While the exact functions of dopamine and serotonin have been elaborated throughout different evolutionary lineages, this basic dichotomy between dopamine and serotonin has been remarkably conserved since the first bilaterians. In species as divergent as nematodes, slugs, fish, rats, and humans, dopamine is released by nearby rewards and triggers the affective state of arousal and pursuit (exploitation); and serotonin is released by the consumption of rewards and triggers a state of low arousal, inhibiting the pursuit of rewards (satiation).
