M₁R signaling and phosphoinositide metabolism. (A) Neuronal M current is mediated by KCNQ2/3 potassium channels (yellow), which require membrane depolarization and PIP₂ to open. (B) Binding of the M₁R agonist Oxo-M facilitates binding of G proteins to the receptor. This binding induces “activation” of G proteins, i.e., nucleotide exchange at the Gα_q subunit from GDP to GTP, and dissociation of Gα_q from Gβγ. Gα_q-GTP activates PLC, which cleaves PIP₂ into DAG and IP₃. The absence of PIP₂ prevents KCNQ2/3 channels from opening. (C) Voltage-gated ion channels are tetramers. Each subunit consists of a four-segment (S1-S4) voltage sensor domain (green and yellow) and a pore-forming domain (dark red). The S4 segment (yellow) contains positive charges, which move upon depolarization. (D) VSPs are monomers. They contain a four-segment voltage sensor and a phosphatase domain. The phosphatase is activated by depolarization and dephosphorylates PIP₂ to PI(4)P. (E) Phosphoinositide metabolism. PI is phosphorylated first by a PI 4-kinase and then by a PIP 5-kinase to yield PI(4,5)P₂. A 4-phosphatase and a 5-phosphatase mediate the reverse reactions. VSP is a 5-phosphatase.