ENDOGENOUS DOPAMINE INHIBITS THE RELEASE OF ENKEPHALIN-LIKE IMMUNOREACTIVITY FROM AMACRINE CELLS OF THE CHICKEN RETINA IN THE LIGHT

The activity of the enkephalin-immunoreactive (ENSLI) amacrine cells of the chicken retina is low in the light and high in the dark, resulting in parallel increases and decreases in the levels of the enkephalins. In vivo, the selective dopaminergic D, antagonist SCH23390 increased the activity of the ENSLI amacrine cells in the light (ED(50); 20 pmol), but had a much lesser effect in the dark, whereas the selective dopaminergic D, antagonist sulpiride had effects only at very high concentrations (ED(50); 39 nmol). In contrast, the non-selective dopamine agonist ADTN hardly affected the activity of the ENSLI amacrine cells in the light, but markedly reduced their activity in the dark. This pattern of effects suggests that dopamine actively inhibits the ENSLI amacrine cells in the light, but exerts much less inhibitory activity in the dark, consistent with the idea that dopamine is released during the exposure of the retina to light. Thus dopaminergic controls over the ENSLI amacrine cells appear to contribute to the light:dark differences in activity of the ENSLI amacrine cells. Results obtained on the dopaminergic control of enkephalin release in vitro were generally consistent with this model, except that ADTN appeared to stimulate the ENSLI amacrine cells in the dark.