Quantitative autoradiography with short-lived positron emission tomographytracers: A study on muscarinic acetylcholine receptors with N-[C-11]methyl-4-piperidylbenzilate

The present work demonstrates quantitative autoradiography by using positro n emission tomography tracers and storage phosphorimaging plates. The uptak e and association of [C-11]N-methyl-4-piperidylbenzilate was measured in ra t brain tissue cryosections of various thicknesses. The signal increased wi th increasing section thickness, but only in 10-mu m-thick sections did the binding reach the steady state during a 50-min observation time. This viol ation of the equilibrium condition, potentially combined with perfusion lim itations, leads to erroneous increased binding-site density and decreased a ffinity in the 25- and 50-mu m-thick sections. For better imaging of recept or distribution it is reasonable to use thicker sections. For quantitative analysis of receptor-binding parameters, the specific properties of ligands at different thicknesses of cryosections need to be considered. Evidence i s provided that the nonselective muscarinic antagonist N-methyl-4-piperidyl benzilate binds preferentially to the M-4 subtype of muscarinic acetylcholi ne receptors.