Leptin is a hormone mainly expressed by adipose tissue (AT) in mammals and/or by liver in birds, at levels that differ according to genetic, physiological, nutritional and environmental factors, as well as hormone treatments (Houseknecht et al., 1998; Barb et al., 2002; Chilliard et al., 2002; Taouis et al., 2002).

AT leptin mRNA level was higher in sheep and goat subcutaneous than visceral tissues, and the opposite was observed in cattle; it was higher in fat than in lean selection line in sheep. Leptin gene expression was decreased by undernutrition and increased by refeeding in AT of cattle and sheep, and decreased during fasting but not after feed restriction in AT of pig. In the chicken, both AT and liver leptin mRNA were decreased by undernutrition. In lactating goats, the replacement of part of the dietary concentrates by soybeans did not change leptin expression. The injection of NPY in sheep, as well as growth hormone (GH) treatment of growing sheep and cattle increased AT leptin mRNA. In the pig, AT leptin mRNA decreased after GH administration and estrogen-induced leptin mRNA increased with age and adiposity. In the chicken, estrogen treatment decreased AT and liver leptin mRNA. In vitro, insulin and glucocorticoïds increased AT leptin mRNA in cattle and pig, and leptin production in sheep. However, these effects on leptin gene expression were inhibited by GH. In the chicken, liver but not AT leptin mRNA was either increased by insulin, glucocorticoïds and GH or decreased by glucagon treatments. Long daylength increased AT lipogenic activities and leptin mRNA, as well as plasma leptin in sheep. In sheep, AT leptin mRNA increased from prebreeding to mid-pregnancy and declined to prebreeding levels during early lactation (Ehrhardt et al., 2001).