# Amplifying and ameliorating light avoidance in mice with photoreceptor targeting and calcitonin gene‐related peptide sensitization

## TL;DR

The study shows that specific eye cells cause mice to avoid certain lights, and a migraine-related chemical increases this avoidance, possibly explaining light sensitivity in migraines.

## Contribution

The study identifies ipRGCs as key in light avoidance and shows CGRP amplifies this behavior in mice.

## Key findings

- Mice avoid blue and red light but prefer UV light due to ipRGC signaling.
- CGRP administration increases light avoidance in mice, mimicking migraine-related photophobia.
- Ablation of ipRGCs eliminates light avoidance and UV preference in mice.

## Abstract

The aim of this study was to determine the photoreceptor basis of light avoidance in mice and assess the effect of CGRP sensitization on this behavior.

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 how spectral targeting of melanopsin or specific cone types in mice produces light avoidance and whether sensitizing mice with calcitonin gene‐related peptide (CGRP) amplifies the avoidance response to ipRGC stimulation.

Light avoidance behavior was measured in a two‐zone chamber illuminated by narrow‐band light‐emitting diodes (LEDs) targeting photopic opsins: 365 nm (ultraviolet [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 wild‐type (WT) C57BL/6J mice to varying contrasts (0.05 to 1.00) of the blue and red LEDs. In a targeted assay, mice were exposed to zones with differing relative contrast levels (0.50, 0.75, or 1.00) for the targeted photoreceptor(s). This was assessed in transgenic mice with: (1) human L‐cone cone knock‐in (HLCKI) or (2) adult‐onset ablation of M1 ipRGCs (Opn4aDTA
). Mice were studied without intervention or following chronic intermittent administration of CGRP with either peripheral CGRP or vehicle (Veh) administration every‐other‐day for 9 days. A primary measure (mean +/− SEM) was the asymptote value (AV) of chamber preference.

WT mice showed greater light avoidance with increasing light contrast. HLCKI mice avoided zones with high melanopsin (1.00: 0.52 ± 0.08; n = 18) and L‐cone (1.00: 0.30 ± 0.11; n = 15) stimulation but showed a preference for the zone with higher S‐cone (1.00: −0.35 ± 0.06; n = 16) stimulation. These effects were contrast‐dependent. The addition of S‐cone stimulation reduced the aversive effect of melanopsin (0.10 ± 0.12; n = 14) or L‐cone (−0.19 ± 0.10; n = 15) contrast. Ablation of ipRGCs in HLCKI x Opn4aDTA
mice eliminated both avoidance of melanopsin stimulation and the preference for S‐cone stimulation, as compared to controls. Nine days of chronic intermittent administration of CGRP led to significantly increased avoidance of melanopsin stimulation (0.58 ± 0.08, n = 21) as compared to Veh administration (0.26 ± 0.09, n = 22) (F (1, 41) = 5.70, p = 0.022).

Our findings support a key role for the ipRGCs in the production of photophobia. This aversive response to light stems from integrated ipRGC signals that combine excitatory intrinsic melanopsin and extrinsic L‐cone inputs and are opposed by extrinsic, inhibitory S‐cone input. Chronic elevation of CGRP levels in migraine may amplify ipRGC signals, leading to photophobia.

To better understand light sensitivity, a common symptom associated with migraine, we studied which cells in the eye produce the signals that cause mice to avoid light. We found that mice avoided blue and red light but preferred ultraviolet (UV) light, which is a result of a special cell type (intrinsically photosensitive retinal ganglion cells [ipRGCs]) in the eye that processes these light signals. We also found that mice who were repeatedly exposed to calcitonin gene‐related peptide (CGRP), a key nervous system messenger in migraine, increased their avoidance of blue light, which may model what happens in people with chronic migraine who experience light sensitivity.