Use of anti-muscarine toxins to facilitate the study of m1 and m4 muscarinic receptors in the striatum

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Molecular and Cellular Pharmacology

First Committee Member

Lincoln T. Potter - Committee Chair


The striatum expresses mRNA primarily for m1 and m4 muscarinic acetylcholine receptors. Studies of striatal m1 and m4 muscarinic receptors have been precluded by the lack of selective antagonists and agonists which can distiguish between m1 and m4 receptors. Investigators in this laboratory have recently isolated two toxins, m1-toxin and m4-toxin, from the venom of the Eastern green mamba, Dendroaspis angusticeps, which show very high selectivity for m1 and m4 receptors respectively. These toxins were the tools necessary to pharacologically separate and separately study m1 and m4 muscarinic receptors in the striatum. m1-Toxin can fully block m1 receptors in the striatum. The number of total muscarinic receptors blocked with m1-toxin can be used to calculate the total concentration of m1 receptors in the striatum of the rat. Forty-two percent of the total striatal muscarinic receptors are m1 receptors. This blockade also facilitates the study of the residual non-m1 receptors. These residual non-m1 receptors bind antagonists as a homogeneous population of receptors with affinities that are the same as the affinities for cloned m4 receptors. The most potent antagonists for m1-toxin-spared striatal receptors are trihexyphenidyl, biperiden, silahexocyclium, and R-(+)-hyoscyamine. Non-m1 striatal muscarinic receptors can be almost fully blocked (by 85%) by increasing amounts of m4-toxin. These data can be interpreted as evidence for 42% m1 receptors, 49% m4 receptors; and 9% other muscarinic receptors. Non-m1 striatal muscarinic receptors bound agonists with an equal proportion of high affinity (K$\sb{\rm H})$ and low affinity (K$\sb{\rm L}$) binding sites. The most potent agonist for striatal m1-toxin-spared (putative m4) receptors was oxotremorine-M. m1-Toxin was used to block m1 receptors and determine the localization of non-m1 receptors. Non-m1 muscarinic receptors were not localized specifically in striosomes or matrix, but were specifically localized in the islands of Calleja of the olfactory tubercle. m1 Receptors were widely distributed throughout the striatum. m1-Toxin and m4-toxin were useful in determining the binding properties of m4 receptors, the quantities of striatal muscarinic receptors, and the distribution of striatal m1 and m4 receptors.


Biology, Neuroscience; Health Sciences, Pharmacology; Biology, Animal Physiology

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