The emergence, localization, and maturation of neurotransmitter systems during development of the retina in Xenopus laevis. III. Dopamine.

The uptake, synthesis, and release of dopamine was studied in retinas of Xenopus laevis. In the tadpole and adult retina, 3H-dopamine is accumulated by cells located in the inner nuclear layer. Retinas preloaded with 3H-dopamine release this compound in response to high K+ concentrations in the medium. This release is probably Ca++-dependent as it is inhibited by Co++ in the medium. Adult retinas are also capable of synthesizing 3H-dopamine from 3H-tyrosine. The appearance and maturation of these dopaminergic properties were followed during retinal development. Our data indicate that synthesis of dopamine can first be detected as early as stage 35/36 whereas uptake of dopamine first occurs at stage 43. K+-stimulated release of preloaded 3H-dopamine from putative dopaminergic neurons is, however, not evident until stage 46. These results show that similar to the development of GABA-ergic and glycinergic properties, the uptake, synthesis, and release mechanisms for dopamine emerge at different stages during retinal differentiation in Xenopus Laevis.