The main findings from our studies on the emergence and maturation of neurotransmitter systems in the rabbit retina are that, during normal development: (1) the commitments for certain neurons to become GABAergic, glycinergic, or dopaminergic are made prenatally; the maturation of these neurons, however, occurs postnatally; (2) specific accumulations of exogenously supplied transmitters by presumed retinal neurons were first observed autoradiographically around embryonic day 22 (E22) for gamma-aminobutyric acid (GABA), E25 for glycine, and E27 for dopamine, suggesting that putative GABAergic, glycinergic, and dopaminergic neurons are determined phenotypically at least by these days; (3) based upon the three transmitter-specific properties--uptake, synthesis, and release--the uptake property is generally the first to emerge, while the appearance of the other properties follows a precise and distinct temporal pattern for each of the transmitters studied; (4) for GABAergic and glycinergic systems, the emergence of K+-stimulated, Ca2+-dependent release mechanisms occurs many days after the first appearance of the uptake properties; in contrast, the mechanisms for dopamine uptake and release emerge at about the same time; and (5) by our criteria, putative GABAergic and glycinergic amacrine cells are mature between postnatal day 10 (P10) and P12, around the time when the eyes first open and direction-selective ganglion cells can first be recorded; however, the dopaminergic neurons probably are not mature until about P24.

The emergence and maturation of dopaminergic neurons during postnatal development of the rabbit retina have been followed using high affinity uptake, content, synthesis, storage, metabolism, and release of dopamine as transmitter-specific physiological probes. Autoradiographic and histochemical studies have shown that dopamine-containing neurons in the rabbit retina belong to a class of amacrine cells whose processes ramify mainly in the most distal region of the inner plexiform layer. These neurons contain high concentrations of dopamine, take up dopamine by a high affinity mechanism, and release the accumulated dopamine by a Ca2+ -dependent mechanism upon depolarization of the retina with high extracellular K'. In addition, the rabbit retina contains significant activities of tyrosine 3-hydroxylase (TH, EC 1.14.16.2) and monoamine oxidase (EC 1.4.3.4), the rate-limiting enzymes for the biosynthesis and degradation of dopamine, respectively. In the present study, we show that certain neurons in the newborn retina already possess a specific mechanism for dopamine uptake. The position, density, morphology, and ramification of these cells in the developing retina strongly suggest that they will become dopaminergic neurons in the adult retina. In addition, the ability of the newborn retina to release the accumulated dopamine upon Ca"+-dependent K+ stimulation is qualitatively similar to that of the adult retina. These putative dopaminergic neurons are, however, probably immature at birth because newborn retinas contain very low levels of TH activities and endogenous dopamine. The activities of retinal TH are extremely low between days 0 and 6 after birth, increasing slowly to 30% of the adult level by day 18. There is then a drastic rise in TH activity, reaching the adult level by day 25. The concentration of dopamine in the developing retinas follows closely the increase in TH activity, rising in the same biphasic pattern and reaching the adult level at about 25 days after birth.Taken together, our results indicate that, in the rabbit retina, the commitment for certain neurons to be dopaminergic is made prenatally. This is similar to our earlier findings that putative GABAergic and glycinergic neurons also are determined prenatally. for y-aminobutyric acid (GABA), glycine, and dopamine as neurotransmitters (Ames and Pollen, 1969;Brandon et al., 1980;Dowling and Ehinger, 1978;Graham, 1974;Kong et al., 1980;Lam, 1976;Lam et al., 1979). The neurons which use these substances as transmitters have been shown to contain high concentrations of these substances and their synthetic enzymes, to possess high affinity uptake mechanisms for these transmitters, and Vol. 1, No. 10, Oct. 1981 to release them in response to appropriate depolarizing stimuli. We have recently used these specific properties as physiological probes to follow the development of the GABAergic, glycinergic, and dopaminergic systems in identified neurons during differentiation and maturation of Xenopus and rabbit retinas. This is the third pap...