EFFECT OF HEMICHOLINIUM-3 ON CHOLINE DISTRIBUTION 1N VW0 IN THE CANINE CAUDATE NUCLEUS*, t

The effect of hemicholinium (HC-3) on choline distribution was studied in aivo in the subcellular fractions of the canine caudate nucleus. Both HC-3 and choline were given intraventricularly to locally anesthetized, decamethonium-paralyzed dogs maintained on artificial ventilation. At various time intervals after intraventricular injection of methyl-W-choline alone and in various HC-3/methyl-W-choline ratios, the animals were sacrificed by air embolus and the caudate was removed and subjected to subcellular fractionation. It was found that methyl-W-choline accumulated primarily in the microsomal (Mic) and the nerve ending, mitochondria, lysosome (NEML) fractions. There was greater accumulation at 4 hr than at 1 hr. The combined injection of HC-3 and methyl-W-choline in ratios of 10, 100, 500 and 2500 progressively increased the uptake of choline by the subcellular elements; an extract of the watersoluble components of fraction NEML showed that HC-3 reduced the conversion of choline to acetylcholine. On the other hand, HC-3 increased the labeling of phosphorylcholine and, particularly, of cytidine diphosphocholine. The levels of free choline in the caudate were not affected by:HC-3. ALTHOUGH the role of acetylcholine as a neurotransmitter in the central nervous system is now widely recognized,‘-3 not much is known about the metabolic origin of its choline moiety. Only recently has this topic received attention,d-12 and it is now clear that the synthesis of most of the choline does not occur in the brain, but rather, that choline is supplied to the brain from extracerebral sources such as the liver. Most experiments in which the uptake of choline by the central nervous system was examined have used tissue slices. For example, Schuberth et al.5 examined the uptake of choline by mouse cortical slices; they concluded that, although choline entered the slice by an active transport mechanism, it was not “significantly metabolized by this tissue”. More recently Chakrin and Shideman’ studied the conversion of methyl-sHlabeled choline to acetylcholine by cat cerebral cortex, while Browning and SchulmaG studied the use, for acetylcholine synthesis, of other possible precursors such as ethanolamine, methionine and serine as well as acetate or glucose. Choline uptake by isolated guinea pig cortex nerve endings has also been described.* * A preliminary report was presented at the Federation Meetings in Atlantic City, April, 1969; Fedtt Proc. 28, 292 (1969). t Supported in part by U.S. Public Health Service Grants MH-11846 (E.F.D.) and NB-06294 (O.Z.S.).