Differential effects of low versus high doses of apomorphine on retinal dopamine metabolism in light- and dark-adapted rabbits

Abstract Previous electrophysiologic results from this laboratory indicate that apomorphine exerts a differential dose-related effect on rabbit electroretinograms, with low doses increasing the b-wave and higher doses decreasing this parameter. Results were interpreted as reflecting apomorphine's agonistic properties at two different receptors: 1.0 mg/kg acting at the postsynaptic site, and the lower dose, 0.01 mg/kg, preferentially stimulating inhibitory autoreceptors. The purpose of this experiment was to investigate further this hypothesis by determining retinal levels of dopamine, dihydroxyphenylacetic acid, and homovanillic acid in retinas of light- or dark-adapted rabbits treated with saline, 1.0, 0.1, or 0.01 mg/ kg apomorphine intravenously. Results indicate that in dark-adapted rabbits only the highest dose tested, 1.0 mg/kg, decreased dopamine concentrations. In animals exposed to light, the lowest dose tested, 0.01 mg/kg, significantly reduced dopamine and metabolite levels, whereas the highest dose unexpectedly increased retinal dopamine turnover. Results are discussed in terms of receptor sites and the influence of lighting conditions