD.Item Open Access Opioid modulation of intrinsically photosensitive retinal ganglion cells(Colorado State University. Libraries, 2019) Cleymaet, Allison Marie, author; Powell, Cynthia, advisor; Vigh, Jozsef, advisor; de Linde Henriksen, Michala, committee member; Hentges, Shane, committee memberWidespread opioid use and abuse has resulted in an opioid epidemic in the United States and worldwide. Among several adverse effects of this drug class, opioids disrupt the sleep/wake cycle. While sleep induction and regulation is complex, and opioid receptors are known to be located in central sleep regulatory nuclei, it has not been specifically studied if opioids affect photoentrainment of circadian rhythm and thus the sleep/wake cycle. Intrinsically photosensitive retinal ganglion cells (ipRGCs) are the exclusive conduits for non-image forming visual functions, such as the aforementioned photoentrainment of systemic circadian rhythms, including the drive to sleep, and the pupillary light reflex (PLR). Systemically applied opioids cross the tight blood/retina barrier and thereby might alter the activity of retinal neurons. It has been recently shown that ipRGCs express μ-opioid receptors (MORs) and exogenously applied opioids inhibit the firing of ipRGCs. The current work aimed to identify the mechanism by which opioids inhibit ipRGC firing as well as downstream behavioral consequence of such inhibition at the organism level, specifically as manifested by modulation of PLR. Through the use of transgenic mice, electrophysiology including multi-electrode array recordings and patch clamp in whole and dissociated retinas, and immunohistochemistry, we have documented the following: (1) In the rodent retina M1-M3 types of intrinsically photosensitive ganglion cells (ipRGCs) express μ-opioid receptors (MORs). (2) Light-evoked firing of ipRGCs is attenuated by the MOR-specific agonist DAMGO in a dose-dependent manner. (3) MOR activation reduces ipRGC excitability by modulating IK and reducing the amplitude of non-inactivating ICa. Additionally, we explored the effect of modulation of ipRGC signaling via MORs on the murine PLR using transgenic mice and pupillometry. Our main findings were: (1) In WT mice but not in systemic μ-opioid receptor knockout mice (MKO) or mice in which μ-opioid receptors were selectively knocked out of ipRGCs alone (McKO), intraocular application of the MOR selective agonist DAMGO strongly inhibited rod/cone driven PLR and slowed melanopsin- driven PLR. (2) Intraocular application of a MOR selective antagonist CTAP enhanced rod/cone driven PLR in the dark-adapted retina and melanopsin driven PLR under photopic conditions in WT mice. In summary, these results identify both a novel site of action, MORs on ipRGCs, and a mechanistic description of a novel neural pathway by which exogenous and potentially endogenous opioids might alter light driven behavior, including the PLR, which may serve as a biomarker of systemic opioid effect.Item Open Access S-nitrosylation mediates synaptic plasticity in the retina(Colorado State University. Libraries, 2015) Tooker, Ryan E., author; Vigh, Jozsef, advisor; Tamkun, Michael, committee member; Hentges, Shane, committee member; Hoke, Kim, committee memberOver the course of an entire day, our visual system must accommodate intensities of light that can change by a factor of 10¹⁰. In order to do so, the retina adapts to large, daily changes in natural light intensity by shifting its dynamic range of coding. For example, as morning light intensity increases, the retina implements multiple strategies that result in decreases in overall sensitivity in order to avoid saturation. However, adaptation to bright environments poses the inherent risk of losing visual information carried by dim/weak signals in complex natural scenes. Here we studied whether the light-evoked increase in retinal nitric oxide (NO) production is followed by NO-mediated, direct post-translational modification of proteins called S-nitrosylation and if it contributes to the modulation of the dynamic range of vision. In the central nervous system, including the retina, S-nitrosylation has not been considered to be significant under physiological conditions, and instead, has been primarily associated with neurodegenerative diseases. In this study, we provide immunohistochemical and proteomic evidence for extensive S-nitrosylation that takes place in the goldfish and mouse retinas under physiologically relevant light intensities, in an intensity-dependent manner. Functionally, we report a novel form of activity-dependent synaptic plasticity via S-nitrosylation: a “weighted potentiation” that selectively increases the output of Mb-type bipolar cells in the goldfish retina in response to weak inputs but leaves the input-output ratio for strong stimuli unaffected. Importantly, the NO action resulted in a weighted potentiation of Mb output in response to small (≤-30 mV) depolarizations. Our data strongly suggest that in the retina, light-evoked NO production leads to extensive S-nitrosylation and that this process is a significant post-translational modification affecting a wide range of proteins under physiological conditions. S-nitrosylation may function to extend the dynamic range of vision by counteracting the decreases in retinal sensitivity during light adaptation ultimately preventing the loss of visual information carried by dim scotopic signals. Finally, our results may set the framework for exploring the role of S-nitrosylation in certain neurodegenerative retinal diseases that are associated with toxic levels of NO.