How do we distinguish our friends from foes? How does dementia destroy memory? And how can past experience invade the present with destructive force? Scientists are closing in on the biochemical roots of these neurological puzzles.

Thomas Insel describes the profound impact of a small group of neuropeptides on social behavior in animals, from worms to humans. Oxytocin, the hormone which turns on maternal behavior and cognition, turns out to play a large role in determining social memories. Mice whose genes for producing oxytocin are knocked out can’t seem to remember animals they’ve met 30 minutes earlier – what Insel describes as “dense social amnesia.” An area of the brain’s amygdala is particularly rich in oxytocin receptors, and when the peptide is injected into a nearby ventricle, the animals’ social interactions revert more closely to normal behavior. Oxytocin is a useful tool for interrogating the circuitry that enables humans to determine “who’s important to me, who I’d die for, who I’m pair-bonded with, who will take care of me,” says Insel.

Alzheimer’s Disease (AD), which afflicts 20 million people worldwide, begins by literally clogging and tangling the hippocampus, the part of the brain essential for learning and memory. Li-Huei Tsai and other researchers have found “compelling evidence” that a small protein may be critically important in activating AD’s awful atrophy of memory. By manipulating specific enzymes, Tsai has managed to model in animals “all the pathological hallmarks of Alzheimer’s Disease,” and zero in on the source of the plaques and tangles seen in human Alzheimer’s patients. Tsai foresees drug interventions that inhibit these enzymes. But, she says, a big task remains “even after we’re successful in halting a deleterious process--how can we restore learning and retrieve lost memory in AD patients?”

Why is it that only some people exposed to a shocking event develop post-traumatic stress disorder (PTSD)? Kerry Ressler’s research posits that some kind of learning must take place in the brain’s amygdala -- its fear response center—that cannot readily be extinguished. Researchers have tracked down a molecular factor that increases “after learning of fear or extinction of fear.” He believes that if this molecule is somehow blocked from doing its job, then someone suffering from PTSD cannot extinguish fear. In a fortuitous medical convergence, the drug D-cycloserine, which has been approved for years to treat tuberculosis, proves very effective in enhancing the effects of the molecule, and reducing fear of all kinds. One example: When people with fear of heights were given D-cycloserine as they took rides in elevators, they reported a significant, long-lasting reduction in their phobias.