Bold zebrafish (Danio rerio) express higher levels of delta opioid and dopamine D2 receptors in the brain compared to shy fish

Abstract

Individual variation in coping with environmental challenges is a well-known phenomenon across vertebrates, including teleost fish. Dopamine is the major transmitter in the brain reward networks, and important for motivational processes and stress coping. Functions of the endogenous opioid system are not well studied in teleosts. However, in mammals the activity in the brain reward networks is regulated by the endogenous opioid system. This study aimed at investigating if there was a correlation between risk-taking behavior and the expression of dopamine and opioid receptors in the zebrafish (Danio rerio) brain. Risk-taking behavior was assessed in a novel tank diving test, and the most extreme high risk taking, i.e. bold, and low risk taking, i.e. shy, fish were sampled for qPCR analysis of whole brain gene expression. The expression analysis showed a significantly higher expression of the dopamine D2 receptors (drd2a and drd2b) and the delta opioid receptor (DOR; oprd1b) in bold compared to shy fish. Besides reward and reinforcing properties, DORs are also involved in emotional responses. Dopamine D2 receptors are believed to be important for active stress coping in rodents, and taken together the results of the current study suggest similar functions in zebrafish. However, additional experiments are required to clarify how dopamine and opioid receptor activation affect behavior and stress coping in this species.

Ecotoxicity Assessment of Fe3O4 Magnetic Nanoparticle Exposure in Adult Zebrafish at an Environmental Pertinent Concentration by Behavioral and Biochemical Testing

“…In comparison to the control group, a significant reduction of the dopamine level in the brain was detected in 10 ppm MNPs exposed group ( p = 0.01). Dopamine is the major neurotransmitter in the brain for coping with stress [62]. The low level of dopamine after MNPs exposure might be related to an observed steep decrease in the exploratory behavior of zebrafish in the novel tank test and aggressiveness test.…”

Coping styles in European sea bass: The link between boldness, stress response and neurogenesis

“…responses, including bolder behavior [81][82][83]. Nevertheless, it is important to note that our study was performed on the whole brain, therefore we cannot conclude that the differences between bold and shy fish are representative of what would be measured in region-specific areas.…”

Zebrafish Danio rerio shows behavioural cross‐context consistency at larval and juvenile stages but no consistency between stages

“…This could also be attributed to the previous experience and manipulation of the fish at 8 dpf or smaller sample size at this experimental age. Such a behavioural syndrome, higher activity in bold individuals, is indeed well described in the literature, including in fish (Castanheira et al ., 2017; Geffroy et al ., 2015; Moretz et al ., 2007; Réale et al ., 2010), whereas the link between boldness and thigmotaxis is less well documented and still debated (Ariyomo et al ., 2013; Burns, 2008; Dahlbom et al ., 2011; Thörnqvist et al ., 2019). In this study, bold fish showed higher thigmotaxis.…”