The Chemogenetic Receptor Ligand Clozapine N-Oxide Induces Neuroreceptor Occupancy and Reduces Striatal Glutamate Levels.

Chemogenetic studies with the ligand clozapine N-oxide (CNO) are predicated upon the assumption that CNO is devoid of actions at natural neuroreceptors. However, recent evidence shows that CNO may be converted back to clozapine (CLZ) , which could yield plasma concentrations that may be sufficient to occupy inter alia dopamine D and serotonin 5HT receptors in living brain. To test this phenomenon, we measured striatal dopamine D receptor occupancy with [F]fallypride PET and serotonin 5HT occupancy using [F]MH.MZ. We found a CNO dose-dependent effect on the availability of both neuroreceptor sites. In parallel MR spectroscopy experiments, we found that CNO reduced creatine + phosphcreatine (Cr+PCr) and increased -acetylaspartate + -acetylaspartylglutamate (NAA+NAAG) signals in the prefrontal cortex, and also reduced the glutamate signal in dorsal striatum, with peak effect at 2 mg/kg. Thus, our findings suggest that conversion of CNO to CLZ in living rats imparts significant occupancy at endogenous neuroreceptors and significant changes to neurometabolite levels.