Leptin and the adaptations of lactation in rodents and ruminants.

Lactation markedly increases nutrient requirements in both rodents and ruminants. This is met mostly by increased food intake, but there are also adaptations to increase metabolic efficiency. Despite such changes, lactating animals usually experience periods of negative energy balance. This is not due to a physical constraint on food intake, at least in the rat. Leptin, a hormone secreted by adipocytes, plays an important role in the regulation of appetite and energy balance. During lactation, serum leptin concentration is decreased in both rodents and ruminants, and the nocturnal rise in concentration is lost in rats. Hypoleptinaemia in lactation is primarily a result of negative energy balance. There is also increased clearance of serum leptin, and the attenuation of the nocturnal rise in leptin in rats is at least partly due to the suckling stimulus. Hypoleptinaemia is not the major factor driving hyperphagia in lactating rats, but it probably facilitates the increased food intake. Leptin may play a more important role in this respect in lactating ruminants. Leptin is probably involved in other adaptations that increase metabolic efficiency during lactation. The ability of hypothalamic neuropeptides to respond to leptin does not appear to be altered by lactation in either rodents or ruminants. The reason why lactating animals do not respond to hypoleptinaemia with a further increase in appetite, thereby achieving energy balance, appears to be due to a failure to respond to changes in neuropeptides which mediate the effects of leptin.