Abstract

Hemicholinium (HC-3), a neuromuscular blocking agent which inhibits the synthesis and the release of acetylcholine (ACh) was investigated for its effects both on [¹⁴C]ACh synthesis from [2-¹⁴C]pyruvate or [6-¹⁴C]glucose and on [³H]choline uptake in purified rat striatal synaptosomes. The synthesis of the transmitter was reduced to 15% of control by 1 µM HC-3 in the presence of eserine (170 µM). The drug produced half-maximal inhibition at 0.06 µM; this effect was totally reversed by 30 µM choline. Choline concentrations as low as 10 µM stimulated significantly the synthesis of [¹⁴C]ACh in a diluted suspension of synaptosomes. The existence of a high-affinity uptake system for choline, recently observed by Yamamura and Snyder, was confirmed in our preparation: its K_m was found to be 3.5 µM. Eserine (170 µM) produced only 10% inhibition of the uptake process in the presence of 1 µM choline. HC-3 was found to be a purely competitive inhibitor of the choline high-affinity uptake (K_i = 25 nM). A very good correlation was found between the inhibitory effects of HC-3 on [¹⁴C]ACh synthesis and on [³H]choline high-affinity uptake. The results suggest that the high-affinity system for the transport of choline is present in cholinergic synaptosomes and plays an important role in sustaining and perhaps regulating the synthesis of the transmitter.