Muscarinic M4 receptor

Class

M4 muscarinic positive allosteric modulator

The M4 muscarinic receptor is one of five subtypes of muscarinic acetylcholine receptors, a family of G-protein-coupled receptors that mediate the effects of the neurotransmitter acetylcholine throughout the nervous system. M4 is densely expressed in the striatum, a brain region central to motor control and reward, where it sits on neurons that also respond to dopamine. Because acetylcholine and dopamine signaling counterbalance each other in this circuit, drugs that enhance M4 activity can indirectly dampen dopamine signaling without blocking dopamine receptors directly. This is of interest in psychiatric conditions such as schizophrenia, where excess dopaminergic signaling contributes to symptoms, and traditional antipsychotics that block dopamine receptors directly often cause movement-related side effects. Rather than activating M4 outright, a positive allosteric modulator (PAM) binds a site on the receptor distinct from where acetylcholine binds and amplifies the receptor's response only when acetylcholine is already present. This preserves the natural timing and location of signaling rather than imposing constant activation. The approach is being explored primarily for psychotic and cognitive symptoms, and reflects a broader effort to find antipsychotic mechanisms that act upstream of dopamine receptors themselves, potentially offering a different side-effect profile than conventional therapies.