Implication of Melanopsin and Trigeminal Neural Pathways in Blue Light Photosensitivity in vivo

Photophobia may arise from various causes and frequently accompanies numerous ocular diseases. In modern highly illuminated world, complaints about greater photosensitivity to blue light increasingly appear. However, the pathophysiology of photophobia is still debated. In the present work, we investigated in vivo the role of various neural pathways potentially implicated in blue-light aversion. Moreover, we studied the light-induced neuroinflammatory processes on the ocular surface and in the trigeminal pathways. Adult male C57BL/6J mice were exposed either to blue (400–500 nm) or to yellow (530–710 nm) LED light (3 h, 6 mW/cm2). Photosensitivity was measured as the time spent in dark or illuminated parts of the cage. Pharmacological treatments were applied: topical instillation of atropine, pilocarpine or oxybuprocaine, intravitreal injection of lidocaine, norepinephrine or “blocker” of the visual photoreceptor transmission, and intraperitoneal injection of a melanopsin antagonist. Clinical evaluations (ocular surface state, corneal mechanical sensitivity and tear quantity) were performed directly after exposure to light and after 3 days of recovery in standard light conditions. Trigeminal ganglia (TGs), brainstems and retinas were dissected out and conditioned for analyses. Mice demonstrated strong aversion to blue but not to yellow light. The only drug that significantly decreased the blue-light aversion was the intraperitoneally injected melanopsin antagonist. After blue-light exposure, dry-eye-related inflammatory signs were observed, notably after 3 days of recovery. In the retina, we observed the increased immunoreactivity for GFAP, ATF3, and Iba1; these data were corroborated by RT-qPCR. Moreover, retinal visual and non-visual photopigments distribution was altered. In the trigeminal pathway, we detected the increased mRNA expression of cFOS and ATF3 as well as alterations in cytokines’ levels. Thus, the wavelength-dependent light aversion was mainly mediated by melanopsin-containing cells, most likely in the retina. Other potential pathways of light reception were also discussed. The phototoxic message was transmitted to the trigeminal system, inducing both inflammation at the ocular surface and stress in the retina. Further investigations of retina-TG connections are needed

Vers une meilleure compréhension des douleurs oculaires chroniques

La sécheresse oculaire est un des premiers motifs de consultation en ophtalmologie. Sa prévalence varie de 5 à 35 % chez des sujets âgés de plus de 50 ans. Cette pathologie du segment antérieur de l’œil est caractérisée par des sensations de douleurs variables dans leur intensité, allant du simple inconfort à une douleur oculaire prononcée. Les douleurs oculaires sont très invalidantes et difficiles à traiter et leurs mécanismes physiopathologiques demeurent de nos jours mal connus. Ce constat impose un approfondissement de nos connaissances fondamentales sur l’anatomie du système nociceptif cornéen et sur les mécanismes cellulaires impliqués dans l’initiation et la chronicisation de la douleur oculaire. Cette revue présente dans une première partie l’anatomie et la physiologie de l’innervation cornéenne et les différentes classes de récepteurs cornéens ainsi que les structures centrales mises en jeu dans la transmission du message nociceptif. La seconde partie fait un état des lieux des données précliniques et cliniques sur les mécanismes inflammatoires et neuro-inflammatoires qui ont été identifiés lors de douleurs cornéennes. Enfin, la dernière partie de cette revue décrit les différents dispositifs actuellement utilisés pour évaluer la douleur et l’inflammation oculaire en clinique humaine

Chimiokines et attractivité des cellules myéloïdes dans les douleurs neuropathiques périphériques

La douleur chronique est devenue un vrai sujet de société en raison de la difficulté de son traitement et par le handicap qu’elle engendre au quotidien. La compréhension des bases neurobiologiques et des mécanismes physiopathologiques à l’origine des différents syndromes douloureux ne cesse d’évoluer et rend compte de la complexité de ses mécanismes. Cette complexité rend malheureusement difficile la découverte de traitements efficaces contre certains types de douleurs chroniques, notamment en ce qui concerne les douleurs neuropathiques périphériques. Des études récentes font apparaître que lors de ces douleurs, les médiateurs de l’inflammation (notamment les chimiokines), outre leurs implications dans la modulation du message nociceptif et dans les mécanismes neuro-inflammatoires centraux, jouent un rôle essentiel dans l’orchestration de la réponse immunitaire consécutive à une lésion d’un nerf périphérique. Dans cette revue, après de brefs rappels concernant les chimiokines et la neuromodulation du message nociceptif, nous nous attacherons à définir leurs rôles et leurs fonctions dans la réponse immunitaire associée aux douleurs neuropathiques périphériques. Ainsi, la parfaite compréhension de ces communications moléculaires et cellulaires entre le système nerveux et le système immunitaire permettra à terme le développement de stratégies thérapeutiques nouvelles et innovantes contre ce type de pathologies très invalidantes

Morphological and Functional Changes of Corneal Nerves and Their Contribution to Peripheral and Central Sensory Abnormalities

International audienceThe cornea is the most densely innervated and sensitive tissue in the body. The cornea is exclusively innervated by C- and A-delta fibers, including mechano-nociceptors that are triggered by noxious mechanical stimulation, polymodal nociceptors that are excited by mechanical, chemical, and thermal stimuli, and cold thermoreceptors that are activated by cooling. Noxious stimulations activate corneal nociceptors whose cell bodies are located in the trigeminal ganglion (TG) and project central axons to the trigeminal brainstem sensory complex. Ocular pain, in particular, that driven by corneal nerves, is considered to be a core symptom of inflammatory and traumatic disorders of the ocular surface. Ocular surface injury affecting corneal nerves and leading to inflammatory responses can occur under multiple pathological conditions, such as chemical burn, persistent dry eye, and corneal neuropathic pain as well as after some ophthalmological surgical interventions such as photorefractive surgery. This review depicts the morphological and functional changes of corneal nerve terminals following corneal damage and dry eye disease (DED), both ocular surface conditions leading to sensory abnormalities. In addition, the recent fundamental and clinical findings of the importance of peripheral and central neuroimmune interactions in the development of corneal hypersensitivity are discussed. Next, the cellular and molecular changes of corneal neurons in the TG and central structures that are driven by corneal nerve abnormalities are presented. A better understanding of the corneal nerve abnormalities as well as neuroimmune interactions may contribute to the identification of a novel therapeutic targets for alleviating corneal pain

TRPM8: A therapeutic target for neuroinflammatory symptoms induced by severe dry eye disease

International audienceDry eye disease (DED) is commonly associated with ocular surface inflammation and pain. In this study, we evaluated the effectiveness of repeated instillations of transient receptor potential melastatin 8 (TRPM8) ion channel antagonist M8-B on a mouse model of severe DED induced by the excision of extra-orbital lacrimal and Harderian glands. M8-B was topically administered twice a day from day 7 until day 21 after surgery. Cold and mechanical corneal sensitivities and spontaneous ocular pain were monitored at day 21. Ongoing and cold-evoked ciliary nerve activities were next evaluated by electrophysiological multi-unit extracellular recording. Corneal inflammation and expression of genes related to neuropathic pain and inflammation were assessed in the trigeminal ganglion. We found that DED mice developed a cold allodynia consistent with higher TRPM8 mRNA expression in the trigeminal ganglion (TG). Chronic M8-B instillations markedly reversed both the corneal mechanical allodynia and spontaneous ocular pain commonly associated with persistent DED. M8-B instillations also diminished the sustained spontaneous and cold-evoked ciliary nerve activities observed in DED mice as well as inflammation in the cornea and TG. Overall, our study provides new insight into the effectiveness of TRPM8 blockade for alleviating corneal pain syndrome associated with severe DED, opening a new avenue for ocular pain management

Electrical match between initial segment and somatodendritic compartment for action potential backpropagation in retinal ganglion cells

International audienceThe action potential of most vertebrate neurons initiates in the axon initial segment (AIS) and is then transmitted to the soma where it is regenerated by somatodendritic sodium channels. For successful transmission, the AIS must produce a strong axial current, so as to depolarize the soma to the threshold for somatic regeneration. Theoretically, this axial current depends on AIS geometry and Na+ conductance density. We measured the axial current of mouse retinal ganglion cells using whole cell recordings with post hoc AIS labeling. We found that this current is large, implying high Na+ conductance density, and carries a charge that covaries with capacitance so as to depolarize the soma by ∼30 mV. Additionally, we observed that the axial current attenuates strongly with depolarization, consistent with sodium channel inactivation, but temporally broadens so as to preserve the transmitted charge. Thus, the AIS appears to be organized so as to reliably backpropagate the axonal action potential.NEW & NOTEWORTHY We measured the axial current produced at spike initiation by the axon initial segment of mouse retinal ganglion cells. We found that it is a large current, requiring high sodium channel conductance density, which covaries with cell capacitance so as to ensure a ∼30 mV depolarization. During sustained depolarization the current attenuated, but it broadened to preserve somatic depolarization. Thus, properties of the initial segment are adjusted to ensure backpropagation of the axonal action potential

An Overview of Current Alternative Models in the Context of Ocular Surface Toxicity

International audienceThe 21st century has seen a steadily increasing social awareness of animal suffering, with increased attention to ethical considerations. Developing new integrated approaches to testing and assessment (IATA) strategies is an Organisation for Economic Co-operation and Development (OECD) goal to reduce animal testing. Currently, there is a lack of alternative models to test for ocular surface toxicity (aside from irritation) in lieu of the Draize eye irritation test (OECD guideline No. 405) performed in rabbits. Five alternative in vitro or ex vivo methods have been validated to replace this reference test, but only in combination. However, pathologies like Toxicity-Induced Dry Eye (TIDE), cataract, glaucoma, and neuropathic pain can occur after exposure to a pharmaceutical product or chemical and therefore need to be anticipated. To do so, new models of lacrimal glands, lens, and neurons innervating epithelia are required. These models must take into account real-life exposure (dose, time, and tear film clearance). The scientific community is working hard to develop new, robust, alternative, in silico, and in vitro models, while attempting to balance ethics and availability of biological materials. This review provides a broad overview of the validated methods for analyzing ocular irritation and those still used by some industries, as well as promising models that need to be optimized according to the OECD. Finally, we give an overview of recently developed innovative models, which could become new tools in the evaluation of ocular surface toxicity within the scope of IATAs

Douleur oculaire : du fondamental à la clinique

International audienceChronic ocular pain is very disabling and difficult to treat. Its pathophysiological mechanisms are still poorly understood. The cornea is the most densely innervated tissue in the human body, with an innervation 600 and 40 times higher than that of the skin and dental pulp, respectively. A lesion of corneal nociceptive free nerve endings results in morphological and molecular changes of these free endings, of the corneal neurons located in the trigeminal ganglion but also in brainstem structures (trigeminal sensory complex) involved in corneal nociceptive transmission. Furthermore, various preclinical models of corneal nociception have demonstrated dysfunctions in these anatomical pathways, including peripheral sensitization (hyperactivity of corneal nerves at the level of the terminals and the trigeminal ganglion) and central sensitization (synaptic plasticity, hyperactivity of second-order neurons of the trigeminal sensory complex). Neuroimmune interactions known to shape the excitability of the peripheral and central nervous systems may be the cause of such alterations. Recently, pharmacological studies have identified and validated novel therapeutic targets in different preclinical models of corneal nociception, opening new therapeutic perspectives. In this review, data from basic research are then put into perspective with clinical data, illustrated in particular by the example of dry-eye related pain, a frequent disease encountered in ophthalmological practice. The tools currently used in the clinic to evaluate ocular pain, corneal inflammation and the morphological criteria commonly used to quantify abnormalities (density, tortuosity, neuroma) of corneal nerves will be described. The various therapeutic options for the patient with painful dry eye will also be discussed. In conclusion, research on ocular pain has accelerated in recent years, due to a closer collaboration between researchers and clinicians. The better understanding of chronic ocular pain and therefore improved diagnosis will allow the development of better analgesic strategies tailored to the profile of each patient

Shhedding New Light on the Role of Hedgehog Signaling in Corneal Wound Healing

The cornea, an anterior ocular tissue that notably serves to protect the eye from external insults and refract light, requires constant epithelium renewal and efficient healing following injury to maintain ocular homeostasis. Although several key cell populations and molecular pathways implicated in corneal wound healing have already been thoroughly investigated, insufficient/impaired or excessive corneal wound healing remains a major clinical issue in ophthalmology, and new avenues of research are still needed to further improve corneal wound healing. Because of its implication in numerous cellular/tissular homeostatic processes and oxidative stress, there is growing evidence of the role of Hedgehog signaling pathway in physiological and pathological corneal wound healing. Reviewing current scientific evidence, Hedgehog signaling and its effectors participate in corneal wound healing mainly at the level of the corneal and limbal epithelium, where Sonic Hedgehog-mediated signaling promotes limbal stem cell proliferation and corneal epithelial cell proliferation and migration following corneal injury. Hedgehog signaling could also participate in corneal epithelial barrier homeostasis and in pathological corneal healing such as corneal injury-related neovascularization. By gaining a better understanding of the role of this double-edged sword in physiological and pathological corneal wound healing, fascinating new research avenues and therapeutic strategies will undoubtedly emerge