FIG. 1. Schematic representation of the three known transduction pathways for 5-HT1 receptors: an increase in an inwardly rectifying K+ current (IK); a decrease in a high threshold Ca2+ current (ICa(ht)); and inhibition of adenylyl cyclase (A-cyc). All of these actions are mediated through the pertussis-toxin-sensitive G proteins Gi and/or Go. Although Gi has traditionally been regarded as the G protein that couples to adenylyl cyclase, recent evidence indicates that the form of Go which is found in the brain also couples negatively to adenylyl cyclase (17).