Abstract
The mammalian visual system relies upon light detection by outer-retinal rod/cone photoreceptors and melanopsin-expressing retinal ganglion cells. Gnat1−/−;Cnga3−/−;Opn4−/− mice lack critical elements of each of these photoreceptive mechanisms via targeted disruption of genes encoding rod α transducin (Gnat1); the cone-specific α3 cyclic nucleotide gated channel subunit (Cnga3); and melanopsin (Opn4). Although assumed blind, we show here that these mice retain sufficiently widespread retinal photoreception to drive a reproducible flash electroretinogram (ERG). The threshold sensitivity of this ERG is similar to that of cone-based responses, however it is lost under light adapted conditions. Its spectral efficiency is consistent with that of rod opsin, but not cone opsins or melanopsin, indicating that it originates with light absorption by the rod pigment. The TKO light response survives intravitreal injection of U73122 (a phospholipase C antagonist), but is inhibited by a missense mutation of cone α transducin (Gnat2cpfl3), suggesting Gnat2-dependence. Visual responses in TKO mice extend beyond the retina to encompass the lateral margins of the lateral geniculate nucleus and components of the visual cortex. Our data thus suggest that a Gnat1-independent phototransduction mechanism downstream of rod opsin can support relatively widespread responses in the mammalian visual system. This anomalous rod opsin-based vision should be considered in experiments relying upon Gnat1 knockout to silence rod phototransduction.
Highlights
A well characterised signal transduction cascade links light absorption by rod and cone opsins to photoreceptor hyperpolarisation[1]
We found that a single curve was appropriate for the two wavelengths when normalized for rod (p.0.05) but not either melanopsin (p,0.0001) or medium wavelength sensitive (MWS) cone opsins (p,0.01)
An alternative explanation for the reduction in b-wave amplitude is that U73122 targets some aspect of the ON bipolar cell response. In support of this possibility we found that physiological concentrations of YM298198, an antagonist of the Gaq phospholipase C (PLC)-coupled glutamate receptor mGluR1[14], reduced the TKO b-wave by a similar amount (Fig. 4A & B)
Summary
A well characterised signal transduction cascade links light absorption by rod and cone opsins to photoreceptor hyperpolarisation[1]. Two such transgenic mice (Gnat12/2 which lacks Gnat, the rod specific a transducin[2]; and Cnga32/2, lacking a cone photoreceptor-specific cyclic nucleotide channel subunit[3]), were crossed with a third lacking the melanopsin (Opn4) gene (responsible for inner retinal phototransduction [4]). As each of these knockouts had been shown previously to abolish phototransduction in the target photoreceptor class, it was assumed that combining them to produce Gnat12/2;Cnga32/2;Opn42/2 (herinafter TKO) mice would result in mice lacking functional photoreception[5]. These mice did lack several visual behaviours that are retained in the absence of rods and cones (circadian photoentrainment and photic suppression of wheel running activity), leading to the conclusion that together rods, cones and melanopsin account for all light detection in the mammalian retina[5]
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