Amplifying and ameliorating light avoidance in mice with photoreceptor targeting and calcitonin gene-related peptide sensitization.

OBJECTIVE

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.

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 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.

METHODS

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.

RESULTS

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).

CONCLUSIONS

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.