Effect on second messengers that mediate transmitter action.
Phosphatidylinositol-4,5-biphosphate (P1P2) is the cell membrane precursor of inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). This enzymic reaction is catalyzed by the enzymephospholipase C (PLC). IP3 and DAG are important second messengers for alpha-adrenergic (andmuscarinic) transmission as shown in the diagram below. After P1P2 is converted to the secondmessengers, IP3 + DAG, it must be reconstituted from IP3 via IP2, IP, and inositol (I). Lithium blocks twosteps in the reconstitution process, namely IP2 ÷ IP, and IP1 ÷ I, thus leading to depletion of P1P2. Theeffects of the transmitter on the receptor (R) and consequently on the cell will diminish. G denotes theG-protein involved in coupling the effects of drug combination with the receptor to the enzyme PLC.