Different effects of low Ca 2+ on signal transmission from rods and cones to bipolar cells in carp retina

Abstract

Modulation of signal transmission from rods, red-sensitive (R-) and green-sensitive (G-) cones to bipolar cells by lowering extracellular Ca 2+ was studied in the isolated superfused carp retina using intracellular recording techniques. Low Ca 2+ (nominally Ca 2+-free) potentiated light responses of rod dominant ON bipolar cells (rod-ON-BCs). On the other hand, responses of cone dominant ON bipolar cells (cone-ON-BCs) driven by G-cones were dramatically decreased whereas those driven by R-cones were hardly changed in low Ca 2+. Similar effects were observed in scotopic and photopic electroretinographic (ERG) b waves, which reflect the activities of ON-BCs driven by rods and cones, respectively. IBMX (100 μM), an inhibitor of PDE, whose effects mimic those of low Ca 2+ on phototransduction, increased responses of both rod-ON-BCs and cone-ON-BCs, suggesting that the distinct effects of low Ca 2+ described above are attributable to differential modulation of signal transfer from different types of photoreceptors to BCs. Moreover, scotopic ERG P III responses, reflecting the rod activity, were potentiated both in low Ca 2+ and in the presence of IBMX (100 μM). Low Ca 2+ causes multiple changes in the outer retina, including increase of glutamate release from the photoreceptor terminal, increase of current and voltage responses of photoreceptors to light, alteration of the synaptic gain from photoreceptors to BCs and modulation of mGluR6 pathway in the rod-ON-BCs. Interplay of these changes may account for differential modulation of R-cone and G-cone driven BC responses, as well as the different effects on rod- and cone-ON-BCs.