Amplifying and ameliorating light avoidance in mice with photoreceptor targeting and CGRP sensitization
Objective: To determine the photoreceptor basis of light avoidance in mice and assess the effect of CGRP sensitization on this behavior. Background: Prior studies have suggested that photophobia is mediated by a subset of retinal ganglion cells (RGCs) that contain melanopsin, making them intrinsically photosensitive (ipRGCs). These cells also receive extrinsic input from cones, which can also mediate light sensitivity. Here, we examined whether spectral variation targeting melanopsin or specific cone types in mice could effectively model light sensitivity. Also, we assessed whether sensitizing mice with calcitonin gene-related peptide (CGRP) could amplify ipRGC-mediated light avoidance. Methods: Light avoidance behavior was observed in a two-zone chamber illuminated by narrow-band LEDs targeting photopic opsins: 365 nm (UV; rodent S-cone), 460 nm (blue; melanopsin), and 630 nm (red; human L-cone). In a non-targeted assay, we assessed the degree of light avoidance in wildtype C57BL/6J mice to varying intensities (5 to 100%) of the blue and red LEDs. In a targeted assay, mice were given a choice to spend time between zones with differing relative contrast levels (0.50, 0.75, or 1.00) for the targeted photoreceptor(s). This was assessed in two transgenic mice with: 1) human red cone knock-in (RCKI), or 2) adult-onset ablation of M1 ipRGCs (Opn4aDTA). Mice were studied without intervention or following priming with either peripheral CGRP or vehicle administration every other day for 9 days. A primary measure (mean {+/-} SEM) was the asymptote value (AV). Results: Wildtype mice showed greater light avoidance with increasing light intensity, demonstrating a parametric response. RCKI mice showed avoidance of the high melanopsin (1.00: 0.52 {+/-} 0.08; n = 18) and L-cone (1.00: 0.30 {+/-} 0.11; n = 15) contrast zones but showed a preference for the higher S-cone (1.00: -0.35 {+/-} 0.06; n = 16) contrast zone. These effects decreased with less relative contrast and, thus, contrast dependent. Adding S-cone contrast opposed avoidance to melanopsin (0.10 {+/-} 0.12; n = 14) or L-cone (-0.19 {+/-} 0.10; n = 15) contrast. Ablation of ipRGCs in Opn4aDTA mice attenuated avoidance of melanopsin and preference for S-cone stimulation compared to control littermates. On day 9, CGRP priming led to significantly increased avoidance of melanopsin stimulation (0.58 {+/-} 0.08, n = 21) as compared to vehicle priming (0.26 {+/-} 0.09, n = 22) (F (1,41) = 5.70, p = 0.02). Conclusions: Our findings further support that ipRGCs play a key role in mediating photophobia. This aversive response to light stems from ipRGCs combining excitatory input from intrinsic melanopsin stimulation and extrinsic L-cone input, which can be opposed by extrinsic inhibitory S-cone input. Chronic exposure to CGRP is likely one of many mechanisms in migraine that can amplify ipRGC signals, leading to photophobia.