Figure 3-25 Schematic drawing of classic receptor activation. For years, binding of ligand was thought to cause receptors to change from an inactive state (R) to an activated state (R*). The activated receptor interacted with intermediary guanine nucleotide (G) proteins or directly with second messenger cascades (i.e., effectors [E]), or both. Later information suggests a more complicated equilibrium between R and R*, as shown in Figure 2-26 and Figure 2-27.