Leptin and its role in the central regulation of reproduction in cattle.

Leptin, a 16kDa product of the adipose obese (ob) gene, has been shown to contribute to the regulation of energy metabolism, feeding behavior, and reproduction in several monogastric species, including humans. Recent reports have provided evidence that the leptin gene is functionally relevant in cattle and sheep, and may contribute to an array of important reproductive events, including puberty. Leptin gene expression and circulating leptin increase markedly during sexual maturation in heifers reaching puberty during late spring or early summer. In addition, serum leptin concentrations increased by over 30% from early winter to the summer solstice in mature cows, and also increased with significant changes in adiposity. However, only limited changes in circulating leptin have been observed during the estrous cycle. Short-term fasting of growing peripubertal heifers causes marked reductions in leptin gene expression and circulating leptin, concomitant with declines in LH pulse frequency, and serum concentrations of insulin and IGF-1. Although short-term fasting of mature cows in excellent body condition is without effects on LH pulse frequency, it has remarkably similar metabolic effects to those observed in heifers. Moreover, ICV administration of recombinant oleptin resulted in a marked hypersecretion of LH in fasted cows, and in vitro studies using both hypothalamic and anterior pituitary explants have provided evidence that this effect is at the pituitary level. Paradoxically, ICV administration of oleptin normalized circulating insulin in fasted cows but hleptin was without effect on insulin in estradiol-implanted wethers. Collectively, work in cattle and sheep indicates that leptin can modulate both the hypothalamic-pituitary axis and endocrine pancreas under defined nutritional conditions. Additional work to more fully characterize these roles is clearly warranted and could lead to the development of novel strategies for modifying reproductive potential in food-producing species.