Retinal development: Time and order of appearance of specific neuronal properties

The high affinity uptake, biosynthesis and K⁺-stimulated release of certain neurotransmitter candidates was studied in adult and developing retinas of Xenopus laevis. In the adult retina, ³H-GABA was accumulated predominantly by horizontal cells while ³H-glycine and ³H-dopamine were accumulated by cells located deeper in the inner nuclear layer (possibly a type of amacrine or interplexiform cells). This retina also synthesized GABA, dopamine and acetylcholine from their precursors supplied exogenously. Furthermore, adult retinas preloaded with ³H-GABA, ³H-glycine and ³H-dopamine, released these transmitters in response to increased K⁺-concentration in the medium. We have determined the time of appearance and maturation of these properties during embryonic development. With GABA, the appearance of the high affinity uptake system appeared first, preceding GABA synthesis which was followed by the development of K⁺-stimulated transmitter release mechanism. Similary, ³H-glycine uptake appeared several stages before its release. In the case of dopamine, however, its biosynthesis occurred first, followed by the development of the high affinity uptake system and finally the release mechanism appeared. The initiation of this sequence of events for the three transmitter systems studied occurred at different developmental stages: the GABA-ergic properties appeared first, followed shortly by the glycinergic properties, which preceded the dopaminergic properties.