PROJECT SUMMARY/ABSTRACT The overall objective of this new research project is to understand detailed synaptic mechanisms underlying rod and cone signaling pathways in the mammalian retina. Anatomical studies have suggested that rod and cone channels in the mammalian retina follow a specific set of circuitry rules: rods make synapses only on rod depolarizing bipolar cells (DBCRs), and DBCRs do not make output synapses directly on ganglion cells (GCs), but indirectly through AII amacrine cells (AIIACs). AIIACs send rod-mediated signals to ON and OFF GCs by piggybacking on the cone depolarizing and hyperpolarizing bipolar cells (DBCCs and HBCCs), which synapse on ON and OFF GCs, respectively. In this research proposal, we plan to systematically investigate these mammalian-specific synaptic circuits by using the mouse retina as a model system. In addition to using whole-cell voltage clamp (with dye-filling), anatomical and pharmacological techniques, we will take advantage of four strains of pathway-specific mutant mice to elucidate how mammalian rod and cone signals are transmitted to BCs, ACs and GCs, and to determine whether the mammalian-specific circuitry rules are totally valid, as recent evidence and our preliminary results suggest that the strict rod/cone input rules may not hold for all mammalian retinal neurons. We will test the overall hypothesis that subpopulations of mouse DBCRs receive direct synaptic inputs from cones and subpopulations of mouse DBCCs and HBCCs receive direct synaptic inputs from rods, the rod- and cone-mediated signals are further mixed by AIIACs before sending to GCs, and the AC-mediated inhibitory synaptic inputs to BCs and GCs carry mixed rod/cone signals. This application has 3 specific aims focused on studying rod and cone contributions to light-evoked cation and chloride currents (IC and ICl, representing glutamatergic and GABAergic/glycinergic synaptic inputs, respectively) in: (1) seven types of bipolar cells (HBCMC/Rs, HBCMCs, HBCSCs, DBCC2/MCs, DBCC1/MC/Rs, DBCR2s, DBCR1s); (2) AII amacrine cells; and (3) three types of alpha ganglion cells (ON, sOFF and tOFF GCs) in the dark-adapted mouse retina. Results obtained will increase our understanding of how parallel channels, such as the rod/cone signaling pathways, process, segregate and integrate information in the eye and in the brain. Since many visual disorders are associated with abnormalities in the rod and cone signaling pathways, this research project will help to identify cellular and synaptic sites responsible for the pathogenesis of these eye diseases.