In recent years, considerable evidence has accumulated which indicates that the regulation of the rate of synthesis of acetylcholine (ACh) in response to altered demand is governed, at least in part, by the sodiumdependent high-affinity uptake system for choline (HACU), and that this system is confined exclusively to cholinergic neurons (for review, see Kuhar and Murrin, 1978). Furthermore, in vivo changes in the rate of HACU are evident in in vitro synaptosomal fractions isolated from animals that have had their cholinergic system experimentally manipulated. Therefore, measurement of the rate of in vitro HACU can be used to investigate changes in in vivo cholinergic activity (see Kuhar and Murrin, 1978).In making these measurements, however, uptake values have classically been expressed on the basis of tissue protein. While such experiments have yielded results which appear to reflect cholinergic function, the amount of protein in the crude synaptosomal fraction used for choline uptake is not reliably related to the number of cholinergic synaptosomes within a given preparation, and reflects primarily noncholinergic synaptosomes, glial elements, and other subcellular organelles. A more rational system would be to calculate choline uptake values on the basis of a parameter that is related to the number of cholinergic synaptosomes present in a preparation, rather than on an independent variable, such as total tissue protein.Such a synaptosomal marker for cholinergic neurons is acetyl-CoA:choline 0-acetyltransferase (EC 2.3.1.6.; CAT). Since CAT activity in the CNS is confined exclusively to cholinergic neurons (for review, see Rossier, 1977) the measurement of CAT activity in the subcellular synaptosornal fraction should yield an estimate of the number of intact cholinergic synaptosomes in that fraction, which can then be used to calculate the choline up-take rate. This paper presents results of some experiments in which CAT activity was used as a basis for calculating the rate of HACU. This method is used routinely in our laboratory, and the results presented here were chosen randomly from a large series of experiments being conducted.
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