VESAMICOL RECEPTOR MAPPING OF BRAIN CHOLINERGIC NEURONS WITH RADIOIODINE-LABELED POSITIONAL ISOMERS OF BENZOVESAMICOL

Alzheimer's disease is characterized by progressive cerebral cholinerg ic neuronal degeneration. Radiotracer analogs of benzovesamicol, which bind with high affinity to the vesamicol receptor located on the upta ke transporter of acetylcholine storage vesicles, may provide an in vi vo marker of cholinergic neuronal integrity. Five positional isomers o f racemic iodobenzovesamicol (4'-, 5-, 6-, 7-, and 8-IBVM) were synthe sized, exchange-labeled with iodine-125, and evaluated as possible in vivo markers for central cholinergic neurons. Only two isomers, 5-IBVM (5) and 6-IBVM (10), gave distribution patterns in mouse brain consis tent with cholinergic innervation: striatum much greater than hippocam pus greater than or equal to cortex > hypothalamus much greater than c erebellum. The 24-h tissue-to-cerebellum concentration ratios for 5-IB VM (5) were 3-4-fold higher for striatum, cortex, and hippocampus than the respective ratios for 6-IBVM (10). Neither 8-IBVM (16) nor 4'-IBV M (17) exhibited selective retention in any of the brain regions exami ned. In the heart, only 5-IBVM (5) exhibited an atria-to-ventricles co ncentration ratio consistent with high peripheral cholinergic neuronal selectivity. The 7-IBVM (14) isomer exhibited an anomalous brain dist ribution pattern, marked by high and prolonged retention in the five b rain regions, most notably the cerebellum. This isomer was screened fo r binding in a series of 26 different biological assays; 7-IBVM (14) e xhibited affinity only for the sigma-receptor with an IC50 of similar to 30 nM. Drug-blocking studies suggested that brain retention of 7-IB VM (14) reflects high-affinity binding to both vesamicol and sigma-rec eptors. Competitive binding studies using rat cortical homogenates gav e IC50 values for binding to the vesamicol receptor of 2.5 nM for 5-IB VM (5), 4.8 nM for 6-IBVM (10), and 3.5 nM for 7-IBVM (14). Ex vivo au toradiography of rat brain after injection of (-)-5-[I-123]IBVM ((-)-[ I-123]5) clearly delineated small cholinergic-rich areas such as basol ateral amygdala, interpeduncular nucleus, and facial nuclei. Except fo r cortex, regional brain levels of (-)-5-[I-123]IBVM((I-123)5) at 4 h exhibited correlation (r(2) = 0.99) with endogenous levels of choline acetyltransferase. Conclusion: Vesamicol receptor mapping of cholinerg ic nerve terminals in murine brain can be achieved with 5-IBVM (5) and less robustly with 6-IBVM (10), whereas the brain localization of 7-I BVM (14) reflects high-affinity binding to both vesamicol and sigma-re ceptors.