Tyraminergic modulation of agonistic outcomes in crayfish
Octopamine, a biogenic amine, modulates various behaviors, ranging from locomotion and aggression to learning and memory in invertebrates. Several studies recently demonstrated that tyramine, the biological precursor of octopamine, also affects behaviors independent of octopamine. Here we investigated the involvement of tyramine in agonistic interaction of the male crayfish Procambarus clarkii. When male crayfish fight, larger animals (3–7% difference in body length) are more likely to win. By contrast, direct injection of tyramine or octopamine counteracted the physical advantage of larger animals. Tyramine or octopamine-injected naive large animals were mostly beaten by untreated smaller naive animals. This pharmacological effect was similar to the loser effect in which subordinate larger animals are frequently beaten by smaller animals. Furthermore, loser effects were partly eliminated by either injection of epinastine, an octopamine blocker, or yohimbine, a tyramine blocker, and significantly diminished by injection of a mixture of both blockers. We also observed that tyramine levels in the subesophageal ganglion were remarkably increased in subordinate crayfish after losing a fight. These results suggest that tyramine modulates aggressive levels of crayfish and contributes to the loser effect in parallel with octopamine.
KeywordsTyramine Octopamine Agonistic interaction Loser effect Aggression
Naive large animal
Naive small animal
Subordinate large animal
We are grateful to Kawamura. K for reading of the manuscript. This work was supported by Grants-in-Aid from the Ministry of Education, Science, Sport, and Culture to TN (16K07432). All experiments were carried out in accordance with the Guide for the care and use of Laboratory animals of Yamagata University (Japan).
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