Just when you thought that serotonin was passé, and you’d tossed all your half-used bottles of S.S.R.I.-type antidepressants because the ones that didn’t give you nausea or smother your libido left you wondering whether you were in the placebo arm of a clinical trial, here comes a raft of new discoveries that sweeps the small, evolutionarily ancient and slyly powerful signaling molecule back on to center stage. Researchers lately have learned that serotonin plays an impressive number of critical roles throughout the body, both below the neck and above it, and from the earliest days of prenatal pre-sentience. One team...