AUTORADIOGRAPHIC EVIDENCE THAT QNB DISPLAYS IN-VIVO SELECTIVITY FOR THE M2 SUBTYPE

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype neuroreceptors in cortical and hippocampal regions of the human brain, Emission tomographic study of the loss of m2 recepto rs in AD is limited by the fact that there is currently no available m 2-selective radioligand which can penetrate the blood-brain barrier, W e have previously reported the results of in vivo dissection studies, using both carrier-free and low specific activity [H-3]QNB, which show that [H-3]QNB exhibits a substantial in vivo m2 selectivity, Because of the expense of the radioligand and the long exposure time required for the X-ray film, performing a large number of direct in vivo autora diographic studies using [H-3]QNB is precluded, Therefore, we now conf irm these results autoradiographically by studying the in vivo inhibit ion of radioiodinated (R)-3-quinuclidinyl (S)-4-iodobenzilate ((R,S)-[ I-125]IQNB) binding by unlabeled QNB, In the absence of QNB, (R,S)-[I- 125]IQNB labels brain regions in proportion to the total muscarinic re ceptor concentration; in the presence of 15 nmol QNB, (R,S,)[I-125]IQN B labeling in those brain regions containing predominantly m2 subtype is reduced to background levels, We conclude that QNB is m2-selective in vivo and that a suitably radiolabeled derivative of QNB, possibly l abeled with F-18, may be of potential use in positron emission tomogra phic study of the loss of m2 receptors in AD. (C) 1995 Academic Press, Inc.