Compounds useful as probes for determining the relative number and/or function of specific receptor subtypes. Of particular interest are nicotinic agonists and antagonists (e.g., metanicotine-type compounds and azaadamantane-type compounds) that are selective to certain nicotinic receptor subtypes. Those compounds are labeled with a radioactive isotopic moiety such as C, F, Br, I or I. Central nervous system disorders are diagnosed by administering to a patient a detectably labeled compound, and detecting the binding of that compound to selected nicotinic receptor subtypes (e.g., alpha 7 and/or alpha 4 beta 2 receptor subtypes). The compounds that have been administered are detected using methods such as position emission topography (PET) and single-photon emission computed tomography (SPECT). The present invention is useful in the diagnosis of a wide variety of CNS diseases and disorders, including Alzheimer's disease, Parkinson's disease and schizophrenia.