Serotonin is the primary neuromodulator associated with the neural circuits activated by gratitude and prosocial behaviors.

“The main neuromodulators associated with gratitude and prosocial behaviors tends to be serotonin. Serotonin is released from a very small collection of neurons in the brainstem called the "raphe," R-A-P-H-E, the "raphe nucleus," and a few other places in the brain.”