BINDING OF THE LABELED MUSCARINIC TOXIN I-125 MT1 TO RAT-BRAIN MUSCARINIC M(1) RECEPTORS

The green mamba (Dendroaspis angusticeps) 'muscarinic toxin', MTI, was radioiodinated by the chloramine T method. I-125-MTI labelled the mus carinic M(1) receptor subtype with a very good selectivity in rat brai n. It had no preference for the receptor states with high or low affin ity for agonists, and was not affected by Gpp(NH)p addition to the inc ubation medium. The I-125-MTI binding was reversible, with a half life of 45 min at 25 degrees C. The effect of competitive and allosteric m uscarinic antagonists on I-125-MTI binding and dissociation can be rat ionalized by assuming that the radioiodinated toxin is able to label t he muscarinic (acetylcholine) binding site.