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J Sharkey, MC Ritz, JA Schenden, RC Hanson and MJ Kuhar
Neuroscience Branch, NIDA Addiction Research Center, Baltimore, Maryland.
(-)-Cocaine inhibits M2 muscarinic cholinergic binding measured with [3H]quinuclidinyl benzilate in heart and brain with a Ki of 18.8 microM. The cyclic nucleotide 5'-guanylylimidodiphosphate does not shift the competition curve, suggesting that (-)-cocaine is an antagonist. (-)-Cocaine also reverses the methacholine-induced inhibition of guinea pig atrial contractions at a similar concentration. Although (+)-cocaine is about 8-fold more potent than (- )-cocaine, (+)-cocaine is not present in extracts of the coca plant. Of the many compounds tested, only (-)-cocaine and lidocaine have a higher affinity at M2 muscarinic receptors than at M1 receptors; other compounds such as (+)-cocaine, norcocaine, procaine and dimethocaine are equipotent at the M1 and M2 subtypes. These results indicate that cocaine can act as an antimuscarinic agent, particularly at higher, toxic doses.
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