Striatal M4 Receptors Mediate M4 PAM Antipsychotic-like Efficacy Independently of Normalizing Striatal Dopamine Release

Muscarinic acetylcholine receptor (mAChR) drugs have emerged as a promising new class of antipsychotics, highlighted by the FDA approval of Cobenfy, the first treatment for schizophrenia that does not directly target dopamine receptors. The M4 receptor subtype is thought to be a key mediator of antipsychotic-like efficacy. However, the in vivo circuit mechanisms underlying the antipsychotic-like efficacy observed with M4-selective positive allosteric modulators (PAMs) remain incompletely understood. Here, we used fiber photometry in awake, behaving mice to assess how the M4 PAM VU0467154 (VU'154) affects striatal dopamine (DA) transmission under basal and amphetamine-stimulated conditions. We also generated and validated dorsal striatum-specific M4 knockout mice (DS-M4-KO) to test the role of striatal M4 receptors in regulating DA release and antipsychotic-like effects. VU'154 dose-dependently reduced the amplitude of striatal DA release events under basal conditions without altering event frequency. However, VU'154 had no significant effects on tonic DA levels under basal conditions, as assessed using the D1 PAM DETQ. Despite robustly attenuating AHL, VU'154 produced little to no effect on amphetamine-induced DA levels, indicating that antipsychotic-like efficacy was dissociated from changes in striatal DA levels. Notably, VU'154 efficacy in AHL was absent in DS-M4-KO mice, demonstrating that M4 receptors in the dorsal striatum are required for antipsychotic-like effects. Combined with previous studies implicating a key role for M4 receptors on D1-expressing neurons, these findings suggest that M4 PAMs produce antipsychotic-like efficacy primarily through postsynaptic mechanisms in the dorsal striatum, rather than through broad normalization of amphetamine-induced DA release.