Muscarinic agonist binding and phospholipid turnover in brain

The ability of muscarinic cholinergic agonists to interact with muscarinic receptors in nerve ending preparations and elicit an increased labeling of phosphatidate and phosphatidylinositol from 32Pi has been investigated. Two groups of brain muscarinic agonists are distinguished. Addition of acetylcholine, carbamylcholine, methacholine, or muscarine resulted in a 2-fold stimulation of phosphatidate and phosphatidylinositol labeling, while bethanechol, pilocarpine, arecoline, and oxotremorine were less effective. Simultaneous addition of two agonists from the more effective group did not result in any further increase in stimulated labeling, while the addition of agonists from the less effective group antagonized the stimulatory effect of carbamylcholine. All of the agonists could completely displace binding of [3H]quinuclidinyl benzilate, a muscarinic antagonist. The displacement of the labeled antagonist by the more effective agonists was more complex than that predicted from a simple mass action isotherm and was compatible with the interaction of the agonists with high and low affinity forms of the receptor. Conversely, the displacement data from less effective agonists did not deviate markedly from those predicted for interaction of the agonists with a single affinity form of the receptor. Dose-response curves for stimulated phosphatidate labeling obtained in the presence of acetylcholine, carbamylcholine, and methacholine were predominantly correlated with occupation of the low affinity form of the muscarinic receptor. These results suggest that the enhancement of phosphatidate and phosphatidylinositol turnover in brain is caused by agonist-mediated conformational changes in the muscarinic receptor and that the ability of an agonist to induce this conversion may be predicted by its differential binding to the high and low affinity forms of the receptor.