Serotonin alters decisions to withdraw in fighting crayfish, Astacus astacus: The motivational concept revisited

The biogenic amine serotonin is thought to play an important role in aggression in many species, including man. This paper summarizes experimental approaches which attempt to link this neuromodulator with fighting in a crayfish model for which the complex agonistic behavior is well characterized. Based on a quantitative analysis of fighting we demonstrate that the infusion of small amounts of serotonin into freely-moving crayfish alters fighting behavior by specifically interfering with the timing of a treated animal's decision to withdraw from an encounter. In the presence of added serotonin, fights last considerably longer compared to controls, but no changes were detected in the rules of escalation, the likelihood of initiating an interaction, or its eventual outcome. Attempts to dissect the underlying neuronal mechanisms pharmacologically hinged on fluoxetine as a potent inhibitor of serotonin re-uptake. Although no behavioral changes were associated with acute infusion of fluoxetine alone, in combination with serotonin it effectively prevented the previously observed fight-enhancing effects. Our data strongly support the significance of functional amine re-uptake mechanisms for behavior and continued use of this invertebrate model should prove a promising route to unravel further the complex bases of aggression.