The effects of dopamine on the agonistic behaviour of crayfish were analysed. When dopamine concentrations of 1 µM were injected into large crayfish, individuals were beaten by smaller opponents, despite their physical advantage. Injection of 10 µM dopamine into small animals increased their winning rate against larger opponents. The inhibitory effect of dopamine on larger animals would be mediated by D1 receptors, and the injection of D1 receptor antagonist prohibited the onset of a loser effect in subordinate animals. The facilitating effect of dopamine on small animals would be mediated by D2 receptors, and the injection of D2 receptor antagonist prohibited the onset of a winner effect in dominant animals. Since the inhibitory effect of 1 µM dopamine was similar to 1 µM octopamine and the facilitating effect of 10 µM dopamine was similar to 1 µM serotonin, functional interactions among dopamine, octopamine, and serotonin were analyzed by co-injection of amines with their receptor antagonists in various combinations. The inhibitory effect of 1 µM dopamine disappeared when administered with D1 receptor antagonist, but remained when combined with octopamine receptor antagonist. Octopamine effects disappeared when administered with either D1 receptor antagonist or octopamine receptor antagonist, suggesting the dopamine system was downstream of octopamine. The facilitating effect of 10 µM dopamine disappeared when combined with serotonin 5HT1 receptor antagonist, as well as D2 receptor antagonist. Serotonin effects also disappeared when combined with D2 receptor antagonist, suggesting that dopamine and serotonin activated each other through mutual parallel pathways.

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