The Role of Serotonin in Tritonia diomedea1

SYNOPSIS. The within-swim pattern of cycle periods in Tritonia swimming changed when the behavior was repeatedly elicited suggesting that an excitatory process reaches a ceiling or wanes over repeated trials. Exposure to subthreshold stimuli enhanced swimming in response to a subsequent super-threshold stimulus, perhaps using a similar excitatory process. In reduced preparations, subthreshold stimuli increased action potential activity in identified serotonergic neurons. Finally, stimulating serotonergic neurons enhanced a fictive swimming pattern, much like subthreshold stimuli enhanced the swimming behavior. Both within-swim and across-swim changes in the swimming behavior may be caused by increased activity in identified serotonergic neurons. Comparative study suggests that ancestral serotonergic systems facilitated network oscillations for the production of rhythmic behaviors such as feeding and locomotion. This concept of serotonin as oscillatizer is used to explain the role of serotonergic neurons in Tritonia. Implications for human mental health are discussed.