High affinity transport of choline into synaptosomes of rat brain

—The accumulation of ³H]choline into synaptosome-enriched homogenates of rat corpus striatum, cerebral cortex and cerebellum was studied at ³H]choline concentrations varying from 0.5 to 100 μm . The accumulation of ³H]choline in these brain regions was saturable. Kinetic analysis of the accumulation of the radiolabel was performed by double-reciprocal plots and by least squares iterative fitting of a substrate-velocity curve to the data. With both of these techniques, the data were best satisfied by two transport components, a high affinity uptake system with K_m. values of 1.4 μM (corpus striatum), and 3.1 μM (ceμ(cerebral cortex) and a low affinity uptake system with respective K_m. values of 93 and 33 μM for these two brain regions. In the cerebellum choline was accumulated only by the low affinity system. When striatal homogenates were fractionated further into synaptosomes and mitochondria and incubated with varying concentrations of ³H]choline, the high affinity component of choline uptake was localized to the synaptosomal fraction. The high affinity uptake system required sodium, was sensitive to various metabolic inhibitors and was associated with considerable formation of ³H]acetylcholine. The low affinity uptake system was much less dependent on sodium, and was not associated with a marked degree of ³H]acetylcholine formation. Hemicholinium-3 and acetylcholine were potent inhibitors of the high affinity uptake system. A variety of evidence suggests that the high affinity transport represents a selective accumulation of choline by cholinergic neurons, while the low affinity uptake system has some less specific function.