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Maureen E. Estevez, Patricia M. Fogerson, Marissa C. Ilardi, David M. Berson; Identity and Receptive Field Properties of Melanopsin Ganglion Cells Innervating the Mouse dLGN. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4344.
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© ARVO (1962-2015); The Authors (2016-present)
Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) were originally linked only to reflexive, homeostatic, non-image forming functions, such as circadian photoentrainment and the pupillary light reflex. However, more sensitive methods now reveal five ipRGC types (M1 through M5), collectively projecting to the dorsal lateral geniculate nucleus (dLGN) and superior colliculus, key image-forming nuclei. This has suggested a role for ipRGCs in pattern vision. We sought to determine which types of mouse ipRGCs project to the dLGN and to characterize their receptive field properties.
We used adult Opn4cre/+;Z/EG mice, which express green fluorescent protein (GFP) in M1-M5 ipRGCs. We deposited retrograde tracer (Alexa594-tagged choleratoxin beta subunit; 'CTB') into the dLGN, avoiding the intergeniculate leaflet and ventral LGN. The contralateral retina was isolated and cells co-labeled with GFP and CTB were dye-filled by intracellular injection of Lucifer Yellow or recorded by whole cell voltage clamp in vitro. Receptive fields were probed with diffuse, spectrally narrowband stimuli and computer-generated spots, annuli, and gratings under conditions strongly favoring cone-driven responses.
ipRGCs of types M2, M4 and M5 all projected to the dLGN (n = 6 brains). M5 cells had small receptive fields, as expected from their small, bushy dendritic fields. They exhibited chromatic opponency (short wavelength ON, long wavelength OFF) and strong surround antagonism. M2 and M4 cells had larger receptive fields, moderate surround antagonism, and lacked chromatic opponency. M1 cells were rarely retrolabeled by selective dLGN injections; they lacked obvious surround antagonism. All of these ipRGC types lacked direction-selectivity.
There are at least three channels of ipRGC signaling to the dLGN. Each blends intrinsic melanopsin photoresponses with processed synaptic influence from the outer retina. M5 cells provide a high-resolution chromatic signal reminiscent of primate midget or small-bistratified cells. M2 and M4 cells both contribute a coarser, spectrally broadband signal; these two types do not co-stratify, hinting at functional distinctions as yet unknown.
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