Effects of retinal dopamine depletion on the rabbit electroretinogram

This study evaluated the effects of intravitreal injections of 300 micrograms of 6-hydroxydopamine (6-OHDA) on electroretinogram (ERG) amplitudes and implicit response times of adult pigmented rabbits. One eye was injected intravitreally with 300 micrograms 6-OHDA and 600 micrograms ascorbic acid in a 0.3 ml 0.9% NaCl solution; the fellow eye received a similar solution containing only 600 micrograms ascorbic acid. Following this treatment ERG recordings were performed at 1, 4, and 7 days. After the last recordings, animals were sacrificed and retinas were isolated for biochemical analyses. Significant and progressive reductions in retinal concentrations of dopamine (DA) and its main metabolites homovanillic acid (HVA), and dihydroxyphenylacetic acid (DOPAC) were found in treated retinas. Concentrations of norepinephrine (NE), serotonin (5HT), and 5 hydroxyindoleacetic acid (5-HIAA) were not affected, thus demonstrating the specific neurotoxic action of 6-OHDA on retinal dopaminergic neurons. Concurrently, significant increases in ERG a- and b-wave amplitudes as well as in implicit response times were observed. These electrophysiological changes were progressive reaching a maximum 7 days after intravitreal injections. Changes in b-wave amplitudes and response times were more pronounced at low intensities of stimulation. These results clearly show that, in rabbits, selective decreases in retinal DA concentrations result in pronounced ERG changes, which offer additional evidence supporting a role for this transmitter in lateral inhibition in the retina.