MODULATION OF HIGH-AFFINITY CHOLINE CARRIER ACTIVITY FOLLOWING INCUBATION OF RAT HIPPOCAMPAL SYNAPTOSOMES WITH HEMICHOLINIUM-3

Membrane carriers display structural and functional asymmetry with a s ubstrate binding site which can be oriented alternately, but not simul taneously, to the extracellular and intracellular environment. Hemicho linium-3 is an inhibitor of the high-affinity choline carrier in choli nergic nerve terminals which binds to the transporter at the outer mem brane surface but is not taken up into the cell. In the present study, we investigated the decline in choline transport which occurs during the first few minutes cholinergic nerve terminals are incubated in phy siological salt solutions. Following incubation of rat hippocampal syn aptosomes with hemicholinium-3, samples were washed free of the inhibi tor and high-affinity choline uptake was measured. Choline uptake into hemicholinium-treated nerve terminals was significantly greater than control (132 +/- 4%). This effect appeared not to be due to an increas e in uptake of choline above initial values in the hemicholinium-treat ed synaptosomes, but to a decrease in choline carrier activity in cont rol samples by more than 25% during the first few minutes of incubatio n. Addition of hemicholinium-3 to samples after the preincubation indu ced decrease in choline uptake, followed by a wash period to remove th e inhibitor resulted in elevation of choline uptake levels to initial levels. The effect of hemicholinium-3 was concentration-dependent, req uiring near saturating concentrations of the inhibitor to elicit the e ffect. Measurement of acetylcholine content of synaptosomes at differe nt points during the incubation procedure revealed that there was a tr end for transmitter levels to vary inversely compared to choline uptak e activity, but the differences were not statistically significant dur ing treatments when significant changes in transport activity were mea sured.