Socially-mediated differences in brain monoamines in rainbow trout: effects of trace metal contaminants

Monoaminergic systems play a crucial role in linking behaviour and physiology. Here the physiological and behavioural effects of metal exposure in relation to monoaminergic systems were considered by exposing rainbow trout dyads, demonstrating stable dominance relationships, to cadmium or lead. Fish exposed to 4 μg l−1 cadmium accumulated more cadmium at the gill than fish held in control water. Fish exposed to 7 μg l−1 cadmium had higher gill, liver and kidney cadmium concentrations. No significant lead accumulation was seen after exposure to 46 μg l−1 for 48 h but exposure to 325 μg l−1 lead caused an increase in gill, liver and kidney lead concentrations. Brain accumulation of both cadmium and lead was only seen after exposure to the highest concentrations. Exposure to 4 or 7 μg l−1 cadmium, or 46 or 325 μg l−1 lead for 48 h did not disrupt established dominance hierarchies. As expected with this stable behavioural situation, in control pairs, animals of different social status displayed different physiological profiles. Subordinate fish had higher concentrations of circulating plasma cortisol and telencephalic 5-hydroxyindoleacetic acid/5-hydroxytryptamine (serotonin) (5-HIAA/5-HT) ratios. However, these physiological profiles were affected by metal exposure, with a trend towards higher serotonergic activity in dominant fish. Dominants exposed to 325 μg l−1 lead had significantly higher hypothalamic 5-HIAA/5-HT ratios when compared with subordinates. The results demonstrate that if stable social hierarchies are established in control water they may not be affected by exposure to cadmium and lead although physiological changes may be evident.