Potential Synergism between Stress and Contaminants in Free-ranging Cetaceans

TLDR: It is likely that noise, increasingly encountered by marine mammals, will add to their allostatic load and disturb the delicate balance required by immature and adult mammals to react optimally to stressors.

Abstract: Noise has increased significantly over the last decades in oceans, and this trend is accelerating in large part because of oil exploration and exploitation, both of which are expanding worldwide. Considered together with recent evidence that noise disturbs the behavior, echolocation, navigation and communication of marine mammals, it is likely that noise, increasingly encountered by marine mammals, will add to their allostatic load. Glucocorticoids (GCs) are the major hormones that mediate the long term effects of stress. GCs’ effects depend, among other factors, on the intracellular concentrations of the various isoforms of the glucocorticoid receptors (GR). Tissue and cell-type specificity are also conferred by the presence in target cells of GR ligands such as chaperones, cochaperones and modulatory element binding proteins whose concentrations vary according to tissue, cell types and even to the cell cycle phase. The normal regulation of GCs production in adult life relies on the normal development of the hypothalamus-pituitary adrenal (HPA) axis in uterine and early postnatal life, which in turn depends on the absence of chronic stress imposed to both the mother and newborn during these critical periods. Worldwide, cetacean populations, such as the beluga population inhabiting the St Lawrence Estuary (SLE) in Canada, are exposed to anthropogenic stressors, and are contaminated by persistent lipophilic contaminants of which many are abundantly transferred to newborns during lactation. GCs and certain organochlorine contaminants (OCs), for instance dioxin-related polychlorinated biphenyls (DRPBs), mediate their prolonged and profound effects through nuclear receptors such as aryl hydrocarbon receptors (AhR). These effects are exerted on most organs, especially on the developing brain and lymphoid organs of fetuses and juveniles and on adrenal glands of adult mammals. Multiple interactions have been demonstrated between GCs and OCs, often through interactions between their receptors. These interactions may disturb the delicate balance required by immature and adult mammals to react optimally to stressors.