Direct Rod Input to Cone BCs and Direct Cone Input to Rod BCs Challenge the Traditional View of Mammalian BC Circuitry

Bipolar cells are the central neurons of the retina that transmit visual signals from rod and cone photoreceptors to third-order neurons in the inner retina and the brain. A dogma set forth by early anatomical studies is that bipolar cells in mammalian retinas receive segregated rod/cone synaptic inputs (either from rods or from cones), and here, we present evidence that challenges this traditional view. By analyzing light-evoked cation currents from morphologically identified depolarizing bipolar cells (DBCs) in the wild-type and three pathway-specific knockout mice (rod transducin knockout $[Tr\alpha ^{ - / - } ]$ , connexin36 knockout $[Cx36^{ - / - } ]$ , and transcription factor beta4 knockout $[Bhlhb4^{ - / - } ]$ ), we show that a subpopulation of rod DBCs $(DBC_{r2} s)$ receives substantial input directly from cones and a subpopulation of cone $DBCs(DBC_{c1} s)$ receives substantial input directly from rods. These results provide evidence of the existence of functional $rod - DBC_c $ and $cone - DBC_R $ synaptic pathways in the mouse retina as well as the previously proposed rod hyperpolarizing bipolar-cells pathway. This is grounds for revising the mammalian rod/cone bipolar cell dogma.