Induced interconversion of agonist affinity states in muscarinic receptor from mice brain: Effects of temperature and sugars

In homogenates of mouse medulla-pons and cerebellum, thermal exposure (50°, 5 min.) caused a 10-12 fold decrease in binding affinity of muscarinic agonists with no effect on antagonist binding, but in cortical preparations both agonist and antagonist binding were unaltered. The thermal effect on agonist reflects a conversion of high to low affinity binding sites and could be prevented using sugars in the medium. Incubation of mouse cortex homogenates at 0° for 3 hours decreased the I₅₀ values for agonist binding with no effect on antagonist binding. The relationships between factors affecting agonist affinity states and the mode of action of muscarinic receptors are discussed.