Hemicholinium-3 mustard reveals two populations of cycling choline cotransporters in Limulus

Cholinergic neurons have both a low-affinity and a high-affinity choline transport process. The high-affinity choline transport is sodium dependent and thus it can be referred to as choline cotransport. Choline cotransport has been shown to be up-regulated by neuronal activity. Protein kinase C has also been shown to regulate choline cotransport. Both forms of regulation appear to modulate transport by altering the numbers of choline cotransporters in the nerve terminal membrane. The present study centers on choline cotransporter trafficking in Limulus brain hemi-slice preparations. The competitive, reversible, non-permeant ligand, [3H]hemicholinium-3, was used in binding studies to estimate the relative number of choline cotransporters in plasma membranes. The hemicholinium-3 mustard derivative has been shown to be an irreversible, highly selective, non-permeant ligand for the choline cotransporter, and was also used. Hemicholinium-3 mustard binding to the choline cotransporter blocked [3H]choline transport and [3H]hemicholinium-3 binding. Antecedent elevated potassium exposure of cholinergic tissues has been shown to up-regulate choline transport by the recruitment of additional choline cotransporters to surface membranes. This treatment was also effective in the recruitment of cotransporters following maximal inhibition by hemicholinium-3 mustard of brain hemi-slices. Long-term washout of hemicholinium-3 mustard in hemi-slices resulted in a time-dependent restoration of choline cotransport. Full recovery occurred within 2 h. In uninhibited slice preparations, both staurosporine and chelerythrine, protein kinase C inhibitors, stimulated choline uptake. However, within a 1-h washout recovery of uptake following hemicholinium-3 mustard inhibition, the staurosporine responsive but not chelerythrine responsive transport had returned. On the basis of these findings, we hypothesize the existence of two distinct populations of cycling choline cotransporters, which includes inactive or “silent” transporters.