On the basis of the recent demonstration of enkephalin-like material and opiate binding sites in the retina, and of the relationship between enkephalinergic and dopaminergic systems in various-brain areas, the possible regulation of dopamine neuron activity by opiates was studied in rat retina. 3,4-Dihydroxyphenylacetic acid (DOPAC) concentrations in this area were measured after acute and chronic treatment with neuroleptics, morphine and ethanol, drugs known to interact with the enkephalinergic and dopaminergic systems in various brain areas. Treatment with neuroleptics induces increase of Dopamine metabolism in striatum and retina. If the treatment is repeated, tolerance to this effect develops in striatum but not in the retina. On the other hand, acute treatment with morphine requires a higher dosage in order to increase retinal dopamine metabolism in comparison to striatum. The results may suggest that in the retina the modulation by opiates on dopaminergic neuron is less operant than in striatum.
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