Physiological changes in response to social isolation in male medaka fish
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ORIGINAL ARTICLE Biology
Physiological changes in response to social isolation in male medaka fish Airi Otsuka1 · Moeko Inahata1 · Yuki Shimomura1 · Nao Kagawa1 Received: 10 April 2020 / Accepted: 25 June 2020 © Japanese Society of Fisheries Science 2020
Abstract Fish that display schooling and social behaviors are expected to show stress responses when they are challenged by isolation. Medaka exhibit social behavior in group situations, but little is known about the effects of social isolation on their physiological stress responses. Here, we subjected male medaka to several different housing situations and assessed whether the stress responses differed between grouped and isolated fish. Social isolation decreased plasma cortisol and increased brain serotonin [5-hydroxytryptamine (5-HT)] levels in male medaka irrespective of whether or not the isolated fish could visually perceive another individual. However, heat shock treatment, a grouped situation, and a paired situation increased plasma cortisol and decreased brain 5-HT levels. These results suggest that, in male medaka, physical contact among individuals is more stressful than isolation. Moreover, both grouped and paired fish had lower tryptophan hydroxylase 2 (Tph2) expression levels than isolated fish, whereas heat shock downregulated both Tph1 and Tph2. The present study indicates that, in fish, the magnitude of reduction in brain Tph1 and Tph2 expression levels and serotonin concentration varies with the type of stress. Keywords Medaka · Plasma cortisol · Serotonin · Social isolation · Tryptophan hydroxylase
Introduction For social mammals such as mice, rats, and certain domesticated species, isolation triggers physiological and behavioral stress responses (Brain and Nowell 1971; Serra et al. 2007; Kanitz et al. 2004; Guesdon et al. 2015). In fish, social isolation may occur during aquaculture management practices such as equipment maintenance and the application of individual drug treatments. Under natural conditions, certain fish species exhibit shoaling behavior and socially interact with each other. Therefore, rearing a fish in isolation is expected to alter its stress responses. Isolation-associated physiological changes have been described for various fish taxa such as angelfish (Gomez-Laplaza and Morgan 1991), cichlids (Earley et al. 2006; Galhardo and Oliveira 2014), swordtail (Hannes and Franck 1983), rainbow trout (Sørensen et al.
* Nao Kagawa [email protected] 1
Department of Life Science, Faculty of Science and Technology, Kindai University, Higashiosaka, Osaka 577‑8502, Japan
2012), perch (Heynen et al. 2016), and zebrafish (Parker et al. 2012; Giacomini et al. 2015; Forsatkar et al. 2017). Depending upon the animal species, however, social isolation may either augment or diminish physiological stress responses such as alterations in plasma cortisol and glucose levels and brain monoaminergic activity. Thus, it is useful and informative to determine how an animal species responds to the stress of social isolation even when th
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