The quaternary antagonist gallamine discriminates subpopulations of muscarinic receptors in the forebrain and brainstem of the rat and also modulates the binding of other muscarinic ligands via an allosteric mechanism in both brain regions. The calcium channel antagonist verapamil exhibits a similar allosteric effect in both regions (at 100 μM) but does not differentiate the subpopulations that are defined by gallamine. On the other hand, N-methylatropine does not produce the allosteric effect at 100 μM, but shows a high degree of selectivity (about 1000-fold) for the gallamine-defined subpopulations. These studies indicate that the allosterism and subpopulation specificity of gallamine are separate phenomena and may be examined in isolation by appropriate ligands.
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