Influx and Efflux of N,N,N-Trimethyl-N-Prop-2-Ynylammonium by a Rat Brain Synaptosome Preparation

Abstract

Several analogs of choline (Ch) have been studied as substrates for the Ch uptake systems present in synaptosomes or brain slices (4–7,9,12,13,15). These analogs are monoethylcholine, diethylcholine, triethylcholine, pyrrolidinecholine, homocholine, sulfocholine, l,l-dimethyl-3-hydroxypiperidine, N-(4-hydroxylbutyl)-N,N, N-trimethylammonium (TMHBA) and N,N-dimethylaminoethanol (DJIAE), and all have a free hydroxyl group. With the exceptions of TMHBA (16) and DMAE (15) they are alternative substrates for the high affinity Ch uptake system as well as precursors to false cholinergic transmitters. These findings and the results of studies on Ch analogs as inhibitors of the high affinity uptake of Ch (22) define some, of the structural requirements for alternative substrates. To be recognized as a substrate, it appears that the Ch analog must be a simple alkyl ammonium or sulfonium that has no more than three methylene groups between the quaternary head and terminal hydroxyl group. The observation that acetylcholine (ACh) is not transported by the high affinity Ch uptake system suggests that a free hydroxyl group also is required for a Ch analog to be a substrate for this transport (17,18). To test this notion, a Ch analog devoid of a hydroxyl group, N,N,N-trimethyl-N-prop-2-ynylammonium (TMPYA), was evaluated as a substrate for the Ch high affinity uptake system. TMPYA was chosen on the basis that the electrostatic potential it generates is quite similar to that of Ch (25). On that basis, TMPYA should be recognized by and interact with binding sites for Ch (see 24 for review). Further, the high affinity transport of Ch, monoethylcholine, pyrrolidinecholine, and homocholine by rat brain synaptosomes is so closely linked to acetylation (3,7,28), it is difficult to study this uptake system separately from the associated metabolism of the substrates. Thus it is desireable to have a non- metabolizable analog of the natural substrate for transport studies. The preliminary results of studies on one such analog, TMPYA, are reported here.