Fig. 2. Diversity of morphology and intrinsic light responses of ganglion cells GFP in the Opn4^Cre/+; Z/EG mouse.

(A–C) Intracellular dye filling of representative examples of three subtypes of ipRGCs targeted by their GFP fluorescence in vitro, an M1 cell (A), an M2 cell (B), and an M4 cell (C). All three of these cells were intrinsically photosensitive, as shown by the whole cell voltage clamp recordings below (J, K, L), obtained during pharmacological blockade of retinal synapses. Light pulse was 20 seconds. Each trace in a given panel shows the response to a different light intensity. Values at left are the number of log units of attenuation in stimulus intensity from the maximal (“0 log”; 2.3×10¹³ photons cm⁻² s⁻¹). Light-evoked currents were much larger in the M1 cell (J) than in the M2 cell (K) or M4 cell (L); they also returned more quickly to baseline after the stimulus. Fast downward deflections are presumed action currents resulting from incomplete voltage clamp. (D–I) Immunofluorescence (dotted circle) for melanopsin (E, G, and I) and Lucifer yellow injected cells (D, F, and H) show that M1 and M2 cells that are used for recording are melanopsin positive whereas the M4 cell (C, H, I and L) despite showing an intrinsic photoresponse lacked detectable melanopsin immunofluorescence. Note that this figure is optimized to highlight the recorded cells and hence some melanopsin positive cells appear to lack GFP labeling (more than 95% of melanopsin positive cells express GFP). Top trace in J slightly retouched to eliminate electrical artifact from series resistance test conducted well after the light response. Scale bars in A–C, 100 μm; D–I, 20 μm.