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 dopami-nergic modulation of calcium channel currents (I-Ca) with whole cell patch-clamp techniques. Dopamine (10 mu M) blocked 27% of steady-state I-Ca, an action blunted to 9% in the presence of the L-type Ca channel blocker verapamil (50 mu M). The dopamine type 1 receptor (D1R) agonist SKF38393 (20 mu M) inhibited I-Ca by 24%. The D1R antagonist SCH23390 (20 mu M) reduced dopamine and SKF38393 inhibition. Dopamine slowed I-Ca activation, blocking I-Ca by 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 dopamine inhibition after preincubation with a G beta gamma-blocking peptide suggested involvement of G beta gamma-proteins in the D1R-mediated modulation. When the G protein activator guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) was added intracellularly, I-Ca was smaller and showed the same slowed kinetics seen during D1R activation. With GTP gamma S in the pipette, additional block of I-Ca by dopamine was only 6%. Strong depolarizing voltage prepulses restored the GTP gamma S-reduced early I-Ca amplitude by 36% and 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 G beta gamma proteins in horizontal cells.