The mesolimbic dopamine system is believed to be highly impo

The mesolimbic dopamine system is believed to be highly important for evaluating environmental stimuli in order to generate an adaptive behavioral response. Thus, not surprisingly these functions appear to have been highly conserved across the vertebrate subphylum. Even though teleost fish lack midbrain dopaminergic cells, there is evidence that suggest that forebrain dopaminergic cells have homologous functions to the mammalian mesolimbic system [25]. The mesolimibic dopamine system is interconnected to another neuronal network that has been described as the social behavior network [14], and interaction between these systems are likely to be important for generating intraspecific divergence in behavioral phenotypes. Functions of the endogenous opioid system are not well studied in teleosts. However, in mammals the activity in the mesolimbic dopamine system is regulated by the endogenous opioid system, where endogenous opioids contribute to the release of dopamine in the nucleus accumbens, partly through disinhibition of the of dopaminergic neurons in the ventral tegmental area [26,27]. Individual differences in these processes are important for physiological processes as well as in vulnerability for substance use disorders [[27], [28], [29]], but the mechanisms behind individual differences are far from well understood.
Material & methods
Results
Discussion In the present study, male zebrafish of the AB line were classified as bold and shy, respectively, based on activity and risk-taking behavior in the novel tank diving test, and extreme bold and shy individuals were selected for analysis of whole 9508 gene expression. The results revealed higher relative expression of dopamine D2 receptors (drd2a and drd2b), and delta opioid receptors (oprd1b) in bold relative to shy fish. The expression of drd2b and oprd1b was also correlated to the duration spent at the bottom of the novel tank. Dopamine has a wide range of physiological functions and is implicated in the control of locomotion, cognition, emotion, neuroendocrine secretion, and reward and reinforcement processes [33,34]. Dopaminergic receptors are expressed in the central nervous system of vertebrates including zebrafish [10,11,33]. However, the distribution of dopaminergic neurons in zebrafish differ from that of mammals; zebrafish have some extra telencephalic and diencephalic dopaminergic nuclei, whereas dopaminergic neurons are absent in the mesencephalon [35]. Moreover, while five dopamine receptor subtypes have been identified in mammals, eight dopamine receptor subtypes have been reported in zebrafish. drd1 is the homolog of the mammalian D1-like receptor type, and drd2a, drd2b, drd2c, drd3, drd4a, drd4b and drd4c are homologous to the mammalian D2-like receptor type [10,11]. The finding herein of higher relative expression of dopamine D2 receptors (drd2a and drd2b) in fish with higher risk-taking behavior and locomotor activity, i.e. bold relative to shy fish, are in line with previous findings in rats. Risk-assessment behavior was correlated with inherent dopamine activity in the dorsal striatum. This effect was suggested to be of importance for inhibitory control and decision-making [36]. Moreover, rats with increased novelty-induced locomotion had higher D2/D3 receptor binding in the dorsal striatum and the nucleus accumbens shell, and these rats were also found to be dominant [37]. In a study by Dahlbom et al. [30], a multivariate data analysis showed that zebrafish males were bolder than females, and that the behaviors displayed could be used to predict which individual fish that later would become dominant or subordinate in a dyadic contest. Thus, the conclusion was that bold behavior is positively correlated to dominance in zebrafish [30]. When non-human primates were housed individually, no difference in D2-receptor binding was found between dominant and subordinate monkeys, while social housing resulted in higher D2 receptor expression in the basal ganglia in dominant compared to subordinate monkeys [38]. In the current experiment fish were kept isolated for 15 days and, even though they had visual contact with neighboring fish, they could not interact and form dominance hierarchies. Still, experimental fish were adults raised in groups. Thus, we do not know if the differences in the expression of D2 dopamine receptors between bold and shy males develop through social interaction, or if these differences are innate, resulting in different behavioral traits that possibly predispose the fish for different social positions.