I I . Biochemical Mechanisms in the Phosphatioylinositol Effect

Stimulated labeling of phospholipids from 32p i is a hallmark of activation of a variety of cell surface receptors. In the case of the central nervous system, the response can ref lect muscarinic activation. Recent studies in nerve ending preparations indicate a postsynaptic site of action. Ca 2+ is required for the expression of cholinergic stimulation of labeling in nerve ending preparations, but whether i t plays a regulatory role is not yet known. While i t Is inferred that the receptor-I igand i nteraction Ieads to increased diacylglycerol avai labi l i ty , i ts source is not established. In experiments with muscarinic agents and ionophore added to nerve ending preparations, there is a potentiated loss of labeling from prelabeled polyphosphoinositides. I t is suggested that phosphodiesteratic cleavage of polyphosphoinositides may be an early consequence of muscarinic receptor activation.