DIFFERENTIAL-EFFECTS OF LOW VERSUS HIGH-DOSES OF APOMORPHINE ON RETINAL DOPAMINE METABOLISM IN LIGHT-ADAPTED AND DARK-ADAPTED RABBITS

Previous electrophysiologic results from this laboratory indicate that apomorphine exerts a differential dose-related effect on rabbit elect roretinograms, with low doses increasing the b-wave and higher doses d ecreasing this parameter. Results were interpreted as reflecting apomo rphine's agonistic properties at two different receptors: 1.0 mg/kg ac ting at the postsynaptic site, and the lower dose, 0.01 mg/kg, prefere ntially stimulating inhibitory autoreceptors. The purpose of this expe riment was to investigate further this hypothesis by determining retin al levels of dopamine, dihydroxyphenylacetic acid, and homovanillic ac id in retinas of light- or dark-adapted rabbits treated with saline, 1 .0, 0.1, or 0.01 mg/kg apomorphine intravenously. Results indicate tha t in dark-adapted rabbits only the highest dose tested, 1.0 mg/kg, dec reased dopamine concentrations. In animals exposed to light, the lowes t dose tested, 0.01 mg/kg, significantly reduced dopamine and metaboli te levels, whereas the highest dose unexpectedly increased retinal dop amine turnover. Results are discussed in terms of receptor sites and t he influence of lighting conditions.