Binding of the labelled muscarinic toxin 125I-MT1 to rat brain muscarinic M1 receptors.

The green mamba (Dendroaspis angusticeps) "muscarinic toxin', MT1, was radioiodinated by the chloramine T method. 125I-MT1 labelled the muscarinic M1 receptor subtype with a very good selectivity in rat brain. It had no preference for the receptor states with high or low affinity for agonists, and was not affected by Gpp(NH)p addition to the incubation medium. The 125I-MT1 binding was reversible, with a half life of 45 min at 25 degrees C. The effect of competitive and allosteric muscarinic antagonists on 125I-MT1 binding and dissociation can be rationalized by assuming that the radioiodinated toxin is able to label the muscarinic (acetylcholine) binding site.