Effects of photoperiod and feeding level on perirenal adipose tissue metabolic activity and leptin synthesis in the ovariectomized ewe

Leptin is secreted by adipose tissue and plays a pivotal role in regulating both body energy homeostasis and reproductive function in rodents. Among livestock, sheep is a seasonal breeder whose reproductive period is initiated by short daylength. We show that plasma leptin and leptin gene expression in perirenal adipose tissue were decreased when ovariectomized Lacaune ewes were exposed to short days (8 versus 16 h light/d). This effect of the photoperiod occurred despite the nutritional status, with leptin levels lower in underfed than in refed ewes, and without significant changes in perirenal fat mass and adipocyte size. Plasma prolactin and leptin followed a similar pattern suggesting a relationship between the two hormones. These findings indicate, for the first time, that adipose tissue leptin is modulated by daylength independently of food intake, body fatness and gonadal activity. Furthermore, plasma non-esterified fatty acids of underfed ewes subjected to short days were more elevated than for underfed ewes on long days. On the other hand, refed ewes placed under long daylength tended to have a higher adipose tissue lipogenic activity than refed ewes on short days. We propose that these adaptations of leptin production and lipogenic activity with long photoperiod are of physiological significance for body fat deposition, which naturally occurs during long days when food is abundant. Conversely, a low leptin level during short days may enhance the sensitivity to food deprivation during the natural reproductive season, where any food shortage would decrease plasma leptin under a threshold critical for reproduction. Furthermore, in this situation, the observed enhanced ability to mobilize body fat may be related to the necessity to cope with energy shortage. (C) Inra/Elsevier, Paris.