Ontogenesis of dopaminergic neurons in the post-natal rabbit retina: pre- and post-synaptic elements

The post-natal development of several aspects of the dopaminergic system in the rabbit have been examined. Dopamine (DA) itself was barely detectable from soon after birth up to about 7 days of age. Thereafter, dramatic increases in the retinal content of DA and also of the two major enzymes involved in its biosynthesis were observed, approaching adult values by about 16 days. These changes occur when functional synapses are being formed as judged by electrophysiological and anatomical studies. On the other hand, the post-synaptic target of these neurons, DA-stimulated adenylate cyclase could be easily detected at 3 days of age and did not appear to undergo any dramatic changes in activity during development. The affinity of haloperidol for this receptor did, however, appear to decrease with age suggesting that the receptor is 'born' in a supersensitive state but attains 'normal' properties after the onset of DA neurotransmission. Depletion of DA stores during these early stages by irreversible inhibition of biosynthesis in vivo did not appear to halt the development of tyrosine hydroxylase or have any irrevocable effects on DA neurons. This suggests that the continued presence of neurotransmitter is not necessary for the developmental process to continue that the commitment of particular neurons to become dopaminergic may be made earlier than 5-6 days post-natally.