(A) The cellular circuitry underlying the rod/cone and mRGC contribution to visual responses. Thickness of the filled arrows roughly highlights the relative strength of information flow. (B) Strategy to fluorescently label mRGCs by breeding a mouse carrying a Cre recombinase “knocked-in” to the melanopsin promoter to Z/EG mouse that allows Cre-dependent expression of GFP from chicken beta-actin promoter. (C) Strategy to achieve inducible and specific ablation of mRGCs. Opn4^Cre/+ mouse was bred to a mouse expressing Cre-dependent expression of simian diphtheria toxin receptor. The resulting progeny develop normal mRGCs expressing DTR, which allows specific ablation of these cells by DT administration. Schematic of two targeting vectors used to achieve inducible mRGC ablation are shown in (D). The Cre knock-in cassette for targeted insertion to melanopsin locus also carried coding sequences for CRE dependent expression of βTau-eYFP. However, fluorescence from βTau:eYFP was undetectable in retina from Opn4^Cre/+ mice (data not shown). The targeting vector and generation of R26^iDTR/+ mice are described in [20]. A schematic of the targeting vector is shown here.