Adipose tissue and the reproductive axis: biological aspects.

The discovery of leptin has clearly demonstrated a relationship between body fat and the neuroendocrine axis since leptin influences appetite and the reproductive axis. Since adipose tissue is a primary source of leptin, adipose tissue is no longer considered as simply a depot to store fat. Recent findings demonstrate that numerous other genes, i.e. neuropeptides, interleukins and other cytokines and biologically active substances such as leptin and insulin-like growth factors I and II, are also produced by adipose tissue, which could influence appetite and the reproductive axis. Targets of leptin in the hypothalamus include neuropeptide Y, proopiomelanocortin and kisspeptin. Transsynaptic connection of hypothalamic neurons to porcine adipose tissue may result in a direct influence of the hypothalamus on adipose tissue function. Nutritional signals such as leptin are detected by the central nervous system and translated by the neuroendocrine system into signals which ultimately regulates luteinizing hormone secretion. Furthermore, leptin directly affects gonadotropin-releasing hormone release from the hypothalamus, luteinizing hormone from the pituitary gland and ovarian follicular steroidogenesis. Although leptin is identified as a putative signal that links metabolic status and neuroendocrine control of reproduction, other adipocyte protein products may play key roles in regulating the reproductive axisin the pig.