Agonistic behavior in naive juvenile lobsters depleted of serotonin by 5,7-dihydroxytryptamine

We have been exploring the role of serotonin in fighting behavior in lobste rs using a specific model of agonistic behavior, the establishment of hiera rchical relationships between pairs of socially naive juvenile lobsters. We selected this model because the behavior is easily evoked, readily quantif iable, and the effects of experience are eleminated by using socially naive animals. In these studies we injected a specific neurotoxin, 5,7-dihydroxy tryptamine, into juvenile lobsters over a 4-week period and then measured t he effects on fighting behavior. This treatment reduces the levels of serot onin in the nervous system and immunocytochemical studies show a dramatic r eduction in neuropil staining for the amine. Control animals received vehic le injection alone. All injected animals were paired against larger or smal ler non-injected opponents, and three successive 30-min fights were carried out and statistically analyzed. The results were surprising: As with eleva tions of serotonin, reduced levels of serotonin increased the amount of tim e animals engaged in fighting behavior. No significant effects were seen on who initiated encounters, who retreated first, or who the eventual winner would be. Thus, in this model, elevation or reduction of serotonergic funct ion increases the tendency of animals to engage in agonistic encounters.