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COMPARATIVE STUDIES ON SYNAPTOSOMES: UPTAKE OF [N‐Me‐3H]CHOLINE BY SYNAPTOSOMES FROM SQUID OPTIC LOBES

Abstract— The uptake of [N‐Me‐3H]choline into synaptosomes from squid optic lobes was studied using a Millipore filtration technique. When incubated in an artificial sea water medium at 26°C, but not at 0°C, the synaptosomes rapidly accumulated choline, most of which could be recovered as unchanged free choline. The accumulated choline was readily released by treatment of the synaptosomes with Triton X‐100 or exposing them to hypo‐osmotic conditions. The influx of choline increased with increasing concentrations of choline and could be resolved into saturable and non‐saturable components. Kinetic analysis revealed the presence of two saturable components one of high affinity (Km about 2 μm) and one of lower affinity (Km 25 μm). The rate of choline uptake by these synaptosomes was considerably greater than by mammalian brain synaptosomes. Both high and low affinity systems were Na+‐requiring and inhibited by hemicholinium no. 3, levorphanol and dextrorphan. NaCN, 2,4‐dinitrophenol and ouabain also inhibited choline uptake, the high affinity system being particularly sensitive to these agents. It is suggested that the high affinity system is specific for cholinergic terminals.