Genetic Dissection of Rod and Cone Synaptic Inputs to Light Responses of Retinal Ganglion Cells in the Mouse

Abstract

Purpose: : To understand the relative contributions of the various rod and cone pathways to light responses of retinal ganglion cells (RGCs) in the mammalian retina.

Methods: : Multi-electrode Array (MEA) was used to record light responses from dark-adapted RGCs in wild type (C57/Bl6) mice, connexin36 (Cx36) KOmice, Bhlhb4 KOmice (which have greatly diminished numbers of rod bipolar cells (DBCrs)) and rod transducin_α KO (Tr_α) KO mice. Flashes included a wide range of stimulus intensities, from 10^-2 Rh*/rod to 10⁵ Rh*/rod.

Results: : (1) In wild type mice, we found evidence of four major classes of ON responses. The first is highly sensitive with a narrow dynamic range (Type I). The operating range corresponds to the operating ranges of our previously recorded excitatory currents of wild type DBCrs. The second is less sensitive and also has a narrow dynamic range (Type II). This operating range corresponds to our previously recorded dip in the ERG b-wave in the Cx36KOmice, which we attribute to loss of rod-cone coupling. The third type also has a narrow dynamic range, but is even less sensitive (Type III). This operating range corresponds to what we have attributed to the cone pathway. The fourth type of ON response has a very wide dynamic range (> 4 log units), spanning the operating ranges of the previously mentioned RGCs (Type IV). (2) In the Tr_α KO mice, we observed only the Type III ON response, consistent with the fact that these mice should have no rod pathway responses. (3) In the Bhlhb4 KOmice, we recorded Type III and Type IV responses, consistent with the fact that these mice should have rod pathways independent of the DBCr, either through rod-cone coupling (2° rod pathway), or perhaps, direct rod input to DBCcs (tertiary rod pathway). (4) In the Cx36 KO mice, we recorded Type III and Type IV responses, further supporting anatomical evidence of a tertiary rod pathway.

Conclusions: : Our data indicate that some RGCs receive segregated inputs from the various rod and cone pathways (corresponding to our Type I, II and III responses), while other RGCs receive integrated inputs from these pathways (Type IV responses). Using pathway-specific knockouts, we have shown physiological evidence for a tertiary rod pathway - from the rods to the DBCcs, in turn to the RGCs, which has not been characterized in previous MEA or single unit recordings.

Keywords: ganglion cells • retinal connections, networks, circuitry • gap junctions/coupling