Abstract
The generation of muscarinic acetylcholine receptor (mAChR) subtype-selective compounds has been challenging, requiring use of nonpharmacological approaches, such as genetically engineered animals, to deepen our understanding of the potential that members of the muscarinic receptor subtype family hold as therapeutic drug targets. The muscarinic receptor agonist 'BuTAC' was previously shown to exhibit efficacy in animal models of psychosis, although the particular receptor subtype(s) responsible for such activity was unclear. Here, we evaluate the in vitro functional agonist and antagonist activity of BuTAC using an assay that provides a direct measure of G protein activation. In addition, we employ the conditioned avoidance response paradigm, an in vivo model predictive of antipsychotic activity, and mouse genetic deletion models to investigate which presynaptic mAChR subtype mediates the antipsychotic-like effects of BuTAC. Our results show that, in vitro, BuTAC acts as a full agonist at the M2AChR and a partial agonist at the M1 and M4 receptors, with full antagonist activity at M3- and M5AChRs. In the mouse conditioned avoidance response (CAR) assay, BuTAC exhibits an atypical antipsychotic-like profile by selectively decreasing avoidance responses at doses that do not induce escape failures. CAR results using M2(-/-), M4(-/-), and M2/M4 (M2/M4(-/-)) mice found that the effects of BuTAC were near completely lost in M2/M4(-/-) double-knockout mice and potency of BuTAC was right-shifted in M4(-/-) as compared with wild-type and M2(-/-) mice. The M2/M4(-/-) mice showed no altered sensitivity to the antipsychotic effects of either haloperidol or clozapine, suggesting that these compounds mediate their actions in CAR via a non-mAChR-mediated mechanism. These data support a role for the M4AChR subtype in mediating the antipsychotic-like activity of BuTAC and implicate M4AChR agonism as a potential novel therapeutic mechanism for ameliorating symptoms associated with schizophrenia.