Dopamine D2 receptor blockade differentially affects the light‑adapted turtle and frog electroretinogram.

The effects of dopamine D2-class receptor blockade by sulpiride on the electroretinographic (ERG) b-wave (ON response) and d-wave (OFF response) were investigated in light-adapted turtle (Trachemys scripta elegans) and frog (Rana ridibunda) eyecups. For turtle ERG, sulpiride (240 μM) produced an amplitude increase of the b- and d-waves, while the 40 μM and 120 μM of sulpiride were ineffective. Alternatively, for frog ERG, a well-developed and dose-dependent b- and d-wave amplitude decrease was obtained with 40 μM and 240 μM sulpiride. In both species, 240 μM sulpiride significantly affected the maximal voltage range of the ERG responses without altering their relative sensitivity (determined by the semi-saturation point). The absolute sensitivity of the ON and OFF responses (evaluated by threshold estimation) was not significantly altered for turtle ERG, but it was decreased for frog ERG. The time characteristics of the ERG responses were unchanged in both species. Our results show important differences between dopamine D2-class receptor-mediated pathways in turtle and frog retina (revealed by ERG).