Serotonin has sometimes been called the “police officer” of the brain, regulating the action of other neurotransmitters like dopamine. But this seems to me to be not quite the right analogy. I think of serotonin as the mood music of the brain.
Originally, serotonin was a signal from the gut to the muscles that said, “Whoa, slow down.” When the roundworm C. elegans is on top of a beautiful lawn of yummy bacteria, serotonin produced in the gut says “sit back and stay a while, this here’s good eatin’.” As evolution advanced, serotonin was used in the brain to signal that things were generally kopacetic, that you not only have good food, but good friends, enough gold in your pocket, and it looks like it’s going to be a beautiful day. A lack of dopamine would be the signal that there are clouds on the horizon, the friends are getting surly, and it’s time to move along to greener pastures.
Serotonin sets the mood by which other neurotransmitter signals will be interpreted. Try watching a scary movie with the sound turned down and some amusing music turned on. It’s not so scary anymore. This is like having a lot of serotonin. But now think of movie scenes of an empty dining room, a bedroom with the beds made and flowers on the nighstand, and add the soundtrack from Jaws, Psycho or some other scary movie. Now those innocent scenes seem ominous. Something bad is going to happen. Things look a little too perfect in this house. That’s the condition when there isn’t enough serotonin (or the receptor is not as effective).
Dopamine is like the words of a song, signalling “hey that’s good, do that again,” but serotonin is like the music that conveys the mood. Even if the words are positive, if the music is discordant, they are going to feel off.
Evolution seems to spend a lot of time tinkering to get the right mix of serotonin. Too much and we become such a sunny optimist that when we see a lion we want to pet the kitty. Things seem so awesome that we just want to lie on our backs and look at the sky, even when in reality our food supply is running out and winter is coming on. Que sera, you know? A little less serotonin and we are worried about how we are going to get breakfast tomorrow. A little less than that and we are worried about how we are going to get breakfast next month. And things just don’t seem right unless the knots in our shoelaces are tied perfectly, with evenly sized loops. In other words, we are little neurotic. Less serotonin than that and we get depressed–nothing seems right–and ultimately we are back to sitting around and staring at the sky, only this time it’s because every option for action looks so bad.