Serotonin 5-HT2 and 5-HT1A-like receptors differentially modulate aggressive behaviors in Drosophila melanogaster

The study of aggression in flies has a long history (Jacobs, 1960, 1978); Drosophila exhibits aggressive behavior in the acquisition of food, territory, and mates, and these behaviors can differ among fly strains (Jacobs, 1960, Dow and Schilcher, 1975, Jacobs, 1978, Hoffmann, 1987). Only recently, however, have modern techniques been used to explore this behavior and establish the fly as an accepted model system to study molecular mechanisms underlying aggression. Drosophila exhibit complex multi-component aggressive behaviors that are easily discernable and scored (Chen et al., 2002, Nilsen et al., 2004). These behaviors, along with many others, are often very similar to those observed in mammals (Nichols, 2006). It is known that male flies will fight with each other, but not with females, to establish social dominance (Yurkovic et al., 2006). Interestingly, sexually dimorphic aggressive behavioral patterns have been shown to be mediated by the fruitless gene (Vrontou et al., 2006, Chan and Kravitz, 2007). Recently, the neurotransmitter octopamine also has been found to mediate aspects of aggression in the fly (Hoyer et al., 2008, Zhou et al., 2008). Genetic studies with fly strains bred for hyper- and hypoaggressiveness, combined with microarray analysis, has delineated some of the molecular factors underlying aggression (Dierick and Greenspan, 2006). Although several differentially expressed genes have been identified, only one, for a cytochrome P450 (Cyp6a20), was determined to affect aggression directly. Somewhat surprisingly, serotonin levels in these strains did not differ. Serotonin has been strongly implicated in mediating aggressive behaviors in other animals, ranging from the more evolutionarily related crustacean, to birds, and mammals (Kravitz, 2000, Sperry et al., 2003, Popova, 2006). In the lobster, infusion of serotonin into the hemolymph elicits a socially dominant posture (Livingstone et al., 1980), as well as encouraging subordinate engagement of dominants (Huber et al., 1997). Attempts to alter serotonin levels in crayfish using biosynthetic precursors and inhibitors also have been found to modulate aggressive and social behaviors (Panksepp and Huber, 2002). The effects of serotonin in crustaceans are likely mediated by receptors similar to mammalian 5-HT1A and 5-HT2 receptors, although they are more dissimilar to their mammalian counterparts with regard to ligand binding profiles than the corresponding Drosophila receptors (Spitzer et al., 2008). In mammals, many aspects of the serotonin system including biosynthesis, metabolism, and receptor activation, primarily of 5-HT1A, 5-HT1B, and 5-HT2 receptors, have been demonstrated to modulate aggressive behaviors (Popova, 2006). In the fly, the effect of manipulating serotonin levels on aggression has been studied by Baier and coworkers (Baier et al., 2002), who examined the potential role of serotonin by feeding flies a metabolic precursor to serotonin, 5-HTP, and a biosynthesis inhibitor, pCPA, to raise and lower endogenous levels, respectively. Neither of these treatments, however, was found to alter aggression in their assays. These unexpected results should not be surprising, however, because two of the three families of serotonin receptors in Drosophila are functional orthologs of the mammalian 5-HT1A and 5-HT2 receptors (there is also a 5-HT7 receptor ortholog). In mammalian systems these two receptor subtypes often act to oppose one another functionally. Moreover, in previous crayfish and lobster studies, it was found that the rate of manipulation of serotonin levels (i.e. rapidly or slowly) was more important for modulation of aggressive behaviors than absolute levels (Ma et al., 1992, Panksepp and Huber, 2002, Panksepp et al., 2003), and it may be that a different treatment protocol would have uncovered a role for serotonin. Nevertheless, in the flies bred for hyperaggressivness by Dierick and Greenspan, mentioned above, follow-up studies manipulating 5-HT levels demonstrated that serotonin does modulate aspects of aggression in these flies, implicating serotonin in this behavior (Dierick and Greenspan, 2007). In flies, serotonin has been demonstrated to modulate sleep, circadian behaviors, and sensory processing (Nichols et al., 2002, Yuan et al., 2006, Nichols, 2007). In both flies and mammals the effects of serotonin are primarily mediated through interactions with a number of G-protein coupled receptor proteins to initiate various effector pathways (Nichols and Nichols, 2008). Significantly, the orthologous serotonin receptor families present in Drosophila are the 5-HT2, 5-HT1A-like, and 5-HT7 receptors (Witz et al., 1990, Saudou et al., 1992, Colas et al., 1995). The 5-HT1A-like receptors include the 5-HT1ADro and 5-HT1BDro (formerly the 5-HTdro2A and 5-HTdro2B) receptors, which are believed to be duplicated genes and homologues to the mammalian 5-HT1A receptor (Saudou et al., 1992). Of all the serotonin receptors, the 5-HT1A, 5-HT1B, and 5-HT2, receptors have been the most strongly implicated in mammalian and human behaviors. Here, we have used pharmacological methods to probe individually 5-HT2 and 5-HT1A-like receptor circuitries for their roles in modulating aggression in Drosophila, and show that 5-HT2- and 5-HT1A-like receptors have differential effects on aggressive behaviors in the fly