Serotonin molecules travel like an entourage of ambassadors, opening up channels of communication between neurons. Serotonin is a major player in such involuntary actions as digestion, respiration, and sensory processing, as well as in voluntary actions like the movement of limbs and fine motor functions. Most importantly for Smith, the lack of serotonin around brain synapses has been linked to depression. Chemical treatments for depression pivot around keeping serotonin around the synapses for a longer time. Serotonin is normally removed by proteins called serotonin transporters, which take the molecule back into the cell where it is broken down.
Antidepressants bind the serotonin transporters, making serotonin uptake impossible.
Smith controls a filter, or window discriminator, that refines the electrode's recording signal by blocking out background noise. She hears cells coming because the discriminator is hooked up to a speaker as well as to a visual instrument called an oscilloscope. Watching and listening, Smith picks out a promising cell from its neighbors and starts collecting clues. Interestingly, she has found that antidepressants don't work as well in older brains, especially in the oldest old category. Since many antidepressants work selectively on the serotonin transporter, antidepressant ineffectiveness means that the transporter is not functioning the same as it does in younger subjects.