Synapse selection based on differences in synapse turnover

Rat retinal neurons formed transient synapses with rat muscle cells in culture only during a discrete period in development, from the 20th day of embryonic development to the 7th neonatal day. In contrast, chick embryo spinal cord neurons formed synapses at all developmental stages tested, from the 2nd to the 18th day of embryonic development. The percentage of cells from the spinal cord that formed synapses with muscle cells was maximum at 4 days of embryonic development and decreased thereafter. However, the number of synapses with muscle formed by cells from 8-day embryonic spinal cord did not decrease during 14 days of culture. Under identical conditions, all synapses formed between rat retinal neurons and muscle cells were terminated during 7 days of culture. These results show that differences in the rates of turnover of two populations of cholinergic synapses can result in the selective retention of one population of synapses and the loss of the other, and thereby alter the specificity of synaptic connections.