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Free choline levels in the rat brain

Choline levels in nervous tissue have been investigated by a number of workers in recent years. The values have varied widely from 39.3 nmol/g (Ewetz, Sparf and Söbo, 1970) to 700 nmol/g (Smith and Saelens, 1967). Many of the values published may have been too high for one of the following reasons: (1) post mortem formation of choline, (2) hydrolysis of phospholipids (PL) by extractants and (3) inadequate assay systems. In the past we too have obtained values which we can now with confidence say were too high due to the post mortem formation of choline. In a method which employed bioassay as an end‐step after extraction of choline by acid‐ethanol the values we obtained were 138 ±27 nmol/g. Despite criticism of this method by Ewetzet al. (1970) and Schuberth, Sparf and Sundwall (1970a) we were reasonably sure that the assay system was both sensitive and specific, and that extraction with acid‐ethanol did not lead to liberation of choline from PL, especially since values for plasma choline by this method were in a number of trial extractions as low as 8 nmol/ml. In view of these results we decided to re‐investigate free choline levels in the brain using a method similar to that of Ewetzet al. (1970) in that the living animal (in this case anaesthetized) was frozen in liquid nitrogen before removing the brain, and comparing the results of three different methods of analysis applied to brain extracts prepared in this way.