Analysis of Feedback Signaling from Horizontal Cells to Photoreceptors in Mice

Genetic manipulation of horizontal cells using a Connexin57-iCre mouse (Cx57-iCre) line combined with calcium imaging is proving to be a valuable method to study horizontal cell feedback inhibition onto photoreceptor terminals. While it is accepted that horizontal cells provide lateral inhibitory feedback to photoreceptors, the cellular mechanisms that underlie this feedback inhibition remain only partially elucidated. Feedback inhibition of photoreceptors acts via modulation of their voltage-gated calcium channels at their synaptic terminal. Calcium imaging of photoreceptors in retinal slices, therefore, reflects the impact of inhibitory feedback from horizontal cells. The development of a Cx57-iCre mouse line permits genetic manipulation of horizontal cells. In wild-type mouse retina, depolarization of horizontal cells by kainate provokes a decrease in photoreceptor Ca²⁺_i, whereas hyperpolarization by NBQX elicits an increase in photoreceptor Ca²⁺_i. These responses indicate increased feedback inhibition occurred when horizontal cells are depolarized, and decreased feedback inhibition, when hyperpolarized. This system was used to test the role of GABA release from horizontal cells in feedback inhibition by the selective elimination of VGAT/VIAAT, the inhibitory amino acid transmitter transporter that loads GABA into the synaptic vesicles of horizontal cells. Combined with calcium imaging of photoreceptors in retinal slices, the knockout of specific proteins, e.g., VGAT, provides a robust technique to test the role of GABA in feedback inhibition by horizontal cells.