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      TRPV1 and Endocannabinoids: Emerging Molecular Signals that Modulate Mammalian Vision

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          Abstract

          Transient Receptor Potential Vanilloid 1 (TRPV1) subunits form a polymodal cation channel responsive to capsaicin, heat, acidity and endogenous metabolites of polyunsaturated fatty acids. While originally reported to serve as a pain and heat detector in the peripheral nervous system, TRPV1 has been implicated in the modulation of blood flow and osmoregulation but also neurotransmission, postsynaptic neuronal excitability and synaptic plasticity within the central nervous system. In addition to its central role in nociception, evidence is accumulating that TRPV1 contributes to stimulus transduction and/or processing in other sensory modalities, including thermosensation, mechanotransduction and vision. For example, TRPV1, in conjunction with intrinsic cannabinoid signaling, might contribute to retinal ganglion cell (RGC) axonal transport and excitability, cytokine release from microglial cells and regulation of retinal vasculature. While excessive TRPV1 activity was proposed to induce RGC excitotoxicity, physiological TRPV1 activity might serve a neuroprotective function within the complex context of retinal endocannabinoid signaling. In this review we evaluate the current evidence for localization and function of TRPV1 channels within the mammalian retina and explore the potential interaction of this intriguing nociceptor with endogenous agonists and modulators.

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          Most cited references153

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          TRP channels as cellular sensors.

          TRP channels are the vanguard of our sensory systems, responding to temperature, touch, pain, osmolarity, pheromones, taste and other stimuli. But their role is much broader than classical sensory transduction. They are an ancient sensory apparatus for the cell, not just the multicellular organism, and they have been adapted to respond to all manner of stimuli, from both within and outside the cell.
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            Characterizing light-regulated retinal microRNAs reveals rapid turnover as a common property of neuronal microRNAs.

            Adaptation to different levels of illumination is central to the function of the retina. Here, we demonstrate that levels of the miR-183/96/182 cluster, miR-204, and miR-211 are regulated by different light levels in the mouse retina. Concentrations of these microRNAs were downregulated during dark adaptation and upregulated in light-adapted retinas, with rapid decay and increased transcription being responsible for the respective changes. We identified the voltage-dependent glutamate transporter Slc1a1 as one of the miR-183/96/182 targets in photoreceptor cells. We found that microRNAs in retinal neurons decay much faster than microRNAs in nonneuronal cells. The high turnover is also characteristic of microRNAs in hippocampal and cortical neurons, and neurons differentiated from ES cells in vitro. Blocking activity reduced turnover of microRNAs in neuronal cells while stimulation with glutamate accelerated it. Our results demonstrate that microRNA metabolism in neurons is higher than in most other cells types and linked to neuronal activity. Copyright (c) 2010 Elsevier Inc. All rights reserved.
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              A subpopulation of nociceptors specifically linked to itch

              Itch-specific neurons have been sought for decades. The existence of such neurons is in doubt recently due to the observation that itch-mediating neurons also respond to painful stimuli. Here, we genetically labeled and manipulated MrgprA3+ neurons in dorsal root ganglion (DRG) and found that they exclusively innervate the epidermis of the skin and respond to multiple pruritogens. Ablation of MrgprA3+ neurons led to significant reductions in scratching evoked by multiple pruritogens and occurring spontaneously under chronic itch conditions whereas pain sensitivity remained intact. Importantly, mice with TRPV1 exclusively expressed in MrgprA3+ neurons exhibited only itch- and not pain behavior in response to capsaicin. Although MrgprA3+ neurons are sensitive to noxious heat, activation of TRPV1 in these neurons by noxious heat did not alter pain behavior. These data suggest that MrgprA3 defines a specific subpopulation of DRG neurons mediating itch. Our study opens new avenues for studying itch and developing anti-pruritic therapies.
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                Author and article information

                Journal
                Cells
                Cells
                cells
                Cells
                MDPI
                2073-4409
                12 September 2014
                September 2014
                : 3
                : 3
                : 914-938
                Affiliations
                [1 ]Department of Ophthalmology & Visual Sciences, Moran Eye Institute, University of Utah School of Medicine, Salt Lake City, UT 84132, USA; E-Mails: sarah.redmon@ 123456utah.edu (S.R.); u0751604@ 123456utah.edu (A.O.J.)
                [2 ]Interdepartmental Program in Neuroscience, University of Utah School of Medicine, Salt Lake City, UT 84132, USA
                [3 ]Department of Neurobiology & Anatomy, University of Utah School of Medicine, Salt Lake City, UT 84132, USA
                [4 ]Center for Translational Medicine, University of Utah School of Medicine, Salt Lake City, UT 84132, USA
                Author notes
                [†]

                Those authors contributed equally to this work.

                [* ]Authors to whom correspondence should be addressed; E-Mails: daniel.ryskamp@ 123456gmail.com (D.A.R.); david.krizaj@ 123456hsc.utah.edu (D.K.); Tel.: +1-801-213-2777 (D.K.); Fax: +1-801-587-8314 (D.K.).
                Article
                cells-03-00914
                10.3390/cells3030914
                4197638
                25222270
                f99270db-bcbe-43b7-9e90-85fceb774b0a
                © 2014 by the authors; licensee MDPI, Basel, Switzerland.

                This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license ( http://creativecommons.org/licenses/by/3.0/).

                History
                : 01 July 2014
                : 27 August 2014
                : 05 September 2014
                Categories
                Review

                retinal ganglion cells,trpv1 channels,endocannabinoids,cb1 receptors

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