Tyramine and octopamine have opposite effects on the locomotion of Drosophila larvae

Biogenic amines are believed to play important roles in producing behaviors. Although some biogenic amines have been extensively studied in both vertebrates and invertebrates, little is known about the effects of trace amines like tyramine and octopamine. We investigated how trace amines affect behaviors using quantitative morphometric methods on Drosophila Tbetah(nM18) and iav(N) mutants that have altered levels of tyramine and octopamine. Locomotion of wild-type and mutant third instar larvae was analyzed using Dynamic Image Analysis System (DIAS) software. We found that Tbetah(nM18) mutants, with elevated tyramine levels and reduced octopamine levels, had a severe locomotion phenotype. Mutant larvae spent much more time in pausing episodes than wild-type larvae and displayed a reduction in speed and linear translocation. The locomotion phenotype was partially rescued by feeding Tbetah(nM18) larvae octopamine, an effect that could be nullified with simultaneous feeding of tyramine. Feeding Tbetah(nM18) larvae yohimbine, an agent that inhibits the activity of Drosophila tyramine receptors, also improved some locomotion parameters. Feeding both octopamine and yohimbine further improved rescue efficiency. Simultaneously reducing the octopamine and tyramine levels as in iav(N) larvae, in contrast, led to a less severe behavioral phenotype than that of Tbetah(nM18) mutants. Feeding iav(N) larvae either tyramine or octopamine exerted only a minor improvement in locomotion. These results suggest that tyramine and octopamine have opposite effects on Drosophila larval locomotion regulation and that a balance between the two is important in producing normal behavior.