Melanopsin expression in the cornea

Abstract

A unique class of intrinsically photosensitive retinal ganglion cells in mammalian retinae has been recently discovered and characterized. These neurons can generate visual signals in the absence of inputs from rods and cones, the conventional photoreceptors in the visual system. These light sensitive ganglion cells (mRGCs) express the non-rod, non-cone photopigment melanopsin and play well documented roles in modulating pupil responses to light, photoentrainment of circadian rhythms, mood, sleep and other adaptive light functions. While most research efforts in mammals have focused on mRGCs in retina, recent studies reveal that melanopsin is expressed in non-retinal tissues. For example, light-evoked melanopsin activation in extra retinal tissue regulates pupil constriction in the iris and vasodilation in the vasculature of the heart and tail. As another example of nonretinal melanopsin expression we report here the previously unrecognized localization of this photopigment in nerve fibers within the cornea. Surprisingly, we were unable to detect light responses in the melanopsin-expressing corneal fibers in spite of our histological evidence based on genetically driven markers and antibody staining. We tested further for melanopsin localization in cell bodies of the trigeminal ganglia (TG), the principal nuclei of the peripheral nervous system that project sensory fibers to the cornea, and found expression of melanopsin mRNA ina subset of TG neurons. However, neither electrophysiological recordings nor calcium imaging revealed any light responsiveness in the melanopsin positive TG neurons. Given that we found no light-evoked activation of melanopsin-expressing fibers in cornea or in cell bodies in the TG, we propose that melanopsin protein might serve other sensory functions in the cornea. One justification for this idea is that melanopsin expressed in Drosophila photoreceptors can serve as a temperature sensor.

Blue light exposure in vitro causes toxicity to trigeminal neurons and glia through increased superoxide and hydrogen peroxide generation

“…Thus, we checked for the presence of these non-visual opsins in neural trigeminal culture. By means of immunochemistry, we observed the presence of melanopsin in neuronal cell bodies, in line with Delwig et al [54]. Only some neurons exhibited melanopsin immunoreactivity, in agreement with Matynia et al, according to whom melanopsin is expressed overall in approximately 3% of TG neurons [26].…”

“…It has been recently reported that small number of neurons in the ophthalmic branch of trigeminal ganglia express melanopsin (opn4) and are intrinsically photosensitive [26]. Moreover, another recent study discovered the previously unrecognized localization of this photopigment in nerve fibers within the cornea [54]. Another non-visual photoreceptor, gaining today an increasing attention, is neuropsin (opn5).…”

Melatonin and the control of intraocular pressure

“…Recently, melanopsin presence in the cornea and the crystalline lens was identified (Alkozi et al, 2017d;Delwig et al, 2018). Studies using human crystalline epithelial cells showed that different light conditions were able to produce changes in melatonin levels in the extracellular medium, in fact, blue light activated melanopsin protein leading to melatonin reduction in comparison to cells incubated under constant darkness or red light.…”

Visual Image Quality Impacts Circadian Rhythm-Related Gene Expression in Retina and in Choroid: A Potential Mechanism for Ametropias

“…42,43 Whether light activates melanopsin-expressing trigeminal neurons is equivocal. 41,44 Other potential sources are choroidal blood vessels or melanocytes as melanopsin has been identified in mouse aorta and in cultured chicken melanocytes. 45,46 Regarding potential function, the normal light-induced increase in choroidal thickness in mice is not observed with systemic knockout of melanopsin, 47 suggesting that melanopsin might regulate diurnal or defocus-induced choroidal thickness alterations.…”