In vivo quantification of cerebral muscarinic receptors in normal human aging using positron emission tomography and [11C]tropanyl benzilate.

Regional cerebral muscarinic cholinergic receptor binding was quantified in normal young and elderly subjects employing the muscarinic antagonist radioligand [11C]tropanyl benzilate (TRB). Binding was determined by kinetic analyses of positron emission tomographic (PET) determinations of cerebral activity in conjunction with radial arterial blood sampling following intravenous radiotracer injection. A significant, but minor (8%), loss of frontal cortical receptors relative to whole brain average receptor density was found with advancing age. Parametric estimates of binding suggest small reductions in cerebral cortex binding as well as increases in brain stem and cerebellar binding underlying the observed pattern difference. However, these latter changes did not achieve statistical significance. We conclude that cerebral muscarinic receptor availability, as depicted by antagonist binding, does not undergo a major decline during normal aging of the adult human brain. The cerebral cortical cholinergic dysfunction in elderly subjects, suggested by prior clinical evidence, is not attributable to major loss of total muscarinic cholinoceptive capacity.