Choline uptake by the neuroblastoma x glioma hybrid, NG108-15

The neuroblastoma x glioma hybrid clone NG108-15 is able to release acetylcholine upon depolarization and form cholinergic neuromuscular synapses in culture. Normal functioning of cholinergic synapses is thought to be dependent on the ability of a neuron to take up extracellular choline, since neurons are unable to synthesize choline de novo. For these two reasons it became important to characterize the choline uptake system of NG108-15 cells. The uptake system appears to bear little if any resemblance to the Na+-dependent high-affinity choline uptake system normally associated with cholinergic neurons. Although the cells appear to possess both high- and low-affinity choline uptake systems, neither system is dependent on Na+ and uptake actually is increased about 60% by the substitution of sucrose for NaCl. Acetylcholine synthesis also is not dependent on Na+, since sucrose, substituted for NaCl, also stimulates acetylcholine synthesis. Changes in the concentrations of the other ions in the uptake medium have little effect on uptake, with the exception that elevated Ca2+ or Mg2+ reverses the stimulation of choline uptake produced by substitution of sucrose for NaCl. Choline uptake is inhibited by hemicholinium-3, but only at high concentrations of the drug (IC50 = 30-80 microM). The metabolic poisons cyanide and iodoacetate inhibit uptake by only 30-40%. Growth of the cells in N6,O2' dibutyryladenosine-3',5'-cyclic monophosphate, which promotes functional and morphological differentiation of the cells, decreased slightly the total amount of choline taken up but had no additional effect on the uptake system. Thus, it appears that NG108-15 cells are capable of forming functional cholinergic synapses with muscle cells even though the neuroblastoma does not possess the high-affinity choline uptake system normally associated with cholinergic neurons.