Characterization of [3H]Hemicholinium-3 Binding Sites in Human Brain Membranes: A Marker for Presynaptic Cholinergic Nerve Terminals

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Abstract
We report here on the binding properties of [3H]hemicholinium‐3, a selective inhibitor of the high‐affinity choline uptake process, to human brain membranes. Under the assay conditions described, the binding of [3H]hemicholinium‐3 exhibited a dependency of physiological conditions on pH, temperature, and NaCl concentrations. Striatal binding proved to be specific, to a single site, saturable, and reversible, with an apparent KD of 10 nM and a Bmax of 82 fmol/mg of protein. [3H]Hemicholinium‐3 specific binding exhibited a pharmacological profile and an ionic dependency suggestive of physiologically relevant interactions and comparable with those reported for the high‐affinity choline uptake. Moreover, specific [3H]hemicholinium‐3 binding exhibited an uneven regional distribution: striatum ≫ nucleus basalis > spinal cord ≫ midbrain = cerebellum ≧ hippocampus > neocortex = anterior thalamus > posterior thalamus ⋙ white matter. This distribution closely corresponds to the reported activity of both enzymatic cholinergic presynaptic markers and high‐affinity choline uptake in mammalian brain. There are no significant differences between these results and those previously found in the rat brain using this radioligand. Our results demonstrate, for the first time, the presence of [3H]hemicholinium‐3 binding sites in human brain and strongly support the proposal that this radioligand binds to the carrier site mediating the high‐affinity choline uptake process on cholinergic neurons. Thus, [3H]hemicholinium‐3 binding may be used in postmortem human brain as a selective and quantifiable marker of the presynaptic cholinergic terminals.