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Katherine Elizabeth Davis, Annette E Allen, Robert J Lucas, Neuroscience; Melanopsin-Derived Signals in the dLGN of the Light-Adapted Mouse. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5766.
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© ARVO (1962-2015); The Authors (2016-present)
A subset of melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) project directly to the dorsal lateral geniculate nucleus (dLGN). As ipRGCs are generally considered to be global irradiance detectors, the significance for primary vision remains unclear. To determine the extent to which they contribute to tracking changes in luminance over time and space, we set out to define the sensory characteristics of melanopsin-derived responses under light-adapted conditions.
To examine melanopsin-derived signals in vivo, a multi electrode array was lowered into the dLGN of the urethane-anaesthatised mouse. Extracellular spiking activity was recorded in response to stimulation of the contralateral eye to full field stimuli. To fully isolate the contribution of melanopsin, we used transgenic mice lacking all cone activity (Cnga3 -/-). LEDS generated blue and yellow stimuli that were isoluminant for rods. By switching from yellow to blue for 30s we were able to present a 5-6 fold increase in effective light intensity for melanopsin (75% Mickelson contrast) that was silent for rods. This stimulus was presented over a 5 log unit range of background light intensity.
Approximately 15 % of light responsive dLGN neurons responded to the melanopsin-isolating stimulus with increased firing at irradiances >1.2 x 10^13 melanopic photons/cm2/s (equivalent to mid-photopic illuminance). Latency to peak was long compared to rod/cone derived responses; however, at <1 sec it was faster than those traditionally associated with ipRGCs and/or those recorded in rodless+coneless mice. A second set (17%) of dLGN neurons showed irradiance dependent increases in background firing (a feature associated with ipRGCs), however these did not show a detectable response to the melanopsin step.
We provide the first description of melanopsin-derived responses in the dLGN under light-adapted conditions and in the presence of outer-retinal photoreception. We find cells that encode irradiance but do not track transient changes in melanopsin excitation while a separate population use melanopsin to encode such higher frequency events. The relatively fast kinetics of the latter group allows the possibility that melanopsin contributes information to spatial structure, identifying an alternative/complimentary route to the known rod and cone infrastructure in signalling structure in the primary visual system.
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