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Nina Milosavljevic, Riccardo Storchi, Franck P Martial, Robert J Lucas; Chemogenetic manipulation of ipRGCs reveals a primary role for this ganglion cell class in the visual systems ability to track slow changes in background light intensity. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5563.
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© ARVO (1962-2015); The Authors (2016-present)
Melanopsin expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are thought to provide the retina’s ability to measure background light intensity (irradiance). In order to determine the extent to which this is true for the sorts of gradual but high amplitude changes in irradiance we experience every day, we developed a new chemogenetic method of specifically and acutely inhibiting ipRGC activity. Using this technology we were able to ask whether responses to slow irradiance ramps in the primary visual thalamus (dorsal Lateral Geniculate Nucleus; dLGN) of mice were driven by ipRGCs.
Melanopsin-Cre mice were intravitreally injected with a Cre-recombinase-dependent viral vector to express engineered inhibitory Designer Receptors Exclusively Activated by Designer Drugs (DREADD) receptors in ipRGCs. DREADD receptors, derived from a muscarinic Gi-coupled GPCR (hM4Di), lost the ability to bind to natural ligands but are potently activated by otherwise biologically inert drug, clozapine-N oxide (CNO) (Armbruster et al, 2007.PNAS). Expression of hM4Di was tested by immunohistochemistry staining. To test whether hM4Di receptors were able to suppress ipRGC activity, we assessed the impact of CNO on the pupil light reflex. These mice were then used for recording irradiance responses in the dLGN.
Immunohistochemistry of retinas revealed expression of hM4Di receptors restricted to ipRGCs. Pupillometry screen confirmed that CNO selectively inhibits ipRGCs in vivo as systemic administration of the drug reduced the magnitude of pupil constriction following light exposure of hM4Di treated eyes, but not control eyes. These mice that exhibited successful chemogenetic manipulation of ipRGCs were then used in electrophysiological recordings. Slow irradiance ramps induced widespread increases in firing across neurones in the dLGN. This was significantly disrupted by systemic administration of CNO in hM4Di mice, but not in the control mice that lacked hM4Di expression.
These data indicate that the ability of neurons in the dLGN to track slow changes in irradiance originate with ipRGCs, the small class of ganglion cell specialised for extracting such information. The chemogenetic manipulation that we employ is a useful tool for selectively modulating retinal circuitry.
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