Skin darkening, a potential social signal in subordinate Arctic charr (Salvelinus alpinus): The regulatory role of brain monoamines and pro-opiomelanocortin-derived peptides

Arctic charr were allowed to interact in groups of three for 5 days. Skin d arkness was quantified by measuring the mean brightness of individual fish before and after social interaction. Brain levels of monoamines and monoami ne metabolites and plasma concentrations of cortisol, adrenocorticotropic h ormone (ACTH), N-acetyl-beta-endorphin and alpha-melanocyte-stimulating hor mone (alpha-MSH) were analysed, The results show that social subordination resulted in a significant skin darkening. Furthermore, plasma concentration s of alpha-MSH, ACTH and cortisol were elevated in subordinates, and these fish also displayed elevated levels of 5-hydroxyindoleacetic acid (5-HLAA) in the telencephalon, The ratio of ES-HIAA] to serotonin [5-HT] was increas ed in several brain areas. In addition, the ratio of 3-methoxy-4-hydroxyphe nylglycol (MHPG) to norepinephrine (NE) concentrations was significantly in creased in the optic tectum of subordinate fish. Skin darkness following so cial interaction showed a significant positive correlation with plasma leve ls of alpha- MSH, Plasma levels of ACTH and alpha(-MSH were both positively correlated with that of cortisol. Brain [5-HIAA]/[5-HT] ratios were positi vely correlated with circulating plasma levels of ACTH, and a similar posit ive correlation was seen between [MHPG]/[NE] ratios in the optic tectum and plasma levels of ACTH, alpha-MSH and N-acetyl-beta-endorphin. In contrast, hypothalamic [MHPG]/[NE] ratios displayed a negative correlation with plas ma alpha-MSH concentrations, The present study demonstrates that social str ess induces skin darkening in Arctic charr and that this effect could be me diated by a stress-induced increase in the levels of alpha-MSH in the circu lation. Furthermore, the results suggest that 5-HT and NE in the central ne rvous system could be factors regulating the pituitary release of ACTH and alpha-MSH.