Abstract
We studied the social hierarchy in zebrafish and assessed differences in neurotransmitters and behavior in the F1 generation offspring of dominant and subordinate zebrafish (Danio rerio). We used behavioral assays to study locomotion, ability to complete cognitive tasks, social interaction and aggression. To study the neurochemical changes, we applied quantitative polymerase chain reaction, high pressure liquid chromatography and immunohistochemistry. Social hierarchies were formed both by males and females when animals were kept in same sex pairs in the dyadic dominant-subordinate hierarchy test. The offspring of dominant animals were the leaders in social interactions, however aggression in the mirror-test was not altered in any group. Serotonin and noradrenaline levels were lower in the F1 generation subordinate animals when compared with dominant animals, but not compared with animals that were naïve to social hierarchy. The mRNA level of the rate-limiting enzyme in histamine synthesis, histidine decarboxylase, was significantly lower in dominant and subordinate larval zebrafish when compared with control animals. In the dominant adult zebrafish tyrosine hydroxylase 1 mRNA level was lower compared with control animals, whereas tyrosine hydroxylase 2 mRNA was not different. The result was verified with immunohistochemistry. There were gender specific differences between the dominant and subordinate animals, where the dominant females performed better in cognitive tasks such as the T-maze than subordinate females. This was not observed in males, as the behavior of the dominant and subordinate males did not differ. These results add to the understanding of the plastic nature of the central nervous system and show that neurochemical features in aminergic neurotransmitter systems are associated with social leadership and dominance.
Highlights
Social hierarchy exists within the majority of different animal pop ulations including humans
We found that the offspring of the dominant fish of the P generation, i.e. the F1 generation dominant group, were social leaders, as they had the highest correlation for probability of being in front and leadership score when compared with the controls i.e. fish naïve to the social hierarchy test (Fig. 7a,b, n = 5/group)
Our results suggest that the offspring of dominant zebrafish are the leaders in a social context and that the levels of serotonin and noradrenaline are significantly lower in subordinate animals when compared with dominant animals. mRNA expression of the rate-limiting enzyme for histamine synthesis, hdc, as well as the rate-limiting enzyme for the dopamine synthesis, th1, are both significantly reduced when dominant zebrafish are compared with the fish that are naïve to the social hierarchy test
Summary
Social hierarchy exists within the majority of different animal pop ulations including humans. In rodents specific serotonergic modules affect aggression [7] whereas dopamine D2 receptor plays an important role in social hierarchy [8]. Especially the subordinate status, is associated with increased stress in fish [4]. Other studies have linked the hypothalamic aminergic systems to social hierarchy These neurotransmitter systems send widespread projections throughout the brain and communicate intensely with the PFC in mammals and dorsal telencephalon in fish [16]. The involvement of aminergic signaling in establishment of social hierarchy has been implicated, by significant impairment of the serotonergic system. Females tend to accept the social rank more than the males, a fact which may explain the discrepancy between the sexes in serotonin turnover [22]
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