title: Social stress affects circulating melatonin levels in rainbow trout.
abstract: In salmonid fishes there are indications that socially subordinate individuals avoid competition with larger, dominant fish by adjusting daily feeding and activity cycles. As in other vertebrates, the pineal organ and its hormone melatonin act as synchronizers of daily rhythms to the external light/dark cycle in salmonids. Social defeat may act as a potent stressor; inducing elevated glucocorticoid secretion and a general behavioral inhibition. Here, we show that social stress also affects circulating melatonin levels in rainbow trout, a species known to display strong dominance hierarchies both in the wild and under captive rearing. Subordinate individuals had significantly higher nighttime melatonin levels than dominant fish or controls. There was no effect of social rank on the much lower melatonin levels observed in animals sampled during the day. Correlations between circulating glucocorticoids and melatonin depended on circadian cycles as well as social context. This study suggests that altered melatonin production contributes to the physiological and behavioral profile of subordinate animals. Social status, and other determinants of the stress level of experimental animals, therefore should be taken into consideration as potential factors influencing the results from in vivo research on this hormone.
authors.affiliation: Department of Comparative Physiology, Evolutionary Biology Centre, Norbyvägen 18A, Uppsala University, SE-75236 Uppsala, Sweden.
authors.forename: Earl T
authors.initials: ET
authors.lastname: Larson
authors.forename: Svante
authors.initials: S
authors.lastname: Winberg
authors.forename: Ian
authors.initials: I
authors.lastname: Mayer
authors.forename: Olivier
authors.initials: O
authors.lastname: Lepage
authors.forename: Cliff H
authors.initials: CH
authors.lastname: Summers
authors.forename: Øyvind
authors.initials: Ø
authors.lastname: Øverli
mesh_terms: D000378:Agonistic Behavior / Q000502:physiology; D000818:Animals; D001522:Behavior, Animal / Q000502:physiology; D002940:Circadian Rhythm / Q000502:physiology; D006854:Hydrocortisone / Q000097:blood; D008550:Melatonin / Q000097:blood*; D017686:Oncorhynchus mykiss / Q000097:blood* / Q000502:physiology*; D012930:Social Dominance; D013312:Stress, Physiological / Q000097:blood / Q000662:veterinary*
chemical_list: D008550:Melatonin; D006854:Hydrocortisone