Development and aging of cholinergic synapses. II. Continuous growth of acetylcholine and choline levels in autonomic ganglia and iris of the chick.

We have followed the pattern of variation in the endogenous levels of acetylcholine (ACh) and choline (Ch) in sympathetic (lumbar) and parasympathetic (ciliary) ganglia, and in the iris of the chick from 1 to 7 years of age. Levels of ACh and Ch seem to undergo a continuous process of growth which is initiated in the first week of embryonic development and continues into adulthood up to 2 years of age. The early increase in ACh and Ch levels are most marked in the ciliary ganglion. In the ciliary ganglion, morphological and biochemical observations show that this period corresponds to one of 'continuous synaptic growth'. In a second period starting around 2 years, which can be defined as one of 'synaptic regression', a phase of progressive and marked decrease in ACh and Ch levels is visible in all three organs examined. In all three organs Ch levels parallel rather well the ACh levels, but the Ch/ACh ratio progressively decreased from 1 to 7 years from 3.5 to 1.5 (ciliary ganglia and iris) and from 2.5 to 1 (sympathetic ganglia). The low levels of ACh reached at 7 years in the three organs have clear implications with regard to physiology of aging cholinergic synapses.