Leptin in horses: Relationship with body condition, nutritional status and reproduction

In animal agriculture in recent years, much interest has been devoted to the study of the interactions that exist among the nutrition and reproductive processes. As we examine an animal's ability to reproduce during times when resources are plentiful and suppress reproductive capability during times when resources are sparse, it becomes more critical that we understand both the metabolic and endocrine parameters that influence these processes. In a production setting, horsemen have identified certain horses as “easy keepers” (horses that require fewer nutrients to maintain a healthy body condition) or “hard keepers” (horses that require more than normal quantities of nutrients to maintain a healthy body condition). The underlying metabolic and endocrine components that are involved in these types of maladies are unknown. We have chosen to address both of these areas by examining the endocrine components that are involved in the regulation of appetite and that also influence reproduction. We have focused our research efforts on the protein hormone leptin, which is secreted mainly by white adipose tissue.; We found that concentrations of leptin were highly correlated with body condition scores, such that as horses and ponies increased in body condition score, peripheral concentrations of leptin also increased. When fat ponies were nutritionally restricted and thin ponies were supplemented, we found that concentrations of leptin in the fat-restricted mares decreased over time and concentrations of leptin in the thin-supplemented mares increased over time.; We also observed that concentrations of leptin were greatest in horses during the nighttime. In the same studies, we noted that ponies fasted for 48 hours had significantly lower peripheral concentrations of leptin than ponies that were fed. Following our observations that day time versus night time differences in peripheral concentrations of leptin existed, we set out to determine if the diurnal secretion of melatonin was involved in regulating the circadian pattern of secretion of leptin. When we implanted ponies with melatonin, peripheral concentrations of leptin were not affected and the diurnal secretion of leptin was not different between the melatonin implanted and control animals.; Finally, when we attempted to determine if a high fat diet (16% fat) would significantly alter peripheral concentrations of leptin, we found no significant relationship between the high fat diet and leptin.