A single exposure to social isolation in domestic piglets activates behavioural arousal, neuroendocrine stress hormones, and stress-related gene expression in the brain
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Abstract
Stressful early life events can have short- and long-term effects on neuroendocrine
and behavioural mechanisms of adaptation. Here, we investigated the effects of a single
social isolation (4 h) of domestic piglets on both behavioural alterations in open-field
tests and modifications in the expression of genes regulating glucocorticoid response
in stress-related brain regions at 7, 21 or 35 days of age. The mRNAs of glucocorticoid
receptor (GR), mineralocorticoid receptor (MR), 11ss-hydroxysteroid dehydrogenase
1 and 2 (11ss-HSD1 and 11ss-HSD2) and c-fos were analysed by real-time RT-PCR in the
hypothalamus, hippocampus and amygdala. The social isolation caused both elevated
stress hormone concentrations (e.g. cortisol) and open-field reactivity (e.g. locomotion,
vocalisation) compared to control piglets. The enhanced behavioural and neuroendocrine
activity was associated with distinct changes in gene expression in the limbic system.
The hypothalamic GR, MR and 11ss-HSD1 mRNA expressions and the hippocampal 11ss-HSD1
mRNA was significantly higher in isolated piglets, whereas in the amygdala social
isolation caused a significant decrease in MR mRNA expression. Isolated piglets also
displayed significantly higher c-fos mRNA expression, an estimate of neuronal activation,
in hypothalamus and amygdala. The mRNA alterations as well as the behavioural and
hormonal pattern show an effect of social isolation on days 7 and 21, but no effect
on day 35. In conclusion, a single social isolation in piglets caused age-dependent
neuroendocrine and behavioural changes that indicate increased arousal and experienced
distress. The present results also suggest that psychosocial stress effects should
be considered for the assessment of livestock handling practices with respect to health
and welfare.