Role of M4‐receptor cholinergic signaling in direct pathway striatal projection neurons during dopamine depletion.

Direct pathway striatal projection neurons (dSPNs) are characterized by the expression of dopamine (DA) class 1 receptors (D1R), as well as cholinergic muscarinic M1 and M4 receptors (M1R, M4R). D1R enhances neuronal firing through phosphorylation of voltage‐gate calcium channels (CaV1 Ca2+ channels) activating Gs proteins and protein kinase A (PKA). Concurrently, PKA suppresses phosphatase PP‐1 through DARPP‐32, thus extending this facilitatory modulation. M1R also influences Ca2+ channels in SPNs through Gq proteins and protein kinase C. However, the signaling mechanisms of M4R in dSPNs are less understood. Two pathways are attributed to M4R: an inhibitory one through Gi/o proteins, and a facilitatory one via the cyclin Cdk5. Our study reveals that a previously observed facilitatory modulation via CaV1 Ca2+ channels is linked to the Cdk5 pathway in dSPNs. This result could be significant in treating parkinsonism. Therefore, we questioned whether this effect persists post DA‐depletion in experimental parkinsonism. Our findings indicate that in such conditions, M4R activation leads to a decrease in Ca2+ current and an increased M4R protein level, contrasting with the control response. Nevertheless, parkinsonian and control actions are inhibited by the Cdk5 inhibitor roscovitine, suggesting Cdk5's role in both conditions. Cdk5 may activate PP‐1 via PKA inhibition in DA depletion. Indeed, we found that inhibiting PP‐1 restores control M4R actions, implying that PP‐1 is overly active via M4Rs in DA‐depleted condition. These insights contribute to understanding how DA‐depletion alters modulatory signaling in striatal neurons. Additional working hypotheses are discussed. [ABSTRACT FROM AUTHOR]