Synaptotagmin-9 in mouse retina

Synaptotagmin-9 (Syt9) is a Ca²⁺ sensor mediating fast synaptic release expressed in various parts of the brain. The presence and role of Syt9 in retina is unknown. We found evidence for Syt9 expression throughout the retina and created mice to conditionally eliminate Syt9 in a cre-dependent manner. We crossed Syt9^fl/fl mice with Rho-iCre, HRGP-Cre, and CMV-Cre mice to generate mice in which Syt9 was eliminated from rods (rod^Syt9CKO), cones (cone^Syt9CKO), or whole animals (CMV^Syt9). CMV^Syt9 mice showed an increase in scotopic electroretinogram (ERG) b-waves evoked by bright flashes with no change in a-waves. Cone-driven photopic ERG b-waves were not significantly different in CMV^Syt9 knockout mice and selective elimination of Syt9 from cones had no effect on ERGs. However, selective elimination from rods decreased scotopic and photopic b-waves as well as oscillatory potentials. These changes occurred only with bright flashes where cone responses contribute. Synaptic release was measured in individual rods by recording anion currents activated by glutamate binding to presynaptic glutamate transporters. Loss of Syt9 from rods had no effect on spontaneous or depolarization-evoked release. Our data show that Syt9 acts at multiple sites in the retina and suggest that it may play a role in regulating transmission of cone signals by rods.