Development and Aging of Cholinergic Synapses

Abstract

Choline (Ch) uptake in the iris of the chick has been characterized from 4½ days of incubation to 1 year of age, in conjunction with an electron microscopic ultrastructural study of the innervation of the iris. Uptake has been shown to constitute an early and reliable biochemical marker for the presence of innervation in target organs. The characteristics of Ch uptake, including Na⁺ and temperature dependence and inhibitor sensitivity, change during development, and the V_max increases steadily while the K_m does not vary significantly during the period of study. The relationships among uptake and other cholinergic functions also change with development of the iris. Acetylcholine (ACh) and Ch uptake but not Ch levels appear to develop together, while cholinergic enzyme activities follow a developmental course more closely related to neurotransmission. The significance of these findings in relation to proposed regulatory relationships in cholinergic biosynthetic mechanisms is discussed.