Role of high-affinity choline uptake on extracellular choline and acetylcholine evoked by NMDA

Previous studies have shown that NMDA evokes a calcium-dependent and region-specific increase in extracellular choline that is associated with a reduction of membrane phosphatidylcholine and precedes neuronal cell death. We investigated, using in vivo microdialysis, the contribution of high-affinity choline uptake on the increase in extracellular choline evoked by NMDA. Dialysis was performed in the presence of Neostigmine (0.5 microM), an acetylcholinesterase inhibitor, in prefrontal cortex or hippocampus of freely moving rats. Drugs were administered through the dialysis probe. In cholinergic denervation experiments, rats were subjected to sham or AMPA-induced lesion of cholinergic nuclei at least 2 weeks before microdialysis. Excitotoxic lesion of the medial septum / ventral diagonal band nuclei reduced hippocampal choline acetyltransferase activity by 74%, [(3)H]hemicholinium-3 binding by 32%, and completely abolished potassium-evoked acetylcholine release. Despite this reduction of presynaptic cholinergic function, perfusion of NMDA (300 microM) by retrodialysis produced an increase in hippocampal extracellular choline (249 +/- 22% of basal levels) that was similar to that observed in sham controls (301 +/- 35%). Inhibition of choline uptake with hemicholinium-3 in nonlesioned rats produced a sustained increase in dialysate choline (163 +/- 8%) and reduced acetylcholine to 33 +/- 2% of basal levels, consistent with a depletion of the acetylcholine pool due to precursor deficit. Simultaneous perfusion of hemicholinium-3 and NMDA produced a synergistic increase in dialysate choline (664 +/- 95% of basal levels), indicating that part of the choline released by NMDA is taken up. In contrast, NMDA antagonized the decrease of acetylcholine produced by hemicholinium-3. These results show that NMDA-evoked choline release is not mediated by inhibition of high-affinity choline uptake and indicate that choline released by NMDA can be used to sustain acetylcholine synthesis when there is a precursor deficit secondary to uptake inhibition.