Meclofenamic acid improves the signal to noise ratio for visual responses produced by ectopic expression of human rod opsin

Purpose: Retinal dystrophy through outer photoreceptor cell death affects 1 in 2,500 people worldwide with severe impairment of vision in advanced stages of the disease. Optogenetic strategies to restore visual function to animal models of retinal degeneration by introducing photopigments to neurons spared degeneration in the inner retina have been explored, with variable degrees of success. It has recently been shown that the non-steroidal anti-inflammatory and non-selective gap-junction blocker meclofenamic acid (MFA) can enhance the visual responses produced by an optogenetic actuator (channelrhodopsin) expressed in retinal ganglion cells (RGCs) in the degenerate retina. Here, we set out to determine whether MFA could also enhance photoreception by another optogenetic strategy in which ectopic human rod opsin is expressed in ON bipolar cells.

Methods: We used in vitro multielectrode array (MEA) recordings to characterize the LIGHT responses of RGCs in the rd¹ mouse model of advanced retinal degeneration following intravitreal injection of an adenoassociated virus (AAV2) driving the expression of human rod opsin under a minimal grm6 promoter active in ON bipolar cells.

Results: We found treated retinas were LIGHT responsive over five decades of irradiance (from 10¹¹ to 10¹⁵ photons/cm²/s) with individual RGCs covering up to four decades. Application of MFA reduced the spontaneous firing rate of the visually responsive neurons under light- and dark-adapted conditions. The change in the firing rate produced by the 2 s LIGHT pulses was increased across all intensities following MFA treatment, and there was a concomitant increase in the signal to noise ratio for the visual response. Restored LIGHT responses were abolished by agents inhibiting glutamatergic or gamma-aminobutyric acid (GABA)ergic signaling in the MFA-treated preparation.

Conclusions: These results confirm the potential of MFA to inhibit spontaneous activity and enhance the signal to noise ratio of visual responses in optogenetic therapies to restore sight.