Furthermore it is possible that the reduced forebrain neural plasticity and

Furthermore it is possible that the reduced forebrain

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1998; Ebbesson and Braithwaite, 2012). Furthermore, it is possible that the reduced forebrain neural plasticity and cognitive deficit at the critical smolt stage also affect imprinting, by altering the olfactory-telencephalic plasticity associated with smoltification (Ebbesson et al., 1996a; Ebbesson et al., 2003; Folgueira et al., 2004). Memories of the natal stream formed during imprinting are later used to return as adults (Hasler and Scholz, 1983; Yamamoto et al., 2010), and thus impaired imprinting could have a profound impact on return success. Such behavioral processes are likely to involve the area of the brain involved in spatial learning and memory, namely the dorsolateral area of the telencephalon (Ebbesson and Braithwaite, 2012). We predicted that fish exposed to a prolonged period of elevated Al would be less accurate in the maze task than control fish. While there was an overall decrease in the number of mistakes made as the experiment progressed for fish from both groups, salmon from the Acid-Al treatment made more mistakes than control fish. It is possible that poorer learning in Acid-Al fish is linked to the observed decrease in neural plasticity, in line with a lower level of NeuroD1 mRNA expression in the telencephalon. The relationship between NeuroD1 gene expression and neurogenesis during early development has been identified in fish (Korzh et al., 1998; Mueller and Wullimann, 2003), and it is well established that neurogenesis in the telencephalon is known to occur throughout adult life in fish (Lema et al., 2005; Zupanc, 2008; von Krogh et al., 2010). The telencephalon is one of the most sensitive regions to stress-induced changes in the fish brain (Sørensen et al., 2011). A possible explanation for the reduced NeuroD1 expression in the telencephalon of Acid-Al fish was the increased levels of stress, shown by the higher levels of plasma glucose and cortisol in this group of fish before testing in the maze. The functional consequences of stress are dependent on the magnitude and duration of the stressor, but chronic stress is known to have detrimental effects on learning and memory (Conrad, 2010). Therefore, decreased neural plasticity suggested by a decrease in NeuroD1 mRNA expression may have mediated some of the observed behavioral changes. Both cortisol and plasma glucose levels have been used as indicators of physiological stress in fish (Barton, 2002; Barton and Iwama, 1991; Begg and Pankhurst, 2004; Wendelaar Bonga, 1997). When cortisol is released in response to a stressful event, it mobilizes fuels such as glucose to bring the fish back to homeostasis (Gregory and Wood, 1999). In the present study, Acid-Al fish had higher levels of both plasma glucose and cortisol than control fish, suggesting a higher stress level due to activation of the endocrine hypothalamic-pituitary-interrenal (HPI) axis. The effects of acidification on Atlantic salmon have been studied and, similarly, it was found that low pH was strongly correlated with negative physiological effects (Liebich et al., 2011). Thus in the present study, fish exposed to Al toxicity in acidic water will have experienced
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