Induction of cholinergic enzymes in chick ciliary ganglion and iris muscle cells during synapse formation.

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Abstract
In chick ciliary ganglia and irises, cholineacetyltransferase (ChAc) and acetylcholinesterase (AChE) activities were measured from the 5th day of incubation until 1 wk after hatching. The changes in enzyme activity were correlated in time with previous electrophysiological and morphological findings of synapse formation in these tissues. At Stage 26 (Hamburger and Hamilton, 1951; before synapse formation in the ganglia) low activities of ChAc (12 .+-. 4 [mean .+-. SE] pmol ACh synthesized/h) were measured in the iris nerve terminals, indicating that ganglion cells are biochemically differentiated immediately after cell migration is completed. The specific activities of ChAc and AChE rose during development and these increases were closely related to the onset and maturation of ganglionic and iris synaptic transmission. These increases in enzyme activities can be used in cholinergic synapses an an index of synapse formation. The 200-fold specific increase of ChAc in iris nerve terminals which occurs at Stage 34 probably reflects an increase in the enzyme in ganglion cells and suggests that the formation of the iris neuromuscular junction triggers the enzyme induction. It is implied that the cell responds to a signal ascending the axon from the terminal. The initial increase of AChE specific activity in the ganglion occurs after transmission is established in all cells between Stage 30-34 and is mainly due to enzyme synthesis by the ganglion cells. In the iris there is a 2-fold increase in specific activity after the formation of neuromuscular junctions which probably reflects enzyme induction in the muscle subneural region. The specific induction of AChE in post-junctional cells probably is due to an influence of the prejunctional element. During synaptic formation in the ciliary ganglion, reciprocal interactions between the neurons and their targets result in the induction of ChAc in the prejunctional elements and AChE in the post-junctional cells.