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Molecular Pharmacology, Vol 16, 1084-1088, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Choline: Binding Studies Provide Some Evidence for a Weak, Direct Agonist Action in Brain

JOSE M. PALACIOS 1 and MICHAEL J. KUHAR 1

1 Department of Pharmacology and Experimental Therapeutics and of Psychiatry and Behavioral Biology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

The potential ability of choline to act as a muscarinic agonist in brain was examined. Binding curves for muscarinic agonists are flattened and their Hill coefficients are lower than unity, while antagonists exhibit mass action curves with Hill coefficients equal to unity. Taking into account these findings, we have studied the behavior of choline on brain muscarinic receptors as measured by competition studies with 3H-quinuclidynyl benzilate. Choline displaces the 3H-muscarinic antagonist in cortical and hippocampal homogenates with an IC50, value 100 times greater than that of acetylcholine. In addition, its displacement curve is flattened with a Hill coefficient of 0.77. We conclude that choline might act as a direct agonist on brain muscarinic receptors but probably only at high concentrations. The agonistic action of choline suggests an "inactivation" role for high affinity choline uptake as well as a role in supplying precursor.

Note:
ACKNOWLEDGMENT The authors acknowledge the clerical assistance of Carol Kenyon and Princie Campbell.

Submitted on December 27, 1978
Accepted on June 5, 1979




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J. Pharmacol. Exp. Ther., July 1, 2006; 318(1): 268 - 275.
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