Roles of Cone Photoreceptor Connexin-36 in Light Adaptation and Circadian Regulation of the Photopic ERG

Abstract

Purpose : Connexin-36 (Cx36) is a gap junction protein involved in electrical signaling between cone photoreceptors, between rods and cones, and among several types of inner retinal neurons. Cx36 in photoreceptors is regulated by light, dopamine, and circadian clocks, closing the gap junctions during the daytime. Signaling through rod-cone gap junctions is thought to provide an alternative pathway for rod signaling through the cone bipolar cells in dim light. We tested the hypothesis that Cx36 in mouse cones also plays a role in light adaptation and circadian regulation of the photopic ERG.

Methods : The expression in cones of Gjd2, the gene that encodes Cx36, was disrupted using the cre-lox system with the cone-specific cre-recombinase driver HRGP^cre. The circadian clock gene Bmal1 was disruped in neural retina using a Chx10-cre driver. Standard ERG and immunohistochemistry protocols were used.

Results : HRGP^cre;Gjd2^fl/flmice had a selective reduction in Cx36 protein in the outer plexiform layer. In control mice, circadian rhythms of b-wave amplitude were observed. The mice showed gradual increases in b-wave amplitude as a function of light adaptation time in either the day or the night, but the amplitudes were greater during the daytime. In contrast, HRGP^cre;Gjd2^fl/flmice showed dramatically higher b-wave amplitudes as soon as the background light was applied, no significant day-night difference, and no gradual increase in b-wave amplitudes with light-adaptation time; the b-wave amplitudes were comparable to those of daytime controls that were fully light adapted. These changes appeared to be driven by cones, as the mice had unusually large a-wave amplitudes. This contrasts with the responses of mice with retina-specific disruption of circadian clock, Chx10^cre;Bmal1^fl/flmice, which showed low b-wave amplitudes day and night, and no light-adaptation response.

Conclusions : Cx36 is required for the circadian rhythm in photopic ERG and mice lacking Cx36 in cones appear to be in a fully light-adapted state regardless of time of day. We hypothesize that in the absence of rod-cone gap junctions in Cx36 knockouts or during light adaptation in the controls (which closes gap junctions) the currents of the cones become isolated increasing synaptic strength to the cone ON bipolar cells.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.