A method using L-hyoscyamine for the study of muscarinic acetylcholine receptor binding in vivo

Studies of muscarinic receptor concentration and comparative binding assays of agonists and antagonists are presently done in vitro by incubation and measurement of the binding to tissue homogenates of the radiolabelled potent antagonists 3H-scopolamine or 3H-quinuclidinyl benzilate. We have developed a technique based on gas chromatography-mass spectrometry that allows studies of the muscarinic receptor concentration to be performed in vivo under physiologic conditions. By injecting the optical antipodes of atropine, D- and L-hyoscyamine separately in mice and following their kinetics in different parts of the brain it was possible to separate the specific receptor binding of the active antipode L-hyoscyamine from that of the inactive antipode D-hyoscyamine, representing unspecific binding. Two hrs after the administration of L-hyoscyamine, 2 mg/kg intravenously, its concentration in brain was found to represent "maximum" specific binding. The physiological significance of specifically bound L-hyoscyamine was tested on its blocking effect on oxotremorine induced tremor.