Serotonin alters decisions to withdraw in ®ghting cray®sh, 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 ®ghting in a cray®sh model for which the complex agonistic behavior is well characterized. Based on a quantitative analysis of ®ghting we demonstrate that the infusion of small amounts of serotonin into freelymoving cray®sh alters ®ghting behavior by speci®cally interfering with the timing of a treated animal's decision to withdraw from an encounter. In the presence of added serotonin, ®ghts 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 ̄uoxetine as a potent inhibitor of serotonin re-uptake. Although no behavioral changes were associated with acute infusion of ̄uoxetine alone, in combination with serotonin it e€ectively prevented the previously observed ®ght-enhancing e€ects. Our data strongly support the signi®cance of functional amine reuptake mechanisms for behavior and continued use of this invertebrate model should prove a promising route to unravel further the complex bases of aggression.