purpose. To assess the structural changes in the retina caused by a functional blockade of rods and cones and to document the time course of their degeneration.

methods. Double knockout mice were generated by cross-breeding CNGA3−/− mice with Rho−/− mice. Retinas of mutant and wild-type mice from 3 weeks up to 12 months of age were studied by confocal light and electron microscopy. The retinas were immunostained with cell-type–specific markers and with antibodies against synapse-associated proteins and transmitter receptors.

results. In 3-week-old CNGA3−/− Rho−/− mice, retinal layers showed normal structural organization, and photoreceptors established normal synaptic contacts. Until postnatal week (Pw) 7, presynaptic markers and postsynaptic glutamate receptors were well expressed at the photoreceptor terminals. Photoreceptor degeneration started at Pw4, progressing to an almost complete loss by 3 months. Rod spherules showed an increase in the number of synaptic ribbons and postsynaptic elements during this early stage of degeneration, and horizontal cell processes grew into the outer nuclear layer. At later stages of retinal degeneration, the inner plexiform layer (IPL) was also affected. Rod bipolar cell axon terminals showed morphologic alterations, but the stratification pattern of cone bipolar cell axons and amacrine cell processes appeared unaffected. Transmitter receptors (GlyRα3, GABA A α2, GluR2/3) showed no obvious changes in the distribution and density of their synaptic clusters throughout the IPL at postnatal month 12.

conclusions. The normal structural and synaptic organization of the mutant retina at Pw3 suggests that photoreceptor light responses are not essential for the development of the retinal circuitry. However, functional photoreceptors are necessary for the maintenance of rods and cones and their contacts in the OPL, because they degenerate almost completely by 3 months after birth. Degenerative changes can also be observed in the IPL; however, they appear to have a slower time course and by 12 months of age the IPL circuitry appears to be surprisingly intact.