As a day animal with sensitivity to inflammation similar to that of humans, the sheep may highly outperform the rodent model in inflammation studies. Additionally, seasonality makes sheep an interesting model in endocrinology research. Although there are studies concerning inflammation's influence on leptin secretion and vice versa, a ewe model, with its possible 'long-day leptin resistance', is still not examined enough. The present study aimed to examine whether leptin may modulate an acute inflammation influence on plasma hormones in two photoperiodical conditions. The experiment was conducted on 48 ewes divided into four groups (control, lipopolysaccharide (LPS), leptin, LPS + leptin) during short and long days. Blood sampling started 1 hour before and continued 3 h after LPS/saline administration for further hormonal analysis. The results showed that the photoperiod is one of the main factors influencing the basal concentrations of several hormones with higher values of leptin, insulin and thyroid hormones during long days. Additionally, the acute inflammation effect on cortisol, insulin and thyroid hormones was photoperiod-dependent. The endotoxemia may also exert an influence on leptin concentration regardless of season. The effects of leptin alone on hormone blood concentrations are rather limited; however, leptin can modulate the LPS influence on insulin or thyroxine in a photoperiod-dependent way.
Keywords: cortisol; insulin; leptin; lipopolysaccharide; photoperiod; plasma; sheep; stress; thyroid.