Dopamine D1 receptor modulation of calcium channel currents in horizontal cells of mouse retina

Horizontal cells form the first laterally interacting network of inhibitory interneurons in the retina. Dopamine, released onto horizontal cells under photic and circadian control modulates horizontal cell function. Using isolated, identified horizontal cells from a connexin-57-iCre x ROSA26-tdTomato transgenic mouse line, we investigated dopaminergic modulation of calcium channel currents (I_Ca) with whole cell patch-clamp techniques. Dopamine (10 µM) blocked 27% of steady-state I_Ca, an action blunted to 9% in the presence of the L-type Ca channel blocker verapamil (50 µM). The dopamine type 1 receptor (D1R) agonist SKF38393 (20 µM) inhibited I_Ca by 24%. The D1R antagonist SCH23390 (20 µM) reduced DA and SKF38393 inhibition. Dopamine slowed I_Ca activation, blocking I_Ca 38% early in a voltage step. Enhanced early inhibition of I_Ca was eliminated by applying voltage prepulses to +120 mV for 100 ms, increasing I_Ca by 31% and 11% for early and steady state currents, respectively. Voltage-dependent facilitation of I_Ca and block of DA inhibition after preincubation with a Gβ blocking peptide, suggested involvement of Gβ proteins in the D1R mediated modulation. When the G-protein activator GTPS was added intracellularly, I_Ca was smaller and showed the same slowed kinetics seen during D1R activation. With GTPS in the pipette, additional block of I_Ca by dopamine was only 6%. Strong depolarizing voltage prepulses restored the GTPS-reduced early I_Ca amplitude by 36% and the steady-state I_Ca amplitude by 3%. These results suggest that dopaminergic inhibition of I_Ca via D1Rs is primarily mediated through the action of Gβ proteins in horizontal cells.

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