Quantitative receptor autoradiography: application to the characterization of multiple receptor subtypes

In vitro autoradiographic techniques combined with computer assisted microdensitometry were used to analyze the characteristics and distribution of multiple recognition sites for the neurotransmitters acetylcholine (M1 and M2) and serotonin (5-HT1A and 5-HT1B). For this purpose, binding competition experiments were performed using non-subtype selective 3H-labeled ligands and selective unlabeled compounds. Consecutive tissue sections were incubated in the presence of increasing concentrations of displacers. By using this approach, maximal densities of binding sites, as well as competition profiles of several drugs could be analyzed and quantified in microscopic brain areas. Our results reveal the presence of brain structures enriched in one class of muscarinic or serotonergic-1 recognition sites. This provides a tool for better characterization of the proposed "subtype-selective" ligands and suggests physiological functions for these receptor subtypes. It is concluded that quantitative autoradiographic techniques provide a level of anatomical and pharmacological information on neurotransmitter receptor subtypes, which is difficult to attain using membrane binding studies.