The Effects of In Ovo Cortisol Exposure on Behaviour and Stress Axis Organization in the Threespine Stickleback Gasterosteus aculeatus

Amongst vertebrates, transfer of maternal glucocorticoid (GC) stress hormones to developing embryos through egg yolk (in ovo) or in utero is documented in a variety of species. This phenomenon has attracted attention in recent years because of the significant physiological and behavioural consequences of this early-life GC exposure and their potential relevance to human health and disease progression. Prenatal programming by maternal glucocorticoid exposure has also been proposed to play a role in adaptation, although natural systems in which to test this hypothesis are lacking. In this study, in ovo cortisol treatment of threespine stickleback eggs at physiologically significant levels is used to test the effects of maternal glucocorticoids on juvenile brain expression levels of three candidate stress-axis genes (GR1, GR2, and POMC), and on three stress-linked behaviours thought to be sensitive to maternal GC levels (aggression, boldness, and shoaling). In rats, glucocorticoid receptor (GR) down-regulation leading to reduced glucocorticoid feedback sensitivity and a subsequent increase in stress-axis reactivity is thought to explain decreases in aggression and boldness that often follow maternal GC elevation in that species. I report that, contrary to the bulk of findings in mammalian studies, GR1 and GR2 expression levels in cortisol-treated fish are higher than those seen in untreated fish. Although cortisol treatment influenced gene expression, neither shoaling, aggression nor boldness were strongly influenced by treatment. Sex differences in boldness and aggression levels matched previously described results, which suggests behavioural assaysthat GC-mediated maternal effects on offspring behaviour may not be attributable exclusively to yolk cortisol increases, or perhaps that behavioural changes may have been too subtle to be detected at the dosage used. Ultimately, this study identifies persistent offspring GR up-regulation as a potential consequence of elevated maternal glucocorticoid levels in fish. The diverse roles of GR in physiological and behavioural regulation suggest that the consequences of this up-regulation for survival outcomes are probably complex, a fact that future studies investigating the adaptive significance of glucocorticoid-mediated maternal effects in fish should take care to consider.Science, Faculty ofUnreviewedUndergraduat