Hemicholinium-3 binding sites in rat brain: a quantitative autoradiographic study.

Quantitative in vitro autoradiography was used to study the distribution of [3H]hemicholinium-3 ([3H]HC-3) binding sites in the rat brain. Regional concentrations of HC-3 binding sites were corrected for regional tissue quenching of tritium in a number of brain structures. Specific binding of 10 nM [3H]HC-3 was highest in the interpeduncular nucleus, followed by the caudate-putamen, olfactory tubercle, amygdala, and the medial and lateral habenulae. There was a high positive correlation between regional HC-3 binding and choline acetyltransferase activity in rat brain; however, a novel pattern of the distribution of cholinergic terminals in the subnuclei of the interpeduncular nucleus was discovered. The apparent Kd in the 1-5 nM range and the pharmacological specificity of the HC-3 binding site agreed with data for choline uptake and for the HC-3 binding site as determined in membrane preparations. HC-3 autoradiography appears to be a useful anatomical marker for cholinergic terminals.